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Comb-layered gabbro

Experimental constraints on subduction-related magmatism I: Differentiation of mantle-derived magmas in the crust Peter Ulmer. Comb-layered gabbro. with major contributions from :. Othmar Müntener (University of Lausanne) Ralf Kägi ( PhD , 2001, EZH)

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Comb-layered gabbro

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  1. Experimental constraints on subduction-related magmatism I:Differentiation of mantle-derived magmas in the crust Peter Ulmer Comb-layered gabbro

  2. withmajorcontributionsfrom: • Othmar Müntener (University of Lausanne) • Ralf Kägi (PhD, 2001, EZH) • Raquel Alonso Perez (PhD, 2006, ETH) • Samuel Villiger (PhD, 2005, ETH) • Oliver Jagoutz (MIT Boston) • Andreas Enggist (ETH, Edmonton) • Jon Blundy (Bristol) • Tim Grove (Boston) • Ezio Callegari (Torino) • Volkmar Trommsdorff (ETH) • Max Schmidt (ETH) • andmanymore

  3. ‘Setup’ • Primarily flux melting in the mantle wedge • => ´basaltic´ mantle magmas • Shallow granitoids are • intermediates and acidics • Differentiation by: • Fractional crystallization • Magma mixing • Assimilation and hybridization • Where do the mantle-derived • magmas differentiate and get • there dominantly andesitic to • dacitic character? • What are the cumulates formed? • How can they be potentiallyidentified? And how stable are they at the bottom of the crust?

  4. Topics • Differentiation of mantle-derived primary magmas by equilibrium/fractional crystallizing at the base and at intermediate depth of growing crust in Island Arcs / Active Continental MarginsCrystallizing solid assemblages - derivative liquid compositions • How can this information potentially be used to deduce depth of differentiation of primitive magmas and the generation of the large amount of intermediates to acidics in arcs (convergent plate margin) environments • Consequences for interpretation of geophysical data and composition of juvenile crust formed at convergent margins

  5. ‘hot zones’ in the lower crust Basalt Moho Kohistan, Annen et al. 2006

  6. Petrography/Petrology of volcanic rocks results pre-eruption conditions ≠ conditions of principal differentiation plg cpx Ti-mag cpx ol plg ol Basalt thin-section BSE image of Fogo 1995 Pahoehoe Lava Plutonic rocks: Mineral-paragenesis in plutonics => Final emplacement depth/conditions

  7. Petrology (experimental phase equilibria and thermobarometry) provides pre-eruptive (volcanics) or emplacement (plutonics) conditions • Fe-Ti-oxide thermometry on inclusions in phenocrysts provide pre- and post-mixing temperatures • Al-in-hornblende barometry constraints pressure of magma chamber where mixing occurred (corrected for super-solidus temperatures) • Phase equilibria constraints, e.g. amphibole stability as f(P,T,fH2O) • Pre-eruptive crystallization and mixing conditions can be constrained

  8. Fractionation experiments - strategy Experimental Techniques Equilibrium and fractional crystallization experiments Kägi et al. 2005

  9. Primary liquid (sm) compositions pb: Picrobasalt (20% normative olivine, 3.0% H2O, multiply saturated at 1370°C and 2.8 GPa) ba: High-Mg basaltic andesite (2% normative ol, 5% H2O, multiply saturated at 1230°C, 1.2GPa,Baker et al,1994; Müntener et al., 2001) Picrobasalt-Adamello High-Mg basaltic andesite Mt. Shasta (Calif. / USA)

  10. Picrobasalt 0.5 GPa - Graphite Equilibrium Crystallization • early olivine • plag and cpx and spi • no opx (or late hyp) • late peritectic amp Ulmer unpubl.

  11. Dunite - pyroxenite sequences: Jijal complex (Pakistan)

  12. Wherlite/ ol-clinopyroxenite (Adamello) Cr-spinel

  13. Cpx-mag-gabbro Hbl-gabbro-layer Anothosite Layer Gabbro (cpx) Layer Late Amphibole overgrowth Shallow gabbroic rocks ol - Cr-sp > cpx+plag+mag > amph Ol-Cpx-mag-gabbro

  14. Picrobasalt 1 GPa - NNO Fractional Crystallization • early olivine • cpx • very minor opx • spinel/mag • amph Müntener & Ulmer (2006)

  15. Picrobasalt 1 GPa - NNO Fractional Crystallization • early olivine + Cr-sp • cpx • Opx (enstatitic) • Amp (peritectic) • NO plag to 990°C Müntener & Ulmer (2006)

  16. Cumulate compositions - basaltic andesite - Fractional • Fractional • 30% pyxroxenites • 15% Cpx- hornblendites • 45% of UM cumulates extracted at 1000°C • Density / Vp • r 3.29 - 3.07 • Vp 7.45 - 7.10 Müntener & Ulmer (2006)

