Igneous Petrology

Igneous Petrology  / Origins and Differentiation of Magma

source http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=65EA2ABEB61A2F4CDFF3FE48119B4014?doi=10.1.1.692.4446&rep=rep1&type=pdf
            https://isru.msfc.nasa.gov/igneous-rocks.html
            lecture notes of Prof. Stephen A. Nelson Tulane University
            http://jgs.geoscienceworld.org/content/148/5/825
            IUGS Subcommission on the Systematics of Igneous Rocks

 Igneous Petrology
    Continental Igneous Rocks
    Coastal Igneous Rocks
    Oceanic Igneous Rocks
    Igneous Rock Classification

Origins and Differentiation of Magma
    Origins of Magma
    Magmatic Differentiation

Continental Igneous Rocks

A wide variety of igneous rocks occur in the continental lithosphere, a reflection of its heterogeneous nature compared to oceanic lithosphere.
Because the continents are not subducted and are subject to uplift and erosion, older plutonic rocks are both preserved and accessible to study.

Granitic Rocks

1. Classification of Granitic rocks based on visible pewrcentages  of "Q" quartz / "A" alkalai feldspar  / "P" plagioclase feldspar

Note: true granites have between 10% and 65% of their feldspars as plagioclase, and between 20% and 60% quartz. 
All rocks will likely contain mafic minerals such as biotite, hornblende, and perhaps pyroxenes, along with opaque oxide minerals.

"S", "I" and "A" Type Granitic Rocks
Mechanisms by which large volumes of granitic magma could be produced are:

• Anatexis of metasedimentary/sedimentary rocks to form S-type granitic magmas.
• Anatexis of young crustal basic meta-igneous rocks to form I-type granitic magmas.
• Melting/Assimilation of lower crustal rocks by mantle-derived basic magmas.
• Crystal fractionation/Assimilation of basaltic and andesitic magmas.
• Granitization, wherein high grade metamorphism bordering on melting converts rocks into those that appear texturally and mineralogically similar to granitic rocks.

    S-type Granites.
S-type granites are thought to originate by melting (or perhaps by ultrametamorphism) of a pre-exiting metasedimentary or sedimentary source rock.
These are peraluminous granites [i.e. they have molecular
Al₂O₃ (Na₂O + K₂O)].
Mineralogically this chemical condition is expressed by the presence of a peraluminous mineral, commonly muscovite, although other minerals such as the Al₂SiO₅ minerals and corundum may also occur. Since many sedimentary rocks are enriched in Al₂O₃  as a result of their constituents having been exposed to chemical weathering near the Earth's surface (particularly rocks such as shales that contain clay minerals), melting of these rocks is a simple way of achieving the peraluminous condition.

Many S-type granitoids are found in the deeply eroded cores of fold-thrust mountain belts formed as a result of continent-continent collisions, such as the Himalayas and the Appalachians, and would thus be considered orogenic granites.

    I-type Granites.
I-type granites are granites considered to have formed by melting of an original igneous type source.
These are generally metaluminous granites, expressed mineralogically by the absence of peraluminous minerals, and the absence of peralkaline minerals, as discussed below.Instead these rocks contain biotite and hornblende as the major mafic minerals.
Mesozoic or younger examples of I-type granites are found along continental margins such as the Sierra Nevada batholith of California and Nevada, and the Idaho batholith of Montana.  In these regions the plutonism may have been related to active subduction beneath the western U.S. in Mesozoic times.I-type granites are also found in the Himalayas, related to continent-continent collisions.
Plutonic suites that were emplaced in convergent continental margin settings, show many of the same characteristics as the calc-alkaline volcanic suite that likely erupted on the surface above.  The suites include gabbros, diorites, quartz monzonites, granodiorites, and granites.  They show only mild to no Fe-enrichment, similar to calc-alkaline volcanic rocks, and a range of isotopic compositions similar to the associated volcanic rocks. Nearly all are I-type granitoids.
The rocks were emplaced during the Mesozoic Era.  Exposed rocks are generally older toward the east and southeast.  Kistler and Peterman showed that the Sr isotopic ratios vary across the batholith in a systematic way.  The younger rocks in the western portion of the batholith are mostly quartz diorites with Sr isotopic ratios less than 0.704, ratios expected from melting of the mantle or young crustal rocks. Plutons farther east are mostly quartz monzonites and granodiorites with ratios increasing along with age of the plutons toward the east and southeast.  One interpretation of the data is that the older rocks contain a higher proportion of older crustal material than the younger plutonic bodies.

    A-type Granites.
A-type granites are generally peralkaline in composition [molecular (Na₂O + K₂O) > Al₂O₃].
Minerals like the sodic amphiboles riebeckite and arfvedsonite and the sodic pyroxene aegerine are commonly found in these rocks.  In addition, they tend to be relatively Fe-rich and thus fayalitic olivine may sometimes occur.

These are considered anorogenic granites because they are generally found in areas that have not undergone mountain building events.Instead, they appear to be related to continental rifting events wherein continental lithosphere is thinned as a result of upwelling asthenosphere.The upwelling raises the geothermal gradient resulting in melting.Young peralkaline granites are found in the Basin and Range Province of the western U.S., and older examples are found throughout southeastern Australia

Depth of Emplacement.
Because the conditions under which a magma cools can play an important role in the texture and contact relationships observed in the final rock, plutons can be characterized by the depth at which they were emplaced.  This is because depth, to a large extent controls the contrast in temperature between the magma and its surroundings.
    Catazonal Plutons
The catazone is the deepest level of emplacement, usually considered to be at depths greater than about 11 km.  In such an environment there is a low contrast in temperature between the magma and the surrounding country rock.   The country rock itself is generally high grade metamorphic rock.  Contacts between the plutons and the country rock are concordant (meaning the contacts run parallel to structures such as foliation in the surrounding country rock) and often gradational. The plutons themselves often show a foliation that is concordant with that in the surrounding metamorphic rocks.  Migmatites (small pods of what appears to have been melted rock surrounded by and grading into metamorphic rocks) are common.  Some catazonal plutons appear to have formed by either melting in place or by ultrametamorphism that grades into actually melting.  Others appear to have intruded into ductile crustal rocks.  Most, but not all, Catazonal plutons are S-type granitoids.
    Mesozonal Plutons
The metazone occurs at intermediate crustal depths, likely between 8 and 12 km.  The plutonic rocks are more easily distinguished from the surrounding metamorphic rocks.  Contacts are both sharp and discordant (cutting across structures in the country rock), and gradational and concordant like in the catazone.  Angular blocks of the surrounding country rock commonly occur within the plutons near their contacts with the country rock.  The plutons generally lack foliation and are often chemically and mineralogically zoned.
    Epizonal Plutons
The epizone is the shallowest zone of emplacement, probably within a few kilometers of the surface. In such an environment there is a large contrast between the temperatures of the magma and the country rock. The country rock is commonly metamorphosed, but the metamorphism is contact metamorphism produced by the heat of the intrusion.  Contacts between the plutons and surrounding country rock are sharp and discordant, indicating intrusion into brittle and cooler crust. The margins of the plutons often contain abundant xenoliths of the country rock.
    Pegmatities
Pegmatites are very coarse grained felsic rocks that occur as dikes or pod-like segregations both within granitic plutons and intruded into the surrounding country rock.  They appear to form during the late stages of crystallization which leaves H₂O-rich fluids that readily dissolve high concentrations of alkalies and silica.  Thus, most pegmatites are similar to granites and contain the minerals alkali feldspar and quartz.  But other chemical constituents that become concentrated in the residual liquid, like B, Be, and Li are sometimes enriched  pegmatites.  This leads to crystallization of minerals that are somewhat more rare, such as:

• tourmaline, [(Na,Ca)(Mg,Fe,Mn,Li,Al)₃(Al,Fe⁺³)₆Si₆O₁₈(BO₃)₂(OH)₄], beryl [Be₃Al₂Si₆O₁₈],
• lepidolite [K₂(Li,Al)_5-6Si_6-7Al_2-1(OH,F)₄, and
• spodumene [LiAl₂Si₂O₆],

which are sometimes found.

