Highly Siderophile Elements in Pallasites and Diogenites, Including the New Pallasite, CMS 04071

Pallasites are long thought to represent a metallic core-silicate mantle boundary, where the IIIAB irons are linked to the crystallization history of the metallic fraction, and the HED meteorites may be linked to the silicate fraction. However, measurement of trace elements in individual metallic and silicate phases is necessary in order to fully under-stand the petrogenetic history of pallasites, as well as any magmatic processes which may link pallasites to both IIIAB irons and HED meteorites. In order to achieve this objective, abundances of a suite of elements were measured, including the highly siderophile elements (HSEs), in kamacite, taenite, troilite, schreibersite, chromite and olivine for the pallasites Admire, Imilac, Springwater, CMS 04071. In the diogenites GRO 95555, LAP 91900, and MET 00436, metal, sulfide, spinel, pyroxene, and silica were individually measured

Experimental Insights into the Origin of Defect-Structured Hibonites Found in Meteorites

Hibonite (CaAl12O19) is a primary, highly refractory phase occurring in many Ca-Al-rich inclusions (CAIs). Previous microstructural studies of hibonite in CAIs and their Wark-Lovering (WL) rims showed the presence of numerous stacking defects in hibonites. These defects are interpreted as the modification of the stacking sequences of spinel and Ca-containing blocks within the ideal hexagonal hibonite structure due to the presence of wider spinel blocks [3], as shown by experimental studies of reaction-sintered compounds in the CaO-Al2O3 system. We performed a series of experiments in the CaO-Al2O3-MgO system in order to provide additional in-sights into the formation processes and conditions of defect-structured hibonites found in meteorites

Experimental Crystallization of Yamato 980459

Currently, only two martian meteorites QUE 94201 (QUE) and Yamato 980459 (Y98) have been experimentally shown to me true melt compositions. Most martian meteorites are instead, cumulates or partial cumulates. We have performed experiments on a Y98 composition to assess whether QUE could be related to Y98 by some fractionation process [1]. Y98 is a basaltic shergottite from the SNC (Shergotty, Nakhla, Chassigny) meteorite group. Y98 is composed of 26% olivine, 48% pyroxene, 25% mesostasis, and no plagioclase [2]. The large size of the olivine megacrysts and absence of plagioclase suggest that the parental melt which formed this meteorite had begun cooling slowly until some mechanism, such as magma ascent, caused rapid cooling [3]. Y98 s olivines have the highest Mg content of all the shergottites suggesting that it is the most primitive [4]. Y98 has been determined to be a melt composition by comparing the composition of experimental liquidus olivines with the composition of the cores of Y98 olivines [4]. The liquidus of Y98 is predicted by MELTS [5] and by experimentation [6] to be ~1450 C. Analyses of Y98 show it to be very depleted in LREEs and it has similar depleted patterns as other shergottites such as QUE [7]

Evolution of the Oxidation State of the Earth's Mantle: Challenges of High Pressure Quenching

The oxidation state of the Earth's mantle during formation remains an unresolved question, whether it was constant throughout planetary accretion, transitioned from reduced to oxidized, or from oxidized to reduced. We investigate the stability of Fe3+ at depth, in order to constrain processes (water, late accretion, dissociation of FeO) which may reduce or oxidize the Earth's mantle. Experiments of more mafic compositions and at higher pressures commonly form a polyphase quench intergrowth composed primarily of pyroxenes, with interstitial glass which hosts nearly all of the more volatile minor elements. In our previous experiments on shergottite compositions, variable fO2, T, and P is less than 4 GPa, Fe3+/TotFe decreased slightly with increasing P, similar to terrestrial basalt. For oxidizing experiments less than 7GPa, Fe3+/TotFe decreased as well, but it's unclear from previous modelling whether the deeper mantle could retain significant Fe3+. Our current experiments expand our pressure range deeper into the Earth's mantle and focus on compositions and conditions relevant to the early Earth. Experiments with Knippa basalt as the starting composition were conducted at 1-8 GPa and 1800 C, using a molybdenum capsule to set the fO2 near IW, by buffering with Mo-MoO3. TEM and EELS analyses revealed the run products from 7-8 GPa quenched to polycrystalline phases, with the major phase pyroxene containing approximately equal Fe3+/2+. A number of different approaches have been employed to produce glassy samples that can be measured by EELS and XANES. A more intermediate andesite was used in one experiment, and decompression during quenching was attempted after, but both resulted in a finer grained polyphase texture. Experiments are currently underway to test different capsule materials may affect quench texture. A preliminary experiment using liquid nitrogen to greatly enhance the rate of cooling of the assembly has also been attempted and this technique will be refined in further experiments

