Marie-Josée Janssens

Core formation in the Earth and Shergottite Parent Body (SPB): Chemical evidence from basalts☆

Constraints on processes of core formation in terrestrial planets may be inferred from the abundances of siderophile and chalcophile elements in their mantles. Of particular interest is a comparison of processes of core formation in the Earth and in the Shergottite Parent Body (SPB), a terrestrial planet tentatively inferred to be Mars. To this end, we (i) present new INAA and RNAA analyses of the non-Antarctic SNC meteorites, (ii) infer the composition of the SPB mantle from the compositions of the SNC meteorites, (iii) infer the composition of the Earths mantle from the compositions of terrestrial basalts, and (iv) deconvolve the effects of volatile depletion, core formation, and mineral/melt fractionation on the abundances of siderophile and chalcophile elements in the SPB and the Earth. Element abundances in the mantles of the SPB and the Earth are estimated from element/element correlations observed among basalt samples. In basalts from the SPB (the SNC meteorites), four groups of covariant elements are observed: highly incompatible, moderately incompatible, indifferent and compatible. From correlations within these groups, the SPB mantle is found to be depleted in volatile elements, and strongly depleted in siderophile and chalcophile elements. For the Earth, the element/element correlation method gives a mantle composition similar to previous estimates. Compared to the Earth, the SPB mantle is richer in moderately siderophile elements (e.g., W, P), consistent with its inferred higher oxidation state. Chalcophile elements in the SPB mantle are more depleted than in the Earths mantle, particularly when compared to estimates of the original abundances of volatile chalcophile elements in the two planets. In the SPB mantle, the NiCo ratio is nonchondritic, in contrast to the chondritic ratio in the Earths mantle. Abundances of siderophile and chalcophile elements in the SPB mantle may be modelled by equilibrium with solid metal and metallic sulfide liquid, with some metal and sulfide trapped in the mantle (i.e., homogeneous accretion and inefficient core formation). Neither this model nor a heterogeneous accretion model is satisfactory in explaining element abundances in the Earths mantle, particularly the abundances of Ni, Co, Mo, and W. Nevertheless it appears that core formation in the SPB and the Earth left quite different chemical signatures in their planetary mantles.

Further studies of trace elements in C3 chondrites

Five carbonaceous chondrites (Renazzo C2V, Allende C3V, Omans C3O, Warrenton C3O, and Orgueil Cl) were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs. Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Te, Tl, U and Zn. These data, together with earlier measurements on seven additional C3 s, are interpreted in the light of petrographie studies by MCSWEEN (1977a, b) and revised condensation temperatures (WAI and Wasson, 1977). Elements condensing between ~ 700 and 420 K (Se, Zn, S, Te, Br, In, Bi, Tl) are systematically more depleted than those condensing between 1000 and 900 K (Ge, Ag, Rb), by factors of 1.3 to 2, and the depletion correlates inversely with matrix content and directly with degree of metamorphism. The most plausible explanation appears to be a gas-dust fractionation during condensation, by settling of dust to the median plane of the nebula. In this model, gas/dust ratios relative to the cosmic ratio ranged from 0.7 at 1000 K to 0.5 at 700 K for those C3O s that accreted first (Ornans, Warrenton) and from 1.3 to 0.6 for the last (Kainsaz). There appears to have been no further gas/dust fractionation below 700 K. Abundances of Sb, Au and Cd follow earlier trends. Depletion of Sb and Au correlates with abundance of Fe-poor olivine and seems to reflect greater volatilization upon more prolonged or intense heating during chondrule formation. The 50–100-fold depletion of Cd in C3Os compared to C3Vs suggests condensation in a region where enough Fe was present to buffer the H2S pressure.

