GEOCHEMICAL JOURNAL
Online ISSN : 1880-5973
Print ISSN : 0016-7002
ISSN-L : 0016-7002
Volume 51 , Issue 1
Showing 1-11 articles out of 11 articles from the selected issue
Special Issue: Evolution of the solar system: New advances in cosmochemistry and planetary chemistry
  • Shoichi Itoh, James P. Greenwood, Sara Russell
    2017 Volume 51 Issue 1 Pages 1-2
    Published: January 31, 2017
    Released: February 10, 2017
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  • Kenichi Abe, Naoya Sakamoto, Alexander N. Krot, Hisayoshi Yurimoto
    2017 Volume 51 Issue 1 Pages 3-15
    Published: January 31, 2017
    Released: February 10, 2017
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    We describe the petrographic occurrences, abundances, and compositional variations of symplectically intergrown Fe,Ni-sulfides and 17,18O-rich magnetite (Δ17O ~ 90‰), named cosmic symplectites (COS; Sakamoto et al., 2007), from the Acfer 094 (C3.0) ungrouped carbonaceous chondrite. A total of 314 COS studied in two polished sections of this meteorite are uniformly distributed in its matrix with ~600 ppm surface area abundance. No COS have been identified in the Acfer 094 dark inclusions (chondritic lithic clasts) which appear to have experienced extensive aqueous alteration prior to incorporation into the host meteorite. The structure of COS can be arranged in a hierarchy of four categories (from finer to coarser): (1) symplectite structure composed of nanocrystalline magnetite and Fe,Ni-sulfides, (2) submicron-sized wormy structure composed of nanocrystalline symplectites, (3) micrometer-sized irregular rope-like structure composed of wormy structure, and (4) aggregates of the rope-like structure. COS typically associate with fractured Fe,Ni-sulfides and lack Fe,Ni-metal. Most COS studied have smooth surfaces; four grains contain abundant pores. In a single COS, the pore-rich regions are depleted in sulfur and nickel relative to the pore-free regions, indicating that the former are depleted in Fe,Ni-sulfides. Most COS studied contain similar abundances of Fe,Ni-sulfides and magnetite, whereas Ni/(Fe+Ni) atomic ratio in Fe,Ni-sulfides ranges from 0 to 0.4. The lack of Fe,Ni-metal associated with COS supports the formation process proposed by Seto et al. (2008), i.e., oxidation of Fe,Ni-metal and sulfides by 17,18O-rich water vapor in the outer part of the protoplanetary disk. The similar abundance ratio for Fe,Ni-sulfide and magnetite cannot be simply explained by oxidation processes of Fe,Ni-sulfide precursors after sulfurization of Fe,Ni-metal precursors for COS formation as proposed by Seto et al. (2008).

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  • Yusuke Fukami, Tetsuya Yokoyama
    2017 Volume 51 Issue 1 Pages 17-29
    Published: January 31, 2017
    Released: February 10, 2017
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    We present the tellurium (Te) isotope compositions of the acid leachates and residues from three carbonaceous chondrites, namely, Allende, Murchison, and Tagish Lake. Most of the Te isotope compositions in the acid leachates and residues were indistinguishable from that of the terrestrial standard within analytical uncertainties, indicating a homogeneous distribution of Te isotopes in the solar nebula. Previous studies have reported nucleosynthetic isotope anomalies for Sr, Mo, W, and Os in the leachates and residues from the same meteorites. This suggests that the anomalous Te isotope signatures within the carbonaceous chondrites, including presolar phases, have presumably been nearly completely erased via a temperature-controlled nebular processes that acted on the relatively volatile elements before the onset of parent body formation. In contrast, the final residue of the Allende chrondrite displays a small but resolvable Te isotope anomaly. We performed mixing calculations to reproduce the observed Te isotopic pattern for the Allende final residue, which can be explained by the depletion of a theoretical r-process component. This result suggests that our Allende final residue was depleted in presolar nanodiamonds, which were enriched in the theoretical r-process component, because nanodiamonds are strongly acid resistant and can survive the leaching steps used in this study. The presolar SiC is not responsible for the observed r-process depletion. The discrepancy might instead be attributed to the existence of another presolar phase, including glassy carbon, in the final Allende residue.

