Degree Of Thermal Metamorphism Research Articles

Hiding in the howardites: Unequilibrated eucrite clasts as a guide to the formation of Vesta's crust

Abstract204 howardites in the National Meteorite Collection at the Smithsonian were examined for the presence of fine‐grained eucrite clasts, with the goal of better understanding the formation of the uppermost crust of asteroid 4Vesta. Eight clasts were identified and characterized in terms of their textures and mineral chemistry, and their degree of thermal metamorphism was assessed. The paucity of fine‐grained eucrites, both within the unbrecciated eucrites and as clasts within the howardites, suggests that they originate from small‐scale units on the surface of Vesta, most likely derived from partial melting. Six of the eight clasts described were found to be unequilibrated, meaning that they preserve their original crystallization trends. The vast majority of eucrites are at least partially equilibrated, making these samples quite rare and important for deciphering the petrogenesis of the vestan crust. Biomodal grain populations suggest that eucrite melts often began crystallizing pyroxene and plagioclase during their ascent to the surface, where they were subject to more rapid cooling, crystallization, and later metasomatism. Pyroxene compositions from this study and prior work indicate that the products of both primitive and evolved melts were present at the vestan surface after its formation. Two howardite thin sections contained multiple eucrite composition clasts with different crystallization and thermal histories; this mm‐scale diversity reflects the complexity of the current day vestan surface that has been observed by Dawn.

Petrology and oxygen isotopic compositions of clasts in HED polymict breccia NWA 5232

Northwest Africa (NWA) 5232, an 18.5 kg polymict eucrite, comprises eucritic and exogenic CM carbonaceous chondrite clasts within a clastic matrix. Basaltic clasts are the most abundant eucritic clast type and show a range of textures and grain size, from subophitic to granoblastic. Other eucritic clast types present include cumulate (high-En pyroxene), pyroxene-lath, olivine rich with symplectite intergrowths as a break-down product of a quickly cooled Fe-rich metastable pyroxferroite, and breccia (fragments of a previously consolidated breccia) clasts. A variable cooling rate and degree of thermal metamorphism, followed by a complex brecciation history, can be inferred for the clasts based on clast rounding, crystallization (and recrystallization) textures, pyroxene major and minor element compositions, and pyroxene exsolution. The range in δ18O of clasts and matrix of NWA 5232 reflects its origin as a breccia of mixed clasts dominated by eucritic lithologies. The oxygen isotopic compositions of the carbonaceous chondrite clasts identify them as belonging to CM group and indicate that these clasts experienced a low degree of aqueous alteration while part of their parent body. The complex evolutionary history of NWA 5232 implies that large-scale impact excavation and mixing was an active process on the surface of the HED parent body, likely 4 Vesta.

Spectral reflectance properties of carbonaceous chondrites 4: Aqueously altered and thermally metamorphosed meteorites

We examined the spectral reflectance properties of 26 carbonaceous chondrites (CCs) that show evidence of aqueous alteration and subsequent thermal metamorphism (termed ATCCs). We also reviewed the thermal and aqueous alteration history of these meteorites and searched for trends between spectral parameters and temperature histories in order to uncover spectral–compositional relationships. Aqueous alteration results in the production of phyllosilicates from anhydrous silicate precursors – largely serpentine group phyllosilicates, and increasing amounts of saponite group phyllosilicates with increasing aqueous alteration. Thermal metamorphism results in dehydration of these phyllosilicates and production of abundant amorphous material except at the highest temperatures (≳900°C), as well as alteration of carbonaceous components. ATCCs are a spectrally diverse group in almost all respects. Spectral slopes, as measured by the ratio of reflectance at 2.4μm to the local peak or inflection in the 0.5–0.8μm region and 2.4/1.5μm ratios range from 0.78 to 1.48, and 0.93 to 1.24, respectively (blue-sloped spectra have ratio values of <1). ATCC powder spectra (<75, <100, or <125μm) are generally dark, with maximum reflectance at the local peak or inflection in the 0.5–0.8μm region, or maximum reflectance at any wavelength ranging from 2.6% to 8.9%, and 3.5% to 10.3%, respectively. All ATCC spectra exhibit an absorption feature in the ∼0.8–1.3μm region, with band depths ranging from ∼1% to 8%. This feature is diverse in terms of number of apparent absorption bands. The presence of mixed valence Fe2+–Fe3+ phyllosilicates, as evidenced by an absorption band near 0.7μm with a depth of up to 5%, and Mg-bearing phyllosilicates, as evidenced by an MgOH combination band in the 2.3–2.4μm region, are seen in many of the least thermally metamorphosed ATCC spectra. The depth of the 0.7μm band generally decreases with increasing temperature. Olivine-associated absorption bands in the 0.8–1.3μm region seem to be more prevalent in the more metamorphosed ATCC spectra. However clearly-resolvable olivine absorption bands are not present in ATCC spectra, suggesting that thermal metamorphism did not lead to the production of widespread crystalline Fe2+-bearing olivine. The reddest ATCC powder spectra are generally the darkest, and C content is correlated with decreasing overall reflectance and weakly correlated with spectral slope. When the degree of thermal metamorphism was compared to various spectral measures of slope, band depth, and overall reflectance, no strong correlations emerged. However, it does appear that the most thermally metamorphosed ATCCs have generally flatter spectral slopes. ATCC chip spectra are brighter and less red-sloped than powder spectra, but band depths are generally comparable. Laboratory-heated CIs and CMs generally exhibit the same types of spectral changes seen in naturally thermally metamorphosed ATCCs. For laboratory-heated CM and CI chondrites, and ATCCs for which temperature estimates are available, reflectance generally decreases with increasing temperature to ∼500°C, and then increases to higher temperatures. Silicate absorption band depths are generally least for temperatures of ∼600–800°C. Below this temperature interval, ATCC spectra show more phyllosilicate-like absorption bands. ATCC spectra generally become flatter with increasing temperature above ∼400°C. Temperatures in excess of those experienced by the ATCCs (∼900°C) are required for the appearance of well-resolved olivine absorption bands.

