Engineering topics

Humayun, M.

Publications and source records attributed to Humayun, M..

43 records · Page 3

NASA NTRS2005

A Model for Siderophile Element Distribution in Planetary Differentiation

Planetary differentiation begins with partial melting of small planetesimals. At low degrees of partial melting, a sulfur-rich liquid segregates by physical mechanisms including deformation-assisted porous flow. Experimental studies of the physical mechanisms by which Fe-S melts segregate from the silicate matrix of a molten H chondrite are part of a companion paper. Geochemical studies of these experimental products revealed that metallic liquids were in equilibrium with residual metal in the H chondrite matrix. This contribution explores the geochemical signatures produced by early stages of core formation. Particularly, low-degree partial melt segregation of Fe-S liquids leaves residual metal in the silicate matrix. Some achondrites appear to be residues of partial melting, e.g., ureilites, which are known to contain metal. The metal in these achondrites may show a distinct elemental signature. To quantify the effect of sulfur on siderophile element contents of residual metal we have developed a model based on recent parametrizations of equilibrium solid metal-liquid metal partitioning experiments.

Humayun, M.↗

NASA NTRS2005

Highly Siderophile Elements and Osmium Isotope Systematics in Ureilites: Are the Carbonaceous Veins Primary Components?

Ureilites are an enigmatic group of primitive carbon-bearing achondrites of ultramafic composition. The majority of the ~143 ureilite meteorites consist primarily of olivine and pyroxene (and occasionally chromite) [1]. They are coarse-grained, slowly cooled, and depleted in incompatible lithophile elements. Minor amounts of dark interstitial material consisting of carbon, metal, sulfides, and fine-grained silicates occur primarily along silicate grain boundaries, but also intrude the silicates along fractures and cleavage planes. Variable degrees of impact shock features have also been imparted on ureilites. The prevailing two origins proposed for these rocks are either as melting residues of carbonaceous chondritic material [2], [3], or alternatively, derivation as mineral cumulates from such melts [4], [5], [6]. It has recently been proposed that ureilites are the residues of a smelting event, i.e. residues of a partial melting event under highly reducing conditions, where a solid Fe-bearing phase reacts with a melt and carbon to form Fe metal and carbon monoxide [7]. Rapid, localized extraction and loss of the basaltic component into space resulting from high eruption velocities could preserve unequilibrated oxygen isotopes and produce the observed olivine-pyroxene residues via 25-30% partial melting of chondritic-like precursor material.

Rankenburg, K.↗

NASA NTRS2004

Solid Metal-Liquid Metal Partitioning of Pt, Re, and Os: The Effect of Carbon

If the measured Os isotopic ratios are a signature from the Earth's outer core, understanding them is a unique opportunity to understand more about the Earth's core. The distribution of elements between the Earth's solid inner core and the liquid outer core will depend on their solid metal-liquid metal partition coefficients (D). Solid metal-liquid metal partitioning data are loosely consistent with the needed fractionations between Re-Os and Pt-Os to account for the Os isotopic signature; D(Os) is greater than both D(Re) and D(Pt), and the magnitude of the partition coefficients are similar to those needed [e.g. 7, 8]. The pressure in the core, the composition of the core, and the crystal structure of the solid Fe alloy in the inner core may influence the specific values of the partition coefficients. It may thus be possible to use these sensitivities of the partition coefficients to gain insight into the conditions within the Earth's core. In this abstract, we focus on the compositional influence of C, a potential component of the light element in the Earth's core [9], on the partitioning behaviors of Pt, Re, and Os.

Chabot, N. L.↗

NASA NTRS2003

Laser-Ablation ICP-MS Analyses of Meteoritic Metal Grains in Lunar Impact-Melt Breccias

Lunar impact-melt breccias contain metal grains from the meteorites that formed the breccias. Because the breccias contain clastic material that may derive from older breccias, metal grains from earlier impacts may be present, too. The large subset of moderately mafic (8 - 12% FeO), KREEP-rich ("LKFM") melt breccias is particularly important because: (1) these are the melt breccias most likely to have been produced in basin-forming impacts, (2) it is from these breccias that many of the approx. 3.9 Gyr ages that are so common in lunar samples derive, (3) the breccias contain large proportions of FeNi metal, more than 1% in some types of Apollo 16 breccias, and (4) the metal potentially provides information about the impactors causing the apparent cataclysm at 3.9 Gyr.

Korotev, R. L.↗

NASA NTRS2003

Experimental Determination of Spinel/Melt, Olivine/Melt, and Pyroxene/Melt Partition Coefficients for Re, Ru, Pd, Au, and Pt

Experimental studies have identified spinels as important hosts phases for many of the highly siderophile elements (HSE). Yet experimental studies involving chromite or Cr-rich spinel have been lacking. Experimental studies of partitioning of HSEs between silicate, oxides and silicate melt are plagued by low solubilities and the presence of small metallic nuggets at oxygen fugacities relevant to magmas, which interfere with analysis of the phases of interest. We have circumvented these problems in two ways: 1) performing experiments at oxidized conditions, which are still relevant to natural systems but in which nuggets are not observed, and 2) analysis of run products with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), allowing a combination of high sensitivity and good spatial resolution.

Righter, K.↗

NASA NTRS2001

A Henry's Law Test for Experimental Partitioning Studies of Iron Meteorites

Low-level doped solid metal/liquid metal experiments analyzed by laser ablation ICP-MS allow Henry's Law to be tested. The results indicate Henry's Law is obeyed and the experimental partition coefficients can be applied to iron meteorites. Additional information is contained in the original extended abstract.

Chabot, N. L.↗

NASA NTRS1993

Potassium isotope cosmochemistry, volatile depletion and the origin of the Earth

We report the first results obtained by our techniques for the precise and accurate determination of the isotopic composition of potassium to constrain the mechanism of volatile element depletion in the formation of the Earth, Moon, and meteorites. Our measurements of delta(K-41) for six chondrites and ten terrestrial rocks attained an average precision of the individual measurement of plus or minus 0.4 percent (2 sigma; plus or minus 0.2 percent/a.m.u. and yield a net chondrite-Earth difference unresolved at the 99 percent confidence limit, delta(K-41) = 0.32 plus or minus 0.35 percent (3 delta). This sets a firm upper limit of 1.3 plus or minus 1.4 percent Rayleigh evaporation of terrestrial potassium (using alpha = square root of 41/39), compared with an observed approximately equals 85 percent chemical depletion of K relative to C1 chondrites. Similar conclusions are reached for the SNC meteorites, Shergotty and Zagami, for 15495 (lunar mare gabbro), and for the eucrite Juvinas. Our conclusion is that direct evaporation of volatile elements from planets (e.g. from silicate vapor atmospheres following giant impact) can be ruled out, and the cause of volatile loss must be sought elsewhere, e.g. nebular processes. Our present findings do not support the conclusions of Hinton et al., the discrepancy to be resolved at a later date. We also find lunar soil 64801, delta(K-41) = +4.99 plus or minus 0.53 percent, to be distinctly heavy in accord with Garner et al.

Humayun, M.↗