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1.
Jasmeet K. Dhaliwal James M. D. Day Kimberly T. Tait 《Meteoritics & planetary science》2023,58(2):275-295
New petrography, mineral chemistry, and whole rock major, minor, and trace element abundance data are reported for 29 dominantly unbrecciated basaltic (noncumulate) eucrites and one cumulate eucrite. Among unbrecciated samples, several exhibit shock darkening and impact melt veins, with incomplete preservation of primary textures. There is extensive thermal metamorphism of some eucrites, consistent with prior work. A “pristinity filter” of textural information, siderophile element abundances, and Ni/Co ratios of bulk rocks is used to address whether eucrite samples preserve endogenous refractory geochemical signatures of their asteroid parent body (i.e., Vesta), or could have experienced exogenous impact contamination. Based on these criteria, Cumulus Hills 04049, Elephant Moraine 90020, Grosvenor Range 95533, Pecora Escarpment 91245, and possibly Queen Alexander Range 97053 and Northwest Africa 1923 are pristine eucrites. Eucrite major element compositions and refractory incompatible trace element abundances are minimally affected by metamorphism or impact contamination. Eucrite petrogenesis examined through the lens of these elements is consistent with partial melting of a silicate mantle that experienced prior metal–silicate equilibrium, rather than as melts associated with cumulate diogenites. In the absence of the requirement of a large-scale magma ocean to explain eucrite petrogenesis, the interior structure of Vesta could be more heterogeneous than for larger planetary bodies. 相似文献
2.
Allan H. TREIMAN 《Meteoritics & planetary science》1997,32(2):217-230
Abstract— The cumulate eucrite meteorites are gabbros that are related to the eucrite basalt meteorites. The eucrite basalts are relatively primitive (nearly flat REE patterns with La ~ 8–30 × CI), but the parent magmas of the cumulate eucrites have been inferred as extremely evolved (La to > 100 × CI). This inference has been based on mineral/magma partitioning, and on mass balance considering the cumulate eucrites as adcumulates of plagioclase + pigeonite only; both approaches have been criticized as inappropriate. Here, mass balance including magma + equilibrium pigeonite + equilibrium plagioclase is used to test a simple model for the cumulate eucrites: that they formed from known eucritic magma types, that they consisted only of magma + crystals in chemical equilibrium with the magma, and that they were closed to chemical exchange after the accumulation of crystals. This model is tested for major and rare earth elements (REE). The cumulate eucrites Serra de Magé and Moore County are consistent, in both REE and major elements, with formation by this simple model from a eucrite magma with a composition similar to the Nuevo Laredo meteorite: Serra de Magé as 14% magma, 47.5% pigeonite, and 38.5% plagioclase; Moore County as 35% magma, 37.5% pigeonite, and 27.5% plagioclase. These results are insensitive to the choice of mineral/magma partition coefficients. Results for the Moama cumulate eucrite are strongly dependent on choice of partition coefficients; for one reasonable choice, Moama's composition can be modeled as 4% Nuevo Laredo magma, 60% pigeonite, and 36% plagioclase. Selection of parent magma composition relies heavily on major elements; the REE cannot uniquely indicate a parent magma among the eucrite basalts. The major element composition of Y-791195 can be fit adequately as a simple cumulate from any basaltic eucrite composition. However, Y-791195 has LREE abundances and La/Lu too low to be accommodated within the model using any basaltic eucrite composition and any reasonable partition coefficients. Postcumulus loss of incompatible elements seems possible. It is intriguing that Serra de Magé, Moore County, and Moama are consistent with the same parental magma; could they be from the same igneous body on the eucrite parent asteroid (4 Vesta)? 相似文献
3.
Abstract— Electron and ion microprobe measurements of major, minor, and trace element concentrations were made in individual grains of pyroxene, plagioclase, and Ca phosphates in Pomozdino and Peramiho, two eucrites previously classified as anomalous. Although Pomozdino pyroxene is highly magnesian, minor and trace element concentrations in both pyroxene and plagioclase of this meteorite are similar to those in other noncumulate eucrites. High incompatible element concentrations (similar to those in Stannern) coupled with mg# typical of cumulate eucrites confirm the anomalous character of this meteorite but do not allow us to distinguish unequivocally between different possible modes of origin. Peramiho has mg# and trace element concentrations similar to main group eucrites, indicating that this meteorite most probably belongs to this group. A previously reported low incompatible element concentration for Peramiho may be due to a sampling problem. 相似文献
4.
