The stable carbon isotopes in enstatite chondrites and Cumberland Falls

The carbon isotopic composition of the total carbon in the enstatite chondrites Indarch, Abee, St. Marks, Pillistfer, Hvittis and Daniel's Kuil and the enstatite achondrite Cumberland Falls has been measured. The empirical relationhip between carbon isotopic composition and total carbon content is distinct from that of carbonaceous and ordinary chondrites. Within the enstatite chondrite group the average ¹³C content increases with petrographic type: E4 < E5 < E6. Daniel's Kuil shows the largest ¹³C enrichment in the bulk carbon of any meteorite. The carbon isotopic composition is most clearly correlated with the abundance of the elements Zn, Cd and In. Insofar as these elements may hold the key to the understanding of enstatite chondrites, more detailed combined carbon isotope and trace element studies of these meteorites will play an important role in the deciphering of their history.     

Equilibration temperatures in enstatite chondrites

Equilibration temperatures for enstatite chondrites are calculated using a method suggested by Larimer (1968). The temperatures range from 640° to 840°C. The method yields temperatures which, in principle, are correct on a relative scale but the absolute error may be a large as 150°. There is a good correlation between the calculated temperatures and petrologic type as well as other mineralogic characteristics and bulk composition. Partial pressures of sulfur and oxygen at the time of equilibration were: pS₂ ~ 10^?8?10^?12 atm and pO₂ ~ 10^?28?10^?37.     

The unequilibrated enstatite chondrites

The enstatite chondrites formed under highly reducing (and/or sulfidizing) conditions as indicated by their mineral assemblages and compositions, which are sharply different from those of other chondrite groups. Enstatite is the major silicate mineral. Kamacite is Si-bearing and the enstatite chondrites contain a wide variety of monosulfide minerals that are not present in other chondrite groups. The unequilibrated enstatite chondrites are comprised of two groups (EH3 and EL3) and one anomalous member (LEW 87223), which can be distinguished by differences in their mineral assemblages and compositions. EH3 chondrites have >1.8 wt.% Si in their kamacite and contain the monosulfide niningerite (MgS), whereas EL3 chondrites have less than 1.4 wt.% Si in their kamacite and contain the monosulfide alabandite (MnS). The distinct mineralogies, compositions and textures of E3 chondrites make comparisons with ordinary chondrites (OCs) and carbonaceous chondrites (CCs) difficult, however, a range of recrystallization features in the E3s are observed, and some may be as primitive as type 3.1 OCs and CCs. Others, especially the EL3 chondrites, may have been considerably modified by impact processes and their primary textures disturbed. The chondrules in E3 chondrites, although texturally similar to type I pyroxene-rich chondrules, are sharply different from chondrules in other chondrite groups in containing Si-bearing metal, Ca- and Mg–Mn-rich sulfides and silica. This indicates formation in a reduced nebular environment separate from chondrules in other chondrites and possibly different precursor materials. Additionally the oxygen isotope compositions of E3 chondrules indicate formation from a unique oxygen reservoir. Although the abundance, size distribution, and secondary alteration minerals are not always identical, CAIs in E3 chondrites generally have textures, mineral assemblages and compositions similar to those in other groups. These observations indicates that CAIs in O, C and E chondrites all formed in the reservoir under similar conditions, and were redistributed to the different chondrite accretion zones, where the secondary alteration took place. Thus, chondrule formation was a local process for each particular chondrite group, but all CAIs may have formed in the similar nebular environment. Lack of evidence of water (hydrous minerals), and oxygen isotope compositions similar to Earth and Moon suggest formation of the E chondrites in the inner solar system and make them prime candidates as building blocks for the inner planets.     

Nature of volatile depletion and genetic relationships in enstatite chondrites and aubrites inferred from Zn isotopes

Enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites (aubrites). They are the most reduced group of all meteorites. The oxygen isotope compositions of both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line, which suggests some genetic links between these meteorites and the Earth as well.For this study, we measured the Zn isotopic composition of 25 samples from the following groups: aubrites (main group and Shallowater), EL chondrites, EH chondrites and Happy Canyon (impact-melt breccia). We also analyzed the Zn isotopic composition and elemental abundance in separated phases (metal, silicates, and sulfides) of the EH4, EL3, and EL6 chondrites. The different groups of meteorites are isotopically distinct and give the following values (‰): aubrite main group (−7.08 < δ⁶⁶Zn < −0.37); EH3 chondrites (0.15 < δ⁶⁶Zn < 0.31); EH4 chondrites (0.15 < δ⁶⁶Zn < 0.27); EH5 chondrites (δ⁶⁶Zn = 0.27 ± 0.09; n = 1); EL3 chondrites (0.01 < δ⁶⁶Zn < 0.63); the Shallowater aubrite (1.48 < δ⁶⁶Zn < 2.36); EL6 chondrites (2.26 < δ⁶⁶Zn < 7.35); and the impact-melt enstatite chondrite Happy Canyon (δ⁶⁶Zn = 0.37).The aubrite Peña Blanca Spring (δ⁶⁶Zn = −7.04‰) and the EL6 North West Forrest (δ⁶⁶Zn = 7.35‰) are the isotopically lightest and heaviest samples, respectively, known so far in the Solar System. In comparison, the range of Zn isotopic composition of chondrites and terrestrial samples (−1.5 < δ⁶⁶Zn < 1‰) is much smaller ( [Luck et al., 2005] and [Herzog et al., 2009]).EH and EL3 chondrites have the same Zn isotopic composition as the Earth, which is another example of the isotopic similarity between Earth and enstatite chondrites. The Zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between EL3 and EL6 chondrites is due to volatilization, probably during thermal metamorphism. Aubrites show strong elemental depletion in Zn compared to both EH and EL chondrites and they are enriched in light isotopes (δ⁶⁶Zn down to −7.04‰). This is the opposite of what would be expected if Zn elemental depletion was due to evaporation, assuming the aubrites started with an enstatite chondrite-like Zn isotopic composition. Evaporation is therefore not responsible for volatile loss from aubrites. On Earth, Zn isotopes fractionate very little during igneous processes, while differentiated meteorites show only minimal Zn isotopic variability. It is therefore very unlikely that igneous processes can account for the large isotopic fractionation of Zn in aubrites. Condensation of an isotopically light vapor best explains Zn depletion and isotopically light Zn in these puzzling rocks. Mass balance suggests that this isotopically light vapor carries Zn lost by the EL6 parent body during thermal metamorphism and that aubrites evolved from an EL6-like parent body. Finally, Zn isotopes suggest that Shallowater and aubrites originate from distinct parent bodies.     

Cosmic ray exposure ages of Rumuruti chondrites from North Africa

We analyzed noble gases and determined ³He, ²¹Ne, and ³⁸Ar cosmic ray exposure ages (CREAs) of Rumuruti chondrites from North West Africa (NWA) to rule on potential pairings and/or source pairings of North Africa R chondrite samples. The ²¹Ne exposure ages range between 10 and 74 Ma, with NWA 2897 and 1668 having the highest known exposure ages among R chondrites. We also include other R chondrites from North Africa (Schultz et al., 2005) and, based on their noble gas characteristics and their ²¹Ne CREAs, propose pairings of the following samples: NWA 2198, 5069, 755, 4615, 845, 851, 978, 1471, and possibly DaG 013 belonging to one fall with a CREA of ∼10 Ma, and NWA 753, 4360, 4419, 5606, 1472, 1476, 1477, 1478, and 1566 representing one fall with a CREA of ∼14 Ma. NWA 2821, 2503, 2289, 3364, 3146, 4619, 4392, 3098, and 2446 seem to belong to one single fall with a CREA of ∼20 Ma, and NWA 2897 and 1668 seem to be paired and show a common CREA of ∼66 Ma. Overall, all R chondrite samples from North Africa analyzed for noble gases so far represent ∼16 individual falls. Comparing falls from North Africa to literature CREAs of R chondrites worldwide, it seems possible that a significant number of all R chondrite falls studied for noble gases were ejected from the R chondrite parent body during one large collisional event between 15 and 25 Ma ago. However, the database is still too small to draw definitive conclusions. The large portion of brecciated R chondrites in collections suggests severe impact brecciation of the R chondrite parent body.     

