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Thermal Metamorphism in Enstatite Chondrites As Fundamental

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Thermal Metamorphism in Enstatite Chondrites As Fundamental 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1016.pdf THERMAL METAMORPHISM IN ENSTATITE CHONDRITES AS A FUNDAMENTAL PROCESS IN THE EVOLUTION OF PLANETARY BODIES: INFORMATION FROM ELEMENTAL DISTRIBUTIONS IN THE MINERAL FRACTIONS. Z.A. Lavrentjeva, Lyul A.Yu. V. I. Vernadsky Institute of Geochemistry and Analytical Chemistry, RAS, Moscow, E-mail: [email protected] Introduction. Enstatite chondrites (ECS) are metamorphism [18]. An understanding of metamorphic thought to have formed in highly reducing environ- reactions is useful because it offers insight into parent ment. This inference is supported by the high body processes and means of identifying the most Mg/(Mg+Fe) of olivine and pyroxene, presence of Si primitive chondrites [17]. Extrapolation of the reac- in Fe,Ni-metal, and occurrence of typically lithophile tions that occur during incipient metamorphism in or- elements, such as Ca, Mg, Mn and K, in sulfide miner- dinary and carbonaceous chondrites [2] to enstatite als in enstatite chondrites [1]. ECS are divided into two chondrites is plausible [6], yet different reactions may main groups, EH and EL, based on high and low abun- be significant in light of the distinct mineralogy (and dances of Fe,Ni metal: both groups show a metamor- hence formation conditions) of enstatite chondrites. phic sequence from type 3 to 6, similar to that ob- Both groups EH and EL enstatite chondrites show a served in ordinary chondrites [2]. Enstatite chondrites metamorphic sequence from type 3 to 6 similar to that are well known to be the most reduced chondrites. observed in ordinary chondrites. In contrast, although a However, primitive ECS contain minor FeO-rich (>3 general sense for metamorphic grade in enstatite chon- wt% FeO) silicates which obviously formed under drites has been determined [3-6], a detailed under- more oxidizing conditions than the bulk of the EC ma- standing of metamorphic petrogenesis remains elusive. terial. Whether these different degrees of oxidation Yet, under the low f (O2) conditions characteristic of correspond to variations in space or in time is still a enstatite chondrites metamorphic reactions bound to matter of debate [8-10]. Mn – Cr isotopic data [11] differ from those in other chondrite groups [7]. Gross- support the contention that enstatite chondrites origi- man and Brearley [2] showed that the mobilization of nate from the inner solar nebula [12]. If so, then the Cr in olivine can be used to identify the least meta- widely recognized reduced nature of ECS [13] implies morphosed members of OC and CC groups. Olivine is that reducing conditions prevailed for some time in the a major mineral in OC and CC and its degree of com- inner solar nebula. However, the presence of FeO-rich positional equilibration is widely used as an indicator silicates in primitive ECS indicates that oxidizing con- metamorphic grade [19]. Distinctive exsolution fea- ditions also must have played a role during early tures in the olivine may also prove useful for determin- petrogenesis of the ECS [8, 9, 14]. ing metamorphic grade and understanding the mobili- Discussion: Texturaly the EL chondrites a parent to zation or Cr from olivine into other phases during met- have experienced much higher levels of metamorphic amorphism [6]. Reduction of olivine in type ECS prob- alteration then EH chondrites of similar equilibration ably occurred both during chondrule formation and temperatures [3]. A reconstruction of the thermal histo- during parent body metamorphism. Absence of olivine ry of brecciated EL6 chondrites [15, 16] proposed that in the equilibrated 4-6 ECS chondrites is likely due to accretion and subsequent thermal metamorphism of the reduction of olivine to form enstatite [20]. Enstatite EL6 parent body caused in to be heated to tempera- chondrites represent initial formation and metamor- tures of ca. 1273 K. Grossman and Brearley [2] phism under highly reduced conditions. Like the other showed that Cr in olivine is remobilized during the chondrite groups, enstatite chondrites underwent vari- carlust stages of thermal metamorphism in O and CO ous degrees of thermal metamorphism resulting in dis- chondrites. During thermal metamorphism Cr redis- tinct petrologic types [19]. However, the petrologic tributed from olivine into metal and sulfide phases that types of enstatite chondrites are not always consistent form features in and around the olivine grains. The with the geothermometry or mineral chemistry indicat- earliest stages of thermal metamorphism generate ing that enstatite chondrites underwent complex ther- exsolution features of metal that are linear within the mal histories [21]. Type 3 chondrites the least meta- olivine grains. Distinctive exsolution features in the morphosed type among chondrite groups. For ordinary olivine may also prove useful for determining meta- chondrites Sears et al. [21] subdivided type 3 into ten morphic grade and understanding the mobilization of finer devisions (type3.0 through 3.9) using thermolu- Cr from olivine into other phases during metamor- minescence (TL) sensitivity as an indicator of meta- phism [6]. In higher petrologic types (≥3, 2), olivine morphic grade subserquently, some mineralogical shows a narrower distribution and lower contents of changes with increasing subtype have been identified Cr, suggesting migration of Cr from olivine during [2]. An approach similar to that of [2] has been applied 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1016.pdf to EH3 chondrites [6], however, a systematic under- standing of metamorphic reactios has not been attained and metamorphic sub-types have not been established for EH3 chondrites. It is important to characterize the lowest end of the metamorphic sequence because type 3 chondrites should represent primitive material and first stage of asteroidal metamorphism. The trend is supported by the chemical composition of pyroxene and troilite compositions: primitive EH3S have high FS content in pyroxene and low Ti content in troilite whereas metamorphosed EH3 has lower F content and S higher Ti content in troilite. It suggests that the reduc- Fig.1. CI chondrite–normalized of trace element abun- tion (lower f (O2)) during the thermal metamorphism dance patterns of nonmagnetic “ultra - and fine - as suggested in [8]. In [22] it is shown that moderately grained” fractions from Pillistfer EL6 enstatite chon- metamorphosed EH3 has lower content of Cr in olivine drite: A – fraction 1<d < 25 µm; B - fraction 25<d < than primitive EH3. This is consistent with the results 45 µm; C – fraction 35<d < 45 µm; D - fraction < 45 from Bendersky et al. [6], suggesting the Cr redistribu- µm; tion from olivine into metal and sulfide phases during References: [1] Weisberg M. K. et al. (2009) LPS the thermal metamorphism of EH3 chondrites. Some XXXX, Abstrat # 1886. [2] Grossman J. and Brearly A. of the difficulties of unraveling metamorphic processes J. (2005) MAPS 40, 87-122. [3] Zhang Y.et al. (1995) of the EH chondrites probably result from mixed ef- JGR. 100, 9417-9438. [4] Hicks T. et al. (2000) LPS fects of nebular reactions thermal metamorphism and XXXI, Abstrat #1491. [5] Kimura M. et. al. (2005) shock. Both chondrite classes (EH and EL) contain MAPS 40, 855 - 868.[6] Bendersky C. et al (2007) LPS members of all metamorphic grades, from unequili- XXXVIII, Abstract #2077. [7] El Goresy A. et al. brated EH3 and EL3 types [24, 25] to highly meta- (1988) Proc. NIPR Symp. Ant. Meteorit., l, 65 -101. [8] morphosed EH6 and EL6, which have suffered the Lusby D. et al.(1987) Proc.LPSC 17, E679-E695. [9] combined action of thermal metamorphism and impact Weisberg M. K. et al. (1994) MAPS 29, 362-373 [10] melting [26, 27]. Sahara 97072, one of the most primi- Fagan T. J. et al. (1999) LPS XXX. #1523. [11] Shu- tive ECS [29], contains fragments of chondrules, shock- kolyukov A. and Lungmair G.W. (1998) LPSC XXIX, melt veins of Fe metal and troilite in eutectoid inter- 1208-1209. [12] Kallemeyn G. W. and Wasson J. T. growths and lumps that appear to be impact-melted. (1986) GCA 50, 2153-2164. [13] Keil K. (1968) JGR. The breccia lump consists of fragments much smaller 73, 6945-6976. [14] Rambaldi E. R. et al. (1984) Na- than the average chondrules and metal-sulfide nodules ture, 311, 138-140. [15] Rubin A. E. (1983) Proc. of Sahara 97072. The fragments are cemented by for- Lunar.Planet. Sci. Conf., 14 th, B 293 [16] Rubin A. E. merly molten metal and sulfide. The formation of the (1984) Earth. Planet. Sci. Lett., 67, 273 [17] Fagan T. lump is evidence of shock-heating event that occurred J. et al. (2008) LPS XXXIX, Abstract #1604. [18] prior to the final accretion or lithification of the mete- Grossman J. (2004) LPS XXXV, Abstract #1320. [19] orite. Lehner et al. [30] interpret Sahara 97072 to be a Van Schmus and Wood (1967) GCA 31, 747-765. [20] primitive breccia. Reconstruction of this metamorphic Weisberg M. K. (2005) LPS XXXVI, Abstract #1420. history requires application of different dating methods [21] Sears D.W. et al. (1983) LPS XIV, 682 – 683. [22] characterized by distinct isotopic closure temperatures Komatsu M. et al. (2011) LPS XXXXII, Abstract [23]. The role of nebular versus planetary processes in #1764. [23] Hopp J. et al. (2013) LPS XXXXIV, Ab- the chemical and textural evolution of enstatite chon- stract #1865. [24] Quirico E. et al. (2011) GCA 75, drites have long been debated. The analysis of the 3088 – 3102. [25] Weisberg M. K. and Kimura M. chemical composition of the obtained data [31] showed (2012) Chem. Erde 72, 101 – 115. [26] Rubin A. et al. that: all “ultra- and fine-grained” fractions (Fig.1) [31] (2009) GCA 73, 1523 – 1537. [27] Rubin A. E. and are enriched in lithophilic Na, Ca, Sc, Cr, La, Sm, Eu, Wasson J.
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