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1.
The results of our combined U-Pb, Rb-Sr, and Sm-Nd isotope study of mare basalt 10017 contribute to the understanding of the petrogenetic processes involved in the origin of geochemical diversity in lunar mare basalt sources, as well as the U-Pb isotope systematics of the Moon. The Rb-Sr, Sm-Nd, and 238U-206Pb isotope systems yield concordant crystallization ages of 3.633 ± 0.057 Ga, 3.678 ± 0.069 Ga, and 3.616 ± 0.098 Ga, respectively. The 235U-207Pb isochron yields an older, though still concordant, age of 3.80 ± 0.12 Ga. Neither the 206Pb-207Pb system nor U-Pb concordia system yields an age for 10017 that is concordant with the age determined from the Sm-Nd, Rb-Sr, and 238U-206Pb systems. The initial 87Sr/86Sr of 10017 is 0.69941 ± 7 and the initial εNd is +3.2 ± 0.4. Initial Pb isotopic compositions, determined from the U-Pb isochrons, are 206Pb/204Pbi = 31 ± 11 and 207Pb/204Pbi = 34 ± 15. Together, these initial Pb compositions constrain the μ value of the 10017 source to be 70 ± 30, assuming a single-stage Pb growth model. This is considerably lower than μ values typically estimated for mare basalt sources (∼100-600). Regardless, the μ values calculated for the sources of mare basalts, as well as other lunar samples, show a range that is larger than can be explained by fractionation of U from Pb solely by crystallization of silicate phases and ilmenite during magma ocean solidification and formation of lunar mantle sources. The U-Pb isotope systematics may reflect late-stage formation of a sulfide phase, which strongly fractionates Pb from U but has minimal effect on Rb/Sr or Sm/Nd compositions, during crystallization of the lunar magma ocean.  相似文献   

2.
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 εNd 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 εNd 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   

3.
This work provides unequivocal evidence of the existence of Mesoarchean granulite facies metamorphic event in the Palghat-Cauvery Shear Zone (PCSZ) of South India. Charnockite samples from two prominent hills at Kollaimalai (KM) and Pachchaimalai (PM) as well as from two quarries within the Bhavani Shear Zone (BSZ) have been analyzed for their Sm-Nd and Rb-Sr ages to investigate the existence or otherwise of the Archean granulite facies events within the PCSZ. The Rb-Sr whole-rock isochron ages for massive charnockites from both the hills appear to be contemporaneous at 2.9 Ga with the initial Sr isotopic ratios of 0.7012 and 0.7014, respectively. However, the Rb-Sr data for whole-rock samples of basic granulites from one of the quarries within the BSZ indicate open system behavior, while the charnockites from the other quarry have insufficient spread in 87Rb/86Sr ratios and do not yield any isochron. The Sm-Nd data, on the other hand, do not distinguish between the massive charnockite and the lowland charnockite and yield Depleted Mantle model ages in the range 2.98±0.3 Ga for all of them. The ɛT CHUR for all of these rocks are highly positive. Both the Sr isotopic ratios and positive ɛT CHUR values for these rocks strongly suggest a mantle source for all of them. An upper age limit of ∼3.28 Ga may be assigned to the crustal accretion of the protolith of all these rocks on the basis of their Nd model ages. The Rb-Sr isochron ages of 2.9 Ga for the two massifs could be the age of granulite facies metamorphism. Thus, the metamorphism in the KM and PM Hills took place within ∼100 Ma of crustal accretion of these rocks and probably was part of the same geological event of crust formation and metamorphism. The open system behavior with respect to Rb-Sr isotopes in the basic granulite from Bhavani is possibly due to the migration of Sr isotopes, triggered during the later shearing of these rocks.  相似文献   

