Munir Humayun

Potassium isotope cosmochemistry: Genetic implications of volatile element depletion

Abstract We report high precision (±0.5%o) potassium isotopic determinations on bulk chondrites, achondrites, and lunar samples, on a separated chondrule, and two CAls. We find that potassium shows a remarkable isotopic homogeneity in various solar system bodies, even though there are chemical depletions of a factor of about 30, between C1 chondrites and eucrites and lunar samples. Theories that propose the evaporation of volatile elements from initially condensed (C1 chondrite) material to account for such chemical depletions, necessarily imply the existence of large isotopic mass fractionations, e.g., about +40%o for Earth, +90%o for eucrites and lunar rocks. Volatile loss of potassium (and by implication Na, Rb, Cs, and other elements of similar volatility) during chondrule formation is also ruled out. The high precision of the data place stringent limits of ≤52% on the quantity of potassium that could have been lost by partial volatilization. This is not detectable by standard chemical techniques, which can resolve 5–20% changes in the K/La and K/U ratios. The two-component models proposed by Larimer and Anders (1967) and by Wanke et al. (1984) invoke vaporization of alkalis which is not supported by the potassium isotope results. The chemical depletion of alkalis and other volatiles must have preceded the processes of chondrule, chondrite, and planetary formation, and occurred during the condensation of precursor dust, probably from a hot stage in the solar nebula.

Staphylococcus aureus IsdB Is a Hemoglobin Receptor Required for Heme Iron Utilization

The pathogenesis of human infections caused by the gram-positive microbe Staphylococcus aureus has been previously shown to be reliant on the acquisition of iron from host hemoproteins. The iron-regulated surface determinant system (Isd) encodes a heme transport apparatus containing three cell wall-anchored proteins (IsdA, IsdB, and IsdH) that are exposed on the staphylococcal surface and hence have the potential to interact with human hemoproteins. Here we report that S. aureus can utilize the host hemoproteins hemoglobin and myoglobin, but not hemopexin, as iron sources for bacterial growth. We demonstrate that staphylococci capture hemoglobin on the bacterial surface via IsdB and that inactivation of isdB, but not isdA or isdH, significantly decreases hemoglobin binding to the staphylococcal cell wall and impairs the ability of S. aureus to utilize hemoglobin as an iron source. Stable-isotope-tracking experiments revealed removal of heme iron from hemoglobin and transport of this compound into staphylococci. Importantly, mutants lacking isdB, but not isdH, display a reduction in virulence in a murine model of abscess formation. Thus, IsdB-mediated scavenging of iron from hemoglobin represents an important virulence strategy for S. aureus replication in host tissues and for the establishment of persistent staphylococcal infections.

Platinum group element fractionation in a komatiitic basalt lava lake

Precise PGE (Os, Ir, Ru, Pt, Pd) abundance data for bulk rock samples, and mineral separates, from a deep differentiated komatiitic basalt lava lake in the Vetreny Belt (Baltic Shield) were obtained using an ICP-MS ID technique. The composition of the erupted liquid (MgO 5 15%) is characterized by a highly fractionated PGE pattern, (Pd/Os)N 5 48, with subchondritic (Os/Ir)N 5 0.43. Analysis of separated olivine and chromite revealed that Ru, Os, and Ir were slightly compatible to moderately incompatible in olivine ( D 5 1.7-0.8), and were compatible with chromite (D 5 100-150). Platinum and Pd were highly incompatible with olivine (D 5 0.08-0.03), and moderately compatible with chromite (D 5 1.6-3.3). Bulk solid-liquid partition coefficients for PGEs, Ni, Cr, and Cu, estimated from the whole rock regressions, were: Os 7.9, Ir 6.6, Ru 4.5, Pt 0.53, Pd 0.09, Ni 6.2, Cr 4.4, and Cu 0.01. The highly incompatible behavior of Cu and Pd indicates that there was no separation of FeNiCu immiscible sulfide liquid during the differentiation of the lava lake. Comparison of the bulk partition coefficients with those calculated from the olivine and chromite compositions implies that the fractionating mineral assemblage included ;10 2561 mass fraction of pyrrhotite. The data obtained provide new constraints on the effects of fractional crystallization on the PGE compositions of mafic- ultramafic magmas. Copyright

Platinum group elements in Kostomuksha komatiites and basalts: Implications for oceanic crust recycling and core-mantle interaction

