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The origin of vast accumulations of nickel and platinum in some continental magmatic rocks is still enigmatic, but ultimately linked to silicate-sulfide liquid immiscibility. The exact composition of pristine sulfide melts has proved extremely difficult to document and understand, largely because of the ephemeral, reactive qualities and small quantities of such melts. Here we report the discovery of Fe-Ni sulfide melt globules highly enriched in noble metals (Pt, Pd, Au; 120 ppm total platinum group elements [PGE]) within an unusual high-Mg andesitic glass (8.2 wt% MgO, 57.3 wt% SiO2) dredged from the southern Mid-Atlantic Ridge, near the Bouvet triple junction. The composition of this glass indicates derivation of its parental silicate melt from a garnet pyroxenite mantle source with pronounced “continental” isotopic (Pb, Sr, Nd, Hf, Os, O) signatures. We infer that the chemical properties of this high-temperature (1250 °C) melt, notably high SiO2 and Ni (310 ppm) contents, promoted sulfide saturation at low pressures in a purely oceanic setting, and propose that this unique example, with its likely origin in the continental lithospheric mantle, may be a useful analogue for incipient Ni-PGE-sulfide melt generation and magmatic PGE enrichment.

In the study of geochemical mass balances at subduction zones, the composition of the mantle wedge prior to additions from the slab is a critically important yet poorly constrained parameter. Deconvolving the influence of ancient versus modern enrichments is particularly difficult, especially when considering elements that are highly mobile. Here we provide an alternative approach, using less mobile elements, and a filter to remove the effects of recent slab additions. We provide new Hf isotope data for 30 Mariana Trough (MT) backarc basin lavas. Once filtered, Hf and Nd isotope ratios are highly correlated, of Indian mid-oceanic ridge basalt character, and display variations similar to ocean ridges of comparable lengths. The isotopic variability observed in this “ambient mantle” provides a new paradigm for the interpretation of the varied volcanic products of the arc. Thus, shoshonites associated with the northern termination of the backarc basin rift axis reflect the interaction of a subducted sediment melt with an isotopically enriched mantle source. In contrast, the large volcanoes of the Central Island province have a consistent offset in Nd isotope compositions from the MT array resulting from fluid addition. Existing data for smaller edifices in the submarine portion of the arc have larger variations resulting from fluid addition on a more local scale. We suggest that the similar characterization of ambient mantle elsewhere may help to resolve many conflicting geochemical observations in arc lavas worldwide.

Speleothems are well-proven archives of terrestrial climate variation, recording mean temperature, rainfall, and surface vegetation data at subannual to millennial resolution. They also form within the generally stable environment of caves, and thus may remain remarkably well preserved for many millions of years and, most important, can be dated radiometrically to provide robust chronologies that do not rely on orbital tuning, ice-flow modeling, or estimates of sediment deposition rates. The recent adaptation of the U-Pb dating technique to speleothems has greatly extended their potential as paleoclimate recorders back into the more distant geological past, well beyond the ∼500 k.y. limit previously imposed by U-series techniques, but the opportunities presented by these new methods have yet to be fully explored. As an extreme example, here we report on samples recovered from Permian cave fills, the oldest radiometrically dated speleothems so far documented. Using state of the art analytical techniques it is possible to determine not only their age and state of preservation, but also to extract apparently nearly pristine climate proxy data. Armed with these methods, it now seems reasonable to apply the lessons learned from more recent speleothems to ancient materials, wherever they can be found, and of whatever age, to generate snapshots of paleoclimate that can be used to greatly refine the records preserved within the sediments and fossils of the time.

The unique low-temperature sensitivity of the apatite (U-Th)/He (AHe) technique provides valuable new information relating to the late-stage thermal evolution of metamorphic core complexes in the southern Basin and Range Province. In all the ranges studied, the data show greater complexity than previously seen from other thermochronometers. In the Chemehuevi and northern Sacramento Mountains, a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall. This is followed by essentially age-invariant segments of 15 ± 1 Ma, reflecting an increase in the rate of detachment fault movement. The timing of the change suggested by the data agrees with recently published AHe results from the nearby Harcuvar Mountains, located ∼125 km to the southeast, suggesting that an underlying regional mechanism triggered this change. Plate reconstructions indicate that the Pioneer-Mendocino fracture zone migrated northward through the area at this time, leaving a slab window in its wake. We suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this, combined with the extensional strain regime present, resulted in an increased rate of slip within the actively extending metamorphic core complexes. This agrees well with timing constraints for extension onset in the central Basin and Range Province of southern Nevada, and suggests that weakening of the southern Basin and Range Province at 15 ± 1 Ma was partially responsible for the onset of extension in the central Basin and Range. These findings demonstrate that improved constraints on changes in extension rate within the Basin and Range Province should be possible through the application of the AHe technique to other metamorphic core complex footwalls.

