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Highly Siderophile Elements in H Chondrites GEOL394

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Highly Siderophile Elements in H Chondrites GEOL394 Highly Siderophile Elements in H Chondrites GEOL394 Jonathan Tino Dr. Richard Walker Dr. Katherine Bermingham Greg Archer 1 Abstract Chondritic meteorites are undifferentiated solar system bodies that have not melted since the beginning of solar system formation. This study examines a subset of these meteorites that are known as the H chondrites due to their high iron metal content. Isotope dilution of highly siderophile elements (HSEs) Re, Os, Ir, Ru, Pt, and Pd has been applied to H chondrite samples of three metamorphic grades to determine the extent of equilibration of these trace elements between metal and silicate. Equilibrium is assessed by comparing the measured ratios of the concentration of the HSEs to established equilibrium concentration ratios, or D values, of metals relative to silicates (O’Neill et. al. 1995). The study also assesses whether fine metal grains (<150 µm) contain higher concentrations than coarser grained metals (>150 µm). Thermal Ionization Mass Spectrometry (TIMS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analyses have been used to quantify isotopic ratios and concentrations of HSEs in fine grained metals, coarse grained metals, and silicate material within the Avanhandava (H4), Richardton (H5), and ALHA 78115 (H6) meteorites. Metamorphic grades 4-6 (in increasing degree of metamorphism, with 4 being the least and 6 being the most) have been chosen in order to assess the possibility of a positive correlation between increasing metamorphic grade and increasing concentration ratio. Similar methods have not previously been applied to H5 or H6 chondrites. After obtaining HSE data for fine grained metal, coarse grained metal, and silicates, I conclude that the HSE in metals and silicates are not in chemical equilibrium for any petrologic type. This is based on the observation that the concentration ratios of Richardton H5 fine metal concentration relative to Richardton H5 silicate concentration reaches a factor of 3833/1. This is the highest degree of separation for any element between any two samples, and it falls well short of the previously established concentration ratio indicative of HSE in chemical equilibrium between these two reservoirs. There is a possibility that open system behavior or metal contamination in the silicate fractions gave rise to this result. It is also possible that the complete chemical equilibrium of these highly refractory HSE does not occur until the sample reaches much higher peak metamorphic temperatures, at which point partial melting would begin and the rock would become an H7 chondrite. 2 Introduction Background: Chondritic Meteorites Chondritic meteorites are among the oldest rocks in our solar system. Their components can include a stony matrix, chondrules, metal grains of various size, and refractory inclusions (McSween, 1987). Chondrules, the namesake of chondritic meteorites, are millimeter-scale spherical grains that cooled from the rock’s initially molten state prior to becoming incorporated in the chondrite’s matrix that are composed of silicate minerals as well as metal grains. Refractory inclusions are calcium-aluminum inclusions (CAIs) and amoeboid olivine aggregates (AOAs). CAIs are thought to be the first substances to condense from the primordial dust and gas cloud known as the solar nebula. These inclusions can constitute anywhere from 0.01-10% of the volume of a chondrite (Scott and Krott, 2003). CAIs are made up of spinel, melilite (Ca,Na)2(Al,Mg,Fe2+) [(Al,Si)SiO7], hibonite ((Ca,Ce)(Al,Ti,Mg)12O19) , Al-Ti diopside, and perovskite, all of which are absent in other chondrite components. Amoeboid olivine aggregates contain some Fe-Ni metal, granular massive olivine [(Fe,Mg)2SiO4], spinel [(Mg,Fe) (Al,Cr)2O4], anorthite (CaAl2Si2O8), Al-diopside, and rarely melilite. Metal grains, primarily comprised of Fe-Ni, can be up to 70% of a chondrite by volume (Scott and Krott, 2003). Matrix material, primarily silicates and fine grained Fe-Ni metal, is volatile rich, can contain sulfides, oxides, organic material, and even rare presolar grains, which are created by stars that existed before ours formed (Scott and Krott, 2003). The chondrites commonly comprise up to 80% matrix. The origin of each component is still disputed, but the background knowledge of the solar system suggests that they formed in different areas of the solar nebula (McSween, 1987). Figure 1: Comparison of elemental abundances in C1 carbonaceous chondrites and the solar photosphere demonstrates an approximate 1:1 correlation. Solar atmosphere elemental abundances are derived using spectroscopy. Red arrow indicates a 1:1 correlation line. 3 Additionally, each component is likely to have condensed at a different time in the formation of the solar system. Solar system cooling and varying volatilities of the chemical elements provide good evidence for the oldest materials being highly refractory. The cloud was originally hotter, and so the first solids to form must