Continental Rhyolites
Rhyolites are much more common and voluminous on the continents than in the ocean basins.  They range from small domes and lava flows to much larger centers that have erupted volumes measured in 100s of km³ and emplaced as pyroclastic material.  Most of the preserved volume is represented as pyroclastic flow deposits, often termed "ash flow tuffs" or "ignimbrites.  Large quantities of these deposits were erupted during the middle Tertiary in the western United States, northern Mexico, throughout Central America, and on the western slopes of the Andes mountains.  The composition of these deposits is usually metaluminous although peralkaline varieties are known.  None are peraluminous in composition.  Although the recent examples occur near continental margins. Most seem to be associated with episodes of continental extension, such as in Basin and Range Province of the Western U.S. and Mexico.



Continental Flood Basalts
Like the large submarine plateaus discussed in our large volumes of basaltic magma have erupted on the continents at various times in Earth history

Province	Age	Original Area Covered (km2)	Types of Basalts %
Qtz -Thol.	Oliv. Thol.	Alk. Bas.
Lake Superior	Precambrian	125,000	42	51	7
Siberia	Permo-Triassic	2,500,000	28	69	3
Karoo, S. Africa	Jurassic	2,000,000	57	37	6
Paraná, Brazil	Cretaceous	2,000,000	72	28	0
Deccan, India	Eocene	500,000	55	35	10
Columbia River	Mid-Miocene	163,000	30	70	-

    Chemical Composition
Although each flood basalt province differs somewhat in the composition of magmas erupted, most provinces have erupted tholeiitic basalts. With the exception of a few early erupted picrites in some provinces, the tholeiitic basalts tend to have lower concentrations of MgO (5 - 8%) than would be expected from melts that have come directly from the mantle without having suffered crystal fractionation. Thus, despite their large volume, they are differentiated magmas that are similar in many respects to MORBs. Still, they show incompatible trace element concentrations more similar to EMORBs, and have ⁸⁶Sr/⁸⁷Sr and ¹⁴³Nd/¹⁴⁴Nd ratios that extend from the OIB field toward and overlapping with continental crust.  This latter feature indicates that they have likely suffered some crustal contamination.

Continental Rift Valleys
Continental Rift valleys are linear zones of extension within continental crust.Some of these extensional zones may be eventually become zones along which the continents break apart to form a new ocean basin, however, there are many examples where such break-ups have failed.
The East African Rift which extends from Syria in the north to Mozambique in the south has been active throughout the Cenozoic.  During the initial stages of rifting fissure eruptions produced large volumes of basalt and siliceous ignimbrites.  During the late Miocene and Pliocene these eruptions became more focused, and produced shield volcanoes consisting of basanites, rhyolites and phonolites.  In Plio-Pleistocene times rhyolites were erupted along the main axis of the rift, while basalts continued to be erupted on the plateaus adjacent to the rift.  Quaternary volcanoes along the axis of the central rift zones, in Kenya and Tanzania consist of phonolite, trachyte, or peralkaline rhyolite.  This province illustrates the wide variety of unusual rock types found in continental rifting settings.  Note, however, that parts of the rift along the Red Sea and Gulf of Aden have evolved to oceanic ridges and produce MORBs to form new seafloor.

Coastal Igneous Rocks

The convergent plate margins are the most intense areas of active magmatism above sea level at the present time. Most of world's violent volcanic activity occurs along these zones. In addition, much magmatism also has resulted in (and probably is resulting at present) significant additions to the crust in the form of plutonic igneous rocks.



The "Pacific Ring of Fire" surrounds the Pacific Ocean basin and extends into the Indian Ocean and Caribbean Sea. Active subduction is taking place, along these convergent plate boundaries, as evidenced by the zone of earthquakes, called a Benioff Zone, that begins near the oceanic trenches and extends to deeper levels in the direction of plate motion. Earthquake focal depths reach a maximum of about 700 km in some areas. Volcanism occurs on the upper plate about 100 to 200 km above the Benioff Zone. For this reason, volcanism in these areas is often referred to as subduction-related volcanism.

Two situations occur.

1. In areas where oceanic lithosphere is subducted beneath oceanic lithosphere the volcanism is expressed on the surface as chains of islands referred to as island arcs.  These include the Caribbean Arc, the Aleutian Arc, the Kurile Kamachatka Arc, Japan, the Philippines, the South Sandwich Arc, The Indonesian Arc, the Marianas, Fiji, and Solomon Islands.
   
2. In areas where oceanic lithosphere is subducted beneath continental lithosphere volcanism occurs as chains of volcanoes near the continental margin, referred to as a continental margin arc. These include the Andes Mountains, Central American Volcanic Belt, Mexican Volcanic Belt, the Cascades, the part of the Aleutian arc on Continental crust, and the North Island of New Zealand.
   
   

Within these volcanic arcs the most imposing, and therefore most recognized by early workers, features of the landscape are large stratovolcanoes. These  usually consist of predominantly  andesitic lava flows and interbedded pyroclastic material.  But, in the late stages of volcanism more silicic lavas and pyroclastics like dacites and rhyolites are common.



Many of these stratovolcanoes pass through a stage where their upper portions collapse downward to form a caldera.
These caldera forming events are usually associated with explosive eruptions that emit silicic pyroclastic material in large-volume eruptions.
It is the sudden evacuation of underlying magma chambers that appears to result in the collapse of the volcanoes to form the calderas.
The imposing presence of these large mostly andesitic stratovolcanoes led to an early widespread perception among petrologists that basalts were rare or absent in these environments.In recent years, however, it has become more evident that basalts are widespread, but do not commonly erupt from the stratovolcanoes. 
Instead, they are found in areas surrounding the stratovolcanoes where they erupt to form cinder cones and associated lava flows.
One explanation for this distribution is that the magma chambers underlying the stratovolcanoes intercept the basaltic magmas before they reach the surface and allow the basalts to differentiate to more siliceous compositions before they are erupted. 
Basaltic magmas that are not intercepted by the magma chambers can make it to the surface to erupt in the surrounding areas.

Petrography
Probably the most distinguishing feature of subduction-related volcanic rocks is their usually porphyritic nature, usually showing glomeroporphyritic clusters of phenocrysts. (A textural term used to describe igneous rocks that contain clusters of phenocrysts, which are large crystals in a finer-grained matrix or groundmass)


Basalts commonly contain phenocrysts of olivine, augite, and plagioclase.
Andesites and dacites commonly have phenocrysts of plagioclase, augite, and hypersthene, and some contain hornblende.




The most characteristic feature of the andesites and dacites is the predominance of fairly calcic plagioclase phenocrysts that show complex oscillatory zoning. 
Such zoning has been ascribed to various factors, including:

• Kinetic factors during crystal growth.  As one zone of the crystal is precipitated the liquid immediately surrounding the crystal becomes depleted in the components necessary for further growth of the same composition.  So, a new composition is precipitated until diffusion has had time to renourish the surrounding liquid in the components necessary for the equilibrium composition to form
• Cycling through a chemically zoned magma chamber during convection. As crystals grow, they are carried in convection cells to warmer and cooler parts of the magma chamber.  Some zones are partially dissolved and new compositions are precipitated that are more in equilibrium with the chemical compositions, pressures, and temperatures present in the part of the magma chamber into which the crystal is transported.
• Magma mixing As magmas mix the chemical compositions of liquids and temperatures change during the mixing process.  This could result in dissolution of some zones, and precipitation of zones with varying chemical composition.

Rhyolites occur as both obsidians and as porphyritic lavas and pyroclastics. Phenocrysts present in rhyolites include plagioclase, sanidine, quartz, orthopyroxene, hornblende, and biotite.
In addition to these features, petrographic evidence for magma mixing is sometimes present in the rocks, including disequilibrium mineral assemblages, reversed zoning etc. 
Xenoliths of crustal rocks are also sometimes found, particularly in continental margin arcs, suggesting that assimilation or partial assimilation of the crust could be an important process in this environment.

