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Author(s): Olivier Alard (corresponding author) [1, 2, 3]; Ambre Luguet [4, 5]; Norman J. Pearson [1]; William L. Griffin [1]; Jean-Pierre Lorand [4]; Abdelmouhcine Gannoun [2]; Kevin W. Burton [2]; [...]

The Yangkou garnet-peridotite complex in the Sulu UHP terrane, eastern China, consists of several blocks, tens of meters across, enclosed within gneiss. The blocks consist mainly of garnet lherzolite and garnet clinopyroxenite with small amounts of eclogite. The complex displays a finely layered stRucture from outcrop to thin-section scales. Lherzolitic garnet is generally homogenous in composition ([Prp.sub.64-67][Alm.sub.23-24][Grs.sub.10-12][Sps.sub.1]). Garnet (PrpsI-63) from clinopyroxenite contains more pyrope component than eclogitic garnets ([Prp.sub.29-35]). Porphyroblastic garnet from clinopyroxenite and eclogite contains abundant exsolved rods of futile and apatite ([+ or -]Cpx). Lherzolitic garnet exhibits an unusual sinusoidal REE pattern characterized by a decrease in abundance from La to Pr and an increase from Nd to the HREE, and shows Ba and Pb enrichment and negative Nb and Ni anomalies in primitive mantle-normalized trace-element plots. In contrast, diopside shows a normal REE pattern with decreasing abundances from La to Lu, a peak at Ba and Pb, and depletion in Co and Ni. Garnet from eclogite has a REE pattern with increasing abundances from La to Lu, or a maximum at Eu. Eclogitic clinopyroxene shows humped REE patterns with a peak at Nd. Except for slightly lower [[delta].sup.18][O.sub.VSMOW] of +4.40% for olivine, O-isotopic compositions (+5.03 to +5.53%) of analyzed garnets and clinopyroxenes from all three rock types lie in the range of mantle [[delta].sup.18]O values of +5.1 to +6.6%0. Small O-isotope fractionations ([DELTA]Grt-Cpx: -0.16 to -0.35) in eclogite suggest isotopic equilibrium. On the basis of petrochemical and isotopic data, this garnet-peridotite complex was initially a mantle peridotite cut by mafic veins. These rocks experienced metasomatism and subduction-zone UHP metamorphism at 750 [+ or -] 50 [degrees]C and 40 to 45 kbar with a low thermal gradient. The fine-scale layering probably formed as a result of strong shear deformation during exhumation.

The Hf isotope composition of rutile in mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths from the Kimberley area of South Africa has been analyzed in situ by laser ablation microprobe--multicollector--inductively coupled plasma mass spectrometry to investigate metasomatic processes in the lithospheric mantle. The [sup.176]Hf/[sup.177]Hf ratio shows a wide range, from 0.2811 to 0.2858 ([[epsilon].sub.Hf] = -55 to +110); much of this range is found within single samples and even within single grains. We suggest that the MARID rutiles initially resulted from the interaction of an asthenospheric melt with ancient depleted harzburgitic mantle with low [sup.176]Hf/[sup.177]Hf ratios ( Keywords: MARID xenoliths, mantle rutile, hafnium isotopes, mantle xenoliths, kimberlitic metasomatism, South Africa lithosphere.

Small volumes of garnet peridotite occur within large dunite bodies in the Western Gneiss Region of the Norwegian Caledonides. Previous Sm-Nd analyses of garnet peridotite yield Proterozoic ages close to the crystallization ages of the host gneisses. In situ (laser-ablation multicollector--inductively coupled plasma--mass spectrometry) Re-Os analysis of sulfides in the garnet peridotites gives a range of Proterozoic and Archean model ages. An Archean (2.7-3.1 Ga) protolith age is supported by whole-rock Re-Os data for sulfide-free dunites from several bodies. The Archean ages imply a melt-extraction event in the lithospheric mantle that predates the growth of the known Proterozoic upper crust in the region. Keywords: Norwegian Caledonides, mantle peridotites, sulfide ages, Re-Os ages, Archean lithospheric mantle.

