Modal Abundances of Pyroxene, Olivine, and Mesostasis in Nakhlites: Heterogeneity, Variation, and Implications for Nakhlite Emplacement

Meteoritics & Planetary Science 50, Nr 9, 1497–1511 (2015) doi: 10.1111/maps.12492

Modal abundances of pyroxene, olivine, and mesostasis in nakhlites: Heterogeneity, variation, and implications for nakhlite emplacement

Catherine M. CORRIGAN1, Michael A. VELBEL1,2,*, and Edward P. VICENZI3

1Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th St. and Constitution Ave., NW, Washington, District of Columbia 20013-7012, USA 2Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824, USA 3Museum Conservation Institute, Smithsonian Institution, Suitland, Maryland 20746, USA *Corresponding author. E-mail: [email protected] (Received 20 August 2013; revision accepted 14 June 2015)

Abstract–Nakhlites, clinopyroxenite meteorites from Mars, share common crystallization and ejection ages, suggesting that they might have been ejected from the same place on Mars by the same ejection event (impact) and are different samples of the same thick volcanic ﬂow unit or shallow sill. Mean modal abundances and abundance ranges of pyroxene, olivine, and mesostasis vary widely among different thin-sections of an individual nakhlite. Lithologic heterogeneity is the main factor contributing to the observed modal- abundance variations measured in thin-sections prepared from different fragments of the same stone. Two groups of nakhlites are distinguished from one another by which major constituent varies the least and the abundance of that constituent. The group consisting of Nakhla, Lafayette, Governador Valadares, and the Yamato nakhlite pairing group is characterized by low modal mesostasis and pyroxene-olivine covariance, whereas the group consisting of the Miller Range nakhlite pairing group and Northwest Africa 5790 is characterized by low modal olivine and pyroxene-mesostasis covariance. These two groups sample the slowest-cooled interior portion and the chilled margin, respectively, of the nakhlite emplacement body as presently understood, and appear to be also related to recently proposed nakhlite groups independently established using compositional rather than petrographic observations. Phenocryst modal abundances vary with inferred depth in the nakhlite igneous body in a manner consistent with solidiﬁcation of the nakhlite stack from dynamically sorted phenocryst-rich magmatic crystal-liquid mush.

INTRODUCTION nakhlites are individually numbered finds for which pairing relationships have been proposed. Assuming the Nakhlites are clinopyroxenite meteorites from Mars, pairing of the Yamato nakhlites (Imae et al. 2003) is with radiometric crystallization ages of ~1.3 Ga reliable, there is one Yamato nakhlite now represented (McSween 1994, 2008; Treiman 2005; Bouvier et al. in scientific collections as three found stones. Assuming 2009). Nakhlites known at the time of this writing the previously proposed pairing of the four Miller include Nakhla, Lafayette, Governador Valadares, Range nakhlites (Corrigan et al. 2011; Hallis and Yamato (Y) 000593, Y 000749, Y 000802, Northwest Taylor 2011; Udry et al. 2012) is upheld, there is one Africa (NWA) 817, NWA 998, NWA 5790, Miller Miller range nakhlite that is now represented in Range (MIL) 03346, MIL 090030, MIL 090032, and scientific collections as four found stones. Nakhla, the MIL 090136. Nakhla was a shower fall, and is five individual non-Antarctic finds, and the two represented in scientific collections by many individual Antarctic pairing groups (if upheld) represent eight stones (Grady 2000). Lafayette, Governador Valadares, discrete nakhlites. NWA 817, NWA 998, and NWA 5790 are all individual The eight presently known nakhlites may all finds. The Yamato and Miller Range Antarctic represent samples of different parts of the same igneous

1497 Published 2015. This article is a U.S. Government work and is in the public domain in the USA. 1498 C. M. Corrigan et al.

Fig. 1. Modally abundant and intergrown augite (Aug) and olivine (Ol) phenocrysts with modally subordinate mesostasis (Mss) in Nakhla. Backscattered electron microscope image. Fig. 2. Modally sparse and isolated olivine (Ol) and euhedral augite (Aug) phenocrysts with modally abundant mesostasis (Mss) in MIL 090136,26. Note compositional zoning visible as rock unit on Mars (Treiman 2005). Early research on the enhanced Z-(atomic number-) contrast at augite and olivine three first recognized, longer known, and therefore best crystal boundaries in contact with mesostasis. Note also studied nakhlites (Nakhla, Lafayette, and Governador anhedral fractured augite fragments (both ends of double Valadares) (1) reported high modal abundances and arrow, lower right); zoning expressed as Z-contrast variation preferred orientation of clinopyroxene crystals in left fragment indicates fragmentation occurred before Fe- enrichment of augite rims. Backscattered electron microscope suggesting a cumulus origin (Bunch and Reid 1975; image. Berkley et al. 1980); (2) inferred an origin as one or more flows or shallow sills, and a possible analogy with Theo’s Flow, a terrestrial Archean flow with a thick among nakhlites in compositional zoning or pyroxenite interval (Treiman 1986); and (3) observed homogeneity of olivines and pyroxenes, and in the differences in pyroxene and olivine compositional zoning degree of crystallization of feldspars, Fe-Ti oxides, and profiles among them that might be consequences of sulfides in interstitial igneous mesostasis. These differences in their relative distances (depths) from the variations are interpreted to indicate that different cooling surface of a magmatic body (Harvey and nakhlites sample multiple related and generally cogenetic McSween 1992). Ejection (cosmic-ray exposure) ages of flows or sills (Treiman 1986) or different distances from these nakhlites and the next subsequently recognized the cooling surface(s) of a single flow or sill (Harvey and nakhlites (e.g., Yamato 000593/749/802 and NWA 817; McSween 1992; Mikouchi et al. 2003, 2005, 2006, 2012; Mikouchi et al. 2003) are similar (~10–12 Ma; Fritz McKay et al. 2007). Nakhlites with compositionally et al. 2005; Herzog 2005; Eugster et al. 2006; McSween homogenous olivines and pyroxenes occurring as 2008; as of this writing ejection ages have not yet been modally abundant phenocrysts (Fig. 1) and more determined for the most recently recovered nakhlites). abundant feldspars, Fe-Ti oxides, and sulfides This suggests that most or all eight presently known crystallized from interstitial mesostasis cooled slowly in nakhlite meteorites (along with the chassignites, which the interior of the magma body, whereas nakhlites with have similar crystallization and ejection ages as the modally abundant crystal-poor interstitial mesostasis nakhlites, but are outside the scope of this paper) not and compositionally zoned olivine and pyroxene (Fig. 2) only have essentially identical igneous crystallization are interpreted to have cooled quickly near the upper ages but were also ejected together, most likely from the and possibly lower margins of the magma body same source locality on Mars, at a different time than (Mikouchi et al. 2003, 2005, 2006, 2012; Treiman 2005; the ejection of most other Mars meteorites (used here to McKay et al. 2007). The inferred spatial array of mean “meteorites from Mars” rather than “meteorites solidification products of this magma body is referred to found on Mars”; ejection ages from Fritz et al. 2005; as the “nakhlite stack” or the nakhlite cumulate pile Herzog 2005; Eugster et al. 2006; McSween 2008). (Harvey and McSween 1992; Friedman Lentz et al. Studies of subsequently recovered and/or recognized 1999; Mikouchi et al. 2003, 2005, 2006, 2012; Lentz nakhlite finds revealed broad and systematic variations et al. 2011; Udry et al. 2012; McCubbin et al. 2013). Modal abundances of major constituents in nakhlites 1499

