Forsteritic olivine in EH (enstatite) chondrite meteorites: A record of nebular, metamorphic, and crystal‐lattice diffusion effects

The occurrence of forsteritic olivine in EH enstatite chondrites is indicative of bulk disequilibrium. In MgO‐rich magmatic systems, forsterite can either crystallize as a liquidus phase or be produced during peritectic melting of enstatite. Because diffusion of divalent cations through forsterite is relatively rapid, it records peak melting (i.e., chondrule‐forming events) and is also sensitive to subsequent metamorphism in the EH chondrite parent body. Here, we report the major and minor element geochemistry of olivine in EH chondrites across petrologic types 3 and 4. In all cases, olivine meets the technical definition of forsterite (>90 mole% Mg2SiO4). For unequilibrated EH chondrites, minor elements identify CaO‐Al2O3‐TiO2‐rich (refractory forsterite), MnO‐rich (“LIME” forsterite), and FeO‐bearing (forsteritic olivine) endmember components, the latter with Cr2O3‐rich and Cr2O3‐poor varieties. At higher petrologic type, minor element concentrations become restricted and compositions approach pure forsterite, while grain sizes reduce strongly with peak metamorphic temperatures. These changes reflect diffusive equilibration with enstatitic groundmass and dissolution reaction with free silica. The global geochemical distribution of forsteritic olivine in EH chondrites is, perhaps unexpectedly, more similar to those in low‐FeO type I chondrules and associated objects in carbonaceous chondrites (CCs), rather than equivalent objects in ordinary (H, L, LL), low‐FeO (or HH), or Kakangari (K) chondrites. Among achondrites, there is similarity between pure forsterite in aubrites and EH4 chondrites arising due to subsolidus equilibration in both settings, while Cr2O3‐poor forsteritic olivine in EH3 and CCs is similar to magnesian xenocrystic olivine in angrites. This might reflect CaO‐rich and SiO2‐poor magmatic sources across multiple early solar system reservoirs.


INTRODUCTION
Enstatite chondrite meteorites are chemically reduced aggregates that comprise chondrules, chondrule fragments, and very rare refractory inclusions in a fine-grained matrix, indicating a nebular origin (e.g., Weisberg & Kimura, 2012).Unlike in other chondrites, the matrix and chondrules are dominated by pyroxene and also contain minor olivine, feldspar or feldspathic glass, and free silica, while metal-rich chondrule-like objects are mainly composed of silicon-bearing metals, sulfides, and accessory silicates and graphite (Keil, 1968;Keil & Andersen, 1965;Mason, 1966).The bulk compositions of enstatite chondrites can be divided into groups with high-and low-Fe and other siderophile element contents, referred to as EH and EL (Baedecker & Wasson, 1975;Rambaldi & Cendales, 1980;Sears et al., 1982Sears et al., , 1984; but see also Rochette et al., 2008 for their similar magnetic susceptibilities).A further subdivision into "a" and "b" series on the basis of accessory mineralogy and minor element contents of metals and sulfides has been suggested, that is, EH a , EH b , EL a , and EL b (Weyrauch et al., 2018); some of these minor element trends may reflect postmetamorphic impact processes (Zhang et al., 1995).While mineralogical and chemical variations exist, bulk enstatite chondrites and related enstatite-rich achondrites share nearly identical stable isotope signatures for many elements, such as oxygen isotope ratios, which has led to a generally accepted view that most enstatite-dominated meteorites originated in several parent bodies accreted in nearly the same chemically reduced part of the early Solar Nebula (e.g., Clayton et al., 1991;Clayton & Mayeda, 1984).That these parent bodies maintained separate later histories is indicated by lack of shared regolith fragments or brecciated clasts (Keil, 1989) and the possible existence of a unique metamorphic series within each group (Weyrauch et al., 2018).
Most investigations into the nebula sources of enstatite chondrites have focused on the EH chondrites, because more were historically available in this subgroup at low metamorphic grades (petrologic types 3-5), while the EL group was dominated by higher grade rocks (type 6: Kallemeyn & Wasson, 1986;Sears et al., 1982Sears et al., , 1984)).The strongly reduced character of enstatite chondrites led to generally low FeO concentrations in olivine and pyroxene, so there is some difficulty in transferring classification criteria from ordinary to enstatite chondrites, especially in the use of ferromagnesian zoning of silicate minerals.Therefore, other criteria have been suggested to narrow down petrologic types, such as the degree of structural order of organic matter (Quirico et al., 2011) or the occurrence of silica-rich glassy mesostasis (Kimura et al., 2005;Kimura & Lin, 1999) and remnant olivine (Bendersky et al., 2007;Nehru et al., 1984;Prinz et al., 1985;Weyrauch et al., 2018).In enstatite chondrites, high enstatite content in combination with reduction of FeO to Fe during chondrule-forming events are responsible for high bulk SiO 2 /(FeO + MgO).This has even progressed toward localized production of free silica (Weisberg et al., 1994) which, in the presence of olivine, is a disequilibrium assemblage that is primed for further enstatite production by dissolution of olivine (Binns, 1967;Kitamura et al., 1988).In MgO-rich magmatic systems, forsterite is a liquidus phase but melting of enstatitic pyroxene occurs incongruently (e.g., Bowen & Schairer, 1935) to also form forsterite and a silica-enriched melt.Such a paragenesis might also be responsible for the assemblages observed in enstatite chondrites, with peak temperatures likely to occur during chondrule-forming events.After accretion, metamorphism and slow cooling might produce more enstatite by reaction between olivine and silica, but no new olivine can be formed.In the presence of free silica, the modal abundance of remnant olivine in EH3-4 chondrites is therefore a potentially useful metamorphic indicator (Hicks et al., 2000).
Separately from mineral modal abundances, the relatively rapid diffusion and re-equilibration of trace elements in olivine during thermal metamorphism has been used to infer metamorphic grade in ordinary and carbonaceous chondrites (CCs; Grossman & Brearley, 2005).In enstatite chondrites, an inverse relationship between petrologic type and minor elements in enstatite has also been interpreted as a metamorphic effect (e.g., McKinley et al., 1984).Such an effect could be useful in tracking diffusive modification of remnant olivine at low metamorphic grades (Bendersky et al., 2007;Weisberg et al., 2005).This would be particularly valuable if trace element diffusion through the olivine crystal proceeds faster than dissolution of olivine grains, because it might constitute a long-duration natural diffusion experiment under conditions that do not occur in terrestrial environments (cf.Qian et al., 2010 for an example of such a natural experiment).Here, we report the major and minor element geochemistry of olivine in EH chondrites in order to: (1) improve understanding of early, high temperature nebular condensation processes; (2) track the progression of metamorphism in the EH chondrite parent body; and (3) test whether this series provides a natural olivine crystal lattice diffusion experiment under silica saturated and chemically reducing conditions.
