Analyzing the Chemical Composition and Classification of Miller Range 07273

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1 Alyssa Dolan Analyzing the Chemical Composition and Classification of Miller Range When small grains and debris that were present in the early solar system combine, they form meteoroids. Meteoroids become meteors when they interact with a planet s atmosphere and are heated by the friction of the atmosphere, causing a streak of light to appear in the sky. Most meteors that enter the Earth s atmosphere vaporize and never reach the surface of the planet because of their small size. However, the meteors, or parts of meteors, that do survive the fall and collide with the Earth s surface are called meteorites. Some meteorites are characterized by the presence of chondrules. Chondrules are small, round grains that are typically about one millimeter in diameter and consist of silicate minerals, such as olivine and pyroxene. Chondrules form as either molten or partially molten droplets in space. When chondrules combine in space, they form meteoroids, which may then become meteors or meteorites. Meteorites that are stony and characterized by the presence of chondrules are called chondrites. Chondrites can be classified as carbonaceous (C) chondrites, ordinary chondrites (OC), and enstatite (E) chondrites. C chondrites contain high percentages of water and organic compounds, OC contain varying percentages of iron, and E chondrites contain high percentages of the mineral, enstatite (MgSiO 3 ). Within OC, there are three groups of chondrites: H chondrites, L chondrites, and LL chondrites. H chondrites contain high amounts of iron, L chondrites contain lower amounts of iron, and LL chondrites contain the lowest amounts of iron. Within H chondrites, there are four subgroups of chondrites: petrologic type 3 (H3), type 4 (H4), type 5 (H5), and type 6 (H6). The petrologic type depends on the degree of aqueous alteration and thermal metamorphism. The degree of alteration and metamorphism increases with an increase in petrologic type number. H3 chondrites have no degree of metamorphism and contain

2 olivine and pyroxene homogeneity greater than 5% deviations from the mean, H4 chondrites have a weak degree of metamorphism and contain olivine and pyroxene homogeneity lower than 5% deviations from the mean, H5 chondrites have a moderate degree of metamorphism and contain olivine and pyroxene homogeneity, and H6 chondrites have a strong degree of metamorphism and contain olivine and pyroxene homogeneity. Studying chondrites provides information about the solar system composition. By comparing the chemical composition of the chondrites and thus the classifications of chondrites, we can group specimens that share characteristics on a single parent body, or a meteoroid. In the research conducted at Fordham University, we perform chemical analyses on samples of chondrites, and take our results and compare them with other analyses that are performed by scientists in various parts of the country on different samples of the same chondrites. Our goal is determine the chemical compositions and thus the classification of the specimens in order to establish if the specimens share similar chemical compositions and the same classification. If the specimens do share similar chemical compositions and the same classification, then we can establish that they share a common origin, or parent body. The sample we have analyzed since October 2013 is Miller Range 07273, or MIL MIL was found in the Miller Range in Antarctica in It is classified as an H5-an chondrite, which means that it is an ordinary chondrite from the H group that is petrologic type 5 and has anomalous properties. 1 It is a part of the H group of ordinary chondrites because it has an iron abundance and a high siderophile element content. Siderophile elements often sink into the core when they collide with the Earth s surface because they dissolve readily in iron as either in the molten state or in solid solutions. MIL is designated as a type 5 chondrite because it has been metamorphosed under conditions that blur chondrule outlines and that homogenize

3 pyroxene and olivine. The chondrite is also anomalous because it has specific properties that are unique and distinctive. Previous work in our laboratory resulted in data of MIL that conflicted with that of other scientists, and we have repeated the original analysis to determine if our new results would be consistent. In order to prepare the chondrite for trace element tests, three samples weighing approximately one hundred milligrams were ground into powder with an agate mortar and pestle. The agate material is a special silicate that is mainly composed of SiO 2. This mortar and pestle is used because of its excellence in preparing high purity powders for trace analysis. In addition, it is used to minimize contamination of the sample. The ground samples were then dissolved in a mixture of hydrofluoric acid and nitric acid using a microwave digestion apparatus at 200 C and 185 psi. The remnants from the digestion bombs were transferred to beakers and heated on hotplates for about seven hours until the solutions evaporated completely. The samples were then dissolved in a solution of perchloric acid and water and re-heated on hotplates. We then turned off the hotplates and added a mixture of nitric acid and water to the beakers to allow the sample to go back into solution. The same techniques were implemented on an internal standard, Allende. Allende is the largest C chondrite ever found, and we compare our samples to it because of its well-known concentrations. Allende is used for standardization and procedural monitoring. Once the samples dissolved, we added water and internal standard to each sample and transferred the samples into two different vials one for analysis of major trace elements and the other for analysis of minor trace elements. Once the solutions were vaporized by the nebulizer, we ran the samples through the ICP- MS. An ICPMS combines a high temperature ICP source with a mass spectrometer. Argon gas flows inside the ICP torch, which is connected to a radio frequency generator that produces a