Fig. 3. Enlarged lower right portion (Ol ≤ 40 modal vol%, Px Fig. 4. Enlarged lower right portion (Ol ≤ 40 modal vol%, Px ≥40 modal vol%, Mss ≤60 modal vol%) of ternary diagram ≥40 modal vol%, Mss ≤60 modal vol%) of ternary diagram showing all major-constituent (pyroxene, olivine, and showing mean modal abundances of pyroxene, olivine, and mesostasis) modal abundances from Table S1 (n = 54) for all mesostasis in thin-sections of all known nakhlites (n = 8). The eight presently known nakhlites. The four Miller Range four Miller Range nakhlites are plotted as the proposed nakhlites are plotted as the proposed pairing group, with MIL pairing group, with MIL 03346 as a filled circle and the mean 03346 as filled circles (●; 12 individual analyses of 17 thin- of the three MIL 090 nakhlites as an open circle. Lafayette, sections) and the MIL 090 nakhlites as open circles (○; six Governador Valadares, and Nakhla, all inferred to be low in individual analyses of six thin-sections). the nakhlite stack, dominate the low-mesostasis group. The Yamato-00593-00749 pairing group, commonly considered to be from higher in the stack, plots with the low-mesostasis The modally dominant textural constituents in group. The four nakhlites of the proposed MIL nakhlite nakhlites are clinopyroxene, olivine, and mesostasis pairing group plot as two pairs with the highest mesostasis (Figs. 1 and 2; e.g., Treiman 2005). (In contrast to some and lowest olivine abundances of all nakhlites. NWA 817 and other papers on nakhlite petrography, the term NWA 998, commonly regarded as sampling near-extreme “textural constituent” is used rather than phase sensu margins of the nakhlite stack, have intermediate modal abundances. NWA 5790 has by far the most abundant lato, in recognition of the fact that mesostasis is a mesostasis and least abundant pyroxene of any presently petrographic textural constituent with a modal known nakhlite. abundance but not a phase in a thermodynamic sense.) However, recent work has pointed out that variations in measured modal abundances among allocations within representative ranges of petrographic parameters in samples and/or meteorites can be a result of the large nakhlites. size (mm scale) and sparse distribution of poikilitic The extent of petrographic variation among olivine phenocrysts (Hallis and Taylor 2011). different samples of the same stone and among different Heterogeneity of finer grained lithologies can occur due meteorites of the same class needs to be better to the occurrence of crystal clusters (glomerocrysts), understood in support of improving the observational magmatic enclaves, xenoliths, breccia (including basis for assessing possible pairing among multiple foundered roof-crust) clasts, or other texturally distinct discrete stones of the same class (e.g., Corrigan et al. volumes (e.g., Helz 1987; Schwindinger and Anderson 2011; Hallis and Taylor 2011; Udry et al. 2012). The 1989; Friedman Lentz et al. 1999; Hallis and Taylor purposes of this paper are (1) to examine variations in 2011; Lentz et al. 2011; Udry et al. 2012; Helz et al. modal major-constituent abundances among nakhlites 2014). Two nakhlites may seem to be obviously as evident in newly compiled previously published different on the basis of modal parameter values modal abundances determined by numerous research acquired from a single analysis of each, but when teams (Table S1; previously published individual heterogeneity is identified by additional modal analyses analyses for each nakhlite are shown in Fig. 3, means from different fragments of each meteorite, such for all published analyses of each nakhlite are shown in distinctions may blur. When both meteorites are Fig. 4); (2) to draw inferences about the nature and comparably well studied (e.g., comparable numbers of scales of petrographic heterogeneity in nakhlites from individual modal analyses of samples prepared from these variations; and (3) to use the compiled modal different parent fragments), comparisons of means and abundances of the three volumetrically dominant ranges of modal major-constituent abundances may constituents—pyroxene, olivine, and mesostasis—in all restore the ease of characterizing similarities and presently known nakhlites to test hypotheses of cumulus differences. On the basis of such considerations, Udry origin and analogy with Theo’s Flow and other et al. (2012) suggested that modal analyses of multiple ultramafic emplacement bodies (e.g., compound flows or samples of each nakhlite are necessary to establish flow units sensu Walker [1971] and Hill et al. [1995]; or 1500 C. M. Corrigan et al.

