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Highly siderophile element abundances and 182W/184W and 187Os/188Os were determined for a suite of Mauna Kea lavas from the Hawaiian Scientific Drilling Project phase 2 drill core. The new analyses, combined with previous measurements, compose the largest database for μ182W (the parts-per-million deviation of 182W/184W from a terrestrial standard) for a single volcano (n = 16). Although most lavas analyzed are characterized by negative μ182W values, lavas with values similar to the modern bulk silicate Earth are found throughout the entire stratigraphic column. This suggests that components with normal μ182W are collocated with components that host μ182W deficits in the plume. Negative μ182W values are associated with elevated 3He/4He, as well as elevated Ti and Nb. These correlations may link μ182W anomalies to ancient deep mantle crystal-liquid fractionation processes. Consistent with previously measured 3He/4He (R/RA) in the drill core, the magnitude of negative μ182W values was greatest when Mauna Kea was close to the plume axis then generally decreased over the ~400 kyr captured by the stratigraphic section. The component with anomalous μ182W was either concentrated near the plume axis, or was more effectively sampled by melting near the plume axis where the temperature excess was greatest, suggesting it was less fusible than the dominant plume components. The process leading to the generation of a mantle component with a negative μ182W anomaly could either be related to some form of core-mantle isotopic equilibration, or early-Earth fractionation within the silicate Earth. At present each possibility remains viable. 

The ca. 1.95-Ga Jormua Ophiolite Complex (JOC), Finland, is a rare Paleoproterozoic ophiolite that preserves a record of diverse upper mantle materials and melting processes. Meter-scale grid sampling of four JOC outcrops, as well as non-grid samples, permits evaluation of meter- to kilometer-scale mantle heterogeneity within the JOC. Significant heterogeneity is observed between the four grids, and also among a number of the non-grid samples examined. Variations in the concentrations of fluid-mobile elements are particularly large among different samples and locations. New whole-rock major, lithophile trace, and highly siderophile element data (HSE: Os, Ir, Ru, Pt, Pd, Re), including 187Re–187Os isotopic data, for serpentinized harzburgites indicate the presence of two distinct compositional types and probable modes of origin within the JOC. This is consistent with prior findings. Type 1 is similar to modern refractory abyssal-type mantle. Type 2 is more highly refractory than Type 1, and most likely represents samples from sub-continental lithospheric mantle (SCLM). Type 1 mantle is moderately heterogeneous with respect to major and trace element and Os isotopic compositions at both the meter and kilometer scales. By contrast, Type 2 mantle is considerably more homogeneous than Type 1 grids at the meter scale, but is more heterogeneous at the kilometer scale. The median initial γOs value for Type 1 mantle, calculated for 1.95 Ga, is ~ −2.0 (where γOs is the % deviation in 187Os/188Os relative to a chondritic reference calculated for a specified time). This isotopic composition is consistent with a moderate, long-term decrease in Re/Os relative to the estimate for primitive mantle, prior to JOC formation. The similarity in this γOs value to the value for the modern abyssal mantle, as well as the initial values for several Phanerozoic ophiolites, suggests that the upper mantle achieved a Re/Os ratio similar to the chondritic reference by ~2 Ga, then evolved along a subparallel trajectory to the chondritic reference since then. For this to occur, only limited Re could have been permanently removed from the upper mantle since at least the time the JOC formed. A localized secondary metasomatic event at ~2 Ga, concurrent with the estimated obduction age for the JOC and subsequent Svecofennian Orogeny, affected the HSE systematics of some Type 1 samples. By contrast, late Archean Os TRD model ages for Type 2 rocks indicate a depletion event superimposed upon the long-term Re depletion of the abyssal mantle. This event was established no later than ~2.6 Ga and may have occurred during a period of significant, well-documented crustal production in the Karelia craton at ~2.7 Ga.

