Abyssal peridotite osmium isotopic compositions from cr‐spinel

Abstract

[1] 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, 187 Os/ 188 Os = 0.129 [Meisel et al., 1996]). We have measured 187 Os/ 188 Os in 10 whole rock abyssal peridotites representing non-plume-influenced mid-ocean ridge segments. The 187 Os/ 188 Os 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 187 Os/ 188 Os 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 187 Os/ 188 Os 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 187 Os/ 188 Os 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

Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications

“…In cumulate dunites, Cr-spinel is one of the earliest minerals to crystallize from the magma, and it is enriched in Os relative to Re. Cr-spinel not only may preserve its Os isotopic composition during later contamination of the bulk rock [28][29][30], but also can rapidly and precisely record geochemical information from the convecting upper mantle source. Thus, Cr-spinel can provide a unique insight into the Os isotopic characteristics of oceanic lithospheric mantle.…”

Dismantling the deep earth : geochemical constraints from hotspot lavas for the origin and lengthscales of mantle heterogeneity

“…While HIMU basalts can have positive Nb and Ta anomalies (Weaver et al, 1987;Weaver et al, 1991;Chauvel et al, 1992), they generally have lower anomalies than high 3 He/ 4 He lavas, and can even have negative Nb anomalies (Sun and McDonough, 1989). Importantly, HIMU lavas exhibit flat or negative Ti-anomalies , and thus lack the positive Ti-anomalies observed in high 3 (Hauri et al, 1996) similar to DMM (Standish et al, 2002) (Fig. 2) The lack of large TITAN anomalies in DMM (Fig.…”

“…The 187 Os/ 188 Os of DMM and Primitive Mantle are not very different (0.12-0.13; Standish et al, 2002;Meisel et al, 2001), and thus the isolated, ancient asthenospheric peridotite portions of the subducted plates are assumed to have an intermediate present-day composition (0.125). DMM is also assumed to have had an Os concentration of 3000 ppt over geologic time.…”

Big insights from tiny peridotites: Evidence for persistence of Precambrian lithosphere beneath the eastern North China Craton

“…1 of Ionov et al (2005) for the locality of Tok) exhibit remarkable similarities in chemical compositions (including a large range of Fo values and PGE patterns; Ionov et al, 2005Ionov et al, , 2006a and Os isotope compositions (Ionov et al, 2006a) to the Fuxin peridotites. Given their similar tectonic settings (both close to the Xing-Meng collisional orogen and PaleoPacific plate subduction), it is envisioned that similar lithospheric (Shirey and Walker, 1998 Parkinson et al (1998), Brandon et al (2000), Standish et al (2002), Harvey et al (2006), and Liu et al (2008). thinning and replacement occurred on the southeastern margin of the Siberia Craton during the Mesozoic-Cenozoic era, as suggested by Ionov et al (2006b).…”