Five Type A CAIs from three CV3 chondrites (Vigarano, Northwest Africa 3118, Allende), which differ in age by no more than -105 years, show mineralogical and textural evidence of gradual transition into Type Bs, indicating that Type B inclusions formed by evolution of Type A CAIs in the solar nebula. This model differs from the conventional condensation model in which aggregates of condensate grains form different kinds of CAIs depending on the relative populations of different kinds of grains. In our model the pyroxene forms nearly isochemically by reaction of perovskite with melilite under highly reducing conditions. Anorthite requires the addition of silica from the gas, and originally forms as veins and reaction rims on gehlenitic melilite within Fluffy Type As. Later partial re-melting of these assemblages results in the formation of poikilitic pyroxene and anorthite that enclose rounded (partially melted) tablets of melilite. Oxygen isotopes in four of the CAIs support the formation of Ti-rich 16O-depleted pyroxene from 16O-depleted perovskite, but not in the fifth CAI. An alternative possibility is that Ti-rich 16O-depleted pyroxene is the result of later solid-state exchange that preferentially affects the most Ti-rich pyroxene. Regardless of the origin of the 16O-depleted pyroxene, we give a model for nebular reservoir evolution based on sporadic FU-Orionis flare-ups in which the 16O-rich region near the proto-Sun fluctuated in size depending on whether the protoSun was in flare-up stage or quiescent. Published by Elsevier Ltd.

Measurements of trace element chemistry, mineralogy, and isotope geochemistry are rarely combined with known environmental data to provide a more complete story about how environmental conditions are recorded in biomineral carbonates. Here, cultured (farmed) pearls serve as relatively pristine time capsules to study these geochemical and mineral-based proxies. Cathodoluminescence (CL) imaging and Raman spectroscopic mapping on the & mu;m-scale reveals that heterogeneous crystal bonding environments, geochemistry, and organic contents across the growth history of a freshwater pearl reflect environmental shifts in Kentucky Lake, TN, USA. A major CL peak at 551 nm aligns with increased manganese and organic contents and correlates with lake conductivity and alkalinity data. These CL features are temporally offset from previous 10-& mu;m-scale aragonite nacre oxygen isotope measurements (& delta;18OArg) that record periods of minimum lake temperatures in winter seasons. Thus, we suggest that these trace element and organic features represent autumn or spring rainy seasons that experience lake turnover events and more land runoff rich in Mn and nutrients, increasing aragonite-bound Mn and organic contents in the nacre. This 551 nm signal is absent in Mn-poor saltwater pearl nacre. A second CL peak at 444 nm shows different heterogeneous features likely due to crystal structure shifts, as evidenced by correlations to a Raman map of translational (T): librational (L) mode height ratios typically signaling changes in nacre tablet orientation. Thus, we show that & mu;m-scale CL and Raman mapping may serve as complementary environmental proxies to novel SIMS-based & delta;18OArg temperature proxies in order to capture additional information about local lake environments at seasonal to sub-seasonal temporal resolutions.

Determination of alkali elements is important to Earth scientists, yet suitable and reliable microanalytical reference materials are lacking. This paper proposes a new albite reference material and evaluates the potential for future K-feldspar reference materials. The proposed Piz Beverin albite reference material from Switzerland yields a homogeneous composition at the centimetre- to micrometre-scale for Si, Al and Na with < 2000 mu g g(-1) total trace elements (mostly heterogeneously distributed Ca, K and Sr). EPMA and LA-ICP-MS measurements confirm a composition of 99.5(2)% albite component, which is supported further by bulk XRF measurements. A round robin evaluation involving nine independent EPMA laboratories confirms its composition and homogeneity for Si, Al and Na. In addition, a set of five distinct clear K-feldspar samples was evaluated as possible reference materials. The first two crystals of adular and orthoclase yield unacceptable inhomogeneities with > 2% relative local variations of Na, K and Ba contents. The three other investigated sets of K-feldspar crystals are yellow sanidine crystals from Itrongay (Madagascar). Despite distinct compositions, EPMA confirms they are each homogeneous at the centimetre to micrometre scale for Si, Al and K and have no apparent inclusions; further investigation to find larger amounts of these materials is therefore justified.

The Mighei-like carbonaceous (CM) chondrites, the most abundant carbonaceous chondrite group by number, further our understanding of processes that occurred in their formation region in the protoplanetary disk and in their parent body/bodies and provide analogs for understanding samples returned from carbonaceous asteroids. Chondrules in the CMs are commonly encircled by fine-grained rims (FGRs) whose origins are debated. We present the abundances, sizes, and petrographic observations of FGRs in six CMs that experienced varying intensities of parent body processing, including aqueous and thermal alteration. The samples studied here, in approximate order of increasing thermal alteration experienced, are Allan Hills 83100, Murchison, Meteorite Hills 01072, Elephant Moraine 96029, Yamato-793321, and Pecora Escarpment 91008. Based on observations of these CM chondrites, we recommend a new average apparent (2-D) chondrule diameter of 170 lm, which is smaller than previous estimates and overlaps with that of the Ornans-like carbonaceous (CO) chondrites. Thus, we suggest that chondrule diameters are not diagnostic for distinguishing between CM and CO chondrites. We also argue that chondrule foliation noted in ALH 83100, MET 01072, and Murchison resulted from multiple lowintensity impacts; that FGRs in CMs formed in the protoplanetary disk and were subsequently altered by both aqueous and thermal secondary alteration processes in their parent asteroid; and that the heat experienced by some CM chondrites may have originated from solar radiation of their source body/bodies during close solar passage as evidenced by the presence of evolved desiccation cracks in FGRs that formed by recurrent wetting and desiccation cycles.

