Oxygen in HEDs: Signatures of Multiple Planetoids or Accretionary Stratigraphy in a Planetary Open System? (original) (raw)
Related papers
Oxygen isotope evidence for rapid mixing of the HED meteorite parent body
Earth and Planetary Science Letters, 2004
The 16 O, 17 O and 18 O abundances of howardites, eucrites, and diogenites have been used to assign them to a single 'HED' parent body, thought to be asteroid 4 Vesta. We report the first evidence of oxygen isotopic heterogeneity among HED meteorites indicating incompletely mixed sources. New high-precision oxygen isotope measurements of 34 HED meteorites reveal that most have the same v 17 OP, consistent with a very rapid early history of large-scale mixing on Vesta. However, howardites are on average very slightly enriched in 16 O, whereas Ibitira, Caldera, Pasamonte, and ALHA78132 are 16 O-depleted compared to other investigated eucrites. The v 17 OP of Ibitira is completely different from all other HEDs measured. Some of the results for eucrites and diogenites can be explained by partial melting and rapid mixing of the interior of Vesta. Others require a separate parent body or indicate that parts of the outer layer of Vesta retained some primary isotopic heterogeneity. The oxygen isotopic composition of howardites provides an upper limit for the amount of admixed carbonaceous chondritic material into the HED parent body regolith.
2010
We examined oxygen three-isotope ratios of 48 extraterrestrial chromite (EC) grains extracted from mid-Ordovician sediments from two different locations in Sweden, and one location in south-central China. The ages of the sediments ($470 Ma) coincide with the breakup event of the L chondrite parent asteroid. Elemental compositions of the chromite grains are generally consistent with their origin from L or LL chondrite parent bodies. The average D 17 O (&-deviation from the terrestrial mass-fractionation line, measured in situ from 15 lm spots by secondary ion mass spectrometry; SIMS) of EC grains extracted from fossil meteorites from Thorsberg and Brunflo are 1.17 ± 0.09& (2r) and 1.25 ± 0.16&, respectively, and those of fossil micrometeorites from Thorsberg and Puxi River are 1.10 ± 0.09&, and 1.11 ± 0.12&, respectively. Within uncertainty these values are all the same and consistent with the L chondrite group average D 17 O = 1.07 ± 0.18&, but also with the LL chondrite group average D 17 O = 1.26 ± 0.24& . We conclude that the studied EC grains from correlated sediments from Sweden and China are related, and most likely originated in the same event, the L chondrite parent body breakup. We also analyzed chromites of modern H, L and LL chondrites and show that their D 17 O values coincide with averages of D 17 O of bulk analyses of H, L and LL chondrites. This study demonstrates that in situ oxygen isotope data measured by SIMS are accurate and precise if carefully standardized, and can be used to classify individual extraterrestrial chromite grains found in sediments.
The oxygen-isotopic record in enstatite meteorites
Meteoritics & Planetary Science, 2000
Oxygen-isotopic compositions were determined for a suite of enstatite chondrites and aubrites. In agreement with previous work , most samples have 0-isotopic compositions close to the terrestrial fractionation line (TFL), and there appear to be no significant differences in 0-isotopic compositions between individual EH and EL chondrites and aubrites. Five enstatite meteorites have 0-isotopic compositions that are significantly different fi-om the other samples and >0.2%0 away from the TFL. Two of these have petrographic evidence of brecciation and interaction between other meteorite types; for the other three, similar scenarios are suggested. There appears to be a systematic increase in ~3 1 8 0 from enstatite chondrites (both EH and EL) of petrologic type 3 to those of type 6. There is also good evidence that the EH meteorites do not fall along a mass fractionation line but along a line slope 0.66. At the present time, detailed understanding of the origin of these 0-isotopic systematics remain elusive but clearly point to a complex accretion history, parent-body evolution, or both.
