-Shocked Norite 78235: Primary Textures and Shock Features, A.El ~oresv'),W.v.~ngelhardt'), J. ~rndt'), D. Van~liers2 . 1) : Yax Planck Tnstitut fiir Kernnhvsik, Heidelherg, Germany 2) : Vineralogisches Institut, IJniversi t3t Tuebinpen, Germany, In a recent paner Sclar and Bauer (1975) suggest that in the shocked norite 78235 a four-phase assemblage: metallic iron + chromite + diopside + silica was generated from hronzite by shock-igduced transient heating to subsolidus temperatures UP to 1400 C, resulting in breakdown of the bronzite and reduc- tion of its Fe+2 to Feo. Studies of the thin section 78235,43 and several sections from 78235,34 in transmitted and reflected light and microprobe analyses lead us to different conclusions, Primary Textures. The rock 78235 consists of almost equal amounts of up to 3 mm large g~ainsof plagioclase and orthopyroxene with subophitic texture. Vajor onaque phases are very few irrepu- lar patches of cobaltian Fe-Ni of primarv origin which exclusive- ly occur in the orthopyroxene. Another assemblage of opaaue mine- rals is decribed below. All sections display a layering of ortho- pyroxene and plagioclase, tvvical of cumulus textures. Numerous micronrobe analyses of the coexistin~phases revealed (in agree- ment with McCallum et a1.,1975) that no compositional zonine exists in orthopyroxene and plagioclase. Compositional variation from grain to grain is verv narrow. Plagioclase has the average composition Ang3,s Ab5,4 Oro,6, orthopvroxene En7g.8 Fs17.5 Wo2.7 (Table 1). Shock Features. All sam~lesshow evidence of severe shock. The orthonvroxene is heavilv hrecciated and disnlavs mosaicism. undulatorv extinction and shock lamellae. The plagioclase ihows different grades of shock metamorphism, indicatinp an inhomo- geneous distribution of shock pressures and post-shock tempera- tures inthe rock, with hiqhest temueratures in plagioclase close to the boundaries against pvroxene: Vost plagioclase grains con- tain still birefringent cores with planar features which grade into dianlectic glass (maskelynite), surrounded along the grain boundaries against pyroxene by a narrow rim of melted plagio- clase glass. Birefringent cores and diaplectic glass contain numerous minute rods of metallic iron which are absent in the melt glass, due to dissolution of the metal in the fluid melt. Crystalline plagioclase, diaplectic and melt glass have identical compositions (Table I), but differ in refractive indices (dia- plectic glass: n = 1.56f12; melted glass: n = 1.5668). The fact that - despite the lower melting temperature of pyroxene - only pla~ioclasehas been shock-melted is in accordance with theoretical and experimental results of Ahrens et al. (1975)

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and Gibbons et al. (1975) showin? that shock-melting of orthopv- roxene needs higher shock pressures than melting of anorthite. Coating and Veins of Brown Glass. The rock is coated with a laver of hrown elass which also penetrates the rock along cracks. We inter~retthis glass as a auenched s~lashof shock melt from the same event which produced the shock effects in the minerals of the rock. The comnosition of the glass (Table 1) corresponds with 50% nlagioclase and 50% orthonvroxene to that of the total norite as it was already stated bv VcCallum et al. (1975). The glass is loaded with metallic spherules of various sizes, many of them occurring in schlieren and displving eutectic textures of FeNi, troilite and schreibersite. Along the boundarv of the brown glass and the shocked norite orthopyroxene displavs exsolution of opaque and semiopaaue grains giving the bronzite a cloudy appearance, Close to the glass boundary an extensive recrvstallisation of Dvroxene took place wherebv the opaque ~hasescollected along the grain boundaries of the recrystallized Dyroxene. In this narrow zone rutile with subordinate ilmenite were observed and the majority of the Dyroxene recrystallisate has the average comnosition EngO,g Fs8.8 Wo0,3g in comparison to the primary pyroxene depleted in Al2O3, ~nband Ti02 (Table 1). Very small grains of clino- pyroxene (too small for microprobe analysis) were also encoun- tered. These observations indicate that bv thermal treatment by the melt splash orthopvroxene broke down to enstatite + rutile + ilmenite + clinopvroxene. In contrast to the non-devitrified plagioclase glass the brown glass is partiallv devitrified, especially in patches devoid of metallic spherules. Where the hrown glass penetrates the rocks it can be observed that some incinient mixing took place between the brown melt and the melted plagioclase. This indicates that very probablv both melts were liauid at the same time and cooled at about the same rate. The incomnlete mixinp of the melts and the fact that only the brown class devitrified might be due to the much hiqher viscosity of the ~laqioclasemelt. According to own experiments with a glass fragment from this rock the coolinp rate of the anorthite qlass must have been higher than 0.3°/min, Assemblage Chromite - Troilite - Clinopyroxene - Silica. Within shocked Dyroxenes in the rock and along grain boundaries between pvroxene and plagioclase a few opaque inclusions occur, consisting of the four-phase assemblage chromite-troilite- ctino~yroxene-silica. In several of these patches tranquilli- tyite was positively identified. The following observations are at variance with the hypothesis of Sclar and Bauer (1975) that the four-phase assemblage was produced hy a shock-induced

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El Goresv et al.

breakdown of pyroxene and support the assumption of McCallum et al. (1975) that it is verv prohablv a late stage mesostasis: (1) Occurrence of tranquillitvite, a typical late staqe crvstalli- sation product. (2) Chromite as a member of a late stage mesosta- sis is quite uniaue to this rock (McCallum et al, 1975). However, chromite constitutes 20 - 30% by volume of this assemblage which is far to much accordinc to the low Cr203 content of the ortho- pyroxene (up to 0.62). (3) Troilite and not metallic iron is a consistent member of the assemblage. If the troilite was formed by sulfurization of metallic iron, originating from the break- down of pyroxene, why were the big patches of primary iron in the pvroxene not sulfurized? Table 1 Average chemical compositions 1 2 3 4 5 Si02 45.1 43.9 52.9 56.5 94.2 Ti02 0.04 n.d 0.3 0.14 0.14 2'3 35.3 35.7 0.80 0.08 18.70 - 0.04 - n.d. '2'3 - - 0.57 Cr203 - 0.56 n.d. FeO 0.17 0.06 12.1 6.34 7.73 Yn0 - n.d 0.32 0.03 n.d. *SO 0.10 n.d. 30.9 36.8 14.83 CaO 19,OO 18.23 1.49 0.24 10.20 Na 20 0.61 0.55 0.03 - 0.26 K2° 0.13 0.16 - - 0.02 100,45 95.6 99.45 100.69 101 .R 1: Plagioclase, 2: Iliaplectic plagioclase glass (maskelynite). 3: Orthopyroxene (bronzite). 4: Recrystallized pyroxene (enstatite) 5: Brown glass (50% Ang5.3Ab4,50ro.3 + 50% En76.9F~lo.8W00.3) References: Ahrens, Th.%J. and O'Keefe, J.D. 1972, The Moon 4, p, 214-249; Ahrens Th,J, et al. 1973, Proc, Lunar Sci.Conf. 3rd, p. 2575-2590, Gibbons, R.V. et al. 1975, Lunar Sci. VI, p, 284-286, McCallum, I.S. et al. 1975, Lunar Sci. VI, p. 534- 536. Sclar, C,B. and Rauer, .J.F. 1975, Lunar Sci. VI, p. 730 - 732.

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