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  • Sara S. Russell, Jean-David Bodenan, Natalie A. Starkey, Teresa E. Jef ...
    2017 Volume 51 Issue 1 Pages 31-43
    Published: January 31, 2017
    Released: February 10, 2017
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    We report the mineralogy, petrology, major, minor and trace element geochemistry, O and Si isotopes of a complex compound chondrule from the Allende meteorite. The chondrule contains zones of refractory (Ca, Al-rich) regions along with regions more similar to ferromagnesian chondrules. The bulk silicon isotopic composition of the object is δ30Si = –0.71 ± 0.03‰. The oxygen isotopic composition of the different phases within the compound chondrules fall along the Allende chondrule line and range from Δ17O – 12.5 to –2‰. Rare earth element abundances are enriched compared to chondritic levels by up to 15× CI, and show a nebular condensate signature with depletions in Eu and Yb. Our data show that the object evolved in oxygen isotopes in a nebular environment, most likely due to formation from a mixture of diverse components combined with interaction with nebular gas. In addition, differences in olivine composition across the inclusion suggest isotopically distinct oxygen regions also existed in dust in the protoplanetary disk. This object and other compound objects demonstrate that CAIs were present in the chondrule forming region, however they are not found in the majority of chondrules. We speculate that the bulk of the CAIs may have been added into the CV parent body after initial accretion of the body.

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  • Alexander N. Krot, Kazuhide Nagashima
    2017 Volume 51 Issue 1 Pages 45-68
    Published: January 31, 2017
    Released: February 10, 2017
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    The mineralogy, petrography, and oxygen-isotope compositions of porphyritic chondrules—dominant chondrule type in most chondrite groups—suggest formation by incomplete melting of isotopically diverse precursors during localized transient heating events in dust-rich regions of the protoplanetary disk characterized by 16O-poor compositions (Δ17Odust+gas ~ –7‰ to +4‰) relative to the inferred Sun’s value (Δ17O ~ –28 ± 2‰). The chondrule precursors included Ca,Al-rich inclusions (CAIs), amoeboid olivine aggregates (AOAs), chondrules of earlier generations, fine-grained matrix-like material, and possibly fragments of pre-existing planetesimals. Like porphyritic chondrules, igneous CAIs formed by melting of isotopically diverse precursors during transient heating events, but in an isotopically distinct, solar-like reservoir of the protoplanetary disk (Δ17Odust+gas ~ –24‰), probably near the protoSun. Based on a narrow range of the initial 26Al/27Al ratios inferred from the internal Al-Mg isochrons in igneous CAIs, their melting started at the very beginning of Solar System formation (t0), defined by the CV CAIs with U-corrected Pb-Pb age of 4567.3 ± 0.16 Ma and the canonical 26Al/27Al ratio of (5.25 ± 0.02) × 10–5, and lasted at least 0.3 Ma. The U-corrected Pb-Pb absolute and 26Al-26Mg relative ages of porphyritic chondrules from type 3 ordinary, CO, CV, and CR carbonaceous chondrites (assuming uniform distribution of 26Al in the disk at the canonical level) suggest chondrule formation started at t0 and lasted for about 4 Ma. These observations may preclude formation of the majority of porphyritic chondrules by splashing of differentiated planetesimals and by collisions between planetesimals; instead, they are consistent with melting of dust balls by bow shocks or magnetized turbulence in the disk. Some porphyritic chondrules in equilibrated (petrologic type 4–6) ordinary chondrites contain relict fragments of coarse-grained chromite, ilmenite, phosphates, and albitic plagioclase. The similar mineral assemblage is commonly observed in type 4–6 ordinary chondrites, but is absent in type 3 chondrites, suggesting these chondrules formed by incomplete melting of thermally metamorphosed ordinary chondrite material, possibly by impacts. The CB metal-rich carbonaceous chondrites contain exclusively magnesian non-porphyritic chondrules crystallized from complete melts. These chondrules formed in a gas-melt plume generated by a hypervelocity (≥20 km/s) collision between planetesimals ~4.8 Ma after t0 in a transition or a debris disk. One of the colliding bodies was probably differentiated. The CH metal-rich carbonaceous chondrites contain chondrules formed by different mechanisms. The magnesian non-porphyritic chondrules formed in the CB impact plume ~4.8 Ma after t0. The chemically diverse (magnesian, ferroan, and Al-rich) porphyritic chondrules formed by incomplete melting of isotopically diverse precursors in the protoplanetary disk, most likely prior the CB impact plume event. We conclude that there are multiple mechanisms of chondrule formation that operated over the entire life-time of the disk.