Possible explanation  Calc-alkaline Suite of rocks that are commonly associated with subduction...

• Subduction carries oceanic crust and sediment to depth.  As the pressure and temperature rise, the MORB crust and sediment undergo metamorphism that releases hydrous fluids.
  
• These hydrous fluids carry with them high concentrations of LILE and REE, but leave behind the relatively insoluble HFSE.  They also carry the isotopic signature of the basaltic crust sediment mixture that released the fluids, and thus have higher  ⁸⁷Sr/⁸⁶Sr ratios and lower ¹⁴³Nd/¹⁴⁴Nd ratios, reflecting the isotopic composition of the subducted material.  If the sediments are young, they may contribute ¹⁰Be to these fluids.
  
• The fluids act to metasomatize the overlying mantle wedge, enriching it in LILE, REE, B, ⁸⁷Sr/⁸⁶Sr, and possibly ¹⁰Be and lowering the ¹⁴³Nd/¹⁴⁴Nd ratio of this mantle.
  
• Adding H₂O to the mantle wedge lowers the solidus temperature allowing for partial melting of this metasomatized mantle and generating hydrous basaltic magmas.
  
• This hydrous basaltic magmas become saturated with water at crustal depths and differentiate by crystal fractionation, possibly accompanied by contamination of crustal material, to generate the andesites, dacites, and rhyolites of the calc-alkaline suite.
  

Oceanic Igneous Rocks

The ocean basins cover the largest area of the Earth's surface.Because of plate tectonics, however, most oceanic lithosphere eventually is subducted and thus the only existing oceanic lithosphere is younger than about Jurassic in age and occurs at locations farthest from the oceanic spreading centers.Except in areas where magmatism is intense enough to build volcanic structures above sea level, most of the oceanic magmatism is difficult to access.Samples of rocks can be obtained from drilling, dredging, and expeditions of small submarines to the ocean floor.Numerous samples have been recovered and studied using these methods.Most of the magmatism is basaltic. Still, few drilling expeditions have penetrated through the sediment cover and into the oceanic lithosphere. Nevertheless, we have a fairly good understanding of the structure of the oceanic lithosphere from seismic studies and ophiolites.
Here we will first look at ophiolites, then discuss basaltic magmatism in general, and then discuss the various oceanic environments where magmatic activity has occurred.
Ophiolites
An ophiolite is a sequence of rocks that appears to represent a setion through oceanic crust. Ophiolites occur in areas where obduction (the opposite of subduction) has pushed a section of oceanic lithosphere onto continental crust. During this process, most ophiolite sequences have been highly deformed and hydrothermally altered. Nevertheless, it is often possible to look through the deformation and alteration and learn something of the structure of oceanic lithosphere.
An idealized ophiolite sequence shows an upper layer consisting of deep sea sediments (limestones, cherts, and shales), overlying a layer of pillow basalts. Pillow basalts have a structure consisting of overlapping pillow-shaped pods of basalt.  Such pillow structure is typical of lavas erupted under water. The pillow basalts overly a layer consisting of numerous dikes, some of which were feeder dikes for the overlying basalts.  Beneath the sheeted dike complex are gabbros that likely represent the magma chambers for the basalts.  The upper gabbros are massive while the lower gabbros show layering that might have resulted from crystal settling.


At the base of the layered gabbros, there is a sharp increase in the density of the rocks, and the composition changes to ultramafic rocks. This sharp change in density is correlated with what would be expected at the base of the crust, and is thus referred to as the petrologic moho. At the top of the ultramafic sequence the rock type is harzburgite (Ol + Opx), a rock type expected to be the residual left from partially melting peridotite. The base of the ultramafic layer is composed of peridotite. Because most ophiolites have been hydrothermally altered, most of the mafic rocks have been altered to serpentinite. Note that ophiolite means "snake rock"
Volcanic Settings
Volcanism occurs at three different settings on the ocean floor.

1. Oceanic Ridges - these are the oceanic spreading centers where a relatively small range of chemical compositions of basalts are erupted to form the basaltic layer of the oceanic crust. This chemical type of basalt is referred to as Mid Ocean Ridge Basalts (MORBs).In some areas, particularly Iceland, where there has been a large outpouring of basalts on the oceanic ridge, basalts called Enriched Mid Ocean Ridge Basalts (EMORBs)have been erupted.
2. Oceanic Islands - these are islands in the ocean basins that generally occur away from plate boundaries, and are often associated with hot spots, as discussed previously.  A wide variety of rocks occur in these islands, not all are basaltic, but those that aren't appear to be related to the basaltic magmas.  In general these rocks are referred to as Oceanic Island Basalts (OIBs).
3. Large Igneous Provinces (LIPs) - these are massive outpourings of mostly basaltic lavas that have built large submarine plateaus.  Most are mid-Cretaceous in age. They are not well studied, but most have compositions similar to OIBs, and some may have once had oceanic islands on top, but most of these have since been removed by erosion.

Thus, most oceanic magmatism is basaltic, so we will first discuss basaltic magmas in general.

Basalts
On a chemical basis, basalts can be classified into three broad groups based on the degree of silica saturation. This is best seen by first casting the analyses into molecular CIPW norms (the same thing as CIPW norms except the results are converted to mole % rather than weight %). On this basis, most basalts consist predominantly of the normative minerals - Olivine, Clinopyroxene, Plagioclase, and Quartz or Nepheline. These minerals are in the 4 component normative system Ol-Ne-Cpx-Qtz, shown here as a tetrahedron.In the tetrahedron, plagioclase plots between Ne and Qtz, and Opx plots between Ol and Qtz. The basalt tetrahedron can be divided

• The plane Cpx-Plag-Opx is the critical plane of silica saturation.Compositions that contain Qtz in their norms plot in the volume Cpx-Plag- Opx-Qtz, and would be considered silica oversaturated.Basalts that plot in this volume are called Quartz Tholeiites

• The plane Ol - Plag - Cpx  is the critical plane of silica undersaturation. Normative compositions in the volume between the critical planes of silica undersaturation and silica saturation are silica saturated compositions (the volume Ol - Plag - Cpx - Opx).  Silica saturated basalts are called Olivine Tholeiites
• Normative compositions that contain no Qtz or Opx, but contain Ne are silica undersaturated (the volume Ne-Plag-Cpx-Ol). Alkali Basalts, Basanites, Nephelinites, and other silica undersaturated compositions lie in the silica undersaturated volume.

Note that tholeiitic basalts are basalts that show a reaction relationship of olivine to liquid which produces a low-Ca pyroxene like pigeonite or Opx.Both olivine tholeiites and quartz tholeiites would show such a relationship and would eventually precipitate either Opx or pigeonite.
The critical plane of silica undersaturation appears to be a thermal divide at low pressure.This means that compositions on either side of the plane cannot produce liquids on the other side of the plane by crystal fractionation.To see this, look at the front two faces of the basalt tetrahedron.  These are in the three component systems Ol-Cpx-Qtz and Ol-Cpx-Ne.

Mid Ocean Ridge Basalts (MORBs)
Occurrence
The Oceanic Ridges are probably the largest producers of magma on Earth. Yet, much of this magmatism goes unnoticed because, with the exception of Iceland, it all takes place below the oceans. This magmatism is responsible for producing oceanic crust at divergent plate boundaries.
Magma is both erupted and intruded near the central depressions that form the oceanic ridges.  Thus, both basalts and gabbros are produced.  But, little is known of the gabbros since they are rarely exposed and most oceanic lithosphere eventually is subducted.The main melting mechanism is likely decompression melting as rising convection cells move upward through the mantle beneath the ridges. At most oceanic ridges the basalts that are erupted are tholeiitic basalts sometimes referred to as NMORBs (normal MORBs)


At the oceanic ridges, the basalts erupted range in composition from Olivine tholeiites to Quartz tholeiites.The compositions are by and large restricted to basalt, i.e. less than about 52% SiO₂. The diagram shown here is called an AFM diagram. It is a triangular variation diagram that plots total alkalies at the A corner, total iron at the F corner, and MgO at the M corner.MORBs show a restricted range of compositions that fall along a linear trend extending away from the compositions of Mg-rich pyroxenes and olivines.
his is the trend that would expected from fractional crystallization involving the removal of early crystallizing olivines and pyroxenes from a tholeiitic basaltic liquid.  Note that the trend is often referred to as an Fe-enrichment trend.

