The mineralogy of the Yaringie Hill meteorite A new H5 chondrite from South Australia

Transcription

1 Meteoritics & Planetary Science 44, Nr 11, (2009) Abstract available online at The mineralogy of the Yaringie Hill meteorite A new H5 chondrite from South Australia Ralf TAPPERT 1*, John FODEN 1, and Allan PRING 2 1 University of Adelaide, Geology and Geophysics, School of Earth and Environmental Sciences, Adelaide, South Australia 5005, Australia 2 South Australian Museum, Science Centre, Adelaide, South Australia 5000, Australia * Corresponding author. ralf.tappert@adelaide.edu.au (Received 26 May 2009; revision accepted 24 July 2009) Abstract The Yaringie Hill meteorite is a new H5 ordinary chondrite found in the Gawler Ranges, South Australia. The meteorite, which shows only minor signs of terrestrial weathering, is predominantly composed of olivine (Fa 17.2 ), orthopyroxene (Fs 15.1 Wo 1.1 ), and three distinct phases of nickeliferous iron metal (kamacite, taenite, tetrataenite). Other minerals include troilite, plagioclase (Ab 81 An 16 Or 3 ), clinopyroxene (En 52 Wo 42 Fs 6 ), chlorapatite, merrillite, ilmenite, and native copper. Three types of spinel with distinctive textures (coarse, skeletal aggregates, rounded aggregates) and with compositions close to the join MgAl 2 O 4 -FeCr 2 O 4 are also present. Chondrules within the Yaringie Hill meteorite, which often have poorly defined boundaries, are placed in a recrystallized matrix. Shock indicators suggest that the meteorite experienced only weak shock metamorphism (S3). INTRODUCTION The Yaringie Hill meteorite was found by entomologist Dr. Peter Hudson, during a fauna survey of the South Australian Museum on October 18, 2006, in the Gawler Ranges, South Australia ( S, E; Fig. 1). The discovery site is located about 15 km east of the eastern shore of Lake Acraman, which coincidentally marks the location of a large Neoproterozoic meteorite impact (Williams 1986). The Yaringie Hill meteorite is the first meteorite found within the Gawler Ranges; however, nine other meteorites were recovered from the adjacent Eyre Peninsula to the south (Fig. 1). PHYSICAL DESCRIPTION AND PETROGRAPHY The Yaringie Hill meteorite was recovered as a single mass and was entirely covered by a thin brownish-black fusion crust. The stone is roughly tetrahedral shaped with two roughly planner faces at right angles to each other; the other sides of the tetrahedron are less regular. The approximate dimensions of the meteorite are cm, and the original weight was 5.75 kg. A conspicuous feature of the meteorite is a thin (~0.5 mm) vein of metal that crosscuts the entire specimen (Fig. 2). The interior of the meteorite is light colored, with some patchy brown oxidized areas (Fig. 2). The Yaringie Hill meteorite is a recrystallized chondrite, predominantly composed of olivine, orthopyroxene, and nickeliferous iron metal. Minor phases are plagioclase, troilite, and clinopyroxene. Accessory minerals include chlorapatite, merrillite, ilmenite, spinel, and native copper. Chondrules composed of olivine, pyroxene or both minerals occur throughout the meteorite. The chondrules include types with porphyritic, non-porphyritic (radial pyroxene, barred olivine, cryptocrystalline), and granular textures (see Gooding and Keil 1981). Chondrule sizes range from ~300 µm to a maximum of ~4 mm. The largest chondrule consisted of radially aligned barred olivine with interstitial feldspar (Fig. 3). The boundaries of the chondrules are often poorly defined, and the interchondrule matrix is recrystallized. Weathering features of the Yaringie Hill meteorite are restricted to minor oxide rims around most of the metal and troilite grains, and staining of the adjacent silicate minerals. These weathering characteristics are consistent with stage W1 on the weathering scale devised by Wlotzka (1993). Meteorite finds with similar weathering characteristics, from regions with arid climate conditions comparable to South Australia, were found to have terrestrial ages of less than 5000 years (Jull et al. 1990; Wlotzka 1993). MINERALOGY The composition of the mineral phases were analyzed with a CAMECA SX-51 electron microprobe at the University of Adelaide, using a 15 kv acceleration voltage and a beam current of 20 na. Counting times for each 1687 The Meteoritical Society, Printed in USA.

