Effect of tectonic setting on chemistry of mantle-derived melts

Size: px

Start display at page:

Download "Effect of tectonic setting on chemistry of mantle-derived melts"

Toby Brendan Webster
6 years ago
Views:

composition Composition of the source Partial melting

1 Effect of tectonic setting on chemistry of mantle-derived melts Lherzolite Basalt Factors controlling magma composition Composition of the source Partial melting process Fractional crystallization Crustal assimilation (continental or oceanic)

2 Tectonic contexts of mantle partial melting 1) Ridge volcanism (MORB - "Mid Ocean Ridge Basalts") - creation of oceanic lithosphere 2) Intraplate volcanism - Large Igneous Provinces (LIP) Oceanic Flood Basalts (OFB) Continental Flood Basalts (CFB) - Ocean Island Basalts (OIB) 3) Arc volcanism (oceanic and continental) - destruction of oceanic lithosphere - creation of continental crust?

3 Volcanism beneath mid-ocean ridges 60,000 km of mid-ocean ridges Sample types available: MORB (Mid Ocean Ridge Basalts) Gabbros Abyssal peridotites Ophiolites Gale et al. (2014) MORB Most abundant volcanic rocks on Earth Very large sample set available relative to deeper rock types of ridges But only 10-15% of oceanic crust

4 Generation of oceanic crust (rapid spreading ridge) MORB

5 Why does the mantle melt beneath the ridges? adiabat As the mantle rises beneath the ridge, it follows an adiabatic geotherm, in which the decrease in temperature is linked uniquely to the decrease in pressure (thermodynamic effect). At around 50 to 100 km, this geotherm intersects the peridotite solidus, and partial melting begins.

6 Simplified peridotite melting Forsterite Mg 2 SiO 4 Peridotite Enstatite MgSiO 3 Diopside CaMgSi 2 O 6 First melts Nelson course notes Tulane Univ. website Peridotite melting produces basalts Major element chemistry of MORB may depend on: Source compostion H 2 O content T and P of melting Extent of melting Fractional crystallization Assimilation of pre-existing crust and sediments Given the many factors that control MORB chemistry, how can we make sense of their compositions?

7 Major elements in MORB vary with ridge depth deep Klein and Langmuir (1987) Na 2 O wt % shallow medium depth AAD (Antarctic discordance) deep Tamayo Fracture Zone - medium depth Kolbeinsey Ridge - shallow Concentrations averaged over 100 km ridge segments to filter out small scale variations. 5 6 Increasing fractionation 7 8 MgO wt% 9 10 On each ridge, NaO 2 varies with MgO - consistent with fractional crystallization Between ridges, large differences in Na 2 O at each MgO value. To understand the variations between ridges, must remove effects of fractional crystallization.

8 Typical low pressure fractionation sequence of MORB T ( o C) Olivine Ol+Pl Ol+Pl+Cpx Tamayo Region, East Pacific Rise Ol = olivine Pl = plagioclase Cpx = clinopyroxene Fractional Crystallization Langmuir et al. (1992) Mg (cation %) Olivine is the liquidus phase. Plagioclase and clinopyroxene appear with decreasing temperature

9 Primitive basalt ~ 5.8% Mg (9.7% MgO) ~ 1.8% Na ~ 8.4% Al Effect of fractionation on liquid composition: Example of Mg, Na and Al Olivine (Fo 90 = Mg-rich olivine) ~ 30 wt% Mg ~ 0 wt% Na ~ 0 wt% Al Effect of olivine fractionation on liquid: Mg Na Al Plagioclase (An 90 = Ca-rich plag) ~ 0 wt% Mg ~ 1 wt% Na ~ 19 wt% Al Effect of plagioclase fractionation on liquid: Mg Na Al

10 Fractional crystallization Effect of low pressure fractionation on MORB major element composition increase in TiO 2 and Na 2 O: incompatible in all major phases decrease in Al 2 O 3 and increase in FeO: plagioclase crystallization kink, then decrease in CaO: onset of cpx crystallization Clipperton fraction zone (EPR) Langmuir et al. (1992) W+L : program of Weaver et Langmuir (1990) N : program of Nielsen (1985)