Silicate minerals and Si‐O glass in comet Wild 2 samples: Transmission electron microscopy

Abstract— A transmission electron microscope (TEM) study of seven comet Wild 2 samples shows that three samples consist mainly of olivine and pyroxene and four samples consist of Mg‐Fe‐bearing Si‐O glass with minor amounts of Fe‐Ni sulfide and metal. The olivine in the silicate‐rich samples differs in fayalite content between the samples and shows a wide range of fayalite content within individual samples, indicating that the degree of thermal metamorphism on the comet, if any, was extremely low. One olivine grain has a high density of dislocations with Burgers vector b = [001], suggesting that the Wild 2 particles experienced hypervelocity impacts before capture. The structural type and composition of pyroxene differ between the samples and within individual samples. Both low‐Ca and high‐Ca pyroxenes are present. Enstatite occurs as ortho‐ and clinoenstatite, suggesting that the Wild 2 particles contain materials that went through distinct high‐temperature and cooling histories.One silicate‐rich sample exhibits a zone texture consisting of a core of low‐Ca pyroxene surrounded by an inner rim of Mg‐Fe‐bearing Si‐O‐rich glass and an outer rim of melted aerogel. The texture suggests that the inner rim was formed by the mixing of melted cometary low‐Ca pyroxene and melted aerogel during capture heating.The four Mg‐Fe‐bearing, Si‐O glass‐rich samples show close similarities in mineralogy and texture to the inner rim of the zoned silicate‐rich sample. The four samples are probably secondary products formed by interaction between melted cometary silicates and melted aerogel during the capture process.

Nitrogen isotopes in bulk ordinary chondrites

Nitrogen isotopic compositions of twenty-one ordinary chondrites were measured using a stepwise combustion technique. We observed a good correlation between the abundances of cosmogenic 15N and cosmogenic 38Ar. We obtained a relationship between the abundances of these two cosmogenic nuclides as follows: 15N c(atoms/g) = (16.0 ± 3.7) · 38Ar c(atoms/g) Using this relationship, the cosmogenic nitrogen production rate in L-chondrites is calculated to be (5.0 ± 1.2) × 10 −12 g- 15N/g·myr, assuming the cosmogenic 38Ar production rate given by Eugster (1988). This value is consistent with the production rate on the lunar surface estimated by Becker et al. (1976). Isotopic ratios of trapped nitrogen in ordinary chondrites seem to be heterogeneous, among both unequilibrated and equilibrated meteorites. Heterogeneity among the latter are well observed in this study. The isotopic ratios of the trapped components seem to differ by more than 110%, ranging from −18%, observed in Guaren˜a (H6), to +95%, observed in Jilin (H5). A lack of correlation between nitrogen abundances and isotopic ratios indicates that differences in nitrogen isotopic ratios are not produced by mass-dependent isotopic fractionation. An inverse correlation between nitrogen abundances and the degree of thermal metamorphism is observed among most of the unequilibrated chondrites. Nitrogen abundances decrease from 8–25 p.p.m. to <1 p.p.m. with increasing petrographic subtype from 3.4–3.9. The nitrogen abundance in equilibrated chondrites is generally higher than the amount expected by extending the trend observed among unequilibrated ones. This is probably because contaminating nitrogen masked the trend at small amounts of trapped nitrogen.

