Meteoritics & Planetary Science 41, Nr 7, 979–988 (2006) Abstract available online at http://meteoritics.org
Microtextures and crystal chemistry of pigeonite in the ureilites ALHA77257, RKPA80239, Y-791538, and ALHA81101
Mario TRIBAUDINO
Dipartimento di Scienze della Terra, Università di Parma, Parco Area delle Scienze 157/A, I-43100, Parma, Italy E-mail: [email protected] (Received 29 September 2005; revision accepted 28 March 2006)
Abstract–The microtextures of pigeonite in four ureilites, Allan Hills (ALH) 77257, Reckling Peak (RKP) A80239, Yamato (Y-) 791538, and Allan Hills A81101, chosen to span a range of composition and shock level, were investigated by transmission electron microscopy (TEM); two of the samples were also investigated by single crystal X-ray diffraction to determine Fe2+-Mg cation site partitioning. The low-shock and compositionally homogeneous pigeonites in ALHA77257 and RKPA80329 (Wo 6.4 for both, mg 86.3 and 84.3 respectively) display irregularly spaced, shock-induced stacking faults oriented parallel to (100), and large antiphase domains (50–100 nm). Antiphase domains have no preferential orientation. No evidence of exsolution was observed. The low-shock Y-791538 pigeonite is homogeneous and has higher Ca and mg (Wo 9.4, mg 91.2). TEM investigation showed spinodal decomposition, indicative of incipient exsolution; small ≈ 2+ antiphase domains were observed ( 5 nm). Single crystal refinement yielded R4σ = 5.71%, with Fe - Mg partitioning coefficient kD = 0.077(8) and Tc = 658(35) °C. ALHA81101 has compositionally heterogeneous pyroxenes, with large local variations in Wo and mg (Wo = 4–13, mg = 86–68). No compositional gradients from core to rim of grains were observed, and the heterogeneity is interpreted as related to cation migration during shock. In one relatively Ca-rich region (Wo ≈12), TEM analysis showed augite-pigeonite exsolution lamellae, with spacing 145(20) nm. Results for ALHA77257, RKPA80239, and Y-791538 support a model of rapid cooling following breakup of the ureilite parent body. The presence of exsolution lamellae in ALHA81101 can be related to a local shock-induced Ca enrichment and provides no constraint on the late cooling history.
INTRODUCTION Among pyroxenes the Ca-poor monoclinic P21/c pigeonite is of specific interest. Pigeonite is invariably formed The analysis of microtextures in minerals via at high temperature and during cooling enters the ortho- transmission electron microscopy (TEM) is a powerful clinopyroxene two-phase field. Therefore, pigeonite can be technique for constraining the thermal and shock history of a homogeneous at the atomic scale only after very rapid meteorite. Such analysis relies upon the correlation of cooling. Very slow cooling results in complete inversion to microtextures with experimental or theoretical data, on orthopyroxene and exsolved clinopyroxene. Furthermore, temperature and pressure conditions of formation, and on the there is a series of intermediate microtextures between kinetics of their formation. Pyroxenes are especially suited for homogeneous pigeonite and exsolved orthopyroxene, which such an investigation because 1) exsolution textures, can be related to the cooling rate. antiphase domains, and microtwinning are commonly Microtextural data can be supplemented by single crystal present; 2) a number of theoretical and experimental X-ray diffraction data (XRD) to retrieve Fe2+-Mg investigations have been performed on pyroxenes to constrain intracrystalline partitioning between the M1 and M2 site in the observed textures; and 3) pyroxenes are ubiquitous in pyroxenes and provide additional information on thermal several meteorite groups, which allows interesting history. The obtained partitioning coefficient kD* is related to comparative studies. the closure Tc of cation exchange and can provide a
979 © The Meteoritical Society, 2006. Printed in USA. 980 M. Tribaudino semiquantitative indication of the cooling rate of the host and pyroxenes were performed on thin sections only for meteorite (Ganguly 1982; Ganguly et al. 1994; Tribaudino samples ALHA77257 and ALHA81101. The pigeonites in et al. 1997; Zema et al. 1997; Domeneghetti et al. 2000; RKPA80239 and Y-791538 were analyzed from a few grains Goodrich et al. 2001; Domeneghetti et al. 2005). Structural of the crushed portion embedded in epoxy and polished. This analyses to calibrate cation partitioning mostly have been was done because the thin section of Y-791538 was lost performed on orthopyroxene, which has a simpler chemistry during thinning and because pigeonite was not present in the than pigeonite and shows fewer defects. Cation ordering in RKPA80239 thin section. pigeonite has been investigated only by Pasqual et al. (2000), Microprobe analyses were determined using a SEM who experimentally calibrated the equilibrium kD at different Cambridge S 360 electron microprobe equipped with EDS temperature in two pigeonites, one from natural volcanic Link Analytical QX 2000. Natural oxides and minerals were rhyodacite (BTS308) and the other from the ureilite PCA used as standards. ZAF 4 corrections were applied to the 82506. The Pasqual et