T. J. Barrett J. J. Barnes R. Tartèse M. Anand I. A. Franchi R. C. Greenwood B. L. A. Charlier M. M. Grady 《Meteoritics & planetary science》2016,51(6):1110-1124
Volatile elements play a key role in the dynamics of planetary evolution. Extensive work has been carried out to determine the abundance, distribution, and source(s) of volatiles in planetary bodies such as the Earth, Moon, and Mars. A recent study showed that the water in apatite from eucrites has similar hydrogen isotopic compositions compared to water in terrestrial rocks and carbonaceous chondrites, suggesting that water accreted very early in the inner solar system given the ancient crystallization ages (~4.5 Ga) of eucrites. Here, the measurements of water (reported as equivalent H2O abundances) and the hydrogen isotopic composition (δD) of apatite from five basaltic eucrites and one cumulate eucrite are reported. Apatite H2O abundances range from ~30 to ~3500 ppm and are associated with a weighted average δD value of ?34 ± 67‰. No systematic variations or correlations are observed in H2O abundance or δD value with eucrite geochemical trend or metamorphic grade. These results extend the range of previously published hydrogen isotope data for eucrites and confirm the striking homogeneity in the H‐isotopic composition of water in eucrites, which is consistent with a common source for water in the inner solar system. 相似文献
5.
Abstract— Available evidence strongly suggests that the HED (howardite, eucrite, diogenite) meteorites are samples of asteroid 4 Vesta. Abundances of the moderately siderophile elements (Ni, Co, Mo, W and P) in the HED mantle indicate that the parent body may have been completely molten during its early history. During cooling of a chondritic composition magma ocean, equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0.80, when the convective system freezes and melts segregate from crystals by gravitational forces. The extruded liquids are similar in composition to Main Group and Stannern trend eucrites, and the last pyroxenes to precipitate out of this ocean (before convective lockup) span the compositional range of the diogenites. Subsequent fractional crystallization of a Main Group eucrite liquid, which has been isolated as a body of magma, produces the Nuevo Laredo trend and the cumulate eucrites. The predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites. Thus, eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with Vesta. 相似文献
6.
Abstract– Eucrites, which are probably from 4 Vesta, and angrites are the two largest groups of basaltic meteorites from the asteroid belt. The parent body of the angrites is not known but it may have been comparable in size to Vesta as it retained basalts and had a core dynamo. Both bodies were melted early by 26Al and formed basalts a few Myr after they accreted. Despite these similarities, the impact histories of the angrites and eucrites are very different: angrites are very largely unshocked and none are breccias, whereas most eucrites are breccias and many are shocked. We attribute the lack of shocked and unbrecciated angrites to an impact, possibly at 4558 Myr ago—the radiometric age of the younger angrites—that extracted the angrites from their original parent body into smaller bodies. These bodies, which may have had a diameter of approximately 10 km, suffered much less impact damage than Vesta during the late heavy bombardment because small bodies retain shocked rocks less efficiently than large ones and because large bodies suffer near‐catastrophic impacts that deposit vastly more impact energy per kg of target. Our proposed history for the angrites is comparable to that proposed by Bogard and Garrison (2003) for the unbrecciated eucrites with Ar‐Ar ages of 4.48 Gyr and that for unbrecciated eucrites with anomalous oxygen isotopic compositions that did not come from Vesta. We infer that the original parent bodies of the angrites and the anomalous eucrites were lost from the belt when the giant planets migrated and the total mass of asteroids was severely depleted. Alternatively, their parent bodies may have formed in the terrestrial planet region and fragments of these bodies were scattered out to the primordial Main Belt as a consequence of terrestrial planet formation. 相似文献
7.
J. A. BARRAT J. BLICHERT‐TOFT PH. GILLET F. KELLER 《Meteoritics & planetary science》2000,35(5):1087-1100
Abstract— We report on major and trace element analyses of 17 eucrites, including three cumulate eucrites (Binda, Moore County, and Serra de Magé), determined by, respectively, inductively‐coupled plasma atomic emission spectrometry and inductively‐coupled plasma mass spectrometry. The results obtained for Binda and Moore County are consistent with the model of Treiman (1997) for the formation of cumulate eucrites, which holds that these meteorites were produced from a eucritic melt. Our sample of Serra de Magé contains unusually large amounts of pyroxene and probably an accessory phase rich in heavy rare earth elements and is therefore not representative of this eucrite as known from literature data. Our results for the noncumulate eucrites Bereba, Bouvante, Cachari, Caldera, Camel Donga, Ibitira, Jonzac, Juvinas, Lakangaon, Millbillillie, Padvarninkai, Pasamonte, Sioux County, and Stannern are in good agreement with literature data. The observed decoupling between major and trace elements for noncumulate eucrites can be explained by in situ crystallization during the differentiation of an asteroidal magma ocean. This model can further account for both the Nuevo Laredo and the Stannern trends but has as a consequence that none of the analyzed eucrites represents a primary melt. 相似文献
8.
