⁴⁰Ar-³⁹Ar age of the Shergotty achondrite and implications for its post-shock thermal history     

D.D. Bogard  Liaquat Husain  L.E. Nyquist 《Geochimica et cosmochimica acta》1979,43(7):1047-1055

Analyses of ⁴⁰Ar-³⁹Ar have been made on a whole rock sample and a maskelynite (feldspar) separate of the shocked Shergotty achondrite. The maskelynite gave a plateau age of 254 ± 10 Myr. The whole rock sample gave a complex release with apparent ages between 240 and 640 Myr. The slightly younger Rb-Sr isochron age of 165 Myr for Shergotty (Nyquistet al., 1978) suggests that the maskelynite as well as the whole rock was incompletely degassed. Reasonable Ar diffusion characteristics for Shergotty for shock heating temperatures of <400°C indicate D/a² of 10^?11?10^?13 sec^?1. The time required to lose 95% of the ⁴⁰Ar from the plagioclase would be ~10³–10⁴ yr. When this gas diffusion time is introduced into a thermal model of a cooling ejecta blanket of variable thickness, a post-shock cooling time of ? 10³ yr and a burial depth of ? 300 m are indicated for Shergotty. These conclusions are not seriously affected by uncertainties in the thermal model. Most likely the shock event occurred ~ 165 Myr ago, but no earlier than 250 Myr ago, when the Shergotty parent object experienced a collision in the asteroid belt. As a result of that collision, feldspar was converted to maskelynite, the K-Ar and Rb-Sr ages were completely or nearly completely reset, and the Shergotty meteorite was heated to <400°C and left to cool slowly inside the parent body.  相似文献   

Rb-Sr age of the Shergotty achondrite and implications for metamorphic resetting of isochron ages     

L.E. Nyquist  J. Wooden  B. Bansal  H. Wiesmann  G. McKay  D.D. Bogard 《Geochimica et cosmochimica acta》1979,43(7):1057-1074

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Constraints on the petrogenesis of Martian meteorites from the Rb-Sr and Sm-Nd isotopic systematics of the lherzolitic shergottites ALH77005 and LEW88516     

Lars E. Borg  Larry E. NyquistHenry Wiesmann  Young Reese 《Geochimica et cosmochimica acta》2002,66(11):2037-2053

Detailed Rb-Sr and Sm-Nd isotopic analyses have been completed on the lherzolitic shergottites ALH77005 and LEW88516. ALH77005 yields a Rb-Sr age of 185 ± 11 Ma and a Sm-Nd age of 173 ± 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 ± 10 and 166 ± 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 ± 4, and the initial ε_Nd value is +11.1 ± 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 ± 4 and an initial ε_Nd value of +8.2 ± 0.6. Several of the mineral and whole rock leachates lie off the Rb-Sr and Sm-Nd isochrons, indicating that the isotopic systematics of the meteorites have been disturbed. The Sm-Nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low ¹⁴³Nd/¹⁴⁴Nd ratio that was probably added to the meteorites on Mars. Tie lines between leachate-residue pairs from LEW88516 mineral fractions and whole rocks have nearly identical slopes that correspond to Rb-Sr ages of 90 ± 1 Ma. This age may record a major shock event that fractionated Rb/Sr from lattice sites located on mineral grain boundaries. On the other hand, the leachates could contain secondary alteration products, and the parallel slopes of the tie lines could be coincidental.Nearly identical mineral modes, compositions, and ages suggest that these meteorites are very closely related. Nevertheless, their initial Sr and Nd isotopic compositions differ outside analytical uncertainty, requiring derivation from unique sources. Assimilation-fractional-crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma, if the assimilant has a Sr/Nd ratio near 1 and a radiogenic Sr isotopic composition. Further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it (a) formed at ∼4.5 Ga, (b) has a high La/Yb ratio, (c) is an oxidant, and (d) is basaltic in composition or is strongly enriched in incompatible elements. The composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks. Its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved Martian crustal component or hydrous metasomatism within the Martian mantle.  相似文献   

Chronology and petrogenesis of a 1.8 g lunar granitic clast:14321,1062     

C.-Y. Shih  L.E. Nyquist  D.D. Bogard  J.L. Wooden  B.M. Bansal  H. Wiesmann 《Geochimica et cosmochimica acta》1985,49(2):411-426