4.
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   

5.
Summary Rb-Sr and Sm-Nd isotopic investigations have been carried out on samples forming a sequence from the bottom to the top of a suite of K-rich olivine trachybasalts from the Manning Massif in the northern Prince Charles Mountains, East Antarctica. Several separate flows, each 3.5-6.5 m thick, occur as individual small outcrops and are related to the Lambert-Amery rift. Nd-Sr isotopic features of the volcanics are similar to those for lherzolite xenoliths from the Mesozoic alkaline ultramafic rocks of the adjacent Jetty Peninsula area, but the trace element abundances and patterns are consistent with the occurrence of minor phases such as apatite, amphibole, ilmenite and perovskite in the source region. According to Nd-Sr isotopic characteristics (143Nd/144Nd = 0.512026-0.512269;87Sr/86Sr = 0.707765-0.708046), the trachybasalts were generated from an enriched mantle source. These features are suggested here to result from isotopic mixing between peridotite wall-rocks similar to the Jetty Peninsula xenolith samples and melts derived from ultramafic veins in the lithosphere. A synthesis from isochron and model ages on volcanics and xenoliths indicates formation of the lithosphere at about 1.2 Ga, followed by two episodes of subsequent enrichment at 0.91.0 Ga and ca. 0.6 Ga. The143Nd/144Nd values for the volcanics show a wide range, but are more enriched for the bottom part of the suite, becoming more depleted towards the top. This may be explained by a steadily decreasing vein/wall-rock ratio from 35% to 10% due to progressive heating of the source region. This variation in ENd is not accompanied by a corresponding increase in87Sr/86Sr, which constrains both vein and wall-rock characteristics quite closely: the wall-rock was an enriched peridotite, whereas the vein assemblage comprised dominantly clinopyroxene and amphibole with a lesser, but essential, amount of phlogopite. REE-rich accessory phases such as apatite and perovskite were present in the vein, but were rare, so that the vein as a whole had high Sr/Nd. Rb-Sr age determination on Manning Massif bulk rocks gives an apparent isochron age of 40±1.2 Ma (ISr = 0.70762), although the age is probably falsified by the mixing processes during melting.
Sm-Nd und Rb-Sr Isotopenuntersuchungen an alkalischen Trachybasalten des ostantarktischen Manning Massivs: Entwicklung des Mantels unter dem Lambert-Amery Rift
Zusammenfassung Sm-Nd- und Rb-Sr-Isotope wurden in einer Abfolge von K-reichen OlivinTrachybasalten im Manning Massiv in den nördlichen Prince Charles Mountains, Ost-Antarktis, untersucht. Proben wurden systematisch von mehreren sich überlagernden Lavaströmen genommen, die Mächtigkeiten von 3,5 bis 6,5 m erreichen und dem Lambert-Amery Rift zugeordnet werden. Nd-Sr Isotopenzusammensetzung der Vulkanite sind ähnlich wie in Iherzolithischen Mantelxenolithen aus alkalischen Ultramafiten der Jetty Halbinsel. Allerdings deuten die Spurenelementgehalte sowie die Spurenelementverteilungen auf die Anwesenheit von Apatit, Amphibol, Ilmenit und Perowskit in der Quellenregionhin. Nd-Sr Isotope der Trachybasalte (143Nd/144Nd= 0.512026–0.512269;87Sr/86Sr = 0.707765–0.708046) weisen auf eine angereicherte Mantelzusammensetzung hin. Für die hier gefundenen Werte wird ein Mischungsmodell zwischen einem Peridotit-Nebengestein ähnlich wie in den Jetty Halbinsel Xenolithen und einer Schmelze vorgeschlagen, die aus ultramafischen Gängen innerhalb der Lithosphäre stammte. Die Kombination aus Isochronenalter und Modellalter der Vulkanite und der Xenolithe zeigt, daß nach der Bildung vor 1.2 Ga die Lithosphäre zwei Anreicherungsereignisse vor 0.9-1.0 Ga und 0.6 Ga erfahren hatte. Trotz der hohen Variation der143Nd/144Nd Verhältnisse ergeben sich für die unteren Lavaströme höhere Werte, während die oberen Bereiche an143Nd verarmt sind. Dies kann auf eine stetige Abnahme des Gang/Peridotit Verhältnisses von 35% auf 10% zurückgeführt werden, das auf eine fortschreitende Aufheizung der Quellenregion hinweist. Die Variation im143Nd/144Nd System wird aber nicht von einem Anstieg des87Sr/86Sr Verhältnisses begleitet, was mit ähnlichen Werten sowohl für die Gänge als auch für den angrenzenden Peridotit erklärt werden kann: das Nebengestein bestand aus einem angereicherten Peridotit, während die Gänge hauptsächlich aus Klinopyroxen und Amphibol zusammengesetzt waren. SEE-reiche Phasen wie Apatit und Perowskit kamen in so geringen Mengen vor, daß die Gesamtzusammensetzung der Gänge hohe Sr/Nd-Verhältnisse hatte. Rb-Sr Altersbestimmungen an Gesamtgesteinen der Manning Massiv-Vulkanite ergeben eine scheinbare Isochrone von 40±1.2 Ma (ISr = 0.70762), obwohl diese Altersinformation vermutlich durch die Mischungsprozesse während des partiellen Schmelzens verfälscht ist.
  相似文献   

