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Rhenium and other trace element data were obtained in situ by laser ablation ICP-MS analysis of submarine-erupted volcanic glasses and olivine-hosted melt inclusions from the Valu Fa Ridge, the south tip of the Lau Basin, in the southwestern Pacific Ocean. The chemistry of the Lau Basin basaltic glasses changes systematically from compositions similar to MORB in the Lau Spreading Centers, to more arc-like compositions in the Valu Fa Ridge, providing geochemical profiles both along the Lau Spreading Centers (ridges) and across the Valu Fa Ridge. The east seamount samples of the Valu Fa Ridge have diagnostic trace element ratios (Ba/Nb, Nb/U, Ce/Pb) close to global arc averages, with high Ba/La, indicating addition of considerable amounts of subduction-released fluids. In contrast, samples from the west seamount and the Lau Spreading Centers show a smaller influence from subduction fluids. The variable degrees of subduction influences apparent in the chemistry of these suites provide an ideal means to explore the mechanisms of Re enrichment in undegassed arc magmas. All of the analyzed arc melts have significantly higher Re concentrations than previously published, largely subaerially erupted samples, confirming that high Re is a characteristic of undegassed arc magmas. The east seamount samples are characterized by higher Re and lower Yb/Re than the more MORB-like Lau Spreading Center lavas. The lack of correlation between Yb/Re and Fo of host olivine suggests that low Yb/Re is not due to magmatic differentiation. When the Lau Basin sample suite is plotted together with MORB data, Yb/Re is positively correlated with Ce/Pb and Nb/U, and negatively correlated with Ba/Nb, indicating that Re is much more mobile than Yb during dehydration of subducted slabs. Thus, Re enrichment in arc magmas is likely due to addition of Re via fluids released from subducted slabs; the recognition of high Re in arcs favors arguments for a slab origin of radiogenic Os-187/Os-188 components in arc rocks.

Within the northern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on the southern side of the Isua supracrustal belt, enclaves up to ~500 m long of variably altered ultramafic rocks contain some relics of unaltered dunite–harzburgite. These are associated with mafic supracrustal and plutonic rocks and siliceous metasediments. SHRIMP U/Pb zircon geochronology on non-igneous zircons in altered ultramafic rocks and on igneous zircons from components of the surrounding orthogneisses intruding them, indicate an absolute minimum age for the ultramafic rocks of ca. 3,650 Ma, but with an age of ca. 3,800 Ma most likely. The diverse ultramafic and mafic rocks with rarer metasediment were all first tectonically intercalated and then became enclosed in much more voluminous tonalitic rocks dated at ca. 3,800 Ma. This is interpreted to have occurred during the development of a 3,810–3,790-Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. This northern part of the Itsaq Gneiss Complex is the most favourable for the geochemical study of early Archaean protoliths because it experienced peak metamorphism only within the amphibolite facies with little or no in-situ melt segregation, and contains some areas that have undergone little deformation since ca. 3,800 Ma. Most of the ultramafic enclaves are thoroughly altered, and largely comprise secondary, hydrous phases. However, the centres of some enclaves have escaped alteration and comprise dunite and harzburgite with >95% olivine (Fo89–91) + orthopyroxene (En89) + Al-spinel (Cr8–20) assemblages. The dunites and harzburgites are massive or irregularly layered and are olivine-veined on 5–10-m to 10-cm scales. Their whole rock major and rare earth element, and olivine and spinel compositions differ significantly from xenoliths representing the Archaean cratonic lithospheric mantle, but are typical of some modern abyssal peridotites. The harzburgites and dunites show both LREE depleted and enriched patterns; however, none show the massive REE depletion associated with the modelled removal of a komatiite. They are interpreted as being the products of small degrees of melt extraction, with some showing evidence of refertilisation. These Greenland dunites and harzburgites described here are currently the best characterised 'sample' of the early Archaean upper mantle.

