Explore the vast range of titles available.

A single grain (∼3 micrograms) returned by the Hayabusa spacecraft was analyzed by neutron activation analysis. This grain is mainly composed of olivine with minor amounts of plagioclase, troilite, and metal. Our results establish that the Itokawa sample has similar chemical characteristics (iron/scandium and nickel/cobalt ratios) to chondrites, confirming that this grain is extraterrestrial in origin and has primitive chemical compositions. Estimated iridium/nickel and iridium/cobalt ratios for metal in the Itokawa samples are about five times lower than CI carbonaceous chondrite values. A similar depletion of iridium was observed in chondrule metals of ordinary chondrites. These metals must have condensed from the nebular where refractory siderophile elements already condensed and were segregated.

Evaporation or freezing of water-rich fluids with dilute concentrations of dissolved salts can produce brines, as observed in closed basins on Earth 1 and detected by remote sensing on icy bodies in the outer Solar System 2 , 3 . The mineralogical evolution of these brines is well understood in regard to terrestrial environments 4 , but poorly constrained for extraterrestrial systems owing to a lack of direct sampling. Here we report the occurrence of salt minerals in samples of the asteroid (101955) Bennu returned by the OSIRIS-REx mission 5 . These include sodium-bearing phosphates and sodium-rich carbonates, sulfates, chlorides and fluorides formed during evaporation of a late-stage brine that existed early in the history of Bennu’s parent body. Discovery of diverse salts would not be possible without mission sample return and careful curation and storage, because these decompose with prolonged exposure to Earth’s atmosphere. Similar brines probably still occur in the interior of icy bodies Ceres and Enceladus, as indicated by spectra or measurement of sodium carbonate on the surface or in plumes 2 , 3 . Samples from the asteroid (101955) Bennu, returned by the OSIRIS-REx mission, include sodium-bearing phosphates and sodium-rich carbonates, sulfates, chlorides and fluorides formed during evaporation of a late-stage brine.

The stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U-Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 ± 4 Ma), and on the late Grampian \"Newer Gabbros\" (470 ± 9 Ma) of Insch and Morven-Cabrach in Aberdeenshire. These age data, together with the existing 590 ± 2 Ma age for the Ben Vuirich Granite, provide key radiometric constraints on the evolution of the Dalradian block, and the implications arising from these ages are critically assessed. Three main conclusions are drawn. (1) The entire Caledonian orogeny, although short-lived, is unlikely to have affected sediments of Arenig age and a break probably occurs between those Dalradian sediments of late Proterozoic (<600 Ma) age and the Ordovician rocks of the Highland Border Complex. (2) A period of crustal thickening probably affected some Dalradian rocks prior to 590 Ma. Such an event is indicated by both the polymetamorphic histories of the lower parts of the Dalradian pile and the contact metamorphic assemblages within the aureole of the Ben Vuirich Granite, which are incompatible with sedimentary thicknesses. (3) Age constraints on global Late Proterozoic glacial activity also suggest that the Dalradian stratigraphy is broken into discrete smaller units. Models involving continuous deposition of Dalradian sediments from pre-750 Ma to 470 Ma are rejected.

The Median Tectonic Zone in Eastern Fiordland, SW New Zealand, comprises a tectonically disrupted belt of Mesozoic magmatic are rocks related to subduction along the palaeo-Pacific margin of Gondwana. New ion microprobe (SHRIMP) U-Pb zircon ages confirm that the bulk of the plutonic rocks in eastern Fiordland range from Mid-Jurassic to Early Cretaceous (168-137 Ma) in age. Carboniferous age granitoids occur in SW Fiordland, along the western side of, and within the zone. Triassic plutonic rocks appear to be restricted to the eastern side of the zone. The Mid-Jurassic-Early Cretaceous igneous rocks (collectively referred to as the Darran Suite) are cut by several plutons of Na-rich granitoid (Separation Point Suite) that give ages of c. 124 Ma, slightly older than equivalent rocks in the NW part of the South Island. Early Cretaceous granulite facies orthogneisses (126-119 Ma) in western Fiordland (Western Fiordland Orthogneiss) are considered to be the lower crustal equivalent of the Separation Point plutons. The majority of the Darran Suite rocks are I-type, hornblende-bearing calc-alkaline igneous rocks, most likely derived from melting in the mantle wedge above a subducting slab of oceanic lithosphere. In contrast, the Separation Point-type plutons are Na-rich, alkali-calcic granitoids with high concentrations of Sr (typically >500 ppm and up to 1000 ppm) and low concentrations of Y (≤5 ppm) and heavy REE (<10 times chondritic). Isotopic compositions are primitive, with 87Sr/86Sr initial ratios of c.0.7038, and εNd values of c. +3 at 120 Ma. Their geochemistry is consistent with melting of a mafic protolith of garnet amphibolite mineralogy. Mafic Darran Suite rocks have the appropriate chemical and isotopic compositions to generate the Western Fiordland Orthogneiss and the higher level Separation Point type plutons. We suggest that the sudden appearance of large volumes of Na-rich magma during the Early Cretaceous was triggered tectonically, perhaps by thrusting of the Median Tectonic Zone arc beneath western New Zealand. Melting of basal arc underplate at depths of >40 km would then have generated Na-rich granitoids, leaving residues of garnet+clinopyroxene+amphibole.