Chemical and physical studies of type 3 chondrites—V: The enstatite chondrites

We report instrumental neutron activation analysis determinations of 19 major, minor and trace elements in three enstatite chondrites. Based on these, and literature data on the bulk and mineral composition of enstatite chondrites, we discuss the history of the type 3 or unequilibrated enstatite chondrites, and their relationship with the other enstatite chondrites. The type 3 enstatite chondrites have E chondrite lithophile element abundances and their siderophile element abundances place them with the EH chondrites, well resolved from the EL chondrites. Moderately volatile chalcophile elements are at the low end of the EH range and Cr appears to be intermediate between EH and EL. We suggest that the type 3 enstatite chondrites are EH chondrites which have suffered small depletions of certain chalcophile elements through the loss of shock-produced sulfurous liquids. The oxygen isotope differences between type 3 and other enstatite chondrites is consistent with equilibration with the nebula gas ~30° higher than the others, or with the loss of a plagioclase-rich liquid. The mineral chemistry of the type 3 chondrites is consistent with either low temperature equilibration, or, in some instances, with shock effects.     

Bismuth and 208Pb microdistributions in enstatite chondrites

Polished sections of 5 enstatite chondrites have been irradiated with 30 MeV ⁴He ions to produce the alpha-radioactive nuclei ²¹¹At and ²¹⁰Po from ²⁰⁹Bi and ²⁰⁸Pb, respectively. The distribution of alpha activity can be mapped, using cellulose nitrate as an alpha track detector, to give the corresponding Bi or Pb distributions in the meteorite. No strong localization of Bi or ²⁰⁸Pb was found; relatively uniform track distributions were observed. In particular, metal or sulfide grains are not enriched in Bi or Pb (relative to bulk), which is in agreement with the predictions of nebular condensation calculations. While the track distributions appear uniform, the results of detailed, track-by-track mappings of the Bi detectors indicate that the Bi is not totally randomly distributed; the statistical fluctuations in the observed track density are different for the cases where the Bi is totally randomly distributed and where the Bi is localized in point sources. Assuming that the Bi in a given sample is localized in identical point sources which are uniformly distributed throughout the sample, the observed relative population densities of clusters (‘stars’) of small numbers of tracks (2–5) corresponds to Bi being localized, with ~90% in grains with about 10^?16g-Bi (~3 × 10⁵Biatoms), and with ~10% in 4 × 10^?14 g-Bi sources. If these are elemental Bi, as predicted theoretically, they are ~ 10² Å and 10³ Å in size, respectively.     

Fe-Ni and Mn-Cr isotopic systems in sulfides from unequilibrated enstatite chondrites

In situ measurements of ⁶⁰Fe-⁶⁰Ni and ⁵³Mn-⁵³Cr isotopic systems with an ion microprobe have been carried out for sulfide assemblages from unequilibrated enstatite chondrites (UECs). Evidence for the initial presence of ⁶⁰Fe has been observed in nine sulfide inclusions from three UECs: ALHA77295, MAC88136, and Qingzhen. The inferred initial (⁶⁰Fe/⁵⁶Fe) [(⁶⁰Fe/⁵⁶Fe)₀] ratios show a large variation range, from ∼2 × 10⁻⁷ to ∼2 × 10⁻⁶. The sulfide inclusions with high Fe/Ni ratios yield (⁶⁰Fe/⁵⁶Fe)₀ ratios of ∼(2-7) × 10⁻⁷, similar to most of the (⁶⁰Fe/⁵⁶Fe)₀ values of troilite and pyroxene observed in unequilibrated ordinary chondrites (UOCs). Inclusions with high inferred (⁶⁰Fe/⁵⁶Fe)₀ ratios (∼1-2 × 10⁻⁶) have low Fe/Ni ratios and the magnitude of the ⁶⁰Ni excesses is similar in two MAC88136 assemblages in spite of a difference of a factor of two in their Fe/Ni ratios. The inferred high (⁶⁰Fe/⁵⁶Fe)₀ ratios were probably the result of Fe-Ni re-distribution in the sulfides during later alteration processes.The ⁵³Mn-⁵³Cr system was measured in five of the sulfide assemblages that were examined for their ⁶⁰Fe-⁶⁰Ni systematics. The ⁵³Mn-⁵³Cr isochrons yielded variable initial (⁵³Mn/⁵⁵Mn) [(⁵³Mn/⁵⁵Mn)₀] ratios from ∼(2-7) × 10⁻⁷. There is no obvious correlation between the (⁶⁰Fe/⁵⁶Fe)₀ and (⁵³Mn/⁵⁵Mn)₀ ratios. The variable ⁵³Mn-⁵³Cr isochrons probably also indicate later disturbance to the isotopic systems in these sulfides. Even though no chronological information can be extracted from the ⁶⁰Fe-⁶⁰Ni and ⁵³Mn-⁵³Cr systems in these UEC sulfides, our results indicate that ⁶⁰Fe was present in the enstatite chondrite formation region of the early Solar System.     