A study was undertaken to determine the chronology of a pristine granite clast (1062) from Apollo 14 breccia 14321 using Rb-Sr, Sm-Nd and ³⁹Ar-⁴⁰Ar methods. The genesis of the granite as constrained by the isotopic results and trace element characteristics is discussed.Chronology: The Rb-Sr internal isochron is slightly disturbed and yields an age of 4.09 ± 0.11 AE ($\text{λ(}{}^{87}\text{Rb) = 0.0139}\text{AE}{}^{\mathrm{?1}}$) and an imprecise initial I(Sr) = 0.702 ? .008. If two data are excluded, the age becomes 4.13 ± 0.03 AE and I(Sr) = 0.698 ? .003. The whole rock and mineral separates are extremely radiogenic; they yield model ages which are relatively well-defined. The average model age is 4.12 ± 0.03 AE (relative to BABI = 0.69898). The Sm-Nd internal isochron is also slightly disturbed and gives an age of 4.11 ± 0.20 AE ($\text{λ(}{}^{147}\text{Sm) = 0.00654}\text{AE}$^?1). The ³⁹Ar-⁴⁰Ar average age of the non-magnetic fraction of the sample yields a slightly younger age of 3.88 ± 0.03 AE (K-Ar constants from Steiger and $\text{>a}\text{?}$, 1977). The concordancy of Rb-Sr and Sm-Nd internal isochrons with the Rb-Sr model age strongly suggests that the granitic clast formed at 4.1 AE ago in the shallow crust and was later excavated and brecciated about 3.88 AE ago.Petrogenesis: Isotopic and trace element data of the lunar granite show large K/La and Rb/Sr fractionations, small Sm/Nd fractionation and the distinct V-shaped REE distribution pattern at the time of crystallization. A two-stage model involving crystal fractionation followed by silicate liquid immiscibility (SLI) is proposed for lunar granite genesis. We propose that the granite can be the immiscible acidic liquid produced by SLI from a residual liquid which underwent fractionation of ca, 3% of phases with REE distribution coefficients similar to those of phosphate minerals from a highly evolved parental magma with REE contents about twice those of the 15405,85 quartz monzodiorite (QMD).The extreme scarcity of lunar granitic samples and their young formation ages suggest that they are probably not directly crystallized from the differentiation of the primordial magma ocean. Our isotopic results and trace elements data from other workers suggest that granites, QMD and probably Mggabbronorites may be genetically related and may have formed in a plutonic environment similar to gabbro-granophyre associations in terrestrial layered intrusions such as the Skaergaard Intrusions.  相似文献   

Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites     

D.D. Bogard  L.E. Nyquist  P. Johnson 《Geochimica et cosmochimica acta》1984,48(9):1723-1739

Isotopic concentrations of the noble gases have been measured in several different phases of Elephant Moraine A79001 and in whole rock samples of Zagami and Allan Hills A77005, three meteorites which belong to the rare group of SNC achondrites that may have originated from the planet Mars. Shocked phases of EETA79001 contain a trapped Ar, Kr, and Xe component characterized by ${}^{84}\text{Kr}{}^{132}\text{Xe}$ ～15, ${}^{40}\text{Ar}{}^{36}\text{Ar}\text{> 2000}$, ${}^{129}\text{Xe}{}^{132}\text{Xe ≥ 2}$, and ${}^4\text{He}{}^{40}\text{Ar}\text{≤ 0.1}$. These elemental and isotopic ratios are unlike those for any other noble gas component except analyses of the Martian atmosphere made by Viking spacecraft. The isotopic composition of the trapped Kr shows an approximate 1% per mass unit enrichment of lighter isotopes compared to terrestrial Kr, and the traped Xe may show either a fission component or a fractionated enrichment of heavier isotopes compared to terrestrial Xe. It is hypothesized that these gases represent a portion of the Martian atmosphere which was shock-implanted into EETA79001, and that they constitute direct evidence of a Martian origin for the shergottite meteorites. Cosmic ray-produced gases in the eight known SNC meteorites form three distinct groups with exposure ages of ～11 MY (Chassigny and the nakhlites), ～2.6 MY (Shergotty, Zagami, and ALHA77005), and ～0.5 MY (EETA79001). These ages suggest three distinct events and cannot have been produced by irradiation for a common time under greatly different shielding. Comparison of cosmogenic ${}^3\text{He}{}^{21}\text{Ne}$ measured in EETA79001 with two independent models for the production of this ratio as a function of shielding indicates that this meteorite was irradiated in space as a relatively small object. If the SNC meteorites were ejected from Mars ～ 180 My ago, the shock age of the shergottites, they must have been relatively large objects (>6 meters diameter) which experienced at least three space collisions to initiate cosmic ray exposure. Ejection from Mars by three events at the times of initiation of cosmic ray exposure would permit the ejected objects to have been much smaller (<1 meter diameter), but would require three such events on 1.3 Gy Martian terraine in the past ～10 MY and would not explain the common 180 MY shock age seen in all four shergottites.  相似文献   