have comprised the least volatile elements. Calcium and aluminum form some of the most refractory compounds, and thus they condense at much higher temperatures relative to most elements. The refractory nature of the calcium and aluminum compounds is the reason that CAIs are presumed to be the oldest components both in chondrites and possibly the solar system. The approximate chemical abundance of elements in the solar system can be determined by analyzing C1 carbonaceous chondrites. These are a class of meteorites that contain high amounts of water (3-22% by volume) and organic material, as well as silicates, oxides, and sulfides (Norton, 2002). Data from Ahrens et. al. (1989) suggest that there is an approximate 1:1 correlation between relative abundances, normalized to 106 Si atoms, of the solar atmosphere and C1 carbonaceous chondrites (Fig. 1). The largest variations from the solar abundance are caused by volatility of the elements in question. Abundances of highly refractory elements are closer to a 1:1 ratio between carbonaceous chondrites and the solar atmosphere, and abundances of volatile elements are not as close to a 1:1 ratio. Due to intense stellar winds during the sun’s T-Tauri phase, most inner solar system terrestrial bodies are depleted in volatiles relative both to outer solar system gaseous bodies and the sun (White, 2013). After formation, temperatures in chondritic parent bodies are not thought to have reached the temperatures necessary in order to melt. Therefore, any diffusion of elements between minerals occured through grain boundary diffusion in metamorphic regimes. I have examined the group known as the ordinary chondrites. These chondrites comprise the majority of the undifferentiated meteorite group, and they are the majority of meteorites found on Earth (Scott and Krott, 2003). Specifically, this project’s focus is examining H chondrites. This subgroup of ordinary chondrites is high in iron metal and total iron content, which makes comparisons of trace elements in metals and silicates easier due to their high concentrations compared to other types of chondrites. The chondrites are split into categories based on their petrologic type, which is rated on a scale from 1-6 and is based on the level of metamorphism the meteorite experienced. Levels 1-2 are reserved for aqueous alteration, which is not observed in H chondrites, so H chondrites begin their classification with grade 3. Grades 3-6 are thermal metamorphic grades, with 6 being the highest grade, which are common in H chondrites. H3 chondrites, which are relatively unaltered, are metamorphosed between 400°C and 600°C, H4s are metamorphosed between 600°C and 700°C, H5s form are metamorphosed between 700°C and 750°C, and finally H6s are metamorphosed between 750°C and 950°C (McSween, 1987). One theory, called the Onion Shell Model, suggests that most H chondrites originated from one parent body, or that they were one proto-planetary mass initially before impacts dislodged the pieces that eventually found their way to 4 Earth (Wood, 2003). Heat production within the parent body is thought to come from planetary accretion and radiogenic heat produced by the decay of 26Al=>26Mg++B, a positron decay reaction (McSween, 1987). The model shown in figure 2 illustrates the perceived source of each metamorphic grade among the H chondrites in this proposed parent body. This thermal model is based on the assumption that heat transfers slowly in silicate bodies, and the heat is therefore stratified into distinct temperature layers. However, Kessel et. al. (2007) used the Fe-Mg exchange between the olivine and spinel that constitute H chondrites to determine that the peak metamorphic temperatures for H4 through H6 chondrites are all in excess of ~730°C. Kessel’s study also concludes that the “onion shell” model is not consistent with such a narrow range of peak metamorphic temperatures for grades that are supposed to occur in layers of distinct heat ranges. Figure 2: From Wood (2003), this illustration shows the higher metamorphic grade H chondrites as having originated at a deeper (hotter) location within the parent body than H3 and H4 chondrites. Temperature and pressure are greater within the parent body than they are near the surface. Thin sections demonstrate the high degree of recrystallization in H6 relative to H4. Highly Siderophile Elements The trace elements examined in this study are the highly siderophile elements (HSEs). These elements are notable for partitioning strongly into iron metal relative to silicate phases. The elements utilized in this study are rhenium, osmium, iridium, ruthenium, platinum, and palladium, ordered by increasing volatility from Re to Pd. First, they partition strongly into metals relative to silicates when the system is in equilibrium, with partition coefficients of >104(O’Neill et. al, 1995). These partition coefficients, or D values, indicate the concentration ratio of an element in one reservoir to another, in this case, metal and silicate. Therefore, there are roughly 104 atoms of the element in the metal grain for 5 each 1 in the silicate material.
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