0 MIL 090nnn NWA 5790 -2 MIL 03346 NWA 817 -4

-6 Yamato pairing group Governador Valadares Nakhla -8

-10 Ol vs depth

depth (m) -12 Phenocrysts vs depth Px vs depth -14

-16 Lafayette -18 NWA 998

-20 020406080100 Fig. 5. Enlarged lower right portion (Ol ≤ 40 modal vol%, Px modal vol% ≥40 modal vol%, Mss ≤60 modal vol%) of ternary diagram ● showing modal abundances of pyroxene, olivine, and Fig. 6. Modal abundances of phenocryst phases olivine ( ) ▼ mesostasis in the Miller Range pairing group (MIL and pyroxene ( ), and sum of modal phenocryst abundances ○ 03346 = ●; 12 individual or averaged analyses of 17 thin- ( ), against depth in the nakhlite stack (average of two depth sections; MIL 09 nakhlites = ○; six individual analyses of six estimates per nakhlite as determined by Mikouchi et al. 2012), thin-sections) and Nakhla (▼; 15 individual analyses of 13 for all presently known nakhlites. Note gentle D-shaped thin-sections). MIL 03346 has less olivine than most (but not profile (solid curve) shown by pyroxene and total phenocryst all) Nakhla thin-sections, and more mesostasis than any abundances. Nakhla thin-section. Reproducibility of Modal Petrographic Analyses from the Same Thin-Section multiple related and generally cogenetic flows or sills [Treiman 1986]) as explanations for the emplacement Comparisons among published modal abundances process of the nakhlite stack. must consider the possible role of different analytical methods. A number of analytical methods were WITHIN-METEORITE COMPARISONS OF employed in previous studies, the results of which are PUBLISHED MAJOR-CONSTITUENT MODAL summarized in Table S1 and Figs. 3–6. (Not all reports ABUNDANCES IN NAKHLITES of modal abundances were accompanied by description of methods, especially abstracts.) Treiman (1986, 2005), Modal-abundance variations among different Friedman Lentz et al. (1999), and Dyar et al. (2005) allocations of the same nakhlite are shown below to be used light microscopy and established point counting a consequence of spatial variability among allocations methods, counting 1000 or more points on larger areas, and not a consequence of bias in the quantification and maintaining the same relatively uniform point- À method. Modal abundances of olivine, pyroxene, and count densities of 5–9 points mm 2 even on smaller mesostasis in an individual thin-section, stone, or areas. Imae et al. (2003) and Imae and Ikeda (2007) meteorite are reported as OlxPxyMssz (mineral employed the time-honored method of weighing paper abbreviations from Kretz [1983] and Whitney and cutouts of individual constituents from printed images. Evans [2010]; Mss = mesostasis). Variation among Day et al. (2006), Hallis and Taylor (2011), Needham different specimens within an individual thin-section, et al. (2013), and Tomkinson et al. (2015) manually stone, or meteorite is here characterized by the mean, identified constituents in digital images and elemental the range spanned by all values in the group, and the (X-ray) maps and point- or pixel-counted the manually number (N) of measurements. This section considers identified images. Treiman and Irving (2008) spaced petrographic heterogeneity among published nakhlite electron microprobe spots on a grid with 20 lm modes at three spatial scales (1) variation among serial spacing. Udry et al. (2012) acquired monochrome thin-sections cut from the same fragment of the same (grayscale) reflected-light images of polished thin- stone; (2) variation among thin-sections cut from sections and used image analysis software to determine different parts of the same stone; and (3) variation the proportions of pixels with different gray levels. within an entire meteorite (represented in some cases by Reproducibility of major-constituent modal a single stone, in other cases by a shower fall, and in abundances measured from the same thin-section using still other cases, a pairing group). the same petrographic method as documented in the Modal abundances of major constituents in nakhlites 1501 literature is approximately Æ4%. Modal abundances in 090032 (Hallis and Taylor 2011; Udry et al. 2012) show MIL 03346,94 were reported twice by the same research intrameteorite variations of <2% (Table S1); for each team (Stopar et al. 2005, 2013); the preliminary analysis meteorite, both allocations were prepared from the was Ol2Px78Mss19, whereas the refined analysis was same parent (MIL 090030,12 and MIL 090032,12, Ol2.7Px80.6Mss16. Two independent petrographic point- respectively) but different allocations were analyzed by counts of Lafayette thin-section USNM 1505-1 have been different petrographic methods (see above). These published; Treiman (1986) reported Ol10.0Px80.8Mss9.2, numerous occurrences of similarities in modal and Friedman Lentz et al. (1999) reported abundances among nakhlite serial thin-sections suggest Ol9.0Px74.4Mss16.6; modal abundances of pyroxene and that the use of different petrographic methods in mesostasis differed by almost 7% between the two different studies generally appears not to introduce petrographic analyses of the same thin-section. artificial differences in modal abundances among thin- Reproducibility of major-constituent modal sections prepared from the same petrographically abundances measured from the same thin-section using homogenous parent fragment. different methods is similar. Differences of Æ5 modal % are In contrast to the many nakhlites just described for demonstrated from two analyses of Nakhla BMNH which serial thin-sections from the same potted butt are 1913,26 by different methods. Optical petrography yielded homogenous, petrographic variation (heterogeneity) in Ol16.0Px74.6Mss9.4 (Friedman Lentz et al. 1999), whereas nakhlites has been documented to occur in other quantification from energy dispersive X-ray elemental nakhlites at several spatial scales. Some substantial maps yielded Ol9.5Px83.8Mss6.7 (Needham et al. 2013); these petrographic variability occurs even within some groups analyses differed by >6% Ol and >9% Px. of different serial thin-section allocations prepared from The published replicate modal analyses of the same the same parent fragment. Serial thin-sections of thin-sections, in some cases by the same methods, in Nakhla examined petrographically by different workers other cases by different methods, appear to define the (AMNH 3887-1 Ol4.8Px78.7Mss16.5, Treiman 1986; and 6–9 modal % range (midrange Æ5 modal %) as the AMNH 3887-2 Ol8.2Px83.0Mss8.7, Friedman Lentz et al. reproducibility limit below which modal abundances are 1999, respectively) differ in modal mesostasis by 8%. operationally indistinguishable from one another. Serial thin-sections prepared by the same laboratory Variation larger than this range (e.g., a range of modes from the same parent chip (MIL 090136,12) and >10 modal %) is less likely to indicate poor examined by the same workers, MIL 090136,21 and 22 reproducibility and more likely to represent genuine differ in modal pyroxene and mesostasis by 8–12% (as modal differences among analyses. noted by Udry et al. 2012; Ol5.6Px75.2Mss18.1 and Ol9.5Px64.1Mss26.4), respectively. The nearly 12 modal % Modal Petrographic Homogeneity and Heterogeneity in difference in Px abundances between the two published Serial Thin-Sections from the Same Fragment MIL 090136 analyses exceeds the 7–9 modal % Px differences demonstrated from replicate analyses of the Several examples suggest that many nakhlites are same thin-section by the same (Lafayette USNM 1505- homogenous on the scale of the volume (with linear 1) and by different (Nakhla BMNH 1913,26) methods. dimensions typically on the order of 1 cm) of the potted These constituents occur as smaller volumes (pyroxene butts from which serial thin-sections are prepared. crystals and interstitial mesostasis pockets) than the Narrow ranges of modal olivine (Ol), modal pyroxene sparse, large olivines. Hallis and Taylor (2011) (Px), and modal mesostasis (<3%; Table S1) occur in attributed the heterogeneous modal olivine abundance the serial thin-section pairs Nakhla BM 1911,369(1) in MIL 03346 observed in their measurements and and 369(2) (Ol5.7Px85.2Mss9.2 and Ol4.6Px84.4Mss11.0, previous data to the sparse distribution of large size respectively; interestingly, these are respectively the (mm scale) olivine. The heterogeneous pyroxene and highest modal Px and lowest modal Ol abundances mesostasis abundances among serial thin-sections among the 16 thin-section analyses compiled in observed in this study must occur for a different reason. Table S1) and the thin-section pair Governador Needham et al. (2013) recently reported modal Valadares BM 1975,M16 (1) and M16(2) (Friedman abundances from numerous virtual slices acquired by Lentz et al. 1999) (Ol9.7Px81.2Mss9.1 and Ol7.2Px83.1 computer tomography (CT) of a 6 9 5 9 3mm Mss9.6, respectively), consistent with the <2% range fragment of Nakhla. Their overall (volume averaged) from multiple refinements of data from a single thin- modal abundances (Ol9.5Px79.4Mss11.1) and the extrema section (MIL 03346,94; Stopar et al. 2005, 2013) of the representative individual slice modes they present discussed above. This suggests petrographic (Ol 2.6–28.7 vol%; Px 66.2–87.9 vol%; and Mss 5.1– homogeneity within each fragment from which the serial 15.7 vol%) show ranges even more extreme than the thin-sections were prepared. MIL 090030 and MIL extrema of conventional point counting from 1502 C. M. Corrigan et al. petrographic thin-sections for mineral phases (Table S1; discussed above. These observations suggest that a range Ol, 4.6–18.0 vol%; Px 74.1–85.2 vol%) but less of ~15% modal pyroxene variation can be expected in extreme for mesostasis (Mss 4.7–16.5 vol%). The thin-section-scale samples of even a single nakhlite stone. complementarity of the extrema (wider range of modal Similar ranges in modal pyroxene abundance are abundances determined by CT compared with observed in data from Nakhla and MIL 03346 (~11% petrographic thin-sections for Ol and Px, but narrower [N = 16], and ~16% [N = 15], respectively; Table S1). range for Mss determined by CT relative to optical Hallis and Taylor (2011) reported only average modal petrography) may reflect the CT grayscale thresholds abundances from the four MIL 03346 thin-sections chosen for defining minerals and mesostasis; a slightly (from three different parent fragments) they analyzed, different feldspar-pyroxene threshold would reassign CT but observed what they regarded as wide (>3%) ranges voxels among pyroxene and the feldspar constituent of of modal olivine abundances among them, and invoked mesostasis. Tomkinson et al. (2015) found their CT petrographic heterogeneity. data to be similar to previously reported modal abundances of mesostasis in NWA 5790, but higher in Modal Petrographic Heterogeneity in Thin-Sections from pyroxene and lower in olivine, and their data also show Different Stones of the Same Fall a wider range of modal pyroxene abundances in the CT data than in the corresponding range for pyroxene in Ten modal analyses of Nakhla from approximately petrographic modes (Table S1). Further consideration nine individual stones were reported by Friedman Lentz of phase definition in CT voxels is required before more et al. (1999). Friedman Lentz et al. (1999) noted wide detailed quantitative comparison between these two variations in olivine (4.6–18.0%) and pyroxene (74.1– methods is undertaken. In order to compare like with like 85.2%) modal abundances; the latter range (>11%) is (petrographic data with petrographic data), emerging CT similar to the range in the four pyroxene modes from data (Needham et al., 2013; Tomkinson et al. 2015) are the single Lafayette stone. Again, this is larger than the not included in the compilation presented here. However, ~10% range that could be explained exclusively by some inferences about nakhlite heterogeneity and scale limitations on reproducibility as discussed above. observation drawn from CT data from a single 6 9 5 9 3 mm fragment of Nakhla conform with the Modal Petrographic Homogeneity and Heterogeneity in inferences drawn here entirely from petrographic data: Thin-Sections from Different Stones of a Proposed “Olivine abundance varies over an order of magnitude, Pairing Group from less than 3% to almost 30%, in the CT slices in Table S1 (representative slices at fixed 50 slice intervals); Four modal analyses from the three meteorites of the range can be extended further when slices are the Yamato-000593/749/802 pairing group were specifically selected for their high and low-olivine reported by Imae et al. (2003). Modal analyses of two abundances (Fig. 3). Olivine abundance varies between different Yamato-000593 allocations show between- 4.6% and 16% in thin-sections, demonstrating that section, within-stone differences in the modal intrafragment variations can equal or exceed any abundance of mesostasis and of pyroxene of ~8% variation observed between the different Nakhla (Ol8.0Px75.6Mss15.4 and Ol6.9Px84.2Mss8.2, respectively; fragments at the scale of petrologic sections” (Needham Table S1). These ranges are similar to the ~10 Æ 2% et al., 2013, p. 100). ranges in modal pyroxene and mesostasis abundances observed in serial thin-sections cut from MIL 090136,12 Modal Petrographic Homogeneity and Heterogeneity in (Table S1 and discussion above). When all three Thin-Sections from Different Fragments of the Same Yamato nakhlites are considered, the ranges of modal Stone pyroxene, olivine, and mesostasis abundances (~14%, ~12%, and ~8%, respectively) are similar to the ~15% The ranges of modal constituent abundances range in modal pyroxene in the single Lafayette stone, observed among multiple thin-sections from different and the ~11% range in modal pyroxene among multiple parent fragments suggest some heterogeneity at larger- Nakhla stones (Table S1 and discussion above). This than-fragment scales. Modal pyroxene abundance varies similarity of ranges quantitatively supports the previous by ~15% (Table S1) in four different thin-sections all cut suggestion that these three stones are paired (Imae et al. from within ~10 cm of each other along one edge of the 2003). Once again, modal variability observed among single Lafayette stone and all analyzed by the same analyses of the Yamato nakhlites is larger than the workers using the same method (Friedman Lentz et al. ~10% range that could be explained exclusively by 1999). This is larger than the ~10% range that could be limitations on reproducibility as discussed above, and explained exclusively by limitations on reproducibility as suggests heterogeneity. Modal abundances of major constituents in nakhlites 1503

