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Recycling of oceanic crust through subduction, mantle upwelling, and remelting in mantle plumes is a widely accepted mechanism to explain ocean island volcanism (1). The timescale of this recycling is important to our understanding of mantle circulation rates. Correlations of uranogenic lead isotopes in lavas from ocean islands such as Hawaii or Iceland, when interpreted as model isochrons, have yielded source differentiation ages between 1 and 2.5 billion years (Gyr) (2-5). However, if such correlations are produced by mixing of unrelated mantle components (6) they will have no direct age significance. Re-Os decay model ages take into account the mixing of sources with different histories (7,8), but they depend on the assumed initial Re/Os ratio of the subducted crust, which is poorly constrained because of the high mobility of rhenium during subduction (9). Here we report the first data on [sup.87]Sr/[sup.86]Sr ratios for 138 melt inclusions in olivine phenocrysts from lavas of Mauna Loa shield volcano, Hawaii, indicating enormous mantle source heterogeneity. We show that highly radiogenic strontium in severely rubidium-depleted melt inclusions matches the isotopic composition of 200-650-Myr-old sea water. We infer that such sea water must have contaminated the Mauna Loa source rock, before subduction, imparting a unique 'time stamp' on this source. Small amounts of seawater-derived strontium in plume sources may be common but can be identified clearly only in ultra-depleted melts originating from generally highly (incompatible-element) depleted source components. The presence of 200-650-Myr-old oceanic crust in the source of Hawaiian lavas implies a timescale of general mantle circulation with an average rate of about 2 ([+ or -]1) cm [yr.sup.-1], much faster than previously thought., Melt inclusions in highly magnesian olivine are commonly used as proxies for parental melts (10). In addition, melt inclusions provide unique information on the isotope heterogeneity of the mantle sources [...]

The Earth's mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by [...]

Determination of zircon ages as well as geochemical and Sm-Nd isotope systematics of granitoids in the Khetri Copper Belt of the Aravalli mountains, NW India, constrain the late Palaeoproterozoic crustal evolution of the Aravalli craton. The plutons are typical A-type within-plate granites, derived from melts generated in an extensional tectonic environment. They display REE and multi-element patterns characterized by steep LREE-enriched and almost flat HREE profiles and distinct negative anomalies for Sr, P and Ti. Initial end values range from--1.3 to--6.2 and correspond to crustal sources with mean crustal residence ages of 2.5 to 2.1 Ga. A lower mafic crustal anatectic origin is envisaged for these granitoids, and the heterogeneous [[epsilon].sub.Nd(t)] values are inferred to have been acquired from the magma source region. Zircon Pb-Pb evaporation and U-Pb ages indicate widespread rift-related A-type magmatism at 1711--1660 Ma in the northern Delhi belt and also suggest a discrete older magmatic event at around 1800 Ma. The emplacement ages of the compositionally distinct A-type granitoid plutons, and virtually coeval granulite metamorphism and exhumation in another segment of the Aravalli mountains, further signify that part of the Aravalli crust evolved during a widespread extensional event in late Palaeoproterozoic time. Keywords: Aravalli craton, Palaeoproterozoic, A-type granites, zircon chronometry, Nd isotopes.

Author(s): Alexander V. Sobolev (corresponding author) [1, 2]; Albrecht W. Hofmann [1]; Stephan V. Sobolev [3, 4]; Igor K. Nikogosian [5, 6] The upper mantle consists largely of peridotite with [...]

Author(s): Eric Hellebrand (corresponding author) [1]; Jonathan E. Snow [1]; Henry J. B. Dick [2]; Albrecht W. Hofmann [1] Rocks in the Earth's uppermost sub-oceanic mantle, known as abyssal peridotites, [...]

Author(s): Alexander V. Sobolev (corresponding author) [1, 2]; Albrecht W. Hofmann [1]; Igor K. Nikogosian [1, 3, 2] The hypothesis that mantle plumes contain recycled oceanic crust [1] is now [...]