4 current to create an electric and magnetic field. The elements from the nebulizer interact with the Argon plasma to convert the sample into positive ions that can be separated by their mass-ion ratios. When the ions are separated, they can be identified by a negatively charged detector, which translates the number of ions into an electrical signal that relates to the number of atoms in the sample. The results from the ICP-MS are transferred to a computer that translates the data into interpretable tables and graphs of the chemical composition of the samples. The previous work in our laboratory consisted of analyzing four low Fe-O chondrites: Burnwell, LAP 04757, EET 96031, and MIL 07273, along with 2 original H5 chondrites for comparison: Miller and Forest City. Our team wanted to determine the chemical compositions and thus the classifications of the chondrite specimens in order to decide whether the specimens were from a common parent body. We compared the specimens to two H5 chondrites, the classification previously found by other scientists for the specimens, and wanted to see if the results were consistent. Since the results 2 for Burnwell, LAP 04757, and EET were consistent with results from other scientists, they were published. However, we did not publish the results for MIL due to its inconsistent data. The compositions of the four chondrites were first compared to the compositions with those established for the original chondrites. We quantified trace element abundances and analyzed the chondrites according to such abundances. Our team wanted to establish if the chondrites were H chondrites (high in iron), L chondrites (low in iron), or LL chondrites (low in iron and in metal) since it is rather hard to determine this distinction. It was determined that the trace elements found in the three low Fe-O chondrites were consistent with the two original H5 chondrites (see Table 1). As shown, for example, compared to Forest City and Miller, which were composed of 1.5 μg/g of lithium each, Burnell contained 1.5 μg/g of lithium, LAP contained 8 μg/g, and EET contained 1.3 μg/g. These results

5 were also consistent with the mean composition of lithium in the H chondrite group, which was found to be 1.7 μg/g by research from J.T. Wasson et al. 3 Similarly, while Forest City and Miller were composed of 7.85 and 7.76 μg/g of scandium, respectively, Burnell contained 7.91 μg/g of scandium, LAP contained 9.05 μg/g, and EET contained 7.62 μg/g. These results were again consistent with the mean composition of scandium in H chondrites, which was Table 1: Data of the abundances of trace elements in three low-feo chondrites: Burnwell, LAP 04757, and EET and two H OC: Forest City and Miller. (Published Data by Julianne Troiano et al. in 2011) found to be 7.9 μg/g by Wasson et al. The chemical compositions of these unusual chondrites were consistent with each other, consistent with the OC, and consistent with the mean composition. This proved that the compositions of the three chondrites were consistent with the compositions of the H5 chondrites. Since the specimens shared similar chemical compositions and the same H5 classification, we could conclude that they originated from a common parent body. The data for MIL was not published in the table because its composition was inconsistent with the OC, and thus H5 chondrites.

6 Our team further showed the correlation between the three chondrites and the original H5 chondrites when comparing the mean CI (Orgueil) weight normalized compositions of lithophiles, siderophiles, and moderately volatile elements in the chondrites. We compared our results to those of scientists, G.W. Kallemeyn et al. 4 and S.S. Russell et al. 5, who analyzed the compositions of chondrites, as well (see Fig. 1). To facilitate comparisons, the trace elements were Figure 1: The top graph shows the lithophile composition (Rh Pd) in the chondrites and the moderately volatile elements composition (Li Bi). The bottom graph shows the siderophile composition (Hf Ba). (Published Data by Troiano et al. in 2011) divided into common geo and cosmochemical groups: refractory siderophiles, including Rh, Ir, Mo, Pt, Co, and Pd; moderately volatile elements, including Li through Bi; and refractory lithophiles, including Hf through Ba. As shown on the top graph of Fig. 1, Burnwell, LAP 04757, EET and each group of OC have essentially the same abundances for the CI normalized lithophile abundance data. This concurs with the results of Kallemeyn et al., who found that OC have primarily the same lithophile abundance data as H chondrites, but different abundance data than the L and LL chondrites. Our team also found that the mean moderately volatile abundances were similar to each other for all three chondrites and similar to H chondrites. This data was also consistent with Kallemeyn et al., who

7 found that moderately volatile content is primarily constant across OC. As shown on the bottom graph of Fig. 1, Burnwell, LAP 04757, and EET have siderophile abundances identical to H chondrites. This data was consistent with Russell et al., who concluded from his results that the siderophile abundance of Burnell was extremely similar to that of H chondrites. The findings of Kallemeyn et al. were also consistent with our data for L and LL chondrites. For siderophile abundances, we discovered that L chondrites have a lower CI normalized mean (1.35 ± 0.10) and LL chondrites have an even lower mean (0.94 ± 0.08). 2 Overall, the data showed that Burnwell, LAP 04757, and EET had elemental abundance data exceedingly similar to that of H chondrites. Thus, from a bulk element perspective, the three chondrites proved to be compositionally identical to the H chondrites. 2 Along with trace element results, we used oxygen isotope results and three dimensional petrography data to conclude that the chondrites were chemically similar and H5 chondrites, and thus derived from the same parent body. While the data from Burnwell, LAP 04757, and EET was consistent with that of H5 chondrites, the data from MIL did not correlate with that of H5 chondrites 6 although it was expected to based on prior research from other scientists (see Fig. 2). As shown in Fig. 2, the abundances for MIL are consistent with the abundances for H chondrites for a few elements, such as siderophile elements like Ir and lithophile elements like Ti. However, for a majority of the elements, the abundances for MIL are inconsistent with the abundances for the H, L, or LL chondrites. Since results from prior analyses performed by scientists like Kallemeyn et al. prove that MIL is an H5 chondrite, our team s results were inconsistent with past results. Since the data of MIL did not comply with the data from the other three specimens that were found to be H5 chondrites and chemically similar to each other, we

wondered if MIL 07273 originated from a different parent body than the other three specimens. Due to the inconsistency, the results were not published for MIL 07273.