The Miller Range nakhlites share major petrographic MIL nakhlite (2.6–9.1% Ol), and in the unweighted features and major-constituent abundances with one mean of aggregated data for the entire proposed MIL another (Table S1), and differ individually and pairing group (3.7% Ol), are all below ~9%. Mean collectively (although not as sharply as previously modal olivine abundances in the MIL nakhlites are thus suggested; see next section) from most other presently lower than those reported from any other nakhlite known nakhlites in most of the characteristics considered (except NWA 5790), all reports of which have mean here (Table S1; Figs. 3 and 4). On the basis of modal modal olivine abundances equaling or exceeding 9% analyses, previous workers (Corrigan et al. 2011; Hallis (Table S1, Fig. 4). This is consistent with previous and Taylor 2011; Udry et al. 2012) concluded that MIL modal analyses and inferences. However, the large 03346, MIL 090030, MIL 090032, and MIL 090136 are number of measurements now available for the four paired. Ranges of modal mesostasis abundance among members of the proposed MIL pairing group includes the four MIL nakhlites (Table S1) all overlap with one maximum measured olivine abundances from MIL another over large parts of the range, and no single MIL 090030 and MIL 090136 (Table S1, Figs. 3 and 5) that nakhlite is modally representative of the proposed pairing closely approach or overlap the lowest olivine group as a whole. Petrographic heterogeneity among the abundances reported from NWA 817, NWA 998, and MIL nakhlites (Table S1, Fig. 4) is not a function of the Yamato nakhlite pairing group (Table S1, Fig. 3). sample size with the largest samples showing the widest The compilation assembled here shows that, if MIL range of modal abundances as might be expected; 090030,21 (Ol9.8Px66.3Mss23.9; Udry et al. 2012) or MIL instead, the original mass of MIL 090136 (170.98 g; 090136,22 (Ol9.5Px64.1Mss26.4; Udry et al. 2012) had Table S1) is barely one-third that of the next largest MIL been recovered, processed, and analyzed first, MIL nakhlite, MIL 090030 (452.63 g; Table S1), yet this nakhlites would have been indistinguishable from NWA smallest Miller Range nakhlite sample yielded serial 817 (Ol10Px69Mss20; Sautter et al. 2002) and NWA 998 thin-sections with nearly as large a range of modal (Ol10.4Px74.1Mss14.2 and Ol7.9Px81.0Mss9.6; Treiman and pyroxene abundances (11.1%) as exhibited by the entire Irving 2008) on the basis of modal olivine abundance. population of MIL 03346 analyses (15.6%; Table S1; Nevertheless, other MIL nakhlites (MIL 03346 and Fig. 4). Modal variability observed in MIL 090030 is MIL 090032) do have the lowest mean modal olivine larger than the ~10%, suggesting heterogeneity within the abundances of all presently known nakhlites sample. (Ol2.6Px74.0Mss22.9 and Ol3.4Px74.7Mss21.6, respectively; Table S1, Figs. 3 and 4). BETWEEN-METEORITE COMPARISONS OF Furthermore, despite the small differences of mean PUBLISHED MAJOR-CONSTITUENT MODAL olivine abundances between Nakhla, Governador ABUNDANCES IN NAKHLITES Valadares, and Lafayette, wide variations in olivine modal abundances were noted by Friedman Lentz et al. The Importance of Modal Olivine Abundance (1999). More than 40% of the modal olivine abundances reported from these three nakhlites by The compiled modal analyses (Table S1) indicate a Friedman Lentz et al. (1999) are below 10% (Table S1); smaller role for olivine abundance in distinguishing in terms of olivine abundance, even low-olivine nakhlites from one another than was suggested by nakhlites of the proposed MIL pairing group (e.g., MIL previous work. Low olivine abundance in the first 03346) are not distinct from low-olivine allocations of recovered Miller Range nakhlite, MIL 03346, was Nakhla (Fig. 5). This further illustrates the difficulties regarded as a signature distinction from all other inherent in assuming that an individual modal analysis nakhlites known at the time (Dyar et al. 2005; Treiman or small number of analyses is representative of an 2005; Day et al. 2006; Imae and Ikeda 2007). Hallis and entire meteorite (cf. Udry et al. 2012, who inferred this Taylor (2011) ascribed the wide ranges of modal olivine from modal analyses of the four MIL nakhlites). abundances in different MIL 03346 allocations known Nevertheless, despite the overlapping extrema of at the time (0.8–4.6% Ol) to heterogeneous distribution olivine modal abundances among these meteorites, the of the sparsely distributed, typically large (mm scale) means differ appreciably and systematically (Fig. 4) in a olivine. They further concluded that the even somewhat manner broadly consistent with much previous work. higher values (up to 8.6% Ol) they measured in several Nakhlites for which only small numbers of modal MIL 09 allocations did not suggest major variations in analyses have been reported (the NWA nakhlites) have mineral modes among the proposed MIL pairing group. not been sufficiently well characterized in terms of the The data compiled here (Table S1) include more variation in major-constituent modal abundances to recently published data (Udry et al. 2012), and show firmly establish similarities and differences with other that mean modal olivine abundances in each individual more thoroughly studied nakhlites. Fortunately, 1504 C. M. Corrigan et al. similarities and differences in other parameters that Miller Range 09 nakhlites. Similarly, numerous analyses record more subtle differences in accumulation and do not yet exist for each of several other nakhlites, but cooling conditions (e.g., homogeneity of mineral the data available allow the preliminary comparison of compositions, compositional zoning of olivine and modal-abundance ranges and trends among different pyroxene, extent of crystallization of interstitial nakhlites. mesostasis) are easily determined. Mean major-constituent modal abundances for all previously characterized nakhlites are plotted in Fig. 4, Differences in Covariation of Modal Abundances between and suggest possible groupings of nakhlites by similarity Nakhla and MIL 03346 of major-constituent modal abundances. Nakhla, Lafayette, Governador Valadares, and the Y 000593/ In a system with three dominant textural 749/802 pairing group constitute a group with the constituents, an increase in the modal abundance of one lowest mean modal mesostasis abundances (9.0–10.6%) constituent requires a decrease in the sum of the other and the highest mean modal pyroxene abundances two constituents’ modal abundances. There appear to (75.8–80.0% Px; Table S1; Figs. 1 and 4). All other be two groups of nakhlites distinguished from one nakhlites differ from the aforementioned group in another by the identity and abundance of the major having lower mean modal pyroxene and higher mean constituent that varies the least. One line of evidence modal mesostasis (Table S1; Fig. 4), but relationships comes from different trends in major-constituent modal- among them are less clear from mean modal abundance variations in the two nakhlites with the abundances alone. The proposed MIL pairing group largest number of published modal analyses, Nakhla and NWA 817 are similar to one another in mean and MIL 03346. Fifteen individual analyses of 14 thin- modal pyroxene abundance (mean = 72.8% and 69% sections of Nakhla, and 12 individual or averaged Px, respectively), and have (a) lower mean modal modal analyses of 17 thin-sections of MIL 03346, have pyroxene abundances than in the Nakhla-Lafayette- been presented to date. These analyses are all plotted Governador Valadares/Yamato group (compare Figs. 3 together in Fig. 5. MIL 03346 has less olivine than most and 4), and (b) higher mean modal pyroxene (but not all) Nakhla thin-sections, and more mesostasis abundances than the few analyses of NWA 5790 than any Nakhla thin-section. Olivine abundances in (mean = 54% Px, range 51.2–56.0% Px, N = 3). Mean MIL 03346 are nearly uniform within limits modal mesostasis abundance (Table S1, Fig. 4) in the (mean = 2.6%, range 0.8–4.6%, N = 11) and most proposed MIL pairing group is lower than the mean variation among thin-sections occurs in the ratio of modal mesostasis abundance of NWA 5790 but much pyroxene and mesostasis (Fig. 5). In contrast, in Nakhla higher than in Nakhla, Lafayette, Governador the mesostasis abundance is uniform within several Valadares, and the Yamato pairing group, and modal percent (mean = 9.0, range 4.7–16.5, N = 15) and somewhat higher than in NWA 817 and NWA 998. most variation among thin-sections is in the ratio of pyroxene to olivine (Fig. 5). Friedman Lentz et al. Grouping by Covariation of Modal Abundances (1999) observed that Nakhla, Lafayette, and Governador Valadares are all identical in this regard. In Differences in their ranges of modal mesostasis MIL 03346 the modal abundances of pyroxene and variation may distinguish individual nakhlites or groups mesostasis vary inversely (Fig. 5) and olivine abundance from one another. Ranges of individual modal analyses is uniformly low. In Nakhla the modal abundances of (rather than means) reveal that variation in modal pyroxene and olivine vary inversely and mesostasis mesostasis abundance is greater in the proposed MIL abundance is uniformly low (Fig. 5). pairing group than in other nakhlites. The nearly 20% Few nakhlites have been as thoroughly studied for range for MIL mesostasis abundances (mean = 22.7%, modal abundances as Nakhla and MIL 03346, but the range 15.7–35.0%, N = 18) is much larger than the data compiled in Table S1 allow examination of the corresponding ~10% range in mesostasis abundance entire nakhlite population for the two covariation among Nakhla, Lafayette, Governador Valadares, and trends described in the previous paragraph. The next the Yamato pairing group (range 4.7–16.5%, N = 27; paragraphs explore such possible covariation. For Table S1, Figs. 4 and 5). Only MIL 090032 has a comparison with other nakhlites and discussion of corresponding range of <1%; modal mesostasis heterogeneity within a pairing group, the 18 modal abundance varies by over 8% among analyses of MIL analyses from the Miller Range pairing group are 090136 (range 18.1–26.4%, N = 2), and by over 19% in treated as two groups, one consisting of the large MIL 03346 (range 15.7–35.0%, N = 12). The mean number of MIL 033436 analyses, the other consisting of modal abundances for the four low-mesostasis nakhlites a single aggregated data set of analyses from the three (Nakhla, Lafayette, Governador Valadares, and the Modal abundances of major constituents in nakhlites 1505

