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Lauretta, D. S.

Publications and source records attributed to Lauretta, D. S..

53 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Chondrules and Isolated Grains in the Fountain Hills Bencubbinite

The Fountain Hills (FH) meteorite was recently classified as a Bencubbin-like (CB(sub a)) chondrite, which are part of the CR clan [1]. The FH O-isotopic composition is indistinguishable from CB(sub a) chondrites. Metal and silicate compositions are consistent with the CB(sub a) classification. Significant differences between FH and the other CB(sub a) chondrites were noted. These include abundant porphyritic chondrules and complete lack of sulfide minerals. We are furthering this investigation by analyzing silicate chondrules and isolated grains in FH to determine more about its composition, thermal history, and implications for chondrule formation in the early solar system.

LaBlue, A. R.↗
NASA NTRS2003

Olivine-Orthopyroxene Equilibrium in Metal-rich Systems: Applications to Achondrites and Equilibrated Chondrites

Olivine and orthopyroxene are major minerals in every type of stony meteorite. The majority of achondritic meteorites and silicate-bearing iron meteorites have experienced high temperatures. If these temperatures persisted for an extended period of time then the iron contents of olivine and orthopyroxene should be in equilibrium. In their study of ungrouped clasts and chondritic meteorites, suggested that the equilibrium compositions of olivine and orthopyroxene should fall on a mixing line between LL chondrites and aubrites. Here we show that this is not necessarily the case and that a range of FeO contents in olivine and orthopyroxene can be in equilibrium with each other. The key parameters that determine the equilibrium Fe content in these minerals are temperature, oxygen fugacity (fO2), and silica activity (aSiO2).

Lauretta, D. S.↗
NASA NTRS2002

Fulgurites: A Look at Transient High Temperature Processes in Silicates

Fulgurites result from transient high temperature processes, and some have extremely reduced phases. We performed both modeling and a microprobe analysis of natural fulgurites. The modeling suggests vapor phase C causes reduction of silicate liquid. Additional information is contained in the original extended abstract.

Wasserman, A. A.↗
NASA NTRS2001

Mercury Abundances and Isotopic Compositions in the Murchison (CM) and Allende (CV)Carbonaceous Chondrites

The abundance and isotopic composition of Hg was determined in bulk samples of both the Murchison (CM) and Allende (CV) carbonaceous chondrites using single- and multi-collector inductively coupled plasma mass spectrometry (ICP-MS). The bulk abundances of Hg are 294 6 15 ng/g in Murchison and 30.0 6 1.5 ng/g in Allende. These values are within the range of previous measurements of bulk Hg abundances by neutron activation analysis (NAA). Prior studies suggested that both meteorites contain isotopically anomalous Hg, with d l 96/202Hg values for the anomalous, thermal-release components from bulk samples ranging from 2260 %o to 1440 9/00 in Murchison and from 2620 9/00 to 1540 9/00 in Allende (Jovanovic and Reed, 1976a; 1976b; Kumar and Goel, 1992). Our multi-collector ICP-MS measurements suggest that the relative abundances of all seven stable Hg isotopes in both meteorites are identical to terrestrial values within 0.2 to 0.5 9/00m. On-line thermal-release experiments were performed by coupling a programmable oven with the singlecollector ICP-MS. Powdered aliquots of each meteorite were linearly heated from room temperature to 900 C over twenty-five minutes under an Ar atmosphere to measure the isotopic composition of Hg released fiom the meteorites as a h c t i o n of temperature. In separate experiments, the release profiles of S and Se were determined simultaneously with Hg to constrain the Hg distribution within the meteorites and to evaluate the possibility of Se interferences in previous NAA studies. The Hg-release patterns differ between Allende and Murchison. The Hg-release profile for Allende contains two distinct peaks, at 225" and 343"C, whereas the profile for Murchison has only one peak, at 344 C. No isotopically anomalous Hg was detected in the thermal-release experiments at a precision level of 5 to 30 9/00, depending on the isotope ratio. In both meteorites the Hg peak at ;340"C correlates with a peak in the S-release profile. This correlation suggests that Hg is associated with S-bearing phases and, thus, that HgS is a major Hg-bearing phase in both meteorites. The Hg peak at 225 C for Allende is similar to release patterns of physically adsorbed Hg on silicate and metal grains.

Lauretta, D. S.↗
NASA NTRS2001

Opaque Minerals in the Matrix of the Bishunpur (LL3.1) Chondrite: Constraints on the Chondrule Formation Environment

The chemistry and mineralogy of a group of opaque mineral assemblages in the matrix of the Bishunpur LL3.1 ordinary chondrite provide insight into the nebular environment in which they formed. The assemblages consist of a kamacite (Fe,Ni) core that is rimmed by troilite (FeS) and fayalite (Fe2Si04). Accessory phases in the rims include silica (Si02), chromite (FeCr204), whitlockite (Ca3(P04)2), maricite (FeNaP04), magnetite (Fe304), and tetrataenite (FeNi). We suggest that the metal melted in and equilibrated with an igneous chondrule under high- temperature, reducing conditions. In this environment the molten alloys incorporated varied amounts of Si, Ni, P, Cr, and Co, depending on the oxygen fugacity and temperature of the melt. Some of the metal was subsequently expelled from the chondrule interiors into the surrounding nebular gas. As the temperature dropped, the alloy solidified and volatile elements corroded the metal. The main reaction products were troilite and fayalite. Thermodynamic equilibrium calculations are used to constrain the conditions under which these two phases can form simultaneously in the solar nebula. Kinetic factors are used to place a lower limit on the formation temperature. We determine that the metal corroded between 1173 and 1261 K at a total pressure in the range of 1025.0 to 1024.1 bars and a dust/gas ratio of 302 to 355 x relative to solar composition. These conditions are consistent with our model that the metal corroded in a dust- rich region of the solar nebula that was cooling after a chondrule formation event.

Lauretta, D. S.↗
NASA NTRS1994

Troilite formation kinetics and growth mechanism in the solar nebula

Troilite formation via the reaction Fe(s) + H2S(g) + H2(g) is the major mechanism for S retention in grains in the solar nebula. Thermodynamic calculations predict that troilite condenses from a solar composition gas. We present experimental results on the kinetics and growth of troilite crystals on Fe metal at temperature (450-650 C) and composition (50-1000 ppm H2S in H2) conditions similar to those in the solar nebula. The fraction of Fe reacted (based on gravimetric data) is plotted at 450, 505, 575, and 650 C. The thickness change of unreacted iron (measured by optical microscopy) is plotted at 575 and 650 C vs. time. the weight change per unit area varies as the square root of time at the lower temperatures and varies linearly with time at the highest temperature. The growth behavior along the lower isotherms is due to diffusion. This behavior suggests sulfide diffusion to the metal-sulfide interface and suggests Fe(2+) diffusion to the sulfide-gas interface. The reaction along the highest isotherm appears to be interface controlled. The formation of troilite crystals is a rapid process forming measurable layers in a few hours. The crystalgrowth is complicated. Initially there are intergrowths of troilite into the pure Fe metal. As the reaction progresses two distinct layers of troilite crystals form. One is in contact with the Fe metal and consists of small randomly oriented crystals with pore space between them. The outermost layer contains large crystals that are all oriented in the same direction. The intergrowth layer is much smaller at 650 C than at 575 C. This suggest that FeS nucleation is inhibited at the higher temperature, accounting for the initially slower reaction rate. Once nucleated, the reaction kinetics are apparently controlled by the growth of the crystals at the interface.

Lauretta, D. S.↗