Abstract– Two categories of symplectites have been observed in howardites: three-phase, composed of vermicular intergrowths of ferroan augite, fayalitic olivine, and silica, and two-phase, composed of vermicular intergrowths of orthopyroxene and troilite. Three-phase symplectites have been previously shown to represent the breakdown products of metastable pyroxene. In howardites, they appear to be genetically related to gabbroic eucrites. In some cases and under yet-to-be specified conditions, ferroan clinopyroxene in gabbroic eucrites may undergo only localized decomposition resulting in oriented exsolution-like features. Breakdown phases in those cases are fayalitic olivine, silica, and—depending on the MgO content of the system—orthopyroxene. As opposed to three-phase symplectites, two-phase symplectites are most likely of diogenitic origin. They probably formed via impact-induced localized melting of diogenitic orthopyroxene in the presence of troilite (grain boundary melting). Three-phase symplectites in howardites occasionally contain accessory amounts of ilmenite, troilite, and/or kamacite and are exclusively associated with medium-grained FeO-rich pyroxene, silica, and plagioclase. All minerals involved are late-stage crystallites or mesostasis phases. In general, highly evolved eucritic lithologies constitute only a minor fraction of howardites. However, considering that three-phase symplectites are generated in a low-pressure, i.e., near-surface, environment, FeO- and CaO-rich eucritic rocks may be exposed locally on Vesta’s surface. This, in turn, is highly relevant to the ongoing DAWN mission.
Howardites are polymict achondritic fragmental breccias composed of eucrite and diogenite debris (Mittlefehldt et al. 1998). Eucrites include basaltic and cumulate lithologies as well as monomict and polymict breccias and constitute surface and subsurface rocks (e.g., BVSP, Basaltic Volcanism Study Project 1981; Mayne et al. 2009). In addition to the basaltic and cumulate subtypes, a third, intermediate subcategory can be distinguished (“gabbroic eucrites”; Patzer and McSween, unpublished data). Diogenites, on the other hand, were generated in deeper crustal to upper mantle settings and include ultramafic orthopyroxenites, harzburgites, and, less commonly, dunites (Beck and McSween 2010). Together, howardites, eucrites, and diogenites are known as the HED suite that is most likely derived from the differentiated asteroid 4 Vesta (e.g., Binzel et al. 1997; DeSanctis et al. 2012). Vesta is the third largest and second most massive asteroid in the main belt measuring 525 km in diameter (Russell et al. 2012). Its surface is covered by large quantities of howarditic material (DeSanctis et al. 2012). In addition to eucritic and diogenitic debris, howardites also contain a range of minor xenoliths (e.g., different carbonaceous chondrite materials), impact breccias, impact melt clasts, and varying amounts of impact-injected troilite and kamacite.
In the context of NASA’s current Dawn mission to explore Vesta’s surface (Russell et al. 2004, 2006), we initiated a study looking into rather uncommon but potentially significant lithologies represented in howardites. The general goal of our survey was twofold: (1) pinpointing minor compositional features of howardites to help interpret Dawn data, (2) exploring out-of-the-ordinary but indigenous clasts as to their possible relevance for Vesta’s magmatic evolution. Our attention was soon directed at clasts featuring late-stage mineral assemblages including symplectites—a lithology that has been described before in two eucrites (Buchanan et al. 2000, 2005) and an HED impact melt (Barrat et al. 2003), but not in howardites. Although minor in terms of quantity, this distinct type of mineral assemblage potentially reveals crucial information about HED genesis.
Late-stage mineral assemblages in general are products of fractional crystallization and constitute the endmost phases to precipitate from a cooling magma. Symplectites are defined as the fine- to very-fine-grained intergrowths of two or more phases displaying vermicular or viscous textures. Such intergrowths result from the precipitation of immiscible melts or the decomposition of metastable phases, which in turn may emerge from highly fractionated magmas. Symplectic assemblages have been reported from both terrestrial and extraterrestrial samples. Accounts from extraterrestrial materials so far have been limited to lunar and Martian rocks (e.g., Ware and Lovering 1970; Rubin et al. 2000; Aramovich et al. 2002; Anand et al. 2003; Fagan et al. 2003; Liu et al. 2009), the Acapulco meteorite (El Goresy et al. 2005), a single clast in the polymict eucrite Macibini (Buchanan et al. 2000), the monomict eucrite breccia Y-82202 (Buchanan et al. 2005), and the HED impact melt rock NWA 1240 (Barrat et al. 2003).
