G. Dreibus

Chemistry of Rocks and Soils at Meridiani Planum from the Alpha Particle X-ray Spectrometer

The Alpha Particle X-ray Spectrometer on the Opportunity rover determined major and minor elements of soils and rocks in Meridiani Planum. Chemical compositions differentiate between basaltic rocks, evaporite-rich rocks, basaltic soils, and hematite-rich soils. Although soils are compositionally similar to those at previous landing sites, differences in iron and some minor element concentrations signify the addition of local components. Rocky outcrops are rich in sulfur and variably enriched in bromine relative to chlorine. The interaction with water in the past is indicated by the chemical features in rocks and soils at this site.

Chemical composition and accretion history of terrestrial planets

The high concentrations of moderately siderophile elements (Ni, Co, etc.) in the Earth’s mantle and the similarity of their Cl normalized abundances to those of moderately volatile elements (F, Na, K, Rb) and some elements such as In, which under solar nebula conditions are highly volatile, are striking. To account for the observed abundances, inhomogeneous accretion of the Earth from two components has been proposed. In this model accretion started with the highly reduced component A devoid of all elements more volatile than Na, followed by accretion of more and more oxidized material (component B), containing all elements in Cl abundances. Recent observations have brought almost conclusive evidence that SNC meteorites are martian surface rocks ejected by huge impacts. By assuming that Mars is indeed the parent body of SNC meteorites, the bulk composition of Mars is estimated. The data on the composition of Mars obtained in this way clearly show that the two-component model is also valid for Mars. The striking depletion of all elements with chalcophile character in the martian mantle indicates that, contrary to the Earth, Mars accreted almost homogeneously (H. Wanke, Phil. Trans. R. Soc. Lond. A 303, 287 (1981)).

Volatiles on Earth and Mars: A comparison

Based on our new and previous determinations of halogens in SNC meteorites, the bulk concentrations of halogens in the SPB, which is thought to be Mars, are estimated. The two-component model for the formation of terrestrial planets as proposed byA. E. Ringwood (Geochem. J. 11, 111–135 (1977) andOn the Origin of the Earth and Moon, Springer-Verlag, New York, 1979) andH. Wa¨nke (Philos. Trans. Roy. Soc. London, Ser. A 303, 287–302 (1981) is further substantiated. It is argued that almost all of the H2O added to Mars during its homogeneous accretion was converted on reaction with metallic Fe to H2, which escaped. By comparing the solubilities of H2O and HCl in molten silicates, the amount of H2O left in the mantle of Mars at the end of accretion can be related to the abundance of Cl. In this way an H2O content in the Martian mantle of 36 ppm is obtained, corresponding to an ocean covering the whole planet to a depth of about 130 m. The huge quantities of H2 produced by the reaction of H2O with metallic iron should also have removed other volatile species by hydrodynamic escape. Thus it is postulated that the present atmospheres of Venus, Earth, and Mars were formed by degassing the interiors of the planets, after the production of H2 had ceased, i.e., after metallic iron was no longer available. It is also postulated that the large differences in the amounts of primordial rare gases in the atmospheres of Venus, Earth, and Mars are due mainly to different loss factors. Except for gaseous species, Mars is found to be richer in volatile (halogens) and moderately volatile elements than the Earth. The resulting low release factor of40Ar for Mars is attributed to a low degree of fractionation, leading to a relatively small crustal enrichment of even the most incompatible elements like K.

Chlorine and bromine abundance in MORB: the contrasting behaviour of the Mid-Atlantic Ridge and East Pacific Rise and implications for chlorine geodynamic cycle

