Rare earth and some other elements in acid-residues of unequilibrated ordinary chondrites

Some major, minor and trace elements including rare earth elements (REEs) were determined in bulk and acid (hydrochloric or nitric acid)-residual samples of unequilibrated ordinary chondrites (UOC\u27s), Allan Hills (ALH-) 77299,ALH-78084,Yamato-74191,ALH-77011,ALH-78038,ALH-77252 and ALH-77278. REEs were analyzed by radiochemical neutron activation analysis (NAA), other elements by instrumental NAA. The content of REEs was found to be higher and more variable in acid-residues of UOC\u27s than in acid-residues of equilibrated ordinary chondrites (EOC\u27s). Clnormalized REE abundance patterns of acid-residues of UOC\u27s show a gradual increase from La to Lu but their slopes are less steep than for EOC\u27s. Both ALH-78038 and ALH-77011 show a weak negative Eu anomaly in the REE abundance patterns of acid-residues, which is again in sharp contrast to the large positive Eu anomaly in EOC\u27s. Among the UOC\u27s studies, paired meteorites, ALH-77011 and ALH-78038,are the most primitive UOC\u27s. The amount of REE and the Cl-normalized REE pattern for acid-residues of ALH-77252 are different from those of other UOC\u27s and are essentially the same as those for EOC\u27s, suggesting that this meteorite (or at least the specimen used in this work) is related to EOC\u27s rather than UOC\u27s

Trace element composition and distribution of Yamato-691, an unequilibrated enstatite chondrite

Twenty one trace elements (Re, Ir, Pd, Au, Ag, Se, Te, Zn, In, Cd, Tl, La, Ce, Nd, Sm, Eu, Gd, Tb, Tm, Yb and Lu) were determined by radio-chemical neutron activation analysis for the bulk sample and acid (nitric acid and aqua regia)-soluble and residual phases of the Yamato-691 (EH3) meteorite. No fractionation was confirmed in siderophile elements among Y-691,EH4-5s and EL6s. Among volatile chalcophiles, Zn and In in Y-691 are in the ranges of their abundance variations in EH4-5s, whereas Cd and Tl were observed to be more enriched in Y-691 than in EH4-5s. About 15% of light rare earth elements (REEs) and 30% of heavy REEs were confirmed to be distributed in acid-residual phases (pyroxene-dominant silicates). A small but difinite negative anomaly of Yb was found in acid-residues, but neither Eu nor Ce anomalies were confirmed. Distributions of the REEs with a negative Yb anomaly in acid-residues must be a strong constraint on formation hypotheses for enstatite chondrites

Chemical characteristics of a Martian meteorite, Yamato 980459

As a part of consortium study, we analyzed a Martian meteorite, Yamato (Y) 980459 by prompt gamma-ray analysis (PGA), instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectrometry (ICP-MS). Our data confirmed that Y980459 belongs to olivine-phyric shergottites in chemical composition. Based on mg numbers (molar Mg(Mg+Fe) ratio), olivine-phyric shergottites are divided into two subgroups with high mg number and low mg number. Y980459 is grouped into members with high mg number along with DaG 476 and SaU 005. Abundances of rare earth elements (REEs) in Y980459 are depleted in light REEs (LREEs) and are similar to those of DaG 476 and SaU 005, being different from those for EETA 79001A, Dho 019 and NWA 1068, which are less depleted in LREEs. Based on the REE abundance in Y980459, a value of 0.19-0.25 was derived for the fraction of Martian crust in the Shergotty meteorite

Boron and chlorine abundances in Antarctic chondrites: A PGA study

Boron and chlorine abundances were determined by prompt gamma-ray analysis for Antarctic meteorites of 22 carbonaceous chondrites and 12 ordinary chondrites. Both B and Cl contents of most Antarctic meteorites analyzed in this study are systematically higher than those for non-Antarctic chondrites of corresponding groups, implying that Antarctic meteorites of carbonaceous and ordinary chondrite groups were contaminated with not only Cl (and other halogens like I) but also B. Boron and Cl contents are correlated with each other in Antarctic ordinary chondrites whereas no apparent correlation can be seen for carbonaceous chondrites. Considering that interior portions were used for carbonaceous chondrites while interior and outer portions were randomly sampled for ordinary chondrite used in this study, B and Cl are distributed differently between the two groups of chondrite, suggesting that B and Cl independently behaved following their own chemical properties after contaminant(s) carrying these elements adhered on the surface of meteorites. Sea mist is a probable candidate for such a contaminant. Leaching experiments showed that no B is essentially recovered by water and acetone even from pulverized specimens, from which a fairly large fraction of Cl is recovered, confirming that B and Cl reside independently in chondrite samples and further that both elements behaved differently after sticking to these samples on Antarctica