6.
Trachytes and rhyolites from Salsette Island, north of Bombay, have distinctive trace element and isotope features which mark them out from typical crustal melts. Their highly incompatible trace element and Sr-, Nd and Pb isotope ratios are similar to those of the associated Deccan flood basalts. Thus the rhyolites and trachytes are closely related to the basalts, and a striking compositional gap between 50 and 65% SiO2 suggests that the high SiO2 rocks evolved by 10–15% partial melting followed by variable amounts of fractional crystallisation. The source material could have been basalt within the Deccan Trap, or related gabbroic rocks in deep crustal sill complexes. The rhyolites yield an Rb-Sr whole rock age of 61.5±1.9 Ma, with a slightly high initial 87Sr/86Sr=0.7085±18. It is argued that crustal extension provides a suitable regime for the generation of acid magmas by partial melting of associated basic rocks.  相似文献   

7.
Advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes. Incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the Rb-Sr and Sm-Nd isotope systems in anatectic melts from a muscovite-bearing metasedimentary source. Modeling results show that (1) the Sm/Nd ratios and Nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt, and (2) the relative proportion of micas (muscovite and biotite) and feldspars (plagioclase and K-feldspar) that enter the melt is a key parameter determining the Rb/Sr and 87Sr/86Sr ratios of the melt. Furthermore, these two factors are not, in practice, independent. In general, nonmodal partial melting of a pelitic source results in melts following one of two paths in εNd-87Sr/86Sr ratio space. A higher temperature, fluid-absent path (Path 1) represents those partial melting reactions in which muscovite/biotite dehydration and apatite but not monazite dissolution play a significant role; the melt will have elevated Rb/Sr, 87Sr/86Sr, Sm/Nd, and εNd values. In contrast, a lower temperature, fluid-fluxed path (Path 2) represents those partial melting reactions in which muscovite/biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue; the melt will have lower Rb/Sr, 87Sr/86Sr, Sm/Nd, and εNd values than its source. The master variables controlling both accessory phase dissolution (and hence the Sm-Nd system), and melting reaction (and hence the Rb-Sr systematics) are temperature and water content. The complexity in Sr-Nd isotope systematics in metasediment-derived melts, as suggested in this study, will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component.  相似文献   