We have analyzed MORB glasses from the Mid-Atlantic Ridge (64°N-20°S) and the East Pacific Rise (7–21°N) for chlorine with an electron microprobe and chlorine and bromine by NAA. Fractionation-corrected average concentrations of chlorine for the MAR amount to 49 and 153 ppm for N- and E-MORB, respectively. EPR glasses contain on the average larger amounts of chlorine (280 ppm for N-MORB), while E-MORB are not significantly different (320 ppm). When chlorine is compared to other lithophile and volatile trace elements, EPR samples appear strongly variable (σ = 163% for N-MORB) contrasting with MAR (σ = 68% for N-MORB) where chlorine behaves like a strongly incompatible element. Chlorine to bromine ratios remain nearly constant at 430 ± 130 (1σ) over a Br concentration range of 60–1300 ppb, similar to the exospheric ratio (390), and independent of the basalt type. Along the MAR, chlorine can be described as a conventional incompatible trace element which correlates positively with potassium (or any other strongly incompatible element). The ClK ratio is correlated with 3He4He along the MAR. ClK ratios vary between 4 and 24 × 10−2 and permit to determine regions with specific ClK ratios. The high EPR chlorine concentrations are not found to correlate with any other geochemical index, and on the basis of ClH2O or ClK must be considered as excess chlorine. This excess has been found at all locations studied along the EPR (50°N to 32°S) and for two samples near 22°N on the MAR. These are believed to be generated by incorporation of a brine derived from seawater unmixing at high temperature in the oceanic crust: the observed ClH2O (up to one) can be derived from neither seawater (ClH2O = 1.9 × 10−2) nor altered oceanic crust (< 10−2). Brine generation and incorporation is probably related to the high rate of spreading of the EPR compared to that of the MAR. Bulk silicate Earth chlorine abundance can be constrained by mass balance between exosphere and depleted mantle on the one hand and from the Cl/Th/Ba ratios on the other. Depending on Th and Ba continental crustal abundances, primitive mantle chlorine is estimated at 35 ± 5 ppm from both approaches. Similarly, primitive mantle bromine is estimated at 88 ppb. The primitive mantle ClK ratio of 14 × 10−2 is significantly lower than that measured for the high 3He4He samples, near 61°N on the Reykjanes Ridge (24 × 10−2). The depletion factor for chlorine and bromine relative to C1 chondrites is 0.035 and 0.024, respectively, a measure of their volatility. A first-order model of chlorine transfer from the mantle to the exosphere is consistent with present-day flux at ridges. It is suggested that the more extensive depletion of chlorine compared to barium from the mantle is controlled by the ages of their exospheric reservoirs, the ocean and the continental crust.

Chemistry and accretion history of Mars

Using element correlations observed in SNC meteorites and general cosmochemical constraints, Wänke & Dreibus (1988) have estimated the bulk composition of Mars. The mean abundance value for moderately volatile elements Na, P, K, F, and Rb and most of the volatile elements like Cl, Br, and I in the Martian mantle exceed the terrestrial values by about a factor of two. The striking depletion of all elements with chalcophile character (Cu, Co, Ni, etc.) indicates that Mars, contrary to the Earth, accreted homogeneously, which also explains the obvious low abundance of water and carbon. SNC meteorites and especially the shergottites are very dry rocks, they also contain very little carbon, while the concentrations of chlorine and especially sulphur are higher than those in terrestrial rocks. As a consequence we should expect SO2 and HC1 to be the most abundant compounds in Martian volcanic gases. This might explain the dominance of sulphur and chlorine in the Viking soils. In turn SO2, being an excellent greenhouse gas, may have been of major importance for the warm and wet period in the ancient Martian history. Episodic release of larger quantities of SO2 stored in liquid or solid SO2 tables in the Martian regolith triggered by volcanic intrusions as suggested here could lead to a large number of warm and wet climate periods of the order of a hundred years, interrupted by much longer cold periods characterized by water ice and liquid of solid SO2. Sulphur (FeS) probably also governs the oxygen fugacity of the Martian surface rocks.