al. (2000) calibrations were analyses. Analytical results on pyroxenes are reported in comparable to those obtained in orthopyroxenes, and for both Table 1. samples cation ordering fell within expectations for rapidly A few single crystals of pigeonite from each of the four cooled samples. ureilites were selected on the basis of sharp optical extinction Microtextural and cation ordering investigations in and then tested by X-ray diffractometry; most of the crystals pigeonite are of interest in ureilites. Ureilites are a C-rich selected turned out not to be suitable for single crystal X-ray achondrite group, mostly composed of olivine and diffraction, displaying large and diffuse peak profiles. Full pyroxene(s), namely uninverted pigeonite and more rarely refinements were done for ALHA77257, RKPA80239, and Y- augite and/or orthopyroxene. Pigeonite from ureilites differs 791538 pigeonites, but the RKPA80239 and ALHA77257 from that in most terrestrial volcanic samples in having higher data were discarded because the refinement results did not mg and higher crystallization temperatures (Mittlefehldt et al. permit reliable cation partitioning. As expected by its highly 1998); inversion to orthopyroxene is not observed due to defective shock overprint, ALHA81101 turned out to be rapid cooling from high temperature. completely unsuitable for data collection. The best crystals A number of microtextural investigations have been for each sample were measured with a Siemens P4 four circle performed on ureilite pyroxenes (Chikami et al. 1997; diffractometer. Tribaudino et al. 1997; Goodrich et al. 2001; Weber et al. The data collections were done with the θ-2θ scan type 2003). In two of these studies the microtextural observations technique using graphite monochromatized MoKα radiation were coupled with determination of Fe-Mg cation partitioning (λ = 0.71073 Å), and the reciprocal lattice was explored up to in orthopyroxenes (Tribaudino et al. 1997; Goodrich et al. θ ≤ 30°. Two sets of equivalent reflections were measured. A 2001). For ureilite pigeonite, determination of cation ordering correction according to the ψ scan (North et al. 1968) for coupled with TEM analysis has been done only on PCA absorption did not significantly improve the refinement 82506 (Pasqual et al. 2000). results. It is the purpose of this paper to investigate specifically The structure refinements were completed using the microstructures and cation partitioning in pigeonite in SHELXL-97 (Sheldrick 1997), performed in P21/c space ureilites, to characterize the different pigeonite samples, and group starting from the atomic coordinates of clinoenstatite to assess the potential and limits for gathering information on (Ohashi and Finger 1976). The refinement reached cooling history from pigeonite. Four samples of different convergence before the final full-matrix least-squares with shock level and composition were therefore investigated by chemical constraints was carried out. All Ca and Na were means of a combined TEM-XRD single crystal analysis. ascribed to the M2 site; Cr was constrained to occupy the M1 site, while Al was partitioned between the T and M1 sites with EXPERIMENTAL the condition that Cr3+ + AlVI = Na + AlIV to obtain a neutral crystal formula. After partitioning of Al the cation sum in T The pigeonite-bearing ureilites ALHA77257, and M1 + M2 sites were normalized and used as a basis for RKPA80239, ALHA81101, and Y-791538 were studied. Fe2+-Mg partitioning between the M1 and M2 sites. Mn was Samples ALHA77257,124, RKPA80239,14, and partitioned as Fe2+ between the M1 and M2 sites. For ALHA81101,39, weighing 2.5, 2.2, and 2.4 g, respectively, consistency with previous calbrations, this procedure was were obtained from the NASA curatorial facility at Johnson preferred over the recommendation of Stimpfl (2005) to Space Center; a sample of Y-791538 weighing 1.6 g was ascribe all Mn to the M2 site. The refinement of Mg and Fe2+ obtained from the National Institute of Polar Research site occupancy between the M2 and M1 sites was done with (Japan). A chip of each sample was crushed in order to select the constraints that: 1) total occupancy of M1 and M2 sites is single crystals for diffraction and TEM observation; thin equal to one, and 2) the Fe2+/Mg ratio is equal to the analytical sections for SEM (scanning electron microscope) and value. The error for the chemical constraints was assumed to microprobe analysis were prepared from the remaining part of be 1σ. Standard SHELXL-97 procedures were used for each sample. As it turned out, microprobe analyses of olivine refinement of the Mg and Fe2+ site occupancies; details of the Microtextures and crystal chemistry of pigeonite in ureilites 981
Table 1. Compositions of pyroxenes in the investigated samples by SEM-EDS analysis on polished thin sections. Pigeonite Orthopyroxene ALHA77257 Y-791538 RKPA80239 ALHA81101 ALHA77257 Y-791538