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  • J. D. Gilmour, S. A. Crowther
    2017 Volume 51 Issue 1 Pages 69-80
    Published: January 31, 2017
    Released: February 10, 2017
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    We report a –1.6 ± 2.6 Myr (1σ error) I-Xe age of the ungrouped achondrite NWA 7325 relative to the Shallowater standard. We re-evaluate the calibration of the relative I-Xe dating system against the absolute Pb-Pb chronometer in the light of this and other recently reported analyses, and taking into account revisions to the Pb-Pb system, deriving a new absolute age for the Shallowater standard of 4562.7 ± 0.3 Ma. With this calibration, the oldest chondrule I-Xe ages overlap the oldest Pb-Pb chondrule ages and the Pb-Pb ages of Calcium Aluminium-rich inclusions. Literature data for large aliquots of equilibrated ordinary chondrites suggest iodine loss during metamorphic processing and show some evidence that bulk 129Xe*/I ratios decrease with increasing petrologic type. However, the range of ratios at each petrologic type suggests that thermal evolution was affected by changes in thermal insulation with time, perhaps by impact processing of the parent planetesimals. Literature I-Xe ages for chondrules from Bjurböle (L/LL4) and pyroxene from Richardton (H5) suggest closure shortly after the peak of metamorphism, consistent with a high closure temperature in mafic minerals. The extended range of ages reported for chondrules from the LL3.4 chondrite Chainpur is interpreted as a product of collisional processing of material near the surface of the parent body, and may record a decline in the rate of collisions in the asteroid belt over the first 100 Myr of solar system history.

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  • Minato Tobita, Tomohiro Usui, Tetsuya Yokoyama
    2017 Volume 51 Issue 1 Pages 81-94
    Published: January 31, 2017
    Released: February 10, 2017
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    Geochemical studies of shergottites (Martian basalts) based on Rb-Sr, Sm-Nd, and Lu-Hf isotopic systematics have provided clues to understanding the geochemical evolution of the Martian mantle and identification of the source reservoirs. However, U-Pb isotopic systematics has been used to a limited extent for shergottite petrogenesis, because it is generally difficult to discriminate indigenous magmatic Pb components from secondary Martian near-surface components and terrestrial contamination. This study compiles and reassesses all the available Pb isotopic data of shergottites, as well as their Rb-Sr, Sm-Nd, and Lu-Hf isotope systematics. The Sr-Nd-Hf isotopic systematics suggests that the geochemical variability of the shergottite suite (i.e., enriched, intermediate, and depleted shergottites) reflects a mixture of two distinct source reservoirs. In contrast, the Pb isotopic systematics does not support the two-component mixing model for shergottites, because the geochemically enriched, intermediate, and depleted shergottites do not participate in a two-component mixing array in Pb isotopic space. To reconcile the isotopic signatures of the Sr-Nd-Hf and Pb systems, we propose a new mixing model in which the geochemically enriched, intermediate, and depleted shergottites were derived from compositionally distinct mantle sources that had different μ (238U/204Pb) values. Moreover, a linear mixing trend defined by the enriched shergottites in Pb isotopic space is interpreted as the incorporation of a high-μ Martian crustal component into a parental magma derived from a fertilized Martian mantle source. Our model implies that the geochemical diversity of shergottites reflects heterogeneous mantle sources and an assimilated high-μ crustal component on Mars.

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  • Jack A. Singer, James P. Greenwood, Shoichi Itoh, Naoya Sakamoto, Hisa ...
    2017 Volume 51 Issue 1 Pages 95-104
    Published: January 31, 2017
    Released: February 10, 2017
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    While it is now recognized that the Moon has indigenous water and volatiles, their total abundances are unclear, with current literature estimates ranging from nearly absent to Earth-like levels. Similarly unconstrained is the source of the Moon’s water, which could be cometary, chondritic, or the primordial nebula. Here we measure H2O and D/H in olivine-hosted melt inclusions in lunar mare basalts 12018, 12035, and 12040, part of the consanguineous suite of Apollo 12 olivine basalts that differ primarily because of cooling rate (Walker et al., 1976). We find that the water contents are higher in the more rapidly cooled 12018 (62–740 ppm H2O) compared to the more slowly cooled basalts 12035 (28–156 ppm H2O) and 12040 (27–90 ppm H2O), suggesting that lunar basalts may have been dehydrating during slow cooling. D/H is similar in the olivine-hosted melt inclusions in all three samples, and indistinguishable from terrestrial water (δD = –183 ± 212‰ to +138 ± 61‰). When we compare the D/H of olivine-hosted melt inclusions to D/H of apatite in the same samples, the evolution of δD and water content can be better constrained. We propose that lunar magmas first exchange hydrogen with a low D/H reservoir during cooling, and then ultimately lose their water during extended subsolidus cooling. Due to high diffusion rates of hydrogen in olivine, it is likely that all basaltic olivine-hosted melt inclusions from the Moon exchanged hydrogen with a low D/H reservoir in near-surface magma chambers or lava flows. The most likely source of the low D/H reservoir on the Moon is the lunar regolith, which is known to have a significant solar wind hydrogen component.