11 To see effects of differences in partial melting linked to ridge depth must first correct for fractional crystallization Comparison at 8 wt% MgO (Na 8.0, Fe8.0, Ti8.0, Ca8.0 ) MgO = 8% Use 8% MgO because: 1) only moderate degrees of fractionation 2) most basalt series include 8% MgO samples Langmuir et al. (1992)

12 Alternative fractionation correction Back-correction until magma in equilibrium with mantle olivine (Fo 90 ) (Na 90, Fe 90, Ti 90, Ca 90...) Advantage: Can directly compare results with peridotite melting experiments Disadvantage: Must extrapolate beyond data range Results of two methods are strongly correlated and lead to similar interpretations Gale et al. (2104)

13 After correction for fractional crystallization, a global correlation exists between ridge depth and major element composition of MORB (Klein et Langmuir, 1987). Each point represents the average of basalt values from ~100 km of ridge. Black squares : MORB from "normal" ridges White squares : MORB influenced by hot spots Crosses : MORB next to hot spots Small diamonds : Back-arc basin basalts Langmuir et al. (1992)

14 plumes Correlations confirmed by recent results back-arcs Results from compilation of Gale et al (2014) using three times as much data. Outlined fields: normal ridges (solid), back-arcs (dashed); Langmuir et al. (1992)

15 Crustal thickness and chemical composition also correlate. In general, where the ridge is shallow, the crust is thick. Implication : In each ridge segment, the MORB composition is linked to the quantity of magma produced.

16 Quantity of magma depends on depth at which melting starts. This depth depends on pressure at which peridotite solidus is crossed. Where the potential temperature is high, mantle material rising along an adiabat crosses the peridotite solidus at greater depth, and a larger quantity of magma is produced. Potential temperature = the temperature that an adiabatic geotherm would have at the surface of the Earth

Polybaric partial melting in an upwelling mantle column Residual Mantle Column Ridge Axis Ridge Axis Residual Mantle Column 40% removed Solidus 30% 30% removed 20% 20% removed Melting Regime 10%

17 Polybaric partial melting in an upwelling mantle column Residual Mantle Column Ridge Axis Ridge Axis Residual Mantle Column 40% removed Solidus 30% 30% removed 20% 20% removed Melting Regime 10% Solidus 10% removed 0% removed Cold Mantle Hot Mantle Langmuir et al. (1992) Higher potential temperature partial melting starts at greater depth Two effects : 1) Greater quantity of magma 2) Higher average pressure of partial melting

18 1) Effect of magma quantity on MORB composition Concentration of incompatible element, (e.g. Na) decreases as extent of partial melting increases (by dilution) Na8.0 correlated with ridge depth Models of decrease in magma Na 2 O content with increasing extent of partial melting (F). Langmuir et al. (1992) High Na8.0 concentration Low potential temperature Small quantity of magma Thin oceanic crust Deep midocean ridge

19 2) Effect of melting pressure on magma compositions MgO and especially FeO increase, SiO 2 decreases with melting pressure numbers: P (kb) 15 Pyrolite melts Experimental magma compositions from partial melting of "pyrolite" at various pressures pyrolite = particular fertile peridotite composition taken by many as primitive mantle composition Figure from Langmuir et al. (1992) using data of Jacques and Green (1980). 10 5

20 High Na8.0 concentration Low potential temperature Small quantity of magma Thin oceanic crust Deep midocean ridge Shallow average melting pressure Low Fe8.0 concentration Negative correlation between Na 8.0 and Fe 8.0

21 Crustal thickness Polybaric melting, with melt onset depending on potential temperature can explain negative correlation between Fe8.0 and Na8.0. However, some differences exist between regions and tectonic contexts: probably reflect source variations. Black squares : MORB from "normal" ridges (N-MORB) White squares : MORB influenced by hot spots Crosses : MORB adjacent to hot spots Diamonds : back-arc basin basalts Langmuir et al. (1992)

22 Variations according to tectonic context Ti8.0 Black squares : MORB from "normal" ridges (N-MORB) White squares : MORB influenced by hot spots Crosses : MORB adjacent to hot spots Diamonds : back-arc basin basalts Na8.0 Na8.0 Among all MORB, a rough correlation exists between Ti 8.0 and Na 8.0 as expected from model. But MORB from certain tectonic contexts (back-arc basins, adjacent to hot spots) plot off trend. Mostly reflects variation of source composition.