Oxygen- and magnesium-isotope compositions of calcium-aluminum-rich inclusions from Rumuruti (R) chondrites

We report oxygen- and magnesium-isotope compositions of Ca,Al-rich inclusions (CAIs) from several Rumuruti (R) chondrites measured in situ using a Cameca ims-1280 ion microprobe. On a three-isotope oxygen diagram, δ¹⁷O vs. δ¹⁸O, compositions of individual minerals in most R CAIs analyzed fall along a slope-1 line. Based on the variations of Δ¹⁷O values (Δ¹⁷O = δ¹⁷O − 0.52 × δ¹⁸O) within individual inclusions, the R CAIs are divided into (i) ¹⁶O-rich (Δ¹⁷O ∼ −23-26‰), (ii) uniformly ¹⁶O-depleted (Δ¹⁷O ∼ −2‰), and (iii) isotopically heterogeneous (Δ¹⁷O ranges from −25‰ to +5‰). One of the hibonite-rich CAIs, H030/L, has an intermediate Δ¹⁷O value of −12‰ and a highly fractionated composition (δ¹⁸O ∼ +47‰). We infer that like most CAIs in other chondrite groups, the R CAIs formed in an ¹⁶O-rich gaseous reservoir. The uniformly ¹⁶O-depleted and isotopically heterogeneous CAIs subsequently experienced oxygen-isotope exchange during remelting in an ¹⁶O-depleted nebular gas, possibly during R chondrite chondrule formation, and/or during fluid-assisted thermal metamorphism on the R chondrite parent asteroid.Three hibonite-bearing CAIs and one spinel-plagioclase-rich inclusion were analyzed for magnesium-isotope compositions. The CAI with the highly fractionated oxygen isotopes, H030/L, shows a resolvable excess of ²⁶Mg (²⁶Mg^∗) corresponding to an initial ²⁶Al/²⁷Al ratio of ∼7 × 10⁻⁷. Three other CAIs show no resolvable excess of ²⁶Mg (²⁶Mg^∗). The absence of ²⁶Mg^∗ in the spinel-plagioclase-rich CAI from a metamorphosed R chondrite NWA 753 (R3.9) could have resulted from metamorphic resetting. Two other hibonite-bearing CAIs occur in the R chondrites (NWA 1476 and NWA 2446), which appear to have experienced only minor degrees of thermal metamorphism. These inclusions could have formed from precursors with lower than canonical ²⁶Al/²⁷Al ratio.     

Petrography,mineral chemistry and origin of Type I enstatite chondrites

Optical and cathodoluminescence petrography were coupled with electron microprobe analysis to relate the textures and chemical compositions of minerals in the chondrules and matrix of the Indarch, Kota-Kota, Adhi-Kot and Abee Type I enstatite chondrites. Clinoenstatites fall into two distinct chemical groups with characteristic red or blue luminescence; red crystals are higher in Ti, Al, Cr, Mn and Ca, and lower in Na, than blue ones. Rare forsterites in Indarch and Kota-Kota show distinct compositions associated with orange or blue luminescence. The chemical ranges are indistinguishable for each color type in chondrules of all textural types, and the presence of both color types in a single chondrule or a metal fragment requires mechanical aggregation of both crystals and liquids of both color types. Porphyritic chondrules are ascribed mainly to aggregation of existing crystals because both types of pyroxene and olivine occur in the same chondrule. Large crystals of one color type are surrounded by fine-grained crystals of another type in some barred and radiating chondrules. All types of chondrules are surrounded by fine-grained rims rich in sulfide. The matrix contains many broken chondrules and individual silicate grains but is rich in sulfide and metal. Analyses are given of albite (minor elements and luminescence color vary between chondrites), kamacite, schreibersite, oldhamite and niningerite.Although the mineral assemblages do not fit theoretical condensation sequences in detail, the red pyroxene and orange olivine might result ultimately from near-equilibrium crystallization in which early reduced condensates reacted with a gas, while the blue crystals might result from fractional condensation in which early condensates were removed mechanically from a gas. Subsequent episodes involving mixing, melting, crystallization, condensation, fracturing, and mechanical aggregation would be needed to produce the complex textures.     