  17. High-Pressure Cumulates I (Adamello) Porphyritic hornblende-gabbro (Mt. Mattoni, Adamello)

  18. High-Pressure Cumulates II (Adamello) - TS Olivine – Cpx – Hornblendite (cortlandite) (Mt. Mattoni, Adamello)

  19. Cumulate compositions - Picrobasalt - Equilibrium 1.5 GPa • Equilibrium • 15% dunites • 40% pyxroxenites • 40% Garnet-pyx- hornblendites >90% of UM cumulates extracted at 1000°C Alonso Perez (2007, PHD)

  20. Picrobasalt 1.5 GPa - Fractional Crystallization • ol • cpx + gar • Little opx • amph

  21. Cpx-bearing garnet hornblendites (Jijal, Kohistan Arc, N-Pakistan)

  22. Picrobasalt 1bar-0.5 GPa - Derivative liquids • Graphite Eq/Frac • peritectic ol - opx • Ol – cpx – plag – spi (mag ) metaluminous derivative liquids Cpx-content decreases with increasing pressure, only peritectic reaction is cpx + plg +/- mag + L = amph Ulmer unpubl.

  23. Picrobasalt 1 GPa - Derivative liquids • NNO (AuPd) Eq/Fractional • olivine • dominantly cpx (minor opx) • quartz-normative liquids • peraluminous below 1050°C • andesitic to dacitic (1000°C) • 70% solids extracted at 990°C metaluminous peraluminous Kägi (2000); Müntener & Ulmer (2006)

  24. Basaltic andesite 1-1.2 GPa - Derivative liquids • Equilibrium • quartz-normative liquids • peraluminous (corundum- normative) below 1120°C • andesitic (1050°C) • 50% solids extracted at 1070°C • garnet at lowest temperatures • Fractional • quartz-normative liquids • peraluminous (corundum- normative) below 1050°C • andesitic to dacitic (990°C) • very similar to picrobasalt fractional crystallization • 45% solids extracted at 990°C Kägi 2000

  25. Picrobasalt 1.5 GPa - Derivative liquids • Equilibrium • Graphite -> Ne-normative • NNO -> quartz-normative liquids • metaluminous • basaltic andesite (1000°C) • garnet stable < 1100°C • 80% solids extracted at 990°C • Fractional • quartz-normative liquids • metaluminous • andesitic to dacitic (990°C) • 85% solids extracted at 990°C Alonso Perez (2007, PHD)

  26. Differentiated liquids • Decreasing normative cpx with increasing pressure to 1.2 GPa, • peraluminous at 1.0 GPa • Reversal at 1.5 GPa due to abundant garnet in crystallizing assemblage peraluminous

  27. Results Phase Equilibria • Mantle extraction depth => composition of primary basaltic magmas is not crucial for the composition of differentiates but affects the volume of (ultra)mafic cumulates • Differentiated rocks systematically become more aluminous (less cpx-normative) with increasing depth of differentiation to 1.2 GPa • 45-80% of ultramafic cumulates (dunite- pyroxenite - hornblendite) extracted to produce andesitic/dacitic liquids at 1 GPa • Garnet-bearing cumulates may form >25 km depth (not all information shown)

  28. The basic principle: Dry Tholeiite ‚barometry‘ (East Pacific Rise) Villiger et al. (JGR, 2007) Restricted primary magma composition Cotectic crystallization of same paragenesis / proportions over large pressure Large experimental data set Similar experimental techniques, no Fe, fO2, H2O problems

  29. Cao – MgO relationships for picrobasalt starting material • Increasing pressure earlier onset of cpx crystallization, maximum is reached earlier and at lower CaO content • Same as dry tholeiite, however, not linear because phase assemblage and proportions change as function of P • Peritectica involved • Crossover at lower MgO

  30. Cao – MgO relationships for basalt to basaltic andesite systems • Overall ‘topology’ is the same but absolute numbers (MgO in particular) are not equal • Problems arising from experiments: • Fe-loss, hydrogen loss, fO2 control in H2O-undersaturated melting experiments STRONG bulk (starting) composition control

  31. MB Cascade Range Western USA (N-California – N-Washington State) GP MR MSH MA MH MJ 3S N CL ML S Mt. Shasta LP

  32. Cao – MgO relationships for Cascadian volcanics • Probably qualitative information can be gained • Majority of volcanic rocks obtained their major element composition at rather great depth, but clearly a polybaric process • Any significance of ‘bifurcation’ at 6 and 3.5 wt.% MgO? Lines from basalt to basaltic andesitesystems Data set kindly made available by Jon Blundy

  33. Magmatic processes beneath Shasta Grove et al (2005)

  34. The Adamello Massif Tertiary calc-alkaline batholith: Complex plutons - exhibiting variety of crystallization sequences for mafic/ultramafic cumulitic rocks, pointing towards polybaric differentiation, high H2O, high fO2 Mafic-ultramafic rocks of the Southern Adamello Dikes ‘low’ pressure gabbroics High pressure gabbroics

  35. Cao – MgO relationships for Adamello plutonics • Additional information from petrography and geochemistry available • Amphibole dominated plutonics and dike rocks, rather high pressure (10 kbar) • Second series at shallower pressure consistent with field, mineralogy and geochemistry Not convincing, if you only have one series of differentiated granitoids: Basically impossible to extract differentiation depths as strongly dependant on parental magma composition