23 Variations between ocean basins: N-MORB only Ti8.0 Na8.0 Langmuir et al. (1992) For Ti 8.0, variations among N-MORB from different ocean basins. Reflect subtle differences between the MORB sources beneath each basin.

24 MORB: Message from major elements Melt composition strongly controlled by fractionation Removing fractionation effects reveals correlation between crustal thickness and melt chemistry Likely explanation: polybaric melting during mantle upwelling. Temperature controls both average pressure and extent of melting Removing melting effects reveals subtle source differences between ocean basins and tectonic contexts

25 What can we learn from trace elements in MORB? CHONDRITE NORMALIZED CONCENTRATION Continental crust MORB Moderately incompatible elements have highest normalized abundances in MORB. Hofmann (1988)

26 How are MORB trace element patterns produced? PRIMITIVE MANTLE NORMALIZED CONCENTRATION Magmas produced by partial melting of residual mantle D = concentration in residue concentration in liquid Hofmann (1988) F = fraction of melting The hump in moderately incompatible elements results from partial melting of mantle material that has already been depleted in highly incompatible elements by crustal extraction.

27 MORB vs. bulk and lower crustal concentrations Primitive Mantle normalized Bulk ocean crust MORB Lower crust 0 Rb Pb U Th Ba K La Ce Nb Pr Sr Nd Hf Zr Sm Eu Gd Tb Dy Ho Er Y Tm Lu Cu Sc Co Ni Increasing compatibility Crustal concentrations given by White (2013) Only 10 to 15% of the ocean crust is composed of MORB. Most of crust is composed of gabbro.

28 Factors controlling trace element fractionation during partial melting 1) The melting process "Batch melting" Liquid "Fractional Melting" Liquid Liquid remains in contact with residue until end of melting Moderate depletion in incompatible elements High porosity Liquid removed as soon as it is produced Very strong depletion in incompatible elements Vanishingly small porosity Reality undoubtedly between the two cases

29 Partition coefficients Elemental abundances of partial melt determined by mineralogical composition of rock and partition coefficients of each phase. k di = concentration in phase i concentration in liquid Global partition coefficient, D = Σ k di x i = C solid /C liquid (x i = proportion of phase i ) Phase proportions can change due to loss of most fusible phases (e.g., clinopyroxene) or to pressure changes (garnet to spinel as Al-bearing phase) D changes during melting.

30 Derivation of batch melting equation "Batch melting" Liquid From mass balance: C o = FC L + (1-F)C S By definition: D = C S /C L Substituting for C S : C o = FC L + (1-F)DC L Rearranging terms: C o = (F + D -DF) C L C L 1 C o = D + F(1 -D) F = fraction of liquid C o = original concentration in solid C L = concentration in liquid C S = concentration in residue

31 Non-modal melting: phases do not enter the melt in the same proportions that they are present in the original solid (more realistic than modal melting). P = bulk partition coeffient of phases melting to form liquid P = Σ k di y i (y i = proportion of phase i entering melt) At any point during melting: D = (D o - PF)/(1-F) (D o = original value of D) As developed by Shaw (1970): Modal melting Non-Modal melting MIT OpenCourseWare, based on equations of Shaw (1970)

32 Equations for fractional melting Modal melting Non-Modal melting

33 Concentrations in liquids and residues produced by batch and fractional melting C L : batch melting liquid C l : fractional melting instanteous melt C L : fractional melting aggregated melt C S : batch melting residue C s : fractional melting residue Example: Variation of La concentration during non-modal melting of garnet pyroxenite (La is highly incompatible). MIT OpenCourseWare, based on equations of Shaw (1970)

34 Factors controlling trace element fractionation during partial melting 2) Source mineralogy Lherzolite mineralogy: olivine (55-85%), orthopyroxene (10-35%), clinopyroxene (5-20%), phase rich in Al 2 O 3 (0-10%) As T and P vary with depth, the mineralogy of the Al 2 O 3 rich phase changes Down to ~24 km, plagioclase is stable Between ~24 and ~70 km, spinel is stable Below ~70 km, garnet is stable Most incompatible elements are in clinopyroxene Many trace elements, particularly the HREE, are hosted mostly in garnet These mineralogical changes produce large changes in bulk partition coefficients during partial melting.

clinopyroxene incompatible LREE HREE For garnet, k d increases dramatically

35 Effect of source mineralogy on trace element compositions of melts: Example of REE k d 's of McKenzie and O'Nions (1991) garnet compatible k d clinopyroxene incompatible LREE HREE For garnet, k d increases dramatically with mass of REE. Melting in garnet facies can therefore strongly fractionate REE.