Himalayan uplift and osmium isotopes in oceans and rivers

Previous studies have shown that ¹⁸⁷Os/¹⁸⁸Os in seawater has become increasingly radiogenic over the last 40 Ma in a manner analogous to strontium. This rapid rise in the marine ¹⁸⁷Os/¹⁸⁸Os over the last 17 Ma has been attributed to an increase in the bulk silicate weathering rates resulting from the rise of the Himalayas and/or selective weathering and erosion of highly radiogenic organic rich ancient sediments. The key test of this hypothesis is the ¹⁸⁷Os/¹⁸⁸Os and the total osmium concentration of the Himalayan rivers. We report the concentration and isotopic composition of osmium in the Ganges, the Brahmaputra, and the Indus rivers. The ¹⁸⁷Os/¹⁸⁸Os of the Ganges close to its source (at Kaudiyal, 30°05′N, 78°50′E) is 2.65 and [Os] = 45 fM/kg. A second sample of the lower reaches of the Ganges at Patna (25°30′N, 85°10′E) gives ¹⁸⁷Os/¹⁸⁸Os =1.59 and [Os] = 171 fM/kg. The ¹⁸⁷Os/¹⁸⁸Os of the Brahmaputra at Guwahati (26°10′N, 91°58′E) is 1.07 and [Os] = 52 fM/kg. A sample of the Indus (Besham, 34°55′N, 72°51′E) has a ¹⁸⁷Os/¹⁸⁸Os of 1.2 and [Os] = 59 fM/kg. We infer that the Himalayas do not provide either a high flow of osmium or a highly radiogenic osmium component to the oceans. The overall trend for osmium and strontium could be explained by a regularly increasing input of global continental weathering sources but the Himalayas themselves appear not to be the dominant source.     

Compositional differences in enstatite chondrites based on carbon and nitrogen stable isotope measurements

Carbon and nitrogen abundance and isotopic compositions, from four EH4, one EH5, five EL6 chondrites and one aubrite, were determined by using stepped pyrolysis (N only) and combustion (N and C) extractions in attempts to distinguish the components present. Carbon contents range from 0.15 to 0.70 wt%, with no systematic relationship between carbon content and meteorite group or petrologic type. Whole-rock δ¹³C values range from −28.5 to −4.1 %., Most C occurs as graphite and when temperature steps above 700°C are considered, there is a difference between EH4,5 (δ¹³C = −9.1 to -5.8%.) and EL6 chondrites (δ¹³C = −6.7 to +4.2%.). Carbon in Bustee aubrite is isotopically lighter (δ¹³C = −24%.) than in any enstatite chondrite.

Nitrogen occurs as osbornite, sinoite and in isostructural substitution for oxygen in silicate lattice sites. Nitrogen abundances and isotopic compositions are more variable than C, due to the heterogenous distribution of N-bearing minerals. Three EL6's containing osbornite have higher N concentrations than other type 6 enstatite chondrites. Sinoite, where present, is depleted in ¹⁵N relative to osbornite. Nitrogen in the Bustee aubrite has a similar abundance and δ¹⁵N value to those of EL6's, again dominated by the presence of osbornite.

In addition to the refractory C-and N-bearing minerals there is also organic material (largely terrestrial contamination) and evidence for at least two “exotic” components. The first is a host for Xe (HL) and is characterized by δ¹³C <-−47%. and δ¹⁵N ≤−73%., whereas the second is less well-defined, but is marked by δ¹⁵N = +269%.     

The compositional classification of chondrites: II The enstatite chondrite groups

We present new data from a neutron activation analysis of four enstatite chondrites including the taxonomically important St. Sauveur, and discuss the classification of enstatite chondrites. The enstatite chondrites can be divided into two compositionally distinct sets; in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1.5–2.0× higher than in the other. A well-resolved compositional hiatus separates these two sets. The differences in composition are as great as those between the groups of ordinary chondrites, and therefore it appears best to treat these sets as separate groups. By analogy with the symbols used for ordinary chondrites we propose to designate the high-Fe, high siderophile group EH and the low-Fe, low-siderophile group EL. Known members of the EH group belong to petrologic types 4 and 5, whereas all EL members are petrologic type 6. Within the EH group no correlation is observed between petrologic type and abundance of nonrefractory siderophiles or moderately volatiles or alkalis.Two physical properties show only modest overlap between the EH and EL groups. Cosmic-ray ages for EH chondrites are 0.5–7 Ma, while those for EL chondrites are 4–18 Ma. Relative to Bjurböle, I-Xe formation intervals are $\text{?1.3 ± 0.6}$ Ma for EH chondrites and $\text{2.9 ± 0.5}$ Ma for EL chondrites. The weight of the chemical and physical evidence indicates that the EH and EL groups formed separate bodies at similar distances from the Sun.The available evidence for Shallowater and Happy Canyon, two strongly recrystallized silicate-rich meteorites containing > 40 mg/g Fe-Ni, indicates that the former is an enstatite-clan chondrite altered by loss of sulfide- and plagioclase-rich melts, whereas the latter is intermediate in composition between EL chondrites and the chondritic silicates in the Pine River IAB-anomalous meteorite.     

