Michael E. Lipschutz

The record in the meteorites—IV: Origin of diamonds in iron meteorites☆☆☆

Abstract A number of diamond-bearing Canyon Diablo specimens have been investigated in order to determine whether the diamonds were produced under high gravitational pressures in a meteorite parent body, or upon impact with the earth, as proposed by Urey and Nininger . respectively. Metallographic studies confirm Niningers observation that the diamond-bearing fragments, and only these, were reheated strongly after the formation of the Widmanstatten pattern. The metal phase appears to have been reheated to ~950°C for 1–5 sec, followed by cooling in less than 2 min. The rapid cooling rate implies that the process took place after the meteorite fragments had attained their present, small size. Thermodynamic calculations and structural studies indicate that cohenite (Fe3C), rather than graphite is the likely precursor of diamonds in iron meteorites. This fact severely limits the possible range of conditions for diamond formation. The diamonds are always associated with troilite. which shows signs of having been heated to much higher temperatures than the metal, suggesting that the heating took place by a shock wave upon impact with the earth. The growth of diamonds must have occurred on cooling or pressure drop, but it is not yet clear whether high pressures (e.g. from the compression during impact shock, or localized stresses) were at all required, or whether the diamonds formed as a metastable product at moderate to low pressures. Possible origins of diamonds in other meteorites are discussed. It is shown that in the presence of free iron, formation under high gravitational pressures could take place only under very extraordinary circumstances. Instead, it is considered much more likely that all meteoritic diamonds were produced by catastrophic events: either upon impact with the earth, or during the breakup of the meteorite parent bodies. This mode of origin obviates the need for postulating meteorite parent bodies of lunar or planetary size, with interior pressures of 3 × 104 atm or greater. The laboratory formation of diamond is discussed in terms of metal carbide dissociation equilibria, and several features of the General Electric process arc explained in this manner. Some comments are offered on Moissans experiments.

Labile trace elements in carbonaceous chondrites - A survey

We report radiochemical neutron activation analysis data for Co, Au, Ga, Rb, Sb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In (ordered by increasing putative volatility in primary nebular processes) in 42 C2–C6 chondrites, all but three from Antarctica. From these and literature data for 19 additional chondrites, Cl-normalized concentrations of the nine most volatile elements (Ag → In) are quite constant in most meteorites. Trace element trends in 39 Antarctic and 22 non-Antarctic carbonaceous chondrites are similar: no evidence exists for substantial alteration by weathering of samples in Antarctica, nor do the data reflect modification by open-system, parent body metamorphism at ≥ 500°C. Volatile element concentrations and siderophile ratios (Au/Co and Ga/Co) define continua which correlate at statistically significant levels. Carbonaceous chondrites sample not a few, compositionally distinct parents but rather a compositional continuum in which parent materials forming under more oxidizing conditions incorporated lesser complements of volatiles, essentially unfractionated from cosmic composition. This may well reflect the range of formation conditions (temperature, duration, and water/rock ratios) represented by oxygen isotope variations during preterrestrial aqueous alteration of parent materials.

Chemical studies of H chondrites: 5. Temporal variations of sources

We report 36Cl (301-kyr half-life) data obtained by accelerator mass spectrometry allowing nominal terrestrial ages to be determined for 39 Antarctic H4–6 chondrites for which contents of volatile trace elements are known. The compositional difference between these Antarctic meteorites and 58 non-Antarctic falls increases with terrestrial age and, using multivariate statistical techniques, becomes highly significant for Antarctic samples with ages >50 kyr. The compositional difference is inconsistent with trivial causes such as weathering and seems to reflect differences in thermal histories of parent sources. Temporal source variations for the H chondrite flux on Earth thus exist not only on a short-term, 40 years, basis (Dodd et al., 1993) but also on a long-term, >50 kyr, basis.

