R. Ganapathy

Chemical fractionations in meteorites—VI. Accretion temperatures of H-, LL- and E-chondrites, from abundance of volatile trace elements

Abstract Extending our earlier work on 11 L-chondrites, we have measured 9 volatile elements (Ag, Bi, Cs, In, Rb, Tl, Se, Cd, Zn) by neutron activation analysis in 11 LL- and 10 E-chondrites; the first 6 elements also in 22 H-chondrites. The observed fractionation patterns are consistent with theoretical condensation curves and hence were apparently established during condensation from the solar nebula. Ordinary chondrites seem to have accreted between 420 and 500°K at P ≈ 10 −5 atm; enstatite chondrites, at 460 to 520°K and P ≈ 5 x 10−4 atm. The values for ordinary chondrites agree with O18-based temperatures by Onuma . et al. (1972) and with other characteristics such as Fe2+ content, presence of FeS and absence of Fe3O4. A few detailed trends were noted. Seven of the 54 meteorites seem to contain small amounts of a material enriched in Ag, Bi and especially T1; possibly a late condensate from a region depleted in metal. Silver shows considerable scatter, which suggests inhomogeneous distribution in the meteorites. Xenon correlates with In approximately as expected for equilibrium solubility, with some differences (petrologic type 3; E-chondrites) attributable to mineralogical factors. Meteorites of higher petrologic types are slightly deficient in Xe, probably due to gas losses during metamorphism. Cesium also appears to have been redistributed during metamorphism. Various features of the two-component model are critically examined in the light of the latest evidence. Apparently this model still is an adequate approximation of reality.

Chemical fractionations in meteorites—IV abundances of fourteen trace elements in L-chondrites; implications for cosmothermometry

Abstract A neutron activation procedure for Ag, Au, Bi, Br, Cd, Cs, Cu, Ga, In, Pd, Rb, Te, Tl and Zn has been developed and applied to 12 chondrites (11 L3 to L6 and 1 LL3-4). Correlations between abundance and petrologic type were found for Bi, Br, Cs, In and Tl. These elements correlated with each other and with primordial Ar and Xe. The observed inter-element correlations generally agree with calculated curves for trace element condensation from a cosmic gas at 10−4 ±2 atm. It appears that most fractionations of volatile elements, including noble gases, arose in the solar nebula rather than in meteorite parent bodies. A few elements (Cs, Br and Tl) sometimes are overabundant in L3,4 chondrites relative to other, less volatile elements (Cu, Ga). Either these elements became enriched in the solar nebula toward the end of accretion, or they were fractionated, presumably by volatilization, in the meteorite parent bodies. Accretion temperatures of L-chondrites were estimated from Bi and Tl contents. Petrologic types 3–6 seem to have accreted in a 100-degree interval centered on 530+80−60°K. The error quoted reflects the uncertainties in total nebular pressure (10−4±2 atm) and solubility of Bi and Tl in nickel-iron. Temperatures derived from Bi and Tl contents are generally concordant, as expected for the two-component model of Larimer and Anders , not for the three- and multicomponent models of Tandon and Wasson and Blander and Abdel-Gawad . An equation for the fraction of In condensed (αIn) was derived from the In data, using the accretion temperatures inferred from Bi and Tl contents: log [ α In (1− α In ) ] = (8850 ± 710) T − (18.82 ± 1.40) .

Abundance of 17 trace elements in carbonaceous chondrites

Abstract Seventeen trace elements (Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Rb, Re, Sb, Se, Te, Tl, U and Zn) were measured by neutron activation analysis in 8 C1 samples (1 Alais, 3 Ivuna, 4 Orgueil) and in 3 C2 samples (one each of Mighei, Murchison, Murray). The results show far less scatter than earlier literature data. The standard deviation of a single measurement from the mean of 8 C1 samples lies between 2 and 14 per cent, except for the following 4 elements: Au ±18 per cent, Ag ±22 per cent, Rb ±19 per cent and Br ±33 per cent. The first two probably reflect contamination and sample heterogeneity, the last two, analytical error. Apparently C1 chondrites have a far more uniform composition than some authors have claimed. The new data suggest significant revisions in cosmic abundance for the following elements (old values in parentheses): Zn 1250 (1500), Cd 1.51 (2.12), Ir 0.72 (0.43) atoms/106 Si atoms. The Br value is also lower, 6.8 vs 20.6, but may be affected by analytical error. Relative to C1 chondrites, the C2 chondrites Mighei, Murchison and Murray are depleted in volatile elements by a factor of 0.508 ± 0.038, much more constant than indicated by oldor data. Ordinary chondrites also show a more uniform depletion relative to the new C1 data. The mean depletion factor of Sb, F, Cu, Ga, Ge, Sn, S, Se, Te and Ag is 0.227 ± 0.027 in H-chondrites. This constancy further strengthens the case for the two-component model of chondrite formation.