⁴⁰Ar-³⁹Ar dating of inclusions from IAB iron meteorites     

Sidney Niemeyer 《Geochimica et cosmochimica acta》1979,43(11):1829-1840

Silicate and troilite from IAB iron meteorites were dated by the ⁴⁰Ar-³⁹Ar technique. Silicate from four IAB meteorites gave well-defined apparent-age plateaus which accounted for 71–99% of the released ³⁹Ar. At low temperatures, only Copiapo showed appreciable loss of ⁴⁰Ar, while Mundrabilla and Woodbine released excess ⁴⁰Ar. The plateau ages are: 4.50 Byr for Copiapo, 4.57 Byr for Mundrabilla, 4.57 Byr for Woodbine, 4.54 Byr for unetched Pitts, and 4.57 Byr for etched Pitts; the 1σ error in each case is ± 0.03 Byr. A poorly-defined age plateau for Landes gives an age of 4.48 Byr, while the total K-Ar age (4.55 Byr) is significantly higher. The average $\text{(}{}^{40}\text{Ar/}{}^{36}\text{Ar)}$_trapped ratio for all silicate samples is 0.4 ± 0.4.Simple and undisturbed K-Ar systems are rare for meteorites, yet it appears to be a common feature for IAB silicates. In addition, plateau ages of IAB silicates are as old or older than the mean age of unshocked chondrites (4.47 Byr).Troilite samples yielded complex patterns which were evaluated via ⁴⁰Ar/³⁶Ar vs ³⁹Ar/³⁶Ar plots. Data for Pitts troilite are consistent with silicate and troilite retaining Ar at about the same time initially, but then 4.25 Byr ago nearly all the Ar in troilite was redistributed. The 700–1000°C points for Mundrabilla troilite define a line which gives an age of 6.2 Byr and $\text{(}{}^{40}\text{Ar/}{}^{36}\text{Ar)}{}_{\mathrm{trapped}}\text{= 42}$. This line may be an artifact, perhaps produced by homogenization of Ar and K.Approximate estimates of cosmic-ray exposure ages are 240 Myr for Landes, 130 Myr for Copiapo, 190 Myr for Woodbine, 170 Myr for Mundrabilla troilite, and 60 Myr for Pitts troilite.The I-Xe study of these same samples revealed a good correlation between well-defined I-Xe ages of silicates and Ni contents of metal (Niemeyer, 1979). The poorer resolution of the ⁴⁰Ar-³⁹Ar technique hampers a similar evaluation; nevertheless, plateau ages of the silicates suggest a systematic trend with Ni contents.  相似文献   

Isotopic and petrographic evidence for young Martian basalts     

Erin L. Walton  Simon P. Kelley 《Geochimica et cosmochimica acta》2008,72(23):5819-5837

Radiometric age data for shergottites yield ages of 4.0 Ga and 180-575 Ma; the interpretation of these ages has been, and remains, a subject of debate. Here, we present new ³⁹Ar-⁴⁰Ar laser probe data on lherzolitic shergottites Allan Hills (ALH) 77005 and Northwest Africa (NWA) 1950. These two meteorites are genetically related, but display very different degrees of shock damage. On a plot of ⁴⁰Ar/³⁶Ar versus ³⁹Ar/³⁶Ar, the more strongly shocked ALH 77005 (45-55 GPa) does not yield an array of values indicating an isochron, but the data are highly scattered with the shock melts yielding ⁴⁰Ar/³⁶Ar ratios of 1600-2026. Apparent ages calculated from these extractions range from 374-8183 Ma, with 50% of the data, particularly from the shock melts, yielding impossibly old ages (>4.567 Ga). On the same plot, extractions from igneous minerals in the less shocked NWA 1950 (30-44 GPa) yield a fitted age of 382 ± 36 Ma. Argon extractions from the shock melts are well distinguished from minerals, with the melts exhibiting the highest ⁴⁰Ar/³⁶Ar ratios (1260-1488) and the oldest apparent ages. Laser step heating was also performed on maskelynite separates from NWA 1950 yielding ages of 1000 Ma at the lowest release temperatures, and ages of 360 and 362 Ma at higher temperature steps. Stepped heating data from previous studies have yielded ages of 500 and 700 Ma to 1.7 Ga for ALH 77005 maskelynite separates. If the ages obtained from igneous minerals represent undegassed argon from an ancient (4.0 Ga) rock, then the ages are expected to anticorrelate with the degree of shock heating. The data do not support this inference. Our data support young crystallization ages for minerals and Martian atmosphere as the origin of excess ⁴⁰Ar in the shock melts.The shock features of shergottites are also reviewed in the context of what is known of the geologic history of the Martian surface through remote observation. The oldest, most heavily cratered surfaces of Mars are thought to be ?4.0 Ga; we contend that ancient rocks from Mars (Noachian >3.5 Ga) are likely to record multiple impact events reflecting megaregolith formation and the cumulative effects of erosion and aqueous alteration occurring during or since that era. Young rocks (Late Amazonian, <0.6 Ga) should record a relatively simple history of emplacement and ejection from the near surface. We show that although shergottites are strongly shocked, they are relatively pristine crystalline igneous rocks and not pervasively altered breccias. The petrography of shergottites is at odds with an ancient age interpretation. A model in which young coherent rocks are preferentially sampled by hypervelocity impact because of material strength is considered highly plausible.  相似文献   

Petrology and trace element geochemistry of Robert Massif 04261 and 04262 meteorites, the first examples of geochemically enriched lherzolitic shergottites     

Tomohiro Usui  Matthew Sanborn  Harry Y. McSween Jr. 《Geochimica et cosmochimica acta》2010,74(24):7283-7306

