Oxygen isotopic compositions of chondrules from the metal-rich chondrites Isheyevo (CH/CBb), MAC 02675 (CBb) and QUE 94627 (CBb)

It has been recently suggested that (1) CH chondrites and the CB_b/CH-like chondrite Isheyevo contain two populations of chondrules formed by different processes: (i) magnesian non-porphyritic (cryptocrystalline and barred) chondrules, which are similar to those in the CB chondrites and formed in an impact-generated plume of melt and gas resulted from large-scale asteroidal collision, and (ii) porphyritic chondrules formed by melting of solid precursors in the solar nebula. (2) Porphyritic chondrules in Isheyevo and CH chondrites are different from porphyritic chondrules in other carbonaceous chondrites ( [Krot et al., 2005], [Krot et al., 2008a] and [Krot et al., 2008b]). In order to test these hypotheses, we measured in situ oxygen isotopic compositions of porphyritic (magnesian, Type I and ferroan, Type II) and non-porphyritic (magnesian and ferroan cryptocrystalline) chondrules from Isheyevo and CB_b chondrites MAC 02675 and QUE 94627, paired with QUE 94611, using a Cameca ims-1280 ion microprobe.On a three-isotope oxygen diagram (δ¹⁷O vs. δ¹⁸O), compositions of chondrules measured follow approximately slope-1 line. Data for 19 magnesian cryptocrystalline chondrules from Isheyevo, 24 magnesian cryptocrystalline chondrules and 6 magnesian cryptocrystalline silicate inclusions inside chemically-zoned Fe,Ni-metal condensates from CB_b chondrites have nearly identical compositions: Δ¹⁷O = −2.2 ± 0.9‰, −2.3 ± 0.6‰ and −2.2 ± 1.0‰ (2σ), respectively. These observations and isotopically light magnesium compositions of cryptocrystalline magnesian chondrules in CB_b chondrites (Gounelle et al., 2007) are consistent with their single-stage origin, possibly as gas-melt condensates in an impact-generated plume. In contrast, Δ¹⁷O values for 11 Type I and 9 Type II chondrules from Isheyevo range from −5‰ to +4‰ and from −17‰ to +3‰, respectively. In contrast to typical chondrules from carbonaceous chondrites, seven out of 11 Type I chondrules from Isheyevo plot above the terrestrial fractionation line. We conclude that (i) porphyritic chondrules in Isheyevo belong to a unique population of objects, suggesting formation either in a different nebular region or at a different time than chondrules from other carbonaceous chondrites; (ii) Isheyevo, CB and CH chondrites are genetically related meteorites: they contain non-porphyritic chondrules produced during the same highly-energetic event, probably large-scale asteroidal collision; (iii) the differences in mineralogy, petrography, chemical and whole-rock oxygen isotopic compositions between CH and CB chondrites are due to various proportions of the nebular and the impact-produced materials.     

An unusual clast in lunar meteorite MacAlpine Hills 88105: A unique lunar sample or projectile debris?

Lunar meteorite MacAlpine Hills (MAC) 88105 is a well‐studied feldspathic regolith breccia dominated by rock and mineral fragments from the lunar highlands. Thin section MAC 88105,159 contains a small rock fragment, 400 × 350 μm in size, which is compositionally anomalous compared with other MAC 88105 lithic components. The clast is composed of olivine and plagioclase with minor pyroxene and interstitial devitrified glass component. It is magnesian, akin to samples in the lunar High Mg‐Suite, and also alkali‐rich, akin to samples in the lunar High Alkali Suite. It could represent a small fragment of late‐stage interstitial melt from an Mg‐Suite parent lithology. However, olivine and pyroxene in the clast have Fe/Mn ratios and minor element concentrations that are different from known types of lunar lithologies. As Fe/Mn ratios are notably indicative of planetary origin, the clast could either (1) have a unique lunar magmatic source, or (2) have a nonlunar origin (i.e., consist of achondritic meteorite debris that survived delivery to the lunar surface). Both hypotheses are considered and discussed.     