SiO2 57.3(7) 58.1(7) 55.2(7) 56.1(9) 56.4(5) 57.5(8) TiO2 n.d. n.d. n.d. n.d. n.d. n.d. Al2O3 0.70(9) 0.90(10) 0.62(8) n.d. 0.72(4) 0.99(12) Cr2O3 1.13(12) 1.01(15) 1.13(13) 1.3(4) 1.07(7) 0.85(16) FeO 8.4(3) 5.27(16) 9.6(2) 11.1(4) 7.4(4) 5.01(16) MnO 0.51(2) 0.55(8) 0.43(8) 0.5(2) 0.48(13) 0.4(3) MgO 29.7(4) 30.8(4) 28.8(5) 28(2) 31.3(4) 32.2(4) CaO 3.29(12) 4.9(2) 3.23(14) 4(2) 2.05(18) 2.56(11) Na2O n.d. 0.40(16) n.d n.d. 0.03(2) 0.34(18) Total 101(1) 102(1) 99(1) 101(2) 99.5(6) 100(1) Si 1.999(7) 1.991(6) 1.980(7) 1.983(10) 1.979(6) 1.990(5) Ti n.d. n.d. n.d. n.d. n.d. n.d. Al 0.030(5) 0.037(5) 0.026(5) n.d. 0.030(3) 0.040(5) Cr 0.031(3) 0.027(4) 0.032(4) 0.035(12) 0.030(4) 0.023(4) Fe2+ 0.244(6) 0.151(5) 0.287(7) 0.33(10) 0.218(13) 0.145(5) Mn 0.015(1) 0.016(3) 0.013(3) 0.014(6) 0.010(4) 0.012(9) Mg 1.543(13) 1.574(7) 1.540(18) 1.50(13) 1.649(8) 1.662(9) Ca 0.123(4) 0.178(4) 0.124(4) 0.16(7) 0.077(6) 0.095(3) Na n.d. 0.027(10) n.d n.d. 0.002(1) 0.023(12) Total 3.984(7) 4.001(11) 4.004(9) 4.013(16) 3.987(8) 3.990(9) mg 86.3(5) 91.2(4) 84.3(5) 82(6) 88.3(8) 92.0(8) Wo 6.4(4) 9.4(6) 6.4(4) 8(3) 4.0(4) 5.0(8) N° an 19 14 15 12 4 12 The standard deviation in brackets refers to the average of the analyses performed for each sample, and is related to the sample heterogeneity. The standard deviation is, in the case of fully homogeneous samples, equal to the analytical error. N° an = the number of analytical points used in averaging compositions; mg = 100Mg/(Mg+Fe); n.d. = not detectable. refinement procedure are described elsewhere (Pasqual et al. Table 2. Cell parameters, main structural results, and site 2000). Atomic scattering curves from the International tables occupancies of the Y-791538 pigeonite. for X-ray crystallography (Ibers and Hamilton 1974) were a (Å) 9.689(3) Si site used. A fully anisotropic refinement was performed using all b (Å) 8.882(3) Si 1.982 the unique reflections obtained after merging the equivalent c (Å) 5.212(2) Al 0.018 reflections. A fixed weighting scheme was used for the data at V(Å3) 108.50(3) M1 site the end of the refinement cycles. Selected structural and Nobs (unique) 1239 Mg 0.938(2) 2+ refinement data are reported in Table 2; the data are available Rall (%) 7.81 Fe 0.016(2) in full from the author. R4σ (%) 5.71 Cr 0.027 TEM investigations were performed on pigeonite Goof 1.23 Al 0.018 crystals that were crushed in an agate mortar and deposited on Mn 0.001
Fig. 1. Molar Ca/(Ca + Mg + Fe2+) versus 1 − Mg/(Mg + Fe2+) in pigeonites from ureilites and other samples. Open circles from a compilation by Singletary and Grove (2003); diamonds from this work and Pasqual et al. (2000). Full square = volcanic BTS308 from Paranà rhyodacite (Pasqual et al. 2000) and full circle = synthetic Ca0.15Mg1.85Si2O6 (Tribaudino and Nestola 2002). The pigeonites studied in this work are in bold. For sample ALHA81101 the standard deviation (1σ) of the analytical dispersion is given; in the other samples investigated in this work it is smaller than the symbol. abundant triple junctions, whereas ALHA81101 has a high- boundaries of olivine due to shock was found by Saito and shock overprint with mosaicized olivine and deformed Takeda (1990) in the same meteorite. Heterogeneity of pigeonite. ALHA77257 and RKPA80239 have 5–50 µm wide pyroxene compositions has been found (e.g., Berkley et al. reduction rims on olivine grains. 1980) in most highly shocked ureilites. SEM-EDS analyses of pyroxenes are reported in Table 1. The compositions of the homogeneous pigeonites can be The analytical dispersion of the analyses is shown in brackets. used to estimate crystallization temperatures to within Cation charge excess is within 0.02 charge unit per formula ±20 °C, assuming that they represent olivine-pigeonite- unit, with the exception of ALHA77257, in which it is as liquidus equilibrium (Singletary and Grove 2003). For the much as 0.06, and in ALHA81101, in which Al possibly pigeonites in this work, crystallization temperatures were could have been overlooked. The analytical results reported estimated as 1260, 1275, and 1257 °C respectively for in Table 1 are consistent with previous reports on the ALHA77257, Y-791538, and RKPA80239 using the pigeonites in the meteorites studied. Pigeonite was found in geothermometric formulation given in Singletary and Grove all samples, and in ALHA77257 and Y-791538 (2003). These temperatures are consistent with expectations orthopyroxene was also found. Orthopyroxene is a main based on the Fo contents of olivine (Singletary and Grove constituent in Y-791538, whereas in ALHA77257 it was 2003). The results of this work are also in agreement with the found only as one tiny crystal (about 50 µm) within olivine, estimate of 1240 °C obtained from two-pyroxene confirming the previous observation by Takeda (1987). The thermometry by Takeda (1989) from the pigeonite- chemical compositions of the examined pigeonites compared orthopyroxene couple in sample Y-791538. with other ureilites and with terrestrial volcanic pigeonite are reported in Fig. 1. In general, ureilitic pigeonites have higher Microtextures in Pigeonite mg than terrestrial volcanic ones; in addition, among ureilites there is significant variation in Ca and mg content. As shown ALHA77257 and RKPA80239 later, this significantly affects the microtextures that are TEM observation shows the presence of a lamellar observed, and in part also