8.
We have carried out a comparative Rb-Sr, Sm-Nd and Lu-Hf isotopic study of a progressively deformed hercynian leucogranite from the French Massif Central, belonging to the La Marche ductile shear zone, in order to investigate the respective perturbation of these geochronometers with fluid induced deformation. The one-meter wide outcrop presents a strongly deformed and mylonitized zone at the center, and an asymmetric deformation pattern with a higher deformation gradient on the northern side of the zone. Ten samples have been carefully collected every 10 cm North and South away from the strongest deformed mylonitic zone. They have been analyzed for a complete major, trace element data set, oxygen isotopes, Rb-Sr, Sm-Nd and Lu-Hf isotopic systematics.We show that most of major and trace elements except SiO2, alkaline elements (K2O, Rb), and some metal transition elements (Cu), are progressively depleted with increasing deformation. This depletion includes REE + Y, but also HFS elements (Ti, Hf, Zr, Nb) which are commonly considered as immobile elements during upper level processes. Variations in elemental ratios with deformation, e.g. decrease in LREE/MREE- HREE, Nd/Hf, Th/Sr, increase in Rb/Sr, U/Th and constant Sr/Nd, lead to propose the following order of element mobility: U ? Th > Sr = Nd ? Hf + HREE. We conclude in agreement with previous tectonic and metallogenic studies that trace element patterns across the shear zone result from circulation of oxidizing F-rich hydrothermal fluids associated with deformation. A temperature of the fluid of 470-480 °C can be deduced from the δ18O equilibrium between quartz-muscovite pairs.Elemental fractionation induces perturbation of the Rb-Sr geochronometer. The well-defined 87Rb/86Sr-87Sr/86Sr correlation gives an apparent age of 294 ± 19 Ma, slightly younger than the 323 ± 4 Ma age of leucogranites in this area. This apparent age is interpreted as dating event of intense deformation and fluid circulation associated with mass transfer, and exhumation of the ductile crust shortly after the leucogranite emplacement. Sm-Nd and Lu-Hf isochron-type diagrams do not define any correlation, because of the low fractionated Sm/Nd and Lu/Hf ratios. Isotopic data demonstrate that only the Lu-Hf geochronometer system is not affected by fluid circulation and gives reliable TDM age (1.29 ± 0.03 Ga) and εHf signatures. By contrast, the Sm-Nd geochronometer system gives erroneous old TDM ages of 2.84-4 Ga. There is no positive εNd-εHf correlation, because of decreasing εNd values with deformation at constant εHf values. However, εNd-εHf values remain in the broad εNd-εHf terrestrial array, which strongly indicates that fluid-induced fractionation can contribute to the width of the terrestrial array. The strong εHf negative values of the leucogranite are similar to metasedimentary granulitic xenoliths from the French Massif Central and confirm the generation of the leucogranite by several episodes of reworking of the lower crust.  相似文献   

9.
Sm-Nd and Rb-Sr whole-rock isochron dating of the samples of cumulated gabbro, diabase and andesitic porphyrite from the Armantai ophiolite, Xinjiang, yielded the ages of 561 ± 41 Ma (εNd(t) = 6.1) and 392 ± 17 Ma [(87Sr/86Sr)i = 0.7041], which stand for the ages of creation and later metamorphism of the ophiolites, respectively. The creation environment of the Armantai ophiolites seems to a back-arc basin caused by the southward subduction of the palaeo-Asia Ocean. This project was specially granted by the State under the Key Scientific and Technological Program for the “Eight Five-Year Plan” period.  相似文献   

