TOPIC 3C Structure of the Earth

Structure of the Earth and the Origin of Magmas: 

TOPIC 3 Structure of the Earth and the Origin of Magmas 1

Overview: 

Overview Melting of Mantle Generation of basalts from a chemically uniform mantle Primary magmas A chemically heterogeneous mantle model 2

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Mantle can melt due to: Increasing temperature Lowering pressure Adding volatiles 3 Melting of Mantle Estimate ~3.33 kbar/km (1 GPa = 10 kbar) Figure 1-9 . Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al . (1980), Earth. Rev. Geophys. Space Sci., 18 , 269-311.

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These methods of mantle melting occur in certain areas Increasing temperature and abnormally high geothermal gradients may occur in: Hot spots or ascending areas of convection cells Places where magmas are rising as diapirs. Pressure reduction may be associated with near-adiabatic (no heat transfer) upwelling of mantle material at rifts or associated with any rising material. Volatiles , especially H 2 O and CO 2 can reduce solidus temperatures sufficiently to create melts with normal geotherms but low mantle volatile content limits the quantity of the melt produced. This is more efficient in magma generation at subduction zones . 4 Melting of Mantle

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What sort of magma would be generated by mantle melting? Although mantle composition is ultramafic , complete melting of the mantle to produce ultramafic magma is rare because: the geothermal gradient is not high enough to produce such high melt proportions once partial melts reach 10-20%, they tend to separate and rise leaving a refractory residue that is unlikely to melt further. What about komatiites? These are ultramafic volcanic rocks that are almost completely restricted to the Archean when the geothermal gradient is thought to be much higher and partial melting could occur more. What is the common product of mantle melting? Basaltic magma Experimental models should explain the formation of basalt in the ocean basins at rifts. There are 2 main scenarios: mantle has uniform (homogeneous) composition mantle has heterogeneous composition 5 Melting of Mantle

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If mantle composition is considered to be uniform then only changes in T and P are considered as controlling factors in the variation of magma composition. A P variation at a particular T implies a different geothermal gradient. Low geothermal gradients reach the solidus and initiate melting at high P. Whereas, high geothermal gradients can initiate melting at lower P. Generation of basalts from a chemically uniform mantle 6

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Generation of basalts from a chemically uniform mantle One of the important effects of P is on the mineralogy of mantle. Although the chemical composition of the mantle may be constant, the mineralogical composition is variable with depth. Because the minerals are being melted, the first melt of a garnet lherzolite would not be the same as the first melt of a plagioclase lherzolite. Important point: P changes have different effect on the melting of minerals because compressibility of minerals is different. 7

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Generation of basalts from a chemically uniform mantle This causes a shift in the position of the eutectic minimum (composition of the first melt) resulting in different eutectic melt compositions. Note how the eutectic minimum moves with increasing P from silica-saturated (tholeiitic) to highly undersaturated and alkaline melts. Conclusion: Tholeiites are formed at lower P (depth) but alkaline basalts at higher P (depth). Fig. A 8

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Partial melting experiments of mantle-type rocks (synthetic pyrolite) show: Below 1 GPa: Olivine and two pyroxenes dominate the sub-solidus mineralogy (lherzolite) and plagioclase is the aluminous phase. At higher P, the mineralogy of the pyrolite mantle differs from the lherzolite. The first melt is more undersaturated (alkaline) at 60 km than at 25 km (see previous Fig. A). At 1.8 GPa (60 km), a few tests at different temperatures are shown. 9 Generation of basalts from a chemically uniform mantle Fig. A

Liquids and residuum of melted pyrolite: 

Liquids and residuum of melted pyrolite Figure 10-9 After Green and Ringwood (1967). Earth Planet. Sci. Lett . 2, 151-160. 10

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Lower fractions of partial melting result in more alkaline basalts. This is because alkalis are highly incompatible and enter the early melts. Successive melt increments slowly dilute the early alkali concentrations producing a more silica-saturated tholeiitic basalt. Conclusion: by varying the depth of melting and/or the amount of partial melting, we can generate either alkaline or tholeiitic basalt. 11 Generation of basalts from a chemically uniform mantle