Thin sections and polished chips were imaged by scanning electron microscopy (SEM) using a JEOL model JSM-6400 equipped with a Pioneer Si-Li detector (Vrije Universiteit Brussel, Belgium).Images were obtained using backscatter electron (BSE) and X-ray energy-dispersive spectroscopy (EDS) methods, with color processing performed with the Thermo Scientific NSS 3 software package.Olivine was identified by its slightly higher Mg Kα and lower Si Kα by comparison with the enstatite that makes up the bulk of all samples (Figure 1).The low FeO content of such chondrules and chondrule fragments identifies them as type "IB" or "IAB," or alternatively as porphyritic pyroxene (PP) in which minor olivine occurs as poikilitic inclusions in pyroxene, or as porphyritic pyroxene-olivine (POP) (e.g.,Jones, 1994;Scott & Taylor, 1983).Isolated olivine fragments in the matrix were also investigated.
Quantitative analyses of olivine were made by electron microprobe analysis (EMPA) using a JEOL JXA-8500F field-emission electron microprobe (Museum für Naturkunde, Berlin, Germany) with a fully focused beam of 60 nA current with 15 kV accelerating voltage, utilizing five wavelength-dispersive X-ray spectrometers.New calibrations were made using Astimex and Smithsonian reference materials.Kα peak measurements were made for Si, Mg, Al, P (TAP), Mn, S (PETJ), Ca, Ti (PETH), Cr, Fe (LIFH) with backgrounds measured on both sides of each peak.Spectrometer arrangement and measurement times are given in Table 1, where it can be seen that we have employed relatively long counting times to achieve good precision on minor elements.Matrix effects were corrected using the JEOL in-built ZAF routine.We present Cr as Cr 2 O 3 although a divalent Cr component is likely, that is, significant CrO in olivine requiring no additional charge balance in the same way as Fe, Ca, and Mn.Analyses with totals outside the range of 98-102 wt% were rejected, as were those suggesting detectable sulfur (greater than 0.075 wt% calculated as the oxide SO 3 ) that indicates secondary fluorescence of sulfide.The full analytical data set (n = 205 across 118 unique objects) is given in Supplementary Information S2.

RESULTS
Olivine is relatively common in EH3 chondrites (e.g., Figure 1a,b) but increasingly difficult to find in EH4 chondrites (Figure 1c,d).For comparison, we also investigated the higher petrologic type A-881475 (EH5) but could not find any olivine in this meteorite (Figure 1e, f).Of the EH chondrites in this study, Y-691 has the highest abundance of olivine ($7 vol%; all abundances on whole-rock basis) in olivine-rich or olivine-bearing chondrules, and a single large barred olivine chondrule.EET 87746 has common euhedral olivine ($3 vol%) in pyroxene-and glass-bearing chondrules with occasional smaller olivine-rich chondrules and olivine fragments in the groundmass.ALHA 77295 also has common olivine ($3 vol%) in chondrules which tend to contain more pyroxene than olivine, as well as rare olivine fragments residing in the matrix.Sahara 97079 (Figure 1a,b) and A-881575 each contain olivine as an accessory phase ($1 vol%) occurring as sub-rounded grains contained in pyroxene-dominated chondrules and rare fragments in the matrix (Figure 1a).A-882059 contains rare olivine ($0.1 vol%) as small rounded grains (maximum diameter $50 μm).Olivine in the more highly metamorphosed chondrites Y-74370 (Figure 1c,d) and the Y-791790 and Y-791810 pair is very rare ($0.01 vol %) and small ($10 μm).The recrystallized textures of these latter three chondrites, which are all accepted as EH4, make recognition of chondrule type difficult.Olivine grains are frequently surrounded by pyroxene in EH3 chondrites (e.g., Figure 2a-c), while in EH4 chondrites they are exclusively surrounded by pyroxene and often in association with small metal grains or rimmed by metal (Figure 2d-i).
All olivine can be described as forsteritic (more than $90 mole% Mg 2 SiO 4 ).Except for SiO 2 and MgO, other oxides are present at trace, minor, or lower major oxide level.We will refer to these hereafter as "trace" or "minor" components in EH chondrite olivine, although FeO can be conventionally referred to as a major oxide, at up to $3.6 wt% for unequilibrated EH chondrites in this study (Figure 3a) and occasionally higher levels in some FeO-rich objects (e.g., in ALH 81189 reported by Lusby et al., 1987).In those EH chondrites where olivine is more common (Y-691, EET 87746, ALHA 77295, Sahara 97079, and A-881575), it has wide ranges in all minor element components.The very rare olivine in higher petrologic type EH chondrites Y-74370, Y-791790, and Y-791810 has lower concentrations of all minor elements, with slightly greater range in CaO, Al 2 O 3 , and TiO 2 in the former.Rare olivine in A-882059 appears to share some characteristics of both groups in having low and restricted FeO, CaO, Al 2 O 3 , and TiO 2 contents, but a wider range of MnO and some variation in Cr 2 O 3 .Across the entire data set, we find a correlation between Cr 2 O 3 and FeO (Figure 3a; cf.Weisberg et al., 2005 andBendersky et al., 2007, who reported a similar data distribution in figure form only) and a possible secondary trend parallel to the former, at very low Cr 2 O 3 (lower part of Figure 3a) which includes points for Y-74370 from this study but is dominated by points reported by Ikeda (1989) for Y-691.Loose correlations also exist between the refractory components CaO and Al 2 O 3 (Figure 3b) and probably also TiO 2 , with the latter close to detection limit.The FeO-Cr 2 O 3 and Al 2 O 3 -CaO-TiO 2 trends are loosely negatively correlated with each other (Figure 4a) and both are also negatively correlated with MnO contents (Figure 4b,c).Some complex structure is present in the data at higher concentrations, which is especially well illustrated in the grouping of points in FeO-MnO space (Figure 4b; cf.Weisberg et al., 2005 who reported a similar data distribution in figure form only). Statistical testing of the data set by cluster analysis using all minor and trace elements (standardized to achieve zero mean and unit variance with equal weighting for every element) identified a hierarchy of clustering the data into three to six groups depending upon interpretation (i.e., a subjective decision on the "Rescaled Distance Cluster Combine" axis of the dendrogram in Figure S1 and discussed in detail in Supplementary Information S3).The six groups are summarized in Table 2. Most of these groups span multiple meteorites and reflect the clustering of minor and trace element-poor The data set was also investigated by principal component analysis (PCA), in which extracted components represent combinations of oxides that explain the variance (Supplementary Information S3).PCA finds three significant components with decreasing contributions to variance: C1 (44%), the refractory oxides CaO, Al 2 O 3 , and TiO 2 ; C2 (33%), the oxides of multivalent transition elements FeO and Cr 2 O 3 ; and C3 (12%), MnO with a minor Cr 2 O 3 loading.This confirms the negative correlation between the various minor oxides outlined above.It is not, however, sensitive to local clusters in the data that are clear from the hierarchical cluster analysis.

