186Os and 187Os enrichments and high-3He/4He sources in the Earth’s mantle: Evidence from Icelandic picrites

Abstract

Picrites from the neovolcanic zones in Iceland display a range in 187 Os/ 188 Os from 0.1297 to 0.1381 (γ Os = 0.0 to 6.5) and uniform 186 Os/ 188 Os of 0.1198375±32 (2σ). The value for 186 Os/ 188 Os is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398±16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in 186 Os/ 188 Os and 187 Os/ 188 Os from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core-mantle interaction. While some plumes with high 3 He/ 4 He, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not.A positive correlation between 187 Os/ 188 Os and 3 He/ 4 He from 9.6 to 19 R A in Iceland picrites is best modeled as mixtures of 500 Ma or older ancient recycled crust mixed with primitive mantle, creating a hybrid source region that subsequently mixes with the convecting MORB mantle during ascent and melting. This multistage mechanism to explain these isotope systematics is consistent with ancient recycled crust juxtaposed with more primitive, relatively He-rich mantle, in convective isolation from the upper mantle, most likely in the lowermost mantle. This is inconsistent with models that propose random mixing between heterogeneities in the convecting upper mantle as a mechanism to explain the observed isotopic variation in oceanic lavas or models that produce a high 3 He/ 4 He signature in melt depleted and strongly

Source components and magmatic processes in the genesis of Miocene to Quaternary lavas in western Turkey: constraints from HSE distribution and Hf–Pb–Os isotopes

“…In particular, the samples with low Os abundances (<35 ppt) exhibit a significant variation in 187 Os/ 188 Os values that range from 0.28 to 0.47, requiring distinct behavior of Os compared to Sr, Nd, Hf, and Pb isotopes. Further, the samples are usually characterized by higher 187 Os/ 188 Os than that reported for typical mantle source of intraplate magmas ( 187 Os/ 188 Os = 0.12-0.16; Lassiter and Hauri 1998;Brandon et al 2007;Class et al 2009;Ireland et al 2009), which can most likely be attributed to contamination by high 187 Os/ 188 Os crust.…”

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

“…Recent work suggests that the high 3 He/ 4 He mantle may also host a component of recycled eclogite (Dixon et al, 2001;Brandon et al, 2007). In this paper, we argue that the high 3 He/ 4 He mantle sampled by OIBs hosts a component of recycled, refractory eclogite, and that this component balances Earth's budget for the elements titanium, tantalum and niobium (TITAN).…”

Geochemical bias in drill cutting samples versus drill core samples returned from the Reykjanes Geothermal System, Iceland

“…While problems of cutting homogenization are difficult to avoid, the bias in selective retainment of the more resistant, coarse-grained minerals can in part be addressed by sampling and analyzing the fine-grained cuttings fraction. Brandon et al (2007), Chauvel and Hémond (2000), Hemond et al (1993), Koornneef et al (2012), McDonough and Sun (1995), Nielsen et al (2007) and Skovgaard et al (2001). …”