An appraisal of mantle metasomatism based upon oxidation states, trace element and isotope geochemistry, and fluid/rock ratios in spinel lherzolite xenoliths

The relationship between the oxidation states of spinel lherzolite xenoliths and their trace element and O-isotope geochemistries has been investigated from the standpoint of 'effective' fluid/rock ratios, mantle fluid compositions, and timescales for metasomatic events in the upper mantle. Spinel lherzolite xenoliths from nine localities in the SW USA, Japan, Australia, Germany, and France have been examined. Oxygen fugacities (fO{dollar}sb2{dollar}) were calculated using spinel lherzolite thermobarametery (Nell and Wood, 1989, 1990), and data are presented in log units relative to the quartz-fayalite-magnetite buffer ({dollar}Delta{dollar}QFM). Trace element compositions of clinopyroxene and amphibole were determined by a combination of instrumental neutron activation analysis (INAA) and secondary ion mass spectrometry (SIMS). O-isotope ratios were determined for clinopyroxene, orthopyroxene, and olivine according to the method of Clayton and Mayeda (1963).; Oxygen fugacities for the suites under consideration span a range of {dollar}-{dollar}2.8 to +1.2 log units relative to QFM, with the overall lowest fO{dollar}sb2{dollar}'s observed for the southwestern United States ({dollar}-{dollar}2.8 to +0.3 {dollar}Delta{dollar}QFM). One sample, a wehrlite from Dish Hill (DH140) has a lower fO{dollar}sb2{dollar} ({dollar}-{dollar}3.3 {dollar}Delta{dollar}QFM), but because of the uncertain genetic relationship between wehrlites and lherzolites, it is presented in the data set but is not a factor in the conclusions drawn. The Eifel Volcanic Field (Germany) displayed the most oxidized fO{dollar}sb2{dollar} range (+0.2 to +1.2 {dollar}Delta{dollar}QFM). High oxygen fugacities were not observed to correlate with either trace element or {dollar}sp{lcub}18{rcub}{dollar}O enrichment. The oxidation state of spinel lherzolite xenoliths, instead bear a direct relation to the style of metasomatism and the tectonic environment. The most oxidized suites (Eifel, Dish Hill, Ichinomegata, Mont Briancon) are continental arc or back-arc environments, containing amphibole {dollar}pm{dollar}phlogopite. This is consistent with the contention that dehydration of the subducted slab and progressive metasomatism of the overlying mantle wedge result in oxidation (Wood et al., 1990).; Calculations of the 'effective' fluid/rock ratios for these mantle systems were performed based upon a change in fO{dollar}sb2{dollar} of +2 log units {dollar}Delta{dollar}QFM, and O-isotope disequilibrium. Both calculations indicate that infinitesimal amounts of fluid in the mantle can significantly change the oxygen fugacity, thereby decoupling it from the other parameters of metasomatism.