 

Major and trace element abundances, including highly siderophile elements, and 187Os and 182W isotopic compositions were determined for ca. 89 Ma mafic and ultramafic rocks from the islands of Gorgona (Colombia) and Curaçao (Dutch Caribbean). The volcanic systems of both islands were likely associated with a mantle plume that generated the Caribbean Large Igneous Provence. The major and lithophile trace element characteristics of the rocks examined are consistent with the results of prior studies, and indicate derivation from both a chemically highly-depleted mantle component, and an enriched, or less highly-depleted mantle component. Highly siderophile element abundances for these rocks are generally similar to rocks with comparable MgO globally, indicating that the major source components were not substantially enriched or depleted in these elements. Rhenium-Os isotopic systematics of most rocks of both islands indicate derivation from a mantle source with an initial 187Os/188Os ratio between that of the contemporaneous average depleted mid-ocean ridge mantle and bulk silicate Earth. The composition may reflect either an average lower mantle signature, or global-scale Os isotopic heterogeneity in the upper mantle. Some of the basalts, as well as two of the komatiites, are characterized by calculated initial 187Os/188Os ratios 10-15% higher than the chondritic reference. These more radiogenic Os isotopic compositions do not correlate with major or trace element systematics, and indicate a mantle source component that was most likely produced by either sulfide metasomatism or ancient Re/Os fractionation. Tungsten-182 isotopic compositions measured for rocks from both islands are characterized by variable 182W values ranging from modern bulk silicate Earth-like to strongly negative values. The 182W values do not correlate with major/trace element abundances or initial 187Os/188Os compositions. As with some modern ocean island basalt systems, however, the lowest 182W value (-53) measured, for a Gorgona olivine gabbro, corresponds with the highest 3He/4He previously measured from the suite (15.8 R/RA). Given the lack of correlation with other chemical/isotopic compositions, the mantle component characterized by negative 182W and possibly high 3He/4He is most parsimoniously explained to have formed as a result of isotopic equilibration between the mantle and core at the core-mantle boundary. 

Asteroid Ryugu and Ivuna-type carbonaceous meteorites may have originated from the outskirts of the Solar System. 

The evolution of Earth's major geochemical reservoirs over ~4.5 × 10⁹ years remains a matter of intense study. Geochemical tools in the form of short‐lived radionuclide isotope ratios (¹⁴²Nd/¹⁴⁴Nd and¹⁸²W/¹⁸⁴W) have expanded our understanding of the geochemical variability in both the modern and ancient Earth. Here, we present¹⁴²Nd/¹⁴⁴Nd and¹⁸²W/¹⁸⁴W data from a suite of rocks from the Slave craton that formed over a 1.1 × 10⁹ year time span in the Archean. The rocks have consistently high¹⁸²W/¹⁸⁴W, yet¹⁴²Nd/¹⁴⁴Nd that is lower than bulk mantle and increased over time. The declining variability in¹⁴²Nd/¹⁴⁴Nd with time likely reflects the homogenization of compositional heterogeneities in the silicate Earth that were initially created by differentiation events that occurred prior to 4.2 Ga. The elevated¹⁸²W/¹⁸⁴W recorded in the Slave samples help refine models for the broader W‐isotope evolution of the silicate Earth. Globally, the Archean mantle that formed continental crust was dominated by¹⁸²W/¹⁸⁴W elevated by some 10–15 ppm compared to the value for the modern upper mantle. The Slave craton lacks significant volumes of komatiite yet has elevated¹⁸²W/¹⁸⁴W until 2.9 Ga. This observation, combined with the presence of other komatiite suites that have low¹⁸²W/¹⁸⁴W, suggests that deep‐seated sources contributed low¹⁸²W/¹⁸⁴W in the Archean Earth. The regional variability in¹⁸²W/¹⁸⁴W may be explained by invoking chemical and/or isotopic exchange between a well‐mixed silicate Earth and the core or a portion of the lower mantle whose W‐isotope composition has been influenced by interaction with the core.

 

One requirement for isotope ratio measurement results with small measurement uncertainties is that the element of interest is effectively separated from the sample matrix. Efficient chemical separation of W from matrix components, especially Ti, can be challenging, particularly for large test portion masses (> 1 g). We present a new W separation procedure that takes advantage of the distinct complexation behaviour of Ti and W with citrate ligand in a moderately lowpH, oxidising solution. This preparation procedure can reduce the Ti/W ratio of large (4–10 g) basaltic (i.e., high‐matrix) test portions by a factor of 10⁵, relative to their original compositions, in a two‐step separation procedure. The procedure additionally provides a separate, well‐purified Mo fraction. We show that optimal separation requires precise selection of reagent concentrations and sample load. The procedure was employed to determine the μ¹⁸²W composition ofBHVO‐2 as −6.7 ± 4.2 (2 standard deviation, 2s). The principles derived from this method may prove useful for chemical separation of other elements used for geochemical and cosmochemical applications given an appropriate selection of organic acid. Future successful applications of this method may reveal that the use of organic acids as procedural reagents is a currently under‐utilised tool for efficient chemical separation protocols.