The tectonic evolution of the Belingwe Greenstone Belt in Zimbabwe has been a matter of great controversy. In the most recent contribution to this debate, [Bolhar et al. (2003)][1] reported major element, trace element, and Nd isotope data for mafic volcanic rocks of different stratigraphic units of

In situ Sr isotopic analyses of kimberlitic perovskite are more representative of primary magmatic compositions than conventional bulk-rock analyses of the same samples, because the latter are variably compromised by contamination and alteration processes. Bulk-rock Sr isotopic data obtained for 18 intrusions from the adjacent Narayanpet and Wajrakarur kimberlite fields of the Dharwar Craton, India, exhibit a high degree of scatter (∼0.701-0.709) and have indistinguishable initial isotope ratios. In contrast, laser ablation perovskite results display strikingly uniform and distinct initial 87Sr/86Sr compositions for each field of 0.70312-0.70333 and 0.70234-0.70251, respectively. The increased resolution provided by these new data permits the evaluation of key aspects of kimberlite genesis. It is argued that lithospheric and crustal contamination had a negligible impact on the perovskite Sr isotope compositions, and that these values are representative of the primary melt component in each field. The results cast doubt on models of kimberlite formation that invoke either the small degree melting of metasomatized subcontinental lithospheric mantle, or derivation from unusually enriched asthenospheric mantle. The data are more compatible with a source region for the Dharwar kimberlites that was similar to a common mantle component such as prevalent mantle (PREMA) or focal zone (FOZO). © 2007 The Geological Society of America.

Late Archean volcanic rocks of the Belingwe Greenstone Belt have long been known to contain a small continental crustal component evident in their radiogenic isotope and trace element signatures (Chauvel et al., 1993). Recently, we discovered new geochemical evidence suggesting that all volcanic units contain members with a distinctive crustal fingerprint. It was concluded that the volcanic rocks were erupted through pre-existing continental crust, although a specific tectonic interpretation was deliberately avoided. Hofmann and Dirks’ claim that the new data are also compatible with an oceanic deposition, provided that enriched mantle sources were melted or that dismembered continental crust was present in the oceanic lithosphere. We wish to draw the reader’s attention to the fact that when all available evidence is considered, continental emplacement of the rocks is an unavoidable conclusion.In view of this overwhelming body of evidence, we remain unconvinced by Hofmann and Dirks’ suggestion that these greenstones formed in the oceanic realm. We re-emphasize that the existence of a major metamorphic and structural disconformity has not been verified by subsequent studies (Blenkinsop et al., 1993). The validity of inferring horizontal accretion on the basis of sheared contacts alone appears problematic too, since the extent of tectonic displacement cannot be quantified. Even tectonic transport involving large-scale displacement does not preclude extrusion onto pre-existing crust, because interaction of asthenospheric melts with crust could have occurred prior to horizontal accretion.Finally, we refute Hofmann and Dirks’ suggestion that lithological variation provides a more reliable discriminator of tectonic setting than trace element geochemistry. The remarkably coherent geochemical variations between different stratigraphic units require very similar processes of magma genesis. Once magmas are formed, lithological variation can be imposed by a variety of environmental factors that are common to many tectonic settings.

Late Archean volcanic rocks of the Belingwe Greenstone Belt have long been known to contain a small continental crustal component evident in their radiogenic isotope and trace element signatures ([Chauvel et al., 1993][1]). Recently, we discovered new geochemical evidence suggesting that all

Detailed geologic field work in the Hamersley basin of Western Australia has identified a single horizon that contains predominantly sand-sized spherules similar to those found in impact ejecta such as at the Cretaceous-Tertiary boundary. Evidence suggests that these spherules represent a reworked distal strewn field formed by a large bolide impact in late Archean time. This so-called “spherule marker bed” occurs throughout the main body of the Hamersley basin and in stratigraphically equivalent, but shallower-water lithologies in the northeastern corner. Carbonate constitutes both a matrix component of the spherule marker bed and minor interbeds in the local stratigraphic section. We have utilized carbonate Pb-Pb dating methods to provide, for the first time, an age estimate (2541 +18/−15 Ma) for this important marker bed and therefore of the bolide impact event. Samples from both facies define, within error, an identical isochron age that is also consistent with the few zircon-based age estimates for the Hamersley succession. These findings highlight the great potential of carbonate Pb-Pb geochronology in the dating of Archean and Proterozoic sedimentary rocks.