Shergottites sampled two distinct geochemical reservoirs on Mars. Basaltic and olivine-phyric shergottites individually sampled both geochemically enriched and depleted reservoirs, whereas lherzolitic shergottites are previously known only to exhibit a relatively limited intermediate geochemical signature that may have resulted from the mixing of the two geochemical end-member reservoirs. Here we show that recently discovered shergottites Robert Massif (RBT) 04261 and RBT 04262 are the first examples of lherzolitic shergottites originating from the enriched reservoir.RBT 04261 and RBT 04262, initially identified as olivine-phyric shergottites, are actually lherzolitic shergottites. Both meteorites exhibit nearly identical textures and mineral compositions, suggesting that they should be paired. Each consists of two distinct textures: poikilitic and non-poikilitic. The poikilitic areas are composed of pyroxene oikocrysts enclosing olivine grains; all pyroxene oikocrysts have pigeonite cores mantled by augite. The non-poikilitic areas are composed of olivine, pyroxene, maskelynite and minor amounts of merrillite, chromite and ilmenite. Olivine and pyroxene show the lowest Mg-number, and maskelynite has the lowest anorthite component among the lherzolitic shergottites. Moreover, the modal abundances of maskelynite in these two meteorites are distinctly higher than the other lherzolitic shergottites.The rare earth element (REE) budgets of RBT 04261 and RBT 04262 are dominated by merrillite. The slightly light rare earth element (LREE)-enriched pattern of this mineral is similar to that of merrillite in the geochemically enriched basaltic shergottites Shergotty and Zagami, and unlike the LREE-depleted pattern of merrillite in the other lherzolitic shergottites. The REE patterns of both high- and low-Ca pyroxenes are also similar to those in Shergotty and Zagami. The REE pattern of a melt calculated to be in equilibrium with the core of a pyroxene oikocryst is parallel to that of the RBT 04262 whole-rock as well as whole-rock compositions of other geochemically enriched basaltic shergottites. These observations imply that RBT 04262 sampled an enriched and oxidized reservoir similar to that sampled by some of the basaltic shergottites and are consistent with an oxidizing condition for the formation of RBT 04262 (log fO₂ = QFM-1.6).The petrographic and geochemical observations presented here suggest that RBT 04261 and RBT 04262 represent the most evolved magma among the lherzolitic shergottites and that this magma originated from a geochemically enriched reservoir on Mars. Based on an evaluation of the relationship between petrographic, geochemical and chronological signatures for shergottites including RBT 04261 and RBT 04262, we propose that both geochemically enriched and depleted shergottites were ejected from the same launch site on Mars.  相似文献   

Concordant Rb-Sr, Sm-Nd, and Ar-Ar ages for Northwest Africa 1460: A 346 Ma old basaltic shergottite related to “lherzolitic” shergottites     

L.E. Nyquist  D.D. Bogard  J. Park  Y.D. Reese 《Geochimica et cosmochimica acta》2009,73(14):4288-4309

Multiple lines of evidence show that the Rb-Sr, Sm-Nd, and Ar-Ar isotopic systems individually give robust crystallization ages for basaltic (or diabasic) shergottite Northwest Africa (NWA) 1460. In contrast to other shergottites, NWA 1460 exhibits minimal evidence of excess ⁴⁰Ar, thus allowing an unambiguous determination of its Ar-Ar age. The concordant Rb-Sr, Sm-Nd, and Ar-Ar results for NWA 1460 define its crystallization age to be 346 ± 17 Ma (2σ). In combination with petrographic and trace element data for this specimen and paired meteorite NWA 480, these results strongly refute the suggestion by others that the shergottites are ∼4.1 Ga old. Current crystallization and cosmic-ray exposure (CRE) age data permit identification of a maximum of nine ejection events for Martian meteorites (numbering more than 50 unpaired specimens as of 2008) and plausibly as few as five such events. Although recent high resolution imaging of the Martian surface has identified limited areas of sparsely cratered terrains, the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come, or that the cratering chronology needs recalibration. Time-averaged ⁸⁷Rb/⁸⁶Sr = 0.16 for the mantle source of the parent magma of NWA 1460/480 over the ∼4.56 Ga age of the planet is consistent with previously estimated values for bulk silicate Mars in the range 0.13-0.16, and similar to values of ∼0.18 for the “lherzolitic” shergottites. Initial ε_Nd for NWA 1460/480 at 350 ± 16 Ma ago was +10.6 ± 0.5, which implies a time-averaged ¹⁴⁷Sm/¹⁴⁴Nd of 0.217 in the Martian mantle prior to mafic melt extraction, similar to values of 0.211-0.216 for the “lherzolitic” shergottites. These time-averaged values do not imply a simple two-stage mantle/melt evolution, but must result from multiple episodes of melt extractions from the source regions. Much higher “late-stage” ε_Nd values for the depleted shergottites imply similar processes carried to a greater degree. Thus, NWA 1460/480, the “lherzolitic” shergottites and perhaps EET 79001 give the best (albeit imperfect) estimate of the Sr- and Nd-isotopic characteristics of bulk silicate Mars.  相似文献   

Argon 40-argon 39 chronology of lithic clasts from the Kapoeta howardite     

R.S. Rajan  J.C. Huneke  S.P. Smith  G.J. Wasserburg 《Geochimica et cosmochimica acta》1979,43(7):957-971

