New ureilite Sahara 99201 has been investigated to determine mineralogy, petrography, light stable isotopes and noble gases. The sample is a typical olivine pigeonite ureilite (Fo 78, Wo 10, En 72). C, N, O and Ar are consistent with other ureilites

Franchi, Ian

Grady, Monica

Pillinger, Colin

Smith, C. L.

Verchovsky, Sasha

Wright, Ian

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsAn integrated mineralogical, petrographic, light stableisotope and noble gas investigation of Sahara 99201ureiliteConference or Workshop ItemHow to cite:Smith, C. L.; Franchi, Ian; Wright, Ian; Verchovsky, Sasha; Grady, Monica and Pillinger, Colin (2001). Anintegrated mineralogical, petrographic, light stable isotope and noble gas investigation of Sahara 99201 ureilite. In:32nd Lunar and Planetary Science Conference, 12-16 Mar 2001, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1647.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.uk  AN INTEGRATED MINERALOGICAL, PETROGRAPHIC, LIGHT STABLE ISOTOPE AND NOBLE GAS INVESTIGATION OF SAHARA 99201 UREILITE. C. L. Smith1, 2, I. A. Franchi1, I. P. Wright1, A. B.  Verchovsky1, M. M. Grady2 and C. T. Pillinger1.  1Planetary Space and Space Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK (C.L.Smith@open.ac.uk), 2Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK.    Introduction:  Sahara 99201 (provisional name) is a new ureilite weighing 91g collected from the Sahara desert during 1999.  The sample has been analysed to determine both mineralogy and petrography and also its light element stable isotopic composition. Methodology:  Mineralogical analyses were car-ried out using a Cameca SX100 electron microprobe, with a beam current of 20nA, accelerating voltage of 20kV and a 10µm spot size in WDS analytical mode.  At least 6 analyses were taken from each mineral grain to ensure good experimental precision. A ~200mg chip was taken from an internal portion of a 35g sample (to reduce the effect of contamination and weathering) and crushed in an agate pestle and mortar. Small fractions of powdered sample were loaded into Pt buckets and high resolution stepped combustion experiments were performed from 300°C to 1400°C to determine the yield and isotopic composi-tion of C, N and Ar. C stable isotope analyses were carried out using MS86, a fully automated high-sensitivity static-vacuum mass spectrometer, whilst a separate aliquot of material was analysed for N and noble gases using Finesse, a multi-element static-vacuum mass spectrometer [1,2]. O isotope analyses were carried out on a dual inlet mass spectrometer (VG PRISM) following the method of Miller et al. [3].  Results:  Mineralogy and petrography. The sample is a coarse-grained (average grain size 1-2mm) olivine pigeonite ureilite displaying typical monomict texture.   The sample is relatively unweath-ered compared with other desert ureilites with a weath-ering grade of W0-W1[4].  The sample is also of low shock with undulatory extinction in olivine implying a shock classification of S2 [5].  Olivine grains within the sample are quite fractured although one prominent grain displays very little fracturing.  Reduction rims occur in olivines in two forms: as typical 50-100µm rims at the grain boundaries of the unfractured olivine, and also as rosettes which mainly occur in the more fractured olivine (figure 1).  The rosettes consist of globular masses of metal blebs approximately 100µm in diameter. The rosettes occur in groups adjacent to the grain boundaries of the olivine and can be up to 800µm in thickness.  These rosettes are apparently a primary texture and not related to weathering.  The pigeonite grains do not display reduction rims as seen in the olivine although they do contain trails of metallic inclusions.  These inclusions appear to follow both cleavage planes in the pigeonite and also fracture zones.  A single blade of graphite can be seen at the boundary of a fractured olivine which is surrounded by a ~100µm thick zone of reduction.  Interestingly, C-rich matrix material is apparently lacking in the sec-tion.  Mean mg# from olivine cores is Fo78.2 ± 0.5 (1σ) with mean pyroxene core compositions of Wo 9.6 ± 0.1, Εn 72.4 ± 0.4.    Figure 1. Plane polarised light image of Sahara 99201 showing rosettes of reduction and other features of interest. Carbon isotopes: A sample of 0.215mg was combusted; results are shown in figure 2.  The total C yield for this sample is 3.3 wt% which is comparable with other ureilites [6]. At temperatures below 500°C, 844ppm C is released with a weighted average δ13C(PDB) of -21.5 , indica-tive of organic contamination.  The main release occurs between 500 and 750°C, a temperature characteristic of graphite combustion, where 3.1 wt% C is liberated, of average δ13C(PDB) -2.5 , which is also within the ob-served range of ureilites [6].  At temperatures above 750°C only 424ppm C is released with a mean δ13C(PDB) of -23 . This is close to that of system blank, although it may be associated with carbide or carbon in metal.              Figure 2. Carbon step combustion profile. 050100150200250300350400-30-25-20-15-10-500 200 400 600 800 1000 1200 1400C (ppm/°C)δδδ δ13C  (rel. PDB)T (°C)Lunar and Planetary Science XXXII (2001) 1647.pdfINVESTIGATION OF SAHARA 99201 UREILITE:  C. L. Smith et al.  Nitrogen: A 3.13mg sample was combusted and found to contain 78ppm of N, within the range reported for other ureilites [7].  The stepped combustion profile (figure 3) indicates three main N releases at low, mid and high temperatures. The first, low temperature release of 34ppm N occurs below 500°C with a weighted average δ15N(AIR)  of +26.8  and is presumably related to the low temperature organic contaminant fraction.  The second N release occurs between 550 and 800°C with 19ppm N and δ15N(AIR) of 54 .  This release is re-lated to the main C release associated with the graphite combustion at these temperatures.  The third N release occurs at temperatures above 900°C with 102ppm N being liberated with a weighted average δ15N(AIR)  of     -16  which is similar to reported values for other ureilites [7].  This high temperature release is most likely related to metal and silicate fractions [8].            Figure 3. Nitrogen step combustion profile. Argon: The sample shows a peak 36Ar release (856ccx10-8/g) at temperatures between 500 and 700°C which again is presumably related to the combustion of graphite.  The 36Ar release and its 36Ar/38Ar and 40Ar/36Ar ratios are within values reported for other ureilites [9].  No other 36Ar or Ne releases above levels of system blank were detected. Oxygen: O isotopic composition for this sample is δ18O(SMOW) =+8.07  and δ17O(SMOW) =+3.25  with ∆17O of        -0.946. Accordingly this sample appears to be within sub-group D of Franchi et al. [10].          Figure 4.  Graph to show co-variation of ∆17O and δ13C within ureilite sub-groups. Dashed lines =1σ on mean [10]. Discussion: In terms of its mineralogy and petrog-raphy the sample is fairly typical of monomict ureilites in general, although the low shock and low weathering grade (in terms of desert finds) of the sample is quite unusual.  The rosette reduction is also fairly common and similar reduction textures can be seen in HH126 [11].  It appears that the rosette reduction is a primary feature as opposed to one developed during weather-ing, for example.  It is perhaps relevant that the rosette reduction can only be seen in the fractured olivine grains as opposed to the typical reduction rim as seen on the fresh olivine.  The rosette style of reduction may be related to a shock event.  Perhaps C-rich fluids  were mobilised during the shock event and were able to preferentially permeate the fractured olivine grains thus resulting in a localised but intense reduction.  C yield and isotopic composition is within range of other reported ureilites falling within the heavier of the two groups reported by [6].  A diamond component is apparently lacking from the stepped combustion pro-file.  If a diamond component were present it is likely that it would be detected during stepped combustion analysis.  Diamond has an elevated combustion tem-perature (~700 to 1000°C) compared with graphite (~600 to 900°C) [6], and accordingly a sample that contains diamond would be expected to show a broader release profile than that seen from this sample.  This lack of diamond component in the step combustion profile is consistent with the low shock classification as suggested by the petrography. Total N yield is also within the reported range.  The isotopic composition of the release associated with graphite combustion (δ15N 54 ) is within values reported for similar stepped combustion experiments on bulk ureilite samples [6,7]. The multi-component release profile seen in this sample is also fairly typical to those observed for other ureilites.  36Ar yield is also within range reported for ureilites and its peak release also occurs at the range in tempera-tures for graphite combustion.  This is to be expected, as it is believed that C in ureilites is the carrier phase for planetary type noble gases.    References: [1] Prosser, S. J. et al. (1990) Chem. Geol. 83, 71-88. [2] Verchovsky, A. B. et al. (1997) MAPS, 32, A131. [3] Miller, M. F. et al. (1999) Rapid. Commun. Mass Sp., 13, 1211-1217. [4] Wlotzka, F. (1993) Meteoritics, 28, 460. [5] Stöffler, D. et al. (1991) GCA, 55, 3845-3867. [6] Grady, M. M. et. al. (1985) GCA, 49, 903-915. [7] Yamamoto, T. et al. (1998) MAPS, 857-870. [8]Franchi I. A. et al. (1993) GCA, 57, 3105-3121. [9] Schultz L. & Kruze H. (1989) Meteoritics, 24, 155-172. [10] Franchi I. A. et al. (1998) LPS, XXIX, 1635. [11] Sexton, A. et al. (1996) LPS, XXVII, 1173. 00.020.040.060.080.10.120.140.16-80-60-40-2002040600 200 400 600 800 1000 1200 1400N (ppm/°C)δδδ δ151515 15N  (rel. AIR)T (°C)-1.1-1-0.9-0.8-0.7-0.6-0.5-10 -8 -6 -4 -2 0∆ ∆∆∆17 Oδ13C  (rel. PDB)Sub-group       BSub-group       DSahara 99201Lunar and Planetary Science XXXII (2001) 1647.pdf

An integrated mineralogical, petrographic, light stable isotope and noble gas investigation of Sahara 99201 ureilite

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An integrated mineralogical, petrographic, light stable isotope and noble gas investigation of Sahara 99201 ureilite

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