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Cabral, RA, Jackson MG, Rose-Koga EF, Koga KT, Whitehouse MJ, Antonelli MA, Farquhar J, Day JMD, Hauri EH.  2013.  Anomalous sulphur isotopes in plume lavas reveal deep mantle storage of Archaean crust. Nature. 496:490-+.   10.1038/nature12020   AbstractWebsite

Basaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials(1,2). However, the residence time of these subducted materials in the mantle is uncertain and model-dependent(3), and compelling evidence for their return to the surface in regions of mantle upwelling beneath hotspots is lacking. Here we report anomalous sulphur isotope signatures indicating mass-independent fractionation (MIF) in olivine-hosted sulphides from 20-million-year-old ocean island basalts from Mangaia, Cook Islands (Polynesia), which have been suggested to sample recycled oceanic crust(3,4). Terrestrial MIF sulphur isotope signatures (in which the amount of fractionation does not scale in proportion with the difference in the masses of the isotopes) were generated exclusively through atmospheric photochemical reactions until about 2.45 billion years ago(5-7). Therefore, the discovery of MIF sulphur in these young plume lavas suggests that sulphur-probably derived from hydrothermally altered oceanic crust-was subducted into the mantle before 2.45 billion years ago and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with negative Delta S-33 values, in the mantle source for Mangaia lavas. Our data also complement evidence for recycling of the sulphur content of ancient sedimentary materials to the subcontinental lithospheric mantle that has been identified in diamond-hosted sulphide inclusions(8,9). This Archaean age for recycled oceanic crust also provides key constraints on the length of time that subducted crustal material can survive in the mantle, and on the timescales of mantle convection from subduction to upwelling beneath hotspots.

Cabral, RA, Jackson MG, Koga KT, Rose-Koga EF, Hauri EH, Whitehouse MJ, Price AA, Day JMD, Shimizu N, Kelley KA.  2014.  Volatile cycling of H2O, CO2, F, and Cl in the HIMU mantle: A new window provided by melt inclusions from oceanic hot spot lavas at Mangaia, Cook Islands. Geochemistry, Geophysics, Geosystems. 15(11):4445-4467.   10.1002/2014GC005473   Abstract

Mangaia hosts the most radiogenic Pb-isotopic compositions observed in ocean island basalts and represents the HIMU (high m5238U/204Pb) mantle end-member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end-member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine-hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end-member. We also report major and trace element abundances and Pb-isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances postmantle melting, including diffusive Fe and H2O loss, degassing, and assimilation. H2O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H2O/Ce ratios vary up to 200, and are consistent with significant dehydration of oceanic crust during subduction and long-term storage in the mantle. CO2 concentrations range up to 2346 ppm CO2 in the inclusions. Relatively high CO2 in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO2 for the generation of the HIMU mantle. F/Nd ratios in the inclusions (3069; 2r standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.07960.028) fall in the range of pristine mantle (0.02–0.08).

Chen, H, Meshik AP, Pravdivtseva OV, Day JMD, Wang K.  2019.  Potassium isotope fractionation during the high-temperature evaporation determined from the Trinity nuclear test. Chemical Geology. 522:84-92.   https://doi.org/10.1016/j.chemgeo.2019.04.02   Abstract

Trinitite materials are the post-detonation glassy residues formed from melting and evaporation of arkosic sands during the first nuclear detonation at the Trinity test site, New Mexico on 16th July, 1945. These trinitites provide useful materials for studying elemental and isotopic behaviors associated with high temperature melting and evaporation that is otherwise difficult to be achieved under laboratory conditions. Using a high-precision method, we measured the potassium (K) isotopic compositions of six bulk trinitite samples taken at different distances from the epicenter of detonation of the Gadget (ground zero). 15 leachates and etchates of trinitite samples were also analyzed to examine the distribution of K within the samples. All trinitites but IF_m (taken within 10 m from the epicenter) show no resolvable K loss and span a narrow range in K isotopic compositions (δ41K: -0.42 ± 0.05‰ to -0.48 ± 0.05‰), revealing no discernible K isotopic fractionation from the Bulk Silicate Earth (BSE) value (-0.48 ± 0.03‰). Residues and etchates of the trinitite material are identical in composition to the bulk samples implying that K isotopes were homogeneous with the arkosic sand at the Trinity test site prior to the nuclear detonation. The most strongly melted green trinitite IF_m, is the only trinitite that shows loss of K (~7%) coupled with resolvable heavier K isotope composition (0.2‰ higher in δ41K than the BSE value). This coupled K loss and isotopic fractionation corresponds to a 55 fractionation factor(avapor-melt) between 0.995 and 0.998 during the Trinity nuclear detonation. These results confirm that K isotopic fractionation occurs through evaporation processes at high temperatures. We also show that, compared with Zn isotopes measured in the same samples, the isotopes of K were significantly less fractionated during evaporation, indicating that K is less volatile during processes such as magma ocean degassing, volcanic outgassing, and impact volatile loss with the relative order of sensitivity being Cu > Zn > K. Our findings support the concept that the heavy K isotopic composition observed in lunar mare basalts reflects the primary signature imprinted by the Moon-forming giant impact event.