⁴⁰Ar-³⁹Ar age spectra have been measured on plagioclase separates from three basaltic clasts (A, B, C), a pyroxene separate from clast B, and a total sample of a fourth basaltic clast (ρ) from the Kapoeta achondritic meteorite. The Ar data show that three of the four clasts crystallized ≥4.5 AE ago. Xe measurements indicate all four formed within a 0.1 AE period (Huneke, et al., 1977, Lunar Science VIII, pp. 484–486). Three clasts have suffered various degrees of ⁴⁰Ar loss since that time. The times of ⁴⁰Ar degassing do not cluster about a single time analogous to the lunar cataclysm. The survival of ≥4.5 AE ages contrasts with the general absence of ages ≥4.0 AE on the moon.The Ar retention age of clast B of ≥4.57 AE is atypically older than the Rb-Sr age of 3.6 AE (Papanastassiouet al., 1974, Lunar Science V, p. 583). The 3.5 AE Ar age of clast A is distinctly younger than the Rb-Sr age of 3.9 AE (Papanastassiou et al., 1974). The K-Ar and Rb-Sr systems are clearly not equivalent dating techniques in these instances.The combined evidence of Ar, Xe and Rb-Sr studies suggests the period of volcanism on the Kapoeta parent planet was restricted to the first ~0.2 AE of solar system history. The subsequent thermal metamorphic histories recorded in each of the four clasts after formation are distinctly different. The clasts must have existed as independent fragments at least as recently as 3.5 AE ago. The cosmic ray exposure ages of all the four clasts are similar (~ 3 Myr), and are not significantly different from that of the bulk meteorite. The clasts spent essentially all of the time prior to the formation of Kapoeta at depths greater than a few meters.  相似文献   

The sources of water in Martian meteorites: clues from hydrogen isotopes     

N.Z Boctor  C.M.O’D Alexander  E Hauri 《Geochimica et cosmochimica acta》2003,67(20):3971-3989

H isotope measurements of carbonate, phosphate, feldspathic and mafic glasses, and post-stishovite silica phase in the shergottites Zagami, Shergotty, SaU 005, DaG 476, ALHA 77005 and EETA 79001, as well as in Chassigny and ALH 84001, show that all these phases contain deuterium-enriched water of extraterrestrial origin. The minerals and glasses analyzed may contain an initial primary hydrogen component, but their isotopic composition was modified to varying degrees by three different processes: interaction with a fractionated exchangeable water reservoir on Mars, hydrogen devolatilization by impact melting, and terrestrial contamination. Positive correlations between δD and water abundance in feldspathic glass and post-stishovite silica in Zagami, Shergotty, and SaU 005 is indicative of mixing of a high δD component (3000-4000‰) and a less abundant, low δD component (∼0‰). The high δD component is primarily derived from the Martian exchangable reservoir, but may also have been influenced by isotopic fractionation associated with shock-induced hydrogen loss. The low δD component is either a terrestrial contaminant or a primary “magmatic” component. The negative correlation between δD and water abundances in mafic and feldspathic glasses in ALH 84001, ALHA 77005, and EETA 79001 is consistent with the addition of a low δD terrestrial contaminant to a less abundant high-deuterium Martian component. The low δD of magmatic glass in melt inclusions suggests that the δD of Martian parent magma was low and that the initial H isotope signature of Mars may be similar to that of Earth.  相似文献   

Sm-Nd age of the Stillwater complex and the mantle evolution curve for neodymium     

D.J. DePaolo  G.J. Wasserburg 《Geochimica et cosmochimica acta》1979,43(7):999-1008

An internal isochron determined for a gabbro from the Stillwater complex by the Sm-Nd method yields a precise age of 2701 ± 8 Myr and initial ¹⁴³Nd/¹⁴⁴Nd = 0.508248 ± 12. The initial is close to the CHUR evolution curve but clearly displaced below it by ?_Nd = ?2.8 ± 0.2. A spectrum of total rocks in the Stillwater complex ranging from anorthosite to pyroxenite were found to lie on the same isochron to within experimental error indicating the same age and initial. These data demonstrate that some ancient mantle-derived rocks have initial ¹⁴³Nd/¹⁴⁴Nd which deviate substantially from the CHUR evolution curve at the time of their formation. This implies that there was early layering in the mantle with substantial REE fractionation (~6–12% Nd/Sm enrichment) or that the Stillwater complex was highly contaminated with REE from much older continental crust during emplacement. The results show the necessity of high-precision ages and initial ¹⁴³Nd/¹⁴⁴Nd values in order to properly describe REE fractionation in the mantle. While the Sm-Nd age results show no indication of any irregularities, we have confirmed that the Rb-Sr data for the Stillwater are highly disturbed. This comparison indicates that the Sm-Nd parent-daughter system may be much less susceptible to element redistribution during metamorphism, therefore permitting wide application of this technique to rocks of complex histories.  相似文献   

I-Xe and ⁴⁰Ar-³⁹Ar dating of silicate from Weekeroo Station and Netschaëvo IIE iron meteorites     

