Similar V/Sc Systematics in MORB and Arc Basalts: Implications for the Oxygen Fugacities of their Mantle Source Regions

JOURNAL OF PETROLOGY VOLUME 46 NUMBER 11 PAGES 2313–2336 2005 doi:10.1093/petrology/egi056 Similar V/Sc Systematics in MORB and Arc Basalts: Implications for the Oxygen Fugacities of their Mantle Source Regions CIN-TY AEOLUS LEE1*, WILLIAM P. LEEMAN1, DANTE CANIL2 AND ZHENG-XUE A LI1 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 ADVANCE ACCESS PUBLICATION JUNE 13, 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 fluidmobile 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 f O2s of arc basalts, MORB and peridotites are remarkably similar (
125 to þ05 log units from the FMQ buffer) and disagree with the observation that the barometric f O2s 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. The purpose of this study is to constrain the oxygen fugacity ( f O2) of the asthenosphere—that part of the upper mantle that is mobile enough to undergo adiabatic decompression and is the dominant source region for most juvenile magmas in arcs and in mid-ocean ridges. Oxygen fugacity is defined as the activity (or roughly the partial pressure) of O2 within a system (Carmichael, 1991; Frost, 1991; Kress & Carmichael, 1991) and represents an intensive variable (such as pressure and temperature) that provides a measure of the system’s redox potential at equilibrium. Our interest in f O2 is motivated by the fact that fO2 controls the valence state, and hence speciation, of redox-sensitive elements. For example, f O2 controls the mobility of redox-sensitive trace metals (e.g. first series transition metals and the platinum group elements) and the species composition of volcanic gases emitted to the ocean–atmosphere system. Understanding how fO2 varies in space and time therefore has important implications for studies of ore genesis, atmospheric evolution and trace-element partitioning. In practice, fO2 is determined indirectly by O2 thermobarometry wherein the activities of the different valence states of a redox-sensitive element in minerals and/or glasses are measured. In homogeneous (single phase) systems, e.g. a melt, experimentally calibrated relationships between f O2 and the activities of Fe3þ and Fe2þ in a glass are typically used to obtain the f O2 of a magma just prior to quenching (Christie et al., 1986; Kress & Carmichael, 1991). In a heterogeneous (e.g. multiphase) system, the f O2 of last equilibration is recorded by the distribution of Fe3þ in different system phases. For example, pertinent to peridotitic mantle lithologies, *Corresponding
The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org KEY WORDS: vanadium; scandium; oxygen fugacity; mantle; arcs INTRODUCTION author. Telephone: 1-713-348-5084. E-mail: JOURNAL OF PETROLOGY VOLUME 46 the reaction 6Fe2 SiO4ðOlÞ þ O2ðfluidÞ ¼ 2Fe3 O4ðSpÞ þ 3Fe2 Si2 O6ðOpxÞ ð1Þ where fayalite, magnetite and ferrosilite appear as components in solid solution with olivine, spinel and orthopyroxene, respectively (Mattioli & Wood, 1986; Wood & Virgo, 1989; Ballhaus et al., 1990, 1991), provides an indirect measure of the f O2 of a peridotite just before it cooled through its closure temperature. These methods for estimating the fO2 of last equilibration are referred to here as ‘barometric f O2’. The barometric method of inferring f O2 has now been widely applied to mantle xenoliths and lavas. Mid-ocean ridge basalts (MORB) appear to have barometric f O2s systematically lower than that of arc lavas: relative to the fayalite–magnetite–quartz (FMQ) buffer, MORB have fO2s between
2 and 0 log units from the FMQ buffer (herein referred to as FMQ–2 and FMQ), whereas arc lavas have f O2s ranging between FMQ and FMQþ6 (Christie et al., 1986; Carmichael, 1991). If these magma fO2s directly reflect that of their mantle source regions, it must be concluded that sub-arc mantle is more oxidized than ambient mantle. That sub-arc asthenosphere is oxidized is, arguably, a widely accepted view, and has led to the paradigm that oxidation of the mantle is related, in some manner, to the availability of oxidized fluids or sediments originating from the subducting slab (Carmichael, 1991). Possibilities include overprinting by hydrous fluids characterized by high f O2 (Carmichael, 1991) and/or addition of silicic magmas derived from melting of hot subducting and oxidized oceanic crust (Mungall, 2002). The problem, however, is that true primary arc magmas are extremely rare. This begs the question of whether the f O2s of arc magmas are really representative of their magma source regions. In fact, a number of magmatic differentiation processes can lead to oxidation of a magma. For example, early fractional crystallization of reduced minerals, interaction with oxidizing fluids or wall rock derived from continental crust, or degassing of reduced species of C, H and S (Mathez, 1984) may lead to an increase in fO2. In addition, reaction of H2O with reduced species (e.g. Fe2þ) in conjunction with preferential loss of H2 to the atmosphere can result in auto-oxidation of the magma without any opensystem exchange other than H2 loss (Sato & Wright, 1996; Holloway, 2004). Given these possibilities for increasing oxidation, the range in barometric fO2 of arc lavas is a maximum bound on their source regions, e.g. arc asthenosphere. An alternative approach for determining the f O2 of arc asthenospheric mantle is to study arc mantle xenoliths. Indeed, peridotite xenoliths from arc-related environments, such as those from Simcoe near the NUMBER 11 NOVEMBER 2005 Washington Cascades, Japan, and the Solomon Islands (Brandon & Draper, 1996; Parkinson & Arculus, 1999), appe (...truncated)