Chemical studies of H chondrites: 4. New data and comparison of Antarctic suites

We report data for the trace elements Au, Co, Sb, Ga, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In (ordered by putative volatility during nebular condensation and accretion) determined by neutron activation analysis in 13 H5 chondriles from Victoria Land and 20 H4–6 chondrites from Queen Maud Land, Antarctica. These and earlier results provide Antarctic sample suites of 34 chondrites from Victoria Land and 25 from Queen Maud Land. Treatment of data for the most volatile 10 elements (Rb → In) in these suites by multivariate statistical techniques more robust, as well as more conservative, than conventional linear discriminant analysis and logistic regression demonstrates that compositions differ at marginally significant levels. This difference cannot be explained by trivial (terrestrial) causes and becomes more significant, despite the smaller size of the database, when comparisons are limited to data from a single analyst and when all upper limits arc eliminated from consideration. The Victoria Land and Queen Maud Land suites have different mean terrestrial ages (∼300 kyr and ∼100 kyr, respectively) and age distributions, suggesting that a time-dependent variation of chondritic sources with different thermal histories is responsible. As a result, these two Antarctic suites are, on average, chemically distinguishable from each other. Since H chondrites serve as a paradigm for other meteorite classes, these results indicate that the near-Earth populations of planetary materials varied with time on the 105-year timescale.

Chemical studies of H chondrites: 6. Antarctic/non‐Antarctic compositional differences revisited

We report data for the trace elements Au, Co, Sb, Ga, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, T1, and In (ordered by putative volatility during nebular condensation and accretion) determined by radiochemical neutron activation analysis of 14 additional H5 and H6 chondrite falls. Data for the 10 most volatile elements (Rb to In) treated by the multivariate techniques of linear discriminant analysis and logistic regression in these and 44 other falls are compared with those of 59 H4-6 chondrites from Antarctica. Various populations are tested by the multivariate techniques, using the previously developed method of randomization-simulation to assess significance levels. An earlier conclusion, based on fewer examples, that H4-6 chondrite falls are compositionally distinguishable from the Antarctic suite is verified by the additional data. This distinctiveness is highly significant because of the presence of samples from Victoria Land in the Antarctic population, which differ compositionally from falls beyond any reasonable doubt. However, it cannot be proven unequivocally that falls and Antarctic samples from Queen Maud Land are compositionally distinguishable. Trivial causes (e.g., analyst bias, weathering) cannot explain the Victoria Land (Antarctic)/non-Antarctic compositional difference for paradigmatic H4-6 chondrites. This seems to reflect a time-dependent variation of near-Earth meteoroid source regions differing in average thermal history.

Effect of thermal metamorphic conditions on mineralogy and trace element retention in the Allende meteorite

HEATING carbonaceous chondrite from the Allende meteorite in a low pressure environment causes visible mineralogical alteration at 700–1,000° C but not at T ≤ 600° C. Samples heated for 29 d at 500° C lose trace elements (Bi, In and Tl) more effectively than those similarly heated for 7 d. At 1,000° C with ∼ 10−5 atm initial pressure of O2, H2 or He these elements, Ga and Se are comparatively more completely lost.

Formation conditions of igneous regions in ordinary chondrites: Chico, Rose City, and other heavily shocked H and L chondrites

We report microprobe and radiochemical neutron activation analysis (RNAA) data for hosts and igneous inclusions in the heavily shocked ordinary chondrites Rose City (H5), Yanzhuang (H6), Farmington (L5), Malakal (L5), Chantonnay (L6), Chico (L6), and Tuan Tuc (L6). Based on these analytical results, equilibrium crystallization calculations [Ghiorso and Sack, 1995], phase equilibrium analysis [e.g., Sack and Ghiorso, 1994a,b; Sack et al., 1994], and experimental cooling rate studies [e.g., Walker et al., 1976], we have assessed the metamorphic and magmatic histories of these seven heavily shocked chondrites. We infer that (1) unfractionated chondritic liquids were intruded to depths >0.1 km in the parent asteroid of Rose City; (2) early chondritic liquids experiencing 5–10% olivine fractionation were erupted onto the surface of the parent asteroid(s) of Yanzhuang and Chantonnay; and (3) near-surface crystallization is also indicated for the liquids in Tuan Tue and Farmington, with Chico and Malakal crystallizing at slightly greater depths. In all but Chantonnay, liquids appear to have derived from melting of chondrite types corresponding to their hosts. In the L chondrite Chantonnay, the intrusive liquids derive from melting an H chondrite source region in which chondritic melts were stored for a sufficient time to produce pigeonite in zoned pyroxene xenocrysts. Heating effects are also reflected in the trace element contents of the chondrites. Our RNAA data for Rose City seem to reflect only the siderophile-lithophile fractionation evident in the metal distribution. The Yanzhuang RNAA data are generally similar to those of other H chondrites: severe shock not involving phase transport seems to leave H chondrites unaffected compositionally. Contents of the four most mobile elements (Cd, Bi, T1, In) and Cs in Chico indicate loss, so that this assemblage experienced extended, low-temperature cooling after injection of dike material into the Chico host. RNAA data for the other four L chondrites examined indicate more rapid cooling.