Some thoughts on the origin of MORBs.

• Melting is likely caused by decompression of the mantle as it rises beneath the oceanic ridges as a result of convection.
  
• Primary MORB magmas appear to be produced by partial melting of the mantle at pressures between 15 and 20 kb.
  
• Most MORBs erupted are not primary melts of the mantle, but instead appear to have suffered olivine fractionation.
  
• The small range in composition of MORBs can be explained by crystal fractionation of Olivine + Plagioclase + Cpx at low pressures near the surface.
  
• MORBs appear to be the result of melting of an incompatible element depleted mantle, both in terms of their incompatible trace element compositions and isotopic ratios of Sr and Nd.

Ocean Island Basalts (OIBs)
Oceanic islands are, in general, islands that do not occur along the divergent or convergent plate boundaries in the ocean basins. Nevertheless, EMORBs, such as those that occur in Iceland, as well as the Alkalic basalts of Iceland have much in common with magmas erupted in the oceanic islands. In the Atlantic Ocean, which is a slow-spreading oceanic basin, as well as in the Galapagos Islands of the eastern Pacific Ocean, some of the islands occur close to oceanic ridge spreading centers.
In all cases we must keep in mind that the parts of these islands that are accessible for sampling represent only a fraction of the mass of the volcanic structures which rise from the ocean floor at depths up to 10,000 m. Thus, as with the ocean ridge volcanic rocks, there is a potential sampling problem.
Here we discuss not only the magmatism that has occurred recently at Oceanic Islands, but also the magmatism that produced massive submarine plateaus on the sea floor during the Cretaceous. The latter are often referred to as Large Igneous Provinces (LIPs).
Oceanic Islands
Most oceanic islands appear to be related to ascending plumes of hot mantle. These plumes must be relative narrow features because they appear to operate independent of the main convection cells that ascend beneath the oceanic ridges and descend at subduction zones. Still, in places like Iceland on the ocean ridge, magma production rates are high, and compositions of rocks are similar to those found in oceanic islands. So Iceland could also be considered an oceanic island.
If these rising plumes of hot mantle remain stationary in their positions in the mantle, they produce hot spots, as discussed previously. Hot spots are most recognizable when they occur beneath plates that move with higher velocities. Beneath faster moving plates, like the Pacific Plate, this results in linear chains of islands.
At the position directly over the hotspot,  rising mantle melts to produce magma that erupts on the seafloor, eventually building a volcanic island directly over the hot spot.  As the lithospheric plate moves over the hot spot the volcano eventually is cut off from its source of magma, and becomes extinct, and a new volcano forms on the plate at the location directly above the hot spot.


The volcanoes that have moved away from the hot spot eventually begin to erode until their elevations are reduced below sea level. At this point they are called seamounts.
Such linear chains of islands and sea mounts are most evident in the Pacific ocean. The largest of these is the Hawaiian - Emperor chain. The hot spot that produced this chain is currently located under the position of the big island of Hawaii, which has the only currently active volcanoes in the chain. The bend in the Hawaiian-Emperor chain must have resulted from a change in the direction of plate motion. Volcanic rocks dredged from the sea floor at the location of this bend are about 40 million years old. Thus, prior to 40 million years ago the Pacific Plate was moving in a more northerly direction. The most northerly seamount is dated at about 60 million years.Seamounts older than 60 million have apparently been subducted.



The reason such island/seamount chains are not as evident in the other oceans is because the plate velocity is lower and volcanoes tend to remain over the hot spots for longer periods of time, building elongated groups of islands rather than linear chains.
Large Igneous Provinces (LIP)s
Large igneous provinces are areas where large volumes of magma have been added to the Earth's crust over relatively short periods of time. Although here we discuss these in terms of the ocean basins, it should be noted that they also include the continental areas where large volumes of magma have been erupted as flood basalts. Eruption of large amounts of magma on the surface of the Earth can have drastic consequences.
For example:

• The magmas may release large amounts of CO₂ gas into the atmosphere and force global warming.
  
• Large amounts of magma erupted on the seafloor change the volume of the ocean basins causing higher sea level and flooding large areas of the continents.
  

The mid Cretaceous Period was a time of higher than normal global temperatures and high stands of the oceans.Eruption of magma on the ocean floor at this time might have been the cause of these conditions.  Evidence is preserved on the sea floor in the form of large submarine plateaus that were emplaced during this time period.
In the Pacific Ocean, much of the oceanic lithosphere east of the position of the ridge 80 million years ago has been subducted.Thus, if the submarine plateaus formed at the ridge, then it would be expected that half of each plateau became separated at the ridge and have since been subducted.These probable plateaus are shown in the map, and if they were present would double the original size of the plateau.Thus, for example the largest of the plateaus is the Ontong-Java Plateau, now located in the southeastern Pacific, with a volume of about 50 million km³.But if the other half had been present, the total volume of magma erupted over a 25 million year period would have been over 100 million km³The few studies that have looked at the rocks in these submarine plateaus suggest that their compositions are similar to EMORBs and OIBs.
Unlike the ocean ridges, which have a rather limited range of rock compositions, the oceanic islands have produced a broader range. Basalts are still predominant, but other compositions are part of the series, and the types of rocks produced are variable from one island to the next. The table below shows that variety of rock types found at different oceanic islands. Some produce tholeiitic rocks similar to EMORBs and others produce alkalic basalts that are saturated to undersaturated with respect to silica.

Oceanic Island Rock Suites
Island or Group	Rock Types
Ascension	Oliv. Tholeiite (dominant) + Hawaiite + Mugearite + Trachyte + Peralk. Rhyolite
Azores	Alk. basalt + Hawaiite + Trachyte
Fernando de Noronha	Alk. Basalt + Nephelinite + Trachyte + Alkali Basalt + Trachyte + Phonolite
St. Helena	Alk. Basalt + Mugearite + Hawaiite + Trachyte + Phonolite
Trinadade	Nephelinite + Phonolite (dominant)
Tristan de Cunha	Alk. Basalt + Trachybasalt (dominant) + Trachyte
Gough	Alk. Basalt + Ol Tholeiite + Hawaiite + Trachyte
Réunion	Ol Tholeiite (dominant) + Mugearite
Mauritius	Alk. Basalt (dominant) + Mugearite + Phonolitic Trachyte
Hawaii	Tholeiite (dominant) + Alkali Basalt + Hawaiite + Mugearite + Trachyte
Tahiti	Alk. Basalt + Mugearite + Hawaiite + Trachyte
Galapagos	Tholeiite + Alk. Basalt + Icelandite (minor) + Qtz Trachyte (minor)
Jan Mayen	Alk. Basalt (dominant) + Trachyte

Some thoughts on the origin of OIBs.

• Melting is caused by decompression of the mantle as it rises in narrow plumes beneath the oceanic islands.
  
• Melting probably takes place at relatively low pressure to produce silica saturated tholeiitic magmas, but at high pressure to produce the silica undersaturated varieties such alkali basalts, basanites and nephelinites..
  
• These basic magmas evolve by crystal fractionation to produce a wide variety of other rock types including hawaiites, mugearites, trachytes, and rhyolites.  The more silica undersaturated basanites would fractionate to produce nephelinites and phonolites.
  
• Lower degrees of melting produce the most incompatible element rich magmas, like basanites, while successively higher degrees of melting could produce the alkali basalts and tholeiitic basalts.
  