Comparison of Olivine Compositions with Previous Work on EH Chondrites
The composition of olivine in enstatite chondrites has long been of interest despite its occurrence as a minor or accessory phase, but previous studies have mainly reported "selected" or average compositions (Grossman et al., 1985;Kimura & Lin, 1999;Leitch & Smith, 1982;Lusby et al., 1987;Rambaldi et al., 1983;Schneider et al., 2002;Weisberg et al., 2011).The full EMPA data sets for only two studies are available in the literature.These are from Ikeda (1989;n = 58) for Y-691 and Weisberg et al. (2021;n = 85) for the ALH 81189/85159 pair.Our study, with n = 205 for 118 unique objects across nine EH chondrites (including Y-791790 and Y-791810, which are likely to be paired) considerably expands upon these data sets and is large enough for statistical testing using HCA and PCA.There are, however, differences in the range of compositions found by these studies and it is worth considering whether they arise through methodological bias or sample heterogeneity.
For Y-691, Ikeda (1989) reported a substantial number of olivine analyses with relatively high FeO, particularly in the range 3.0-7.4wt%, with less than $0.3 wt% Cr 2 O 3 and CaO first decreasing and then increasing as FeO contents climb higher than $3.0 wt%.We have not found such compositions.The only other study to report similar FeO-rich olivine in EH chondrites is Lusby et al. (1987) for ALH 81189, who reported somewhat higher Cr 2 O 3 at similar FeO contents and lower CaO concentrations.They also reported some analyses with yet higher FeO in the range $10-20 wt% and no particular relationship with Cr 2 O 3 .By contrast, Weisberg et al. (2021) reinvestigated this meteorite and its pair ALH 85159 but found only olivine similar to that in other EH3 chondrites, with up to $3.1 wt% FeO.A possible explanation for the discrepancies between our study and that of Ikeda (1989) could be our use of both Mg and Si Kα X-ray maps to find olivine, rather than Si alone.If it were the case that Mg counts are less reliable, then this could have biased our search toward more forsteritic olivine.A similar bias might explain the relatively FeO-poor compositions found by Weisberg et al. (2021) for the ALH 81189/85159 pair, but is less convincing when one considers the strongly FeO-rich silicates in this meteorite reported by Lusby et al. (1987).A heterogeneous distribution of FeO-rich silicates in EH chondrites at the scale of polished sections would seem to be a better explanation for the variations reported in these studies.FIGURE 3. Major and minor oxide contents for forsteritic olivine in EH chondrites obtained by EMPA in this study (colored by meteorite; ordered from high to low olivine content) and previously reported literature values (in gray; Grossman et al., 1985;Ikeda, 1989;Kimura & Lin, 1999;Leitch & Smith, 1982;Lusby et al., 1987;Rambaldi et al., 1983;Schneider et al., 2002;Weisberg et al., 2011Weisberg et al., , 2021)).Data for EL olivine are from Schneider et al. (2002).In the above studies, one might question whether the correlation between CaO and Al 2 O 3 in otherwise quite pure forsterite could have arisen simply through secondary fluorescence of a Ca-and Al-rich host phase.Previous work on type I chondrules concluded that it is genuine (e.g., Brearley & Jones, 1998) and the petrographic occurrences of forsteritic olivine in our sample suite are supportive.We have found that enclosing pyroxene grains in both olivinebearing chondrules or near isolated olivine grains in the matrix are enstatite and typically no richer in CaO and Al 2 O 3 than olivine itself (e.g., Schneider et al., 2002;Weisberg et al., 2011Weisberg et al., , 2021)).For our study, we have found only two euhedral forsterite crystals hosted in glass with relatively high CaO and Al 2 O 3 (EET 87746 analysis 91, and Y-691 analyses 01, 02, 03, and 04).This is the most likely situation in which secondary fluorescence might have been induced, or similarly by glass inclusions such as those found in Y-691 (Ikeda, 1988).All other such CaO-and Al 2 O 3 -rich forsterites we have found are hosted in pyroxene or were isolated in fine-grained matrix (number of objects = 15).We conclude that elevated CaO and Al 2 O 3 contents in these forsterites are genuine; similar compositions were found by previous studies and such grains are possibly related to refractory forsterites in other chondrite types (e.g., Steele, 1986;Steele et al., 1985).
Our run table included S as an opportunistic monitor of secondary fluorescence by sulfide.We have excluded analyses in which substantial S was detected (as SO 3 greater than 0.075 wt%) and do not expect any significant contribution of false Fe signals from FeS. Secondary fluorescence by Fe-metal would be harder to detect but should be indicated by high analytical totals, which we have already minimized by only accepting analyses with totals in the range 98-102 wt%.There are no phases in EH chondrites that are particularly rich in Mn, unlike in EL chondrites, in which MnS is present, and while daubréelite (Cr 2 FeS 4 ) is present in EH a chondrites, secondary fluorescence from such phases should correlate with SO 3 so we do not expect significant false signals for these elements.We conclude that the negative correlations between the Ca-Al-Ti, Fe-Cr, and Mn components are robust and the obtained concentrations are accurate.FIGURE 4. Major and minor oxide contents for forsteritic olivine in EH chondrites obtained by EMPA in this study (colored by meteorite; ordered from high to low olivine content) and previously reported literature values (in gray; Grossman et al., 1985;Ikeda, 1989;Kimura & Lin, 1999;Leitch & Smith, 1982;Lusby et al., 1987;Rambaldi et al., 1983;Schneider et al., 2002;Weisberg et al., 2011Weisberg et al., , 2021)).Data for EL olivine are from Schneider et al. (2002).The minor and trace element contents of olivine in moderately metamorphosed EH4 chondrites have been investigated in only a few studies, but where available they were found to be at quite low levels.Pure forsterite was reported for the EH4 chondrite Y-791790 by Kimura and Lin (1999), and quite pure, blue-luminescing forsterites were found in Indarch (EH4) by Leitch and Smith (1982).Similar minor element-poor forsterites were also found in the EH3 Kota-Kota by Leitch and Smith (1982) and Lusby et al. (1987).While Kota-Kota was reported to contain more olivine than Indarch, it has less than most EH3 chondrites (Weisberg et al., 2010), which seems to be on trend.This meteorite could be considered an "EH3 (high)" enstatite chondrite, following the terminology of Kimura et al. (2005).We have found very low minor or trace element concentrations in rare forsterites of the EH4 chondrites Y-74370, Y-791790, and Y-791810 and our contribution greatly expands the available data in this compositional region.For these meteorites, Kimura et al. (2005) assigned "EH4 (low)" to Y-74370 and "EH4 (high)" to Y-791790, on the basis of sulfide chemistry, that is, their (Mg, Mn, Fe)S compositions.The very slightly more diverse minor element chemistry of forsterite in Y-74370 by comparison with low and homogeneous minor element contents of those in Y-791790 and Y-791810 (Figures 3 and 4) is consistent with such "low" and "high" subtypes.Therefore, there is a trend within the EH chondrites of decreasing minor element concentrations in forsteritic olivine with increasing petrologic type.

