1. -3-20 123 4 567  FMQ Arc lavas Arc-related xenoliths Cascades central Mexico Hornblende-rich xenoliths Ichinomegata, Japan Izu-Bonin, Japan Lihir, Papua New Guinea Mexico Arc-related Mantle Oceanic Mantle fO 2 thermobarometry -201234567 Equivalent fO 2 in  NNO units calculated at 1300 o C, 3 GPa Abyssal Peridotites & MORBs 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. fO 2 of a magma can be modified by: Fractional crystallization Degassing Interaction with crust or fluids fO 2 of mantle xenoliths could have been modified by: Secondary metasomatic processes

2. USE VANADIUM TO OBTAIN ORIGINAL MELTING fO 2 (paleo-fO 2 ) IN MANTLE Partitioning of V is redox-sensitive V 2+, V 3+, V 4+ V is not enriched in metasomatizing fluids/melts V retains a memory of original melting fO 2 V norm to Sc (has similar partitioning to V 3+ ) + 0 2 4 6 8 10 12 14 16 18 20 05101520 V/Sc Melts FMQ FMQ+3 FMQ-3 F FMQ-1 Fractionation Partial melting

3. Primitive arc magmas have V/Sc similar to MORBs! No evidence from V/Sc for extremely high fO 2 s in arc magma sources (mantle wedge?) V/Sc Arc basalts (Cascades) V/Sc Ba/Yb FMQ +1 -2  log(fO 2 ) at F=10 wt. % 3 4 5 6 7 8 9 10 11 12 0100200300400500 Washington No. California Oregon Inc fluid inc fO 2 ? V/Sc appears to correlate with fluid mobile elements X-fe3 range:~0.1-0.3