2 1688 R. Tappert et al. Fig 1. Map of the Eyre Peninsula and the Gawler Ranges (South Australia) with the location of the sites of the Yaringie Hill and other meteorite finds. element ranged from 40 to 60 seconds (including background counts). Representative mineral analyses are presented in Table 1. Olivine occurs both within chondrules and as granoblastic matrix grains. The average fayalite component of the olivines is Fa 17.2 (range: Fa 16.6 Fa 19.4, 1σ: 0.45, n = 54) (Fig. 4). Like olivine, orthopyroxene also occurs within chondrules and as granoblastic matrix grains. Orthopyroxenes have an average ferrosilite content of Fs 15.1 (range: Fs 14.5 Fs 15.9, 1σ: 0.34, n = 28), and an average wollastonite content of Wo 1.1 (range: Wo 0.6 Wo 2.9, 1σ: 0.53) (Fig. 4). Despite the fact that plagioclase feldspar comprises a significant part of the matrix and of the interstitial phases of chondrules within the Yaringie Hill meteorite (Fig. 5A), discrete and inclusion-free crystals of sizes >10 µm are uncommon. Most of the very fine-grained to cryptocrystalline feldspar forms intergrowths with other fine-grained minerals. Compositionally the plagioclase feldspar of the Yaringie Hill meteorite is oligoclase with an approximate average composition Ab 81 An 16 Or 3 (Table 1). This feldspar composition falls into the range which is typical for H chondrites (Van Schmus and Ribbe 1968). Clinopyroxene is a rare component of the Yaringie Hill meteorite. It occasionally occurs as distinct grains of up to 50 µm, but more commonly forms thin overgrowths ( 20 µm) of larger orthopyroxene crystals. The average composition of clinopyroxene is En 51.9, Wo 42.0, Fs 6.1. Minor elements include titanium (0.47 wt% TiO 2 ), aluminium (1.65 wt% Al 2 O 3 ), and chromium (1.12 wt% Cr 2 O 3 ) (Table 1). Minerals belonging to the spinel group are compositionally variable and show a range of textures. The maximum size of all types of spinel group minerals is around 200 µm. Most abundant are coarse Cr-spinels (Ramdohr 1973), which are rather uniform in composition (Fig. 6, Table 1), and commonly occur in association or as inclusion in Fe- Ni metal or troilite (Fig. 5B). Aggregate spinels, which often have distinctive skeletal crystal shapes (skeletal aggregate spinels, Fig. 5C) are more variable in composition but have lower chromium and iron and higher aluminium and magnesium contents than coarse Cr-spinels (Fig. 6, Table 1). The skeletal aggregate spinels are commonly rimmed by plagioclase (Fig. 5C). Other spinels, which are also associated with plagioclase, form conspicuous rounded aggregates (rounded aggregate spinels, Fig. 5D). These spinels have even lower chromium and iron and higher aluminium and magnesium contents. Rubin (2003) suggested that the

3 The mineralogy of the Yaringie Hill meteorite A new H5 chondrite 1689 Fig. 2. Photograph of the Yaringie Hill meteorite after cutting. The prominent metal vein (white arrow) and a single large chondrule (black arrow) are visible on the cut surface. Scale bar is in centimeters. Fig. 3. Photomicrograph (crossed polars) of a large chondrule (Ø ~4 mm) from the Yaringie Hill meteorite. The chondrule consists of radially aligned barred olivine, rimmed by coarse recrystallized matrix.