Cosmochemical constraints on the sulfur content in the Earth's core*

Abstract The density of the Earths core (32.5% of the mass of the Earth) is about 8% less than that of pure metallic Fe under similar P, T conditions, requiring the presence of a substantial amount of a light element or light elements in the core ( McQueen and Marsh, 1960 , McQueen and Marsh, 1966 ). Sulfur is a good candidate for reducing the density. On the basis of shock wave experiments, Ahrens (1979) estimated that 9–12% S in the core would be sufficient to account for the observed density difference. This corresponds to a S-content of 2.9–3.9% for the bulk Earth. Here we present an estimate of the bulk Earth S-content which is based on the volatility of S in the solar nebula and the general depletion of volatile elements in meteorites and in the Earth. It is suggested that the CI-normalized S-abundance of the bulk Earth is similar to, or even lower than, the CI-normalized abundance of Zn, an element of similar volatility as S in the reducing environment of the solar nebula. Since the Zn content of the mantle of the Earth is well known and since the core is most likely free of Zn, the bulk Earth S-content can be calculated from the Zn-abundance of the mantle. A maximum bulk Earth S-content of 0.56% and a corresponding maximum S-content of the core of 1.7% are estimated. The S-content of the mantle is so low that the contribution to the bulk Earth S-content is negligible. The upper limit of S in the core as derived from cosmochemical constraints is much too low to produce the required decrease in the cores density. At least one other light element is needed.

Chemical Composition of Rocks and Soils at the Pathfinder Site

As Viking Landers did not measure rock compositions, Pathfinder (PF) data are the first in this respect. This review gives no proof yet whether the PF rocks are igneous or sedimentary, but for petrogenetic reasons they could be igneous. We suggest a model in which Mars is covered by about 50% basaltic and 50% andesitic igneous rocks. The soils are a mixture of the two with addition of Mg-sulfate and -chloride plus iron compounds possibly derived from the hematite deposits.

Chemical systematics of the shergotty meteorite and the composition of its parent body (Mars)

Abstract We report chemical data for 60 elements by INAA and RNAA in two bulk samples, for 30 elements in various mineral separates of Shergotty, and results of leaching experiments with 1M HCl on powdered aliquots of Shergotty and BETA 79001, lithologies A and B. Shergotty is homogeneous in major element composition but heterogeneous with respect to LIL trace elements (~20%). The heterogeneity is even greater for volatile and siderophile trace elements. The mineral data, including three clinopyroxene fractions with variable FeO contents, maskelynite and minor phases (Ti-magnetite, ilmenite, quartz, K-rich phase), show that major minerals do not account for the rare earth elements (REE) in the bulk meteorite. Instead, the REE are to a large extent concentrated in accessory whitlockite and apatite (shown by leaching with 1M HCl): together with the majority of REE (La, 96%, Yb 70%), Cl and Br are quantitatively dissolved by leaching. The REE patterns of the leachate of Shergotty and EETA 79001 are different. The Shergotty leachate may consist of two components. Component l is similar to that of EETA 79001 leachate (whitlockite), component 2 is enriched in light REE and may be responsible for the higher LREE contents of Shergotty in comparison to the other shergottites. There is some evidence that Shergotty was an open system and component 2 was introduced after crystallization. The REE patterns of the residues of Shergotty and EETA 79001 are identical indicating that the parent magmas of both meteorites are compositionally similar. Based on cpx separates with the lowest REE content, the REE pattern in the Shergotty parent magma was calculated. It is enriched in LREE and has a subchondritic Nd Sm ratio. The negative Eu anomaly in the phosphates indicates that at least some plagioclase crystallized before phosphate. Based on several element correlations in SNC meteorites, it was suggested (Dreibus and Wanke, 1984) that both the Shergotty parent body (SPB, very probably Mars), and the Earth accreted from the same two chemically different components: component A, highly reduced and devoid of volatile elements and an oxidized component B containing also volatile elements. The SPB (Mars) mantle is 2–4 times richer in volatile and moderately siderophile elements than the Earth, indicating a higher portion of component B in the SPB. The concentrations of chalcophile elements in the SPB mantle are low, reflecting equilibration with a sulfide phase and subsequent segregation of sulfide into the core. Unlike the Earth (Wanke, 1981), the SPB (Mars) may therefore have accreted almost homogeneously.