Petrology and chemistry of the Miles IIE iron. II: Chemical characteristics of the Miles silicate inclusions

A total of nine silicate inclusions (6 gabbroic, 3 cryptocrystalline) were isolated from slab samples of the Miles IIE iron meteorite. They were studied petrologically and then analyzed by instrumental neutron activation analysis, along with the host metal. Based on the siderophile element abundances in the host metal phase, Miles can be classified as a IIE iron but in some aspects it does not match the siderophile abundances of either IIE or anomalous IIE (IIE-An) irons. It may be a member of a third group of IIE irons, tentatively named fractionated IIE. Compared with the average H chondrite, the highly siderophile elements (Re, Os, Ir) are relatively more depleted than the less refractory siderophiles, except for Cu which is as depleted as the highly siderophile elements. This suggests that the metal phase of Miles was not simply produced by a vaporization/fractional condensation process. Rather, it was produced by the melting of H chondritic materials, followed by melt-solid segregation. At least a part of the immiscible sulfide in which Cu was distributed to a considerable degree, was removed from the metal melt. Silicate inclusions have variable amounts of siderophile elements, but their relative abundances are similar to those of the metal phase. This suggests that the siderophile elements in the silicate inclusions have the same genesis as those in the host metal. Plagiophile elements (Al, Na and K) are all enriched in the silicate inclusions, especially in the cryptocrystalline inclusions, with K being the most abundant, successively followed by Na and Al. This fractionation of the plagiophile elements must have occurred during the fractional crystallization process, when the silicate inclusions formed. Rare earth elements (REE) are fairly enriched in the silicate inclusions (especially in the gabbroic inclusions) but their concentrations are slightly lower than that expected from the degree of partial melting of H chondritic materials. A mechanical loss of Ca-phosphate in sampling is suggested for the inconsistency in the REE abundances. Based on the REE abundance variations in the silicate inclusions of several IIE irons, including Miles, we infer that the degree of differentiation of the parental liquids from which silicate inclusions formed was highly variable; it increased from Watson to Weekeroo Station, with Miles being intermediate

Enclaves in the Mt.Padbury and Vaca Muerta mesosiderites: Magmatic and residue (or cumulate) rock types

Nine eucritic enclaves from mesosiderites and one possible enclave were petrologically and chemically studied. Three are from Mt. Padbury, six from Vaca Muerta, and one eucritic meteorite (AMNH 4627) was found in northern Chile and may be an enclave from Vaca Muerta. The enclaves studied were classified into two groups based on the presence or absence of primary ilmenite : (1) an Il-bearing group ranging in texture from medium-grained ophitic to coarse-grained gabbroic, and (2) an Il-free group from gabbroic to coarse-grained equigranular texture. Pigeonites of the Il-bearing group are more ferroan than those of the Il-free group. The peripheral portions of pigeonite in the Il-free group have remarkable textures not found in HED meteorites. They are replaced by more-magnesian hypersthene and troilite with a minor amount of silica-mineral, indicating that reduction of pigeonite took place by H_2S under subsolidus conditions prior to formation of the mesosiderite, metal-silicate mixing. The major element compositions of the ten whole rock samples indicate that the Il-bearing group is similar to the non-cumulate eucrites, but the Il-free group differs from the cumulate eucrites. The Il-free group is significantly enriched in silica-mineral component in comparison with the cumulate eucrites. INAA and RNAA analyses for five whole rock samples were performed. Two Il-free enclaves have low REE contents with remarkable positive Eu-anomalies (chondrite-normalized Eu/Sm=13.6), and two Il-bearing enclaves have higher REE contents with slight positive Eu-anomalies (chondrite-normalized Eu/Sm=1.88-2.22). The high Eu/Sm ratios are partly due to terrestrial alteration of whitlockites, and the original Eu/Sm ratios of the Il-bearing and Il-free enclaves might be similar to those of non-cumulate eucrites and cumulate eucrites, respectively. These suggest that the Il-bearing enclaves were formed in the similar way to the non-cumulate eucrites, and the Il-free enclaves formed as cumulates from silica-rich magmas or as residues by partial melting of silica-bearing cumulate eucritic sources. A eucritic sample (AMNH 4627) has the highest REE content with no Eu-anomaly and is similar to the non-cumulate eucrites, and may be an independent eucrite