Metamorphism of EH Chondrites and a Natural Olivine Diffusion Experiment
The occurrence of olivine in enstatite chondrites has been suggested as a useful defining characteristic for petrologic type 3 in these chondrites (Bendersky et al., 2007;Hicks et al., 2000;Nehru et al., 1984;Prinz et al., 1985); other possibilities include the structural state of organic matter (Quirico et al., 2011), the type of silica polymorph (Kimura et al., 2005), and the chemistry of sulfide and metal phases (Weyrauch et al., 2018).The available modal proportions of olivine and other phases in enstatite chondrites are scattered throughout the literature.Similarly, the FeO contents of olivine in EH chondrites have received attention not just because of the importance of the common solid solution between Mg and Fe endmembers, but because of the use of Fe as a redox indicator.In Figure 5, we compile the modal abundance and minor oxide contents of olivine in EH chondrites from the literature and our study.There are substantial available data for FeO but far fewer data available for other minor elements.In each subfigure of Figure 5, it can be seen that there is a relationship between olivine abundance and minor element chemistry.The generally high FeO contents of olivine grains in olivine-rich EH3 chondrites are consistent with a suspected origin of some as transported, nebular relicts (note: the term "relict" has been used in some of the enstatite chondrite literature for what we prefer to call "remnant" forsterites that survived metamorphism in EH4 chondrites; this brings the terminology for enstatite chondrites and other chondrites into agreement).Ferroan, possibly relict olivines in EH3 chondrites are far from chemical equilibrium with the chemically reduced, silica-rich host materials and cannot form by peritectic melting of enstatite or even ferroan pyroxene precursors (Bowen & Schairer, 1935).With petrologic type increasing to EH4, the abundance of olivine decreases very strongly and the FeO content drops to a minor element level so that such grains can be described as nearly pure forsterite (Figure 5a).Ikeda (1988) reported the reconstructed compositions of now decomposed, formerly FeO-rich olivine in Y-691.It may be that during exposure to a chemically reducing environment, whether in some part of the solar nebula or in the EH chondrite parent body, the breakdown of FeOrich olivine or impure forsteritic olivine might progress faster than for more nearly pure forsterite, as suggested by the occurrence of dusty forsteritic olivine (e.g., Rambaldi et al., 1983).This seems consistent with the relationship between modal abundance and FeO contents of olivine in Figure 5a.For those relatively pure forsteritic grains that survived the longest, FeO contents have converged to a value of $0.5 wt%.Historically, the fO 2 in metal-silicate mixtures has often been approximated relative to the iron-w üstite buffer (IW) by the ratio of Fe in metal to FeO in silicate, for example, for the primitive terrestrial mantle, ΔIW = 2 × log (X FeO /X Fe ), giving a value about two log units below IW ($ΔIWÀ2; e.g., Righter & Ghiorso, 2009).A yet lower value for FeO content from silicates in enstatite chondrites suggests fO 2 conditions several log units below the iron-w üstite redox buffer.Assuming that 0.5 wt% FeO in olivine is representative of the aFeO and in quasiequilibrium with Fe-rich metal (X Fe ≈ 0.9), such an approximation gives $ΔIWÀ4.5 for EH4 chondrites.This compares favorably with the calculated value of ΔIWÀ3.9 from the formulation developed by Tenner et al. (2015), and is somewhat lower than the determination by Brett and Sato (1984) who found ΔIWÀ2.6 for the EL6 Hvittis.For other minor oxides Cr 2 O 3 , MnO, and CaO (in Figure 5b-d, respectively), the relationship with olivine modal abundance is somewhat less precisely defined.Unlike FeO, the decrease in concentration with modal abundance does not seem to converge on an equilibrium value.
In detail, there are a few finer features of the compositional ranges for olivine that may be used to infer whether they represent a metamorphic series.For A-881575 (EH3), we have not found any olivine that is rich in CaO or Al 2 O 3 , but some do contain high FeO,  Binns (1967), Leitch and Smith (1982), Nehru et al. (1984), Lusby et al. (1987), Ikeda (1989), Kimura and Lin (1999), and Weisberg et al. (2011Weisberg et al. ( , 2021)).Modal abundances of olivine are from Binns (1967), Okada (1975), McKinley et al. (1984), Nehru et al. (1984), Prinz et al. (1984Prinz et al. ( , 1985)), Mason (1968), Weisberg and Prinz (1998), Kimura and Lin (1999), and this study.For Indarch, Binns (1967) stated that olivine is "very rare," for which we assign a value of 0.03%, guided by their report of 0.1% olivine in Kota-Kota.The curved arrow indicates an exponential fit to olivine modal abundances and average olivine oxide concentrations.Cr 2 O 3 , and MnO concentrations.This seems to be evidence for a slightly different population of olivine in their protolith, that is, lacking the refractory aspect of Group 2 (Table 2) but otherwise representative of EH3 chondrites.By contrast, forsterite in A-882059 (EH3) has very low FeO, lower CaO, and moderately variable Cr 2 O 3 , and a wide range of MnO concentrations that are not substantially different from other EH3 chondrites.These characteristics were found by the HCA to contribute to Group 5 (Table 2).It appears that olivines in this meteorite were quite strongly modified to near forsteritic compositions, but not completely, and from their lesser abundance, they seem to be remnant leftovers after some dissolution.FeO and Cr 2 O 3 may have quite rapidly approached the value found in EH4 chondrite forsterites, while CaO contents were not changed so much, and MnO not at all.This may be consistent with reduction as the primary agent of metamorphism.For convenience, A-882059 might then be classified as an EH3 (high) chondrite, following Kimura et al. (2005).Finally, some of the olivine in Y-74370 (EH4) has rather higher Al 2 O 3 , FeO, or lower CaO.The high Al 2 O 3 may be consistent with slower diffusion of this trivalent element under some conditions, for example, as found in terrestrial rocks (Milman-Barris et al., 2008), in some achondrites (McKibbin et al., 2013), in dedicated experiments (Zhukova et al., 2017), and finally in type I chondrules (Marrocchi et al., 2018).The experiments of Zhukova et al. (2017) suggest that diffusion of Al is suppressed under conditions of low aSiO 2 as is the case in mantle-related samples (McKibbin et al., 2013;Milman-Barris et al., 2008) rather than high aSiO 2 as in enstatite chondrites.Perhaps, very local conditions might sometimes exert control over diffusion in individual crystals or at the hand-specimen scale, such as where grains are in contact with either enstatite and metal or enstatite and silica.As well as diffusive effects, such variations may also change the relative influence of fO 2 or aSiO 2 on dissolution of olivine.Nevertheless, this situation lends further support to the EH4 (low) classification given by Kimura et al. (2005) from sulfide chemistry, through which precursors must pass in order to achieve EH4 (high), for example, Y-791790 and Y-791810, and beyond (EH5).