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  • James P. Greenwood, Naoya Sakamoto, Shoichi Itoh, Paul H. Warren, Jack ...
    2017 Volume 51 Issue 1 Pages 105-114
    Published: January 31, 2017
    Released: February 10, 2017
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    The last liquids of the lunar magma ocean, known as urKREEP, should be highly enriched in volatiles, such as water, fluorine, and chlorine. We find chlorine-rich glasses in two pristine KREEP basalts from the Moon and calculate the volatile contents of the urKREEP component, and use this to estimate the fluorine and chlorine content of the lunar magma ocean. The Cl/Nb and F/Nd of KREEP imply that the lunar magma ocean was depleted in fluorine and chlorine by an order of magnitude compared to the Earth’s mantle. The extremely dry nature of most lunar samples is simply a result of partial melting of magma ocean cumulates that had already lost their volatiles to the urKREEP layer. The volatile-rich KREEP component may have helped lower the solidus of high-temperature magma ocean cumulates that were melted to form the Mg-suite rocks of the highlands, and also aided the dissemination of the KREEP signature into the upper crust. The chlorine-rich KREEP glasses also demonstrate that the large chlorine isotope anomaly found in lunar samples is likely an early lunar signature.

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  • Yoshinori Higashi, Shoichi Itoh, Minako Hashiguchi, Shuhei Sakata, Tak ...
    2017 Volume 51 Issue 1 Pages 115-122
    Published: January 31, 2017
    Released: February 10, 2017
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    Many studies in the past decade have sought to explore the origin and evolution of water in planetary bodies based on the hydrogen isotopic compositions of apatite. However, no investigation has studied hydrogen diffusivity in apatite. This work reports hydrogen diffusion experiments using a natural Durango fluorapatite carried out under a saturated 2H2O/O2 vapor flow at temperatures of 500–700°C. Diffusion depth profiles for 1H and 2H were measured using secondary ion mass spectrometry (SIMS), indicating that 2H diffusion occurred by an exchange reaction between the original 1H and 2H during annealing. Hydrogen diffusion coefficients were obtained by the fitting of diffusion profiles of 2H using Fick’s second law; they followed an Arrhenius-type relationship. The temperature dependence of hydrogen diffusion parallel to the c-axis at 500–700°C can be expressed as

    Hydrogen diffusion coefficients in apatite are several orders of magnitude greater than those of other elements. Hydrogen diffusion in apatite occurs at relatively low temperatures (below 700°C). This study indicates that the hydrogen isotopic compositions of apatite are readily affected by the presence of water vapor through the 1H-2H exchange reaction without changing the total water content in the crystal.

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Express Letter
  • Hiroshi Amakawa, Akira Usui, Koichi Iijima, Katsuhiko Suzuki
    2017 Volume 51 Issue 1 Pages e1-e7
    Published: January 13, 2017
    Released: February 10, 2017
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    The neodymium (Nd) isotopic composition of the surface layers of eleven ferromanganese crust samples collected from the Takuyo-Daigo Seamount (northwest Pacific Ocean) was determined. This is the first systematic study of the Nd isotopic composition of ferromanganese crusts collected by remotely operated vehicles (ROVs) from a single seamount over water depths of 1000–5400 m, representing intermediate to deep water masses in the ocean. We found that the depth profile of ferromanganese crusts is similar to the vertical seawater profile reported for a station close to the seamount. This similarity suggests that Nd in the surface layer of the ferromanganese crust is directly supplied by ambient seawater and reflects the oceanic water mass structure in the region. These samples are suitable for determining time-series Nd isotopic data to study past ocean circulation of intermediate to deep water masses in the northern Pacific.

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