4 1690 R. Tappert et al. Table 1. Representative compositions of minerals from the Yaringie Hill meteorite (analyses given in wt%). Mineral Olivine (n = 54) Orthopyroxene (n = 28) Clinopyroxene Feldspar Apatite Merrillite Ilmenite Spinel (coarse) Spinel (skeletal agg.) Spinel (rounded agg.) P 2 O SiO TiO Al 2 O V 2 O Cr 2 O MgO CaO MnO FeO NiO ZnO Na 2 O K 2 O F Cl Total P Si Ti Al V Cr Mg Ca Mn Fe Ni Zn Na K Cation tot [O] association of such texturally variable, Al 2 O 3 enriched spinels and plagioclase in ordinary chondrites is the result of shock melting processes. Molar Mg/(Mg+Fe) and Cr/(Cr+Al) ratios demonstrate that all spinel types in the Yaringie Hill meteorite fall close to the join chromite (FeCr 2 O 4 ) spinel s.s. (MgAl 2 O 4 ) (Fig. 6). Concentrations of minor elements in spinel group minerals, which include titanium, vanadium, and zinc, were found to correlate with the FeCr 2 O 4 (chromite) component of the spinels (Table 1). Magnesian ilmenite is a rare constituent of the Yaringie Hill meteorite and is present as anhedral grains (~50 µm) in close contact with metal grains. Chlorapatite with chlorine concentrations of up to 5.5 wt% (Table 1) is present in the Yaringie Hill meteorite as subhedral to anhedral crystals of up to 200 µm. An additional phosphate mineral in the Yaringie Hill meteorite is merrillite, which also occurs as subhedral to anhedral crystals of up to 200 µm. Merrillite is often found adjacent to metal or troilite grains. Merrillite also occurs closely associated with chlorapatite, which is not uncommon in ordinary chondrites (Van Schmus and Ribbe 1969, Jolliff et al. 2006). Chlorapatite and merrillite in the Yaringie Hill meteorite are present in approximately equal abundance. Troilite is present as isolated grains of up to 0.5 mm within the matrix of the meteorite or, more commonly, associated with metal phases. Smaller grains of troilite were also observed as inclusions in chondrules. Fe-Ni metal occurs as abundant anhedral grains of up to ~0.5 millimeters and forms the prominent vein which crosscuts the meteorite (Fig. 2). Three distinct metal phases can be distinguished based on their nickel contents, which have been determined for the cores of randomly selected metal grains. Kamacite is the most abundant metal phase and has an average nickel content of 6.12 wt% (n = 47). The prominent metal vein, which undulates around larger chondrules, is also primarily comprised of kamacite, but

5 The mineralogy of the Yaringie Hill meteorite A new H5 chondrite 1691 Fig. 4. Average fayalite component of olivine versus average ferrosilite component of orthopyroxene of the Yaringie Hill meteorite and other ordinary chondrites from the Eyre Peninsula (Mason 1974; Wallace and Pring 1991a, 1991b). Compositional fields for H, L, and LL ordinary chondrites are based on Keil and Fredriksson (1964). Error bars represent the standard deviation (1σ) for individual point analyses. contains occasional troilite, coarse Cr-spinel, and fragments of silicates. A less abundant metal phase in the Yaringie Hill meteorite is taenite, with nickel contents in the range 29.0 to 35.3 wt%. A third metal phase, with markedly higher nickel contents ( wt% Ni), has been identified as tetrataenite (Clarke and Scott 1980). No obvious zoning or intergrowths within individual metal grains have been noted. Native copper is a rare component of the Yaringie Hill meteorite, occurring as small isolated grains ( 10 µm), either surrounded by silicates or within metal phases. SHOCK METAMORPHISM The result of shock metamorphism is evident in the undulatory extinction of olivine, pyroxene and plagioclase, as well as in the presence of planar fractures in olivine crystals. Based on the classification scheme of Stöffler et al. (1991), this indicates that the Yaringie Hill meteorite is weakly shocked, i.e., experienced shock stage S3. The presence of spinelplagioclase