Sidney Niemeyer 《Geochimica et cosmochimica acta》1980,44(1):33-44

Silicate inclusions from two IIE iron meteorites were dated by the I-Xe and ⁴⁰Ar-³⁹Ar techniques. Weekeroo Station, a ‘normal’ IIE iron, shows no loss of radiogenic ⁴⁰Ar at low temperature, and the well-defined ⁴⁰Ar-³⁹Ar plateau yields an age of 4.54 ± 0.03 Byr. The xenon data define a good I-Xe correlation with an age of +10.9 ± 0.5 Myr relative to Bjurböle [the monitor error (±2.5 Myr) is not included].^Despite its relatively young age, Weekeroo Station's $\text{(}{}^{129}\text{Xe}{}^{132}\text{Xe)}{}_{\mathrm{trappad}}$ ratio (= 0.84 ± 0.05) lies significantly below the solar value. Netschaëvo silicate has a chondritic composition, unlike ‘normal’ IIE silicate which is more differentiated. Nevertheless Netschaëvo gives a ⁴⁰Ar-³⁹Ar plateau-age of only 3.79 ± 0.03 Byr, with the xenon data failing to define an I-Xe isochron.Only irons from the IAB and IIE groups contain silicate inclusions, but these two groups differ in many other respects, mostly suggesting that IAB meteorites are more primitive. The I-Xe chronology supports this suggestion inasmuch as Weekeroo Station formed well after (8–15 Myr) IAB silicates. In terms of Scott and Wasson's (1976) model, ages for Weekeroo Station date the shock event which formed ‘normal’ IIE irons by mixing the low-melting fraction of the parent silicate with shock-liquefied metal. Scott and Wasson's suggestion that Netschaëvo represents IIE parent material, however, is contradicted by Netschaëvo's 3.8 Byr age.The four silicate-bearing IIE irons which have now been dated can be subdivided into distinct pairs: Weekeroo Station and Colomera formed near the beginning of the solar system, while Netschaëvo and Kodaikanal both formed only 3.8 Byr ago. A review of other properties of these meteorites generally support this subdivision.This work underscores the complexity of the history of IIE meteorites; in particular, an adequate model must account for the formation of two IIE irons at 3.8 Byr without disturbing rare gases in Weekeroo Station. All formation models are quite speculative, but the one which seems best to fit the available evidence postulates two parent bodies: the 3.8 Byr old silicate formed on one parent body, all other IIE material resided in a separate body, and subsequent collision(s) mixed the young silicate with IIE metal.  相似文献   

Experimental study of polybaric REE partitioning between olivine, pyroxene and melt of the Yamato 980459 composition: Insights into the petrogenesis of depleted shergottites     

Alexandra Blinova  Christopher D.K. Herd 《Geochimica et cosmochimica acta》2009,73(11):3471-19164

A synthetic composition representing the Yamato 980459 martian basalt (shergottite) has been used to carry out phase relation, and rare earth element (REE) olivine and pyroxene partitioning experiments. Yamato 980459 is a sample of primitive basalt derived from a reduced end-member among martian mantle sources. Experiments carried out between 1-2 GPa and 1350-1650 °C simulate the estimated pressure-temperature conditions of basaltic melt generation in the martian mantle. Olivine-melt and orthopyroxene-melt partition coefficients for La, Nd, Sm, Eu, Gd and Yb (D_REE values) were determined by LA-ICPMS, and are similar to the published values for terrestrial basaltic systems. We have not detected significant variation in D-values with pressure over the range investigated, and by comparison with previous studies carried out at lower pressure.We apply the experimentally obtained olivine-melt and orthopyroxene-melt D_REE values to fractional crystallization and partial melting models to develop a three-stage geochemical model for the evolution of martian meteorites. In our model we propose two ancient (∼4.535 Ga) sources: the Nakhlite Source, located in the shallow mantle, and the Deep Mantle Source, located close to the martian core-mantle boundary. These two sources evolved distinctly on the ε¹⁴³Nd evolution curve due to their different Sm/Nd ratios. By partially melting the Nakhlite Source at ∼1.3 Ga, we are able to produce a slightly depleted residue (Nakhlite Residue). The Nakhlite Residue is left undisturbed until ∼500 Ma, at which point the depleted Deep Mantle Source is brought up by a plume mechanism carrying with it high heat flow, melts and isotopic signatures of the deep mantle (e.g., ε¹⁸²W, ε¹⁴²Nd, etc.). The plume-derived Deep Mantle Source combines with the Nakhlite Residue producing a mixture that becomes a mantle source (herein referred to as “the Y98 source”) for Yamato 980459 and the other depleted shergottites with the characteristic range of Sm/Nd ratios of these meteorites. The same hot plume provides a heat source for the formation of enriched and intermediate shergottites. Our model reproduces the REE patterns of nakhlites and depleted shergottites and can explain high ε¹⁴³Nd in depleted shergottites. Furthermore, the model results can be used to interpret whole rock Rb-Sr and Sm-Nd ages of shergottites.  相似文献   

Sm-Nd systematics of a tonalitic augen gneiss and its constituent minerals from northern Michigan     