We report precise PGE (Os, Ir, Ru, Pt, and Pd) concentrations for komatiite-basalt lava sequences of the Kostomuksha greenstone belt, and for the komatiite standards KAL-1 (Abitibi) and WITS-1 (Barber- ton). The flowtop breccia komatiites (MgO 5 27.5%) from the Kostomuksha sequence, which represent the composition of the primary komatiite liquid, have Os 5 1.5 6 0.1 ppb, chondritic (Os/Ir)N 5 1.06 6 0.05, and moderately fractionated PGE patterns, (Pd/Os)N 5 6.5 6 0.4. Based on the KAL-1 standard, the Alexo primary liquid had Os 5 1.8 6 0.1 ppb, (Os/Ir)N 5 1.05 6 0.05, and (Pd/Os)N 5 8.5 6 1.0. PGEs in the Kostomuksha komatiites exhibit incompatible behavior during magmatic differentiation, with bulk D values of Os, Ir 5 0.75, Pt 5 0.52 and Pd 5 0. PGE abundances from Kostomuksha komatiites and basalts were used to calculate the Pt/Os and Re/Os composition of a magnesian Archean oceanic crust. The Pt-Re-Os isotopic evolution of this crust was modeled, indicating that coupled 186,187 Os enrichments observed in some modern plumes require a non-crustal source. A previously proposed model for core-mantle interaction by physical admixture was evaluated and was shown to produce mantle sources with high PGE abundances (Os 5 6 to 14 ppb, Pd 5 140 ppb for 1% outer core addition) and (Os/Ir)N

Origin and age of the earliest Martian crust from meteorite NWA 7533

1, depending on the assumptions made. These features were not observed in the Kostomuksha komatiites (g 187 Os(T) 51 3.6), shown here to be derived from a reservoir with ;25% lower absolute PGE abundances than primitive mantle. If the radiogenic initial 187 Os/ 188 Os in the Kosto- muksha komatiite is an outer core signature, then core-mantle interaction cannot have involved physical admixture of outer core material into mantle plume sources. Instead, it must have occurred in the form of isotopic equilibration between liquid outer core and solid mantle at the core-mantle boundary. Copyright

186 Os^ 187 Os systematics of Gorgona Island komatiites: implications for early growth of the inner core

The ancient cratered terrain of the southern highlands of Mars is thought to hold clues to the planet’s early differentiation, but until now no meteoritic regolith breccias have been recovered from Mars. Here we show that the meteorite Northwest Africa (NWA) 7533 (paired with meteorite NWA 7034) is a polymict breccia consisting of a fine-grained interclast matrix containing clasts of igneous-textured rocks and fine-grained clast-laden impact melt rocks. High abundances of meteoritic siderophiles (for example nickel and iridium) found throughout the rock reach a level in the fine-grained portions equivalent to 5 per cent CI chondritic input, which is comparable to the highest levels found in lunar breccias. Furthermore, analyses of three leucocratic monzonite clasts show a correlation between nickel, iridium and magnesium consistent with differentiation from impact melts. Compositionally, all the fine-grained material is alkalic basalt, chemically identical (except for sulphur, chlorine and zinc) to soils from Gusev crater. Thus, we propose that NWA 7533 is a Martian regolith breccia. It contains zircons for which we measured an age of 4,428 ± 25 million years, which were later disturbed 1,712 ± 85 million years ago. This evidence for early crustal differentiation implies that the Martian crust, and its volatile inventory, formed in about the first 100 million years of Martian history, coeval with earliest crust formation on the Moon and the Earth. In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically.

Siderophile element constraints on the formation of metal in the metal-rich chondrites Bencubbin, Weatherford, and Gujba

The presence of coupled enrichments in 186 Os/ 188 Os and 187 Os/ 188 Os in some mantle-derived materials reflects longterm elevation of Pt/Os and Re/Os relative to the primitive upper mantle. New Os data for the 89 Ma Gorgona Island, Colombia komatiites indicate that these lavas are also variably enriched in 186 Os and 187 Os, with 186 Os/ 188 Os ranging between 0.1198397 7 22 and 0.1198470 7 38, and with QOs correspondingly ranging from +0.15 to +4.4. These data define a linear trend that converges with the previously reported linear trend generated from data for modern Hawaiian picritic lavas and a sample from the ca. 251 Ma Siberian plume, to a common component with a 186 Os/ 188 Os of approximately 0.119870 and QOs of +17.5. The convergence of these data to this Os isotopic composition may imply a single ubiquitous source in the Earth’s interior that mixes with a variety of different mantle compositions distinguished by variations in QOs. The 187 Os- and 186 Os-enriched component may have been generated via early crystallization of the solid inner core and consequent increases in Pt/Os and Re/Os in the liquid outer core, with time leading to suprachondritic 186 Os/ 188 Os and QOs in the outer core. The presence of Os from the outer core in certain portions of the mantle would require a mechanism that could transfer Os from the outer core to the lower mantle, and thence to the surface. If this is the process that generated the isotopic enrichments in the mantle sources of these plume-derived systems, then the current understanding of solid metal^liquid metal partitioning of Pt, Re and Os requires that crystallization of the inner core began prior to 3.5 Ga. Thus, the Os isotopic data reported here provide a new source of data to better constrain the timing of inner core formation, complementing magnetic field paleointensity measurements as data sources that constrain models based on secular cooling of the Earth. Published by Elsevier Science B.V.