Geochimica et Cosmochimica Acta, 2004
The timescale of accretion and differentiation of asteroids and the terrestrial planets can be constrained using the extinct 182 Hf-182 W isotope system. We present new Hf-W data for seven carbonaceous chondrites, five eucrites, and three shergottites. The W isotope data for the carbonaceous chondrites agree with the previously revised 182 W/ 184 W of chondrites, and the combined chondrite data yield an improved ⑀ W value for chondrites of Ϫ1.9 Ϯ 0.1 relative to the terrestrial standard. New Hf-W data for the eucrites, in combination with published results, indicate that mantle differentiation in the eucrite parent body (Vesta) occurred at 4563.2 Ϯ 1.4 Ma and suggest that core formation took place 0.9 Ϯ 0.3 Myr before mantle differentiation. Core formation in asteroids within the first ϳ5 Myr of the solar system is consistent with the timescales deduced from W isotope data of iron meteorites. New W isotope data for the three basaltic shergottites EETA 79001, DaG 476, and SAU 051, in combination with published 182 W and 142 Nd data for Martian meteorites reveal the preservation of three early formed mantle reservoirs in Mars. One reservoir (Shergottite group), represented by Zagami, ALH77005, Shergotty, EETA 79001, and possibly SAU 051, is characterized by chondritic 142 Nd abundances and elevated ⑀ W values of ϳ0.4. The 182 W excess of this mantle reservoir results from core formation. Another mantle reservoir (NC group) is sampled by Nakhla, Lafayette, and Chassigny and shows coupled 142 Nd-182 W excesses of 0.5-1 and 2-3 ⑀ units, respectively. Formation of this mantle reservoir occurred 10-20 Myr after CAI condensation. Since the end of core formation is constrained to 7-15 Myr, a time difference between early silicate mantle differentiation and core formation is not resolvable for Mars. A third early formed mantle reservoir (DaG group) is represented by DaG 476 (and possibly SAU 051) and shows elevated 142 Nd/ 144 Nd ratios of 0.5-0.7 ⑀ units and ⑀ W values that are indistinguishable from the Shergottite group. The time of separation of this third reservoir can be constrained to 50-150 Myr after the start of the solar system. Preservation of these early formed mantle reservoirs indicates limited convective mixing in the Martian mantle as early as ϳ15 Myr after CAI condensation and suggests that since this time no giant impact occurred on Mars that could have led to mantle homogenization. Given that core formation in planetesimals was completed within the first ϳ5 Myr of the solar system, it is most likely that Mars and Earth accreted from pre-differentiated planetesimals. The metal cores of Mars and Earth, however, cannot have formed by simply combining cores from these pre-differentiated planetesimals. The 182 W/ 184 W ratios of the Martian and terrestrial mantles require late effective removal of radiogenic 182 W, strongly suggesting the existence of magma oceans on both planets. Large impacts were probably the main heat source that generated magma oceans and led to the formation metallic cores in the terrestrial planets. In contrast, decay of short-lived 26 Al and 60 Fe were important heat sources for melting and core formation in asteroids.
Molecular and Isotope Analyses of Organic Matter in a Primitive Clast in the Zag H Chondrite
Japan Geoscience Union, 2017
The Zag meteorite is a halite-bearing H3-6 chondrite [1]. The Zag contains xenolithic clast with abundant organic matter which was proposed to be originated from Ceres [2,3]. Here we report coordinated organic analyses by STXM-XANES and NanoSIMS, in order to understand the nature and origin of the organic matter. Our systematic research of the Zag clast may also provide an important linkage to the recent remote sensing observations obtained by the DAWN mission to Ceres [e.g., 4,5]. Carbon-rich areas were located in the clast grains separated from the Zag meteorite with SEM-EDS, and then lift-out sections were prepared with a FIB instrument. C, N, OX ray absorption near-edge structure (C,N,O-XANES) spectra of the sections (~100 nm-thick) were obtained using scanning transmission X-ray microscopes (STXM) on beamline 5.3.2.2 at Advanced Light Source, Lawrence Berkeley National Laboratory, and BL-13A at the Photon Factory, KEK. Subsequently, H, C, N, O isotopic images were collected using a CAMECA NanoSIMS 50L ion microprobe. The STXM elemental map of C-rich region of the Zag clast shows that sub-micrometer organic grains were scattered over the FIB section, some of which have a vein-like structure. The organic matter was somewhat associated with Fe (probably Fe-sulfides). The Fe (+Ni) and C association was also observed in the clasts in Sharps (H3.4) chondrite, suggesting a potential of catalytic gas-solid reactions such as Fischer-Tropsch type (FTT) synthesis [6,7]. C-XANES spectra of the organic grains showed large peaks at 285.2 eV assigned to aromatic carbon, and at 290.3 eV assigned to carbonate (either organic or inorganic), with some features at 287.4 eV (enol C=C-OH), and 287.9 eV (aliphatic), and 288.8 eV (carboxyl). The C-XANES spectra have some similarity with organic matter from Comet Wild 2, rather than with primitive chondritic IOM [8], except for the abundant carbonate in the Zag clast. NanoSIMS isotope imaging analyses revealed that δ 15 N and δD have highly heterogeneous distributions within the organic matter. The average δ 15 N value was 393 ±82 ‰with a hot spot (2639 ±722 ‰), and the average δD value was 813 ±206 ‰with a hot spot (4,150 ±1,710 ‰). The δ 15 N was similar to the value of insoluble organic matter (IOM) from Bells (an unusual CM chondrite) and CRs, although δD was less than these IOM [9]. It may indicate that some hydrogen have been exchanged with isotopically light water in the clast parent body. Both molecular structure and isotopic signatures indicated highly pristine (less altered) nature of organic matter in the clast, and it may be related to cometary organics and/or primitive chondritic IOM.