Typically, lunar or Martian symplectites include ferroan clinopyroxene, fayalitic olivine, and a SiO2-rich phase. Most of them have been interpreted to be breakdown products of metastable pyroxferroite upon cooling under low pressure (Rost et al. 2009). In some instances, direct late-stage crystallization from a Fe-rich melt has been considered (Fagan et al. 2003, 2008). Among HED meteorites, the monomict eucrite Y-82202 and a single clast in the polymict eucrite Macibini (Clast A) have been found to contain three-phase symplectite assemblages (Buchanan et al. 2000, 2005). Yamato-82202 is made of fine- to medium-grained eucritic plagioclase and pyroxene and displays unusual, impact-induced glass veins. Clast A from Macibini is a mm-sized eucrite fragment with unequilibrated composition and subophitic texture. In both cases, symplectites consist of iron-rich augite (Mg# 32), fayalitic olivine (Fa78–90), and silica and emerge along the edges of zoned pyroxene (Mg# 70-20). The only other occurrence of symplectites in HED meteorites has been reported for a find from Northwest Africa (NWA 1240). The sample is an HED impact melt and contains pigeonite crystals that exhibit iron-rich rims (Barrat et al. 2003). Symplectic intergrowths of iron-rich augite, ferrosilite, troilite, ilmenite, plagioclase, and silica sometimes formed within these rims. As opposed to all lunar, Martian, and HED occurrences, Acapulco—a primitive achondrite—displays rare vermicular intergrowths of orthopyroxene and troilite (El Goresy et al. 2005). These two-phase symplectites were probably produced through quenching of a silicate-sulfide partial melt.
Samples were examined by applying optical microscopy and electron microprobe analysis. Optical parameters studied include textural, weathering, and shock features as well as the general occurrence and abundance of opaque minerals. The chemical composition of individual phases was determined by means of a Cameca SX-100 electron probe microanalyzer at the University of Tennessee. Operating conditions were: 1–5 μm spot size, 15 kV accelerating voltage, 10–15 nA sample current, and 20 s count time. For element calibration, the following natural and synthetic standards were used: diopside (Si, Ca), spinel (Mg, Al), hematite (Fe), albite (Na), orthoclase (K), spessartite (Mn), rutile (Ti), sphalerite (S), metallic chromium (Cr), metallic nickel (Ni), and fluor-apatite (P). All data reported here are based only on analyses that returned wt%-totals between 98.5 and 101.0 and stoichiometric cation totals ±0.1. Backscattered electron (BSE) images were taken at 15 kV and 15 nA.
We studied six thin sections of five Antarctic howardites. Classifications and general properties have been addressed in the Antarctic Meteorite Newsletter (AMN). Detailed information and chemical compositions of mentionable lithologies other than late-stage mineral assemblages detected in 10 new howardites including the samples listed below will be published in a separate report.
The most intriguing and informative meteorite of our suite bearing in-situ symplectic mineral assemblages, meaning clasts containing symplectites within (some of) their original geologic setting, is LaPaz Ice Field (LAP) 04838. We studied sections #19 and #27. The sample consists of two distinct lithologies: gabbroic eucrite and howardite. The gabbroic portion is highly fractured and displays numerous pockets of incipient melting or decomposition. Major phases are medium-grained lath-shaped plagioclase and medium-grained pyroxene. Mesostasis occurs in abundant, porous-looking patches and includes inclusion-riddled silica, minor ilmenite, chromite, and troilite. Another minor phase is fayalitic olivine which percolates through pyroxene grains and appears as a fine-grained interstitial phase. Its distribution is heterogeneous, ranging from 0 to about 10 area% in a given 1 × 1 cm frame. The howardite fraction of the sample is a highly diverse breccia. Among others, we found several mm-sized subophitic basalt fragments, an astonishing spectrum of symplectic and symplectite-bearing clasts, impactites, and secondary breccias (breccia-in-breccia).
The lithological variety displayed in Miller Range (MIL) 05085 (sections #2 and #11) is very similar to the howardite part of LAP 04838. Observed components include basalts with various textures, diogenite fragments, secondary breccias, impact-produced rocks, glass spherules, and symplectic clasts. All clasts are set in a poorly sorted matrix of comminuted pyroxene and plagioclase. Opaque phases comprise accessory ilmenite, chromite, Fe-Ni metal, and troilite. The meteorite’s polymict and relatively loose texture in conjunction with the observed lithological diversity including melt spherules suggest a regolith origin.
Miller Range (MIL) 07007,8 contains a large number of sub-mm and mm-sized monomineralic fragments. Most of them are orthopyroxene. Large olivine fragments occur sporadically. Plagioclase has only been detected as μm-sized debris (groundmass). Polymineralic clasts are rare and include a handful of sub-mm-sized clasts with late-stage assemblages. Among the accessory opaque minerals, we found predominant chromite, some sulfide, little ilmenite, and very little Fe-Ni metal. In terms of shock features, cataclastic fabrics, shock twinning, and mosaicism are omnipresent.