the reliability of the determined texture in both samples. The lamellae are oriented parallel to cation partitioning. (100), are irregularly spaced (on average 8 nm, but with The pyroxenes from the low-shock ureilites were found lamellae up to 100 nm wide), and induce a streaking of the to be compositionally homogeneous, without core-rim zoning diffraction patterns (Figs. 2a–2c). They are interpreted as a (Table 1). In contrast, the pigeonite in ALHA81101 is pervasive twinning, presumably related to shock, in heterogeneous, with an average composition similar to that of agreement with previous observations by Mori and Takeda the other samples but large local variations in Wo and mg (4– (1988) on the ALHA77257 ureilite. Twinning locally occurs 13 for Wo, and 86–68 for mg). A possible explanation for this at the unit cell scale; a superstructural periodicity of 1.8 nm heterogeneity is strong elemental mobilization at high and an orthopyroxene-like slab is then observed (Fig. 2b). temperature due to impact strain. Migration of Ca at grain Repetition of twinning each three or more unit cells is also Microtextures and crystal chemistry of pigeonite in ureilites 983
Fig. 2. Microtextures in the RKPA80239 and ALHA77257 pigeonites: a) selected area electron diffraction (SAED) pattern in RKPA80239, view along [010]; note streaking along the a* axis, due to stacking disorder; b) lattice image showing 1.8 nm orthopyroxene-like periodicity within 0.9 nm periodicity of pigeonite; c) stacking disorder in ALHA77257, dark field image g = 400; the diffraction pattern is equivalent to that in (a); d) antiphase domains in ALHA77257; dark field g = 012. observed. TEM-EDS analysis did not show significant Y-791538 chemical differences between twinned and untwinned areas. In the Y-791538 pigeonite, pervasive lamellae parallel to No exsolution texture, not even in the form of a nonperiodic (100) are not present; consequently, selected area diffraction modulation, was found in these samples; furthermore, peak patterns do not show significant streaking along a* (Fig. 3a). splitting indicative of exsolution was never found, confirming Diffraction spots show a peak splitting (inset of Fig. 3a), and that the observed lamellae have a crystallographic rather than dark field imaging with h+k even reflections shows a compositional origin. modulation, most prominent parallel to (100) but also parallel Antiphase domains are present in both samples, sized to (001) (Fig. 3c). The presence of satellites along a* about 150 nm (Fig. 2d), as previously observed in another (Fig. 3a), although they are rather diffuse, indicates that the section of ALHA77257 by Mori and Takeda (1988). modulations can be interpreted as incipient spinodal 984 M. Tribaudino
Fig. 3. Microtextures in Y-791538 pigeonite: a) SAED pattern viewed along [010] (as in Fig. 2a). Note the lack of streaking along a* and the splitting of diffraction peak. In the inset the (204) diffraction peak is enlarged; note the presence of satellites of the main peak (arrowed); b) small-sized antiphase domains; dark field g = 102; c) compositional modulation related to spinodal decomposition; dark field, g = 202. The images in (a–c) were taken on the same grain. decomposition textures (Weinbruch et al. 2003). The (Fig. 4a), and by TEM-EDS microprobe analysis of the modulations along the c* axis are, in contrast, nonperiodic, phases. The TEM-EDS analysis of the grain in Fig. 4 reveals and in diffraction patterns they are revealed by streaking a Ca/Si ratio of 0.09 for the host pigeonite, 0.30 for the augite (Figs. 3a–3c). Elongated antiphase domains 10 nm in size lamella, 0.12 for the bulk crystal, and between 86 and 88 for (Fig. 3b), i.e., significantly smaller than in ALHA77257 and mg. An alternative determination of lamellar composition RKPA80239, are observed. Smaller-sized domains in was performed measuring in selected area diffraction patterns connection with compositional modulations have been the differences in β parameter, which is extremely sensitive to observed in samples with mottled textures (Tribaudino 2000; the Ca content (Turncock et al. 1973, Gordon et al. 1981) but Weinbruch et al. 2001) and interpreted as a result of a pinning insensitive to mg. The obtained ∆β between the Ca-rich and effect of the Ca-rich region of the modulation to Ca-enriched Ca-poor phase is 1.26°, which corresponds to a composition domain boundaries (Tribaudino et al. 2003). The presence of of Wo 31 for the clinopyroxene. Application of two-pyroxene antiphase domains indicates that the pigeonite crystallized in thermometry from QUILF (Andersen et al. 1993) to the two- C2/c symmetry, above the transition to P21/c. This is in good pyroxene pair compositions obtained by TEM-EDS agreement with thermometric estimates: the critical microprobe analysis gives an exsolution T = 1253(12) °C. temperature for the transition can be estimated, for the Owing to the small amount of pigeonite available, a composition of the Y-791538 pigeonites, to be near 1000 °C systematic analysis on the sample was not performed, and (Arlt et al. 2000; Tribaudino et al. 2005). further investigation may be needed.