Estimation of groundwater residence time using the 36Cl bomb pulse

We propose a methodology for estimating the residence time of groundwater based on bomb-produced (36)Cl. Water samples were collected from 28 springs and 2 flowing wells located around Mt. Fuji, Central Japan. (36)Cl/Cl ratios in the water samples, determined by accelerator mass spectrometry (AMS), were between 43 × 10(-15) and 412 × 10(-15). A reference time series of the above-background (i.e., bomb-derived) (36)Cl concentration was constructed by linearly scaling the background-corrected Dye-3 data according to the estimated total bomb-produced (36)Cl fallout in the Mt. Fuji area. Assuming piston flow transport, estimates of residence time were obtained by comparing the measured bomb-derived (36)Cl concentrations in spring water with the reference curve. The distribution of (36)Cl-based residence times is basically consistent with that of tritium-based estimates calculated from data presented in previous studies, although the estimated residence times differ between the two tracers. This discrepancy may reflect chlorine recycling via vegetation or the relatively small change in fallout rate, approximately since 1975, which would give rise to large uncertainties in (36)Cl-based estimates of recharge for the period, approximately since 1975. Given the estimated ages for groundwater from flowing wells, dating based on a (36)Cl bomb pulse may be more reliable and sensitive for groundwater recharged before 1975, back as far as the mid-1950s.     

Forsterite‐bearing type B refractory inclusions from CV3 chondrites: From aggregates to volatilized melt droplets

Abstract– Detailed petrologic and oxygen isotopic analysis of six forsterite‐bearing Type B calcium‐aluminum‐rich inclusions (FoBs) from CV3 chondrites indicates that they formed by varying degrees of melting of primitive precursor material that resembled amoeboid olivine aggregates. A continuous evolutionary sequence exists between those objects that experienced only slight partial melting or sintering through objects that underwent prolonged melting episodes. In most cases, melting was accompanied by surface evaporative loss of magnesium and silicon. This loss resulted in outer margins that are very different in composition from the cores, so much so that in some cases, the mantles contain mineral assemblages that are petrologically incompatible with those in the cores. The precursor objects for these FoBs had a range of bulk compositions and must therefore have formed under varying conditions if they condensed from a solar composition gas. Five of the six objects show small degrees of mass‐dependent oxygen isotopic fractionation in pyroxene, spinel, and olivine, consistent with the inferred melt evaporation, but there are no consistent differences among the three phases. Forsterite, spinel, and pyroxene are ¹⁶O‐rich with Δ¹⁷O ～ ?24‰ in all FoBs. Melilite and anorthite show a range of Δ¹⁷O from ?17‰ to ?1‰.     

The isotopic composition and fluence of solar‐wind nitrogen in a genesis B/C array collector

Abstract– We have measured the isotopic composition and fluence of solar‐wind nitrogen in a diamond‐like‐carbon collector from the Genesis B/C array. The B and C collector arrays on the Genesis spacecraft passively collected bulk solar wind for the entire collection period, and there is no need to correct data for instrumental fractionation during collection, unlike data from the Genesis “Concentrator.” This work validates isotopic measurements from the concentrator by Marty et al. (2010, 2011) ; nitrogen in the solar wind is depleted in ¹⁵N relative to nitrogen in the Earth’s atmosphere. Specifically, our array data yield values for ¹⁵N/¹⁴N of (2.17 ± 0.37) × 10^?3 and (2.12 ± 0.34) × 10^?3, depending on data‐reduction technique. This result contradicts preliminary results reported for previous measurements on B/C array materials by Pepin et al. (2009) , so the discrepancy between Marty et al. (2010, 2011) and Pepin et al. (2009) was not due to fractionation of solar wind by the concentrator. Our measured value of ¹⁵N/¹⁴N in the solar wind shows that the Sun, and by extension the solar nebula, lie at the low‐¹⁵N/¹⁴N end of the range of nitrogen isotopic compositions observed in the solar system. A global process (or combination of processes) must have operated in interstellar space and/or during the earliest stages of solar system formation to increase the ¹⁵N/¹⁴N ratio of the solar system solids. We also report a preliminary Genesis solar‐wind nitrogen fluence of (2.57 ± 0.42) × 10¹² cm^?2. This value is higher than that derived by backside profiling of a Genesis silicon collector ( Heber et al. 2011a ).     