Iodine abundances in oceanic basalts: implications for Earth dynamics

Abstract Iodine analyses by neutron activation have been performed on 32 oceanic basalts glasses, 1 phonolite and 3 subaerial arc basalts. The world-wide sample set encompasses all typical geodynamic settings [ridges (MORB), oceanic islands (OIB), arcs (IAB) and back-arc basins (BABB)] and the diversity of oceanic basalt types [depleted (N), intermediate (T) and enriched (P)]. Most basalts, including all N-types, all but one T-type, and some P-types, exhibit low iodine concentrations (2.5–13 ppb). Very high I concentrations (up to 363 ppb) in a small number of samples (all P-types) are interpreted to be the result of a recycled component which includes organic matter of sedimentary origin (sediment organic matter accounts for about 80% of total terrestrial iodine). Iodine appears to be the most incompatible element after the noble gases. Mass balance considerations permit the mantle iodine concentration and hence the bulk silicate abundance of the Earth to be constrained to 9–24 ppb, with a preferred value of 10 ppb. The low terrestrial iodine abundance, coupled with a chondrite-like chlorine/iodine ratio, strongly favours the late veneer model of Earth accretion. The scenario proposed to explain the terrestrial iodine distribution includes heterogeneous accretion, iodine extraction from the mantle simultaneous with (or even before) continent formation, and depleted mantle homogenization. As iodine is not recycled into the mantle by oceanic crust, heterogeneities in the mantle should result from organic sediment (C) recycling, of which iodine may be a good tracer.

Lunar highland meteorites and the composition of the lunar crust

Abstract Major, minor, and trace element data obtained by neutron activation techniques and by spark source mass spectrometry (SSMS) on two lunar meteorites MAC88104 and MAC88105 are reported. Both MAC samples were also analysed for their contents and isotopic compositions of rare gases. Additional SSMS-data were obtained on four lunar highland meteorites previously found in Antarctica: ALHA81005, Y791197, Y82192, and Y86032. MAC88104 and MAC88105 are very similar in chemistry, suggesting that they are pieces of a single fall event. The bulk chemical composition of MAC88104/5 is not very different from the other lunar highland meteorites: highly aluminous with relatively low contents of REE and siderophile element concentrations slightly above 1% of a CI-chondritic level. However, mafic element concentrations (Mg, Cr, Mn, etc.) are slightly lower in MAC88104/5 than in the other lunar highland meteorites. The contents of solar rare gases in the two MAC samples are low, indicating only a small regolith contribution in agreement with rare petrographically identifiable regolith components. The MAC samples and also Y82192 and Y86032 are classified as fragmental breccias with negligible regolith components, in contrast to ALHA81005 and Y791197 which are regolith breccias with high solar wind derived rare gas contents. There is no correlation among lunar meteorites between peak shock pressures and solar gas contents, indicating that peak shock pressures of up to 25 GPa do not lead to gas loss. A low 26Al activity ( Vogt et al., 1990) and high contents of cosmogenic rare gases in MAC88104/5 suggest a long exposure (400,000 years) in the lunar sub-surface. K-Ar ages are in excess of 3.9 by. Lunar highland meteorites and compositionally similar granulitic rocks from the Apollo 16 and 17 landing sites contain about 1% of a CI-chondritic component, according to siderophile and volatile element contents, but independent of the amount of regolith components. Apparently, the major fraction of meteoritic elements in these rocks was not provided by micrometeorites impacting the regolith. The abundances of siderophile (e.g., Ir) and volatile elements may therefore reflect the last spike of accretion of the Moon after the formation of the anorthositic crust. Lunar meteorites of highland origin are chemically different from the bulk of the Apollo 16 highland samples in having higher Fe Mg ratios and lower contents and less fractionated patterns of incompatible and siderophile elements. Since lunar highland meteorites are associated with at least three but probably four different fall events, and since they are not derived from chemically exotic front-side terranes, they may represent a better sampling of the average chemical composition of the lunar crust than previous estimates based on returned lunar samples and remote sensing data. A comparison between an average highland composition derived by Taylor (1982) and an estimate based on lunar highland meteorites shows that the Taylor composition contains higher concentrations and more fractionated incompatible elements mainly because of a substantial amount of KREEP (a trace element rich, highly fractionated component from the front side of the Moon not present at the sites from which the lunar meteorites come).