U-Pb and Hf isotope analyses of zircons from felsic granulite xenoliths in Mesozoic volcanics reveal Early Archean ([greater than or equal to] 3.6 Ga) lower crust beneath the younger ( Keywords: North China craton, Hf isotopes, zircon U-Pb dating, lower crustal granulites, Archean crustal evolution.

The Os isotope compositions of sulfides in mantle xenoliths from the Penghu Islands, Taiwan Strait, reveal the presence of Proterozoic subcontinental lithospheric mantle beneath the highly extended southeast margin of the South China block. Both [T.sub.RD] (Re depleted) model ages for individual sulfides and model ages estimated from the initial [sup.187]Os/[sup.188]Os ratios of Re-Os mixing lines require that some volumes of the subcontinental lithospheric mantle formed prior to 2.3-1.9 Ga. Later events in the subcontinental lithospheric mantle may be recorded by [T.sub.RD] model ages of 1.5-1.2 Ga and ca. 0.9 Ga. The events recognized in the subcontinental lithospheric mantle are consistent with those known in the crust of the mainland South China block. The sulfide Os isotope data show that Proterozoic lithosphere beneath the South China block has survived the extensive Mesozoic Yanshanian magmatism on the continental margin and has not been delaminated even during the severe lithospheric extension that led to the subsidence of the Taiwan Strait. Keywords: lithospheric mantle, Re-Os, sulfides, Taiwan Strait, Proterozoic, South China block.

Quenched carbonate-silicate inclusions in lherzolitic clinopyroxene macrocrysts, derived from 200 km beneath the Slave craton in northern Canada, are interpreted as natural samples of mantle carbonatites. Oxygen, carbon, and strontium isotope data provide evidence for the involvement of subducted crustal material in the origin of these carbonatites, supporting suggestions that carbon recycling by subduction is an important prerequisite for carbonatite magmatism. The compositional range of the inclusions suggests that the parent melt was decreasing in silica content as it was trapped in the host crystal, a trend that is predicted experimentally. Isotopic disequilibrium between the carbonatitic inclusions and the host clinopyroxene indicates that they were trapped shortly before kimberlite eruption, suggesting a temporal link between the entrapment of the carbonatite in the host and the Paleocene eruption of the kimberlite. Keywords: carbonatites, isotopes, subduction, Slave province, mantle melts, xenoliths.

Author(s): Olivier Alard (corresponding author) [1]; William L. Griffin [1, 2]; Jean Pierre Lorand [3]; Simon E. Jackson [1]; Suzanne Y. O'Reilly [1] The abundances of highly siderophile (iron-loving) elements [...]

Pb, Sr, and Nd isotope data for Cenozoic basalts in eastern Australia indicate that mantle isotope signatures of Pacific Ocean mid-oceanic ridge basalts (MORB) type characterize the lava-field basalts (55-14 Ma) in southeastern Australia, whereas mantle isotope signatures of Indian Ocean MORB type characterize younger basalts (6-0 Ma) from northeastern Australia. This discovery further constrains the secular distribution of major asthenospheric mantle reservoirs represented by the Pacific and Indian Oceans MORB sources during and following the breakup of eastern Gondwana and tracks, for the first time, the locus of the boundary of the two reservoirs beneath the Australian continent. These new data fill the gap between previous Indian Ocean MORB-Pacific Ocean MORB boundary locations determined from backarc basin basalts in the southwestern Pacific Ocean and ocean-floor basalts in the Southern Ocean. We propose that the Indian Ocean MORB source is a long-term asthenospheric reservoir beneath most of eastern Gondwana and that the westward migration of the Pacific Ocean MORB source may have been associated with the opening of the Tasman Sea (ca. 85-60 Ma) along a broad front southeast of the Australian continent.