This meteorite was recovered from the Pecora Escarpment (PCA) in 2002 (section #14). The rock is relatively porous and of predominantly cataclastic texture. Large areas feature a poorly recrystallized matrix with rounded olivine, lathy plagioclase, and interstitial pyroxene. Locally and in minor volume, the typical assemblage of comminuted pyroxene and plagioclase can be found. The list of identified clasts includes debris from basaltic and cumulate eucrites, secondary breccias, some late-stage assemblages, and two mm-sized monomineralic fragments (olivine and feldspar, respectively). Many mineral components exhibit mosaicism, zoning, patchy exsolution, and clouding. In addition, the metal/sulfide content clearly lies above average. Several large patches of Fe-Ni metal, ranging from 200 to 500 μm in size, and many smaller interstitial grains of Fe-Ni metal and sulfide are present. Terrestrial weathering led to macroscopically visible limonitic staining. Based on the observed lithological properties, PCA 02066 appears to have been generated in a large impact where eucrite and diogenite material was mixed with partial melt and injected with liquid metal/sulfide.
Queen Alexandra Range (QUE) 94200,16 consists of sub-mm to mm-sized orthopyroxene clasts set in a matrix of comminuted plagioclase and pyroxene. Impact breccia and melt clasts constitute a minor ingredient (see also Mittlefehldt and Lindstrom 1998). In addition, a single mm-sized cumulate eucrite fragment was found. Opaque minerals include small fragments of mostly chromite and occasional ilmenite as well as accessory troilite and kamacite. Both, sulfide and Fe-Ni metal predominantly occur as specks in impact breccia or melt clasts and as inclusions in pyroxene and olivine. Discrete grains of troilite or kamacite in the matrix are very rare.
LAP 04838 contains an above-average quantity of clasts displaying symplectic textures or other properties affiliated to late-stage precipitation. In addition, the array of textures in those clasts is remarkably broad. We observed the classic form of symplectite with vermicular and sometimes partially viscous appearance, sublamellar inclusions, and oriented stubby lamellae (Fig. 1). Involved minerals include ferroan augite, fayalitic olivine, and silica as well as accessory ilmenite, troilite, and/or kamacite.
While most of the smaller symplectic clasts reveal nothing or little of their original geological setting, some of the larger (>200 μm) fragments contain symplectites in primary contact with medium-grained pyroxene, silica, and/or plagioclase (Figs. 2 and 3). Furthermore, single vermicular fragments are not only embedded in the rock’s matrix but also occur as debris within secondary breccias: One of the impact melt breccias sampled in this meteorite encloses a small (75 μm), embayed symplectic grain. Finally, a large clast of feldspar features a peculiar set of trapped late-stage minerals (Fig. 4). As distinct from the instances illustrated in Figs. 1–3, phases present in pockets of vermicular intergrowths are troilite and silica.
Miller Range 05085,2 contains two larger symplectic clasts—approximately 600 and 250 μm, respectively—as well as a few (unstudied) smaller fragments with grain sizes below the given analytical limit. The two larger clasts are compositionally similar: main constituents are ferroan augite, fayalitic olivine, and silica. However, textural aspects vary.
The 600 μm fragment features a fine-grained symplectic center bordered by several larger, inclusion-rich grains (pyroxene, plagioclase, and silica; Fig. 5a). In terms of proportions, augite and olivine predominate the symplectic area (approximately 70 area%). Augite forms slightly larger grains (up to 75 μm) while olivine and silica exhibit a more vermicular intergrowth (grain sizes up to 50 μm). A single 45 μm grain of ilmenite and two <10 μm grains of plagioclase are present as well. Elemental concentrations of symplectic augite and olivine yield little variation. The large pyroxene grain on the left edge of the clast, however, is clearly zoned; it grows more Mg- and Ca-rich toward the symplectic center. In addition, it displays small oriented, sublamellar inclusions of silica, minor fayalitic olivine, and occasional troilite. These inclusions are reminiscent of features observed in LAP 04838 (Fig. 1d). Other large grains include anorthitic plagioclase and silica. The latter is riddled with troilite inclusions and also hosts two small grains (<10 μm) of apatite.
The 250 μm symplectic clast in MIL 05085,2 is very fine-grained (grain sizes ≤10 μm) with roughly equal proportions of vermicular ferroan augite, fayalitic olivine, and silica (Fig. 5b). Elemental abundances for both, augite and olivine appear very homogeneous. Compared with the 600 μm fragment, they are slightly more FeO-rich. Neighboring phases were not sampled.
In addition to the symplectic fragments, MIL 05085,2 carries an augite clast showing sublamellar inclusions of silica and fayalitic olivine (Fig. 6a). Textural details are comparable to those detected in LAP 04838 (Fig. 1d). Furthermore, compositions of associated plagioclase in both meteorites are similar. Other mineralogical and chemical aspects, however, differ: Host pyroxene and olivine inclusions in the clast from MIL 05085 are more MgO-rich; kamacite is absent; and ilmenite occurs as accessory phase.
In MIL 05085,11, we found a 350 μm fragment of pyroxene exhibiting stubby, multidirectional lamellae (Fig. 6b) very similar to those in LAP 04838 (Fig. 1b). General mineralogical attributes coincide as well. Like in LAP 04838, host pyroxene in this clast from MIL 05085,11 is ferroan augite. Olivine lamellae are homogeneous and fayalitic. They are rimmed by or closely associated with silica. As distinct from LAP 04838, however, pyroxene and olivine are more FeO-rich (see Figs. 1b and 6b). In addition, a neighboring pyroxene grain was sampled as well. This grain contains yet more FeO but significantly less CaO.