ALHA81101 DISCUSSION In ALHA81101, the major finding was the observation of augite exsolution lamellae in pigeonite (Figs. 4a and 4b). The Crystal Structure and Fe2+-Mg Order Disorder average size of the pigeonite lamellae is about 120 nm, that of augite 20 nm, the wavelength 140 nm. The exsolution occurs Fe2+-Mg site partitioning in Y-791538, as determined by along (001) as confirmed by the analysis of selected area single crystal X-ray diffraction, is reported in Table 2. The kD diffraction patterns, showing clear splitting along the c* axis partitioning coefficient of the reaction: FeM2 + MgM1 = MgM2 Microtextures and crystal chemistry of pigeonite in ureilites 985
Fig. 4. Exsolution in ALHA81101: a) SAED pattern showing peak splitting related to exsolution lamellae along 001; b) bright field image of exsolution lamellae in ALHA81101.
− + FeM1 is also reported, calculated as kD* = XFe*(M1)[1 orthopyroxenes from FRO 90054 and Hughes 009 XFe*(M2)] / XFe*(M2)[1 − XFe*(M1)], where XFe*(M1) = (Tribaudino et al. 1997; Goodrich et al. 2001), and the (Fe + Mn)M1 / (Fe + Mn + Mg)M1 and XFe*(M2) = (Fe + pigeonite from PCA 82506 (Pasqual et al. 2000). Mn)M1 / (Fe + Mn + Mg)M1 (Pasqual et al. 2000). As in any The presence of a disordered (broadly speaking) pyroxene, there is a preferential partitioning of Fe2+ in the M2 configuration is indicative of a rather fast cooling rate. In site and of Mg in the M1 site, and cation disorder has to be Hughes 009 orthopyroxene (Goodrich et al. 2001), the considered in a relative way. A general rule to interpreting cooling rate was estimated at the closure T to be about 7°/h, cation partitioning is that higher disorder is related to faster based on the analysis of single crystals of orthopyroxene. cooling; however, the same kD in different samples may not a priori indicate the same cooling rate. The effect of different Microtextures and Cooling History compositions and microstructures also has to be addressed. Following an ideal model, a linear equilibrium relation The various pigeonites examined in this study have lnkD* = a/T + b (Ganguly 1982) was established and has been different compositions and display different microstructures. calibrated for several samples of different mg in The ALHA77257 and RKPA80239 pigeonites have a low orthopyroxene and pigeonite. The differences between average Ca content (Wo 6.4) and contain twin lamellae related orthopyroxene and pigeonite are rather small (Stimpfl et al. to stacking disorder and antiphase domains, but no exsolution 1999; Pasqual et al. 2000), permitting at least a general textures. The lack of exsolution is related to the relatively Ca- comparison between them. Namely, the calibration for poor composition and rapid cooling. These pigeonites orthopyroxenes with mg between 89 and 80 (Stimpfl et al. crystallized in the single phase field at high temperature, and 1999) for orthopyroxene and that of Pasqual et al. (2000) on during cooling intercepted the solvus of Ca-rich and Ca-poor pigeonite sample from ureilite PCA 82506 (mg = 80) were clinopyroxene. Due to the low Ca content the chemical solvus used in the comparative Fig. 5 (in Zema et al. 1997). The was actually intercepted, but not the spinodal, and exsolution calibration by Pasqual et al. (2000) was used to determine the could take place only through nucleation and growth (Fig. 6). Tc of the refined sample. Nucleation and growth is more kinetically demanding than In Fig. 5, comparison with terrestrial volcanic and spinodal decomposition and could not occur in ALHA77257 metamorphic orthopyroxenes and the volcanic pigeonite and RKPA80239 pigeonites due to relatively fast cooling. The BTS308 shows that Y-791538 falls within the field of rapidly same interpretation can be applied to the lack of exsolution in cooled terrestrial volcanics, close, within error, to the PCA 82506, which has a similar Ca content, and to the 986 M. Tribaudino
Fig. 5. -lnk versus 1/T in pigeonites (open diamonds) and orthopyroxenes (full squares) in ureilites; k * = XFe*(M1)[1 − XFe*(M2)]/ D − D XFe*(M2)[1 XFe*(M1)], where XFe*(M1) = (Fe + Mn)M1/(Fe + Mn + Mg)M1 and XFe*(M2) = (Fe + Mn)M2/(Fe + Mn + Mg)M2, was determined from single crystal refinement. The range observed in orthopyroxenes in other achondrites and in terrestrial rocks is reported for comparison; the small diamond refers to the BTS308 pigeonite in rhyodacite (Pasqual et al. 2000). The dashed line indicates the calibration for orthopyroxene by Stimpfl et al. (1999); the range of terrestrial volcanic and metamorphic orthopyroxenes is shown in bold. The two pigeonites lie slightly off the line, as the 1/T was obtained by the calibration of Pasqual et al. (2000).