Ries impact crater, southern Germany: search for meteoritic material

Abstract Twenty-three samples from the Ries crater, representing a wide range of shock metamorphism, were analyzed for seven siderophile elements (Au, Ge, Ir, Ni, Os, Pd, Re) and five volatile elements (Ag, Cd, Sb, Se, Zn). Taking Ir as an example, we found siderophile enrichments over the indigenous level of 0.015 ppb Ir occur in only eight samples. The excess is very modest; even the most enriched samples (a weakly shocked biotite gneiss and a metal-impregnated amphibolite) have Ir, Os corresponding to ~4 × 10 −4 C1 chondrite abundances. Of five fladle glasses analyzed only one shows excess Ir. Suevite matrix and vesicular glass have slight enrichment, but homogenous glass from the same rock does not. In fladle glasses, Ni and Se are strongly correlated and apparently reside in Ir, Os-poor Sulfides [pyrrhotite, chalcopyrite, pentlandite(?)]of terrestrial, probably sedimentary, origin. The Ir, Os and Ni enrichments of the metal-bearing amphibolite are compatible with chondritic ratios, but these are ill-defined because of uncertainty in Ni. In the other samples enriched in siderophiles Ir(Os), Ni and Se are mutually correlated; Ni Ir and Ni Os ~ 11 × C1 and are much higher than any chondritic ratios; Se Ni ~ 2 × C1 and suggests a sulfide phase, rather than metal may be the host of the correlated elements. Lacking a plausible local source, this material is apparently meteoritic in origin. The unusual elemental ratios, coupled with the very low enrichments, tend to exclude chondrites and most irons as likely projectile material. Of the achondrites, aubrites seem slightly preferable. Ratios of excess siderophiles in Ries materiel match tolerably those of an aubrite (possibly atypical) occurring as an inclusion in the Bencubbin meteorite, Australia. The Hungaria group of Mars-crossing asteroids may be a source of aubritic projectiles.

Enstatite chondrites - Trace element clues to their origin

We have analyzed by RNAA 3 EH and 3 EL chondrites for 20 trace elements. Interelement correlations were examined visually and by factor analysis, to assess the effects of nebular fractionation and metamorphism. Refractory siderophiles (Ir, Os, Re) correlate with “normal siderophiles” (Ni, Pd, Au, Sb, and Ge) in ELs but not EHs; presumably these two element groups originally condensed on separate phases (CAI and metal), but then concentrated in metal during metamorphism. Sb and Ge are more depleted than the other three elements of the “normal” group, presumably by volatilization during chondrule formation. Volatiles are consistently more depleted in ELs than EHs, by factors >10× for the more volatile elements. Some of the stronger correlations are found for In-Tl, Tl-Bi, and Zn-Cd-In. These correlations are about equally consistent with predicted condensation curves for the solar nebula (especially for host phases with negative heats of solution, or for P = 0.1−1 atm) and with volatilization curves for artificially heated Abee, as determined by M E. Lipschutz and coworkers at Purdue. No decisive test between these alternatives is available at present, but the close correlation of Zn, Cd, In may eventually provide a crucial test. Factor analysis shows that 3 factors account for 93% of the variance; they seem to reflect volatile (F1), siderophile (F2), and chalcophile (F3) behavior. The element groupings agree largely with those recognized visually; they are listed with the inferred host phases. F1 (minor sulfide, probably ZnS): Zn, Cd, In, Br; F2 (CAI, later metal): Ir, Os. Re; F1, F2 (metal): Ni, Pd, Au, Ge, Sb; F3, F1 (FeS): Se, Te, Bi, Tl. These correlations differ to some extent from those obtained by Shaw (1974) in an earlier factor analysis, presumably because the new data are more homogeneous and extensive, especially for siderophiles. The new correlations also show that the cosmochemical behavior of some volatiles in E-chondrites differs from that predicted for ordinary chondrites, so that condensation curves for the latter are not strictly applicable.