Chemical fractionations in meteorites—V. Volatile and siderophile elements in achondrites and ocean ridge basalts

Eighteen achondrites and 4 terrestrial basalts (3 ocean ridge, 1 continental) were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Co, Cs, Cu, Ga, In, Ir, Rb, Se, Tl and Zn. Samples included 7 eucrites, 5 howardites, 2 nakhlites, 2 shergottites, an angrite, and an aubrite. Light and dark portions of the gas-rich meteorites Kapoeta and Pesyanoe were analyzed separately. Nakhlites and shergottites have volatile element abundances similar to those in ocean ridge basalts; eucrites, howardites and angrites show greater depletions by an order of magnitude and less similar abundance patterns. In terms of a two-component model of planetary accretion, the parent planets contained the following percentages of low-temperature material: eucrites 0.8, nakhlites 38, shergottites 28. Nominal accretion temperatures (oK) inferred from Tl contents were: eucrites 432, nakhlites 438, shergottites 433, all for an assumed nebular pressure of 10−5 atm. These data appear to be consistent with the oxygen isotope composition of these meteorites. Shergottites may be genetically related to L-chondrites. The siderophile element pattern of achondrites resembles that of the Moon, but with less extreme depletions. Terrestrial basalts, on the other hand, show a different pattern, with a steep decline in the order Ag > Au > Ir. The dark portion of Kapoeta seems to contain 1–2 % chondritic material, compositionally similar to Cl chondrites. No such enrichment was found for our samples of Pesyanoe.

Trace elements in the Allende meteorite. II - Fine-grained, Ca-rich inclusions

Abstract Nine fine-grained feldspathoid-, grossular-, spinel-, pyroxene-bearing inclusions from the Allende meteorite were analysed by instrumental neutron activation analysis. On the average, these inclusions are enriched in the refractory lithophile elements Ca, Sc, Ta and the rare earths by factors of 5–30 relative to Cl chondrites but are depleted in the refractory and volatile siderophiles, Ir, Co and Au. The volatile elements Fe, Cr and Zn are present at levels of 3.38–8.51%, 326–2516 ppm and 308–1376 ppm, respectively. Textural, mineralogical and chemical data suggest that the fine-grained inclusions formed in the solar nebula by the simultaneous condensation of volatiles and refractory lithophile elements which failed to condense into the coarse-grained, high-temperature condensate inclusions. The marked differences in the enrichment factors for different refractories in the fine-grained inclusions are caused by relatively small differences in their accretion efficiencies into the coarse-grained ones. The trace element data indicate that the refractories in the fine- and coarse-grained inclusions can only be the cosmic complements of one another if the fine-grained ones represent no more than ~ 20% of the most abundant refractory elements.

The simultaneous determination of 20 trace elements in terrestrial, lunar and meteoritic material by radiochemical neutron activation analysis

Abstract A radiochemical neutron activation method has been developed and was applied to the determination of Ag, Au, Bi, Br, Cd, Co, Cs, Cu, Ga, Ge, In, Ir, Ni, Rb, Re, Sb, Te, Tl, U, and Zn in 45 terrestrial, 230 lunar and 70 meteoritic samples. The inherent accuracy and precision for most elements is generally 10% or better, as is demonstrated by the results for well-mixed powdered rocks, for example, the U.S.G.S. standard basalt BCR-1.

Meteoritic material on the moon

Three types of meteoritic material are found on the Moon: micrometeorites, ancien planetesimal debris from the ‘early intense bombardment’, and debris of recent, crater-forming projectiles. Their amounts and compositions have been determined from trace element studies.The micrometeorite component is uniformly distributed over the entire lunar surface, but is seen most clearly in mare soils. It has a primitive, C1-chondrite-like composition, and comprises 1-1.5% of mature soils. Apparently it represents cometary debris. The mean annual influx rate is 2.4 × 10−9 g cm−2 yr−1. It shows no detectable time variation or dependence on selenographic position.The ancient component is seen in highland breccias and soils more than 3.9 AE old. It has a fractionated composition, with volatiles depleted relative to siderophiles. The abundance pattern does not match that of any known meteorite class. At least two varieties exist (LN and DN, with Ir/Au, Re/Au 0.25-0.5 and > 0.5 the C1 value). Both seem to represent the debris of planetesimals that produced the mare basins and highland craters during the first 700 Myr of the Moons history. It appears that the LN and DN objects impacted at less then 10 km s−1, had diameters less than 100 km, contained more than 15% Fe, and were not internally differentiated. Both were depleted in volatiles; the LN objects also in refractories (Ir, Re). This makes it unlikely that the LN bodies served as important building blocks of the Moon.The crater-forming component has remained elusive. Only a possible hint of this component has been seen, in ejecta from Dune Crater and Apollo 12 KREEP glasses of Copernican (?) origin.