Yamato pairing group) are distributed along the ~10% of nakhlites based on multiple measures. The nakhlites mesostasis isopleth (Fig. 4) in the same manner as the cluster into very similar groups when grouped by individual analyses from Nakhla (Fig. 5). Friedman modal-abundance variation trends (this paper) as when Lentz et al. (1999) showed the same to be true for sorted by patterns of volatile abundances in apatite Lafayette and Governador Valadares. (McCubbin et al. 2013). The two nakhlite subgroups Nakhla, Lafayette, Governador Valadares, and the defined here from patterns of covariation among major- Yamato pairing group form a group in which (1) constituent modes (CALM, consisting of Nakhla, mesostasis abundance is uniform within several modal Lafayette, Governador Valadares, and the Yamato percent near 10% and (2) most variation among pairing group; and CALO, consisting of the Miller multiple thin-sections of an individual meteorite (e.g., Range nakhlites, and NWA 5790) conform to two of Figs. 3 and 5) and among the meteorites (Figs. 3 and 4) the three nakhlite subgroups defined by McCubbin is in the ratio of pyroxene to olivine (means plotted in et al. (2013) based on volatiles in apatite. Their Fig. 4; individual analyses for Nakhla plotted in Fig. 5; population 2 apatites occur in our CALM group individual analyses for Nakhla and most other nakhlites (Nakhla, Lafayette, and Governador Valadares) in in this group are plotted and show the same trend in which modal mesostasis abundance is uniformly low, Friedman Lentz et al. 1999; their fig. 6). This is the and olivine and pyroxene modal abundances covary “near-constant and low-mesostasis” (CALM) nakhlite inversely; our results suggest the Yamato group (which group. they did not analyze) is part of their population 2 In contrast, because of the range of variation group. Their population 3 apatites occur in our CALO among different analyses of MIL 03346 and MIL group (the Miller Range pairing group and NWA 5790) 090136, the Miller Range nakhlite pairing group has in which modal olivine abundance is uniformly low and olivine abundances that are nearly uniform, and most mesostasis and pyroxene modal abundances covary variation among thin-sections of the same meteorite is inversely; their results suggest NWA 817 also belongs to in the ratio of pyroxene and mesostasis (Figs. 3 and 5). this group. Their population 1 apatites occur only in This “near-constant and low-olivine” (CALO) style of NWA 998, which is not clearly identifiable with either modal-abundance variation distinguishes any individual modal-variation group (Figs. 3 and 4). Future work MIL nakhlite, and the group as a whole, from the must establish whether there exist other compositional CALM group. Emerging modal analyses of NWA 5790 properties that vary between the CALM group with its suggest it resembles the Miller Range pairing group in population 2 apatites and the CALO group with its this attribute. Too few individual modal analyses are population 3 apatites. available for the three presently known NWA nakhlites (particularly NWA 998 and NWA 817) to determine Inferred Depth Relations among Nakhlites whether their major-constituent modal abundances vary in a manner consistent with the CALO or CALM Mikouchi et al. (2012) estimated the depth of each group. However, their distribution relative to one nakhlite within a hypothesized cooling unit from another and relative to the two variation-groups measured profiles of compositional zoning in olivine. suggests NWA 817 is likely to have low variation in Cooling rates and corresponding burial depth were modal olivine abundance and high variation in modal inferred by fitting diffusion-profile models to the mesostasis abundance. If so, then most nakhlites of the observed compositional zoning profiles of major and CALO group vary mainly in pyroxene and mesostasis minor elements in olivine. Absolute depths inferred for with olivine abundances varying much less, around any given nakhlite from Ca diffusion profiles in 10%. compositionally zoned olivine are two to six times greater than the depths inferred from Fe-Mg interdiffusion, but Comparison: Grouping of Nakhlites by Covariation of the relative sequences of inferred burial depths are very Modal Abundances with Grouping by Other Measures similar (Mikouchi et al. 2012, their table 2). Modal abundance of mesostasis appears to be Many of the nakhlites (most NWA nakhlites, and broadly related to inferred depth in the nakhlite stack. the MIL 09 trio) were recognized or recovered so Lafayette, Governador Valadares, and Nakhla, all recently that they are not yet incorporated into inferred to have solidified at slightly different depths comparative studies with the previously recognized deep in the nakhlite stack (Harvey and McSween 1992; nakhlites. One recent paper (McCubbin et al. 2013) Mikouchi et al. 2012), dominate the low-mesostasis examined six of the eight presently known nakhlites group (Fig. 6). The four nakhlites of the proposed MIL (excluding only the Yamato pairing group and NWA nakhlite pairing group and NWA 5790, both inferred to 817), providing the opportunity to compare groupings have cooled rapidly near the top of the nakhlite stack 1506 C. M. Corrigan et al.