Thick successions of turbidites are widespread in the Ross–Delamerian and Lachlan orogens and are now dispersed through Australia, Antarctica and New Zealand. U-Pb detrital zircon age patterns for latest Precambrian, Cambrian and Ordovician metagreywackes show a closely related provenance. The latest Neoproterozoic–early Palaeozoic sedimentary rocks have major components, at c. 525, 550, and 595 Ma, i.e. about 40–80 million years older than deposition. Zircons in these components increase from the Neoproterozoic to Ordovician. Late Mesoproterozoic age components, 1030 and 1070 Ma, probably originate from igneous/metamorphic rocks in the Gondwanaland hinterland whose exact locations are unknown. Although small, the youngest zircon age components are coincident with estimated depositional ages suggesting that they reflect contemporaneous and minor, volcanic sources. Overall, the detrital zircon provenance patterns reflect the development of plutonic/metamorphic complexes of the Ross–Delamerian Orogen in the Transantarctic Mountains and southern Australia that, upon exhumation, supplied sediment to regional scale basin(s) at the Gondwana margin. Tasmanian detrital zircon age patterns differ from those seen in intra-Ross Orogen sandstones of northern Victoria Land and from the oldest metasediments in the Transantarctic Mountains. A comparison with rocks from the latter supports an allochthonous western Tasmania model and amalgamation with Australia in late Cambrian time.

Trace elements from Archaean lavas known as komatiites were found to be similar to those found in modern intraplate basalts. Findings suggest that komatittes are ancient analogues of modern plume-related magmas.

THE formation of the high-pressure, high-temperature rocks known as granulites has traditionally been attributed to metamorphism in the deeper parts of continental collision zones and magmatic arcs 1-3 . More recently, greater consideration has been given to the possibility of granulite formation during continental rifting 4-6 , and in particular4 to whether granulites are forming today beneath the North American Basin and Range province. Here we report thermobarometric data and single-zircon U-Pb ages for an analogous, but much more deeply exhumed, extensional terrain in Fiord-land, New Zealand 7,8 , where Early Cretaceous granulites underwent renewed granulite-facies metamorphism at significantly shallower crustal levels following the extensional collapse of magmatically thickened continental crust. Our data show that granulite-facies metamorphism can take place during continental extension, and demonstrate that in western New Zealand it was accompanied at higher structural levels by the emplacement of one or more metamorphic core complexes 7-9 , in keeping with the model postulated 4 for the Basin and Range province.

The Indus Suture Zone is defined as the plate boundary between India and Eurasia. Here we document geochronological data that suggest that Indian rocks outcrop to the north of this suture zone. The inherited age spectrum of zircons from mylonitic gneiss collected in the southern part of the Karakorum Batholith is similar to those obtained from the Himalayan Terrane, the Pamir and is apparently Gondwanan in its affinity. These data are taken to indicate that the Karakorum Terrane was once a component of Gondwana, or at least derived from the erosion of Gondwanan material. Several continental ribbons (including the Karakorum Terrane) were rifted from the northern margin of Gondwana and accreted to Eurasia prior to India‐Eurasia collision. Many therefore consider the Karakorum Terrane is the southern margin of Eurasia. However, we do not know if rifting led to the creation of a new microplate(s) or simply attenuated crust between Gondwana and these continental ribbons. Thus there is a problem using inherited and detrital age data to distinguish what is “Indian” and what is “Eurasian” crust. These findings have implications for other detrital/inherited zircon studies where these data are used to draw inferences about the tectonic history of various terranes around the world. Key Points We present new SHRIMP dates from a mylonitic gneiss of the Karakorum Terrane The age spectrum is similar to the Himalayan Terrane and the Pamir The Karakorum Terrane was derived from material torn off or shed from Gondwana

We report the first tungsten isotopic measurements in stardust silicon carbide (SiC) grains recovered from the Murchison carbonaceous chondrite. The isotopes 182W, 183W, 184W, 186W and 179Hf, 180Hf were measured on both an aggregate (KJB fraction) and single stardust SiC grains (LS+LU fraction) believed to have condensed in the outflows of low-mass carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The SiC aggregate shows small deviations from terrestrial (=solar) composition in the 182W/184W and 183W/184W ratios, with deficits in 182W and 183W with respect to 184W. The 186W/184W ratio, however, shows no apparent deviation from the solar value. Tungsten isotopic measurements in single mainstream stardust SiC grains revealed lower than solar 182W/184W, 183W/184W, and 186W/184W ratios. We have compared the SiC data with theoretical predictions of the evolution of W isotopic ratios in the envelopes of AGB stars. These ratios are affected by the slow neutron-capture process and match the SiC data regarding their 182W/184W, 183W/184W, and 179Hf/180Hf isotopic compositions, although a small adjustment in the s-process production of 183W is needed in order to have a better agreement between the SiC data and model predictions. The models cannot explain the 186W/184W ratios observed in the SiC grains, even when the current 185W neutron-capture cross section is increased by a factor of two. Further study is required to better assess how model uncertainties (e.g., the formation of the 13C neutron source, the mass-loss law, the modelling of the third dredge-up, and the efficiency of the 22Ne neutron source) may affect current s-process predictions.