Trace elements in the Allende meteorite-III. Coarse-grained inclusions revisited

Abstract New RNAA determinations of Ba, Sr, Zr, U, Re, Pd, Ag, Zn and Se and INAA measurements of Lu are added to published data for 21 other elements in the same suite of ten samples. On the average, 21 refractory elements are not significantly fractionated from one another. The mean of their enrichment factors relative to Cl chondrites is 17.5 ± 0.4, indicating that the high-temperature condensate inclusions represent 5.7 wt% of the total condensable matter. Os, Ir, Ru, Re and most of the W condensed in one or more refractory siderophile element alloys along with small fractions of the Pd, Co, Au and Ag. The bulk of the Eu and Sr condensed in solid solution in melilite. Sc, Zr, Hf, Ta, U and the remaining REE condensed in a phase whose abundance in the inclusions is negatively correlated with that of melilite, either diopside or one or more minor or trace phases, including perovskite. Ba condensed in a different phase, separately from all these elements. In individual inclusions, fractionations are common between elements which were carried in by different condensate phases. Smaller fractionations are also observed for elements which condensed together. These may be due to variable proportions of them in a common condensate phase in response to different nebular equilibration temperatures or to multiple condensate phases containing different proportions of these elements. Available evidence indicates that some trace elements no longer reside in the phases which carried them into the inclusions, indicating a post-accretion thermal event which redistributed some of them. From the minimal variation of the Zr/Hf ratio in the inclusions, the solar system ratio is estimated to be 29.6 ± 1.8. From the mean U content of the inclusions and estimates of the bulk terrestrial and lunar U abundances, the Earth and Moon are estimated to contain 21% and 22–30% high-temperature condensates, respectively.

Trace Elements and Radioactivity in Lunar Rocks: Implications for Meteorite Infall, Solar-Wind Flux, and Formation Conditions of Moon

Lunar soil and type C breccias are enriched 3-to 100-fold in Ir, Au, Zn, Cd, Ag, Br, Bi, and Tl, relative to type A, B rocks. Smaller enrichments were found for Co, Cu, Ga, Pd, Rb, and Cs. The solar wind at present intensity can account for only 3 percent of this enrichment; an upper limit to the average proton flux during the last 4.5 x 109 years thus is 8 x 109 cm-2 yr-1. The remaining enrichment seems to be due to a 1.5 to 2 percent admixture of carbonaceous-chondritelike material, corresponding to an average influx rate of meteoritic and cometary matter of 2.9 x 10-9 g cm-2 yr-1 at Tranquility Base. This is about one-quarter the terrestrial rate. Type A, B rocks are depleted 10-to 100-fold in Ag, Au, Zn, Cd, In, Tl, and Bi, relative to terrestrial basalts. This suggests loss by high-temperature volatilization, before or after accretion of the moon. Positron activities due mainly to 22Na and 26Al range from 90 to 220 β+ min-1 kg-1 in five small rocks or fragments (9 to 29 g). The higher activities presumably indicate surface locations. Th and U contents generally agree with those found by the preliminary examination team.

Chemical fractionations in meteorites. X - Ureilites

Abstract Four ureilites (Dyalpur, Goalpara, Havero, and Novo Urei) were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Rb, Re, Sb, Se, Te, Tl, and U. An attempt has been made to resolve the data into contributions from the parent ultramafic rock and the injected, carbon- and gas-rich vein material. Interelement correlations, supported by analyses of separated vein material (WANKE et al , 1972), suggest that the vein material is enriched about 10-fold in refractory Ir and Re over moderately volatile Ni and Au, and is low in volatiles except Ge, C, and noble gases. It appears to be a refractory-rich nebular condensate that precipitated carbon by surface catalytic reactions at ~500K and trapped noble gases but few other volatiles. The closest known analogue is a Cr- and C-rich fraction from the Allende meteorite, highly enriched in heavy noble gases and noble metals. By analogy with Allende, the gas-bearing phase in ureilites may have been an Fe, Cr-sulfide. The ultramafic rock contains siderophiles and chalcophiles (Ni, Au, Ge, S, Se) at ~0.05 of Cl chondrite level, and highly volatile elements (Rb, Cs, Bi, Tl, Br, Te, In, Cd) at ~0.01 Cl level. It probably represents the residue from partial melting of a C3V-like chondrite body, under conditions where phase separation was incomplete so that some liquid was retained. The vein material was injected into this rock at some later time.