Trace Element Microanalysis in Iron Meteorites by Laser Ablation ICPMS

Laser ablation inductively coupled plasma mass spectrometry was used to measure abundances of P, Cr, Fe, Co, Ni, Cu, Ga, Ge, As, Mo, Ru, Rh, Pd, Sn, Sb, W, Re, Os, Ir, Pt, and Au in metal grains in the Bencubbin-like chondrites Bencubbin, Weatherford, and Gujba to determine the origin of large metal aggregates in bencubbinites. A strong volatility-controlled signature is observed among the metal grains. The refractory siderophiles Ru, Rh, Re, Os, Ir, and Pt are unfractionated from one another, and are present in approximately chondritic relative abundances. The less refractory elements Fe, Co, Ni, Pd, and Au are fractionated from the refractory siderophiles, with a chondritic Ni/Co ratio and a higher than chondritic Pd/Fe ratio. The moderately volatile siderophile elements Ga, Ge, As, Sn, and Sb are depleted in the metal, relative to chondritic abundances, by up to 3 orders of magnitude. The trace siderophile element data are inconsistent with the following proposed origins of Bencubbin-Weatherford-Gujba metal: (1) condensation from the canonical solar nebula, (2) oxidation of an initially chondritic metal composition, and (3) equilibration with a S-rich partial melt. A condensation model for metal-enriched (10 7 CI) gas is developed. Formation by condensation or evaporation in such a high-density, metal-enriched gas is consistent with the trace element measurements. The proposed model for generating such a gas is protoplanetary impact involving a metal-rich body. Copyright

Origin of zoned metal grains in the QUE94411 chondrite

A laser ablation microanalysis system has been developed that can analyze trace elements with a sensitivity in the ppb range, using a CETAC LSX-200 laser ablation system with a Finnigan Element. This capability has been applied to a set of iron meteorites to demonstrate the laser microprobes analytical capability for the determination of platinum group elements (PGEs) with a spatial resolution of ∼20 μm, comparable to that of dynamic secondary ion mass spectrometry (SIMS). The laser is shown to provide an accurate means of solid sampling for magnetic sector inductively coupled plasma mass spectrometry (ICPMS), allowing the determination of bulk metal composition, chemical zoning within the sample, and depth profiling. Recovery of the chemical zoning in taenite lamellae was achieved for Ru, Rh, and Pd, which was not previously possible using SIMS. The methods presented here show that magnetic sector ICPMS can be successfully coupled to a laser ablation system, providing the advantages of higher sensitivity of the sector instrument, low background count rates (<0.1 counts/s), and flat-topped spectral peaks, while minimizing tradeoff against the speed of data acquisition required to handle the transient signals from the laser ablation system.

Elemental fractionation during LA-ICP-MS analysis of silicate glasses: implications for matrix-independent standardization

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure distributions of the siderophile elements P, V. Cr, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, and Pt in metal grains in the metal-rich chondrite QUE94411 with a spatial resolution of similar to 30 mum. The platinum group elements (PGEs), except Pd, exhibit radial zoning in these grains that mimics that previously observed in Ni and Co; the concentrations of these elements decreases from the cores to the rims of the grains. The PGE distributions support a condensation origin for the enhanced refractory element abundances in the zoned grains; the lack of zoning in Pd refutes an origin by a redox-controlled process, and none of the PGE-Ni relationships support an origin by fractional crystallization from a metallic melt. Several models of grain formation were explored, including equilibrium fractional condensation, which failed to yield the correct radial zoning. The zoning may be the product of a nonequilibrium fractional condensation process, in which the refractory siderophiles remained supersaturated in the cooling solar nebula, or of diffusion between refractory-enhanced Fe-Ni cores and other Fe-Ni metal that may have been deposited later from the solar nebula. Copyright (C) 2001 Elsevier Science Ltd.