Experimental constraints on subduction-related magmatism : Hydrous Melting of upper mantle perdotites Modified after a ppt by Peter Ulmer (Blumone, Adamello, Italy) Garnet-peridotite in kimberlite

Topics Mantle composition? How do we constrain them? Dry Melting of mantle peridotites Hydrous Melting: Basic concepts Hydrous Mantle Melting: P-T-f-x relationships Conclusion: Arc primary mantle magmas are: “basaltic”, representing relatively large melt fractions wet (hydrous) hot (>1200°C) oxidized (NNO – NNO+2)

Arc-Genesis Model I

P-T Lherzolite (Dry) Melting

Evidence for Mass transfer: Metasomatic Peridotites Phlogopite-Peridotite in kimberlite Carbonatite-globule phl olivine CO 2 -rich fluid inclusions Mantle-cpx in basanite

Dry Lherzolite Melting Fundamental Principles (Phase Equilibria) Pressure effects on melting and composition of primary melts Temperature effects on melting and compositions of primary melts

Fundamentals: Forsterite – SiO 2 Pressure < 1.4 kbar

Fundamentals: Forsterite – SiO 2 Pressure < 1.4 kbarPressure > 1.4 kbar PeritecticCotectic (thermal max)

Anhydrous Peridotite Melting: Melt fraction as a function of pressure and source composition Ulmer (2001)

Anhydrous Peridotite Melting: Solidus temperatures and melt compositions as a function of source composition at 3 GPa Hirschmann (2003)

Hydrous Lherzolite Melting Fundamental principles (phase equilibria) Pressure – H 2 O effects on melting and composition of primary melts Temperature effects on melting and compositions of primary melts Geochemical signatures of “Arc” magmas

Diopside – Peridotite – H 2 O - Melting

H 2 O – solubility in basalt and albite liquids at 1100°C

Schematic diagram showing melting phase relations for a system containing Anhydrous minerals (A) Hydrous mineral (H) H 2 O (V) Important (univariant) curves: H 2 O-saturated solidus (A) Dehydration solidus (V) Dry solidus (V) (low right)

Peridotite – H 2 O Melting ACMA: Average Current Mantle Adiabat (diamond symbols: multiply saturated primary liquids = extraction depth?)

multiple saturation (ol+opx±cpx) of primitive arc magmas peridotite = mantle orthopyroxene olivine clinopyroxene spinel primitive arc magma

Inverse – multiple saturation experiments on primary arc basalts

Picrobasalt (3 wt.% H 2 O) phase diagram with multiple saturation

Hydrous Peridotite Melting: Melt fraction as function of MELT H 2 O-content Ulmer (2001)

Effects of small amounts of H 2 O (in source) on melt-fractions

Katz et al (2003) Parameterization (mostly thermodynamically based, including PMELTS)

Evidences for hydrous nature of “arc” magmas and geochemical characteristics of supra-subduction magmas  Violent, explosive, gas-rich (H 2 O) eruptions typical for differentiated magmas (andesite – rhyolite)  Melt inclusions (up to >10 wt.% H 2 O in primitive Olivine inclusions (e.g. Shasta, Hess, Grove, Sisson and co-workers)  Early amphibole (and biotite) saturation indicating > 4 wt% H 2 O at time of crystallization  Geochemical characteristics of supra-subduction magmas (major and traces) => “Calc-alkaline” and “arc trace element signature” of magmas and their ( metasomatized ) mantle sources  high fO 2 probably related to oxidation by slab-derived fluids (Fe- isotopes indicate reduced arc mantle prior to fluid metasomatism)

“Spiderdiagram” of Island Arc Basalts (IAB) HFSE – depletion, LILE and LREE enrichment, fluid mobile elements, residual rutile and garnet to retain HFSE and HREE in “slab source” during dehydration

Spiderdiagram of Philippine Mantle Xenoliths

Major Element composition of MORB - IAB Arcs: Silica enrichment and FeO-suppression due to late plag, early amph and mag

Mantle melting trend to high-SiO 2 - low FeO*/MgO is controlled by reaction relations during ascent to the base of the crust Opx = Olivine + Liquid (SiO 2 -component) Grove et al. (03)

Composition of primitive arc magmas wt. % Picro-Olivine-SiO 2 -richHigh-MgBoninite BasaltTholeiite Andesite SiO TiO Al 2 O Fe 2 O FeO MgO CaO Na 2 O K2OK2O x Mg max. Press:30kb18kb12kb7-10kbca.10

Oxygen Fugacity from Ol-Spinel oxybarometry

Oxygen Fugacity from volcanic glasses

Mature Island Arc (after Ringwood, 1974)

4 points to remember:  Presence of H 2 O during melting leads to enormous solidus depression (function of pressure => solubility)  However, geochemistry (major elements) and experimental constraints indicate significant melt fractions (10-20%) generated at conditions close to the mantle adiabat (>1200°C)  Arc magmas are more siliceous at a given pressure compared to dry tholeiites (MORB, OIB) => “calc-alkaline*  Arc magmas carry particular signatures (trace elements, fO 2, fH 2 O) that can be linked to slab-derived components => Primary mantle melts are: basaltic, hot, wet, oxidized

Groves chlorite solidus

Grove’s “primitive “ melts???????

Ulmer Conclusion: Arc primary mantle magmas are: “basaltic”, representing relatively large melt fractions “basaltic”, representing relatively large melt fractions wet (hydrous) wet (hydrous) hot (>1200°C) hot (>1200°C) oxidized (NNO – NNO+2) oxidized (NNO – NNO+2)

Grove Small fractions Can be as high in silica as 60% Wet Low temps ( C) Oxidized Oxidized

Conclusions for us Lets assume that wet basalts are in fact the most common compositions coming out of the mantle beneath arcs Will have to test this against observational data to see Need to look at primitive basalts from various deep crustal arc sections.

Deep crustal sections Salinia Sierra Valle Fertil Southern Sierra Parts of the Cascades

Chapman et al., 2014

Famatinia at Valle Fertil Tilted section Down to 30 km Up to 10 km mafic section Upper crust: granodiorite From Walker et al., 2015

CONCLUSIONS Primitive arc magmas are basalts NOT andesites if periditotite is the source Wet basalts They stall in the mid to lower crust and fractionate hb, cpx and opx Basalt fractionation does not lead to anything more than a quartz diorite Lifting the SiO2 above 55-58% requires upper crustal contributions