(Mikouchi et al. 2012), have the highest mesostasis Valadares), but more homogenous than those in NWA abundances of all presently known nakhlites (Fig. 6). 817 and MIL 03346 (Mikouchi et al. 2006). The widely different ranges of major-constituent modes NWA 817 and NWA 998, commonly regarded as for pyroxene and mesostasis in serial thin-sections (MIL sampling different near-extreme margins of the nakhlite 090136,21 and 22) prepared from the same parent chip stack (Mikouchi et al. 2003, 2005, 2006; McKay et al. of MIL 090136,12 of one of the top-most nakhlites in the 2007), differ from each other in major-constituent modal stack suggest that some parent fragments incorporate abundances, and are intermediate between the CALO sharp contacts or abrupt transitions between texturally and CALM groups (Fig. 4). Previously suggested different lithologies. Different lithologies may be differences between NWA 817 and NWA 998 (Mikouchi juxtaposed due to preservation of accumulated crystal et al. 2006, 2012) are supported by the modal analyses. clusters (Helz 1987; Schwindinger and Anderson 1989; NWA 998 is more modally similar to Nakhla, Lafayette, Friedman Lentz et al. 1999; Vernon 2004; Simura and Governador Valadares, and the Yamato pairing group, Ozawa 2006), discrete magmatic enclaves (Vernon 2004), all of which share narrow ranges of olivine composition, incorporation of xenoliths (Vernon 2004), brecciation than to NWA 817 (Figs. 3 and 4) with its wide range of including rubbly flow-surface breccias and foundered olivine compositions (Mikouchi et al. 2006). Further roof-crust fragments (Moorhouse 1970; Walker 1971; comparisons involving NWA 817 must await further MacKenzie et al. 1982; Lentz et al. 2011; Helz et al. analyses of this meteorite for which only two modal 2014), or other causes. McCubbin et al. (2013) recently analyses have been published to date. proposed that escaping fluids concentrated in the nakhlite In contrast to the modest modal-abundance magma chamber by crystallization of cumulus olivine differences between NWA 817 and NWA 998, NWA and pyroxene drove eruption of magma containing 5790 has far more mesostasis than any previously entrained cumulus minerals from the top of the nakhlite recognized nakhlite (Figs. 3 and 4), extending the pile onto the surface. This would account for a number of parameter space that needs to be considered for properties of nakhlites from near the top of the stack. nakhlites well beyond the range previously considered Turbulent mixing of phenocryst-rich crystal-liquid mush for the nakhlite stack (Jambon et al. 2010; Weisberg during such an eruption may have contributed to the et al. 2010; Mikouchi et al. 2012; Corrigan et al. 2013, observed heterogeneity of modal parameters in 2014; Tomkinson et al. 2015). some serial-section allocations (e.g., MIL 090136,21 and 22) from the proposed Miller Range nakhlite pairing Emplacement of the Nakhlite Stack group. Some previously proposed distinctions and The emplacement mode of the nakhlites has been groupings among nakhlites are not evident in the major- debated for decades. For example, the hypothesis of constituent modal-abundance data compiled in Table S1. nakhlite emplacement as a crystal-liquid mush has been The two trios of nakhlites (one consisting of Nakhla, considered and rejected by some (e.g., Berkley et al. Governador Valadares, and NWA 817; the other of 1980) but embraced by others (e.g., Imae and Ikeda Lafayette, the Yamato nakhlite pairing group, and NWA 2007). Phenocryst modal-abundance versus depth 998) that share properties indicating each trio might be relationships enable testing various hypothetical from a separate flow (Lentz et al. 2005) are not grouped emplacement processes for the nakhlite igneous body. or distinguished on the basis of major-constituent modal Latypov (2003) cites numerous previously published abundances (Fig. 4; Table S1). However, the modal examples of S-, D-, and I-shaped distributions of major-constituent abundances support the uniqueness of phenocryst modal-abundance variations with depth MIL 03346 relative to the nakhlites of the two proposed (while in many cases interpreting them differently than trios as recognized by Lentz et al. (2005). the original authors). Figure 6 shows the modal The Yamato-00593/749/802 pairing group plots abundances of the phenocryst phases olivine and (Fig. 4) in a manner consistent with it being intermediate pyroxene, and the sum of modal phenocryst between the low-mesostasis group commonly regarded as abundances, plotted against depth in the nakhlite stack occurring deep within the pile and the low pyroxene (averaged for each meteorite from the burial depths group commonly considered to be from higher in the estimated by Mikouchi et al. [2012] using the two nakhlite stack (Mikouchi et al. 2003, 2005, 2006, 2012; different diffusing species summarized above), for all Imae et al. 2005; Imae and Ikeda 2007; McKay et al. presently known nakhlites. The compiled data 2007). This intermediate character is consistent with the (Table S1; Fig. 6) do not show the S-shaped phenocryst range of olivine compositions present in the Yamato abundance versus depth profile expected for crystal nakhlites, more varied than olivines in the low- settling under the influence of gravity in the presence of mesostasis group (Nakhla, Lafayette, and Governador solidification fronts (Moore and Evans 1967; Gibb and Modal abundances of major constituents in nakhlites 1507