Adam Robert Sarafian Michael F. Roden Alberto E. Patiño‐Douce 《Meteoritics & planetary science》2013,48(11):2135-2154
Apatite was analyzed by electron microprobe in 3 cumulate and 10 basaltic eucrites. Eucritic apatite is fluorine‐rich with minor chlorine and hydroxyl (calculated by difference). We confirmed the hydroxyl content by measuring hydroxyl directly in apatites from three representative eucrites using secondary ionization mass spectroscopy. Overall, most eucritic apatites resemble fluorine‐rich lunar mare apatites, but intriguing OH‐ and Cl‐rich apatites suggest a role for water and/or hydrothermal fluids in the Vestan interior or on other related differentiated asteroids. Most late‐stage apatite found in mesostasis has little hydroxyl or chlorine and is thought to have crystallized from a degassed magma; however, several apatites exhibit atypical compositions and/or textural characteristics. For example, the isotopically anomalous basaltic eucrite Pasamonte has apatite in the mesostasis with significant OH. Apatites in Juvinas also have significant OH and occur as veinlets crosscutting silicates. Euhedral apatites in the Moore County cumulate eucrite occur as inclusions in pyroxene and are also hydroxyl‐rich (0.62 wt% OH). The OH was confirmed by SIMS analysis and this apatite clearly points to the presence of water, at least locally, in the Vestan interior. Portions of Elephant Moraine (EET) 90020 have large and abundant apatites, which may be the product of apatite accumulation in a zone of melt‐rock reaction. Relatively chlorine‐rich apatites occur in basaltic eucrite Graves Nunataks (GRA) 98098 (approximately 1 wt% Cl). Particularly striking is the compositional similarity between apatite in GRA 98098 and apatites in lunar KREEP, which may indicate the presence of residual magmas from an asteroid‐wide magma ocean on Vesta. 相似文献
9.
Haruka Ono Atsushi Takenouchi Takashi Mikouchi Akira Yamaguchi 《Meteoritics & planetary science》2019,54(11):2744-2757
Some eucrites contain up to 10 vol% silica minerals; however, silica minerals have not been studied in detail so far. We performed a mineralogical study of silica minerals in three cumulate eucrites (Moore County, Moama, and Yamato [Y] 980433). Monoclinic tridymite was common in all three samples. Moama contained orthorhombic tridymite as lamellae within monoclinic tridymite grains. Y 980433 included quartz around an impact melt vein. The presence of orthorhombic tridymite in Moama indicates that Moama cooled more rapidly than the other two samples at low temperatures (<400 °C). This result is different from the slower cooling rates of Moama (?0.0004 °C yr?1) than that of Moore County (>0.3 °C yr?1, after the shock event) at high temperatures (>500 °C) estimated from compositional profiles of pyroxene exsolution lamellae. The difference of the cooling rates may reflect their geological settings. Y 980433 cooled slowly at low temperature, as did Moore County. Quartz in Y 980433 could be a local product transformed from monoclinic tridymite by a shock event. We suggest that silica minerals in meteorites record thermal histories at low temperatures and shock events. 相似文献
10.
Abstract— This study explores the controls of oxygen fugacity and temperature on the solubilities of Fe, Ni, Co, Mo, and W in natural eucritic liquids to better constrain the formation of eucritic melts. The solubilities of all five elements in molten silicate in equilibrium with FeNiCo‐, FeMo‐, and FeW‐ alloys increase with increasingly oxidizing conditions and decrease with decreasing temperatures. In applying these data to formation scenarios of the eucrite parent body, we find that the siderophile element abundances in eucrites (meteoritic basalts) cannot be explained by a single‐step partialmelting process from a chondritic, metal‐containing source. The Ni content of the partial melt is too high, and the W and Mo contents are too low compared to the abundances in eucritic meteorites. But Fe, Ni, and Co concentrations in eucrites can be modeled by metal‐silicate equilibrium during more or less complete melting of the eucrite parent body with subsequent fractional crystallization of olivine and orthopyroxene. However, the computed values of Mo are still too low and those of W too high when compared with Mo and W abundances in eucritic meteorites. One possibility is that the Mo and W partition coefficients strongly depend on pressure, although the howardite‐eucrite‐diogenite (HED) parent body only had a minimal pressure gradient (maximum interior pressure = 0.1 GPa). Alternatively, sulfides may have played some role in establishing Mo abundances. 相似文献
11.