Miller Range 07007,8 hosts a limited number of small, but diverse clasts featuring vermicular or late-stage mineral assemblages. Most notably, we detected a 170 μm pocket of intimately intertwined orthopyroxene and troilite set in medium-grained orthopyroxene (Fig. 7a). A different fragment shows a well-rounded contour and is composed of typical mesostasis phases including major silica and troilite as well as minor ilmenite, plagioclase, and apatite (Fig. 7b). Additional occurrences comprise a 470 μm-sized heterogeneous augite clast displaying sublamellar inclusions of orthopyroxene and olivine (Fig. 7c) and a texturally similar, 200 μm-sized augite fragment with elongated inclusions of fayalitic olivine and small patches of silica, pigeonite, and ilmenite. The clast in Fig. 7c also contains a few small (<15 μm) inclusions of iron metal (98.5 wt% Fe, 0.2 wt% Co, no significant Ni).
In PCA 02066,14, several sub-mm-sized clasts of clinopyroxene containing irregular patches of silica, subhedral ilmenite, and small blebs of troilite and plagioclase were found (Fig. 8a). A second type of late-stage assemblage is represented by a single fragment composed of fine-grained silica, augite, troilite, and kamacite (Fig. 8b).
Queen Alexandra Range (QUE) 94200,16 principally exhibits two types of late-stage assemblages. The first category consists of fine-grained troilite (and occasional Fe-Ni metal) and silica emerging as compound inclusions in clinopyroxene (Fig. 9a). Inclusion shapes vary from patchy or elongated to crisscrossing thin veins. The second group comprises fragments of vermicular troilite and orthopyroxene occurring either as individual grains (Fig. 9b), as patches set in medium-grained orthopyroxene, or as patches associated with medium-grained olivine (Fig. 9c).
To gain additional insight into the primary geological context of symplectic clasts, we examined for zoning two medium-grained fragments containing symplectites. In Fig. 3, spot analyses revealed that the MgO content of host augite close to the symplectic intergrowth changes from a relatively low level (En7–8) to clearly higher proportions (En30) at the far end of the clast. Concomitantly, the CaO concentrations drop from Wo39 to Wo22. The ferroan augite on the opposite side of the medium silica grain is also very MgO-poor (En5). Elemental mapping of the respective clast (“example 1”) effectively visualizes the distribution and sharp zoning of Ca, Mg, and Fe (Fig. 10).
The second clast selected for elemental mapping is located in MIL 05085 (“example 2,”Fig. 11). Like example 1, the medium-grained pyroxene grows more Ca-rich toward the symplectic area. Unlike example 1, however, Ca zoning proves to be significantly more gradational. Also, Fe and Mg follow exactly opposite trends from what is seen in the first example. Even though restricted to a relatively narrow band along the common border, Fe is now less and Mg more abundant in close proximity to the symplectite.
For all symplectites involving pyroxene, olivine, and silica, the pyroxene phase is ferroan augite with individual compositions covering a relatively broad range of CaO and FeO concentrations (Table 1, Fig. 12). Associated olivine proves to be significantly more homogeneous. Fayalite contents vary only from 83 to 94 mol% (Mg# 6–17). Symplectic silica typically contains an appreciable amount of FeO (up to 2.2 wt%). Quite distinct from these three-phase symplectites are vermicular intergrowths consisting of troilite and orthopyroxene. Detected contents of ferrosilite in pyroxene span the delimited spectrum known from diogenites (Beck and McSween 2010).
Table 1. Average compositions of pyroxene and olivine in symplectic and related phases.
All quantities in mol%. Abbreviations: px = pyroxene, ol = olivine, En = enstatite, Fs = ferrosillite, Wo = wollastonite. * Fragments I + II = symplectites, fragment III = partially symplectic pyroxene, fragment IV = exsolved pyroxene similar to Fig. 7c.
Three-Phase Symplectites Related to the Breakdown of Metastable Pyroxene
Many of the observed symplectic assemblages are texturally and compositionally similar to the three-phase symplectites reported for the Los Angeles Martian meteorite (Rubin et al. 2000). They consist of ferroan augite, fayalitic olivine, and silica (probably tridymite: Rost et al. 2009). Ware and Lovering (1970)—referring to findings from lunar rocks at that point—were the first to interpret these three-phase symplectites as breakdown products of metastable pyroxferroite (see also Lindsley et al. 1972). Almost four decades later, Rost et al. (2009) applied sensitive ToF-SIMS techniques to accurately determine the chemical composition of the involved, μm- to sub-μm-sized phases and test various hypotheses as to their formation. They confirmed former results and concluded that pyroxferroite decomposition in Los Angeles occurred under low pressure (i.e., shallow placement of the igneous parent rock) and rapid cooling (probably below 990 °C in less than 3 days [Lindsley et al. 1972]). Pyroxferroite precipitation took place under nonequilibrium conditions. The metastable phase crystallized after the host pyroxene was formed. The host pyroxene exhibits a so-called forbidden composition and achieved stable crystal configuration through exsolution into augite and pigeonite on a nanometer scale (Rost et al. 2009).