Rocks exposed at the Earth's surface provide an incomplete record of the varied and complex processes that have acted to form and modify the continents. The continental lithospheric mantle (CLM), sampled as xenoliths carried in explosive volcanics, is an alternative reservoir of age information that has only recently begun to be tapped. Data from southeastern Australia, a technically complex region, illustrate that the CLM can record important aspects of continental evolution that are not indicated by the exposed crustal rocks. Re-Os isotopic data and the resultant model ages for 15 spinel peridotite xenoliths from five localities in southeastern Australia show that parts of the CLM are ∼ 1960 Myr old, some 1400 Myrs older than the oldest crust exposed immediately in the sampled region. As the five sampled localities are east of the presumed eastern edge of the Precambrian interior of Australia, the preservation of Proterozoic ages in the lithosphere indicates that the supposed Precambrian edge of the Australian continent (the Tasman Line) is a surficial feature rather than a lithospheric scale boundary. This conclusion is consistent with recent Skippy seismic tomography models for Australia. The mantle xenolith age distribution also suggests a significant age change from Proterozoic to dominantly Phanerozoic, which appears coincident with the surface expression of the boundary between two major fold belts (Lachlan and Delamerian) and with a deep lithospheric boundary previously proposed on the basis of differences in Sr and Pb isotopic systematics of southeast Australian basalts.

Core addition and crustal recycling models that seek to explain the radiogenic Os isotopic compositions of primitive Hawaii tholeiites predict distinctive geochemical consequences for chalcophile and siderophile element abundances in the mantle plume. To test these models and to improve our understanding of compositional variability in the Hawaiian plume, the platinum group element (PGE) and Re contents of primitive shield picrites from several Hawaiian volcanoes were measured. PGE abundances span a large range, from similar to MORB for a picrite from Koolau, to compositions similar to those of basaltic komatiites for picrites from Kilauea and Loihi. Re concentrations range from 0.25 to 0.95 ng/g and with a mean of 0.73 ng/g, higher than previously compiled global averages for ocean island basalts (OIB) (0.38 ng/g) and closer to average MORB (0.98 ng/g) than previously recognised. Some subaerial tholeiites, notably from Kilauea and Mauna Kea, have anomalously low Re abundances and high Cu/Re ratios, possibly reflecting Re loss upon eruption or during degassing of shallow magma chambers. These data show that the PGE and primary Re contents of primitive Hawaiian picrites are well correlated with isotopic compositions of these lavas, linking the PGE and Re characteristics directly with source features of the mantle plume. However, mixing models that describe the isotopic effects of core addition and crustal recycling do not account for the PGE and Re abundances. The range of PGE and Re contents in these lavas does not appear to reflect abundance variations in the plume components, but some aspect of the melting process that is linked to source characteristics of the plume. One possibility is that the PGE and Re characteristics of Hawaiian tholeiites may reflect variable amounts or compositions of residual sulphide during melting. In this scenario, the high PGE and Re contents of Kilauea and Loihi picrites may be indicating a relatively small amount of residual sulphide during melting, whereas the low PGE and Re contents of Koolau primitive magmas may be indicating greater amounts of residual sulphide in the plume. The systematic compositional variations of PGE and Re in primitive tholeiites must be accounted for by any model for the origin of the Hawaiian plume.