• OIBs (including EMORBs) appear to be the result of melting of an incompatible element enriched mantle, both in terms of their incompatible trace element compositions and isotopic ratios of Sr and Nd.
  

Igneous Rock Classification

Based on the IUGS Subcommission on the Systematics of Igneous Rocks
  The main QAPF classification for plutonic and volcanic rocks which is based on the modal mineral proportions of quartz (Q), alkali feldspar (A) and plagioclase (P) or of alkali feldspar (A), plagioclase (P) and feldspathoids (F). Rocks with mafic content >90% have their own classification.
This is followed by the classification separates and individually classifies the pyroclastic, carbonatitic, melititic, lamprophyric and charnockitic rocks before entering If the mineral mode cannot be determined as is often the case for volcanic rocks, then a chemical classification of total alkalis versus silica (TAS) is used. The nomenclature for these classifiations necessitates only 297 rock names out of the about 1500 that exist

    Ten principles of  IUGS Igneous Rock Classification 

1. use descriptive attributes;
2. use actual properties;
3. ensure suitability for all geologists;
4. use current terminology;
5. define boundaries of rock species;
6. keep it simple to apply;
7. follow natural relations;
8. use modal mineralogy;
9. if mode not feasible, use chemistry;
10. follow terminology of other IUGS advisory bodies

     Main QAPF classification









Pyroclastic Rocks



Carbonatitic Rocks



Melititic Rocks



Lamprophyric Rocks



Charnockitic Rocks

Origins and Differentiation of Magma

Magmas must require special circumstances in order to form and do not form just anywhere in the crust.
The Different Layers of the Earth Have Differing Chemical Compositions

Crust - variable thickness and composition
• Continental 10 - 70 km thick, underlies all continental areas, has an average composition that is andesitic.
  
• Oceanic 8 - 10 km thick, underlies all ocean basins, has an average composition that is basaltic.

Mantle - 3488 km thick, made up of a rock called peridotite (Olivine + Opx + Cpx). Evidence comes from Seismic wave velocities, experiments, and peridotite xenoliths (foreign rocks) brought to the surface by magmas. Experimental evidence suggests that the mineralogy of peridotite changes with depth (or pressure) in the Earth. At low pressure, the mineral assemblage is Olivine + Cpx + Opx + Plagioclase (plagioclase peridotite).   At higher pressure the assemblage changes to Olivine + Cpx + Opx + Spinel [(Mg,Fe⁺²) (Cr, Al, Fe⁺³)₂O₄] (spinel peridotite). At pressures above about 30 kilobars, the assemblage changes to Olivine + Cpx + Opx + garnet (garnet peridotite).  This occurs because Al changes its coordination with increasing pressure, and thus new minerals must form to accommodate the Al.

At greater depths, such as the 400 km discontinuity and the 670 km discontinuity, olivine and pyroxene likely change to high pressure polymorphs.  Despite these changes in mineral assemblage, the chemical composition of the mantle does not appear to change much in terms of its major element composition.

Core  - 2883 km radius, made up of Iron (Fe) and small amount of Nickel (Ni). Evidence comes from seismic wave velocities, experiments, and the composition of   iron meteorites, thought to be remnants of other differentiated planets that were broken apart due to collisions.
Lithosphere - about 100 km thick (up to 200 km thick beneath continents), very brittle, easily fractures at low temperature.  Note that the lithosphere is comprised of both crust and part of the upper mantle. The plates that we talk about in plate tectonics are made up of the lithosphere, and appear to float on the underlying asthenosphere.
Asthenosphere  - about 250 km thick - solid rock, but  soft and flows easily (ductile).  The top of the asthenosphere is called the Low Velocity Zone (LVZ) because the velocities of both P- and S-waves are lower than the in the lithosphere above.  But, not that neither P- nor S-wave velocities go to zero, so the LVZ is not completely liquid.
Mesosphere - about 2500 km thick, solid rock, but still capable of flowing.
Outer Core  - 2250 km thick -  liquid.  We know this because S-wave velocities are zero in the outer core.  If  V_s = 0, this implies m = 0, and this implies that the material is in a liquid state.
Inner core- 1230 km radius,solid



Origins of Magma

Magmas are not likely to come from the only part of the Earth that is in a liquid state, the outer core, because it does not have the right chemical composition.  The outer core is made mostly of Fe with some Ni, magmas are silicate liquids.
In the ocean basins, magmas are not likely to come from melting of the oceanic crust, since most magmas erupted in the ocean basins are basaltic.  To produce basaltic magmas by melting of the basaltic oceanic crust would require nearly 100% melting, which is not likely.
In the continents, both basaltic and rhyolitic magmas are erupted and intruded. Basaltic magmas are not likely to have come from the continental crust, since the average composition is more siliceous, but more siliceous magmas (andesitic - rhyolitic) could come from melting of the continental crust.Basaltic magmas must come from the underlying mantle.
Thus, with the exception of the continents, magmas are most likely to originate in the mantle from melting of mantle peridotite.
Origin of Magmas
Temperature varies with depth or pressure in the Earth along the geothermal gradient.  The normal geothermal gradient is somewhat higher beneath the oceans than beneath the continents, at least at shallow levels.If we compare the normal geothermal gradients with the experimentally determined phase diagram for peridotite containing little water or carbon dioxide, we find that the peridotite solidus temperature is everywhere higher than the normal geothermal gradients.Thus, under normal conditions the mantle is solid, as we would suspect from the seismic evidence.



Thus, in order to generate a melt, either we must find a way to increase the geothermal gradient so that it is above the peridotite solidus or reduce the temperature of the peridotite solidus.  In either case note that all we have to do is get the temperature in some part of the Earth, as expressed by the geothermal gradient, into the field of partial melt.  Partial melting is the most likely case because it requires less of an increase in temperature or less of a decrease in the peridotite solidus.   Once a partial melt has formed, the liquid portion can be easily separated from the remaining solids since liquids are more mobile and, in general, have a lower density than solids.

Raising the Geothermal Gradient
Radioactive Heat - Elements like U, Th, K, and Rb have radioactive isotopes.  During radioactive decay, sub-atomic particles are released by the decaying isotope and move outward until they collide with other atomic particles.   Upon collision, the kinetic energy of the moving particles is converted to heat. If this heat cannot be conducted away, then the temperature will rise.  Most the heat within the Earth is generated by radioactive decay, and this is the general reason why temperature increases with depth in the Earth. But most the radioactive isotopes are concentrated in the crust.  Although there are areas in the continental crust where high concentrations of radioactive elements have locally raised the temperature, at least high enough to cause metamorphism, this is a rare occurrence. It is even more unlikely that areas of high concentration develop within the mantle.  Thus, concentrations of radioactive elements is not likely to cause melting.
Frictional Heat
In areas where rocks slide past one another, such as at the base of the lithosphere, on at subduction zones, heat could be generated by friction.If this heat cannot be conducted away fast enough, then it may cause a localized rise in temperature within the zone where the sliding or shearing is taking place.This could cause a localized spike on the geothermal gradient that could cause local temperatures to rise above the solidus.



Decompression due to Convection
Convection is a form of heat transfer wherein the heat moves with the material.Convection can be induced if the temperature gradient is high enough that material at depth expands so that its density is lower than the material above it. This is an unstable situation and the hotter, lower density material will rise to be replaced by descending cooler material in a convection cell.
The rate of convection depends on both on the temperature gradient and the viscosity of the material (note that solids convect, but the rate is lower than in liquids because solids have higher viscosity). In the Earth, temperature gradients appear to be high enough and viscosity low enough for convection to occur.  Plate tectonics appears to be driven by convection in some form.


