Journal of Petrology Advance Access originally published online on June 13, 2005
Journal of Petrology 2005 46(11):2313-2336; doi:10.1093/petrology/egi056
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Similar V/Sc Systematics in MORB and Arc Basalts: Implications for the Oxygen Fugacities of their Mantle Source Regions
1 DEPARTMENT OF EARTH SCIENCES, MS-126, RICE UNIVERSITY, 6100 MAIN STREET, HOUSTON, TX 77005, USA
2 SCHOOL OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF VICTORIA, PETCH BUILDING, ROOM 280, 3800 FINNERTY ROAD, VICTORIA, BC, CANADA V8W 3P6
RECEIVED JULY 15, 2004; ACCEPTED MAY 3, 2005
V/Sc systematics in peridotites, mid-ocean ridge basalts and arc basalts are investigated to constrain the variation of fO2 in the asthenospheric mantle. V/Sc ratios are used here to ‘see through’ those processes that can modify barometric fO2 determinations in mantle rocks and/or magmas: early fractional crystallization, degassing, crustal assimilation and mantle metasomatism. Melting models are combined here with a literature database on peridotites, arc lavas and mid-ocean ridge basalts, along with new, more precise data on peridotites and selected arc lavas. V/Sc ratios in primitive arc lavas from the Cascades magmatic arc are correlated with fluid-mobile elements (e.g. Ba and K), indicating that fluids may subtly influence fO2 during melting. However, for the most part, the average V/Sc-inferred fO2s of arc basalts, MORB and peridotites are remarkably similar (–1·25 to +0·5 log units from the FMQ buffer) and disagree with the observation that the barometric fO2s of arc lavas are several orders of magnitude higher. These observations suggest that the upper part of the Earth's mantle may be strongly buffered in terms of fO2. The higher barometric fO2s of arc lavas and some arc-related xenoliths may be due respectively to magmatic differentiation processes and to exposure to large, time-integrated fluid fluxes incurred during the long-term stability of the lithospheric mantle.
KEY WORDS: vanadium; scandium; oxygen fugacity; mantle; arcs
* Corresponding author. Telephone: 1-713-348-5084. E-mail: ctlee{at}rice.edu
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