Sunflower Seeds in Rations for Beef Cattle

In 1981, North Dakota was the US leader in sunflower seed production. Prior research on sunflower seeds did not indicate the nutritional value of hybrid flowers with oil content. The article covers three trials. Steers that received sunflowers gained weight more quickly. Their coats were richer and fuller. However, there is a cap of 1 pond of oil seeds per day without inferring with their digestive systems. Those heifers fed three pounds of sunflower seed never consumed as much feed. It was concluded that sunflowers should be fed as an energy supplement rather than a protein supplement. While high in protein, high oil sunflower seeds were a poor protein supplement for cattle

The Combined Strength of Thermodynamics and Comparative Planetology: Application of Activity Models to Core Formation in Terrestrial Bodies

Recent models for accretion of terrestrial bodies involve metal-silicate equilibrium as the metallic core formed during growth. Most elements considered are either refractory or well studied elements for which effects of pressure, temperature, oxygen fugacity, and metallic liquid composition are well known. There are a large number of elements that are both siderophile and volatile, whose fate in such models is unknown, largely due to a lack of data at comparable conditions and com-positions (FeNi core with light elements such as S, C, Si, and O). We have focused on Ge, In, As, Sb and determined the effect of Si and C on metal-silicate partitioning, and developed a thermo-dynamic model that allows application of these new data to a wide range of planetary bodies. New experiments: We have previously carried out experiments with FeSi metallic liquid at C-saturated conditions at 1600 and 1800 C [4]. In a new series of experiments we investigate the effect of Si in carbon-free systems at 1600 C for comparison. Experiments were carried out at 1 GPa in MgO capsules using the same basaltic starting composition as in previous studies. The MgO capsule reacts with the silicate melt to form more MgO-rich liquids that have 22-26 wt% MgO. Experimental met-als and silicates were analyzed using a combination of electron microprobe analysis and laser ablation ICP-MS. Results: The new results can be interpreted by considering Ge as an example, in the simple exchange equilibrium Fe + GeO = FeO + Ge, where the equilibrium constant Kd can be examined as a function of Si content of the metal. The slope of lnKd vs. (1-XSi) for this new series allows derivation of the epsilon interaction parameter for each of these four elements and Si (both C-saturated and C-free).All four elements have positive epsilon values, indicating that Si causes a decrease in the partition coefficients; values are 6.6, 6.5, 27.8 and 25.2 for In, Ge, As, and Sb, respectively, at 1 GPa and 1600 C. As an example of how large the effect of Si can be, these epsilon values correspond to activity coefficients (gamma) for As of 0.01 when XSi = 0, and up to gamma = 23 when XSi = 0.2. Combining these new results with previous determinations [5,6] of epsilon parameters for S and C for these elements allows us calculate activity of Ge, In, As, and Sb in Fe-Ni-Si-S-C-O metallic liquids. We apply this new model to sever-al terrestrial bodies such as Earth (Si-rich core), Mars (S-rich core), Moon (S-, C-, and Si-poor core), and Vesta, and examine the resulting core and mantle concentrations of these elements. Mantle concentrations of these four elements are well explained for Earth and Mars in models that call for mid-mantle equilibration between Si-bearing and S-bearing FeNi cores, respectively. Modeling results for the Moon and Vesta will also be presented

Liquidus Phases of the Richardson H5 Chondrite at High Pressures and Temperatures

