T153 Interdisciplinary Investigations of Mantle-Crustal Mass Transfer

Tuesday, 6 November 2018
8:00 AM - 12:00 PM
Room 140

CU-ISOTOPE HETEROGENEITY IN THE LITHOSPHERIC MANTLE: A ROLE FOR SUBDUCTION-DERIVED FLUIDS?

Paper number: T153-8
Program: Technical Sessions
Time: 9:50 AM – 10:05 AM

Authors

Abstract

The Cu-isotope composition of the mantle is thought to be homogeneous (0.06 ± 0.20‰), and high temperature mantle processes are not believed to significantly fractionate Cu isotopes. However, metasomatized peridotites exhibit a significantly larger range of δ65Cu values than mantle-derived basalts and unmetasomatized mantle lithologies (‒0.6 to +1.8‰). To better understand the origin of this heterogeneity, we are investigating a suite of ultramafic xenoliths hosted in Plio-Pleistocene-age alkali basalts from the Geronimo Volcanic Field, SE Arizona. The suite includes both Type I (i.e. Cr- and Mg-rich spinel lherzolite, harzburgite, and websterite) and Type II (Fe-, Al- and Ti-rich clinopyroxenite, wehrlite, and amphibole clinopyroxenite) lithologies. Evidence for both cryptic and modal metasomatism exists within the xenolith suite. The Cu isotope compositions of 18 mantle peridotites and 5 alkali basalt have been determined using both an aqua regia (AR) leaching technique and full wholerock (WR) dissolution. The WR δ65Cu value for one unmetasomatized Type I lherzolite (+0.2‰) is within the published range for bulk silicate earth, whereas metasomatized Type I rocks range to heavier values (+0.01 to +1.12‰); AR leaching results for the same samples are much lighter (‒0.8 to ‒0.4‰). Type II xenoliths exhibit a much wider range in compositions than Type I (WR = ‒0.05 to +1.44‰; AR = ‒0.29 to +3.88‰). These compositional variations are also observed at the hand specimen scale. AR leaching results for one composite xenolith (a Type I lherzolite crosscut by a Type II clinopyroxenite vein) yield heavier δ65Cu in the vein (+2.2‰) than in the lherzolite wall rock (‒0.8‰). Host basalts have lighter δ65Cu than most xenoliths (‒1.3 to ‒0.23‰) and so cannot be responsible for the spread to heavy δ65Cu seen, particularly in Type II peridotites. These data suggest that the lithospheric mantle is heterogeneous with respect to δ65Cu, particularly among Fe-Al-Ti-rich Type II rocks. The heavy isotope ratios observed suggest a role for recycling of Cu via subduction of hydrothermally altered oceanic lithosphere, which has been reported to have δ65Cu values up to +5‰. Future work will focus on identifying the mineral host(s) for the Cu in Type II xenoliths and the origin of the sample-scale heterogeneity observed.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-318235

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