Fig. 6. Phase diagram showing the chemical solvus (solid line) between one-phase pigeonite (Pig) and two-phase pigeonite + augite (Pig + Aug) fields, and the coherent spinodal solvus (dashed line). The chemical solvus was calculated using Andersen et al. (1993) for mg ∼92, as in Y-791538 pigeonite. A decrease in mg to the values by ALHA77257 and RKPA80239 pigeonites decreases the solvus by about 30 °C. Symbols used: diamonds = Y-791538 (Y79, on the right), ALHA77257 and RKPA80239 (Al77/RK80, on the left, overlapping one another) pigeonites at the crystallization T determined by Singletary and Grove (2003); error bar = compositional range of ALHA81101 pigeonite at the T determined by pyroxene exsolution (see text); circle = bulk composition of the grain displaying exsolution in ALHA81101. Microtextures and crystal chemistry of pigeonite in ureilites 987 pigeonite of Jalanash and HaH 064 ureilites (Wo 7.8 and 4.5, Another point is stacking disorder, which was found in respectively [Weber et al. 2003]). The Y-791538 pigeonite the relatively Ca-poor pigeonites, in ALHA77257, and had a higher average Ca content (Wo 9.4); at its formation RKPA80239 in this work, and also in Jalanash and HaH 064 temperature, it is within the single phase stability field, but (Weber et al. 2003). One interpretation is that stacking during cooling the spinodal solvus is intercepted at a disorder results from mild shock (Takeda 1987). However, temperature and at a cooling rate sufficient to promote cation although both ALHA77257 and Y-791538 were mildly diffusion in Ca-richer and Ca-poorer areas. shocked, stacking disorder is very high only in the former. In It must be stressed that in the above interpretation, fact, a chemical constraint is likely: stacking disorder leads to different cooling rates for Ca-poorer, exsolution-free, and the formation of orthopyroxene lamellae, with a structure Ca-richer, exsolution-bearing pigeonite are not required. In locally unfavorable to Ca. In relatively Ca-rich pigeonites the the analysis of microtextures as related to cooling history, formation of stacking faults during shock is inhibited; care must be taken to compare compositionally similar however, in relatively Ca-poor pyroxenes subjected to the samples. same degree of shock, stacking disorder will occur. In the case of Y-791538, the lack of experimental calibration of the onset and the evolution of the wavelength of Acknowledgments–The author wishes to thank the Antarctic spinodal decomposition in pigeonite limits cooling rate Meteorite Working Group in the U.S. and the National calculations, but end-member conditions can be still defined. Institute of Polar Research in Japan for providing the At first, assuming that the chemical and spinodal solvi have meteorite samples. The revision of the text and help in the same relation as in the Ca-rich region, it can be tentatively choosing the appropriate samples by C. A. Goodrich are assumed that the spinodal solvus is crossed at about 1100 °C acknowledged. Reviews by F. Camara, M. C. Domeneghetti, for Y-791538 (Fig. 6, based on Jantzen 1984). This can be D. H. Lindsley, S. J. Singletary, and I. Weber significantly confirmed by the size and shape of antiphase domains, which improved the paper. as discussed above, require a pinning of local Ca-enrichment; the enrichment should be already present at the transition. As Editorial Handling—Dr. Cyrena Goodrich ≈ the P21/c–C2/c transition occurs at T 1000 °C (Arlt et al. 2000; Tribaudino et al. 2005), the beginning of spinodal REFERENCES decomposition is constrained to a higher temperature. An indication as to the cooling rate of Y-791538 may Andersen D. J., Lindsley D. H., and Davidson P. M. 1993. QUILF: A come from continuous cooling experiments by Weinbruch PASCAL program to assess equilibria among Fe-Mg-Ti oxides, et al. (2001). In their experiments, a pigeonite of composition pyroxenes, olivine, and quartz. Computers in Geosciences 19: similar to Y-791538 displayed modulated structure only when 1333–1350. cooled slower than 50°/h. This condition is consistent with Arlt T., Kunz M., Stoltz J., Armbruster T., and Angel R. J. 2000. P- T-X data on P21/c