Most asteroidal igneous rocks are eucrite-like basalts and gabbros, composed mostly of ferroan low- and high-Ca pyroxenes and calcic plagioclase, plus smaller amounts of silica (most commonly tridymite), ilmenite, chromite, troilite, Ca-phosphate, metal and sometimes ferroan olivine. Eucrite-like mafic rocks are fragments of the crusts of differentiated asteroids, and most are likely from 4 Vesta

Barrett, T. J.

Berger, E. L.

Le, L

Mittlefehldt, D. W.

Peng, Z. X.

Ross, D. K

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsFine-Scale Variations in Eucritic Pyroxene FeO/MnO:Process vs. Provenance.Conference or Workshop ItemHow to cite:Mittlefehldt, D. W.; Barrett, T. J.; Le, L; Peng, Z. X.; Berger, E. L. and Ross, D. K (2018). Fine-ScaleVariations in Eucritic Pyroxene FeO/MnO: Process vs. Provenance. In: 49th Lunar and Planetary Science Conference(LPSC2018), 19-23 Mar 2018, The Woodlands, Houston, Texas, USA.For guidance on citations see FAQs.c© 2018 The AuthorsVersion: Version of RecordLink(s) to article on publisher’s website:https://www.hou.usra.edu/meetings/lpsc2018/pdf/2700.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 FINE-SCALE VARIATIONS IN EUCRITIC PYROXENE FEO/MNO: PROCESS VS. PROVENANCE.  D. W. Mittlefehldt1, T. J. Barrett2, L. Le3, Z. X. Peng3,4, E. L. Berger5 and D. K. Ross3,6, 1NASA Johnson Space Center, Houston TX, USA (david.w.mittlefehldt@nasa.gov), 2The Open University, Milton Keynes, UK, 3Jacobs JETS-NASA Johnson Space Center, Houston TX, USA, 4California State University, Northridge CA, USA, 5GCS-Jacobs JETS-NASA Johnson Space Center, Houston TX USA, 6UTEP-CASSMAR, El Paso TX, USA.   Introduction:  Most asteroidal igneous rocks are eucrite-like basalts and gabbros, composed mostly of ferroan low- and high-Ca pyroxenes and calcic plagio-clase, plus smaller amounts of silica (most commonly tridymite), ilmenite, chromite, troilite, Ca-phosphate, metal and sometimes ferroan olivine. Eucrite-like mafic rocks are fragments of the crusts of differentiat-ed asteroids, and most are likely from 4 Vesta [1]. Oxygen isotopic compositions of meteorites are important indicators of genetic relationships [2]. Eu-crite-like basalt Northwest Africa (NWA) 011 is anomalous in O isotopic composition compared to other eucrites and is thought to be from different aster-oid [3]. Current analytical methods allow smaller O isotopic differences to be resolved [4, 5]; for example, Ibitira differs from the eucritic 17O mean by 0.171 ‰, 20 times the uncertainty of the mean (Fig. 1). Some eucrite-like mafic rocks have even smaller O-isotopic anomalies (Fig. 1). Here, “normal” eucrites are indis-tinguishable from the group mean O isotopic composi-tion; “anomalous” eucrites are distinguishable. -0.25 -0.20 -0.15 -0.100510NWA 011: 17O = -1.8 ‰excluding labeled meteorites:17O = -0.2400 ± 0.0087 ‰ (n = 60)PCA 91007,PasamontePecora Escarpment(PCA) 82502EmmavilleA-881394number of meteorites17O (‰)IbitiraBunburra Rockhole,Elephant Moraine(EET) 92023 Figure 1. O-isotopic compositions of eucrite-like mafic meteorites; Open University data from [5-10]. The FeO/MnO ratios of planetary basalts can be used to fingerprint the source body. This ratio is rela-tively invariant during most igneous processes and is inherited from its source region with little modification [11]. Low-Ca pyroxenes in Ibitira and NWA 011 have FeO/MnO ratios that are resolvable from those of most eucritic-like basalts, supporting formation on different asteroids [3, 12]. The efficacy of the FeO/MnO ratio as a diagnostic of source body was demonstrated through comparison of basalts from the Earth, Moon, Mars, Vesta and the angrite