Kiyoto Futa 《Geochimica et cosmochimica acta》1981,45(7):1245-1249

The Sm-Nd isotopic system of a tonalitic augen gneiss and its constituent minerals from northern Michigan was disturbed during metamorphism. Sm-Nd zircon ages are lower than the wholerock Sm-Nd model age. However, closely associated pairs of minerals (for example, sphene and biotite or apatite and plagioclase) retain their apparent metamorphic ages. The Sm-Nd model age for the tonalitic augen gneiss of 3919 ± 30myr, appears to reflect open system behavior during metamorphism. A mineralogically different gneiss from the same location has a Sm-Nd model age of 3520 ± 70 myr. The two whole rocks differ in their Sm-Nd and Rb-Sr systematics and in their chondrite-normalized rare earth element (REE) patterns. The whole-rock-normalized mineral REE patterns show the contribution of the major and trace minerals to the REE content of the whole rock. The trace minerals contain a significant amount of the total REE.  相似文献   

相山铀矿田铀多金属成矿时代与成矿热历史   总被引：1，自引：1，他引：0  

林锦荣  胡志华  王勇剑  张松  陶意 《岩石学报》2019,35(9):2801-2816

相山铀矿田的铀多金属矿化主要可划分为碱性铀矿化、酸性铀矿化、铅锌银铜矿化和金矿化四种类型。通过沥青铀矿和矿化岩石U-Pb等时线、黄铁矿Rb-Sr等时线、绢云母~(40)Ar-~(39)Ar同位素年龄测定,结合铀多金属成矿特征研究,厘定了相山铀矿田铀多金属成矿时代,确定铀多金属矿化的成矿时序为:碱性铀矿化、铅锌银铜矿化、金矿化、酸性铀矿化。锆石裂变径迹研究表明,相山矿田铀多金属矿化样品的锆石裂变径迹峰值年龄与U-Pb、Rb-Sr和~(40)Ar-~(39)Ar同位素年龄一致性良好,裂变径迹年龄(峰值年龄)可以限定热液铀多金属成矿热事件时代。碱性铀成矿热事件的锆石裂变径迹峰值年龄为119. 8～125. 6Ma;金成矿热事件和铅锌银铜多金属成矿热事件的锆石裂变径迹峰值年龄为106. 1～113. 8Ma;酸性铀成矿热事件的锆石裂变径迹峰值年龄为86. 7～100. 0Ma;新发现一期锆石裂变径迹峰值年龄为66. 4～78. 6Ma的热事件,该期热事件可能为相山矿田最晚一期酸性铀成矿热事件。相山矿田66. 4～78. 6Ma的铀成矿热事件,与华南花岗岩型热液铀矿床的区域成矿热事件时代耦合,该发现对华南火山岩型铀矿成矿时代的重新认识,对火山岩型、花岗岩型铀矿床成矿统一性认识具有重要意义。  相似文献   

Constraints on the U-Pb isotopic systematics of Mars inferred from a combined U-Pb, Rb-Sr, and Sm-Nd isotopic study of the Martian meteorite Zagami   总被引：1，自引：0，他引：1  

Lars E. Borg  Jennifer E. Edmunson 《Geochimica et cosmochimica acta》2005,69(24):5819-5830

Uranium-lead, Rb-Sr, and Sm-Nd isotopic analyses have been performed on the same whole-rock, mineral, and leachate fractions of the basaltic martian meteorite Zagami to better constrain the U-Pb isotopic systematics of martian materials. Although the Rb-Sr and Sm-Nd systems define concordant crystallization ages of 166 ± 6 Ma and 166 ± 12 Ma, respectively, the U-Pb isotopic system is disturbed. Nevertheless, an age of 156 ± 6 Ma is derived from the ²³⁸U-²⁰⁶Pb isotopic system from the purest mineral fractions (maskelynite and pyroxene). The concordance of these three ages suggest that the ²³⁸U-²⁰⁶Pb systematics of the purest Zagami mineral fractions have been minimally disturbed by alteration and impact processes, and can therefore be used to constrain the behavior of U and Pb in the Zagami source region. The μ value of the Zagami source region can be estimated, with some confidence from the ²³⁸U-²⁰⁶Pb isochron, to be 3.96 ± 0.02. Disturbance of the U-Pb isotopic systems means that this represents a minimum value. The μ value of the Zagami source is significantly lower than the μ values estimated for most basaltic magma sources from Earth and the Moon. This is surprising given the high initial ⁸⁷Sr/⁸⁶Sr ratio (0.721566 ± 82) and low initial ε_Nd value (−7.23 ± 0.17) determined for Zagami that indicate that this sample is derived from one of the most highly fractionated reservoirs from any known planetary body. This suggests that Mars is characterized by a low bulk planet U/Pb ratio, a feature that is consistent with its relatively volatile-rich nature.The leachates contain terrestrial common Pb that was probably added to the meteorite during handling, curation, or sawing. The mineral fractions, particularly those with significant amounts of impact melt glass, contain a second contaminant. The presence of this contaminant results in Pb-Pb ages that are older than the crystallization age of Zagami, indicating that the contaminant is characterized by a high ²⁰⁷Pb/²⁰⁶Pb ratio. Such a contaminant could be produced by removal of single-stage Pb from a relatively high μ martian reservoir before ∼1.8 Ga, and therefore could be an ancient manifestation of hydrous alteration of martian surface material.  相似文献   