Abstract— The isotopic composition and concentrations of noble gases were measured in the eucrites Bereba, Cachari, Caldera, Camel Donga, Chervony Kut, Ibitira, Jonzac, Juvinas, Millbillillie, Moore County, Padvarninkai, Pasamonte, Pomozdino, Serra de Magé, Sioux County, and Vetluga. The distribution of 81Kr-Kr exposure ages shows “clusters” at (7 ± 1) Ma, (10 ± 1) Ma, (14 ± 1) Ma, (22 ± 2) Ma, and (37 ± 1) Ma that agree with those for howardites, eucrites, and diogenites (HED) at (6 ± 1) Ma, (12 ± 2) Ma, (21 ± 4) Ma, and (38 ± 8) Ma. This most likely indicates a common origin of HED meteorites. Correlation equations for the shielding-sensitive cosmogenic ratios 78Kr/83Kr, 80Kr/83Kr, 82Kr/83Kr, and 124Xe/131Xe were obtained. Comparison with data from simulation experiments suggests that most eucrites were exposed to the cosmic radiation as somewhat large meteoroids with diameters of ~1 m or more. The shielding-dependence of the 78Kr and 126Xe production rates was found to be small, with a few exceptions the variations aren <10%–15%. Concentrations of spallogenic 3He indicate diffusive losses of up to 70% that can be, in first approximation, described by a model of quasi-continuous losses during the exposure to the cosmic radiation with a loss rate of the order of ~3 × 10?8 a?1. Radiogenic 4He shows additional substantial losses that occurred at the time of, or prior to, the separation of the meteoroids from their parent body. Typical 40Ar retention in eucrites is 50%–60% which corresponds to a 40Ar-K retention age of 3.4–3.6 Ga. In all analyzed unbrecciated eucrites, the retention is distinctly larger (70%–100%). The 244Pu fission ratio (86Kr/136Xe)Pu, was evaluated from the data on Pomozdino samples to be 0.039 ± 0.014. 相似文献
12.
DEREK W. G. SEARS STEVEN J. K. SYMES J. DAVID BATCHELOR D. GLEN AKRIDGE PAUL H. BENOIT 《Meteoritics & planetary science》1997,32(6):917-927
Abstract— We report induced thermoluminescence (TL) data for separates from three howardite, eucrite and diogenite (HED) meteorites and the Vaca Muerta mesosiderite. The results of thermal modeling of the surface of their parent body are also described. The TL sensitivities for matrix samples from the LEW 85300, 302 and 303 paired eucrites and the Bholghati howardite are lower than the TL sensitivities for the clasts, which is consistent with regolith working of the matrix in fairly mature regoliths. Within an isochemical series of HED meteorites, TL sensitivity reflects metamorphic intensity, but clast-to-clast variations in the TL sensitivities of the Vaca Muerta mesosiderite and clasts in the EET 87509, 513 and 531 paired howardite primarily reflect differences in mineralogy and petrology. Thermoluminescence peak temperatures indicate that all the components from the LEW 85300, 302 and 303 paired eucrites experienced a reheating event involving temperatures >800 °C, which is thought to have been due to impact heating, and therefore that the event was concurrent with or postdated brecciation. The Vaca Muerta clasts are essentially unmetamorphosed, but the induced TL data indicate that the remaining howardite, eucrite, dioenite and mesosiderite (HEDM) meteorites experienced metamorphism to a variety of intensities but involving temperatures <800 °C. Laboratory heating experiments show that temperatures >800 °C cause a change in TL peak temperature. Feldspars from a variety of terrestrial and extraterrestrial sources show this behavior, and x-ray diffraction and kinetic studies suggest that it is indirectly related to Al, Si disordering. Cooling rates are not consistent with autometamorphism following the initial igneous event or with heating by subsequent eruptions of lava onto the surface of the HED parent body. Instead, our thermal models suggest that the metamorphism occurred within a regolith ejecta blanket of up to a few kilometers thick, with different levels of metamorphism corresponding to different thicknesses of blanket, between essentially 0 and ~2 km, rather than different burial depths in a regolith of uniform thickness. We argue that metamorphism occurred 3.9 Ga ago and was associated with the resetting of the Ar-Ar system for the HED meteorites. 相似文献
13.