parent asteroid, where large dif-ferences in pyroxene and/or olivine FeO/MnO are pre-sent [11]. Modifications imposed by igneous processes are small secondary effects in this comparison. Our analytical protocol allows precision of better than 2% (1σ standard deviation) on FeO/MnO to be achieved for homogeneous eucritic pyroxenes [12]. Our data have resolvable differences in pyroxene FeO/MnO for some anomalous eucrite-like mafic rocks, but not all [8, 10, 13]. For example, Emmaville differs from the eucritic 17O mean by 0.087 ‰, roughly 10 times the uncertainty of the mean (Fig. 1), but its pyroxene FeO/MnO is indistinguishable from those of normal eucrites [8]. Further, some normal, eucrite-like mafic rocks have distinct pyroxene FeO/MnO ratios, whilst others show internal variations in FeO/MnO [14]. Here we begin to address the utility and limitations of using FeO/MnO ratios as source-body vs. formational process indicators when differ-ences in this ratio are small. FeO/MnO:  Divalent Fe and Mn are homologous species; their identical charge and similar ionic radius (78 vs. 83 pm; octahedral coordination) allow them to substitute for each other in mineral structures with similar levels of distortion to the crystal structure. They have similar mineral/melt partition coefficients for ferromagnesian silicates [15]. For the degrees of fractionation  typical of basaltic suites, little change to magma FeO/MnO is incurred. Eucrite-like basalts were formed at an oxygen fugacity near the iron-wüstite buffer, and had variable, but poorly document-ed, S contents [1, 15]. Small differences in fO2 or fS2 could lead to resolvable differences in pyroxene FeO/MnO as Fe, but not Mn, is partitioned between the silicate, metal and sulfide phases. Redox and sili-cate-sulfide interactions leave clear petrological signa-tures that allow us to distinguish secondary, parent-body processes, from primary differences due to prov-enance. Two examples: high-S magmas result in loss of FeO to FeS coupled with systematically decreasing pyroxene FeO/MnO with crystallization (normal eu-crite Queen Alexandra Range (QUE) 94484 [16]); subsolidus redox results in pyroxenes with very low FeO/MnO (18.9 ± 0.4) and high modal abundances of Fe metal and silica (normal eucrite Elephant Moraine (EET) 87542 [14]). Eucritic Pyroxenes:  Eucrite-like basalts crystal-lized pigeonite on their liquidi, which evolved to au-2700.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) gite with fractionation [15]. The majority of a eucrites’ Fe and Mn are in pyroxene. Comparison of pyroxene FeO/MnO can, therefore, be a robust proxy for bulk-rock FeO/MnO. The large Ca2+ cation only occupies the pyroxene M2 site, whilst Fe2+ and Mn2+ preferen-tially fill the M2 site, with the filling order of the latter being Mn2+ > Fe2+. The slightly smaller radius of Fe2+ indicates that the filling order for the M1 site is Fe2+ > Mn2+ [17]. Thus, Fe2+ and Mn2+ partition differently in low-Ca and high-Ca pyroxenes. Most eucrite-like mafic rocks underwent variable degrees of thermal metamorphism engendering exsolution of high-Ca pyroxene from pigeonite, resulting in augite or diop-side lamellae in a low-Ca pyroxene host for the most metamorphosed rocks [18]. This can lead to fractiona-tion of FeO/MnO between the phases (e.g., Asuka (A-) 881394, Fig. 2). We compare only low-Ca pyroxene compositions in order to minimize problems associated with FeO/MnO partitioning between pyroxene types. 