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fO 2 thermobarometry. Equivalent fO 2 in D NNO units calculated at 1300 o C, 3 GPa. -2. -1. 0. 1. 2. 3. 4. 5. 6. 7. Oceanic Mantle. Abyssal Peridotites & MORBs. Arc-related Mantle. Arc lavas. central Mexico Hornblende-rich xenoliths. Cascades. Arc-related xenoliths.

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D FMQ


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    1. fO2 thermobarometry Equivalent fO2 in DNNO units calculated at 1300 oC, 3 GPa -2 -1 0 1 2 3 4 5 6 7 Oceanic Mantle Abyssal Peridotites & MORBs Arc-related Mantle Arc lavas central Mexico Hornblende-rich xenoliths Cascades Arc-related xenoliths Ichinomegata, Japan Izu-Bonin, Japan Lihir, Papua New Guinea Mexico 1 2 3 5 6 7 -3 -2 -1 0 4 DFMQ Why don’t “arc” xenoliths match the lavas in arcs? Fugacity Haiku Oxygen fugacity, like temperature, intensive parameter. Fugacity bears no mem’ry, no sign of a past, uh only what it sees last. • fO2 of a magma can be modified by: • Fractional crystallization • Degassing • Interaction with crust or fluids • fO2 of mantle xenoliths could have been modified by: • Secondary metasomatic processes

    2. 100 Canil Vanadium 10 Melts 1 D 20 0.1 V/Sc 18 FMQ+3 OPX 16 Cpx+Ol+Plag Olivine 0.01 CPX 14 10 Olivine 0.001 12 Spinel 9 FMQ+3 10 0.0001 FMQ FMQ+1 8 -5 -4 -3 -2 -1 0 1 2 3 4 5 8 FMQ -1 6 V/Sc FMQ-1 DlogfO2(FMQ) 4 7 2 FMQ -3 FMQ-3 6 0 0 5 10 15 20 F FMQ -5 5 4 4 5 6 7 8 9 10 11 12 13 14 15 MgO USE VANADIUM TO OBTAIN ORIGINAL MELTING fO2 (paleo-fO2) IN MANTLE • Partitioning of V is redox-sensitive • V2+, V3+, V4+ • V is not enriched in metasomatizing fluids/melts • V retains a memory of original melting fO2 • V norm to Sc (has similar partitioning to V3+) + Fractionation Partial melting

    3. Arc basalts (Cascades) V/Sc appears to correlate with fluid mobile elements +1 inc fO2 ? 12 11 V/Sc FMQ Dlog(fO2) at F=10 wt. % 10 X-fe3 range:~0.1-0.3 9 -1 Inc fluid 8 7 -2 6 Ba/Yb 5 4 -2 FMQ +2 +3 -3 -1 +1 3 0.35 0 100 200 300 400 500 Basalts 0.30 MORB 8-12 wt. % MgO 0.25 Relative Frequency 0.20 0.15 Washington No. California Arc Literature data (GEOROC & RidgePetDB) 0.10 Oregon 0.05 0.00 0 5 10 15 20 • Primitive arc magmas have V/Sc similar to MORBs! • No evidence from V/Sc for extremely high fO2s in arc magma sources (mantle wedge?) V/Sc