Exnovae and nova shells are generally faint and difficult to observe. Only a few have been studied, and even less of them in sufficient detail. We give a progress report on our survey of exnovae and nova shells, especially on the results of a spectral survey of novae in the southern Milky Way. The three-dimensional structure in the light of H derived for the nebula of GK Per is displayed.Based on observations obtained at the European Southern Observatory, La Silla, Chile, and at the Centro Astronomico Hispano-Aleman Calar Alto, Spain, operated by the Max-Planck-Institut für Astronomie, Heidelberg.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986. 相似文献
14.
We report and discuss solar systemN
R
abundances for nuclidesA>70, obtained as differences between measured solar system abundances and calculatedS-process contributions. The abundance peak atA163 in the rare Earth element region reveals properties which are similar to those of theR-process peaks corresponding to magic neutron numbersN=82 andN=126. We observe that systematic differences in theN
R
abundances of even-A and odd-A nuclides are restricted to specific mass regions. We discuss possible interpretations and conclude that these differences are most probably related to the properties of nuclear species during – to the stability valley.... A genesis of the elements such as is sketched out would not be confined to our little Solar System, but would probably follow the same general sequence of events in every center of energy now visible as a Star. Sir William Crookes (1886)Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献
15.
Noah Brosch 《Astrophysics and Space Science》1992,188(2):289-298
We examined the radial distribution of 1227 blue knots in the rectified planes of 32 galaxies and showed that the method is equivalent to mapping the distribution ofHIi regions from narrow-band imaging in H. The blue knots show less detail in their distribution, as compared with Hii regions or with a parameter measuring the local efficiency of star formation. This is interpreted as showing the diffusion of OB stars out of the regions where they formed. 相似文献
16.
Abstract— Eucrite meteorites are igneous rocks that derived from a large asteroid, probably 4 Vesta. Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ≥800°C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new 39Ar‐40Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic‐brecciated eucrites. Precise Ar‐Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ± 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar‐Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb‐Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4–4.515 Gyr, and 147Sm‐143Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41–4.55 Gyr. Similar Ar‐Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K‐Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar‐Ar ages at ?4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ?4.48 Gyr ago, which quickly cooled the samples and started the K‐Ar chronometer. A large (?460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb‐Pb and Sm‐Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar‐Ar age of 4.48 Gyr, and the few older Pb‐Pb and Sm‐Nd ages may reflect an isotopic closure before the large cratering event. One cumulate eucrite gives an Ar‐Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar‐Ar ages of 3.4–3.7 Gyr; and 2 unbrecciated eucrites give Ar‐Ar ages of ?3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar‐Ar impact‐reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5–4.1 Gyr. Among these, Piplia Kalan, the first eucrite to show evidence for extinct 26Al, was strongly impact heated ?3.5 Gyr ago. When these data are combined with eucrite Ar‐Ar ages in the literature, they confirm that several large impact heating events occurred on Vesta between ?4.1–3.4 Gyr ago. The onset of major impact heating may have occurred at similar times for both Vesta and the moon, but impact heating appears to have persisted for a somewhat later time on Vesta. 相似文献
17.
Rhiannon G. MAYNE Jessica M. SUNSHINE Harry Y. McSWEEN Timothy J. McCOY Catherine M. CORRIGAN Allison GALE 《Meteoritics & planetary science》2010,45(7):1074-1092
Abstract– We investigate the relationship between the petrology and visible–near infrared spectra of the unbrecciated eucrites and synthetic pyroxene–plagioclase mixtures to determine how spectra obtained by the Dawn mission could distinguish between several models that have been suggested for the petrogenesis of Vesta’s crust (e.g., partial melting and magma ocean). Here, we study the spectra of petrologically characterized unbrecciated eucrites to establish spectral observables, which can be used to yield mineral abundances and compositions consistent with petrologic observations. No information about plagioclase could be extracted from the eucrite spectra. In contrast, pyroxene dominates the spectra of the eucrites and absorption band modeling provides a good estimate of the relative proportions of low‐ and high‐Ca pyroxene present. Cr is a compatible element in eucrite pyroxene and is enriched in samples from primitive melts. An absorption at 0.6 μm resulting from Cr3+ in the pyroxene structure can be used to distinguish these primitive eucrites. The spectral differences present among the eucrites may allow Dawn to distinguish between the two main competing models proposed for the petrogenesis of Vesta (magma ocean and partial melting). These models predict different crustal structures and scales of heterogeneity, which can be observed spectrally. The formation of eucrite Allan Hills (ALH) A81001, which is primitive (Cr‐rich) and relatively unmetamorphosed, is hard to explain in the magma ocean model. It could only have been formed as a quench crust. If the magma ocean model is correct, then ALHA81001‐like material should be abundant on the surface of Vesta and the Vestoids. 相似文献
18.