Considering the similar compositional distributions of olivine in most EH3 chondrites, and especially the range of subgroups illustrated in Figure 4 and identified by HCA that are given in Table 2, there is little evidence for much difference in protoliths among the various EH chondrites.The only convincing exception is the absence of CaO-Al 2 O 3 -TiO 2 -rich olivine in A-881575, even though it contains the other types.We conclude that the trend toward minor element-free forsterites at higher petrologic type was driven by metamorphism in their parent body, rather than reflecting a nebular source for EH4 chondrites containing inherently minor elementpoor olivine.
Some constraints on the peak temperature of metamorphism are available from accessory phase mineralogy of EH4 chondrites, especially their sulfide chemistry and observations from experimental work.The FeS contents of niningerites in most EH4 chondrites indicate closure temperatures of $800°C, with a slightly higher value of $900°C for Y-791790 from sphalerite (Kimura & Lin, 1999;Zhang & Sears, 1996).Additionally, the presence of the high-silica accessory phase roedderite in most EH4 chondrites indicates that peak temperatures did not exceed the solidus (Kimura & Lin, 1999).For EH4 assemblages, this is found at $1000°C (e.g., Levin et al., 1964;McCoy et al., 1999;Schairer & Yoder Jr, 1961).From the minor elementpoor geochemistry of olivine, it seems that the Y-791790 and Y-791810 meteorites are the most strongly metamorphosed EH chondrites in our study.These characteristics would be consistent with heating of EH chondrites to variable temperatures on the basis of their burial depths.A peak temperature of around 900°C was achieved by the very last surviving forsterite grains before their final dissolution, followed by rapid cooling, perhaps during impact excavation or breakup of their parent body (e.g., Kimura et al., 2005;Kimura & Lin, 1999).
As well as being an informative metamorphic record, this situation effectively acts as a natural diffusion experiment for minor elements in olivine.Such environments are very rare, with one such example reported by Qian et al. (2010), in which olivine xenocrysts had been residing unstably in dioritic magma with dissolution occurring more slowly than trace elements could diffuse through the crystal, thereby modifying their compositions.This occurred at intermediate magmatic temperatures ($950-1000°C) under typical terrestrial fO 2 (about 0.5-1.0log units more oxidizing than the fayalitemagnetite-quartz buffer) and water contents (probably a few wt% H 2 O) over a time scale of $100 years or less.By contrast, in EH chondrites, dissolution and minor element modification occurred under quite different conditions that were very unlikely to have ever occurred in terrestrial environments.Although temperatures may have been similar or slightly lower ($800-900°C), the environment was very strongly reducing (ΔIW-4.5 or thereabouts), free of H 2 O and probably lacking in any other fluid, and heating was conducted over a considerably longer time scale of many thousands of years.
Looking closely at the abundance and geochemistry of forsteritic olivine in each of the EH chondrites, a progression can be seen from what might be called the "primitive" EH3 chondrites (A-881575, ALHA 77295, EET 87746, Sahara 97079, and Y-691), through "EH3 (high)" A-882059, to "EH4 (low)" Y-74370 and "EH4 (high)" Y-791790 and Y-791810 (following Kimura et al., 2005).As mentioned above, the suggested EH3 (high) classification for A-882059 is made on the basis of its slightly less diverse olivine geochemistry and only moderate olivine abundance.Unlike for FeO, the olivine Cr 2 O 3 , MnO, and CaO concentrations in A-882059 (Figure 5b-d) seem to be above trend for its olivine modal abundance (0.1 vol%).For EH4 chondrites Y-74370, Y-791790, and Y-791810, the situation seems reversed, with Cr 2 O 3 , MnO, and CaO contents somewhat lower than expected from the trend (at 0.01 vol% olivine).As is also the case for the process of olivine dissolution, variations in chemistry may indicate microscale influences of fO 2 or aSiO 2 on diffusion of minor elements through olivine.

Forsteritic Olivine as a Tracer of Nebular Sources
The wide range of olivine geochemistry in primitive EH3 chondrites is consistent with inheritance of olivine and other precursor materials from a range of environments.However, the elemental evidence in olivine for differences in bulk-rock protolith mixtures among EH3 chondrites is very limited, being only that we did not find any refractory CaO-rich forsterites in A-881575.As discussed above, the trend toward pure forsterite with increasing petrologic type (i.e., into EH4) is likely due to metamorphism, and the original diverse olivine compositions probably reflect nebular sources feeding into the EH chondrite region.Because of the peritectic melting relationship between forsterite, enstatite, and silicate melt (Bowen & Schairer, 1935), the forsteritic olivine in EH3 chondrites is likely to preserve information about peak melting temperatures, that is, chondrule-forming events, and minor elements might be used to distinguish their sources.
For comparison with other nearby regions in the early Solar Nebula, we have gathered compositional data from the literature for forsteritic olivine in meteorites, and consider certain further criteria that might be suggestive of some relationship with EH chondrites.While rare FeO-rich pyroxenes and olivines do exist in EH chondrites (e.g., Lusby et al., 1987), the vast majority of olivines in EH chondrites are forsteritic in the technical sense (>90 mole% Mg 2 SiO 4 ).Similarly, reconstructed compositions for decomposed olivine indicate a small proportion of much more oxidized FeO-rich materials, but such olivine seems prone to breakdown (Ikeda, 1988;Lusby et al., 1987;Rambaldi et al., 1983).Ferroan or previously ferroan relicts, while isotopically related to enstatite chondrites (as found, e.g., as FeO-bearing pyroxenes reported by Weisberg et al., 2011) could not have formed during melting at the forsterite-enstatite peritectic under reducing conditions but must be inherited from a more oxidized, FeO-bearing reservoir (e.g., Jacquet et al., 2015).Such a reservoir may have become oxidized during a yet earlier period of chondrule formation followed by cooling at constant pO 2 , a principle well understood in thermodynamic and experimental petrology (e.g., Kilinc et al., 1983) and recently suggested for chondrules in CCs (Libourel et al., 2023).In chondrites more broadly, forsteritic olivines occur either in type I chondrules, as relict grains in type II chondrules, as isolated grains in the matrix, or in less common objects such as amoeboid olivine aggregates (AOAs; e.g., Frank et al., 2014).There is a tendency in the literature to divide forsteritic from more ferroan olivine in chondrites because at $10 wt% FeO, there is a frequently observed change from negative to positive correlation between FeO and CaO, and there can also be breaks in FeO-MnO correlations (Frank et al., 2014;Pack et al., 2004;Schrader et al., 2015;Scott & Taylor, 1983).These are likely to be a useful guide for assessing potential relationships between forsteritic olivine in EH chondrites and other types of meteorites.
