Abyssal peridotite osmium isotopic compositions from cr‐spinel

Abstract

Abyssal peridotites, fragments of residual upper oceanic mantle, are believed to have less radiogenic Os compositions and higher Os concentrations than primitive upper mantle (PUM, 187Os/188Os = 0.129 [Meisel et al., 1996]). We have measured 187Os/188Os in 10 whole rock abyssal peridotites representing non‐plume‐influenced mid‐ocean ridge segments. The 187Os/188Os ratios range from 0.1183–0.1582. This large range in Os composition, to both less radiogenic and more radiogenic values than primitive upper mantle, can be attributed to ancient melting and subsequent sequestering of isotopic signatures, melt‐rock reaction, or secondary alteration, or a combination of any of the three. Petrographic, electron microprobe, and hand‐sample inspection show the peridotites to have experienced varying amounts of serpentinization and weathering, accompanied by heterogeneity in 187Os/188Os values. In addition, a majority of the peridotites studied here are spatially associated with gabbros having equally, if not more highly, heterogeneous Os isotopic compositions and N‐MORBs with homogeneous Sr, Nd, and Pb isotopic signatures. This suggests secondary seawater alteration as the dominant influence on present‐day Os signatures. We show that careful separation, leaching, and analysis of Cr‐spinel from abyssal peridotite largely removes the radiogenic seawater Os isotopic signature, allowing a more accurate assessment of the Os signature of depleted mid‐ocean ridge basalt mantle (DMM). Cr‐spinel is a highly refractory mantle mineral and commonly host to tiny sulfide inclusions, which are carrier phases for Os. These sulfides are well protected by the Cr‐spinel from high‐temperature serpentinization and low‐temperature seafloor weathering, thereby preserving nonradiogenic DMM‐like Os isotopic signatures. Our analyses of treated Cr‐spinel fractions finds 187Os/188Os ratios that are dominantly less radiogenic (0.1238–0.1482) than corresponding whole rock compositions. Nonmagnetic Cr‐spinels show more highly variable Os compositions and concentrations than magnetic Cr‐spinels, overlapping with the more radiogenic whole rock values. Magnetic Cr‐spinels are dominantly less radiogenic than PUM, with high but variable Os concentrations, possibly resulting from a sulfide “nugget” effect. The systematically lower 187Os/188Os in Cr‐spinels compared to whole rocks shows that whole rock abyssal peridotites are largely compromised by radiogenic seawater interaction, often obscuring the mantle signature. Measurements of treated abyssal peridotite Cr‐spinels in some cases can circumvent this seawater alteration problem allowing more straightforward interpretation of isotopic values and thus further constraining the Os isotopic signature for the depleted upper mantle. Our results suggest that the 187Os/188Os for the upper mantle lies in the range 0.120–0.125.