$peridotite. Strong fractionation of REE, in liquids and residues. At ~ 70 to 24 km, spinel replaces garnet as Al 2 O 3 bearing phase.$

36 spinel facies liquid Effect of pressure, and thus mineralogy, on REE in melt C L /C o or C S /C o garnet facies reisdue garnet facies liquid spinel facies reisdue At > 70 km, garnet is stable in peridotite. Strong fractionation of REE, in liquids and residues. At ~ 70 to 24 km, spinel replaces garnet as Al 2 O 3 bearing phase. REE partitioning controlled mostly by clinopyroxene, so very little fractionation of HREE. Strong fractionation of HREE implies deep melting Assumptions: 8% melting, partition coefficients of McKenzie & O'nions (1981), non-modal batch melting model. Phase proportions garnet facies: 55%ol, 20%opx, 15%cpx, 10% gt; melting proportions: 50%cpx, 50%gt; phase proportions spinel facies: 55%ol, 22%opx, 18% cpx, 5% sp; melting proportions: 5% opx, 95% cpx.

1000 Calculated melts from depleted mantle MORB generation mainly occurs in spinel facies (< 70 km) Calculated liquid/chondrites 100 10 melting in spinel

37 1000 Calculated melts from depleted mantle MORB generation mainly occurs in spinel facies (< 70 km) Calculated liquid/chondrites melting in spinel facies melting in garnet facies 1 La Ce Pr NdSmEu Gd Tb DyHo Er TmYb Lu Depleted mantle values of Salters and Stracke (2004) used for mantle source White (2009)

$Factors controlling trace element fractionation during partial melting 3)$ $fractionation between incompatible trace elements because nearly all are$

38 Factors controlling trace element fractionation during partial melting 3) Extent of melting Spinel Facies 1% 1% Garnet Facies C L /C o C L /C o 10% 10% At large extents of melting typical of MORB magmatism, very little fractionation between incompatible trace elements because nearly all are found in melt phase.

Trace element ratios in MORB: Useful source tracers Partial melting creates only limited trace element variation in MORB because of high extents of melting Fractional crystallization has almost no

39 Trace element ratios in MORB: Useful source tracers Partial melting creates only limited trace element variation in MORB because of high extents of melting Fractional crystallization has almost no effect on incompatible trace element ratios Incompatible trace element ratios in MORB can be used to trace source variations. E-MORB: "Enriched" (La/Sm) N > 1.8 Requires an enriched source N-MORB: "Normal" (La/Sm) N < 1 T- MORB: "Transitional" (between the others) Schilling et al. (1983)

40 Trace element and isotopic variations along mid-atlantic Ridge Ulrich et al. (2012) Implication : MORB source is not completely uniform at a ~100 km length scale

$Radiogenic isotopes Ideal source tracers because not fractionated by melting process$

41 Radiogenic isotopes Ideal source tracers because not fractionated by melting process White (2009) MORB are isotopically quite homogeneous compared to OIB and crustal magmas

42 Still, some systematic variations are observed between ocean basins (Sr, Nd, Pb ) Continental crust Indian Ocean MORB Evidence of an enriched component: recycled pelagic sediments? White (2009)

43 MORB - Points to remember After effects of fractionation removed, MORB major element compositions correlate with crustal thickness. Probably explained by polybaric melting, with onset of melting dependant on temperature. Relative enrichment in moderately incompatible elements observed, resulting from remelting of a mantle depleted by crustal extraction, ie, MORB source is depleted in incompatible elements. Trace element abundances suggest MORB produced by high degrees of melting in spinel facies. MORB source relatively homogeneous compared to other Earth reservoirs. However major and trace elements and isotopes all indicate subtle variations at large and small length scales.

Worked Example of Batch Melting: Rb and Sr

Worked Example of Batch Melting: Rb and Sr Basalt with the mode: Table 9.2. Conversion from mode to weight percent Mineral Mode Density Wt prop Wt% ol 15 3.6 54 0.18 cpx 33 3.4 112.2 0.37 plag 51 2.7 137.7