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182W and HSE constraints from 2.7 Ga komatiites on the heterogeneous nature of the Archean mantle

dc.contributor.authorPuchtel, Igor S
dc.contributor.authorBlichert-Toft, Janne
dc.contributor.authorTouboul, Mathieu
dc.contributor.authorWalker, Richard J.
dc.description.abstractWhile the isotopically heterogeneous nature of the terrestrial mantle has long been established, the origin, scale, and longevity of the heterogeneities for different elements and isotopic systems are still debated. Here, we report Nd, Hf, W, and Os isotopic and highly siderophile element (HSE) abundance data for the Boston Creek komatiitic basalt lava flow (BCF) in the 2.7 Ga Abitibi greenstone belt, Canada. This lava flow is characterized by strong depletions in Al and heavy rare earth elements (REE), enrichments in light REE, and initial epsilon143Nd = +2.5 ± 0.2 and initial epsilon176Hf = +4.2 ± 0.9 indicative of derivation from a deep mantle source with time-integrated suprachondritic Sm/Nd and Lu/Hf ratios. The data plot on the terrestrial Nd-Hf array suggesting minimal involvement of early magma ocean processes in the fractionation of lithophile trace elements in the mantle source. This conclusion is supported by a mean mu142Nd = -3.8 ± 2.8 that is unresolvable from terrestrial standards. By contrast, the BCF exhibits a positive 182W anomaly (mu182W = +11.7 ± 4.5), yet is characterized by chondritic initial gamma187Os = +0.1 ± 0.3 and low inferred source HSE abundances (35 ± 5% of those estimated for the present-day Bulk Silicate Earth, BSE). Collectively, these characteristics are unique among Archean komatiite systems studied so far. The deficit in the HSE, coupled with the chondritic Os isotopic composition, but a positive 182W anomaly, are best explained by derivation of the parental BCF magma from a mantle domain characterized by a predominance of HSE-deficient, differentiated late accreted material. According to the model presented here, the mantle domain that gave rise to the BCF received only ~35% of the present-day HSE complement in the BSE before becoming isolated from the rest of the convecting mantle until the time of komatiite emplacement at 2.72 Ga. These new data provide strong evidence for a highly heterogeneous Archean mantle in terms of absolute HSE abundances and W isotopic composition, and also indicate slow mixing, on a timescale of at least 1.8 billion years. Additionally, the data are consistent with a stagnant-lid plate tectonic regime in the Hadean and Archean, prior to the onset of modern-style plate tectonics.en_US
dc.description.sponsorshipThis work was supported by NSF Petrology and Geochemistry grant EAR 1447174 (USA) to ISP, NSF-CSEDI grant EAR 1265169 (USA) to RJW, ANR grant 10-BLANC-0603 M&Ms – Mantle Melting – Measurements, Models, Mechanisms (France) to JBT, and NSF-IF grant EAR 0549300 (USA), which provided partial support for the Triton mass-spectrometer used for most of the measurements in this study.en_US
dc.publisherElsevier - Geochimica et Cosmochimica Actaen_US
dc.subject182Hf-182W; 187Re-187Os; 176Lu-176Hf; 146,147Sm-142,143Nd; Highly siderophile elements; Abitibi greenstone belt; Boston Creek; Komatiite; Stochastic late accretion; Mantle mixingen_US
dc.title182W and HSE constraints from 2.7 Ga komatiites on the heterogeneous nature of the Archean mantleen_US
dc.relation.isAvailableAtCollege of Computer, Mathematical & Natural Sciencesen_us
dc.relation.isAvailableAtDigital Repository at the University of Marylanden_us
dc.relation.isAvailableAtUniversity of Maryland (College Park, MD)en_us

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