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Conclusions: Tholeiites are favored by shallower melting 25% melting at <30 km ® Olivine tholeiite (silica-saturated) 25% melting at 60 km ® Olivine basalt Tholeiites are favored by greater % partial melting 20 % melting at 60 km ® Alkaline basalt (silica undersaturated incompatibles (alkalis) ® initial melts 30 % melting at 60 km ® Olivine tholeiite (silica-saturated) 12 Generation of basalts from a chemically uniform mantle

Primary magmas: 

Primary magmas Definitions: Igneous province : Is a field area where igneous activity has occurred over a distinct time period. Igneous rock suite : Is a group of igneous rocks collected from an igneous province. These rocks may or may not be genetically related. Igneous rock series : Is a group of igneous rocks that are genetically related (co-magmatic) and which were collected from a relatively small area (an igneous province). Rocks of an igneous rock series therefore display a continuous gradation in chemical composition from the most mafic to the most felsic in the group. A parental magma : Is a magma capable of producing all rocks belonging to an igneous rock series by differentiation. 13

Primary magmas: 

Primary magmas Definitions: Magmatic differentiation is a complex process where a single melt can produce a wide variety of different igneous rocks. Some degree of differentiation typically develops across space and time in exposed magma bodies (intrusive or extrusive). Most melts are thought to develop in the lower crust or in the asthenosphere (upper mantle) and have a primitive mafic or basaltic composition. Whereas, melts developing in the upper crust have a higher initial silica content. Regardless of depth of formation, melts ultimately produce igneous rocks that depend on the composition of the original melt, on the properties of wall rocks encountered during ascent, and on rate/s of cooling. The main processes involved in differentiation are assimilation , exchange of volatiles, fractional crystallization, and magmatic mixing. 14

Primary magmas: 

Primary magmas Differentiation: Assimilation is the process of magmatic differentiation where ascending magmas evolve chemically by incorporating easily melted or dissolved components ( fusibles ) from the walls of their conduits. Ascending magmas pick up volatiles, silica, trace elements, and occasionally fragments of wall rock. The heat that the melt gains by leaving behind quick-freezing refractories (an exothermic process) is typically sufficient to compensate for heat lost in the endothermic reactions required for the assimilation (melting) of country rock components. This trade-off ensures that assimilation can proceed without causing the melt to freeze (solidify). Any wall rock fragments that survive more or less intact, without completely melting or dissolving into the magma, are xenoliths. Surviving wall rock crystals are called xenocrysts. Together, xenoliths and xenocrysts provide invaluable information about rarely exposed lower crust and mantle levels by carrying these materials up within the ascending magma. 15

Primary magmas: 

Primary magmas Differentiation: Volatiles found in varying amounts in nearly all wall rocks and magmas are important also in the magmatic differentiation process of assimilation. These volatiles include CO 2 , SO 2 , O 2 , Cl 2 , and particularly H 2 O. Water is available in wall rocks of the mid-crust, both as free water and within the hydrated minerals commonly found at depths. Some assimilated water enters hydration reactions with predominantly anhydrous melt components, but most water simply accumulates in the ever-shrinking, residual silicate melt. When a melt has taken on sufficient water, that magma will develop a water-saturated silicate fraction and a separate water-based fluid phase. 16

Primary magmas: 

Primary magmas Differentiation: Fractional crystallization (fractionation) is that process of magmatic differentiation that accompanies the failure of early-forming crystals to react to the melt that remains. The process of fractional crystallization is responsible for the bulk of differentiation that is occurs in igneous rocks. As ascending melts cool and react with country rock, those minerals in the melt that have the highest melting points or the lowest solubilities (quick-freezing refractories, like olivine and pyroxene) crystallize out first, leaving minerals with the lowest melting points or solubilities (quick-melting fusibles, like silica) behind in the melt to freeze out last. 17

Primary magmas: 

Primary magmas Differentiation: Gravitative differentiation is the commonest form of fractionation, and results from the phenomenon that most solid minerals are denser than their parent melts. As denser crystals settle to the bottom of the magma body, they become segregated from the residual melt. Rocks that are formed by settling crystals are termed cumulates , and the rocks are often zoned, with the densest, first-formed crystals accumulated at the base of the magma chamber. Cumulates formed by the lighter crystals occasionally float to the top, with the lightest at the very top. This process produces layering in igneous rocks. The crystals of cumulate rocks are typically cemented by residual magmatic fluids. 18

Primary magmas: 