10.
The Dargawan gabbros intrusive into the Moli Subgroup of Bijawar Group, yielded Rb-Sr whole rock isochron age of 1967 ± 140 Ma. Based on the oldest age from overlying Lower Vindhyan (1.6Ga) and the underlying youngest basement ages (2.2 Ga), the time range of Bijawar sedimentation may be assigned as 2.1–1.6 Ga (Paleoproterozoic). Sm-Nd Model ages (TDM), obtained, for Dargawan gabbros, is c. 2876–3145 Ma. High initial 87Sr/ 86Sr ratio of 0.70451 (higher than the contemporary mantle) and negative ɛNdi (at 1.9 Ga) value of −1.5 to − 4.5, indicate assimilation of Archaean lower crustal component by the enriched mantle source magma at the time of gabbroic intrusion. The dolerite, from Damdama area, which is intrusive into the basement and overlying sediments of Chandrapur Group in the central Indian craton, yielded Rb-Sr internal isochron age of 1641 ± 120 Ma. The high initial 87Sr/86Sr ratio of 0.7098 and ɛNdi value of −3.5 to −3.7 (at 1.6 Ga) is due to contamination of the mantle source magma with the overlying sediments. These dolerites have younger Sm-Nd Model ages (TDM) than Dargawan gabbros as c. 2462–2675 Ma, which is similar to the age of the Sambalpur granite, from which probably sediments to this part of Chattisgarh basin are derived. Hence mixing of sediments with the Damdama dyke during its emplacement, gives rise to high initial 87Sr/86Sr and low initial 143Nd/144 ratios for these dykes. The c. 1600 Ma age indicates minimum age of onset of the sedimentation in the Chandrapur Group of Chattisgarh basin. Both the above mafic intrusions might have taken place in an intracratonic rift related (anorogenic) tectonic setting. This study is the first reliable age report on the onset of sedimentation in the Chandrapur Group. The total minimum time span of Chandrapur and Raipur Group may be 1.6 Ga to 1.0 Ga (Mesoproterozoic). The unconformably underlying Shingora Group of rocks of Chhattisgarh Supergroup thus indicates Paleoproterozoic age (older than 1.6 Ga). Most part of the recently classified Chattisgarh Supergroup and Bijawar-Vindhyan sequence are of Mesoproterozoic-Paleoproterozoic age and not of Neoproterozoic-Mesoproterozoic age as considered earlier. Petrographic study of basic dykes from Damdama area (eastern margin of Chattisgarh Supergroup) indicated presence of primary uranium mineral brannerite associated with goethite. This is the evidence of mafic intrusive providing geotherm and helping in scavenging the uranium from the surrounding and later alterations causing remobilisation and reconcentration of pre-existing uranium in host rocks as well as in mafic dyke itself otherwise mafic rocks are poor source of uranium and can not have primary uranium minerals initially. It can be concluded that mafic dykes have role in uranium mineralisation although indirectly.  相似文献   

11.
Rubidium-strontium and samarium-neodymium isotopes of lunar meteorite LaPaz Icefield (LAP) 02205 are consistent with derivation of the parent magma from a source region similar to that which produced the Apollo 12 low-Ti olivine basalts followed by mixing of the magma with small amounts (1-2 wt%) of trace element-enriched material similar to lunar KREEP-rich sample SaU 169. The crystallization age of LAP 02205 is most precisely dated by an internal Rb-Sr isochron of 2991 ± 14 Ma, with an initial 87Sr/88Sr at the time of crystallization of 0.699836 ± 0.000010. Leachable REE-rich phosphate phases of LAP 02205 do not plot on a Sm-Nd mineral isochron, indicating contamination or open system behavior of the phosphates. Excluding anomalous phases from the calculation of a Sm-Nd isochron yields a crystallization age of 2992 ± 85 (initial ε143Nd = +2.9 ± 0.8) that is within error of the Rb-Sr age, and in agreement with other independent age determinations for LAP 02205 from Ar-Ar and U-Pb methods. The calculated 147Sm/144Nd source ratios for LAP 02205, various Apollo 12 and 15 basalts, and samples with strong affinities to KREEP (SaU 169, NWA 773, 15386) are uncorrelated with their crystallization ages. This finding does not support the involvement of a common KREEP component as a heat source for lunar melting events that occurred after crystallization of the lunar magma ocean.  相似文献   