The ReOs systematics of a suite of spinel peridotite xenoliths from southeast Australia provide insight into the effects of melt extraction and metasomatism on Re and Os and place strong constraints on the evolution and long-term stability of post-Archean lithospheric mantle in a tectonically complex region. Data from variably melt-depleted and non-modally metasomatised xenoliths demonstrate that Re abundances are largely controlled by melt extraction, with Re similarly distributed to Os. Ratios of ReOs correlate strongly with indices of melt extraction (e.g. Al2O3, Ni and Yb), and with the calculated bulk partition coefficient of Re, comparable to that of Yb over a large range of melt extraction (∼ 4–20%). Hence, if Re is controlled by sulfide phases, sulfur:clinopyroxene ratios should remain essentially constant over large degrees of melt extraction. Eight of the 24 samples analysed were wehrlites or apatite-bearing peridoties, representing residual peridotite which has interacted with a carbonatitic melt. In comparison with the non-modally metasomatised xenoliths, these samples show no evidence for disturbance of Os isotopic composition, or addition of Re or Os during metasomatism. The entire suite provides a 220 km long, WNW-ESE lithospheric mantle transect, east of, and perpendicular to, the presumed Australian Precambrian shield margin. The Os model ages indicate at least three episodes of mantle depletion: ca. 1960 Ma, 800–1000 Ma and < 500 Ma. The older age is found only in the two westernmost localities where a subset of four samples define a ReOs age of 1959 ± 100 Ma, with an initial γOs = +0.2. Although the oldest exposed rocks in the region are of Cambrian age, and the presence of early Proterozoic basement is highly contentious, the Os isotopic data require that early Proterozoic basement extends some 400 km further east than the easternmost exposed early Proterozoic crust. Model ages of 800–1000 Ma are common to all but one locality, indicating at least two melt extraction events in the western localities. Paleozoic ages are only identified in the eastern localities, suggesting the lithospheric mantle becomes younger to the east. Importantly, this and other ReOs isotopic studies provide increasing evidence for the long-term stability and persistence of lithospheric mantle of Proterozoic as well as of Archean age.

Syenites from the Barrel Spring pluton were emplaced in the Early Proterozoic Mojave crustal provine of southeastern California at 1.42 Ga. All rocks, even the most mafic, are highly enriched in incompatible elements (e.g. K2O 4–12 wt%, Rb 170–370 ppm, Th 12–120 ppm, La 350–1500xchondrite, La/Ybn 35–100). Elemental compositions require an incompatible element-rich but mafic (or ultramafic) source. Trace element models establish two plausible sources for Barrel Spring magmas: (1) LREE enriched garnet websterite with accessory apatite±rutile (enriched lithospheric mantle), and (2) garnet amphibolite or garnet-hornblende granulite with enriched alkali basalt composition, also with accessory apatite±rutile (mafic lower crust). Nd and Pb isotopic ratios do not distinguish a crust vs mantle source, but eliminate local Mojave province crust as the principal one, and indicate that generation of the enriched source occurred several hundred million years before emplacement of the Barrel Spring pluton. 1.40–1.44 Ga potassic granites are common in southeastern California, suggesting a genetic link between the Barrel Spring pluton and the granites; however, although the same thermal regime was probably responsible for producing both the granitic and syentic magmas, elemental and isotopic compositions preclude a close relationship. Isotopic similarity of the Barrel Spring pluton to 1.40–1.44 Ga granites emplaced in the Central Arizona crustal province to the east may imply that a common component was present in the lithosphere of these generally distinct regions.

Ma to 3760 Ma metadiorites and tonalites, and older mafic inclusions contained within these rocks from southern West Greenland, have a range of initial end values from + 4.5 to --4.5. The extremely positive initial values determined for nine of the fourteen early Archean samples demonstrate that they were derived from a LREE depleted mantle reservoir which had an end value of ~ +4 prior to 3800 Ma. These rocks provide the best constrained evidence for the existence of highly LREE fraetionated mantle reservoirs in the early Earth. In contrast, previous studies of younger 3700 and 3730 Ma tonalites from the same region show that the latter have lower initial ENd values ranging from -4.6 to + 2.0. The highest initial ENj values determined for 3450 Ma komatiites from western Australia and southern Africa are +2 to +4, and for worldwide ca. 2700 Ma greenstone belts are + 2 to + 5. These values are well below the values for eNd(3450 Ma) (> + 8) and eyd(2700 Ma) (> + 15) that would be expected for the continued evolution of a highly depleted reservoir with Sm/Nd similar to that which produced the ca. 3.8 Ga Greenland metadiorites and tonalites. Thus, if the source region for the oldest Greenland gneisses was the prevalent upper mantle composition, it must have been an ephemeral feature later modified either by mixing with less-depleted mantle or with recycled LREE enriched, negative ENd crust. The generation of highly positive end values by 3.8 Ga requires differentiation of an extremely LREE fractionated reservoir very early in Earth's history. The Archean Nd isotope data may record the isolation, depletion by crustal extraction and subsequent partial rehomogenization of limited portions of the upper mantle, or alternatively may reflect transient large-scale differentiation processes unrelated to crustal extraction such as might occur in a terrestrial magma ocean.