Primary magmas Differentiation: Magmatic mixing in areas of active magmatism is the process where adjacent magma bodies can develop transient subsurface communications before their eruption or final subsurface emplacement. Anatexis-related magmatic mixing involves the secondary melting of mid- to lower crustal rocks upon contact with much hotter, rising mafic melts of mantle origin, and produces felsic (feldspar- and quartz-rich) magmasin arc and continental rift settings. Such melts may reach high crustal levels carrying both mantle heat and mantle material. 19

Primary magmas: 

Primary magmas Primary magmas are the "first melt" produced by partial melting at depth within the mantle and have not yet been modified by differentiation such as fractional crystallization or assimilation. Modified magmas are called derivative or evolved magmas . A primary magma may therefore evolve into a parental magma by differentiation. Parental magma : Because it is difficult to determine if a magma in an area is truly primary, the most primitive magma type in a spectrum of magmas at a given locality is called parental magma. Parental magma (whether primary or derivative itself) is then considered to be the immediate source of the more evolved magmas in the series. 20

Primary magmas: 

Primary magmas For a melt to qualify for the definition of primary magma, it must fulfill the following conditions: (1) have a higher liquidus T compared to its differentiation products, (2) be richer in minerals removed by fractional crystallization compared to its differentiation products, (3) have a composition in equilibrium with the mantle phases from which it was produced by partial melting at high pressure. All primary magmas must have > 10% MgO by weight. 21

Primary magmas: 

Primary magmas There are criteria that are used to evaluate if a magma is primary or not. These criteria cannot prove that the magma is truly primary but can be used to determine if they are not primary. If we don’t see these criteria, then that magma is not primary. If we see these criteria, the magma could be primary or derivative (evolved). 22

Primary magmas: 

Primary magmas Criteria that are used to evaluate if a magma is primary or not: The magma plots at the extreme end of a differentiation index such as low %SiO 2 , high Mg/(Mg+Fe) , low alkalies, etc. However, these criteria only indicate that this magma is parental not primary. Multiply saturated: they are saturated in several phases at once. Most basalts when erupted are close to their liquidus temperature (they either contain phenocrysts or phenocrysts form with a little more cooling). Also all of the major minerals begin to crystallize within a narrow temperature interval suggesting that they are close to being multiply saturated. NOTE: Multiply saturated magma at low pressure are modified by fractional crystallization during ascending and therefore is not a primary magma. We need to do some experimental work on the sample to prove if the magma was multiply saturated at high pressure. In this case, it would be primary. 23

Primary magmas: 

Primary magmas Criteria that are used to evaluate if a magma is primary or not: Magmas that contain dense dunite and peridotite nodules are believed to have risen rapidly in order to keep the nodules suspended. Such magmas would have less time to undergo fractionation and may be primary. 24

Primary magmas: 

Primary magmas Criteria that are used to evaluate if a magma is primary or not: Olivine in residual mantle peridotite nodules is usually in the narrow composition range: Fo 86 to Fo 81 . Basaltic magmas in equilibrium with such olivines should have a ratio of MgO /( MgO+FeO ) in the range of 0.66 to 0.75. So we can use major element geochemistry to see if a magma is possibly primary. 25

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26 Fo% = [Mg/(Mg+Fe)] x 100 = 1.140/2.010 = 56.7% = Fo 56.7 Fa% = [Fe/(Mg+Fe)] x 100 = 0.870/2.010 = 43.3% = Fa 43.3

Primary magmas: 

Primary magmas Criteria that are used to evaluate if a magma is primary or not: Trace element geochemistry: Cr > 1000 ppm Ni > 400-500 ppm 27

Summary from the homogeneous mantle model: 

Summary from the homogeneous mantle model A chemically homogeneous mantle can yield a variety of basalt types. Alkaline basalts are favored over tholeiites by deeper melting and by low % partial melting. Fractionation crystallization at moderate to high depths can also create alkaline basalts from tholeiites . In spite of this initial success, there is evidence to suggest that such a simple approach is not realistic, and that the mantle is chemically heterogeneous. Although just by changing conditions such as P, %partial melting, T, liquid (magma) composition and fractional crystallization, it is possible to form various types of basalts from a single mantle composition, there are some facts that can not be explained by a homogeneous model. 28