Hf-W isotope systematics of chondrites, eucrites, and martian meteorites: Chronology of core formation and early mantle differentiation in Vesta and Mars     

T. Kleine  K. Mezger  H. Palme 《Geochimica et cosmochimica acta》2004,68(13):2935-2946

The timescale of accretion and differentiation of asteroids and the terrestrial planets can be constrained using the extinct ¹⁸²Hf-¹⁸²W isotope system. We present new Hf-W data for seven carbonaceous chondrites, five eucrites, and three shergottites. The W isotope data for the carbonaceous chondrites agree with the previously revised ¹⁸²W/¹⁸⁴W of chondrites, and the combined chondrite data yield an improved ?_W value for chondrites of −1.9 ± 0.1 relative to the terrestrial standard. New Hf-W data for the eucrites, in combination with published results, indicate that mantle differentiation in the eucrite parent body (Vesta) occurred at 4563.2 ± 1.4 Ma and suggest that core formation took place 0.9 ± 0.3 Myr before mantle differentiation. Core formation in asteroids within the first ∼5 Myr of the solar system is consistent with the timescales deduced from W isotope data of iron meteorites. New W isotope data for the three basaltic shergottites EETA 79001, DaG 476, and SAU 051, in combination with published ¹⁸²W and ¹⁴²Nd data for Martian meteorites reveal the preservation of three early formed mantle reservoirs in Mars. One reservoir (Shergottite group), represented by Zagami, ALH77005, Shergotty, EETA 79001, and possibly SAU 051, is characterized by chondritic ¹⁴²Nd abundances and elevated ?_W values of ∼0.4. The ¹⁸²W excess of this mantle reservoir results from core formation. Another mantle reservoir (NC group) is sampled by Nakhla, Lafayette, and Chassigny and shows coupled ¹⁴²Nd-¹⁸²W excesses of 0.5-1 and 2-3 ? units, respectively. Formation of this mantle reservoir occurred 10-20 Myr after CAI condensation. Since the end of core formation is constrained to 7-15 Myr, a time difference between early silicate mantle differentiation and core formation is not resolvable for Mars. A third early formed mantle reservoir (DaG group) is represented by DaG 476 (and possibly SAU 051) and shows elevated ¹⁴²Nd/¹⁴⁴Nd ratios of 0.5-0.7 ? units and ?_W values that are indistinguishable from the Shergottite group. The time of separation of this third reservoir can be constrained to 50-150 Myr after the start of the solar system. Preservation of these early formed mantle reservoirs indicates limited convective mixing in the Martian mantle as early as ∼15 Myr after CAI condensation and suggests that since this time no giant impact occurred on Mars that could have led to mantle homogenization. Given that core formation in planetesimals was completed within the first ∼5 Myr of the solar system, it is most likely that Mars and Earth accreted from pre-differentiated planetesimals. The metal cores of Mars and Earth, however, cannot have formed by simply combining cores from these pre-differentiated planetesimals. The ¹⁸²W/¹⁸⁴W ratios of the Martian and terrestrial mantles require late effective removal of radiogenic ¹⁸²W, strongly suggesting the existence of magma oceans on both planets. Large impacts were probably the main heat source that generated magma oceans and led to the formation metallic cores in the terrestrial planets. In contrast, decay of short-lived ²⁶Al and ⁶⁰Fe were important heat sources for melting and core formation in asteroids.  相似文献   

40Ar-39Ar and Rb-Sr geochronology of high-pressure metamorphism and exhumation history of the Tavsanli Zone, NW Turkey     

Sarah Sherlock  Simon Kelley  Simon Inger  Nigel Harris  Aral Okay 《Contributions to Mineralogy and Petrology》1999,137(1-2):46-58

Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects ⁴⁰Ar-³⁹Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; ⁴⁰Ar-³⁹Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger ⁴⁰Ar-³⁹Ar ages in the cores of coarser white micas, which in conjunction with ⁴⁰Ar-³⁹Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma⁻¹ and exhumation to the surface via thrusting. The ⁴⁰Ar-³⁹Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks. Received: 12 May 1998 / Accepted: 21 April 1999  相似文献   

Ar-Ar ages of martian nakhlites     

Jisun Park  Donald D. Bogard 《Geochimica et cosmochimica acta》2009,73(7):2177-2189

We report ³⁹Ar-⁴⁰Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of ³⁹Ar recoil and variable amounts of trapped ⁴⁰Ar in the samples. Thus, we examine possible Ar-Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar-Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2σ errors). For NWA 998 and MIL 03346 the Ar-Ar ages are within uncertainties of preliminary Rb-Sr isochron ages reported in the literature. These Ar-Ar ages for Y-000593 and Nakhla are several Ma older than Sm-Nd ages reported in the literature. We conclude that the major factor in producing Ar-Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial ⁴⁰Ar on weathered grain surfaces that was degassed along with the first several percent of ³⁹Ar. A total K-⁴⁰Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2σ). We emphasize the precision of this isochron over the value of the isochron age. Our Ar-Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of ∼12 Ma is also similar to CRE ages for other nakhlites.  相似文献