Henderson 1992; Marsh 1996; Simura and Ozawa 2006, immediately beneath the upper cooling front, and an 2011). Olivine modal abundances show a slight S-shaped phenocryst abundance versus depth monotonic increase with depth (Fig. 6) that resembles distribution (Marsh 1996; Simura and Ozawa 2006), either (1) the I-shaped profile of initially phenocryst-free contrary to the observations from nakhlites. In igneous magmas in which all phenocrysts crystallized and were bodies with S-shaped phenocryst abundance versus rapidly incorporated within the solidification front depth profiles, the tops of cumulus zones are in the before settling (Marsh 1996) or (2) olivine modal- interiors of the bodies, where cooling rates are slow and abundance versus depth profiles reported in some olivine crystals are homogenous and unzoned (e.g., cumulate zones (e.g., Simura and Ozawa 2006). Moore and Evans 1967; Helz et al. 2014). The extensive Pyroxene modal abundances (and, because compositional zoning observed in olivines from the pyroxene is the dominant phenocryst phase, total modal inferred upper part of the nakhlite stack (NWA 5790 phenocryst phases) show D-shaped profiles with depth and the Miller Range nakhlites) is inconsistent with, (Fig. 6). D-shaped profiles in dikes, where largely and thus militates against, the hypothesis that the symmetrical D-shaped lateral variation in mineral nakhlites are the sampled lower zone of a layered (e.g., modes cannot be attributed to gravitational settling, are Theo’s Flow-like) cumulus body for which the upper explained by dynamic sorting (the Bagnold effect; phenocryst-poor zone remains unsampled; such a Bagnold 1954) of a crystal-rich magmatic suspension cumulus layer would have unzoned olivines at the top (“mush”) (Simkin 1967; Gibb 1968; Gibb and of the pyroxenite zone, which is not the case for the Henderson 1992). Komar (1972, 1976) presents brief uppermost nakhlites. On the other hand, dynamic treatments of one quantitative approach, modeling fluid sorting by grain-dispersive pressures (Komar 1972, motion and grain-dispersive pressure to produce across- 1976) in a crystal-rich magma will result in higher dike phenocryst concentration-sorting profiles similar to modal phenocryst abundances with increasing distance those observed by Simkin (1967) and Gibb (1968). from a flow-bounding cooling surface (Simkin 1967; Phenocryst-rich bodies with D-shaped phenocryst Gibb 1968; Gibb and Henderson 1992), as observed in abundance versus depth profiles are produced during the nakhlites (Fig. 6). The combined D-shaped emplacement (Gibb and Henderson 1992) in “flow- phenocryst versus depth relationships (Fig. 6), modal differentiated magma where during ascent the dominance of pyroxene over olivine as the phenocryst phenocrysts have been concentrated in the axial region phase in nakhlites (Figs. 3–6; Table S1), and cooling of the supply conduit and have settled en masse behind rates inferred from compositional zoning of olivine in the leading edge of the ascending magma” (Marsh 1996, the nakhlites (Mikouchi et al. 2012) are inconsistent p. 15). This distribution persists when such phenocryst- with a number of other scenarios reviewed and rich flow-differentiated crystal-liquid mushes move at presented by Latypov (2003). shallower dips, producing D-shaped profiles (vertically The observations (Fig. 6) favor a crystal-liquid asymmetrical in some occurrences) of phenocryst modes mush (consisting of remobilized cumulus magma) over a and cooling in horizontal magmatic bodies such as sills direct cumulus origin for the nakhlites, and support the or flows (Simkin 1967; Gibb 1968; Gibb and Henderson crystallization and emplacement scenario proposed by 1992; Marsh 1996). Phenocryst-depleted border Imae et al. (2005) and Imae and Ikeda (2007). The D- lithologies, especially a possible chilled upper margin, shaped profile of modal phenocryst abundance with remain unsampled by nakhlites, as suggested by Day inferred distance from the cooling surface(s) is et al. (2006). However, the low phenocryst abundance consistent with observations (Simkin 1967; Gibb 1968; in the recently recognized NWA 5790 nakhlite Gibb and Henderson 1992; Marsh 1996) and models (Tomkinson et al. 2015; Figs. 3, 4, and 6) approaches (Komar 1972, 1976) favoring control of phenocryst the level expected for the formerly missing “fast abundances by grain-dispersive pressure in a flowing quenched zone A” upper interval of the nakhlite stack crystal-liquid mush. Vigorous grain–grain interactions inferred to exist by Day et al. (2006, their fig. 12). are suggested by anhedral augite fragments in The crystal-liquid mush hypothesis as stated here, mesostasis-rich, phenocryst-poor nakhlites (Fig. 2). and observations and models of the modal phenocryst Compositional zoning expressed as Z-contrast variation abundance versus depth profile produced by vertically in fragmented augite (Fig. 2) indicates that augite converging solidification fronts in a flow or sill (e.g., fragmentation occurred before late Fe-enrichment of Marsh 1996; Simura and Ozawa 2006), both predict augite rims, possibly during magma migration prior to that some phenocrysts will be present in an upper the rapid postemplacement cooling stage during which chilled margin of a D-shaped phenocryst versus depth the Fe-enriched augite rims formed. The observed profile (Fig. 6). However, crystal settling would produce preferred orientation of clinopyroxene crystals, long both depletion of phenocrysts from the zone taken as evidence of crystal settling and compaction 1508 C. M. Corrigan et al. during a cumulus origin of the nakhlites (Berkley et al. phenocryst abundances and complementary low modal 1980; Friedman Lentz et al. 1999), is equally consistent mesostasis abundances previously interpreted as evidence with flow-produced alignment of elongate crystals in a of a cumulus origin and compaction of the nakhlites moving crystal-liquid mush. (Berkley et al. 1980; Friedman Lentz et al. 1999). The high modal phenocryst abundances of nakhlites Finally, although several sites on Mars have been are often invoked as evidence against emplacement as proposed as the source craters from which the nakhlites mush (e.g., Day et al. 2006), on the grounds that they were ejected (e.g., Harvey and Hamilton 2005), Zumba exceed the threshold value (>55%) above which the crater on the southern flanks of Arsia Mons, proposed as magma behaves as a rigid network of crystals with the nakhlite source crater by Tornabene et al. (2006), is interstitial liquid (Marsh 1996). However, the observed in a region of elongate volcanic flow landforms with lava high modal phenocryst abundances previously channels and lobate (possibly breakout) margins. The interpreted as evidence of a cumulus origin and evidence favoring a crystal-liquid mush precursor of the compaction of the nakhlites (Berkley et al. 1980; nakhlites refines the scenario proposed by Imae et al. Friedman Lentz et al. 1999) need not have existed at the (2005) and Imae and Ikeda (2007) by specifying the time of mush flow. Using the higher Z (atomic number)- processes that occurred (1) during the migration of contrast of Fe-rich rims on pyroxenes to distinguish the crystal-liquid mush from the cumulus zone after the overgrowth rims from cores, Imae et al. (2003, 2005) accumulation and concentration of cumulus minerals and Imae and Ikeda (2007) determined that, while total early in the crystallization sequence and (2) before and modal pyroxene abundances in the Yamato pairing during the extrusion or emplacement of the crystal-liquid group and MIL 03346 were 77.0 and 67.7 vol% mush as a flow unit within which nakhlite crystallization (respectively), the modal abundances of pyroxene cores went to completion as interstitial melt was expressed. (total minus overgrowth rims) in the same two nakhlites Modal phenocryst abundances in a hypothetical were 61.1 and 56.5 vol%, respectively. Given that the future recovery or recognition of one or more nakhlites range of modal pyroxene abundance in a given nakhlite more rapidly cooled than NWA 5790 would provide can be constrained at best