0 10 20 30 4026283032343638 A-881394 GRO 06059 Ibitira PCA 91007FeO/MnO (molar)Wo (mole%)  Figure 2. FeO/MnO vs. Wo for pyroxene analyses of cumulate gabbro A-881394, and basalts Grosvenor Mountains (GRO) 06059, Ibitira and Pecora Escarp-ment (PCA) 91007. Fe/Mn = 31.8Fe/Mn = 34.9200 µmFe/Mn x‐ray maplow highFe/Mn X‐ray intensity ratio Figure 3. Variation in FeO/MnO within a single grain fragment in GRO 06059. Resolvable differences in low-Ca pyroxene FeO/MnO can occur within a given eucrite-like mafic rock. We have found that the majority of low-Ca py-roxene grains in the normal basaltic eucrite GRO 06059 have FeO/MnO of 31.5 ± 0.3, within the range of normal eucrites, whilst a subset of analyses has FeO/MnO of 34.9 ± 0.4, in between those of anoma-lous basalts PCA 91007 (33.3 ± 0.5) and Ibitira (36.3 ± 0.4) (Fig. 2) [10, 12]. The different pyroxene FeO/MnO ratios can be found within individual grain fragments. Elemental mapping shows that the differ-ences are caused by irregular zoning patches in the pyroxenes (Fig. 3). These high FeO/MnO zones are more magnesian (mg# 39.0 ± 0.3 vs. 37.6 ± 0.4), less calcic (Fig. 2), have higher Al (0.0118 ± 0.0016 vs. 0.0089 ± 0.0015 apfu), lower Mn (0.0336 ± 0.0003 vs. 0.0371 ± 0.0007 apfu), but are indistinguishable in Fe. Discussion:  We have identified several cases where variations in pyroxene FeO/MnO within normal eucrites is equivalent to, or larger than, the difference between normal and anomalous eucrites such as PCA 91007 (Fig. 2) [10]. In all such cases, detailed petro-logical study allows us to identify the cause and assess the likelihood that a given eucrite-like mafic rock is distinct from normal eucrites in FeO/MnO. Neverthe-less, there are issues that require consideration. For example, high-Ca pyroxene lamellae in A-881394 have much lower FeO/MnO than the low-Ca pyroxene hosts (Fig. 2). The primary pyroxene would have had an FeO/MnO intermediate between these, making firm conclusions regarding provenance from comparisons with eucrite-like rocks that are less metamorphosed somewhat uncertain. Key Point:  Precise, low-Ca pyroxene FeO/MnO ratios can be used to distinguish eucrite-like mafic rocks from different source asteroids even when varia-tions in this ratio are small as long as petrologic obser-vations are used to understand any variations caused by parent asteroid processes. References: [1] Mittlefehldt D. W. (2015) Chem. Erde Geochem., 75, 155. [2] Clayton R. N. et al. (1976) Earth. Planet. Sci. Lett., 30, 10. [3] Yamaguchi A. et al. (2002) Science, 296, 334. [4] Wiechert U. H. et al. (2004) Earth Planet. Sci. Lett., 221, 373. [5] Greenwood R. C. et al. (2005) Nature, 435, 916. [6] Greenwood R. C. et al. (2017) Chem. Erde Geo-chem., 77, 1. [7] Scott E. R. D. et al. (2009) Geochim Cosmochim. Acta, 73, 5835. [8] Barrett T. J. et al. (2017) Meteoritics & Planet. Sci., 52, 656. [9] Bland P. A. et al. (2009) Science, 325, 1525. [10] Mittlefehldt D. W. et al. (2016) LPS XLVII, Ab-stract #1240. [11] Papike J. J. (2003) Am. Mineral., 88, 469. [12] Mittlefehldt D. W. (2005) Meteoritics & Planet. Sci., 40, 665. [13] Mittlefehldt D. W. et al. (2017) LPS XLVIII, Abstract #1194. [14] Mittlefehldt D. W. et al. (2015) LPS XLVI, Abstract #1933. [15] Stolper E. (1977) Geochim. Cosmochim. Acta, 41, 586. [16] Mittlefehldt D. W. et al. (2015) 78th Met. Soc., Abstract #5342. [17] Cameron M. and Papike J. J. (1981) Am. Mineral., 66, 1. [18] Takeda H. and Graham A. L. (1991) Meteoritics, 26, 129. 2700.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)

Fine-Scale Variations in Eucritic Pyroxene FeO/MnO: Process vs. Provenance.

Barrett, Thomas

The University of Manchester - Institutional Repository

Fine-Scale Variations in Eucritic Pyroxene FeO/MnO: Process vs. Provenance.

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