Anywhere there is a rising convection current, hotter material at depth will rise carrying its heat with it.  As it rises to lower pressure (decompression) it will cool somewhat, but will still have a temperature higher than its surroundings. Thus, decompression will result in raising the local geothermal gradient. If this new geothermal gradient reaches temperatures greater than the peridotite solidus, partial melting and the generation of magma can occur. This mechanism is referred to as decompression melting



Lowering the Solidus Temperature
Mixtures of components begin melting at a lower temperature than the pure components.In a two component system addition of a third component reduces both the solidus and liquidus temperatures.This suggests that if something can be added to the mantle, it could cause the solidus and liquidus temperatures to be lowered to the extent that the solidus could become lower than the geothermal gradient and result in partial melting, without having to raise the geothermal gradient. Such a melting mechanism is referred to as flux melting
It's difficult to imagine how solid components could be added to the mantle.But volatile components, for example H₂O and CO₂, because of the high mobility, could be added to the mantle, particularly at subduction zones.

• Oceanic crust is in contact with sea water, thus water could be in oceanic crust both due to weathering, which produces hydrous minerals like clay minerals, and could be in the pore spaces in the rock.
  
• Oceanic sediments eventually cover the basaltic oceanic crust produced at oceanic ridges.  Much of this sediment consists of clay minerals (which contain water) and carbonate minerals (which contain carbon dioxide).
  
• As the oceanic lithosphere descends into the mantle at a subduction zone, it will be taken to increasingly higher temperatures as it gets deeper.  This will result in metamorphism of both the basalt and the sediment.  As we will see later in our discussion of metamorphism, metamorphism is essentially a series of dehydration and decarbonation reactions, i.e. chemical reactions that transform hydrous and carbonate minerals into nonhydrous minerals and give up H₂O and CO₂ as a fluid phase.
  
• Addition of this fluid phase, either to the subducted lithosphere or the mantle overlying the subducted lithosphere could lower the solidus and liquidus temperatures enough to cause partial melting.



Crustal Anatexis
In the continental crust, it is not expected that the normal geothermal gradient will be high enough to cause melting despite the fact that hydrous and carbonate minerals occur in many continental rocks.Furthermore, because continental rocks are at low temperature and have a very high viscosity, convective decompression is not likely to occur.Yet, as we will see, there is evidence that continental crustal rocks sometimes melt.  This is called crustal anatexis.  The following scenario is one mechanism by which crustal anatexis could occur.

• Basaltic magmas, generated in the mantle, by flux melting, decompression melting or frictional heat, rise into the crust, carrying heat with them.
  
• Because basaltic liquids have a higher density than crust, they may not make it all the way to the surface, but instead intrude and cool slowly at depth.
  
• Upon cooling the basaltic magmas release heat into the crust, raising the geothermal gradient (increasing the local temperature).
  
• Successive intrusions of mantle-derived mantle into the same area of the crust may cause further increases in temperature, and eventually cause the geothermal gradient to become higher than the wet solidus of the crustal material, resulting in a partial melt of the crust.



Magmatism and Plate Tectonics
From the discussion above it should be obvious that magmatism is closely related to plate tectonics.The diagram below summarizes melting mechanisms that occur as a result of plate tectonics and may be responsible for the generation of magmas in a variety of plate tectonic settings, such as oceanic ridges, near subduction zones, and at rift valleys.



Diverging Plate Boundaries
Diverging plate boundaries are where plates move away from each other.   These include oceanic ridges or spreading centers, and rift valleys.
Oceanic Ridges are areas where mantle appears to ascend due to rising convection currents.  Decompression melting could result, generating magmas that intrude and erupt at the oceanic ridges to create new oceanic crust.
Iceland is one of the few areas where the resulting magmatism has been voluminous enough to built the oceanic ridge above sea level.





Continental Rift Valleys or Extensional Zones are areas, usually located in continental crust where extensional deformation is occurring.  These areas may be incipient spreading centers and may eventually evolve into oceanic ridges, such as has occurred in the Red Sea region.  Whether or not they develop into spreading centers, they are likely caused by mantle upwelling below the zone of extension.Mantle upwelling may result in decompression melting of the mantle, and could induce crustal anatexis. A good example of a continental rift valley is the East African Rift Valley. Another example is the Rio Grande Rift in Colorado and New Mexico, which is part of a larger region of extension that includes much of the western U.S. and is called the Basin and Range Province (Eastern California]
Converging Plate Boundaries
Converging plate boundaries are where plates run into each other.The most common type are where oceanic lithosphere subducts. Several mechanisms could contribute to the generation of magmas in this environment (see diagram at top of this section).


1. Frictional heating is likely to occur along the boundary between the subducted plate and the overlying mantle wedge.
2. Flux melting of either the subducted lithosphere or the overlying mantle wedge could occur as a result of the release of volatiles as the subducted plate heats and metamorphoses producing water and/or carbon dioxide fluids.
   
3. The process of subduction may drag the overlying mantle wedge down with it.  In order to replace the mantle dragged down in this process, part of the mantle wedge will have to rise.This upwelling of the mantle could result in decompression melting.

If an oceanic lithospheric plate subducts beneath another oceanic lithospheric plate, we find island arcs on the surface above the subduction zone



If an oceanic plate subducts beneath a plate composed of continental lithosphere, we find continental margin arcs If magma generated near the subduction zone intrudes and cools in the crust, it could induce crustal anatexis.


In areas where two continental lithospheric plates converge fold-thrust mountain ranges develop as the result of compression.  If water-bearing crustal rocks are pushed to deeper levels where temperatures are higher, crustal anatexis may result.



Intraplate Magmatism & Hot Spots
There are a few areas where magmatism does not appear to be related to converging or diverging plate boundaries.These areas occur in the middle of plates, usually far from the plate boundaries.This phenomenon is referred to as intraplate magmatism. Intraplate magmatism is thought to be caused by hot spots formed when thin plumes of mantle material rise along narrow zones from deep within the mantle.The hot spot remains stationary in the mantle while the plate moves over the hot spot.
Decompression melting caused by the upwelling plume produces magmas that form a volcano on the sea floor above the hot spot.The volcano remains active while it is over the vicinity of the hot spot, but eventually plate motion results in the volcano moving away from the plume and the volcano becomes extinct and begins to erode.



Because the Pacific Plate is one of the faster moving plates, this type of volcanism produces linear chains of islands and seamounts, such as the Hawaiian - Emperor chain, the Line Islands, the Marshall-Ellice Islands, and the Austral seamount chain.





Magmatic Differentiation


Rocks emplaced in any given restricted area during a short amount of geologic time were likely related to the same magmatic event.  Evidence for some kind of relationship between the rocks, and therefore between the magmas that cooled to form the rocks came from plotting variation diagrams.
A variation diagram is a plot showing how each oxide component in a rock varies with some other oxide component.  Because SiO₂  usually shows the most variation in any given suite of rocks, most variation diagrams plot the other oxides against SiO₂ as shown in the diagram to right, although any other oxide could be chosen for plotting on the x-axis.Plots that show relatively smooth trends of variation of the components suggested that the rocks might be related to one another through some process.  Of course, in order for the magmas to be related to one another, they must also have been intruded or erupted within a reasonable range of time.  Plotting rocks of Precambrian age along with those of Tertiary age may show smooth variation, but it is unlikely that the magmas were related to one another.If magmas are related to each other by some processes, that process would have to be one that causes magma composition to change. 



Any process that causes magma composition to change is called magmatic differentiation. Over the years, various process have been suggested to explain the variation of magma compositions observed within small regions.Among the processes are:
1. Distinct melting events from distinct sources.
2. Various degrees of partial melting from the same source.
3. Crystal fractionation.
4. Mixing of 2 or more magmas.
5. Assimilation/contamination of magmas by crustal rocks.
6. Liquid Immiscibility.