to within a range of 10–15 further insight into the distribution of phenocrysts at modal vol% (see discussion above of Figs. 3–6and the time of migration and emplacement of the nakhlite Table S1), the modal abundances (~55–60%) of magma. It is possible that an aphyric nakhlite consisting pyroxene cores in the Yamato and MIL 03346 nakhlites entirely of chilled mesostasis as predicted for the top of are indistinguishable from the upper limit of the range of the nakhlite stack by Day et al. (2006) may eventually crystal abundances (25–55 vol%) in liquid within which be recognized. Such a discovery would militate against the crystal-liquid mixture behaves as a viscous fluid the hypothesis presented here, which predicts that even mush (Marsh 1996). Thus, the widely reported modal the most rapidly cooled nakhlite should contain some total pyroxene and olivine phenocryst abundances for precooling (mush) phenocrysts even if their abundance nakhlites, regarded as being prohibitively large for has been depleted from the magma margin by dynamic magma movement as mush (e.g., Day et al. 2006), in sorting. Until such a meteorite is actually described, fact include overgrowth material that likely solidified evidence for this line of refutation of the mush only after the mush movement ceased, and therefore hypothesis does not yet exist. overestimate the volume fraction of solid present at the At the presently understood depth resolution of the time the mush flowed. available nakhlites and their modal analyses, a single Furthermore, the modal abundance of mesostasis in emplacement event (such as that proposed by Friedman nakhlites may represent less than the full inventory of Lentz et al. 1999; and Lentz et al. 2011) is sufficient to liquid present between crystals during mush flow. A account for the observed depth profiles of major- mush consisting of crystals (not yet enlarged by Fe-rich constituent modal abundances in the inferred nakhlite overgrowths) in a larger amount of liquid than remains stack. However, Gibb and Henderson (1992) hypothesize represented by the mesostasis may have lost liquid but that the commonality of D-shaped profiles with local left crystals behind if emplaced as, for example, an irregularities (observed over depth intervals smaller than inflated flow. Removal of interstitial liquid (possibly the inferred depth resolution of the presently available related to the “missing component” inferred by Day nakhlites) is a consequence of emplacement of bodies et al. 2006) as breakout lobes at the flow margins (e.g., with D-shaped phenocryst profiles by multiple pulses of Hallis and Taylor 2011) would leave behind close-packed crystal-rich magma mushes (e.g., in an inflated flow, crystals and interstitial liquid that constitutes only a compound flow, or flow unit consisting of multiple fraction of the liquid present during flow. Such related and generally cogenetic flows, or a shallow sill; postemplacement presolidification depletion of Walker 1971; Treiman 1986; Hill et al. 1995), with later interstitial liquid could account for the high modal and more phenocryst-rich mushes intruded between Modal abundances of major constituents in nakhlites 1509 cooler and more viscous liquids from earlier pulses partly analyses of multiple samples of each nakhlite are cooled at the body’s margins, or between two converging necessary to establish representative ranges of solidification fronts. The CALM group (Nakhla, petrographic parameters in order to characterize Lafayette, Governador Valadares, and the Yamato possible genetic relationships among nakhlites (e.g., pairing group) constitute the entire central part of the D- relative position in the nakhlite stack). shaped phenocryst abundance versus depth profile and Existing modal analyses suggest the existence of two part of the bottom margin of the profile (Fig. 6), whereas nakhlite groups distinguished from one another by the the CALO group (the Miller Range pairing group and identity and abundance of the major constituent that NWA 5790) constitute the uppermost part of the varies the least. The “near-constant and low-mesostasis” phenocryst depth profile sampled to date by presently group consists of Nakhla, Lafayette, Governador known Mars meteorites (Fig. 6). Following the Valadares, and the Yamato pairing group; in this group, hypothesis proposed by Gibb and Henderson (1992) for most of the variation among thin-sections is in their explaining discontinuities in modal abundances in similar ratios of pyroxene to olivine, and mesostasis abundance igneous bodies, the two groups of nakhlites distinguished is near 10%. The “near-constant and low-olivine” group here on petrographic evidence may represent the consists of the Miller Range pairing group and NWA solidification products of two pulse-intruded magma 5790; in each of these meteorites, most of the variation mushes, with higher modal phenocryst abundances (and among thin-sections of any specific meteorite is mainly possibly other differences not readily discerned in the ratio of pyroxene to mesostasis, and olivine petrographically) in the later emplaced more central abundance is uniform and low relative to the other region. As noted above, the two groups of nakhlites group. These two groups, defined by modal abundances identified here are similar to groupings already revealed of major constituents, appear to be related to recently by McCubbin et al. (2013). Further comparisons of these proposed nakhlite groups independently established two groups will shed additional light on the matter of using compositional rather than petrographic how closely their parent magmas were related. Future observations. Profiles of phenocryst abundance with work examining other measurable properties of nakhlites inferred depth in the nakhlite stack are consistent with beyond petrographic modal abundances may also solidification of the nakhlite stack from dynamically establish whether one emplacement of crystal-liquid sorted phenocryst-rich magmatic crystal-liquid mush. mush is sufficient, or multiple pulses of crystal-liquid- mush intrusion are required, to account for Acknowledgements—We thank Andrew R. Konicek for compositional variations among the nakhlites. helpful comments on earlier drafts of this work, and Rosalind T. Helz, Tony Irving, Thomas A. Vogel, SUMMARY Tyrone O. Rooney, Susan Krans, Brandon Chiasera, Ben Brugman, Matt Karl, Kaitlyn Trestrail, and Brian Modal abundances of major phases vary among Wade for helpful conversation during preparation of different allocations of the same nakhlite stone, with later versions. Comments by Editor Tim Jull, Associate multiple modal analyses of different allocations from Editor Akira Yamaguchi, reviewer Lydia Hallis, and an some stones spanning wider ranges of values than other anonymous reviewer were very helpful and are greatly stones. Serially cut individual polished thin-sections appreciated. M. Velbel is most grateful to his prepared from the same parent fragment are commonly Smithsonian coauthors for hosting his visit as a identical within a few percent modal abundance for Smithsonian Senior Fellow at the Division of major constituents regardless of the different methods Meteorites, Department of Mineral Sciences, National used to determine the modal abundances. However, Museum of Natural History, Smithsonian Institution, there are exceptions to the commonly observed within- during which visit this paper was written. parent homogeneity; in MIL 090136, even successively numbered serial thin-sections from the same parent Editorial Handling—Dr. Akira Yamaguchi differ in modal major-constituent abundances by up to 12%. Such large variations, occurring similarly among REFERENCES reported modal abundances for different allocations of the same stone, exceed the range of modal abundances Bagnold R. A. 1954. 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SUPPORTING INFORMATION Table S1: Compilation of published modal pyroxene, olivine, and mesostasis abundances in nakhlites; Additional supporting information may be found in arranged as nakhlite stack, top down. the online version of this article: