Bland, P.A.

D'Orazio, M.

Folco, L.

Franchi, I.A.

Rocchi, S.

English

Open Research Online 

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsDar Al Gani 896: A unique picritic achondriteConference or Workshop ItemHow to cite:Folco, L.; Bland, P.A.; D’Orazio, M.; Franchi, I.A. and Rocchi, S. (2002). Dar Al Gani 896: A unique picriticachondrite. In: 65th Annual Meteoritical Society Meeting, 21-26 Jul 2002, Los Angeles, California, 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/metsoc2002/pdf/5176.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.ukDAR AL GANI 896:  A UNIQUE PICRITIC ACHONDRITE.  L. Folco1, P. A. Bland2, M. D’Orazio3, I. A. Franchi2, and S. Roc-chi3, 1Museo Nazionale Antartide, Via Laterina 8, 53100 Siena, Italy, 2Planetary and Space Sciences Research Institute, Open Uni-versity, Milton Keynes, MK7 6AA, U.K., 3Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy, (folco@unisi.it)  DaG 896 is the provisional name assigned to a dark, 50% fusion crusted, stony fragment of 22.6 g found in the Dar al Gani region of the Libyan Sahara (~27°08’N – 16°05’E) in November 2000. Mineralogy:  DaG 896 is hypocrystalline with porphyritic tex-ture (porphyritic index  ~70). Abundant fine-grained (avg. grain-size 100 µm), skeletal forsterite crystals (avg. Fo82) are set in a ground-mass consisting mainly of rhyolitic glass plus quench microlites of pigeonite (En54 Wo9). Minor mineral constituents include enstatite (En83 Wo2), augite (En37Wo30) and traces of chromite, troilite, Fe-Ni metal and phosphates. Veinlets up to 15 µm thick of secondary car-bonates are most likely due to terrestrial weathering. Mineral mode (vol. %) is forsterite 69, glass 17, clinopyroxene (pigeonite >> augite) 8, enstatite 4, carbonates 1, others 1. Strong undulose extinc-tion and planar fracturing in olivine and mm-thick interconnecting veinlets indicate moderate shock S4, after [1].  Fe/Mn a.p.f.u. ratios for olivine and pyroxenes (35 and 18, respectively) are chondritic [2]. No achondrite with the above texture and mineral composition is documented in literature [e.g., 2]. Similar textures and mineral compositions are in turn observed in some igneous inclusions in ordinary chondrites [e.g. 3]. Bulk chemistry:  XRF and ICP-MS major and trace element concentrations indicate an high-Mg (mg# 79.2) picritic IUGS com-position (SiO2 = 47.62, Na2O = 0.16, K2O = 0.24, MgO = 30.3 wt %) with a chondritic [Mn/Mg] = 7.5 *10-3 ratio. The overall compo-sition is similar to H-chondrites [4] (e.g., Mg/Si = 0.83, Al/Si = 0.073, Ti/Si = 0.006) except for a strong depletion in siderophile and chalcophile elements (e.g., Fe/Si = 0.50, Fe/Ni = 254 and S/Si = 0.009). A close major element compositional match is found with the harzburgitic igneous inclusion in the Y-794046 H5 chondrite [5], [6] and [7] and the komatiite-like lithic fragment in the Eva H4 chondrite [3]. Anomalously high concentrations of Cs, Rb, Sr and Pb suggest some terrestrial contamination. Oxygen isotopes:  Mean data from two replicates are δ17O = 2.55, δ18O = 3.50 and ∆17O = 0.726 ± 0.007 ‰. The ∆17O value is typical for equilibrated H-chondrites [8], although the δ18O ratio is slightly offset to the light side for this grouping.  Conclusions:  i) DaG 896 is a unique picritic achondrite; some  igneous inclusions in ordinary chondrites  are the closest existing analogs. ii)  DaG 896 formed through rapid cooling of a high-temperature melt likely representing the silicate fraction of H-chondritic source material, which underwent severe heating, melting and metal plus sulfide loss. References:  [1] Stöffler D. et al. (1991) GCA, 55, 3845-3867. [2] Papike J. J., ed. (1998) Rev. Mineral., 36. [3] Ruzicka et al. (1998) GCA 62, 1419-1442. [4] Jarosewich E. (1990) Meteoritics 25, 323-337. [5] Nakamura N. et al. (1994) Proc. NIPR Symp. Ant-arct. Meteorites, 7, 125-143. [6] Sack R. O. et al. (1994) JGR, 99, E12, 26029-26044. [7] Mittlefehldt D. W. et al. (1995) Proc. NIPR Symp. Antarct. Meteorites, 8, 251-271. [8] Clayton R. N. et al. (1991) GCA, 55, 2317-2337. 65th Annual Meteoritical Society Meeting (2002) 5176.pdf