Oxygen Isotope Variations in Main Group Pallasites and HEDs
Introduction: Pallasites are stony-iron meteorites mainly composed of cm-sized olivine crystals disseminated in a network of Fe-Ni metal. Based on the oxygen isotope signatures of pallasite olivines and silicates from IIIAB iron meteorites [1] it has been suggested that main group pallasites may represent the coremantle boundary and IIIAB irons may represent the core of the Howardite-Eucrite-Diogenite (HED) parent body 4 Vesta [1]. The HED suite of achondrites have a close resemblance to terrestrial igneous rocks, and are thought to originate from the shallow to deep regions of the asteroid 4 Vesta on the basis of spectral analyses [2] and data from the DAWN mission [3]. The triple oxygen isotope analyses of the major minerals (plagioclase and pyroxene) in HEDs is consistent with an igneous origin for these meteorites [1]. Eventually, after the original work of [1] it was generally accepted that pallasites and HEDs belong to the same parent body. Recently, there has been interest in distinguishing differentiated meteorites including main group pallasites (MG), HEDs and mesosiderites on the basis of 17 O [4][5][6][7][8]. We have contributed to this effort by studying the offset ( 17 O) of oxygen isotopes from the terrestrial fractionation line (TFL) to distinguish differentiated meteorites including MG pallasites and HEDs. The resolution of 17 O among these meteorites is of utmost importance to understand the genetic processes that happened on the parent bodies.
A novel organic-rich meteoritic clast from the outer solar system
Scientific Reports
the Zag meteorite which is a thermally-metamorphosed H ordinary chondrite contains a primitive xenolithic clast that was accreted to the parent asteroid after metamorphism. the cm-sized clast contains abundant large organic grains or aggregates up to 20 μm in phyllosilicate-rich matrix. Here we report organic and isotope analyses of a large (~10 μm) oM aggregate in the Zag clast. the X-ray microspectroscopic technique revealed that the oM aggregate has sp 2 dominated hydrocarbon networks with a lower abundance of heteroatoms than in IoM from primitive (CI,CM,CR) carbonaceous chondrites, and thus it is distinguished from most of the oM in carbonaceous meteorites. the oM aggregate has high D/H and 15 N/ 14 N ratios (δD = 2,370 ± 74‰ and δ 15 N = 696 ± 100‰), suggesting that it originated in a very cold environment such as the interstellar medium or outer region of the solar nebula, while the oM is embedded in carbonate-bearing matrix resulting from aqueous activities. thus, the high D/H ratio must have been preserved during the extensive late-stage aqueous processing. It indicates that both the oM precursors and the water had high D/H ratios. Combined with 16 o-poor nature of the clast, the oM aggregate and the clast are unique among known chondrite groups. We further propose that the clast possibly originated from D/p type asteroids or trans-Neptunian objects. Xenolithic clasts are often found in a wide variety of meteorite groups 1-9 , some of which contain exotic organic matter (OM) 10. Xenolithic clasts have been protected in host meteorites that are typically more metamorphosed and thus are physically strengthened by thermal annealing via heating processes occurring prior to the incorporation of the clasts. Hence, such clasts can contain primitive and fragile materials that would not have survived parent body alteration processes and atmospheric entry. The Zag meteorite is a H3-6 chondrite which fell in Morocco on August 1998, and is known to contain xenolithic, fluid inclusion-bearing halite crystals and a centimeter-sized carbonaceous chondrite-like clast 1,11. The clast in the Zag meteorite consists of saponite, serpentine, CaFe -Mg carbonates, Fe-Ni sulfides, magnetite, halite, minor olivine and pyroxene, as well as abundant large OM grains or aggregates up to 20 μm, indicating that the Zag clast has been subjected to significant aqueous alteration 12. We analyzed the molecular structure and isotope chemistry of a focused ion beam (FIB) section obtained from an OM aggregate using scanning transmission X-ray microscopy (STXM) coupled with X-ray absorption near edge structure (XANES) and nanoscale secondary ion mass spectrometry (NanoSIMS), as well as bulk O-isotopic analyses.
The study of planetary materials m chondritlc meteorites constrains the compositional dwerslty of materials in different nebular environments and provides reformation on the degree of &fferentiation of early planetary bodies. We stu&ed a unique microgabbro fragment from the Parnallee (LL3) unequihbrated ordinary chondrite. The fragment, which was originally identified by its ophltic to sub-ophltlC texture, exhibits features characteristic of lunar and terrestrial tholelitic basalts--extreme compositional zoning in chnopyroxene (Wot0En65Fs25 to WolsEnEFss3) , a multiply saturated major element composition similar to mid-ocean ridge basalt with 3.1 wt% Na20 and 0.15 wt% K20 , and uniformly enriched rare earth elements (c. 10 × C1). A high bulk MnO/FeO ratio (0.064) distinguishes the microgabbro from other basaltic rocks and suggests the precursor material formed or reached equilibrium in a reducing environment. However, the absence of Fe metal and the extreme enrichment of FeO (up to 40 wt%), in late crystallizing pyroxferroite, requires the last crystallizatmn event to have occurred m a relatively oxidizing environment. We suggest the mlerogabbro formed by partml melting in a planetary body after removal of metalhc Fe. Examination of possible planetary source materials, such as alkali-rich eucrmc material, a volatile-depleted C1 carbonaceous chondrite or H-group chondrite, shows that multiple-stage fractionation is required to produce a melt with the FeO/MgO ratio of the mlcrogabbro from these materials. The increasing number of planetary igneous fragments observed m unequilibrated ordinary chondrites (UOC) suggests the picture of UOC as primitive assemblages of unprocessed material is overly simplistic. 0012-821X/92/$05.00
Geochimica et Cosmochimica Acta, 2009
In order to derive constraints on planetary differentiation processes, and ultimately the formation of the Earth, it is required to study a variety of meteoritic materials and to investigate their melting relations and elemental partitioning at variable pressures, temperatures, and oxygen fugacities (f O2). This study reports the first high pressure (HP) and high temperature (HT) investigation of an enstatite chondrite (Indarch). Four series of experiments exploring various f O2 conditions have been carried out at 1 GPa in a piston-cylinder apparatus using the EH4 chondrite Indarch. We show that temperature and redox conditions have important effects on the phase equilibria of the meteorite: the solidus and liquidus temperatures of the silicate portion increase with decreasing f O2 , and the stability fields of various phases are modified. Olivine and pyroxene are stable around 1.5 log f O2 unit below the iron-wü stite buffer (IWÀ1.5), whereas quartz and pyroxene is the stable assemblage under the most reducing conditions, between IWÀ5.0 and IWÀ4.0, due to reduction of the silicate. While these changes are occurring in the silicate, the metal gains Si from the silicate, (Fe, Mg, Mn, Ca, Cr)-bearing sulfides are observed at f O2 less than IWÀ4, and the partitioning of Ni and Mo are both affected by the presence of Si in Fe-S-C liquids. The f O2 has also a significant effect on the liquid metal-liquid silicate partitioning behavior of Si and S, two possible light elements in planetary cores, and of the slightly siderophile elements Cr and Mn. With decreasing f O2 , S becomes increasingly lithophile, Si becomes increasingly siderophile, and Cr and Mn both become strongly siderophile and chalcophile. The partitioning behavior of these elements places new constraints on models of core segregation for the Earth and other differentiated bodies.