Distribution of halogens in an Antarctic ordinary chondrite, Y-74014 (H6)

One of the striking features in Antarctic meteorites is overabundances of halogens, especially iodine. In order to investigate how these halogens are distributed in constituent mineral phases of Antarctic chondrites, the outer portion of Yamato (Y)-74014 H6 chondrite (Y-74014,101) was subjected to fractional dissolution using several solutions with different chemical properties. Pulverized meteorite sample was successively leached by acetone, (hot) water, EDTA and nitric acid, and individual leachates as well as the acid residue were analyzed for halogens by neutron activation analysis. About 10% of iodine was recovered in acetone whereas only less than 2% of Cl and Br are leached, suggesting that overabundant iodine is partly present in the form of acetone-soluble, possibly organic compound. Dissolution patterns of Cl and I with water are similar to each other. This implies that contaminated Cl and I both reside in a common phase, presumably in akaganeite, a corrosion product of Fe-Ni alloy with chlorine attracted from the environment. Either iodine was involved in this mineral when corrosion reaction occurred or iodine was later migrated into the mineral by exchanging with Cl^- and/or OH^-. For the chronological studies using long-lived nuclides such as ^Cl and ^I, it is essential to use indigenous halogens in meteorites. For such purposes, the acid-residual fraction can be used even for Antarctic meteorites having overabundant halogens. In addition, Cl, Br and I contents in bulk Allende and Bruderheim chondrites are also obtained in this study and their abundances are briefly discussed

Chemical composition of Yamato (Y) 000593 and Yamato 000749: Neutron-induced prompt gamma-ray analysis study

Neutron-induced prompt gamma-ray analysis (PGA) was applied to Antarctic nakhlites, Yamato (Y) 000593 and Y000749 together with Nakhla for the non-destructive determination of most major and minor elements (plus some trace elements). In addition to analyses of lump samples of these three meteorite, powder samples representing six different portions within Y000593 also were analyzed for discussing the chemical heterogeneity. As a result, it was confirmed that most major elements are homogeneously distributed among Y000593 within an error of ±10%. Chemical compositions of Y000593 and Y000749 are essentially identical within error limits, supporting the suggestion that these meteorites are paired. Compositional similarity is also confirmed between Nakhla and Y000593 (and Y000749), verifying that Y000593 (and Y000749) belong to nakhlites. Based on our PGA data coupled with literature data for Martian meteorites, we propose a diagram used for classifying achondrites and further for grouping into individual groups of Martian meteorites. Although there are some scatterings in B and Cl data, we would propose their abundances in nakhlites to be 3.33 ppm and 80 ppm, respectively. Our H data are systematically higher than their literature data. Although further refinements in our analytical procedure of PGA are required for getting more reliable and accurate values of H in Martian meteorites, it is suspected that some literature values of H contents in Martian meteorites are a little too small

Petrology and chemistry of the Miles IIE iron. I: Description and petrology of twenty new silicate inclusions

A petrologic study of twenty new silicate inclusions (15 gabbroic, 5 cryptocrystalline) in the Miles IIE iron was undertaken in order to help clarify the petrogenesis of this meteorite and its relationship to other IIE irons with silicates, as well as H-group chondrites. Nine of the inclusions were analyzed by the INAA method, which is presented in a companion paper. The results indicate that Miles was formed by shock events on an H-group chondrite parent body. It formed silicate melt and Fe-Ni-S-P melt by about 25% partially melting, and left behind olivine-orthopyroxene residues. The silicate melt was a crystal mush which consisted of phenocrysts of high-Ca pyroxene, orthopyroxene and plagioclase, and residual melt. The phenocryst-enriched mush formed irregular-shaped gabbroic inclusions, which also contain minor olivine, inverted pigeonite, antiperthite, K-feldspar, tridymite, glass, chromite, rutile, ilmenite, armalcolite, chlorapatite, whitlockite, kamacite, taenite, schreibersite, troilite and pentlandite. The residual melt formed round- to ellipsoidal-shaped cryptocrystalline inclusions, which contain microphenocrysts of alkali feldspar or phenocrysts of pyroxene. The crystal mush was mixed with abundant Fe-Ni-S-P melt, from which most of the sulfur escaped, and from which some of the phosphorus reacted with the silicate melts, resulting in their reduction. This reduction produced Ca-poor magnesian orthopyroxene and Ca-poor plagioclase in the gabbroic inclusions. The degree of reduction was, however, more intense in the cryptocrystalline inclusions than in the gabbroic inclusions. The remaining phosphorus in the solid host metal exsolved as schreibersite around the silicate inclusions