assemblages is also consistent with a shock stage of at least S3 (Rubin 2003). The peak pressure for shock stage S3 is estimated to be in the range GPa (Stöffler et al. 1991). Other effects of shock metamorphism, possibly related to localized P-T excursions, include the presence of rare opaque melt veins. The prominent metal vein, which cross-cuts the entire meteorite may also be the result of this shock metamorphism. CLASSIFICATION Based on the magnesium-rich composition of the ferromagnesian silicates olivine (Fa 17.2 ) and orthopyroxene (Fs 15.1 Wo 1.1 ) the Yaringie Hill meteorite belongs to the H- group of ordinary chondrites (Keil and Fredriksson 1964) (Fig. 4). This classification is also consistent with the high modal abundance of metal phases in the meteorite, the composition of the plagioclase feldspar (Van Schmus and Ribbe 1968), and the composition of the coarse Cr-spinels (Bunch et al. 1967). The distinctive composition of the ferromagnesian silicates suggests that the Yaringie Hill meteorite is not related to any of the other meteorites described from the Eyre Peninsula (Fig. 4). The microstructure of the Yaringie Hill meteorite, which shows chondrules with indistinct rims in a crystalline matrix, indicates that the meteorite experienced a pre-terrestrial metamorphic overprint. The scarcity of discrete plagioclase crystals, however, rules out that the Yaringie Hill meteorite belongs to the highest metamorphic class for ordinary chondrites (type 6), but rather suggests a type 5 classification in the petrographical classification scheme of Van Schmus and Wood (1967). The type 5 classification of the Yaringie Hill meteorite is also consistent with the relatively low average wollastonite content of the orthopyroxenes (Wo 1.1 ), since the wollastonite content was shown to correlate with the degree of metamorphic overprint and hence the petrographic

6 1692 R. Tappert et al. Fig. 5. Backscattered electron (BSE) images of the Yaringie Hill meteorite. A) Chondrule with porphyritic orthopyroxene (light) and interstitial feldspar (dark). B) Coarse Cr-spinel inclusions in troilite. C) Skeletal spinel aggregate with plagioclase rim in matrix of ferromagnesian silicates. D) Rounded aggregate spinel with plagioclase in matrix of ferromagnesian silicates. Fig. 6. Mg/(Mg + Fe) versus Cr/(Cr/Al) of accessory spinel in the Yaringie Hill meteorite. Fig. 7. Isotherm diagram for spinels from the Yaringie Hill meteorite, based on Wlotzka (2005).

7 The mineralogy of the Yaringie Hill meteorite A new H5 chondrite 1693 type of chondrites (Scott et al. 1986). Chondrites belonging to type 6 are expected to have average wollastonite contents of >1.6. The variability in the composition of olivine and orthopyroxene provides additional support for the type 5 classification of the Yaringie Hill meteorite, because it indicates some degree of unequilibration. Because of the unequilibrated nature of the silicates in the Yaringie Hill meteorite, it was not possible to gain meaningful temperature information using two-pyroxene thermometry (e.g., Wells 1977; Lindsley 1983; Bertrand and Mercier 1985). However, temperatures have been determined using the olivine-spinel thermometer of Wlotzka (2005). The resulting temperatures, which are in the range C, depend strongly on the textural type of the spinels (Fig. 7). Coarse Cr-spinels indicate relatively low equilibration temperatures of C. Temperature estimates for skeletal aggregate spinels ( C) and rounded aggregate spinels ( C) are significantly higher (Fig. 7). Similar temperature discrepancies of C between chromium-rich coarse spinel and more aluminous spinel assemblages have been observed in other meteorites (Wlotzka 2005). Assuming aluminous spinel-plagioclase assemblages are the result of shock induced melting, as proposed by Rubin (2003), the higher temperature estimates for these spinels may reflect the temporarily