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Although mantle only includes limited rock types (lherzolite, harzburgite and dunite), the variety is too important. Although major elements seem to be uniform, their variation allows us to separate samples into 2 groups: So fertile xenoliths have more incompatible elements so they can form magma before becoming more refractory like depleted samples. Garnet and spinel lherzolites are the most fertile samples and dunites are the most depleted . Fertile or enriched xenoliths Depleted xenoliths Al, Ca, Ti, Na, K > < Mg/( Mg+Fe ) and Cr/( Cr+Al ) 29 A Chemically heterogeneous mantle model

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Trace element patterns are not the same for all mantle-derived basaltic magmas. Negative sloping for ocean island basalt (OIB) is a typical enriched pattern and can be explained by: Partial melting of peridotite . Fractional crystallization of a peridotite-derived melt in which the more incompatible elements are concentrated in the liquid (OIB) fraction. Positive sloping for middle ocean ridge basalt (MORB): this cannot be explained by partial melting or fractional crystallization of chondrite-like mantle that incorporates the HREE and other relatively compatible elements into the liquid in preference to the less compatible elements. The only way a partial melt can have a pattern with a positive slope is to melt a significant proportion of a solid that is already depleted in LREE and incompatible elements. 30 A Chemically heterogeneous mantle model

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Because these diagrams are chondrite normalized, and undepleted mantle should be close to chondrite, it should plot at rock/chondrite =1 and have a slop of zero. In order to become depleted in LREE and incompatible elements, these elements must be extracted from the mantle and incorporated into melts before the formation of the MORB. 31 A Chemically heterogeneous mantle model

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Conclusions: The most common magma on the planet (i.e. MORB) must be derived from a mantle that has been previously depleted (probably by the earlier extraction of melts to form oceanic and continental crust). The other common magma (OIB), shows no such pattern and appears to be derived from a non-depleted (fertile) mantle source. 32 A Chemically heterogeneous mantle model

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143 Nd/ 144 Nd and 87 Sr/ 86 Sr isotopic data of a variety of oceanic basalts and mantle xenoliths show an array (mantle array) consistent with a range of enriched to depleted mantle material. The upper-left part of the array (MORB plots) has high 143 Nd/ 144 Nd and low 87 Sr/ 86 Sr which is characteristic of a depleted source and the lower right is less depleted. The mantle array is actually a mixing line: mixing between a depleted MORB-like source and the other is either non-depleted or slightly enriched. More depleted Less depleted 33 A Chemically heterogeneous mantle model

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Interpretation: mantle is stratified into 2 major levels. Boundary: 660km The upper level is depleted: If partial melting happened throughout geologic time, then the upper mantle may be heterogeneous containing both fertile and depleted lherzolite as well as refractory harzburgite and dunite . Convection should re-homogenize these differences to a large degree in the sublithospheric upper mantle. In contrast, the rigid lithospheric mantle should preserve many of these irregularities imposed upon it since it was formed. The lower level is less depleted (if not enriched) 34 A Chemically heterogeneous mantle model

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Quiz: The highest rate of ascending magma is: 4 km/hour 40km/year 40km/hour 10cm/year X 35

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Quiz: Examples for HFS and LFS elements respectively: K, Zr Hf, Ce Nb, Rb Sr, Ba X 36

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Quiz: Which element package is compatible? Cr, Ni, Co, U Cu, Pd, Pt, Ni K, Cr, U, Th Cs, REE. U, Fe X 37

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Quiz: Ni would fractionate into which mineral? OPX CPX Hornblende Olivine X 38

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Quiz: Which element behaves both compatibly and incompatibly? Ni Fe Mg Sr X 39

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Quiz: Which sentence is correct (comparison between high and low geothermal gradients)? Low gradients reach the solidus at lower P high gradients reach the solidus at deeper settings Low gradients reach the solidus at higher P high gradients reach the solidus at higher P X 40

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Quiz: By increasing pressure, the eutectic point for mantle melting would move toward … in Ne-Fo-Si system. Ne Fo Si None (i.e., would not change) X 41

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Quiz: Which sentence is correct? Tholeiites are favored by less % partial melting Tholeiites are favored by deeper melting Alkali basalts are favored by shallow melting Alkali basalts are favored by less partial melting X 42

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Quiz: The magma that comes directly from mantle and shows no sign of fractional crystallization and assimilation is called: Derivative magma parental magma primary magma original magma X 43