Chemical studies of H chondrites 8. On contemporary meteoroid streams

Using date and time of fall and petrographic classification as criteria, many equilibrated H chondrites that fell during September and October from 1812 to the present form four significant clusters, denoted as Cluster 2 through Cluster 5, on day-year plots. Using radiochemical neutron activation analysis, we determined 15 trace elements, U, Au, Co, Sb, Ga, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In (ordered by increasing putative volatility during nebular condensation), in 27 members of these four clusters. We used model-dependent and model-independent multivariate statistical techniques to compare contents of the 10 most volatile elements separately in the four clusters with those of a 33-member suite of random H chondrite falls (from 1773 to 1970). The Clusters 2 and 5 suites (that fell in September 1880–1991 and October, 1919–1984, respectively), each of which is represented by 10 H chondrite falls, are not compositionally distinguishable from the suite of random falls. However, the 17-member combined suite of Clusters 3 and 4 chondrites (that fell during September–October, 1812–1992) proves compositionally distinguishable from random falls at moderate to strong significance levels of 0.01–0.001. This 17-member suite is less readily distinguished from random falls than are the previously reported suite of Cluster 1 falls (May 1855–1895), or Antarctic H chondrites with nominal terrestrial ages >50 kyr, each of which is highly significant at <0.001 levels. All suites are genomict and exhibit a range of cosmic ray exposure ages with a plurality having 6–8 Ma ages. Inconclusive results are obtained in the cases of Clusters 2 and 5. However, three H chondrite suites (Clusters 1,3, and 4) distinguishable from the random background by one property (time of fall) are also distinguishable by another (contents of volatile trace elements or thermal history). Temporal change of H chondrite sources sampled by Earth are indicated by these data.

Igneous inclusions from ordinary chondrites: High temperature cumulates and a shock melt

We report microprobe, instrumental neutron activation analysis, and radiochemical neutron activation analysis data for three large igneous inclusions in the Yamato (Y-)75097, Y-793241, and Y-794046 ordinary chondrites. The inclusions in the first two chondrites are troctolitic cumulates that have undergone appreciable reactions with their hosts either during emplacement and/or cooling. Olivine-spinel Fe-Mg exchange pairs in these two inclusions record equilibration temperatures of about 710°C, and these temperatures are similar to those exhibited by mineral pairs in the Y-75097 and Y-793241 hosts. The inclusion in Y-794046 is texturally unique, consisting of fine-grained, randomly distributed olivines, coarse (∼2 mm) fascicular pyroxene laths, and angular pockets of maskelynite/plagioclase feldspar. The phase compositions are readily interpreted as having resulted from extremely rapid, essentially isochemical cooling to temperatures 1670°C. We suggest that this igneous inclusion formed in situ by shock.

Search for Diamonds in the Chondrite Ghubara

IT is generally agreed that meteoritic diamonds are a critical key to understanding the pressure history of meteorites. They are known to exist in four meteorites, one being the crater-forming Canyon Diablo coarse octahedrite (iron), and the others the three known members of the ureilite class of stony meteorites1–4,6. Recent papers5 have summarized the arguments for and against the formation of diamonds by hydrostatic or shock processes.