Abstract— The Sm-Nd systematics of whole-rock and mineral separate samples from nakhlite Governador Valadares define a good 147Sm-143Nd mineral isochron age of 1.37 ± 0.02 Ga. This age is in excellent agreement with the 39Ar-40Ar and Rb-Sr ages obtained previously for this meteorite. However, the Rb-Sr isotopic data for our sample show that the isotopic system is disturbed. The lack of isotopic equilibrium is probably caused by the weathering of the sample as indicated by the presence of secondary alteration phases. The whole-rock and acid-washed mineral data yield a Rb-Sr age of 1.20 ± 0.05 Ga, which probably represents a lower limit to the crystallization age of the rock. The petrographic evidence indicates that this meteorite is a clinopyroxene cumulate that probably crystallized in a subsurface sill (McSween, 1994). Thus, the Sm-Nd isotopic age probably represents the age of such a magmatic event. The initial ε143Nd value determined for the rock at 1.37 Ga is +17 ± 1, indicating that the parent magma of the rock came from a light-rare-earth-element-depleted source of 147Sm/144Nd = ~0.237 based on a simple two-stage evolution model. Results of the same model calculation for the initial 87Sr/86Sr ratio of the rock suggest that its source material was depleted in 87Rb/86Sr by ~50% relative to the estimated martian value at 1.37 Ga. Both the high Sm/Nd and low Rb/Sr values support a clinopyroxene-rich cumulate source for the genesis of the nakhlite Governador Valadares. Furthermore, our Sm-Nd age and ε143Nd data and the previously published ε142Nd datum for the rock (Harper et al., 1995) are consistent with early differentiation of the parent planet, formation of cumulate sources ~4.56 Ga ago, and late melting of the sources and formation of the rock ~1.37 Ga ago. The good agreement of isotopic ages and petrographic features among Governador Valadares, Nakhla, and Lafayette strongly suggests that all three nakhlites have undergone similar evolutionary histories. The nakhlite age data suggest that isotopic heterogeneity in the martian mantle sources existed up to ~1.37 Ga ago and early mantle structures probably have not been disturbed for a significant portion of martian history. 相似文献
19.
Meenakshi WADHWA Ernst K. ZINNER Ghislaine CROZAZ 《Meteoritics & planetary science》1997,32(2):281-292
Abstract— We measured with a secondary ion mass spectrometer Mn/Cr ratios and Cr isotopes in individual grains of Mn-bearing sulfides (i.e., sphalerites, ZnS; alabandites, MnS; and niningerites, MgS) in nine unequilibrated enstatite chondrites (UECs). The goals were to determine whether live 53Mn (half-life ~3.7 Ma) was incorporated in these objects at the time of their isotopic closure and to establish whether Mn-Cr systematics in sulfides in UECs can be used as a high-resolution chronometer to constrain formation time differences between these meteorites. Sulfide grains analysed in four of these UECs, MAC 88136 (EL3), MAC 88184 (EL3), MAC 88180 (EL3), and Indarch (EH4), have clear 53Cr excesses. These 53Cr excesses can be very large (δ53Cr/52Cr ranges up to ~18,400%, the largest 53Cr excess measured so far) and, in some grains, are well correlated with the Mn/Cr ratios. Thus, they were most likely produced by the in situ decay of 53Mn in the meteorite samples. In the remaining five meteorites, no detectable excesses of 53Cr were found, and only upper limits on the initial 53Mn/55Mn ratios could be established. The four meteorites with 53Cr excesses show variations in the inferred 53Mn/55Mn ratios in various sulfide grains of the same meteorite. The Mn-Cr systematics in these sulfides were disturbed (during and/or after the decay of 53Mn) by varying degrees of reequilibration. Provided 53Mn was homogeneously distributed in the region of the early solar system where these objects formed, the data suggest that the time of the last isotopic equilibration of sulfides in EL chondrites occurred at least 3 Ma after a similar episode in EH chondrites. 相似文献
20.