Compound ultrarefractory CAI‐bearing inclusions from CV3 carbonaceous chondrites

Two compound calcium‐aluminum‐rich inclusions (CAIs), 3N from the oxidized CV chondrite Northwest Africa (NWA) 3118 and 33E from the reduced CV chondrite Efremovka, contain ultrarefractory (UR) inclusions. 3N is a forsterite‐bearing type B (FoB) CAI that encloses UR inclusion 3N‐24 composed of Zr,Sc,Y‐rich oxides, Y‐rich perovskite, and Zr,Sc‐rich Al,Ti‐diopside. 33E contains a fluffy type A (FTA) CAI and UR CAI 33E‐1, surrounded by Wark‐Lovering rim layers of spinel, Al‐diopside, and forsterite, and a common forsterite‐rich accretionary rim. 33E‐1 is composed of Zr,Sc,Y‐rich oxides, Y‐rich perovskite, Zr,Sc,Y‐rich pyroxenes (Al,Ti‐diopside, Sc‐rich pyroxene), and gehlenite. 3N‐24’s UR oxides and Zr,Sc‐rich Al,Ti‐diopsides are ¹⁶O‐poor (Δ¹⁷O approximately ?2‰ to ?5‰). Spinel in 3N‐24 and spinel and Al‐diopside in the FoB CAI are ¹⁶O‐rich (Δ¹⁷O approximately ?23 ± 2‰). 33E‐1’s UR oxides and Zr,Sc‐rich Al,Ti‐diopsides are ¹⁶O‐depleted (Δ¹⁷O approximately ?2‰ to ?5‰) vs. Al,Ti‐diopside of the FTA CAI and spinel (Δ¹⁷O approximately ?23 ± 2‰), and Wark‐Lovering rim Al,Ti‐diopside (Δ¹⁷O approximately ?7‰ to ?19‰). We infer that the inclusions experienced multistage formation in nebular regions with different oxygen‐isotope compositions. 3N‐24 and 33E‐1’s precursors formed by evaporation/condensation above 1600 °C. 3N and 33E’s precursors formed by condensation and melting (3N only) at significantly lower temperatures. 3N‐24 and 3N’s precursors aggregated into a compound object and experienced partial melting and thermal annealing. 33E‐1 and 33E avoided melting prior to and after aggregation. They acquired Wark‐Lovering and common forsterite‐rich accretionary rims, probably by condensation, followed by thermal annealing. We suggest 3N‐24 and 33E‐1 originated in a ¹⁶O‐rich gaseous reservoir and subsequently experienced isotope exchange in a ¹⁶O‐poor gaseous reservoir. Mechanism and timing of oxygen‐isotope exchange remain unclear.     

Cosmic ray sidereal diurnal variation of galactic origin observed by neutron monitors

We have analyzed the sidereal diurnal variation of cosmic rays, using 620 station-years of neutron monitor data during the period 1958–1979. The sidereal variation averaged over the period for all the stations in the Northern Hemisphere is different from the corresponding variation in the Southern Hemisphere. The difference is statistically significant and can be identified with the spurious sidereal variation produced from the stationary anisotropy of solar origin, responsible for the solar semi-diurnal variation. The variation common to both hemispheres is also exceptionally significant from the statistical point of view and could be regarded as being due to a uni-directional galactic anisotropy. This variation has an amplitude of 0.0204 ± 0.0015% and a phase of 6.8 ± 0.3 h and is clearly different from that ( ～ 0.05%, 0 ～ 3 h) observed in the high rigidity region (500 ～ 10⁴ GV). The physical meaning of the variation is discussed from the standpoint of the heliomagnetospheric modulation of galactic anisotropy.