  36. 80% metaluminous 80% peraluminous REE pattern of tonalites from amphibole dominated vs cpx-plg-magnetite dominated suites in the S-Adamello Alonso-Perez (PhD, 2006)

  37. Sr/Y vs Y of tonalites from amphibole dominated vs cpx-plg-mag dominated suites of the S- Adamello La/Yb = 33, 80% of granitoids peraluminous La/Yb = 12, 80% of granitoids metaluminous

  38. Trace element modeling: Ingredients • Starting composition => Picrobasalt S- Adamello • Modal and chemical compositions of fractionating phases => Fractional x-talisation experiments at 1.0 GPa starting from P-bas • Kdfor trace elements for the fractionating phases at corresponding temperature and composition (solid and liquid) => new cpx/liq and amph/liqKd along Liquid-Line-of-Descent plus literature • Target: Leuco-tonalite associated with Ol-Cpx-Hornblendites

  39. Cpx/Liquid Partition Coefficients at 1.5 GPa along the LLD • Increasing D with decreasing temperature and increasing SiO2 • at 1020°C parabola much narrower (E increased to 380 GPa) • Equilibrium crystallization muss less temperature sensitive (closed system, less composition variation in coexisting liquids) • Rather strong Sr/Y fractionation: D(Sr)/D(/Y) = 0.15-0.07 (decreasing T) • D(La)/D(Yb) = 0.09-0.15-0.03 Alonso-Perez (PhD, 2006)

  40. Amph/Liquid Partition Coefficients at 0.8-1.2 GPa • Strong dependence on SiO2 liquid content • Y compatible, Sr incompatibleD(Sr/Y) = 0.12-0.15 • Relatively strong REE fractionationD(La/Yb) = 0.08-0.17

  41. Fractional Crystallization Calculation: REE • Strong increase of (La/Yb)N from2.4-6.3; Leucotonalite 11.0 but more differentiated then 990°C fractional melt. • Amphibole in latest fractionation steps responsible for “trough” in HREE abundances

  42. Fractional Crystallization Calculation: Traces • Leucotonalite associated with ol-cpx hornblendites (and hornblende-gabbros) fits pattern of modeled melt composition (astonishingly) well, confirming that trace element modeling can successfully be utilized to constrain the fractionating assemblage and, thus, the pressure of principal differentiation of basic to acidic derivative compositions. • PS: Some people call melts with such characteristics ADAKITES:However, no garnet involved! (Sr/Y = 70, (La/Yb)N = 11, Y=9 ppm, Sr=600 ppm)

  43. Trace Modeling Results Phase equilibria constraints at high pressure (experimentally and/or thermodynamically derived) along the liquid lines of descent combined with appropriate trace element partition coefficients and reasonably well constraint primary (parental) magma compositions are promising tools to extract P-T(-fO2-fH2O) conditions of principal differentiation of igneous rock suites in hydrous, calc-alkaline systems at convergent plate margins from rock (plutonic and volcanic) compositions.

  44. Density – seismic velocity of “arc cumulates”

  45. Cumulate compositions - Picrobasalt - Fractional • Fractional • 20%dunites & wherlites • 40% pyroxenite (sp) • 10% Cpx-hornblendites • 70% UM cumulates extracted at 1000°C • Density / Vp • at 800°C / 1GPa • (Hacker & Abers, 2003) • r3.31 - 3.50 - 3.15 • Vp 8.01 - 7.15 Lz/Hz • r 3.30 / 3.26 • Vp 7.80 / 7.94 Müntener & Ulmer (2006)

  46. Cumulate compositions - basaltic andesite - Fractional • Fractional • 30% pyxroxenites • 15% Cpx- hornblendites • 45% of UM cumulates extracted at 1000°C • Density / Vp • r 3.29 - 3.07 • Vp 7.45 - 7.10

  47. Cumulate compositions - Picrobasalt - Equilibrium 1.5 GPa • Equilibrium • 15% dunites • 40% pyxroxenites • 40% Garnet-pyx- hornblendites >90% of UM cumulates extracted at 1000°C • Density / Vp • r 3.26 - 3.31 -3.20 • Vp 8.00 - 7.25 Müntener & Ulmer (2006)

  48. Cumulate compositions - andesite (differentiated) • Equilibrium variable H2O • garnetites • gar-hornblendites • Gar-gabbros • Hbl-gabbros • Density / Vp • r 3.81 - 2.91 • Vp 8.68 - 6.86 Müntener & Ulmer (2006)

  49. Calculated Densities and p-wave velocities a continuum of Seismic velocities calculated after Hacker & Abers (2003) with 800°C at 1.0 and 1000°C at 1.5 GPa Müntener & Ulmer (2006)

  50. Southern Andean (Chile) Cross Section (23°S) Transitional MOHO - sign of UM-cumulates at base of crust? Graeber & Asch, JGR, 1999

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