Iodine-Xenon dating of chondrules from the Qingzhen and Kota Kota enstatite chondrites

Initial ¹²⁹I/¹²⁷I values (I-Xe ages) have been obtained for individual mineralogically characterized chondrules and interchondrule matrix from the enstatite chondrites Qingzhen (EH3) and Kota Kota (EH3). In view of the absence of aqueous alteration and the low-peak metamorphic temperatures experienced by these meteorites, we suggest that the I-Xe ages for the chondrules record the event in which they were formed. These ages are within the range recorded for chondrules from ordinary chondrites, demonstrating that chondrules formed during the same time interval in the source regions of both ordinary chondrites and enstatite chondrites. The timing of this chondrule-forming episode or episodes brackets the I-Xe closure age of planetesimal bodies such as the Shallowater aubrite parent body. Although chondrule formation need not have occurred close to planetesimals, the existence of planetesimals at the same time as chondrule formation provides constraints on models of this process. Whichever mechanisms are proposed to form and transport chondrules, they must be compatible with models of the protosolar nebula which predict the formation of differentiated bodies on the same timescale at the same heliocentric distance.     

A reappraisal of the metamorphic history of EH3 and EL3 enstatite chondrites

The thermal history of a series of EH3 and EL3 chondrites has been investigated by studying the degree of structural order of the organic matter (OM) located and characterized in matrix areas by Raman micro-spectroscopy. By comparison with unequilibrated ordinary chondrites (UOCs) and CO and CV carbonaceous chondrites, the following petrologic types have been assigned to various E chondrites: Sahara 97096 and Allan Hills 84206: 3.1-3.4; Allan Hills 85170 and Parsa: 3.5; Allan Hills 85119: 3.7; Qingzhen, MacAlpine Hills 88136 and MacAlpine Hills 88184: 3.6-3.7. The petrologic type of Qingzhen is consistent with the abundance of the P3 noble gas component, a sensitive tracer of the grade of thermal metamorphism. The petrologic types are qualitatively consistent with the abundance of fine-grained matrix for the whole series. No significant effects of shock processes on the structure of OM were observed. However such processes certainly compete with thermal metamorphism and the possibility of an effect cannot be fully discarded, in particular in the less metamorphosed objects. The OM precursors accreted by the EH3 and EL3 parent bodies appear to be fairly similar to those of UOCs and CO and CV carbonaceous chondrites. Raman data however show some slight structural differences that could be partly accounted for by shock processes. The metamorphic history of EH3 and EL3 chondrites has often been described as complex, in particular regarding the combined action of shock and thermal metamorphism. Because OM maturity is mostly controlled by the temperature of peak metamorphism, it is possible to distinguish between the contributions of long duration thermal processes and that of shock processes. Comparison of the petrologic types with the closure temperatures previously derived from opaque mineral assemblages has revealed that the thermal history of EH3 and EL3 chondrites is consistent with a simple asteroidal onion shell model. Thermal metamorphism in enstatite chondrites appears to be fairly similar to that which takes place in other chondrite classes. The complex features recorded by mineralogy and petrology and widely reported in the literature appear to be mostly controlled by shock processes.     