Buchanan et al. (2000) offered a generally similar explanation for their observations in Clast A of Macibini. They, too, concluded that rapid cooling caused the decomposition of metastable pyroxene and generation of three-phase symplectites. In detail and based on bulk compositional data, however, Buchanan et al. (2000) pointed out that the CaO content of Clast A symplectites was significantly higher than that of pyroxferroite and, consequently, stated that the precursor phase probably was iron-rich subcalcic augite. They further suggested that this mineral broke down in a two-step process: First, it exsolved into host augite containing vermicules and lamellae of low-CaO pyroxene. Subsequently and upon further cooling, the vermicules and lamellae decomposed into fayalite and silica. The symplectic assemblages detected in NWA 1240 possibly froze in stage 1 due to a significantly higher cooling rate (Barrat et al. 2003).
Occasionally, three-phase symplectites in the Los Angeles meteorite have been noticed in association with vermicular intergrowths of only fayalitic olivine and silica or accessory merrillite (Aramovich et al. 2002). In MIL 05085, we also found phosphates (most likely apatite) in spatial association with a symplectic area, but only as inclusions in medium-grained silica (see Fig. 5a). The only rare phases sporadically detected within three-phase symplectites are plagioclase, ilmenite, troilite, and Fe-Ni metal (most likely kamacite). All of those latter phases and apatite represent late-stage minerals and were probably trapped as mesostasis inclusions in the metastable precursor pyroxene and accompanying medium-grained silica.
In two examples (Figs. 1a, 4b), fayalitic olivine formed continuous rims around 30–125 μm-sized grains of ilmenite in sharp contact with a very-fine-grained symplectic area. The textural detail may imply the presence of relatively large ilmenite grains (as inclusions) prior to the decomposition of metastable pyroxene.
Original Geological Setting of Three-Phase Symplectites
Host phases, i.e., mineral fragments bearing symplectic assemblages, and associated minerals include sub-mm to mm-sized clinopyroxene, anorthitic plagioclase, and silica. Solely based on grain size, these occurrences suggest a gabbroic or cumulate provenance. Strong support for a link to the former rock type can be drawn from the elemental compositions of associated medium-grained plagioclase (up to An93) which match those of gabbroic eucrites (Patzer and McSween, unpublished data). In most instances and in agreement with evidence from the Los Angeles meteorite, host and associated clinopyroxenes are FeO-rich augite. As opposed to them, one fragment in LAP 04838—displaying a random array of irregular symplectic veins (Fig. 1b)—and another clast from MIL 07007 (Fig. 7c) are made of low-Ca ferroan clinopyroxene (Wo12–19; Fig. 12). Apparently, the compositional range of host pyroxenes is broader than that documented for Los Angeles.
The distribution patterns of Ca, Fe, and Mg in medium-grained pyroxene of two clasts with symplectites were unveiled by elemental mapping. In example 1 (Fig. 10), Ca concentrations are clearly lower, Mg levels clearly higher, and Fe abundances slightly elevated toward the symplectite. This signature changes sharply about half-way through the pyroxene grain. Also, spot analyses revealed slightly heterogeneous concentrations of particularly Fe and Ca between vermicular and medium-grained pyroxene. The trends observed probably reflect primary igneous zoning (see also Buchanan et al. 2000). First, a stable and relatively Mg-rich pyroxene (about En30) precipitated. Due to the fractionated state of the magma, its composition then abruptly changed to Mg-poor (En7–8) until finally, metastable pyroxene crystallized in the periphery of the grain. The “cut-off” composition in the present case seems to have been reached at En5. Upon cooling, the metastable portion broke down into a three-phase symplectite.
The elemental maps in Fig. 10 emphasize that the key components leading to metastable pyroxene formation (and subsequent breakdown) are Ca and Mg. The entire pyroxene grain displays a FeO-rich, “forbidden” composition that is probably compensated by nanometer-scale exsolution into augite and pigeonite (see Rost et al. 2009). Metastable pyroxene formation, however, required Mg levels <En5 before stabilization through exsolution became unattainable. This finding is consistent with the empirical formula of pyroxferroite (Ca0.15Fe0.85SiO3) allowing Mg substitution at a factor of at least 1/7 (Lindsley and Burnham 1970).