Initially, researchers attempted to show that one or the other of these process acted exclusively to cause magmatic differentiation.  With historical perspective, we now realize that if any of them are possible, then any or all of these processes could act at the same time to produce chemical change, and thus combinations of these processes are possible.  Still, we will look at each one in turn in the following discussion.
Distinct Melting Events
One possibility that always exists is that the magmas are not related except by some heating event that caused melting.In such a case each magma might represent melting of a different source rock at different times during the heating event.  If this were the case, we might not expect the chemical analyses of the rocks produced to show smooth trends on variation diagrams.  But, because variation diagrams based on a closed set of numbers (chemical analyses add up to 100%), if the weight% of one component increases, then the weight percent of some other component must decrease.  Thus, even in the event that the magmas are not related SiO₂ could increase and MgO could decrease to produce a trend.  The possibility of distinct melting events is not easy to prove or disprove.
Various Degrees of Partial Melting
When a multicomponent rock system melts, unless it has the composition of the eutectic, it melts over a range of temperatures at any given pressure and during this melting the liquid composition changes.   Thus, a wide variety of liquid compositions could be made by various degrees of partial melting of the same source rock.



For example...A simple in a three component system containing natural minerals, the system Fo - Di - SiO₂, shown in simplified form here. A proxy for mantle peridotite, being a mixture of Ol, Cpx, and Opx would plot as shown in the diagram.  This rock would begin to melt at the peritectic point, where Di, En, Ol, and Liquid are in equilibrium. The composition of the liquid would remain at the peritectic point (labeled 0% melting) until all of the diopside melted. This would occur after about 23% melting. The liquid would then take a path shown by the dark curve, first moving along the En - Ol boundary curve, until the enstatite was completely absorbed, then moving in a direct path toward the peridotite composition.
Fractional Melting
Note that above some of the liquid must be left behind.If all of the liquid is removed, then we have the case of fractional melting, which is somewhat different.
In fractional melting some all of the liquid is removed at each stage of the process.  Let's imagine that we melt the same peridotite again, removing liquids as they form.  The first melt to form again will have a composition of the peritectic, labeled "Melt 1" in the diagram.  Liquids of composition - Melt 1 can be produced and extracted until all of the Diopside is used up.  At this point, there is no liquid, since it has been removed or fractionated, so the remaining solid consists only of Enstatite and Forsterite with composition "Solid 2".  This is a two component system.  Thus further melting cannot take place until the temperature is raised to the peritectic temperature in the two component system Fo- SiO₂. 



Crystal Fractionation
In our discussion of phase diagrams we saw how liquid compositions can change as a result of removing crystals from the liquid as they form.  In all cases, except a eutectic composition, crystallization results in a change in the composition of the liquid, and if the crystals are removed by some process, then different magma compositions can be generated from the initial parent liquid.  If minerals that later react to form a new mineral or solid solution minerals are removed, then crystal fractionation can produce liquid compositions that would not otherwise have been attained by normal crystallization of the parent liquid.
Bowen's Reaction Series
Norman L. Bowen, an experimental petrologist in the early 1900s, realized this from his determinations of simple 2- and 3-component phase diagrams, and proposed that if an initial basaltic magma had crystals removed before they could react with the liquid, that the common suite of rocks from basalt to rhyolite could be produced.This is summarized as Bowen's Reaction Series



Bowen suggested that the common minerals that crystallize from magmas could be divided into a continuous reaction series and a discontinuous reaction series.

• The continuous reaction series is composed of the plagioclase feldspar solid solution series.  A basaltic magma would initially crystallize a Ca- rich plagioclase and upon cooling continually react with the liquid to produce more Na-rich plagioclase.  If the early forming plagioclase were removed, then liquid compositions could eventually evolve to those that would crystallize a Na-rich plagioclase, such as a rhyolite liquid.
• The discontinuous reaction series consists of minerals that upon cooling eventually react with the liquid to produce a new phase.  Thus, as we have seen, crystallization of olivine from a basaltic liquid would eventually reach a point where olivine would react with the liquid to produce orthopyroxene.  Bowen postulated that with further cooling pyroxene would react with the liquid, which by this time had become more enriched in H₂O, to produce hornblende.  The hornblende would eventually react with the liquid to produce biotite.  If the earlier crystallizing phases are removed before the reaction can take place, then increasingly more siliceous liquids would be produced.

This generalized idea is consistent with the temperatures observed in magmas and with the mineral assemblages we find in the various rocks.  We would expect that with increasing SiO₂ oxides like MgO, and CaO should decrease with higher degrees of crystal fractionation because they enter early crystallizing phases, like olivines and pyroxenes. Oxides like H₂O, K₂O and Na₂O should increase with increasing crystal fractionation because they do not enter early crystallizing phases.  Furthermore, we would expect incompatible trace element concentrations to increase with fractionation, and compatible trace element concentrations to decrease.  This is generally what is observed in igneous rock suites.   Because of this, and the fact that crystal fractionation is easy to envision and somewhat easy to test, crystal fraction is often implicitly assumed to be the dominant process of magmatic differentiation.
Mechanisms of Crystal Fractionation
In order for crystal fractionation to operate their must be a natural mechanism that can remove crystals from the magma or at least separate the crystals so that they can no longer react with the liquid.Several mechanisms could operate in nature.

• Crystal Settling/Floating - In general, crystals forming from a magma will have different densities than the liquid.
  • If the crystals have a higher density than the liquid, they will tend to sink or settle to the floor of the magma body. The first layer that settles will still be in contact with the magma, but will later become buried by later settling crystals so that they are effectively removed from the liquid.
  • If the crystals have a lower density in the magma, they will tend to float or rise upward through the magma.  Again the first layer that accumulates at the top of the magma body will initially be in contact with the liquid, but as more crystals float to the top and accumulate, the earlier formed layers will be effectively removed from contact with the liquid.
    
    
• Inward Crystallization - Because a magma body is hot and the country rock which surrounds it is expected to be much cooler, heat will move outward away from the magma.   Thus, the walls of the magma body will be coolest, and crystallization would be expected to take place first in this cooler portion of the magma near the walls.  The magma would be expected then to crystallize from the walls inward.  Just like in the example above, the first layer of crystals precipitated will still be in contact with the liquid, but will eventually become buried by later crystals and effectively be removed from contact with the liquid.
  
  
• Filter pressing - this mechanism has been proposed as a way to separate a liquid from a crystal-liquid mush.  In such a situation where there is a high concentration of crystals the liquid could be forced out of the spaces between crystals by some kind of tectonic squeezing that moves the liquid into a fracture or other free space, leaving the crystals behind. It would be kind of like squeezing the water out of a sponge.   This mechanism is difficult to envision taking place in nature because (1) unlike a sponge the matrix of crystals is brittle and will not deform easily to squeeze the liquid out, and (2) the fractures required for the liquid to move into are generally formed by extensional forces and the mechanism to get the liquid into the fractures involves compressional forces.  Filter pressing is a common method used to separate crystals from liquid in industrial processes, but has not been shown to have occurred in nature.

Magma Mixing
If two or more magmas with different chemical compositions come in contact with one another beneath the surface of the Earth, then it is possible that they could mix with to produce compositions intermediate between the end members.  If the composition of the magmas is greatly different (i.e. basalt and rhyolite), there are several factors that would tend to inhibit mixing.


• Temperature contrast - basalts and rhyolites have very different temperatures.  If they come in contact with one another the basaltic magma would tend to cool or even crystallize and the rhyolite would tend to heat up and begin to dissolve any crystals that it had precipitated.
• Density Contrast- basaltic magmas have densities on the order of 2600 to 2700 kg/m³, whereas rhyolitic magmas have densities of 2300 to 2500 kg/m³.   This contrast in density would mean that the lighter rhyolitic magmas would tend to float on the heavier basaltic magma and inhibit mixing.
• Viscosity Contrast- basaltic magmas and rhyolitic magmas would have very different viscosities. Thus, some kind of vigorous stirring would be necessary to get the magmas to mix.


Despite these inhibiting factors, there is evidence in rocks that magmas do sometimes mix.  The smaller the difference in chemical composition between two magmas, the smaller will be the contrasts in temperature, density, and viscosity.
If magmas of contrasting composition come in contact and begin to mix some kind of stirring mechanism would first be necessary.Such stirring could be provided by convection, Evidence for Mixing


• Mingling of magmas
  If,in the initially stages of such mixing, the magma were erupted, then we might expect to find rocks that show a "marble cake" appearance, with dark colored mafic rock intermingled with lighter colored rhyolitic rock.This, however, is mingling of magmas.Note that differences in color are not always due to differences in composition, so even in rocks that show this banding, mingling of magmas may not have occurred.
  