8 wondered if MIL originated from a different parent body than the other three specimens. Due to the inconsistency, the results were not published for MIL The inconsistency of the MIL results with prior results from other scientists was due to a sampling issue. Since the MIL sample used was significantly less than 100 mg and thus very minute, there was not a large representation of the sample. In order for all the trace elements to be accounted for in the analysis, a large Figure 2: The top graph shows the lithophile composition (Rh Pd) in the chondrites and the moderately volatile elements composition (Li Bi). The bottom graph shows the siderophile composition (Hf Ba). (Unpublished Data by Troiano et al. in 2011) sample of about 100 mg is necessary. However, since the sample size was small and unrepresentative, the analysis was erroneous from the beginning leading to flawed and unexpected results that failed to conclude that MIL was an H5 chondrite. In October 2013, we reanalyzed the composition of the chondrite. From our second analysis, we found MIL to be an H5 chondrite. Comparing our past trace element results with the new results, in addition to Wasson s results on H5 chondrites, our team found that our results were consistent with those of the H5 chondrites. Thus, our new results on MIL were inconsistent with our prior unsound results on MIL To show

9 the comparison in trace element results between the past results, the new results, and Wasson s H5 chondrite sample, we comprised a table consisting of lithium and scandium data (see Table 2). While the new MIL sample and Wasson s Table 2: Data of the abundances of trace elements, Li and Sc, in the 2011 MIL sample, the 2013 MIL sample, and Wasson s H chondrite sample. H5 chondrite sample contained similar amounts of lithium and scandium, the composition of the previous sample MIL07273 significantly differed from the other two. Thus, the composition of the new sample of MIL was chemically similar to the composition of the H5 chondrites. The consistency between our Figure 3: The top graph shows the trace element composition (Zr Ba) in MIL and an original chondrite. The bottom graph shows the trace element composition (Re Zn) in MIL and H, L, and LL chondrites. new data and Wasson s data showed that MIL was indeed an H5 chondrite. Further proof that MIL was an H chondrite is shown in Fig. 3. While the top graph shows the correlation between MIL data and OC data, the bottom graph shows the correlation between MIL data and H chondrite data with respect

10 to trace elements. This data further proves that MIL is an H chondrite, as opposed to the previous findings. From the ICP-MS results on the re-analyzed MIL sample, we found that the chemical composition of MIL was chemically similar to that of H5 chondrites, and thus MIL was an H5 chondrite. In addition, we found that Burnwell, LAP 04757, and EET were also H5 chondrites and chemically similar to each other in the original analysis. Since MIL Burnwell, LAP 04757, and EET shared similar chemical compositions and the same H5 classification, we were able to conclude that all four specimens originated from a common parent body. This was consistent with previous results from other scientists. Our first set of results for MIL were faulty because the sample was too small, and this sampling issue led to an unrepresentative sample of MIL contributing to erroneous results. When we reanalyzed the results, the large enough sample size of MIL that was used allowed us to obtain consistent results. It is clear that the use of trace element analysis is exceedingly valuable. From trace element analysis of chondrites, we were able to determine the chemical composition of the chondrites and associate specific chondrites with a group of chondrites: H, L, or LL and with a type of chondrites: type 3, type 4, type 5, or type 6 in order to successfully determine the existence of a common parent body that the four H5 specimens originated from. 1 The Meteoritical Society. International Society for Meteorites and Planetary Science Troiano, Julianne, Rumble D, Rivers L., Friedrich, J. Compositions of three low-feo ordinary chondrites: Indications of a common origin with the H chondrites. Geochimica et Cosmochimica Acta. (2011):

11 3 Wasson, J.T. and Kallemeyn G.W. Compositions of Chondrites. Philosophical Transactions of the Royal Society of London. (1988): 325, Kallemeyn G. W., Rubin A. E., Wang D. and Wasson J. T. Ordinary chondrites bulk compositions, classification, lithophile-element fractionations, and composition-petrographic type relationships. Geochimica et Cosmochimica Acta. (1989): 53, Russell S. S., McCoy T. J., Jarosewich E. and Ash R. D. The Burnwell, Kentucky, low-feo chondrite fall: description, classification and origin. Meteoritics & Planetary Science. (1998): 33, Troiano, Julianne, Rumble D, Rivers L., Friedrich, J. Compositions of four low-feo ordinary chondrites: Is a new chondritic meteorite parent body necessary? Unpublished. (2011).

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