12.
The Nd and Sr isotopic ratios on a suite of continental alkali basalts from Marie Byrd Land, West Antarctica, define a change in the source over the range of K/Ar dates between 1 and 28 m.y. ago. The 87Sr/86Sr isotopic ratios (0.7026 to 0.7031) are unusually low for continental alkali basalts, although the corresponding 143Nd/144Nd ratios (0.51283 to 0.51299) are similar to previously reported values. On a 87Sr/86Sr vs. 143Nd/144Nd diagram, they define a trend on the low 87Sr/86Sr side of the “mantle array”, which has a slope steeper than the mantle array. An explanation for the light rare earth elements (LREE) enrichment of the alkali basalts, with high 143Nd/144Nd ratios and low 87Sr/86Sr ratios, is suggested by a model which modifies the source region with a mantle-derived, CO2-enriched metasomatic fluid.  相似文献   

13.
The Ethiopian continental flood basalt (CFB) province (∼30 Ma, > 3 × 105 km3) was formed as the result of the impingement of the Afar mantle plume beneath the Ethiopian lithosphere. This province includes major sequences of rhyolitic ignimbrites generally found on top of the flood basalt sequence. Their volume is estimated to be at least 6 × 104km3, which represents 20% of that of the trap basalts. Their phenocryst assemblage (alkali feldspar, quartz, aegyrine-augite, ilmenite ± Ti-magnetite, richterite, and eckermanite) suggests temperatures in the range of 740 to 900°C. Four units were recognized in the field (Wegel Tena, Jima, Lima Limo, and Debre Birhan areas), each with its own geochemical specificity. Zr/Nb ratios remain constant between basalt and rhyolite in each area, and rhyolites associated with high-Ti or low-Ti basalts are, respectively, enriched or depleted in titanium. Their trace element and isotope (Sr, Nd, O) signatures (high 143Nd/144Nd and low 87Sr/86Sr ratios, compared to those of rhyolites from other CFB provinces) are clearly different from those of typical crustal melts and indicate that the Ethiopian rhyolites are among the most isotopically primitive rhyolites. Their major and trace element patterns suggest that they are likely to be derived from fractional crystallization of basaltic magmas similar in composition to the exposed flood basalts with only limited crustal contribution. Since Ethiopian high-Ti basalts have been shown to form from melting of a mantle plume, it is likely that Ethiopian ignimbrites, at least those that are Ti-rich, also incorporated material from the deep mantle.Rb-Sr isochrons on whole rocks and mineral separates (30.1 ± 0.4 Ma for Wegel Tena and 30.5 ± 0.4 Ma for Jima ignimbrites) show that most of the silicic volcanism occurred within < 2 Ma during the Oligocene. Ignimbritic eruptions resumed in the Miocene during two episodes dated at 15.4 ± 0.2 Ma and 8.0 ± 0.2 Ma for the Debre Birhan area. The Rb-Sr isochron ages of ignimbrites (both Oligocene and Miocene rhyolites) are indistinguishable within uncertainties from the 40Ar/39Ar ages of the underlying flood basalts. The Oligocene ignimbrites and the underlying trap basalts are synchronous with a shift in the oxygen composition of foraminifera recorded in Indian and Atlantic Ocean cores. The temporal coincidence of Ethiopian Oligocene volcanism, which released immense volumes of S (> 1.4 × 1015 mol) and Cl (6.4 × 1015 mol) into the atmosphere over a short time span, with the global cooling event at 30.3 Ma suggests that this volcanism might have accelerated the climate change that was already underway.  相似文献   

14.
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   

15.
Two major episodes are evident in the metamorphic and igneous Precambrian basement of the Llano Uplift, central Texas. Dynamothermal metamorphism was accompanied by minor basaltic and tonalitic syntectonic plutonism. This was followed by a second period of thermal overprinting accompanying emplacement of high-K2O, high-level major granite plutons. Extensive isotopic age work by Zartman, published in the mid-1960s, suggests that development of the basement complex, spanning an interval of 150 m.y. or more, began with deposition of Valley Spring Gneiss (the lowest unit) and terminated about 1,050 m.y. ago with final postmetamorphic cooling (indicated by retention ages of Ar and Sr in biotite). We have supplemented these data with more than 50 new K-Ar and Rb-Sr analyses.Two foliated plutons in the southeast are 1,167±12m.y. (2) old, with distinctly different initial 87Sr/86Sr ratios. Field relationships and isotopic data indicate that these plutons are the earliest yet known in the Uplift. Metamorphosed basalt dikes and gabbro bodies were emplaced immediately preceding and following the syntectonic plutons. Eleven of these rocks had extremely uniform initial 87Sr/ 86Sr=0.7029±0.0005. A Rb-Sr whole-rock isochron of the unfoliated Enchanted Rock pluton indicates an age of 1,048±34 m.y. with initial 87Sr/86Sr= 0.7048±0.0007. One of the northern unfoliated granites, the Lone Grove pluton, gives a whole-rock isochron age of 1,056±12 m.y., with initial 87Sr/86Sr = 0.7061±0.0003. All of the intrusive rocks have initial 87Sr/86Sr ratios consistent with a source in the mantle or lower crust, but not in ancient remobilized continental crust. Six K-Ar hornblende ages from metabasalts are 1,078±19 m.y. (1), in general agreement with K-Ar and Rb-Sr mineral ages elsewhere in the eastern Llano Uplift. A metasedimentary Valley Spring Gneiss sample from the western Uplift has a whole rock-muscovite Rb-Sr age of 1,129±9 m.y. Field and isotopic data are now sufficiently numerous to permit a moderately detailed reconstruction of the Precambrian history of the area.  相似文献   

16.
The northern Noorabad area in western Iran contains several gabbro and basalt bodies which were emplaced along the Zagros suture zone. The basalts show pillow and flow structures with amygdaloidal textures, and the gabbroic rocks show massive and foliated structures with coarse to fine-grained textures. The SiO2 contents of the gabbros and basalts are similar and range from 46.1–51.0 wt.%, and the Al2O3 contents vary from 12.3–18.8 wt.%, with TiO2 contents of 0.4–3.0 wt.%. The Nb concentrations of some gabbros and basalts are high and can be classified as Nb-enriched arc basalts. The positive εNd(t) values (+3.7 to +9.8) and low 87Sr/86Sr(initial) ratios (0.7031–0.7071) of both bodies strongly indicate a depleted mantle source and indicate that the rocks were formed by partial melting of a depleted lithospheric mantle and interaction with slab fluids/melts. The chemical composition of trace elements, REE pattern and initial 87Sr/86Sr-143Nd/144Nd ratios show that the rocks have affinities to tholeiitic magmatic series and suggest an extensional tectonic regime over the subduction zone for the evolution of these rocks. We propose an extensional tectonic regime due to the upwelling of metasomatized mantle after the late Cretaceous collision in the Harsin-Noorabad area. These rocks can be also considered as Eocene back arc magmatic activity along the Zagros suture zone in this area.  相似文献   

17.
Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the 87Sr/86Sr ratio between the basalts and their chilled glasses and less significant difference in ?Nd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear.  相似文献   

18.
The Costelloe Murvey Granite is a chemically evolved, high heat production, leucocratic component of the 400 Ma old Galway Granite batholith and is host to hydrothermal fluorite-quartz-calcite veins. A previously reported clinopyroxene 40Ar-39Ar age of 231±4 Ma obtained from a pre-mineralization dolerite dyke is reinterpreted as dating this mineralization. The hydrothermal fluid extensively altered its granite wallrocks, leading to lower Sm and Nd and higher Rb concentrations in altered granite, disturbing both its Rb-Sr and Sm-Nd isotopic systems. The 87Sr/86Sr ratio of the hydrothermal fluid from which fluorite and calcite precipitated ranged from 0.7101 to 0.7139. These ratios are very much lower than in the Costelloe Murvey Granite at the time of mineralization, precluding the granite as a source for more than 2% of the hydrothermal Sr. The initial 143Nd/144Nd ratio varies between fluorite in different veins due to Nd derivation from local wallrocks, and between fluorite of petrographically distinct growth phases within a single hand specimen, highlighting the difficulty of Sm-Nd isochron dating of fluorite in cases where there are multiple sources of hydrothermal Nd. It is proposed that fluorite and calcite precipitated where hot, dilute fluids rising through the granite mixed with cooler, more saline fluids of basinal origin migrating through Lower Carboniferous limestone which then overlay the granite. Received: 3 August 1995 / Accepted: 11 April 1996  相似文献   