Elemental maps of example 2 (Fig. 11) yield different results. Zoning proves to be more gradational and Mg and Ca are relatively enriched toward the symplectite while Fe is depleted. Furthermore, Ca, Mg, and Fe concentrations at the inner edge of the medium-grained pyroxene closely agree with those of the symplectic augite. As opposed to example 1, this pattern suggests that the clast underwent a thermally driven redistribution of major elements. While temperatures were high enough, intragranular chemical differences of the cumulate pyroxene were erased. Redistribution proved to be particularly efficient for Fe and Mg. While the original Ca enrichment was partially retained along the symplectic contact zone, Fe and Mg concentrations were reversed and adapted to the respective element abundances within the vermicular augite.
Compositional Homogeneity of Three-Phase Symplectites
Major element distributions in three-phase symplectites of the Los Angeles shergottite, Clast A in Macibini, and NWA 1240 have been reported as prevailingly homogenous (Buchanan et al. 2000; Barrat et al. 2003; Rost et al. 2009). The same conclusion can be drawn for most cases addressed here. However, considering all data collected, it turns out that—from one symplectic occurrence to another—fayalitic olivine can vary significantly (Mg# 6–17). Involved ferroan clinopyroxenes exhibit even more heterogeneity (En8–18Fs41–58Wo27–44; Table 1, Fig. 12). Focusing on single clasts, the most homogeneous distribution of major elements appears to have developed in the small symplectic fragment from MIL 05085 (Fig. 5b): Variations in FeO and CaO prove negligible. The broadest spread of compositions is observed for a symplectic clast in LAP 04838 (Fig. 1c). Here, FeO and CaO contents of vermicular pyroxenes scatter by >30% and >50%, respectively. In addition to the compositional variability, the latter clast’s texture is distinctly viscous and displays two general directions of strike. Clearly, in this case, high-resolution elemental mapping that lay beyond the capabilities of the instrument used for this study would shed more light on existing elemental distribution patterns and gradients. Based on the data collected, we can only speculate that the viscous appearance of vermicular phases and the heterogeneity of respective pyroxenes are co-genetic and linked to each other. Possibly, cooling took place while the rock unit was still moving, therefore preserving a primary “turbulent” chemical signature.
A Comparison of Views
Taking into account data presented by Buchanan et al. (2000, 2005) and Barrat et al. (2003), we chose not to determine bulk compositions for the individual symplectic clasts discussed here. Judging from the observed textural diversity, any bulk data would scatter over a significant range of elemental abundances. In addition, the restricted 2D exposure and very fine grain sizes make such data problematic. In our opinion, previous work on three-phase symplectites has sufficiently demonstrated the general compositional context of these assemblages (i.e., the existence of a precursor pyroxene with “forbidden” composition; see also Fig. 12). Whether the precursor was actually pyroxferroite or an iron-rich pyroxenoid or pyroxene (see Buchanan et al. 2000; Barrat et al. 2003), we deem secondary. Instead, it appears important to us to focus on the implications and discuss earlier interpretations in the light of our results.
Buchanan et al. (2000) suggested a two-step process that produced three-phase symplectites via exsolution of an iron-rich subcalcic augite into stable and metastable pyroxene and subsequent breakdown of the metastable portion into olivine and silica. We concur with those authors and assume that actual pyroxferroite may have been generated only rarely, if at all. In one of our cases (Fig. 3), it also appears that three-phase symplectites evolved in the rim area of zoned pyroxene, similar to what was observed in Macibini’s Clast A. Our study, however, emphasizes that settings and textures of three-phase symplectites in HED meteorites are far more variable. Most importantly, we find evidence for a close relationship to mesostasis phases or, in more general terms, late-stage precipitation. Rare earth element data reported by Buchanan et al. (2000) are consistent with the progressive crystallization of a eucritic melt and lend strong support to our idea that three-phase symplectites in HED meteorites represent a late stage of melt evolution. Furthermore, our data suggest that three-phase symplectites in HEDs are related to medium-grained gabbroic eucrites that formed from the undisturbed fractionation of a MgO-rich eucritic magma (see Bartels and Grove 1991).
Two-Phase Symplectites of Diogenitic Origin
Occasionally, two-phase symplectites involving orthopyroxene and troilite were observed (MIL 07007 (Fig. 7a) and QUE 94200 (Fig. 9b)). This particular type of lithology can also be found in diogenite LEW 88679 (Beck, personal communication). LEW 88679 is a polymict breccia of harzburgite and magnesian orthopyroxenite (Beck and McSween 2010). In the context of this study, we revisited the sample described by Beck and McSween and specifically examined the rock portion displaying vermicular orthopyroxene-troilite intergrowths (Fig. 13). The symplectic areas are associated with orthopyroxene as major constituent, abundant olivine, minor plagioclase, and accessory chromite.
We also observed two small areas (150 and 200 μm, respectively) of two-phase symplectites in diogenite MET 01084,5 and several small fragments thereof (typically 100–200 μm) in howardites QUE 97001,49 and 94200,16. In one case from QUE 97001, vermicular orthopyroxene and troilite were associated with medium-grained olivine (Mg# 58). The same spatial and textural relationship of orthopyroxene, troilite, and olivine was detected in a clast from QUE 94200 (Fig. 9c).