In gathering a representative collection of meteoritic forsteritic olivine, the ordinary chondrites (OCs) and CCs are of obvious interest, but this phase is also found in some of the smaller chondrite groups as well as achondrites that have isotopic compositions close to those of EH chondrites.Such close isotopic relationships with EH chondrites can be most clearly identified by modern high-precision three-oxygen isotope analyses, for which we take advantage of gaps in the compositional space to outline a region close to the bulk enstatite chondrites.This isotopic space is likely to be especially important in terms of transport of forsteritic olivine grains or their precursor materials from one meteorite forming region to another.In isotope geochemistry, the ratios 18 O/ 16 O and 17 O/ 16 O are frequently expressed in δ notation relative to a reference composition (McKinney et al., 1950).For example, 18 O/ 16 O is expressed in partsper-thousand (per mille) as With δ 17 O similarly defined for 17 O/ 16 O, and variations in δ 17 O with respect to δ 18 O further expressed by deviation from a reference line (Clayton & Mayeda, 1984): where a value of 0.52 is commonly selected for λ to approximate mass-dependent fractionation behavior Forsteritic olivine in EH (enstatite) chondrites (e.g., Clayton & Mayeda, 1983).For most applications, Vienna Standard Mean Ocean Water (VSMOW) is used as a reference point defining the origin (Lin et al., 2010) and to anchor the terrestrial fractionation line (TFL; Clayton & Mayeda, 1984).We illustrate bulk meteorite compositions in Figure 6 (data from various sources, e.g., Greenwood et al., 2017Greenwood et al., , 2020;;and references therein).For isotopically heterogeneous materials such as chondrites, the use of secondary ion mass spectrometry (SIMS) to investigate grain-scale variations in threeoxygen isotope ratios has also generated a significant body of lower precision data, including for forsteritic olivine in enstatite chondrites (Weisberg et al., 2010(Weisberg et al., , 2021)).Using the reported δ 18 O and Δ 17 O values with their 1σ uncertainties, we generated a two-dimensional probability distribution plot (contoured at 5% intervals of total peak height) of forsteritic olivine in enstatite chondrites and overlay this onto Figure 6, which shows a close correspondence with bulk enstatite chondrite chondrules (Clayton et al., 1991;Clayton & Mayeda, 1985;Tanaka & Nakamura, 2017) in the area near the TFL.
Guided by the locations of well-populated meteorite groups, an area can be outlined with δ 18 O and Δ 17 O in the range +2.5‰ to +7.0‰ and À0.7‰ to +0.7‰, respectively, which encompasses a number of relevant materials.This isotopic space is likely to capture most sources of relict olivine falling into the enstatite chondrite regions.Although objects with compositions from outside this region certainly contributed incidentally to the EH chondrite inventory, such as CC, ordinary, or Rumuruti chondrite-related objects, the area outlined in Figure 6 contains the majority of the relevant SIMS data, that is, 51 of 60 olivine analyses with compositions in the near-terrestrial region (Weisberg et al., 2010(Weisberg et al., , 2021)).A similar distribution is available for pyroxene, which interestingly includes a few ferroan compositions (Kimura et al., 2003;Weisberg et al., 2011).Among the chondrite and primitive achondrite groups, there is a FIGURE 6. Three-oxygen isotope ratios of meteorites and selected meteorite components near the enstatite meteorite and terrestrial compositions.Data are presented in delta-notation (δ 18 O and Δ 17 O), that is, per mille (‰) variations relative to VSMOW and the Terrestrial Fractionation Line (TFL).Enstatite chondrite chondrule compositions are given by black squares (Clayton et al., 1991;Clayton & Mayeda, 1985;Tanaka & Nakamura, 2017) and all bulk meteorites are in gray squares (Greenwood et al., 2017(Greenwood et al., , 2020;;and references therein).Major meteorite groups are indicated either by a line showing the average Δ 17 O of achondrites (from Greenwood et al., 2017) or the general areas indicated for chondrites.Lower precision compositions of forsteritic olivine in enstatite chondrites (Weisberg et al., 2010(Weisberg et al., , 2021) ) are given by a two-dimensional probability distribution plot with contours at 5% intervals of total peak height.The Young and Russell (Y&R; Young & Russell, 1998), Primitive Chondrite Minerals (PCM; Ushikubo et al., 2012), and Carbonaceous Chondrite Anhydrous Minerals (CCAM; Clayton et al., 1977) lines are also given for reference.The rectangle encompasses compositions that are most likely to have some petrogenetic relationship to enstatite chondrites.Forsteritic olivine with less than $10 wt% FeO occur in most meteorite groups within the defined region, that is, the low-FeO (or HH) chondrites, Kakangari (K) chondrites, IAB complex and winonaites, and angrites (as xenocrysts), but not in the IIIAB-MGP, HED, IIE, or brachinite meteorites.Forsteritic olivine also occurs in most chondrites (e.g., type I chondrules in ordinary and CCs).
tendency toward chemically reduced and enstatite-rich assemblages on approach to the TFL (e.g., Rubin & Wasson, 1995;Sanders et al., 2017).For evolved achondrites, even though many have three-oxygen isotope ratios that are close to those of enstatite-bearing meteorites, some contain particularly FeO-rich silicates that are far outside the compositional ranges reported for EH chondrite materials.Examples of such materials include olivine-rich brachinites or the groundmass minerals in angrites; in these cases, we consider any genetic connection to be very speculative.