H-chondrites - Trace element clues to their origin

We have analyzed 10 H-chondrites for 20 trace elements, using RNAA. The meteorites included 4 of petrologic type 4 and 2 each of types 3, 5 and 6. The data show that H-chondrites are not isochemical. H3s are depleted by some 10% not only in Fe (Dodd, 1976), but also in the siderophiles Os, Re, Ir, Ni, Pd, Au, and Ge. Moreover, the abundance pattern of siderophiles varies systematically with petrologic type. As similar fractionations of REE have been observed by Nakamura (1974), it appears that both the proportions and compositions of the main nebular condensates varied slightly during accretion of the H-chondrites. Thus the higher petrologic types are independent nebular products, not metamorphosed descendants of lower petrologic types. Abundances of highly volatile elements (Cs, Br, Bi, Tl, In, Cd, Ar36) correlate with petrologic type, declining by ≤ 10−3 from Type 3 to Type 6. The trends differ from those for artificially heated Type 3s (Ikramuddinet al., 1977b; Herzoget al., 1979), but agree passably with theoretical curves for nebular condensation. Apparently the low volatile contents of higher petrologic types are a primary feature, not the result of metamorphic loss. The mineralogy of chondrites suggests that they accreted between 405 K (absence of Fe3O4) and 560 K (presence of FeS), and the abundances of Tl, Bi, and In further restrict this interval to 420–500 K. Accretion at 1070 ± 100 K, as proposed by Hutchisonet al. (1979, 1980), leads to some extraordinary problems. Volatiles must be injected into the parent body after cooling, which requires permeation of the body by 1011 times its volume of nebular gas. This process must also achieve a uniform distribution of the less volatile elements (Rb, Cu, Ag, Zn, Ga, Ge, Sn, Sb, Se, F), without freezeout in the colder outer layers. Factor analysis of our data shows 3 groupings: siderophiles (Os, Re, Ir, Ni, Pd, Au, and Ge), volatiles (Ag, Br, In, Cd, Bi, and Tl) and alkalis (Rb and Cs). The remaining 5 elements (U, Zn, Te, Se, and Sb) remain unassociated.

Ureilites: trace element clues to their origin

Carbonaceous vein separates from Kenna and Havero, as well as bulk Kenna, were analyzed by RNAA for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Pd, Os, Rb, Re, Sb, Se, Te, Tl. U, and Zn. The data are reviewed together with four earlier Chicago analyses of bulk ureilites. Linear regressions confirm the presence of two metal components, with the following Cl-normalized ratios: Ir/Ni = 14.6, ≤ 1; Ge/Ni = 5.4, 2.4; Au/Ni = 2.3, 0.9. The high-Ir component is enriched in vein separates and hence belongs to veins; the lowIr component belongs to the ultramafic rock. Vein material is enriched in all elements analyzed by us except Zn, and accounts for most of the C, noble gases, and presumably siderophiles in the meteorite. Most of the properties of ureilites apparently can be explained by the cumulate model of Berkley et al. (1980), with certain modifications. Comparison of ureilites with three other ultramafic rocks from different planets (Earths mantle, lunar dunite, and Chassigny) suggests that the ureilite parent body had a primitive chondritic composition, similar to C3V chondrites but richer in metal and carbon. It melted, causing depletion of incompatibles to a mean abundance of ~0.02 × Cl and incomplete segregation of metal, FeS, and C. Fractional crystallization or melting of metal in the presence of S and C apparently can explain the fractionations of Ir, Re, Ni, Au, and perhaps Ge, obviating the need for extraneous sources of vein metal or unusual parent-body compositions. Noble gases from the parent material may have been retrapped in carbon during magmatism, provided the system was closed.

A carbonaceous inclusion from the Krymka LL-chondrite - Noble gases and trace elements

Abstract A black inclusion from the Krymka LL3 chondrite was analyzed for 20 trace elements and five noble gases, by radiochemical neutron activation and mass spectrometry. The trace element pattern somewhat resembles that of C1 or C2 chondrites, but with several unique features. Elements of nebular condensation T ≳ 1000 K (U, Re, Os, Ir, Ni, Pd, Au, Sb and Ge) are essentially undepleted, as in C1 chondrites, but Re Ir is 1.49 × higher than the characteristic Cl value. Among elements condensing below 1000 K, Cs, Se, Te, and In are depleted to approximately C2 levels (~0.6 × C1), whereas Ag, Bi, Tl are enriched to ~ 1.6 × C1. Such enrichments are thought to be characteristic of late nebular condensates. The noble-gas pattern also is unique. Gas contents are higher than in C1s, by factors of 2.6 to 19 for Ne through Xe. The Ar 36 Xe 132 ratio of 500 is higher than mean values for C1s or C2s (109 or 89) and exceeds even the highest value seen in C3Os, 420, whereas the He 4 Ne 20 ratio of 62 is much lower than the values for C1s and C2s (200–370). The Xe 129 Xe 132 and Xe 136 Xe l32 ratios of 1.040 and 0.320 resemble those of C1 chondrites, and seem to imply typical proportions of radiogenic Xe129 and ‘fissiogenic’ xenon. It appears that the inclusion represents a new primitive meteorite type, similar to C-chondrites, but probably a late condensate from a region of higher nebular pressure.