Apart from the obvious textural similarities, chemical compositions of all two-phase symplectites observed in howardites are consistent with those of the respective phases in diogenites (Beck and McSween 2010). Thus, it seems very likely that the fragments in question are of diogenitic origin. Furthermore, based on all cases observed so far, vermicular orthopyroxene-troilite intergrowths are restricted to harzburgitic lithologies.
Interestingly, orthopyroxene-troilite symplectites have also been reported for the Acapulco meteorite (El Goresy et al. 2005). In Acapulco, these symplectites always emerge in direct vicinity to primary orthopyroxene and large troilite grains. El Goresy et al. (2005) inferred rapid cooling of a partial, immiscible silicate-sulfide melt be the circumstance of formation. In analogy, it may be possible that two-phase symplectites in diogenite LEW 88679 were also produced via localized melting and subsequent quenching. Diogenites are cumulates that were generated deep in Vesta’s crust or upper mantle. High-velocity impacts were necessary for their excavation and likely also caused localized melting. Localized melts may be produced at the scale of mineral constituents at intermediate shock pressures (Keil et al. 1997). In particular, they may be produced at grain boundaries of high density and low density minerals. Within this scenario, it is conceivable that impact-induced localized melting and subsequent rapid cooling of LEW 88679 and other diogenites (of the harzburgitic kind) may have led to the formation of orthopyroxene (low density)–troilite (high density) symplectites.
The interpretation of howarditic fragments displaying two-phase symplectites in association with medium-grained olivine proves less straightforward. While the forsterite content of diogenitic olivine is known to range from 61 to 78 mol% (e.g., Beck and McSween 2010), olivine in the detected clasts shows significantly lower abundances (Mg# 54–58). Nevertheless, we advocate a diogenitic provenance and suggest that the chemical change is due to thermally induced redistribution of Fe: After being excavated, brecciated, and mixed with eucritic material, the respective howarditic rock unit was buried, for instance, under a hot impact melt sheet. Thermal metamorphism then led to the diffusion of Fe from symplectic orthopyroxene (or, alternatively, more fractionated [eucritic] lithologies [Beck, personal communication] to diogenitic olivine clasts. This hypothesis could also explain the discrepancy in MgO content between vermicular and associated orthopyroxene in LEW 88679 (Fig. 13). Finally, the same location was exposed to another impact launching the QUE 94200 and 97001 meteoroids into space.
In addition to three-phase symplectites generated from the breakdown of metastable pyroxene, howarditic late-stage mélanges may comprise typical mesostasis minerals: plagioclase, silica, ilmenite, and troilite as well as occasional Fe-Ni metal, apatite, and hedenbergite.
In one remarkable case—a mm-sized fragment of plagioclase in LAP 04838—a range of highly fractionated phases formed an intricate triangular inclusion. Involved minerals are major silica, troilite, and plagioclase, minor ilmenite and ferroan augite, and accessory hedenbergite (Fig. 4). Troilite and silica developed an intimate intergrowth, reminiscent of symplectites, due to their immiscibility (see also Fig. 7b). Additional, smaller inclusions in the same feldspar grain consist of major silica and troilite and accessory apatite and hedenbergite. A vein of ferroan augite plus minor ilmenite and hedenbergite connects to the polymineralic interstice. All phases trapped in the cumulate feldspar clearly qualify as mesostasis crystallites.
In PCA 02066, we detected clinopyroxene clasts containing very irregular patches of silica, blebs of troilite, some minor plagioclase and relatively large subhedral ilmenite (e.g., Fig. 8a). Appearance and composition of the inclusions are consistent with late-stage precipitation. The inclusions were probably trapped in the crystallizing augite toward the end of the source rock’s formation.
Another, granular clast is composed of augite, silica, and above-average amounts of Fe-Ni metal (PCA 02066, Fig. 8b). No troilite was detected. The recrystallized texture of the fragment implies thermal processing. The presence of abundant silica and Fe-Ni metal suggests an affinity to the category of mesostasis assemblages. Troilite may have been an additional premetamorphic component but was possibly mobilized and drained during the thermal event.
In OUE 94200, we came across a relatively large FeO-rich augite fragment exhibiting very fine, faint exsolution and sublamellar inclusions of major silica and troilite plus minor Fe-Ni metal and augite (Fig. 9a). In this case, too, we propose a mesostasis origin for the inclusions and suggest that their textural alignment, which runs parallel to the exsolution lamellae, took place at the time of exsolution.
Fayalitic Olivine and Silica Inclusions in Pyroxene: Genetically Linked to Metastable Pyroxene?