The detection and quantification of metal sulfides in host rocks by electrical measurements have been priorities for field and laboratory studies, motivated by mineral prospecting and fundamental interest in the mantle structure or core/mantle differentiation, among other reasons. Here, we reanalyze electrical data for a dunite host with added FeS or Fe-S-Ni (Saxena et al. 2021), and report additional experimental runs along with electron microprobe analyses. The applied pressure was 2 GPa; impedance spectra were acquired while annealing at 1023 K (below the metal-sulfide solidus), and while varying temperature from 570 to 1650 K. Addition of 6.5 or 18 vol% FeS strongly enhances conductivity of the bulk sample compared with that of the dunite host, though values are 100-100 000 times less than those of pure FeS. These results indicate that most metal sulfide content is not part of a viable conductive path, even for the 18 vol% quantity. Nevertheless, the relatively high conductivity and weak temperature dependence of the 18 vol% sample reveal that contiguous paths of solid or molten FeS span the electrodes. The sample with 6.5 vol% sulfide also exceeds the percolation threshold for temperatures as low as similar to 100 K below the eutectic melting point, likely because FeS softens. Conductivity is nearly unchanged upon crossing the eutectic temperature, however a decline over 1400-1500 K reveals that the 6.5 vol% molten FeS forms a fragile electrical network in dunite. Samples with Fe50S40Ni10 or Fe40S40Ni20 (at%) are less conductive than pure dunite at temperatures below similar to 1450 K. This surprising result, likely caused by a reducing influence of Fe or Ni metal, does not support the use of FeS as an analog for compositions with nickel or excess metal. Our findings suggest that probing the electrical network of metal sulfides as solids complements other studies focused on connectivity of molten metal sulfides.

Probability density Probability density .0 0.8 0.6 0.4 0.2 0.0 0.6 0.4 0.2 0.0 142 Nd-10-9-8 182 W MORB Nd standard OIB-7-6-5-4-3-2-1 0 1 2 3 OIB MORB W standard-10-9-8-7-6-5-4-3-2-1 0 1 2 3 New high precision Nd and W isotopic compositions were obtained on the same basalt samples from the Pacific-Antarctic Ridge. These provide the best estimate so far for the mu 142 Nd and mu 182 W values of the depleted mantle source of mid-ocean ridge basalts known as DMM. The PAR basalts yield a mean mu 142 Nd = - 1.6 +/- 5.0 (2 s.d.) and mu 182 W = - 1.9 +/- 3.5 (2 s.d.), which together with the literature data allow the isotope composition of the DMM to be constrained. The present-day DMM mu 182 W is 10 - 20 ppm lower than that of the Archean mantle. This decrease could be related to the broad incorporation of mantle plume material into the upper mantle, starting between 2.4 and 3 billion years ago, due to the onset of deep cold slab subduction, and its attendant return mantle flow.

Nitrous oxide (N2O) is a potent greenhouse gas and ozone depletion agent, with a significant natural source from marine oxygen-deficient zones (ODZs). Open questions remain, however, about the microbial processes responsible for this N2O production, especially hybrid N2O production when ammonia-oxidizing archaea are present. Using 15N-labeled tracer incubations, we measured the rates of N2O production from ammonium (NH4+), nitrite (NO2-), and nitrate (NO3-) in the eastern tropical North Pacific ODZ and the isotopic labeling of the central (alpha) and terminal (beta) nitrogen (N) atoms of the N2O molecule. We observed production of both doubly and singly labeled N2O from each tracer, with the highest rates of labeled N2O production at the same depths as the near-surface N2O concentration maximum. At most stations and depths, the production of 45N2O alpha and 45N2O beta were statistically indistinguishable, but at a few depths there were significant differences in the labeling of the two nitrogen atoms in the N2O molecule. Implementing the rates of labeled N2O production in a time-dependent numerical model, we found that N2O production from NO3- dominated at most stations and depths, with rates as high as 1600 +/- 200 pM N2O d-1. Hybrid N2O production, one of the mechanisms by which ammonia-oxidizing archaea produce N2O, had rates as high as 230 +/- 80 pM N2O d-1 that peaked in both the near-surface and deep N2O concentration maxima. Based on the equal production of 45N2O alpha and 45N2O beta in the majority of our experiments, we infer that hybrid N2O production likely has a consistent site preference, despite drawing from two distinct substrate pools. We also found that the rates and yields of hybrid N2O production were enhanced at low dissolved oxygen concentrations ([O2]), with hybrid N2O yields as high as 20 % at depths where [O2] was below detection (880 nM) but nitrification was still active. Finally, we identified a few incubations with [O2] up to 20 mu M where N2O production from NO3- was still active. A relatively high O2 tolerance for N2O production via denitrification has implications for the feedbacks between marine deoxygenation and greenhouse gas cycling.