Dar Al Gani 896: A unique picritic achondrite

Folco, L

Bland, PA

D'Orazio, M

Franchi, IA

Rocchi, S

Spiral - Imperial College Digital Repository

DAR AL GANI 896:  A UNIQUE PICRITIC ACHONDRITE.  L. Folco1, P. A. Bland2, M. D’Orazio3, I. A. Franchi2, and S. Roc-chi3, 1Museo Nazionale Antartide, Via Laterina 8, 53100 Siena, Italy, 2Planetary and Space Sciences Research Institute, Open Uni-versity, Milton Keynes, MK7 6AA, U.K., 3Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy, (folco@unisi.it)  DaG 896 is the provisional name assigned to a dark, 50% fusion crusted, stony fragment of 22.6 g found in the Dar al Gani region of the Libyan Sahara (~27°08’N – 16°05’E) in November 2000. Mineralogy:  DaG 896 is hypocrystalline with porphyritic tex-ture (porphyritic index  ~70). Abundant fine-grained (avg. grain-size 100 µm), skeletal forsterite crystals (avg. Fo82) are set in a ground-mass consisting mainly of rhyolitic glass plus quench microlites of pigeonite (En54 Wo9). Minor mineral constituents include enstatite (En83 Wo2), augite (En37Wo30) and traces of chromite, troilite, Fe-Ni metal and phosphates. Veinlets up to 15 µm thick of secondary car-bonates are most likely due to terrestrial weathering. Mineral mode (vol. %) is forsterite 69, glass 17, clinopyroxene (pigeonite >> augite) 8, enstatite 4, carbonates 1, others 1. Strong undulose extinc-tion and planar fracturing in olivine and mm-thick interconnecting veinlets indicate moderate shock S4, after [1].  Fe/Mn a.p.f.u. ratios for olivine and pyroxenes (35 and 18, respectively) are chondritic [2]. No achondrite with the above texture and mineral composition is documented in literature [e.g., 2]. Similar textures and mineral compositions are in turn observed in some igneous inclusions in ordinary chondrites [e.g. 3]. Bulk chemistry:  XRF and ICP-MS major and trace element concentrations indicate an high-Mg (mg# 79.2) picritic IUGS com-position (SiO2 = 47.62, Na2O = 0.16, K2O = 0.24, MgO = 30.3 wt %) with a chondritic [Mn/Mg] = 7.5 *10-3 ratio. The overall compo-sition is similar to H-chondrites [4] (e.g., Mg/Si = 0.83, Al/Si = 0.073, Ti/Si = 0.006) except for a strong depletion in siderophile and chalcophile elements (e.g., Fe/Si = 0.50, Fe/Ni = 254 and S/Si = 0.009). A close major element compositional match is found with the harzburgitic igneous inclusion in the Y-794046 H5 chondrite [5], [6] and [7] and the komatiite-like lithic fragment in the Eva H4 chondrite [3]. Anomalously high concentrations of Cs, Rb, Sr and Pb suggest some terrestrial contamination. Oxygen isotopes:  Mean data from two replicates are δ17O = 2.55, δ18O = 3.50 and ∆17O = 0.726 ± 0.007 ‰. The ∆17O value is typical for equilibrated H-chondrites [8], although the δ18O ratio is slightly offset to the light side for this grouping.  Conclusions:  i) DaG 896 is a unique picritic achondrite; some  igneous inclusions in ordinary chondrites  are the closest existing analogs. ii)  DaG 896 formed through rapid cooling of a high-temperature melt likely representing the silicate fraction of H-chondritic source material, which underwent severe heating, melting and metal plus sulfide loss. References:  [1] Stöffler D. et al. (1991) GCA, 55, 3845-3867. [2] Papike J. J., ed. (1998) Rev. Mineral., 36. [3] Ruzicka et al. (1998) GCA 62, 1419-1442. [4] Jarosewich E. (1990) Meteoritics 25, 323-337. [5] Nakamura N. et al. (1994) Proc. NIPR Symp. Ant-arct. Meteorites, 7, 125-143. [6] Sack R. O. et al. (1994) JGR, 99, E12, 26029-26044. [7] Mittlefehldt D. W. et al. (1995) Proc. NIPR Symp. Antarct. Meteorites, 8, 251-271. [8] Clayton R. N. et al. (1991) GCA, 55, 2317-2337. 65th Annual Meteoritical Society Meeting (2002) 5176.pdf

Dar al Gani 896:  A unique picritic achondrite.

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Dar Al Gani 896: A unique picritic achondrite

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