Part of early mantle evolution may include a magma ocean, where core formation began before the proto-Earth reached half of its present radius. Temperatures were high and bombardment and accretion were still occurring, suggesting that the proto-Earth consisted of a core and an at least partially liquid mantle, the magma ocean. As the Earth accreted, pressure near the core increased and the magma ocean decreased in volume and became shallower as it began to cool and solidify. As crystals settled, or floated, the composition of the magma ocean could change significantly and begin to crystallize different minerals from the residual liquid. Therefore, the mantle may be stratified following the P-T phase diagram for the bulk silicate Earth. To understand mantle evolution, it is necessary to know liquidus phase relations at high pressures and temperatures. In order to model the evolution of the magma ocean, high pressure and temperature experiments have been conducted to simulate the crystallization process using a range of materials that most likely resemble the bulk composition of the early Earth

Majorite-Garnet Partitioning of the Highly Siderophile Elements: New Results and Application to Mars

HSE and Os isotopes are used to constrain processes such as accretion, mantle evolution, crustal recycling, and core-mantle mixing, and to constrain the timing and depth of differentiation of Mars. Although showed that the HSE contents of the martian mantle could have been established by metal-silicate equilibrium in early Mars, the role of a cooling magma ocean and associated crystallization in further fractionating the HSEs is unclear. Garnet is thought to have played an important role in controlling trace element concentrations in the martian mantle reservoirs. However, testing these models, including Os isotopes, has been hindered by a dearth of partitioning data for the HSE in deep mantle phases - majorite, wadsleyite, ringwoodite, akimotoite - that may be present in the martian mantle. We examine the partitioning behavior of HSEs between majorite garnet (gt), olivine (oliv), and silicate liquid (melt)

Evolution of the Oxidation State of the Earth's Mantle

The oxidation state of the Earth's mantle during formation remains an unresolved question, whether it was constant throughout planetary accretion, transitioned from reduced to oxidized, or from oxidized to reduced. We investigate the stability of Fe3(+) at depth, in order to constrain processes (water, late accretion, dissociation of FeO) which may reduce or oxidize the Earth's mantle. In our previous experiments on shergottite compositions, variable fO2, T, and P less than 4 GPa, Fe3(+)/sigma Fe decreased slightly with increasing P, similar to terrestrial basalt. For oxidizing experiments less than 7GPa, Fe3(+)/sigma Fe decreased as well, but it's unclear from previous modelling whether the deeper mantle could retain significant Fe3(+). Our current experiments expand our pressure range deeper into the Earth's mantle and focus on compositions and conditions relevant to the early Earth. Preliminary multi-anvil experiments with Knippa basalt as the starting composition were conducted at 5-7 GPa and 1800 C, using a molybdenum capsule to set the fO2 near IW, by buffering with Mo-MoO3. TEM and EELS analyses revealed the run products quenched to polycrystalline phases, with the major phase pyroxene containing approximately equal to Fe3(+)/2(+). Experiments are underway to produce glassy samples that can be measured by EELS and XANES, and are conducted at higher pressures

Stacking Defects in Synthetic and Meteoritic Hibonites: Implications for High-Temperature Processes in the Solar Nebula

Hibonite (CaAl12O19) is a primary, highly refractory phase occurring in many Ca-Al-rich inclusions (CAIs) from different chondrite groups, except CI chondrites. Hibonite is predicted to be one of the earliest minerals to condense during cooling of the solar nebula at higher temperatures than any other major CAI mineral. Therefore, hibonite has great potential to reveal the processes and conditions of the very early, high-temperature stages of the solar nebular evolution. Previous microstructural studies of hibonite in CAIs and their Wark-Lovering (WL) rims showed the presence of numerous stacking defects in hibonite. These defects are interpreted as the modification of the stacking sequences of spinel and Ca-containing blocks within the ideal hexagonal hibonite structure, as shown by experimental studies of reaction-sintered ceramic CaO-Al2O3 compounds. We performed preliminary experiments in the CaO-Al2O3-MgO system to understand the formation processes and conditions of defect-structured hibonite found in meteorites