increased heat flux during the shock event. Coarse spinels, on the other hand, are more likely to reflect steady-state conditions during metamorphism. Acknowledgments We thank Dr. Hudson for bringing the meteorite to the attention of the South Australian Museum, and the Waterhouse Club of the South Australian Museum who funded the survey on which the meteorite was found. Additional financial support was provided by the Australian Research Council (ARC), the Department of Primary Industries and Resources of South Australia (PIRSA), and Flinders Mines Ltd. The manuscript benefited from helpful reviews by A. Bevan and A. Ruzicka. Editorial Handling Dr. Alex Ruzicka REFERENCES Bertrand P. and Mercier J. C The mutual solubility of coexisting ortho- and clinopyroxene: Toward an absolute geothermobarometer for the natural system? Earth and Planetary Science Letters 76: Bunch T. E., Keil K., and Snetsinger K. G Chromite composition in relation to chemistry and texture of ordinary chondrites. Geochimica et Cosmochimica Acta 31: Clarke R. S. and Scott E. R. D Tetrataenite ordered FeNi, a new mineral in meteorites. American Mineralogist 65: Gooding J. L. and Keil K Relative abundances of chondrule primary textural types in ordinary chondrites and their bearing on conditions of chondrule formation. Meteoritics 16: Jolliff B. L., Hughes J. M., Freeman J. J., and Zeigler R. A Crystal chemistry of lunar merrillite and comparison to other meteoritic and planetary suites of whitlockite and merrillite. American Mineralogist 91: Jull A. J. T., Wlotzka F., Palme H., and Donahue D. J Distribution of terrestrial age and petrologic type of meteorites from western Libya. Geochimica et Cosmochimica Acta 54: Keil K. and Fredriksson K The iron, magnesium, and calcium distribution in coexisting olivines and rhombic pyroxenes of chondrites. Journal of Geophysical Research 69: Lindsley D. H Pyroxene thermometry. American Mineralogist 68: Mason B Notes on Australian meteorites. Records of the Australian Museum 29: Ramdohr P The opaque minerals in stony meteorites. Amsterdam: Elsevier. 245 p. Rubin A. E Chromite-plagioclase assemblages as a new shock indicator; implications for the shock and thermal histories of ordinary chondrites. Geochimica et Cosmochimica Acta 67: Scott E. R. D., Taylor G. J., and Keil K Accretion, metamorphism, and brecciation of ordinary chondrites: Evidence from petrologic studies of meteorites from Roosevelt County, New Mexico. Proceedings, 17th Lunar and Planetary Science Conference. Journal of Geophysical Research 91(B13):E115 E123. Stöffler D., Keil K., and Scott E. R. D Shock metamorphism of ordinary chondrites. Geochimica et Cosmochimica Acta 55: Van Schmus W. R. and Ribbe P. H The composition and structural state of feldspar from chondritic meteorites. Geochimica et Cosmochimica Acta 32: Van Schmus W. R. and Ribbe P. H Composition of phosphate minerals in ordinary chondrites. Geochimica et Cosmochimica Acta 33: Van Schmus W. R. and Wood J. A A chemical-petrologic classification for the chondritic meteorites. Geochimica et Cosmochimica Acta 31: Wallace M. and Pring A. 1991a. Geological note: The Streaky Bay meteorite, a new (L4) chondrite from South Australia. Australian Journal of Earth Sciences 38: Wallace M. and Pring A. 1991b. The Mangalo meteorite, a new (L6) olivine-hypersthene chondrite from South Australia. Transactions of the Royal Society of South Australia 115: Wells P. R. A Pyroxene thermometry in simple and complex systems. Contributions to Mineralogy and Petrology 46: Williams G. E The Acraman impact structure: Source of ejecta in Late Precambrian shales, South Australia. Science 233: Wlotzka F A weathering scale for the ordinary chondrites. Meteoritics 28:460. Wlotzka F Cr spinel and chromite as petrogenetic indicators in ordinary chondrites: Equilibration temperatures of petrologic types 3.7 to 6. Meteoritics & Planetary Science 40: