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1.
Samarium-neodymium isotopic analysis of the martian meteorite Dar al Gani 476 yields a crystallization age of 474 ± 11 Ma and an initial εNd143 value of +36.6 ± 0.8. Although the Rb-Sr isotopic system has been disturbed by terrestrial weathering, and therefore yields no age information, an initial 87Sr/86Sr ratio of 0.701249 ± 33 has been estimated using the Rb-Sr isotopic composition of the maskelynite mineral fraction and the Sm-Nd age. The Sr and Nd isotopic systematics of Dar al Gani 476, like those of the basaltic shergottite QUE94201, are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system. Nevertheless, Dar al Gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the QUE94201 source region. This is not consistent with the fact that the parental magma of Dar al Gani 476 is significantly more mafic than the parental magma of QUE94201, and underscores a decoupling between the major element and trace element-isotopic systematics observed in the martian meteorite suite.Combining the εNd142-εNd143 isotopic systematics of the martian meteorites yields a model age for planetary differentiation of 4.513+0.033−0.027 Ga. Using this age, the parent/daughter ratios of martian mantle sources are calculated assuming a two-stage evolutionary history. The calculated sources have very large ranges of parent/daughter ratios (87Rb/86Sr = 0.037-0.374; 147Sm/144Nd = 0.182-0.285; 176Lu/177Hf = 0.028-0.048). These ranges exceed the ranges estimated for terrestrial basalt source regions, but are very similar to those estimated for the sources of lunar mare basalts. In fact, the range of parent/daughter ratios calculated for the martian meteorite sources can be produced by mixing between end-members with compositions similar to lunar mare basalt sources. Two of the sources have compositions that are similar to olivine and pyroxene-rich mafic cumulates with variable proportions of a Rb-enriched phase, such as amphibole, whereas the third source has the composition of liquid trapped in the cumulate pile (i.e. similar to KREEP) after ∼99% crystallization. Correlation between the proportion of trapped liquid in the meteorite source regions and estimates of fO2, suggest that the KREEP-like component may be hydrous. The success of these models in reproducing the martian meteorite source compositions suggests that the variations in trace element and isotopic compositions observed in the martian meteorites primarily reflect melting of the crystallization products of an ancient magma ocean, and that assimilation of evolved crust by mantle derived magmas is not required. Furthermore, the decoupling of major element and trace element-isotopic systematics in the martian meteorite suite may reflect the fact that trace element and isotopic systematics are inherited from the magma source regions, whereas the major element abundances are limited by eutectic melting processes at the time of magma formation. Differences in major element abundances of parental magma, therefore, result primarily from fractional crystallization after leaving their source regions.  相似文献   

2.
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   

3.
Northeastern Queensland, a part of the Phanerozoic composite Tasman Fold Belt of eastern Australia, has a Paleozoic to Mesozoic history dominated by subduction zone processes. A suite of 13 peridotite xenoliths from the <3 Ma Atherton Tablelands Volcanic Province, predominantly from Mount Quincan, comprise fertile (1.8-3.4 wt.% Al2O3 and 38.7-41.9 wt.% MgO) spinel lherzolites free from secondary volatile-bearing phases and with only weak metasomatic enrichment of incompatible trace elements (SmN/YbN = 0.23-1.1; LaN/YbN = 0.11-4.9). The suite is isotopically heterogeneous, with measured Sr (87Sr/86Sr = 0.7027-07047), Nd (143Nd/144Nd = 0.51249-0.51362), and to a lesser extent, Os (187Os/188Os = 0.1228-0.1292) compositions broadly overlapping MORB source mantle (DMM) and extending to more depleted compositions, reflecting evolution in a time-integrated depleted reservoir. Major and rare earth element systematics are consistent with mantle that is residual after low to moderate degrees of melt extraction predominantly in the spinel facies, but with a few samples requiring partial melting at greater pressures in the garnet field or near the garnet-spinel transition. In contrast to most previously studied suites of continental lithospheric mantle samples, the incompatible trace element contents and Sr and Nd isotopic systematics of these samples suggest only minimal modification of the sampled lithosphere by metasomatic processes.Five of six Mount Quincan xenoliths preserving depleted middle to heavy REE patterns form a whole rock Sm-Nd isochron with an age of ∼275 Ma (εNdi = +9), coincident with widespread granitoid emplacement in the overlying region. This isochron is interpreted to indicate the timing of partial melting of a DMM-like source. Xenoliths from other Atherton localities scatter about the isochron, suggesting that the sampled mantle represents addition of DMM mantle to the lithosphere in the Permian, when the region may have broadly been within a subduction zone setting. A sixth middle to heavy REE-depleted Mount Quincan xenolith has a distinct Nd and Os isotopic composition consistent either with an earlier, possibly Precambrian melt extraction event, or with Permian derivation from a mantle source with a less depleted (time-averaged lower Sm/Nd) Nd isotopic composition, but a more depleted (low Re/Os) Os isotopic composition.The range in measured whole rock Os isotopic compositions cannot solely be the result of time-integrated effects of variable melt extraction, especially considering the coherent Sm-Nd systematics of the suite. The Os heterogeneity more likely reflects either a heterogeneous ∼275 Ma DMM source that would have a present-day Os composition (187Os/188Os ∼ 0.1265-0.1287) overlapping both abyssal peridotites and chondrites, or significant and variable enrichment within the lithospheric mantle by secondary sulfides carrying radiogenic Os in a cryptic chalcophile enrichment event. Regardless of the origin of the Os isotopic variability, these data highlight the mantle Re-Os isotopic heterogeneity that may be present over small length scales where the lithophile Sm-Nd system may be relatively homogeneous.  相似文献   

4.
A study was undertaken to determine the chronology, petrogenesis and relationships among the shergottites, Shergotty and Zagami and the unique achondrite ALHA77005. These meteorites are the product of a variety of complex processes.Petrogenesis: Chondrite-normalized abundance patterns of Shergotty and Zagami are very similar and show pronounced depletions of both the light REE (La-Nd) and heavy REE (Dy-Lu) relative to Sm-Gd. These characteristic depletions are even more pronounced for ALHA77005. The light REE depletion is qualitatively consistent with the presence of cumulus pyroxene and/or olivine in these meteorites, but trace element models show that the parental magmas of all three meteorites were probably also light REE depleted. Both trace element model calculations and combined Rb-Sr and Sm-Nd isotopic systematics show that the meteorites could not have been co-magmatic nor can ALHA77005 be representative of the source material of the shergottites. Light REE depletion of the parental magmas also implies light REE depletion of the source material. The Sm-Nd systematics of the shergottites require a time-averaged sub-chondritic (light REE enriched) Sm-Nd ratio since 4.6 AE ago. The Sm-Nd systematics of ALHA77005 permit a time-averaged super-chondritic (light REE depleted) Sm/Nd ratio if its crystallization age is less than TICE = 0.72 AE.Chronology. Rb-Sr internal isochrons for all three meteorites and a Sm-Nd internal isochron for Zagami are concordant at ~ 180 Myr. 39Ar-40Ar plateau ages of Shergotty and Zagami maskelynite are ~250–260 Myr. These ages apparently reflect resetting of these isotopic systems by shock metamorphism which converted the feldspar to maskelynite. The concordance of these ages suggests a single shock event during which the meteorites were in close physical proximity. The time of this event is most precisely given by the Rb-Sr age of 180 ± 4 Myr for Zagami.The crystallization ages of the meteorites were not precisely determined. Extreme upper limits are determined by Sm-Nd model ages relative to an eucrite initial 143Nd144Nd = 0.505835 at 4.6 AE ago. These model ages for Shergotty, Zagami and ALHA77005 are 3600, 3500 and 2850 Myr, respectively. The Sm-Nd whole rock age of 1340 ± 60 Myr for the three meteorites gives the crystallization age if the Sm/Nd ratios of the precursor materials were always the same. We consider this 1340 Myr age as a “best estimate” upper limit. “Best estimate” lower limits for Shergotty and Zagami are taken from the average 39Ar-40Ar ages of 1200 and 900 Myr of pyroxene separates. The average 39Ar-40Ar age of a whole rock sample of ALHA77005 was 1600 Myr and can be partitioned between a low temperature (feldspar) phase and a high temperature (olivine + pyroxene + inclusions) “phase”. The average apparent 39Ar-40Ar age of the low temperature phase is ~1050 Myr, which is chosen as the “best estimate” lower limit to the age. The crystallization ages of Shergotty, Zagami and ALHA77005 probably lie within the ranges of 1200–1300, 900–1300 and 1000–1300 Myr, respectively. The Rb-Sr whole rock age of 4400 ± 400 Myr and single-stage BABI model ages of ~4800–5100 Myr are interpreted as reflecting differentiation of the parent body at ~4600 Myr ago.The complex geochemical and isotopic evolution recorded by these meteorites suggests a geologically active parent body capable of sustaining melting at two or more epochs in its history.  相似文献   

5.
Coupled with a petrographical study, I carried out an ion probe study of rare earth element microdistributions in mineral phases of silicate inclusions from the Colomera IIE iron meteorite. Most mineral grains have homogeneous REEs, but show considerable inter-grain variations by a factor of 2 to 100. The whole rock REE abundances for Colomera,estimated by combining REE data with modal abundances, are relatively LREE-enriched with REEs of～10’CI, which suggest that Colomera silicates were highly differentiated and might represent a low degree partial melt (-10%) of a chondritic source. REE geochemistry of Colomera silicate inclusions points to an origin that involves differentiation,dynamic mixing, remelting, reduction, recrystallization, and subsequent rapid cooling near the surface of a planetary body.  相似文献   

6.
We report the results of a Sm-Nd isotopic, major element and rare earth element (REE) study of the Older Metamorphic Group (OMG) tonalite-amphibolite association of the eastern Indian Craton. The Older Metamorphic Tonalite Gneisses (OMTG) have been previously dated to be 3.8 Ga using Sm-Nd isotope systematies, and 3.2–3.4 Ga by Rb-Sr and Pb-Pb dating. The results of this study indicate that the protoliths of the OMG amphibolites are 3.3 Ga isochron age=3.30±0.06 Ga, Nd= +0.9 ± 0.7), and therefore, the OMTG, which intrude into the associated amphibolites, cannot be any older than 3.3 Ga. The amphibolites display light REE enrichment ((Ce/Yb)N=2.2–6.7; La=30–100 x chondrite) and nearly flat heavy REE patterns ((Tb/Lu)N=1.2–1.9); the basaltic parents of the amphibolites were probably generated by the partial melting of a spinel lherzolite mantle. Strong linear relationships between the amphibolites and tonalites in 147Sm/144Nd-143Nd/144Nd space (isochron age =3.29±0.04 Ga, Nd= +0.8 ± 0.8) imply that they are genetically related. The tonalites display fractionated REE patterns (La=100–300 x chondrite) with moderate heavy REE depletions ((Tb/Lu)N=1.9–3.4). The isotopic, major element and REE data are consistent with the derivation of the OMTG from partial melting of OMG amphibolites or equivalent rocks at amphibolegarnet stabilization depths. An initial Nd(t) value of +0.9±0.7 for the amphibolites indicates the presence of a slightly depleted mantle source at 3.3 Ga with 147Sm/144Nd. between 0.20 and 0.22. It is suggested that the growth of continental crust in the eastern Indian craton occurred in response to magmatic underplating in a plume setting.  相似文献   

7.
《Precambrian Research》1987,36(2):99-126
The Western Namaqua Province (WNP) of southern Africa is composed of several supracrustal assemblages and intrusive granitoid suites that were formed during the mid-Proterozoic 2.0-1.0 Ga ago. The Bushmanland Group (BG) has been the subject of intense study recently because of its considerable strategic metal potential, hosting several giant stratiform base metal deposits (e.g., Gamsberg and Aggeneys). The BG contains amphibolites which represent metamorphosed tholeiitic basalts with MgO (9-4%) and enriched in Ni relative to modern basalts, a feature reminiscent of Archaean tholeiites. Post depositional mobility of elements is probably widespread and hampers recognition of parental magma composition and source region characteristics. However, the amphibolites have yielded a Sm-Nd isochron age of 1649 ± 90 Ma (ϵNd(T) = −0.48 ± 0.89), which is interpreted as the time of extrusion of the basalt precursors, and by inference a minimum sedimentation age for the underlying metalliferous BG sequence. It follows that the BG sequence evolved during an intermediate period in the history of the Namaqua Province, after major crustal stabilisation around 2.0-1.9 Ga and before the last major tectonothermal event around 1.2-1.1 Ga. Results of a parallel Rb-Sr and U-ThPb study of the BG amphibolites indicate strong metamorphic resetting 1.2-1.1 Ga ago, thereby confirming earlier work on the WNP. Characteristics of the source region to the BG basalts have been inspected with MORB normalised trace element abundances, with the salient result being the BG basalts require derivation from a source that experienced variable geochemical modification similar to that proposed for the mantle wedge overlying modern subduction zones. Such a process is currently preferred over crustal contamination, since the Sm-Nd isochron for the BG amphibolites is remarkably linear, but independent confirmation of the Sm-Nd age is still required before this alternative model can be discounted. To reconcile the requirement of subduction related metasomatism in the source to tholeiitic basalts erupted within what appears to be a cratonic sedimentary basin, we suggest that back are continental extension could provide a satisfactory tectonic setting, as such an environment is intrinsically linked to subduction.  相似文献   

8.
We have determined the Nd3+ diffusion kinetics in natural enstatite crystals as a function of temperature, f(O2) and crystallographic direction at 1 bar pressure and applied these data to several terrestrial and planetary problems. The diffusion is found to be anisotropic with the diffusion parallel to the c-axial direction being significantly greater than that parallel to a- and b-axis. Also, D(//a) is likely to be somewhat greater than D(//b). Diffusion experiments parallel to the b-axial direction as a function of f(O2) do not show a significant dependence of D(Nd3+) on f(O2) within the range defined by the IW buffer and 1.5 log unit above the WM buffer. The observed diffusion anisotropy and weak f(O2) effect on D(Nd3+) may be understood by considering the crystal structure of enstatite and the likely diffusion pathways. Using the experimental data for D(Nd3+), we calculated the closure temperature of the Sm-Nd geochronological system in enstatite during cooling as a function of cooling rate, grain size and geometry, initial (peak) temperature and diffusion direction. We have also evaluated the approximate domain of validity of closure temperatures calculated on the basis of an infinite plane sheet model for finite plane sheets showing anisotropic diffusion. These results provide a quantitative framework for the interpretation of Sm-Nd mineral ages of orthopyroxene in planetary samples. We discuss the implications of our experimental data to the problems of melting and subsolidus cooling of mantle rocks, and the resetting of Sm-Nd mineral ages in mesosiderites. It is found that a cooling model proposed earlier [Ganguly J., Yang H., Ghose S., 1994. Thermal history of mesosiderites: Quantitative constraints from compositional zoning and Fe-Mg ordering in orthopyroxene. Geochim. Cosmochim. Acta 58, 2711-2723] could lead to the observed ∼90 Ma difference between the U-Pb age and Sm-Nd mineral age for mesosiderites, thus obviating the need for a model of resetting of the Sm-Nd mineral age by an “impulsive disturbance” [Prinzhoffer A, Papanastassiou D.A, Wasserburg G.J., 1992. Samarium-neodymium evolution of meteorites. Geochim. Cosmochim. Acta 56, 797-815].  相似文献   

9.
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 εNd 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” εNd 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   

10.
This work presents isotope Sm-Nd data obtained for bulk samples of granites of all 8 emplacement phases of the Raumid granite massif, which occurred 35 Ma ago at a hypabyssal depth during the orogenic stage of development of Southern Pamir fold system. The 147Sm/144Nd ratio in studied collection of granite samples ranges between 0.091 and 0.323; the εNd(T) value is–4.0. The Sm-Nd isotope study results suggest that all granite varieties distinguished in the Raumid massif are comagmatic formations and contamination and hybridization processes did not play any role in REE distribution in granites. At this, the source of parental magma did not change during granite generation. We assume that the only process, resulted in the trace element evolution in granites, was differentiation of three batches of magma sequentially uplifted from the source.  相似文献   

11.
The Betts Cove ophiolite includes the components of typical ocean crust: pillow lavas, sheeted dikes, gabbros and ultramafics. However, the trace element geochemistry of basaltic rocks is unusual. Three geochemical units are recognized within the lava and dike members. Within the pillow lavas, the geochemical units correspond to stratigraphic units. Upper lavas have ‘normal’ (i.e., typical for ocean floor basalts) TiO2 contents (0.75 to 2.0 wt%), heavy rare earth elements (HREE) values in the range 6–20× chondrites and chondrite-normalized REE patterns with relative LREE depletion. Intermediate lavas have TiO2 contents between 0.30 and 0.50 wt%, HREE contents from 4–7× chondrites and extreme relative LREE depletion. Lower lavas have anomalously low TiO2 contents (<0.30 wt%) and unusual convex-downwards REE patterns with REE abundances around 2–5 × chondrite. These geochemical differences can be explained if the three groups were derived from different mantle sources. Independent mantle sources for the three units are consistent with their different 143Nd144Nd ratios varying at 480 m.y.B.P. from 0.51222 in a lower lava to 0.51238 in an upper lava. The upper lavas may be partial melts of a source similar in composition to that of modern MORB, the intermediate lavas may be from a very depleted oceanic mantle (second stage melt), and the lower lavas may have formed by melting an extremely depleted mantle that had been invaded by a LREE-enriched fluid. A possible tectonic environment where these different sources could be juxtaposed is a back-arc or inter-arc basin.  相似文献   

12.
《Precambrian Research》1987,37(4):323-342
A suite of early Proterozoic basic to granitic rocks exposed near St. Cloud, Minnesota are cut by northeast-trending basaltic dikes. Pb isotope data for all of these rocks overlap and plot about an 1800 Ma PbPb correlation line. The basic rocks, which are light REE enriched, have ϵNd values between +0.4 and −4.8. Petrogenetic considerations suggest that the basic rocks were derived from a light REE enriched source which had an Fe/Mg ratio greater than that for pyrolite. The enrichment in Fe/Mg is probably a result of the addition of basic melts to the source. The light REE enrichment may have a mantle origin by the addition of mantle-derived, light REE enriched basic melts or fluids; or a sedimentary origin by the addition of light REE enriched fluids or melts derived from subducted sediments with an early Proterozoic provenance. In either case, the Nd isotopes suggest that the light REE enriched component existed since c. 2300 Ma. The igneous complex may have formed at a convergent margin. An anorthositic gabbro near Mora, has an ϵNd of +5 indicating that it was derived from a mantle source with a history of light REE depletion. This gabbro may have been part of an early Proterozoic ocean crust.  相似文献   

13.
The Bovedy meteorite fell on 25 April 1969 in Northern Ireland; the main mass of 4·94 kg was found at Bovedy (54°57′N, 06°37′W). It is an L3 chondrite with abundant chondrules clearly visible in hand specimen. Bulk chemical analyses are presented, the total Fe content being 22·5%. The olivines are homogeneous (Fa24) but the pyroxenes are not equilibrated (Fs8–28). Brown glass is common within chondrules but a clear glass of composition An85 is present interstitially in a few orthopyroxene-rich (Fs17–28) chondrules. A bleb, 2 mm across, of clear glass, again of composition An85 was found in one stone of the meteorite and in the glass five REE (rare earth elements), (La, Sm, Eu, Yb, Lu) were determined. The low REE abundances coupled with a large positive Eu anomaly are characteristic of plagioclase, but the finer details of the pattern suggest that this glass has a closer affinity to the lunar anorthosites than to plagioclases from lunar mare basalts or eucritic meteorites. There is also evidence that the magnitude of the Eu anomaly for plagioclases and anorthosites from extra-terrestrial sources is inversely related to trivalent REE content. The existence of anorthositic material and, as a consequence, a differentiated planetary body prior to the formation of the Bovedy meteorite is suggested.  相似文献   

14.
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 εNd 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   

15.
Eucrites are extraterrestrial plagioclase-pigeonite basalts. Experimental studies suggest that they were produced by partial melting of an olivine (Fo65)-pigeonite (Wo5En65)-plagioclase (An94)-spinel-metal source region. Quantitative modeling of the evolution of REE abundances in the eucrites indicates that the main group of eucrites (e.g. Juvinas) may be produced by approximately 10% equilibrium partial melting of a source region with initial REE abundances which were chondritic relative and absolute. Other eucrites appear to represent greater (e.g. Sioux County—15%) or smaller (e.g. Stannern—4%) degrees of melting. Moore County and Serra de Magé appear to be cumulates of pyroxene and plagioclase produced by fractional crystallization of a Juvinas-like melt. Nuevo Laredo may represent a residual liquid after such fractional crystallization. Our calculations are consistent with the conclusion that the eucrites were derived from a single type of source region. The close correspondence of the age of the eucrites (? 4.6 AE) to the age of the solar system appears to preclude the possibility of extensive chemical differentiation of the eucrite parent body prior to the event which produced the eucritic melts. Thus our calculations have yielded not only the mode of the source region but, assuming homogeneous accretion, the mode and hence the bulk composition of the eucrite parent body as well. We are unable to estimate quantitatively the ratio of metal to olivine in the parent body. If no metal is present, the bulk composition (in oxide wt%) is Na2O—0.04, MgO—29.7, Al2O3—1.8, SiO2—39.0, CaO—1.2, FeO—28.3. If, in contrast, the parent body contained 30% metal, the bulk composition of the silicate portion of the eucrite parent body is Na2O—0.06, MgO—28.0, Al2O3—2.6, SiO2—41.3, CaO—1.9, FeO—26.3. Relative abundances of the meteorites suggest that the eucrite parent body is still intact. The solar system object most closely resembling the eucrites is asteroid 4 Vesta. Because Vesta is unique among the asteroids, we have license to conclude that it is the source of the eucrites and its bulk composition is close to the analyses given above.  相似文献   

16.
Primary basanitoids from Ross Island, Antarctica have REE patterns and Pb isotope ratios similar to those for primary alkali basalts and nephelinites on ocean islands. The lead data from all volcanics on Ross Island have a spread of 4% in the 206/204 ratio and give a two-stage model lead age of 1500 m.y. The age is interpreted to be the time since the development of the chemical heterogeneity of the mantle source, presumably during an earlier melting process. Comparison of REE, K, Rb, Sr, Ba and P2O5 concentrations for alkali basalts and nephelinites shows that the chondrite normalized mantle source is enriched in light REE with average La/Sm=3.4, Ce/Sm=2.6, Nd/Sm=1.6. Assuming a mantle source with heavy REE abundances of three times chondrites, nephelinites require 3 to 7% partial melting of the mantle source and alkali basalts require 7 to 15% partial melting. The patterns of K, Cu, V and Ti abundances suggest that phlogopite is a residual mineral for most nephelinite, but not alkali basalt mantle sources, and that a sulfide phase and a titanium-rich mineral are in the residual mantle source for both alkali basalts and nephelinites. Small positive Eu anomalies (2–5%) in near primary alkali basalts and nephelinites suggest that the xxx of the mantle sources is 10?6 to 10?9 atm. The progressive enrichment of light REE and incompatible elements in the mantle sources for nephelinites and alkali basalts is proposed to result by intrusion of veins of basaltic melt due to very low percentages of melting 1 000 to 3 000 m.y. ago when this part of the deeper mantle was previously involved in convection and partial melting.  相似文献   

17.
Major, trace and rare earth element (REE) compositions of upper Proterozoic metavolcanic and metasedimentary rocks from the Tsaliet and Tembien Groups in the Werri district of northern Ethiopia were determined to examine their tectonic setting of eruption, provenance and source area weathering conditions. Tsaliet Group metavolcanic rocks in the Werri area have sub-alkaline chemistry characterized by low to intermediate SiO2 contents, high Al2O3, low MgO and very low Cr and Ni. High field strength element (HFSE) abundances are highly variable. ∑REE abundances vary from 66.7 to 161.3 ppm, and chondrite-normalized REE patterns are moderately fractionated, with LaN/YbN values of between 3.1 and 9.0. Europium anomalies are variable (Eu/Eu* 0.80–1.21) but are generally positive (average Eu/Eu* 1.06). On tectonic discrimination diagrams, most samples have either volcanic-arc chemistry or fall in the overlap field with mid-oceanic ridge basalt (MORB). However, primitive mantle-normalized trace element abundances are comparable with sub-alkaline basalts from developed island arcs. 147Sm/144Nd ratios range from 0.1167 to 0.1269 (n = 3), yielding initial εNd(800 Ma) of +3.8 to +4.9 and mean TDM model age of 0.96 Ga, indicative of derivation from juvenile Neoproterozoic mantle. Metasediments from three locations (Werri1, Werri2 and Tsedia) in the Werri and Tsedia Slates have similar Al2O3, TiO2 and HFSE contents but variable and low Na2O, CaO and K2O. Cr and Ni are slightly enriched in the Werri2 and Tsedia suites. SiO2 is very variable, with average values of 70.75, 72.2 and 66.4 wt.% in the Werri1, Werri2 and Tsedia suites, respectively. ∑REE abundances in the metasediments (14.74–108.1) are lower than in the metavolcanics, and are slightly less fractionated, with LaN/YbN ratios of 0.8–5.9. Europium anomalies vary (Eu/Eu* 0.80–1.21) but are insignificant on average (Eu/Eu* 0.96). High values for the Chemical Index of Alteration (generally 70–90), and Plagioclase Index of Alteration (>75) in the Werri metasediments indicate moderate to severe chemical weathering in their source. Average major and trace element compositions of the metasediments and their REE patterns are comparable with the metavolcanics. 147Sm/144Nd ratios of the metasediments range from 0.1056 to 0.1398 (n = 4), with initial εNd(800 Ma) of +3.4 to +5.0 and mean TDM model age of 0.97 Ga, indicating derivation from juvenile Neoproterozoic crust similar to the underlying metavolcanics, with minimal (4–10%) contribution from older crust. The most sensitive tectonic setting discriminators indicate the Werri metasediments represent developed oceanic island arc sediments. The chemical similarity of the Werri metavolcanics to the nearby Adwa metavolcanics, Nakfa terrane in Eritrea, and volcanic units in central Saudi Arabia imply that juvenile Neoproterozoic Arabian Nubian Shield crust extended south at least as far as the Werri area of northern Ethiopia. The comparable geochemistry of the metasediments and their underlying lithologies attests to their derivation from this juvenile crustal material.  相似文献   

18.
The lunar meteorite Northwest Africa (NWA) 032 is a low-Ti basalt that has incompatible-element abundances and Th/Sm ratios characteristic of the involvement of late stage magma ocean crystallization products (urKREEP) in its petrogenesis. This sample is very fine-grained and contains terrestrial weather products. A progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain Rb-Sr and Sm-Nd ages. Although many of the leachates, as well as the unleached mineral and whole rock fractions contain terrestrial alteration products, selected residue fractions yield concordant Rb-Sr and Sm-Nd ages. Rubidium-Sr isotopic analyses yield an age of 2947 ± 16 Ma with an initial 87Sr/86Sr of 0.700057 ± 17. These characteristics indicate NWA 032 is derived from a source region with an 87Rb/86Sr ratio of 0.044 ± 0.001. This value is higher than all but those determined for KREEP basalts, and suggests that NWA 032 is derived from a source region that has higher incompatible-element abundances than other low-Ti basalts. Samarium-neodymium isotopic analysis yield a concordant age of 2931 ± 92 Ma and an initial εNd of +9.71 ± 0.74 corresponding to a source region with 147Sm/144Nd ratio of 0.246 ± 0.004. The initial Nd isotopic composition stands in contrast to the initial Sr isotopic composition by requiring NWA 032 to be derived from a source with lower incompatible-element abundances than most low-Ti basalts. The source of NWA 032 is therefore unlike those of other lunar basalts.Modeling of magma ocean cumulate formation demonstrates that unlike other low-Ti basalt source regions the NWA 032 source is a mixture of olivine, pigeonite, and clinopyroxene bearing cumulates and only a small amount of urKREEP. Furthermore, unlike other mare basalt sources, the NWA 032 source does not contain appreciable quantities of plagioclase. Partial melting models demonstrate that the incompatible-element characteristics of the NWA 032 result from formation by smaller degrees of partial melting than other mare basalts. Thus, the incompatible-element geochemical signature that is observed in NWA 032 appears to reflect the combined effects of generation from an unusual plagioclase-free incompatible-element-depleted source region by very small degrees of partial melting. This study demonstrates that both the presence of urKREEP in the source region and small degrees of partial melting generate magmas with similar, but not identical, incompatible-element characteristics. In addition, it underscores the fact that there is significantly more geochemical diversity on the Moon than is represented by samples collected by the American and Soviet lunar missions.  相似文献   

19.
Northeast Africa 003 (NEA 003) is a lunar meteorite found as a two paired stones (6 and 118 g) in Libya, 2000 and 2001. The main portion (∼75 vol%) of the 118 g meteorite, used for this study, (NEA 003-A) consists of mare-basalt and a smaller adjacent portion (∼25 vol%) is a basaltic breccia (NEA 003-B). NEA 003-A has a coarse-grained magmatic texture consisting mainly of olivine, pyroxene and plagioclase. The late-stage mineral association is composed mainly of elongated plagioclase, ilmenite, troilite, fayalite, Si-K-rich glass, apatite, and a rare SiO2 phase. Other accessory minerals include ulvöspinel, chromite, and trace Fe-Ni metal. Olivine and pyroxene contain shock-induced fractures, and plagioclase is completely converted into maskelynite.The Fe/Mn values of the whole rock, olivines and pyroxenes, and the bulk-rock oxygen isotopic composition provide evidence for the lunar origin of NEA 003-A meteorite. This is further supported by the presence of Fe-Ni metal and the anhydrous mineral association.NEA 003-A is geochemically and petrographically distinct from previously described mare-basalt meteorites and is not paired with any of them. The petrography and major element composition of NEA 003-A is similar to the composition of low-Ti olivine mare basalts from Apollo 12 and olivine-normative basalts from Apollo 15. The NEA 003-A meteorite shows obvious geochemical similarities in trace elements contents with Apollo 15 olivine-normative basalts and could represent a yet unknown geochemically primitive member of the olivine-normative basalt series. The meteorite is depleted in rare earth elements (REE) and incompatible trace elements indicating a primitive character of the parental magma. The bulk-rock chemical composition demonstrates that the parent melt of NEA 003-A was not contaminated with KREEP components as a result of magma mixing or assimilation processes. Results of crystallization modelling and low minimum cooling rate estimates (∼0.07 °C/h) suggest that the parent melt of NEA 003-A crystallized in the lower part of a lava flow containing cumulate olivine (∼10%) and was probably derived from more primitive picritic magma by fractional crystallization processes.Sm-Nd dating yields an age of 3.09 ± 0.06 Ga which corresponds to the period of lower Eratosthenian lunar volcanic activity, and the near-chondritic εNd value of −0.4 ± 0.3 indicates that the meteorite could be derived from a slightly enriched mantle source similar to the Apollo 15 green glasses. Ar-Ar step release results are inconsistent with Sm-Nd ages suggesting that NEA 003-A was exposed to one or more impact events. The most extensive event took place at 1.8 Ga and the shock intensity was likely between 28 and 45 GPa. The absence of solar Ar suggests that NEA 003-A has not been directly exposed at the lunar surface but the cosmic ray exposure age of 209 ± 6 Ma suggests that NEA 003-A resided in the upper regolith for part of its history.  相似文献   

20.
鞍山地区太古代岩石同位素地质年代学研究   总被引：23，自引：4，他引：23       下载免费PDF全文
乔广生 《地质科学》1990,(2):158-165
鞍山本溪地区太古代变质岩可分为三套,即含铁的表壳岩建造、侵入于铁建造中的花岗质片麻岩和铁架山奥长花岗质-花岗质片麻岩,后者为表壳岩的基底。原划为上鞍山群樱桃园组(齐大山矿带)和山城子组(歪头山-北台矿带)的斜长角闪岩分别获得2729Ma和2724Ma的Sm-Nd等时线年龄。这就为有争议的鞍本地区铁建造属于同一时代提供了依据,并讨论了表壳岩中的变质沉积岩以及铁架山基底片麻岩的同位素年代。  相似文献   

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