Pyroxene fragments that contain aligned silica and olivine inclusions reminiscent of exsolution lamellae can be grouped into a fourth category. One of the clasts in question (Fig. 1d) is composed of the same phases recognized in three-phase symplectites but differs from the latter in textural terms: Ferroan augite constitutes the host phase displaying abundant sublamellar inclusions of silica and fayalitic olivine. The clast also stands out because of a relatively high content of Fe-Ni metal inclusions. Similarly, the medium-grained pyroxene in Fig. 5a displays small oriented inclusions of silica and minor fayalitic olivine (see also Fig. 6a). In two other instances, we found augite fragments exhibiting stubby lamellae of fayalitic olivine and silica (Figs. 1b and 6b). Finally, we observed a bimodal augite clast in MIL 07007 showing sublamellar inclusions of fayalitic olivine, orthopyroxene, and minor silica (Fig. 7c).
As opposed to trapped mesostasis, the concurrence of fayalitic olivine and silica as oriented inclusions in ferroan augite suggests a genetic link to metastable pyroxene. All clasts listed above, however, deviate from the established scheme in missing the vermicular texture and the familiar phase proportions. Yet, the common denominator appears to be decomposition—here: spatially very restricted decomposition. This conclusion is supported by the occurrence of intermediate cases where augite clasts display irregular vein-like features consisting of vermicular augite+olivine+silica (e.g., Fig. 1c). Additional differences from three-phase-symplectites, like the relatively high abundance of Fe-Ni metal inclusions in Fig. 1d, are probably incidental and simply represent compositional variables of the late-stage magma.
In some cases and seemingly independent from the given composition of the host augite, stubby lamellae of fayalitic olivine with minor silica were formed (Fig. 14). In a third species, breakdown phases include not only fayalitic olivine and silica but also orthopyroxene. The emergence of orthopyroxene is obviously attributable to the relatively high MgO content of the host pyroxene. At relatively high levels of endmember enstatite in the host, orthopyroxene inclusions may actually become more abundant than silica (Fig. 7c). Also, the FeO content of concurrent olivine decreases.
Given a distinct, yet-to-be determined set of pressure and temperature parameters, it appears possible to generate the observed variety of assemblages via decomposition of relatively FeO-rich augites. A direct genetic link to three-phase symplectites, i.e., the reconfiguration of three-phase symplectites into solid augites with exsolution-like textures during a later metamorphic event, seems unnecessarily complicated and more unlikely. In view of the complex thermal history recorded in Vesta’s lithological units, however, the latter scenario cannot be fully excluded.
Three-phase symplectites (ferroan augite + fayalitic olivine + silica), previously only known from terrestrial, lunar, and Martian rocks as well as the HED impact melt NWA 1240, the monomict eucrite Y-82202, and a single clast from the polymict eucrite Macibini, have now been found to occur in several howardites. They were generated from the breakdown of iron-rich metastable pyroxene (possibly pyroxferroite), i.e., a highly fractionated precursor that decomposed—probably in a two-step process (Buchanan et al. 2000)—upon cooling under low pressure in the order of temperatures below 990 °C within less than 3 days.
In addition to three-phase symplectites, howardites may contain fragments of two-phase symplectites (orthopyroxene + troilite). This particular type of lithology is most likely derived from diogenites. It was generated by localized impact-induced melting (grain boundary melting) and subsequent quenching of an immiscible silicate-sulfide melt. The same type of two-phase symplectite has also been observed in the Acapulco meteorite.
Some larger clasts in howardites reveal (some of) the geological context necessary to understand the evolution of three-phase symplectites. Based on our findings, three-phase-symplectites are associated with gabbroic eucrite fragments that are relatively rich in silica and ilmenite, i.e., late-stage minerals. Thus, based on the FeO-rich state of three-phase symplectites and their host pyroxenes as well as the spatial relationship of three-phase symplectites with late-stage crystallites, gabbroic eucrites bearing those symplectites are to be placed on the most fractionated end of eucrite formation.
Implications of the above inference are: The original bulk composition and pressure-temperature setting of the respective eucritic magma allowed extensive precipitation of increasingly differentiated gabbroic rocks up to the formation of metastable pyroxene. The medium-grained host lithology invokes a history of slow cooling while the breakdown of metastable pyroxene requires low-pressure cooling from temperatures above 990 °C in less than 3 days. Consequently and after an extended, undisturbed crystallization period at deeper crustal levels, the gabbroic rock units that harbor three-phase symplectites must have been suddenly placed in a relatively shallow position (e.g., impact-related excavation or exposure). In addition, the occurrence of three-phase symplectites on Earth, the Moon, Mars, and Vesta (as the presumed HED parent body) suggests that these symplectites are a minor but not uncommon feature of highly fractionated igneous rocks on differentiated solar system bodies.
Acknowledgments— We would like to particularly thank the following people: Allan Patchen for technical assistance with the microprobe and thoughtful comments on the subject, Kevin Righter for allocating and providing samples from the Antarctic Meteorite Collection at JSC, as well as Brian Balta, Andrew Beck, Paul Buchanan (reviewer), and Carle Pieters (Associate Editor) for their valuable feedback.
This study was funded through German Research Foundation (DFG) grant PA1970/1-1 (Research Fellowship, AP) and NASA Cosmochemistry grant NNX10AG48G (HYM).