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Genealogy of Iva Iron and Pallasite Meteorites: the Implications for Planetesimal Differentiation Processes in the Early Solar System

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Genealogy of Iva Iron and Pallasite Meteorites: the Implications for Planetesimal Differentiation Processes in the Early Solar System 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1780.pdf GENEALOGY OF IVA IRON AND PALLASITE METEORITES: THE IMPLICATIONS FOR PLANETESIMAL DIFFERENTIATION PROCESSES IN THE EARLY SOLAR SYSTEM. M. E. Sanborn1, Q.-Z. Yin1, and K. Ziegler, 1Department of Earth and Planetary Sciences, University of California-Davis, Davis, CA 95616 (E-mail: [email protected]), 2Institute of Meteoritics, University of New Mexico, Albuquerque, NM Introduction: Melting and differentiation of as- differentiation on planetesimals in the early Solar Sys- teroids in the emerging early Solar System are two of tem. the key processes in the evolution of planetesimals into Analytical Methods: Four meteorites were inves- the planetary bodies we have today. Information into tigated in this study: two pallasites (Brenham and Mil- surface and near surface magmatic processes during ton) and two IVA iron meteorites (Steinbach and São differentiation are provided to us by stony achondrite João Nepomuceno). For the IVA iron meteorites and meteorites. However, insight into differentiation pro- the pallasite Milton, silicate material was removed cesses at greater depths requires a different set of plan- manually and then leached in 6N HCl to remove any etary materials, stony-iron and iron meteorites, provid- adhered metal. After leaching, the silicate materials ed through collisional breakup of differentiated plane- from each sample were placed in individual Parr tesimals. bombs and heated in a 3:1 mixture of HF:HNO3 at In most cases, the parent body bulk composition of 190°C for 96 h. For Brenham, a single large chromite stony-iron and iron meteorites are not known. There- grain was isolated from the metal matrix. A subsample fore, in order to fully understand differentiation pro- of this chromite was placed directly into a Parr bomb cesses occurring in the early Solar System, it is neces- using the same procedure as the silicate fractions. sary to reconstruct the genealogy of the iron and stony- Chromium was separated from the dissolved sam- iron meteorites and their relationships and origins with ples using a 3-column chemistry procedure described other planetary materials in hand. By doing so, it is by [7]. The Cr isotopic composition was measured possible to link evidence of deep differentiation pro- using a Thermo Triton Plus thermal ionization mass cesses (core formation and cooling, mantle crystalliza- spectrometer at the University of California, Davis. A tion) with processes recorded in the crust of the same total of 12 µg of Cr was analyzed for each sample by planetesimals. Alternatively, the stony-iron and iron loading onto outgassed W filaments (3 µg per fila- meteorites may be originating from distinct geochemi- ment). Details on the mass spectrometry procedures cal reservoirs for which no stony meteorite counter- can be found in [8]. Every sample was bracketed by parts are known. Distinguishing between these scenar- measurement of the NIST SRM 979 Cr standard. All ios is important for their use in modelling and inter- the Cr isotope ratios are reported as parts per 10,000 preting planetary differentiation and evolution. deviation (e-notation) from the measured Cr standard. Potential connections between at least some stony- Results and Discussion: The Cr isotopic composi- iron and iron meteorite groups and stony meteorite tion of each of the four meteorites analyzed in this counterparts have been proposed. Such potential con- study are shown plotted in Figure 1 in Cr-O isotope nections include IVA iron meteorites with ordinary space. chondrites [1], the anomalous Eagle Station pallasite The anomalous pallasite Milton has a positive with CV chondrites [2], and IIIAB irons and main e54Cr, similar to the values seen among carbonaceous group pallasites (MPG) [3]. The connection of stony- chondrite, specifically plotting near the CV-type chon- iron and iron meteorites to chondritic counterparts has drites. Milton is not an isolated achondrite in this re- strong implications for planetesimal evolution. Their gion of Cr-O space with several stony meteorites: genetic relationship would provide support for various Northwest Africa (NWA) NWA 1839 (CO7, originally differentiation models, such as the “onion-shell” struc- L7), NWA 3133 (CV7 achondrite), and NWA 10503 ture proposed for the ordinary chondrite parent bodies (CV metachondrite) having similar Cr and O [9,10]. [4,5] or partially differentiated carbonaceous chondrite As such, Milton could potentially be sampling the parent bodies [6]. core-mantle boundary of a partially differentiated CV- Here, we focus on investigating what geochemical type object for which these stony meteorites are shal- reservoir within the