Publication Type:Journal Article
Source:Geochimica et Cosmochimica Acta, Elsevier, New York, NY, International, Volume 74, Number 4, p.1448-1470 (2010)
Keywords:alloys, asteroids, basalts, carbonaceous chondrites, chemical composition, chondrites, core, cosmochemistry, enrichment, geochemistry, ICP mass spectra, igneous rocks, Isotopes, magmas, magmatic differentiation, major elements, mantle, mass balance, mass spectra, melting, metals, meteorites, mid-ocean ridge basalts, ophiolite, osmium, peridotites, planetary interiors, planetology, platinum group, plutonic rocks, radioactive isotopes, spectra, stony meteorites, time scales, Trace elements, tungsten, ultramafics, upper mantle, volcanic rocks
Tungsten is a moderately siderophile high-field-strength element that is hydrophile and widely regarded as highly incompatible during mantle melting. In an effort to extend empirical knowledge regarding the behaviour of W during the latter process, we report new high-precision trace element data (W, Th, U, Ba, La, Sm) that represent both terrestrial and planetary reservoirs: MORB (11), abyssal peridotites (8), eucrite basalts (3), and carbonaceous chondrites (8). A full trace element suite is also reported for Cordilleran Permian ophiolite peridotites (12) to better constrain the behaviour of W in the upper mantle. In addition, we report our long-term averages for a number of USGS (BIR-1, BHVO-1, BHVO-2, PCC-1, DTS-1) and GSJ (JA-3, JP-1) standard reference materials, some of which we conclude to be heterogeneous and contaminated with respect to W. The most significant finding of this study is that many of the highly depleted upper mantle peridotites contain far higher W concentrations than expected. In the absence of convincing indications for alteration, re-enrichment or contamination, we propose that the W excess was caused by retention in an Os-Ir alloy phase, whose stability is dependent on fO (sub 2) of the mantle source region. This explanation could help to account for the particularly low W content of N-MORB and implies that the lithophile behaviour of W in basaltic rocks is not an accurate representation of the behavior in the melt source. These findings then become relevant to the interpretation of W-isotopic data for achondrites, where the fractionation of Hf from W during melting is used to infer the Hf/W of the parent body mantle. This is exemplified by the differentiation chronology of the eucrite parent body (EPB), which has been modeled with a melt source with high Hf/W. By contrast, we explore the alternative scenario with a low mantle Hf/W on the EPB. Using available eucrite literature data, a maximum core segregation age of 1.2 + or - 1.2Myr after the closure of CAIs is calculated with a more prolonged time between core formation and mantle fractionation of ca. 2Myr. This timeline is consistent with most recent published chronologies of the EPB differentiation based on the (super 53) Mn- (super 53) Cr and (super 26) Al- (super 26) Mg systems. Abstract Copyright (2010) Elsevier, B.V.
GeoRef, Copyright 2018, American Geological Institute.<br/>2010-045480<br/>Al/Mg<br/>Mn/Cr