Clay, PL, Burgess R, Busemann H, Ruzie-Hamilton L, Joachim B, Day JMD, Ballentine CJ.  2017.  Halogens in chondritic meteorites and terrestrial accretion. Nature. 551:614-618.   10.1038/nature24625   Abstract

Volatile element delivery and retention played a fundamental part in Earth’s formation and subsequent chemical differentiation. The heavy halogens—chlorine (Cl), bromine (Br) and iodine (I)—are key tracers of accretionary processes owing to their high volatility and incompatibility, but have low abundances in most geological and planetary materials. However, noble gas proxy isotopes produced during neutron irradiation provide a high-sensitivity tool for the determination of heavy halogen abundances. Using such isotopes, here we show that Cl, Br and I abundances in carbonaceous, enstatite, Rumuruti and primitive ordinary chondrites are about 6 times, 9 times and 15–37 times lower, respectively, than previously reported and usually accepted estimates1. This is independent of the oxidation state or petrological type of the chondrites. The ratios Br/Cl and I/Cl in all studied chondrites show a limited range, indistinguishable from bulk silicate Earth estimates. Our results demonstrate that the halogen depletion of bulk silicate Earth relative to primitive meteorites is consistent with the depletion of lithophile elements of similar volatility. These results for carbonaceous chondrites reveal that late accretion, constrained to a maximum of 0.5 ± 0.2 per cent of Earth’s silicate mass2,3,4,5, cannot solely account for present-day terrestrial halogen inventories6,7. It is estimated that 80–90 per cent of heavy halogens are concentrated in Earth’s surface reservoirs7,8 and have not undergone the extreme early loss observed in atmosphere-forming elements9. Therefore, in addition to late-stage terrestrial accretion of halogens and mantle degassing, which has removed less than half of Earth’s dissolved mantle gases10, the efficient extraction of halogen-rich fluids6 from the solid Earth during the earliest stages of terrestrial differentiation is also required to explain the presence of these heavy halogens at the surface. The hydropilic nature of halogens, whereby they track with water, supports this requirement, and is consistent with volatile-rich or water-rich late-stage terrestrial accretion5,11,12,13,14.

Combs, LM, Udry A, Howarth GH, Righter M, Lapen TJ, Gross J, Ross DK, Rahib RR, Day JMD.  2019.  Petrology of the enriched poikilitic shergottite Northwest Africa 10169: Insight into the martian interior. Geochimica et Cosmochimica Acta. 266:435-462.   https://doi.org/10.1016/j.gca.2019.07.001   Abstract

The martian meteorite Northwest Africa (NWA) 10169 is classified as a member of the geochemically enriched poikilitic shergottites, based on mineral composition, Lu-Hf and Sm-Nd isotope systematics, and rare earth element (REE) concentrations. Similar to other enriched and intermediate poikilitic shergottites, NWA 10169 is a cumulate rock that exhibits a bimodal texture characterized by large pyroxene oikocrysts (poikilitic texture) surrounded by olivine-rich interstitial material (non-poikilitic texture). Olivine chadacrysts and pyroxene oikocrysts have higher Mg#s (molar Mg/Mg + Fe) than those in the interstitial areas, suggesting that the poikilitic texture represents early-stage crystallization and accumulation, as opposed to late-stage non-poikilitic (i.e., interstitial material) crystallization. Calculated oxygen fugacity values are more reduced (FMQ −2.3 ± 0.2) within the poikilitic regions, and more oxidized (FMQ −1.1 ± 0.1) within the interstitial areas, likely representing auto-oxidation and degassing during magma crystallization. Calculated parental melt compositions using olivine-hosted melt inclusions display a dichotomy between K-poor and K-rich melts, thus possibly indicating mixing of parental melt with K-rich melt. The 176Lu-176Hf crystallization age for NWA 10169 is 167 ± 31 Ma, consistent with the ages reported for other enriched shergottites. Based on the isochron initial 176Hf/177Hf value, the modeled source 176Lu/177Hf composition for NWA 10169 is 0.02748 ± 0.00037, identical within uncertainty to the source compositions of the enriched shergottites Shergotty, Zagami, LAR 06319, NWA 4468, and Roberts Massif (RBT) 04262, suggesting a shared, long-lived geochemical source, and distinct from the source of other enriched shergottites Los Angeles, NWA 856, and NWA 7320. This study reveals that at least two sources are responsible for the enriched shergottites, and that the martian mantle is more heterogeneous than previously thought. Additionally, the enriched shergottites, which share a source with NWA 10169, have consistent crystallization ages and magmatic histories, indicating that a common magmatic system on Mars is likely responsible for the formation of this group.