Comparison with Other Chondrites
Among the chondritic materials, forsteritic olivine frequently occurs in OCs and CCs in type I chondrules, as relict grains in type II chondrules, as isolated grains in the matrix, and as more exotic objects such as AOAs.Geochemically, "low-iron manganese-enriched" (LIME) forsterite, and CaO-rich "refractory" forsterite are particularly conspicuous.As for EH chondrites, the refractory, CaO-rich and Al 2 O 3 -rich forsterites in OCs and CCs are lacking in other minor elements (Figure 7).For crystallization of the most CaO-rich refractory forsterites ($1 wt% CaO), the condensed parental melt requires more than $20 wt% CaO, while moderately refractory forsterites ($0.3 wt% CaO) still require in the range $10-15 wt% CaO in the melt (Pack & Palme, 2003).Such CaO concentrations are suggestive of melts that also coexist with anorthite and diopside (e.g., Libourel et al., 1989), or extensive gas-solid reaction in the nebula (Jacquet et al., 2021).Nonrefractory forsterites with lower CaO values around $0.1 wt% are probably indicative of melts formed under conditions typical of enstatite chondrite-type environments.In similar fashion to CaO-rich forsterites, the MnO-rich forsterites in EH chondrites are also similar to those in OCs and CCs.However, generally speaking, the forsteritic olivines in EH chondrites are distinguished from those in OCs by their lower MnO (Figure 7a), and from both OCs and CCs by lower Cr 2 O 3 (Figure 7b), for any particular FeO content.The tendency toward higher MnO and Cr 2 O 3 concentrations in forsteritic olivine of OCs in particular reflects the higher concentrations of these oxides in bulk samples of these meteorites compared with ECs and CCs (Jarosewich, 1990;Shukolyukov & Lugmair, 2004;Wasson & Kallemeyn, 1988).
Of the minor chondrite groups with three-oxygen isotope ratios close to EH chondrites, forsteritic olivine is found in low-FeO (or "HH") chondrites (Pourkhorsandi et al., 2017;Pratesi et al., 2019), Kakangari (K)-type chondrites (Barosch et al., 2020;Nagashima et al., 2015), and forsterite (F)-type chondritic clasts found in the Cumberland Falls aubrite (Neal & Lipschutz, 1981; Figure 7).One can also add winonaites to this list (although we will discuss it in more detail in the next section), as these primitive achondrites likely represent metamorphosed chondrites (Tomkins et al., 2020).The forsteritic olivine in low-FeO chondrites, K chondrites, and winonaites is rather FeO-and MnO-rich but almost devoid of CaO and Cr 2 O 3 .The forsterites in the F-chondritic clasts are also quite poor in CaO and Cr 2 O 3 but also FeO and MnO, except for one (Cumberland Falls 604-6 in Neal & Lipschutz, 1981) which is very CaO-rich, similar to refractory forsterites in other chondrites.The occurrence of one such grain is consistent with previous suggestions that refractory CaO-rich forsterites occur in all chondrite groups, including those that are not well studied (e.g., Pack et al., 2004).For K chondrites, the single average composition available for LEW 87232 has lower FeO and MnO than any of the analyses reported for the well-studied Kakangari meteorite (Barosch et al., 2020;Berlin, 2009;Nagashima et al., 2015).This places it nearer the middle of the EH chondrite range than the low-FeO chondrites and suggests a hint of some similarity in petrogenesis between K and EH chondrites, but requires further study and perhaps discovery of more K chondrites.

Comparison with Achondrites
Among achondrites with three-oxygen isotope ratios similar (although not identical) to EH chondrites, forsteritic olivine is a primary constituent of aubrites, winonaites, IAB complex irons, and also occurs as xenocrysts in angrites.As might be expected, the overall geochemical trends of olivine in these meteorites are quite unlike those in EH3 chondrites and most chondrites in general.Rather than displaying the trimodal distribution of CaO, MnO, or FeO-Cr 2 O 3 enriched compositions, forsteritic olivine in achondrites generally displays FeO and MnO increasing together, at very low CaO and Cr 2 O 3 .While fewer data are available for the latter oxides, it is clear that Cr 2 O 3 levels in achondritic forsteritic olivine are very low (usually below detection limits) and unrelated to FeO content.The trends in achondritic olivine are mainly defined by the winonaites, and are rather scattered, especially in the case of FeO-MnO systematics, which is consistent with their status as primitive achondrites and likely origin as a metamorphosed minor chondrite group (e.g., Tomkins et al., 2020).At very low minor element oxide levels, the very pure forsterites in aubrites are similar to the remnant pure forsterites in EH4 chondrites.Such compositions are consistent with slowly cooled plutonic or metamorphic environments.
A small number of unusual olivine analyses reported for achondrites are similar to some of the endmembers that we have identified in EH3 chondrites.One analysis each from the winonaite Tierra Blanca (King et al., 1981) and the LEW 87007 L87I basalt vitrophyre found in an aubrite (Fogel, 2005) are quite MnO-rich with few other minor elements.This is similar to MnO-rich olivine in EH3 chondrites, although it should be noted that Cr 2 O 3 in the basalt vitrophyre forsterite is rather low, at 0.1 wt % (the Cr 2 O 3 content is unavailable for most forsteritic olivines in achondrites).While winonaites are primitive achondrites containing relict chondritic components indicating that they are highly metamorphosed chondrites, the basalt vitrophyres and their aubrite hosts are evolved achondrites and are not immediately related to chondritic materials.A relict LIME component might therefore be responsible for MnO-rich forsterite in the former, but almost certainly not in the latter, which is purely igneous.
Surprisingly, at higher FeO contents, some compositional similarity can be found between those forsteritic olivines in EH chondrites with lower CaO, MnO, and Cr 2 O 3 contents, and olivine xenocrysts in angrite NWA 1670 (with FeO as low as $5 wt%).These xenocrysts are the most magnesian found in angrites and are notable for their low MnO and CaO with modest levels of Cr 2 O 3 ($0.05%MnO, $0.1% CaO, and $0.2% Cr 2 O 3 , respectively; Mikouchi et al., 2011).This similarity occurs because with increasing FeO, forsteritic olivine in EH chondrites maintains moderate CaO and MnO contents while Cr 2 O 3 diverges into higher and lower varieties, with the lower similar to angrite xenocrysts.This is quite unlike the trend shown by ordinary, low-FeO, and K chondrites, and it also does not occur among common achondrites such as winonaites.Some of this trend is defined by the reconstructed "decomposition" olivines that Ikeda (1988) interpreted as breakdown products, but grains with this composition were found in other studies (Grossman et al., 1985;Lusby et al., 1987;Weisberg et al., 2021; and a single point in this study).
In chondritic materials, the Fe/Mn ratio of silicates is especially informative for understanding redox-and magmatic fractionation relationships between minerals and bulk samples.Most FeO-poor olivine found in EH chondrites, type I chondrules, and other related materials have, very broadly speaking, Fe/Mn in the range from $1 to 10, while most forsterites have moderately superchondritic values as in type II chondrules in OCs, around Fe/Mn $50.Due to the trimodal distribution of minor element data between refractory, LIME, and oxidized endmembers, the FeObearing and MnO-, CaO-poor forsteritic olivines in EH chondrites have Fe/Mn effectively in accord with those of type II chondrules in CCs, and especially CI and CO varieties, at $100 (Anders & Grevesse, 1989;Berlin et al., 2011;Frank et al., 2014).Such ratios have also been recognized for angritic materials and seem to indicate complete oxidation of available Fe-metal in their sources.This yields abundant FeO for crystallization of olivine which inherits an effectively unmodified Fe/Mn ratio near the bulk CI ratio of $100.In magmatic systems, such high FeO contents lead to secondary effects such as a higher modal olivine fraction, silica undersaturation driving enhanced CaO in olivine via low aSiO 2 , and eventually stabilization of spinel that can buffer Cr 2 O 3 in olivine to moderate levels (e.g., Jurewicz et al., 1993;McKibbin & O'Neill, 2018).The low Cr 2 O 3variety of forsteritic olivine in carbonaceous and EH chondrites exhibiting CaO increasing modestly with FeO suggests that such environments were a minor, but substantial, contributor of materials during magmatic or chondrule-forming events in the EH chondrite region.