clinopyroxenes and their displacive phase previous determinations on ureilites (Goodrich et al. 2001; transitions. Contributions to Mineralogy and Petrology 138:35– Weber et al. 2003), which are on the order of 20°/h at 1100 °C, 45. and with the proposed model of a breakup followed by deep Berkley J. L. 1986. Four Antarctic ureilites—Petrology and burial in a rubble pile from a single parent body proposed by observations on ureilite petrogenesis. Meteoritics & Planetary Goodrich et al. (2004). Science 21:169–189. Berkley J. L., Taylor G. J., Keil K., Harlow G., Prinz M. 1980. The The relatively long exsolution wavelength in nature and origin of ureilites. Geochimica et Cosmochimica Acta ALHA81101 indicates slow cooling, on the order of years 44:1579–1597. (Weinbruch and Müller 1995), and is in any case not Chikami J., Michouchi T., Takeda H., and Miyamoto M. 1997. compatible with fast cooling after breakup. In ureilitic Mineralogy and cooling history of the calcium-aluminum- pyroxene coarse exsolution lamellae (≅50,000 nm) were chromium enriched ureilite, Lewis Cliff 88774. Meteoritics & Planetary Science 32:343–348. found in the LEW 88774 ureilite (Chikami et al. 1997; Domeneghetti M. C., Molin G. M., Triscari M., and Zema M. 2000. Goodrich and Keller 2000). These lamellae were interpreted Orthopyroxene as a geospeedometer: Thermal history of as related to exsolution at high temperature of a single Kapoeta, Old Homestead 001, and Hughes 002 howardites. pyroxene of intermediate composition. However, LEW 88774 Meteoritics & Planetary Science 35:347–354. ∼ Domeneghetti M. C., Zema M., and Tazzoli V. 2005. Kinetics of is one of the most ferroan ureilites (Fo 75) and therefore is 2+ Fe -Mg order-disorder in P21/c pigeonite. American probably formed at much greater depth than the Mg-richer Mineralogist 90:1816–1823. ureilites investigated in this work (Goodrich et al. 2004). Ganguly J. 1982. Mg-Fe order-disorder in ferromagnesian silicates: In ALHA81101, the bulk composition is within the II. Thermodynamics, kinetics, and geological applications. In compositional field of pigeonite, but shock-induced Advances in physical geochemistry, vol. 2, edited by Saxena S. K. mobilization produced local Ca-enrichments that pushed the New York: Springer-Verlag. pp. 58–99. Ganguly J., Yang H., and Ghose S. 1994. Thermal history of composition towards the two-phase field. In the more Ca-rich mesosiderites: Constraints from compositional zoning and Fe- areas of ALHA81101, prolonged heating at high temperature Mg ordering in orthopyroxenes. Geochimica et Cosmochimica after a shock event possibly promoted local exsolution. Acta 58:2711–2723. 988 M. Tribaudino
Goodrich C. A. and Keller L. P. 2000. Transmission electron Stimpfl M., Ganguly J., and Molin G. M. 1999. Fe2+-Mg order- microscope investigation of a silicate mineral/melt reaction disorder in orthopyroxene: Equilibrium fractionation between texture in ureilite Lewis Cliff 88774 (abstract). Meteoritics & the octahedral sites and thermodynamic analysis. Contributions Planetary Science 35:A60. to Mineralogy and Petrology 136:297–309. Goodrich C. A., Fioretti A. M., Tribaudino M., and Molin G. M. Takeda H. 1987. Mineralogy of Antarctic ureilites and a working 2001. Primary trapped melt inclusions in olivine in the olivine- hypothesis for their origin and evolution. Earth and Planetary augite-orthopyroxene ureilite Hughes 009. Geochimica et Science Letters 81:358–370. Cosmochimica Acta 65:621–652. Takeda H. 1989. Mineralogy of coexisting pyroxenes in magnesian Goodrich C. A., Scott E. R. D., and Fioretti A. M. 2004. Ureilitic ureilites and their formation conditions. Earth and Planetary breccias: Clues to the petrologic structure and impact disruption Science Letters 93:181–194. of the ureilite parent asteroid. Chemie der Erde 64: 283–327. Takeda H. and Yanai K. 1982. Mineralogical examination of the Gordon W. A., Peacor D. R., Brown P. E., Essene E. J., and Allard L. Yamato-79 achondrites polymict eucrites and ureilites. Memoirs F. 1981. Exsolution relationships in a clinopyroxene of average of the National Institute of Polar Research 25:97–123. composition Ca0.43Mn0.69Mg0.82Si2O6: X-ray diffraction and Tribaudino M. 2000. A transmission electron microscope analytical electron microscopy. American Mineralogist 66:127– investigation on the C2/c-P21/c phase transition in 141. clinopyroxenes along the diopside-enstatite (CaMgSi2O6- Ibers J. A. and Hamilton W. C., eds. 1974. International tables for X- Mg2Si2O6) join. American Mineralogist 85:707–715. ray crystallography. Birmingham, Alabama: Kynoch Press. pp. Tribaudino M. and Nestola F. 2002. Average and local structure in 99–101. P21/c clinopyroxenes along the join diopside-enstatite Jantzen C. M. 1984. On spinodal decomposition in Fe-free (CaMgSi2O6-Mg2Si2O6). European Journal of Mineralogy 14: pyroxenes. American Mineralogist 69:277–282. 549–555. Mittlefehldt D. W., McCoy T. J., Goodrich C. A., and Kracher A. Tribaudino M., Fioretti A. M., Martignago F., and Molin G. M. 1997. 1998. Non-chondritic meteorites from asteroidal bodies. In Transmission electron microscope texture and crystal chemistry Planetary materials, edited by Papike J. J. Washington, D. C.: of coexisting ortho- and clinopyroxene in the Antarctic ureilite Mineralogical Society of America. pp. 1–195. Frontier Mountain 90054: Implications for thermal history. Mori H. and Takeda H. 1988. Stress-induced transformation of Meteoritics & Planetary Science 32:671–678. pigeonites from achondritic meteorites. Physics and Chemistry of Tribaudino M., Nestola F., Meneghini C., and Bromiley G. D. 2003. Minerals 15:252–259. The high temperature P21/c-C2/c phase transition in Fe-free Ca- Nakamuta Y. and Aoki Y. 2000. Mineralogical evidence for the origin rich P21/c clinopyroxenes. Physics and Chemistry of Minerals of diamond in ureilites. Meteoritics & Planetary Science 35:487– 30:527–535. 494. Tribaudino M., Nestola F., and Meneghini C. 2005. The structure North A. C. T., Phillips D. C., and Mathews F. S. 1968. A semi- behaviour of intermediate pyroxenes along the diopside-enstatite empirical method of absorption correction. Acta join. Canadian Mineralogist 43:1411–1421. Crystallographica A 24:351–359. Turncock A. C., Lindsley D. H., and Grover J. E. 1973. Synthesis and Ohashi Y. and Finger L. W. 1976. The effect of Ca substitution on the unit cell parameters of Ca-Mg-Fe pyroxenes. American structure of clinoenstatite. Carnegie Institution of Washington Mineralogist 58:50–59. Year Book 75:743–746. Weber I., Bishoff A., and Weber D. 2003. TEM investigation on the Pasqual D., Molin G. M., Tribaudino M. 2000. Single-crystal monomict ureilites Jalanash and Hammadah al Hamra 064. thermometric calibration of Fe-Mg order-disorder in pigeonites. Meteoritics & Planetary Science 38:145–156. American Mineralogist 85:953–962. Weinbruch S. and Müller W. F. 1995. Constraints on the cooling rates Saito J. and Takeda H. 1990. Information of elemental distributions of chondrules from the microstructure of clinopyroxene and in heavily shocked ureilites as a guide to deduce the ureilite plagioclase. Geochimica et Cosmochimica Acta 59:3221–3230. formation process (abstract). 21st Lunar and Planetary Science Weinbruch S., Müller W. F., and Hewins R. H. 2001. A transmission Conference. pp. 1063–1064. electron microscope study of exsolution and coarsening in iron Sheldrick G. M. 1997. SHELXL-97. Program for crystal structure bearing clinopyroxene from synthetic analogues of chondrules. refinement. Goettingen University, Germany. Meteoritics & Planetary Science 36:1237–1248. Singletary S. J. and Grove T. L. 2003. Early petrologic processes on Weinbruch S., Styrsa V., and Müller W. F. 2003. Exsolution and the ureilite parent body. Meteoritics & Planetary Science 38:95– coarsening in iron-free clinopyroxene during isothermal 108. annealing. Geochimica et Cosmochimica Acta 24:5071–5082. Stimpfl M. 2005. The Mn, Mg-intracrystalline exchange reaction in Zema M., Domeneghetti M. C., Molin G. M., and Tazzoli V. 1997. 2+ donpeacorite (Mn0.54Ca0.03Mg1.43Si2O6) and its relation to the Cooling rates of diogenites: A study of Fe -Mg ordering in fractionation behavior of Mn in Fe, Mg-orthopyroxene. orthopyroxene by X-ray single-crystal diffraction. Meteoritics & American Mineralogist 90:155–161. Planetary Science 32:855–862.