U-Pb zircon geochronology and Nd isotope geochemistry have been used in the northwestern Yilgarn Craton region of Western Australia to map gneiss units of different ages and to provide a model for late Archean crustal evolution, despite the inherent difficulties of minimal exposure and variations in the appearance of units brought about by heterogeneous strain. The 3300–3730 Ma gneisses and intercalated metasedimentary units crop out in a ≤100 km wide tract, bounded on both sides by areas containing 2920–3000 Ma gneisses. Between 2750 and 2620 Ma, several generations of granitoids were emplaced throughout the region. Regardless of whether the granitoids intruded the 3300–3730 Ma or 2920–3000 Ma gneisses, modeling of their initial Nd isotopic compositions shows that they were most likely formed by partial melting of predominantly 2920–3000 Ma gneisses; only a minor input from early Archean sources is indicated for granitoids cutting the 3300–3730 Ma gneisses and intercalated metasediments. Some of the granitoids that intrude the 3300–3730 Ma gneisses contain 2920 Ma inherited zircons, supporting the Nd evidence that their source is dominated by 2920–3000 Ma gneisses. The 3300–3730 Ma gneisses are interpreted as belonging to an allochthonous terrain emplaced over a younger terrain comprising the 2920–3000 Ma gneisses. Subsequently, partial melting concentrated in the underlying 2920–3000 Ma terrain gave rise to the late Archean granitoids.

The central Transantarctic Mountains of Antarctica are divided into distinct lithotectonic packages of variable age, deformation style and metamorphic character. In the Miller Range (83°S, 155°E), the boundary between two such lithotectonic units is a low-angle, thrust-type ductile shear zone that carried high-grade gneisses of the East Antarctic craton to the southeast over a lower-grade metasedimentary sequence. Broad constraints limit the age of ductile deformation as late Early Proterozoic to Late Cambrian. In addition to a reverse component of movement, this zone also records along-strike motions in a direction subparallel to the present orientation of dominant Beardmore- and Ross-age orogenic trends. We postulate that major pre-Ordovician crustal shortening which occurred along this part of the Gondwana continental margin began in the Middle to Late Proterozoic as an early phase of long-lived convergence. Furthermore, orogen-parallel displacements indicate that oblique plate interactions may have played an important role in the early evolution of this active margin.

[1] To investigate the nebular history of material contributing to the terrestrial planets, and search for evidence of a high-temperature origin of the Moon, we measured Mg isotopic compositions of primitive olivines from the Earth, Moon, Mars, and pallasite parent body using laser-ablation multi-collector ICPMS. No temporal variation in the Earth's mantle since at least 3.8 Ga, and only limited variations in the compositions of mantle sources from diverse tectonic settings were found. Earth, Moon, Mars, and differentiated asteroids appear to have formed from a nebular reservoir that was homogeneous with respect to Mg isotopes. This implies either a minor role for evaporation-condensation in the inner solar system, or a limited variation in the proportion of refractory CAI-like material contributing to the terrestrial planets. The Mg isotopic composition of the Moon is identical to the Earth's mantle, placing strong constraints on any volatility-related fractionation that occurred during formation of the Moon.