Extinct I 129 in C3 chondrites

Eight C3 chondrites were examined by the I 129Xe 129 dating method, to see whether their IXe “ages” (better, initial I 129 I 127 ratios ≡ R 0) correlate with any other properties. The R 0's range from 1.60 × 10 −4 to 1.09 × 10 −4, corresponding to IXe ages from 2.0 Myr before to 6.7 Myr after Murchison magnetite. Three C3O's (Lancé, Felix, Ornans) have essentially indistinguishable R 0's of (1.41 ± 0.13) to (1.17 ± 0.10) × 10 −4; the fourth C3O, Warrenton, is undatable owing to homogenization of radiogenic and trapped Xe. Four C3V's show a distinct spread: Vigarano and Grosnaja are highest [ R 0 = (1.60 ± 0.07) and (1.57 ± 0.14) × 10 −4], Mokoia is intermediate, and Kaba is lowest [ R 0 = (1.38 ± 0.06) and (1.09 ± 0.10) × 10 −4]. Literature values for Allende place it near Kaba. These R 0's correlate inversely with 4 other properties: I-, Br-, and Cd-content, and olivine composition, both percent mean deviation (PMD) and proportion of iron-poor olivine grains (≤2% fayalite). It is difficult to accept the ∼9 Myr spread in R 0 as a true age, reflecting either nebular or parent-body processes. This time span is more than an order of magnitude longer than the lifetime of the solar nebula inferred from astronomical evidence. Nor does the degree of thermal metamorphism, which is slight for C3's anyway, correlate with R 0. A more plausible interpretation is that the variations in R 0 reflect mainly isotopic heterogeneity of iodine. The simplest model that accounts for the correlations with R 0 involves mixing of two iodine components in the solar nebula, associated with gas and grains, respectively. The second, of lower I 129 I 127 ratio, predominated at later times and thus became enriched in late-formed meteorites, along with other volatiles such as Cd and Br. The low Fe content and large PMD of olivine may reflect either less metamorphism owing to shallow location in the parent body, or greater reduction of Fe 2+ during chondrule formation.

A preliminary study on mineralogical and chemical compositions of some chondrites falling in China

Samples are available from 37 stony meteorites falling in China. Twenty-two chondrites are examined in terms of chemical and mineral compositions, cosmogenic nuclides, formation and exposure ages, impact effect and chondrule textures. On the basis of chemical-petrologic features these chondrites are classified asE 4 (Qingzhen),H 5 (Jilin, Changde, Shuangyang, Anlong, Xinyi and Yangjang),L 6 (Renqiu, Junan, Heze, Rugao and Nei Monggol) andLL 6 (Dongtai).E4 is characterized by high iron and sulfur, with the former occurring mainly as Feo and FeS. FromH5 throughH6 toLL6, iron and nickel decrease gradually while FeO and the ratio of Fe silicate to total iron increase gradually. indicating a general increase in the orderE-H-L-LL in the degree of oxidation at the time of formation.E4 consists mainly of enstatite and, to much less extent, free SiO2 but olivine is hardly to be found. The olivine proportions amount to 29.07, 41.98 and 51.36 percent inH5,L6 andLL6 respectively, with Fa increasing from 17 to 27 percent.Recrystallization has been noticed to different degrees inH 4,H 5,L 6 andLL 6 chondrites. The extent to which the original structure disappears and the boundaries of chondrules become indistinct decreases from type 6 through type 5 to type 4, reflecting different degrees of thermal metamorphism. Major minerals in the meteorites all exhibit signs of low to medium shock metamorphism.Specific activity, depth effect and orbit effect are also measured on some chondrites that have fallen in recent years and some new information has been obtained with respect to the orbit and source region for meteorite parent bodies in space. This results show that the environment of formation ofE group may be nearer to Mars than that ofO group.Each chemical group of chondrites has its own evolutionary history, and chondritets of different chemical groups may have originated from parent bodies of different compositions. Or owing to the differentiation caused by thermal melamorphism, various kinds of meteorites may be derived from a common parent body. From this argument it is suggested that five stages may be recognized during the formation process of chondrites.