CONCLUSIONS
In this study, we have investigated nine EH3 and EH4 chondrites by petrographic and chemical methods to determine modal abundances and minor element geochemical trends in primary and remnant forsteritic olivine.We have found that, in EH chondrites, the abundance of forsteritic olivine decreases with increasing petrologic type, and the compositions of remnant grains approach quite pure forsterite.This occurred because with increasing metamorphism, olivine was susceptible to dissolution by reaction with free silica, as well as simultaneous extraction of minor elements due to their higher compatibility in groundmass enstatite.The modal abundance and minor element geochemistry of olivine in EH chondrites are therefore very useful in classification of petrologic type for these meteorites.For olivine-rich unequilibrated EH chondrite protoliths, the global trends are also informative to compare with other meteorite groups.The minor element geochemistry of EH3 olivines is quite unlike those in all other non-CC meteorites.Among forsteritic olivines in OCs, low-FeO chondrites, K chondrites, and winonaites, we find that FeO and MnO are strongly correlated, but this is not the case for EH or CCs.We also find that refractory forsterites with elevated CaO seem to occur in all chondrite groups, and that olivines in K-chondrites might have some relationship to those in EH chondrites with moderate minor element contents, but only a few K-chondrites are known at present.Of all chondrite materials, the minor element distribution in unequilibrated EH chondrites most closely resembles that of forsterite in type I chondrules, relict grains, or AOAs in CCs, despite their quite separate isotopic identities and likely origins in different parts of the solar system (e.g., probable inner and outer solar system sources for noncarbonaceous and CC materials, respectively; Trinquier et al., 2007;Warren, 2011).
Among achondrites, the relatively abundant, pure forsterite in aubrites is geochemically similar to remnant forsterite in EH4 chondrites, due to both having long histories of slow cooling with extraction of minor elements by enstatitic groundmass under reducing conditions.A few forsteritic olivine grains in other achondrites show unexpected similarities with some of the EH3 olivine clusters, that is, (1) at high MnO, forsterite in winonaite Tierra Blanca and the aubriteassociated LEW 87007 L87I basalt vitrophyre are similar to the MnO-rich branch of EH3 chondrite olivines, but they seem unlikely to be related to the "LIME" component found in chondrites; and (2) at high FeO and moderate CaO, MnO, and Cr 2 O 3 contents, the most magnesian olivine xenocrysts in angrites have some unexpected similarity to FeO-rich, Cr 2 O 3 -poor olivines in EH3 and CCs.Therefore, the geochemistry of forsteritic olivines in EH chondrites seems to indicate not only a range of fO 2 conditions in their sources, as has long been recognized from FeO contents and olivine breakdown products, but also a high degree of silica undersaturation leading to elevated CaO in FeO-bearing grains that stand in contrast to the SiO 2 -rich reservoir responsible for the formation of other phases in enstatite chondrites.

FIGURE 1 .
FIGURE 1. (a, c, e) Backscatter electron (BSE) and (b, d, f) corresponding X-ray energy-dispersive spectroscopy (EDS) images of selected enstatite chondrites in this study.Colors in EDS images were assigned by element resulting in a false-color phase map corresponding to mineralogy, where bright blue = olivine; pale blue = pyroxene; bright yellow = silica; dark yellow = plagioclase; bright green = Fe-Ni metal; dark green = weathering effects; orange = troilite (FeS); red = oldhamite (CaS); purple = niningerite (MgS).Field of view for all images is 2 mm.Samples include: (a) and (b) EH3 chondrite Sahara 97079; (c) and (d) EH4 chondrite Y-74370; and (e) and (f) EH5 chondrite A-881475.(Color figure can be viewed at wileyonlinelibrary.com) olivine and minor and trace element-rich olivine that are negatively correlated across the data set.Group (1) comprises analyses of olivine with low concentrations of all minor and trace elements, that is, quite pure forsterite, dominated by analyses from the EH4 chondrites Y-74370, Y-791790, and Y-791810.The other groups are controlled by olivine in A-881575, ALHA 77295, EET 87746, Sahara 97079, and Y-691.These groups comprise olivine with characteristics being: (2) CaO-rich; (3) FeO-CaO-rich and Cr 2 O 3 -bearing; (4) FeOrich and MnO-Cr 2 O 3 -bearing; (5) MnO-rich and Cr 2 O 3bearing; and (6) geochemistry intermediate to all the other groups.
FIGURE 3. Major and minor oxide contents for forsteritic olivine in EH chondrites obtained by EMPA in this study (colored by meteorite; ordered from high to low olivine content) and previously reported literature values (in gray;Grossman et al., 1985;Ikeda, 1989;Kimura & Lin, 1999;Leitch & Smith, 1982;Lusby et al., 1987;Rambaldi et al., 1983;Schneider et al., 2002;Weisberg et al., 2011Weisberg et al., , 2021)).Data for EL olivine are fromSchneider et al. (2002).(a) FeO and Cr 2 O 3 (cf.Weisberg et al., 2005 and Bendersky et al., 2007, who reported a similar data distribution in figure form only).(b) Al 2 O 3 and CaO.Some data at higher values are not shown at these scales.(Color figure can be viewed at wileyonlinelibrary.com) FIGURE 4. Major and minor oxide contents for forsteritic olivine in EH chondrites obtained by EMPA in this study (colored by meteorite; ordered from high to low olivine content) and previously reported literature values (in gray;Grossman et al., 1985;Ikeda, 1989;Kimura & Lin, 1999;Leitch & Smith, 1982;Lusby et al., 1987;Rambaldi et al., 1983;Schneider et al., 2002;Weisberg et al., 2011Weisberg et al., , 2021)).Data for EL olivine are fromSchneider et al. (2002).(a) CaO and FeO.(b) FeO and MnO (cf.Weisberg et al., 2005 who reported a similar data distribution in figure form only). (c) CaO and MnO.Some data at higher values are not shown at these scales.(Color figure can be viewed at wileyonlinelibrary.com)

TABLE 2 .
Hierarchical cluster groupings for olivine in EH chondrites with average compositions and SDs.