Investigating the variations in carbon and nitrogen isotopes in carbonaceous chondrites

The carbonaceous chondrites contain significant amounts of carbon- and nitrogen-bearing components, the most abundant of which is organic matter. Stepped combustion data of whole rock and HF/HCl residues of carbonaceous chondrites reveal that the organic material can be subdivided operationally into three components: (1) free organic matter (FOM), which is readily extractable from whole-rock meteorites and is enriched in ¹³C and ¹⁵N; (2) labile organic matter (LOM), which has a macromolecular structure but is liberated by hydrous pyrolysis; LOM is the parent structure for some FOM and is also enriched in ¹³C and ¹⁵N; and (3) refractory organic matter (ROM), which is also macromolecular but is virtually unaffected by hydrous pyrolysis and is relatively depleted in ¹³C and ¹⁵N. The macromolecular entities (LOM and ROM) are by far the most abundant organic components present, and as such, the relative abundances of the ¹³C- and ¹⁵N-enriched LOM and the ¹³C- and ¹⁵N-depleted ROM will have a major influence on the overall isotopic composition of the whole-rock meteorite. Laboratory experiments designed to simulate the effects of parent body aqueous alteration indicate that this form of processing removes LOM from the macromolecular material, allowing ROM to exert a stronger influence on the overall isotopic compositions. Hence, aqueous alteration of macromolecular materials on the meteorite parent body may have a significant control on the stable isotopic compositions of whole-rock carbonaceous chondrites. The enstatite chondrites are also carbon rich but have been subjected to high levels of thermal metamorphism on their parent body. Stepped combustion data of HF/HCl residues of enstatite chondrites indicate, that if they and carbonaceous chondrites inherited a common organic progenitor, metamorphism under reducing conditions appears to incorporate and preserve some of the ¹³C enrichments in LOM during graphitisation. However, when metamorphism is at its most extreme, the ¹⁵N enrichments in LOM are lost.     

多接收器等离子体质谱（MC-ICP-MS）测定Mg同位素方法研究

采用"样品-标准"交叉技术,以纯Mg试剂、水样和矿物岩石样品作为实验材料,尝试建立高精度多接收器等离子体质谱(MC-ICP-Ms)测定Mg同位素方法.应用质谱仪上窄的进样狭缝,将来自Ar载气、空气和酸中的C 2、C2H 、C2H2、CN 、NaH 等分子对Mg同位素的影响减至最小.当标准Mg浓度为3×10-6时,保持样品与标准的浓度比在0.5～2之间,对Mg同位素比值测量没有影响.大量实验表明,不同基质的行为各异:Na、Fe和Al的基质效应使δ25.Mg和δ26Mg偏负;Ca的基质效应使δ25.Mg和δ26Mg偏正;Cr的基质效应使δ25Mg和δ26Mg波动.控制[元素]/[Mg]的浓度比在0.05范围内,可以忽略同质异位素的干扰和基质效应.通过对本实验室的两个工作标准CAGS1-Mg和CAGS2-Mg的长期测量,估计出Mg同位素测量的外精度(2SD)对于δ26Mg可达0.18‰,对于δ25Mg可达0.090‰.在25Mg/24Mg对26Mg/24Mg的同位素比值图上,所有样品的Mg同位素值都落在斜率约0.5的质量分馏线上,意味着建立的MC-ICP-MS测定Mg同位素方法既精确又无干扰.相对于DSM3国际标准,样品的Mg同位素组成大致变化范围是δ26Mg值为2.790‰,δ25Mg值为1.282‰.其中,CAGS1.Mg的δ26Mg值最大,为0.399,来自新疆喀呐斯湖水的26MG值最小,为-2.091.     

MC-ICP-MS高精度测定Li同位素分析方法

以不同浓度Li元素标准样品和K、Ca、Na、Mg、Fe单元素标准样品的混合溶液为研究对象,采用3根阳离子交换树脂（AG 50W X8,200~400目）填充的聚丙烯交换柱和石英交换柱对Li进行分离富集,淋洗液分别为28 mol/L HCl、015 mol/L HCl以及05 mol/L HCl 30%乙醇,淋洗液体积小,仅为38 mL。分离回收率高,均大于976%。国际标样AGV 2（相对于IRMM 016）、BHVO 2（相对于IRMM 016）和IRMM 016（相对于L SVEC）的δ7Li值分别为(513±094)‰(2σ,n=10)、(408±060)‰(2σ,n=4)和(0038±073)‰(2σ,n=10),与前人分析结果吻合,分析精度与国际同类实验室水平相当。并对比了马里兰大学同位素实验室和笔者实验室对同种岩石矿物样品的分析结果,在误差范围内具有很好的一致性。此外,对美国地质调查局提供的准标样NKT 1霞石岩（相对于IRMM 016）给出了定值,δ7Li值为(871±046)‰(2σ,n=4)。因此,本方法可用于测定天然样品的Li同位素组成。