solar nebula generated the IVA lower components. This parent body would be distinct iron meteorites and pallasites (main group pallasites - from the one for Eagle Station based on their different MGP and anomalous ones) using Cr isotopes. Using Cr and O compositions. Eagle Station, which contrary this information, we further look at the proposed to the suggestion as being similar to CV-type chon- petrogenetic linkages to stony meteorite counterparts drites [2], actually plots closer to CK- and CO-type and what implications this may have in how we view chondrites (Fig. 1). As with Milton, there is another 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1780.pdf example of a stony meteorite, NWA 8186 (CK7), that as opposed to thermally metamorphosed type-6 chon- plots in the same region of Cr-O space as Eagle Station drite comprising the interior. Recently, physical evi- and the CO- and CK-type chondrites [11,12]. If Milton dence for large scale break-up of the L-chondrite par- and Eagle Station were two distinct objects, then the ent body has been reported based on massive meteorite complete disruption of these early differentiating bod- fluxes during the Ordovician [20,21] and observed L ies in the carbonaceous chondrite forming region of the chondrite fall with lower intercept isotopic disturbance nebula was not a rare occurrence. at ~470 Ma [22]. This indicates that large scale plane- In contrast to the anomalous pallasites, the MGP tesimal-wide disruption was occurring in the region of Brenham has a negative e54Cr, similar to the only other the nebula where the ordinary chondrite parent bodies reported MGP, Krasnojarsk, and the howardites- formed. Such a breakup event could provide the eucrites-diogenites (HEDs) [13,14]. However, the D17O mechanism to excavate the IVA irons from the interior. for all MGP including Brenham plots >2s above the The ubiquity of melting and differentiation of plan- normal HED mean [15, 16]. This may indicate the etesimals in the early Solar System is evident from the MGP originated from a Vesta-like object with similar chucks of basalts as a result of surface eruptions to the Cr, yet distinct O isotopic composition to the normal range of stony-iron and iron meteorites of deeper inte- HEDs points to separate parent body. riors. Now with the new Cr isotopic finger printing 2.00 approach (cosmic forensics, so to speak), we can relate SJN Steinbach 1.00 these diverse melt products to specific parent body OCs Mars GRO 95551 Angrites Brenham (SNC) Grouplet composition that have been sampled by other primitive 0.00 CI (Orgueil) Earth, ECs, and Aubrites Vesta planetary materials in hand (revealing planetary gene- (HEDs) -1.00 Acapulcoite- CR (Renazzo) alogy), thus allowing us to begin to assemble a more Ureilites lodranites robust, full picture of the geochemical evolution of -2.00 CB (Bencubbin) O (‰) CM (Murchison) 17 asteroids after their initial accretion. As such, our view 0.00 CM (Sutter’s Mill) - 3.00 Milton NWA 6074 - 0.05 Ibitira NWA 3133 of planetary melting and differentiation processes can CV (Allende) NWA 10503 - 0.10 Asuka 881394 Krasnojarsk EET 92023 NWA 1839 -4.00 - 0.15 Bunburra Rockhole NWA 7822 now be further expanded to both carbonaceous and O (‰) Brenham NWA 8186 17 Eagle Station - 0.20 Moama Pasamonte PCA 91007 CO (Lance) non-carbonaceous regions of the solar nebula. -5.00 - 0.25 CK (Karoonda) NWA 1240 - 0.30 - 0.90 - 0.70 - 0.50 - 0.30 - 0.10 Acknowledgements: We thank Tim McCoy and 54Cr -6.00 the Smithsonian for the two IVA samples, Institute of -1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 2.00 54Cr Meteoritics at UNM for Milton silicate, and the ASU Fig. 1. D17O-e54Cr diagram of Milton (anomalous pallasite), Center for Meteorite Studies for Brenham chromite. Brenham (MGP), São João Nepomuceno (SJN) and Stein- References: [1] Ruzicka A. and Hutson M. (2006) bach (both IVA irons) with other meteorite groups. Inset MAPS, 41, 1959-1987. [2] Shukolyukov A. and Lugmair G. shows a zoomed in view of the HED region with Brenham W. (2006) EPSL, 250, 200-213. [3] Franchi I. A. (2013) (red), Krasnojarsk (blue), normal eucrites (black squares), MetSoc76, A5326. [4] Göpel C. et al. (1994) EPSL, 121, and anomalous eucrites (grey squares). Literature data are 153-171. [5] Trieloff M. et al. (2003) Nature, 422, 502-506. from [9,10,13,14,18] and references therein. [6] Elkins-Tanton L. T. et al. (2011) EPSL, 305, 1-10. [7] The previous suggestion for IVA irons being simi- Yamakawa A. et al. (2009) Anal Chem, 81, 9787-9794. [8] lar to ordinary chondrites was based on the similarity Sanborn M. E. and Yin Q.-Z. (2014) LPS XLV, A2018. [9] in their oxygen isotopic composition [1]. The e54Cr Sanborn M. E. et al. (2015) LPS XLVI, A2259. [10] Irving A. determined for the silicate inclusions from Steinbach J. et al. (2016) 79th MetSoc Meeting, A6461. [11] Srinivasan and São Joao Nepomuceno plot within the range pre- P. et al. (2015) LPS XLVI, A1472. [12] Yin Q.-Z. and viously determined for ordinary chondrites as well.
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