• Disequilibrium Mineral Assemblages
  If convective stirring progresses beyond the point of mingling, some evidence might still be preserved if the crystals present in one of the magmas does not completely dissolve or react.  This might leave disequilibrium mineral assemblages.For example, if a basaltic magma containing Mg-rich olivine mixed with a rhyolite magma containing quartz and the magma was erupted before the quartz or olivine could be redissolved or made over into another mineral, then we would produce a rock containing mineral that are out of equilibrium.
• Reverse Zoning in Minerals
  When a mineral is placed in an environment different than the one in which it originally formed it will tend to react to retain equilibrium.  Instead of dissolving completely or remaking their entire composition, solid solution minerals may just start precipitating a new composition that is stable in the new chemical environment or at the new temperature.  This can result in zoned crystals that show reversals of the zoning trends. for Example:
  Mg-Fe solid solution minerals normally zone from Mg-rich cores to Fe-rich rims. If a Fe-rich olivine or pyroxene is mixed into a Mg-rich magma that is precipitating Mg-rich olivine or pyroxene, it may precipitate the more Mg-rich composition on the rims of the added crystals.Analyses of such crystals would reveal a reversal in zoning.Similarly, if a Na-rich plag. originally crystallizing from a rhyolitic magma were mixed into a basaltic magma precipitating a Ca-rich plag., a Ca-rich rim may be added to the Na-rich plagioclase.
  

Glass Inclusions
Crystal growth from liquids is sometimes not perfect.   Sometimes the crystal grows incompletely trapping liquid inside.  If that liquid is quenched on the surface and a thin section is cut through the crystal this trapped liquid will be revealed as glass inclusions in the crystal.
Since the glass inclusions should represent the composition of the magma that precipitated the crystal, chemical analysis of glass inclusions give us the composition of the liquid in which the crystal formed.  The groundmass may also contain glass representing the composition of the liquid in which the crystal resided at eruption. If the composition of glass inclusions is different from glass in the groundmass, and if the groundmass composition is not what is expected from normal crystallization of the minerals present, this provides evidence of magma mixing.



Chemical Evidence
 If the mixing process proceeds to the point where other evidence is erased, evidence for mixing will still be preserved in the composition of the mixed magmas.  On oxide-oxide variation diagrams mixtures will lie along a straight line. Thus if diagrams show a group of rocks that lie along the same straight line, and the proportional distances are the same on all diagrams, one could hypothesize that the chemical variation resulted from magma mixing.
Crustal Assimilation/Contamination
Because the composition of the crust is generally different from the composition of magmas which must pass through the crust to reach the surface, their is always the possibility that reactions between the crust and the magma could take place.If crustal rocks are picked up, incorporated into the magma, and dissolved to become part of the magma, we say that the crustal rocks have been assimilated by the magma.If the magma absorbs part of the rock through which it passes we say that the magma has become contaminated by the crust.Either of these process would produce a change in the chemical composition of the magma unless the material being added has the same chemical composition as the magma.
In a sense, bulk assimilation would produce some of the same effects as mixing, but it is more complicated than mixing because of the heat balance involved.  In order to assimilate the country rock enough heat must be provided to first raise the country rock to its solidus temperature where it will begin to melt and then further heat must be added to change from the solid state to the liquid state. The only source of this heat, of course, is the magma itself.
Evidence for Assimilation/Contamination
As magma passes upward through the crust pieces of the country rock through which it passes may be broken off and assimilated by the magma.Just as in magma mixing, various stages of this process may be preserved in the magma and rock that results. Xenoliths (meaning foreign rock) are pieces of rock sometimes found as inclusions in other rocks.The presence of xenoliths does not always indicate that assimilation has taken place, but if the xenoliths show evidence of having been disaggregated with their minerals distributed thought the rest of the rock it is likely that some contamination of the magma has taken place.This may result in disequilibrium mineral assemblages and reversely zoned minerals, just as in the case of magma mixing.  And, if the assimilation goes to completion, with all of the xenoliths being dissolved in the magma, the only evidence left may be chemical, and again similar to the straight line mixing patterns produced by mixing.
Perhaps the best evidence of assimilation/contamination comes from studies of radiogenic isotopes.  Here we give an example using the systematics of the Rb - Sr system.


• ⁸⁷Rb is a radioactive isotope that decays to ⁸⁷Sr with a half life of   billion years.
  
• Because Rb is an incompatible element, it has been extracted from the mantle by magmas and added to the crust.  Thus the concentration of Rb in the crust (avg. about 100 ppm) is much higher than it is in the mantle (avg. about 4 ppm).
  
• ⁸⁶Sr is a stable, non radiogenic isotope whose concentration does not change with time.
  
• Because ⁸⁷Rb decays to produce ⁸⁷Sr and because there is more Rb in the crust than in the mantle, the ⁸⁷Sr/ ⁸⁷Sr of the crust has, over time changed to much higher values than the ⁸⁷Sr/⁸⁷Sr ratio in the mantle.
  
• The ⁸⁷Sr/⁸⁷Sr ratio of the mantle is generally in the range between 0.702 - 0.705.  Thus rocks derived from melting of the mantle should have ⁸⁷Sr/⁸⁷Sr ratios in this range.
  
• ⁸⁷Sr/⁸⁷Sr ratios of crustal rocks will depend on their age and concentration of Rb.  Older crustal rocks will have high values of ⁸⁷Sr/⁸⁷Sr in the range 0.705 - 0.720, younger crustal rocks having been recently derived from the mantle will ⁸⁷Sr/⁸⁷Sr ratios more similar to the mantle.
  
• If mantle derived magmas assimilate or are contaminated by older crustal rocks, then we would expect to find ratios of ⁸⁷Sr/⁸⁷Sr in these contaminated rocks that are higher than those found in the mantle and extend up to values found in older crustal rocks.
  
  
Liquid Immiscibility
Liquid immiscibility is where liquids do not mix with each other. We're all familiar with this phenomenon in the case of oil and water/vinegar in salad dressing.We've also discussed immiscibility in solids, for example in the alkali feldspar system.Just like in the alkali feldspar system, immiscibility is temperature dependent.
For example, in a two component system if there is a field of immiscibility it would appear as in the diagram to shown here.Cooling of a liquid with a composition of 25%B & 75%A would eventually result in the liquid separating into two different compositions.Further with further cooling these liquids one liquid would become more enriched in A and the other more enriched in B.Eventually both liquids would reach a temperature where crystals of A would start to form.Note that both liquids would be in equilibrium with crystals of A at the same temperature.   Further cooling would result in the disappearance of the A-rich liquid.



This points out two important properties of immiscible liquids.
1. If immiscible liquids are in equilibrium with solids, both liquids must be in equilibrium with the same solid compositions.
   
2. Extreme compositions of the two the liquids will exist at the same temperature.
   

Liquid immiscibility was once thought to be a mechanism to explain all magmatic differentiation.  If so, requirement 2, above, would require that siliceous liquids and mafic liquids should form at the same temperature.  Since basaltic magmas are generally much hotter than rhyolitic magmas, liquid immiscibility is not looked upon favorably as an explanation for wide diversity of magmatic compositions.  Still, liquid immiscibility is observed in experiments conducted on simple rock systems.
Combined Processes
As pointed out previously, if any of these process are possible, then a combination of the process could act to produce chemical change in magmas.  Thus, although crystal fractionation seems to be the dominant process affecting magmatic differentiation, it may not be the only processes. As we have seen, assimilation is likely to accompanied by crystallization of magmas in order to provide the heat necessary for assimilation.If this occurs then a combination of crystal fraction and assimilation could occur.   Similarly, magmas could mix and crystallize at the same time resulting in a combination of magma mixing and crystal fractionation.In nature, things could be quite complicated.


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