19.
Strontium and neodymium isotopes, measured on diverse mud and sand fractions of sediment in transit along all major Nile branches, identify detritus sourced from Precambrian basements, Mesozoic strata, and Tertiary volcanic rocks exposed along the shoulders of the East African rift and in Ethiopian highlands. Sr and Nd isotopic ratios reflect the weighted average of detrital components generated in different catchments, allowing us to discriminate provenance, calculate sediment budgets, and investigate grain-size and hydraulic-sorting effects.87Sr/86Sr and 143Nd/144Nd range, respectively, from as high as 0.722 and as low as 0.5108 for sediment derived from Archean gneisses in northern Uganda, to 0.705 and 0.5127 for sediment derived from Neoproterozoic Ethiopian and Eritrean basements. 87Sr/86Sr and 143Nd/144Nd, ranging 0.705-0.709 and 0.5124-0.5130 for Blue Nile tributaries, are 0.704-0.705 and 0.5127-0.5128 for largely volcaniclastic sediments of River Tekeze-Atbara, and 0.705-0.706 and 0.5126-0.5127 for main Nile sediments upstream Lake Nasser.Model mantle derivation ages (tDM), oldest in Uganda where sediment is principally derived from the Congo Craton (3.4-3.0 Ga for Victoria and Albert Nile), progressively decrease northward across the Saharan Metacraton, from 2.6 Ga (Bahr el Jebel in South Sudan), to 2.4-2.2 Ga (Bahr ez Zeraf across the Sudd), and finally 1.6-1.3 Ga (White Nile upstream Khartoum). Instead, tDM ages of Sobat mud increase from 0.9 to 1.5 Ga across the Machar marshes. TDM ages are younger for sediments shed by Ethiopian (1.2-0.7 Ga) and Eritrean basements (1.5-1.2 Ga), and youngest for sediments shed from Ethiopian flood basalts (0.3-0.2 Ga).Integrated geochemical, mineralogical, and settling-equivalence analyses suggest influence on the Nd isotopic signal by volcanic lithic grains and titanite rather than by LREE-rich monazite or allanite. Because contributions by ultradense minerals is subordinate, intrasample variability of Sr and Nd ratios is minor. In Blue Nile, Atbara and main Nile sediments of mixed provenance, however, the Nd ratio tends to be higher and tDM ages lower in largely volcaniclastic mud than in mixed volcaniclastic/metamorphiclastic sand.The complete geochemical database presented here, coupled with high-resolution bulk-petrography and heavy-mineral data, provides a key to reconstructing erosion patterns and detrital fluxes across the whole Nile basin, and to investigate and understand how sources of sediment have changed in the historical and pre-historical past in relation to shifting climatic zones across arid northern Africa.  相似文献   

20.
《Gondwana Research》2000,3(1):91-104
The mafic granulites of Schirmacher region, East Antarctica, the rocks under study, occur more or less as concordant sills or as lenses or as boudinage structures within the felsic rocks, charnockites or metapelites of the region. They show variation from garnet bearing two-pyroxene granulites and garnet free pyroxene granulites to transitional amphibolite-pyroxene granulites. Their major, trace, REE and isotopic chemistry are not distinct from each other and they represent Mg-basalts with MgO >7% and Al2O3 <16%.The majority of the analyzed samples plot in the tholeiitic field or show tholeiitic trends, suggesting their metatholeiitic nature as well as general preservation of original composition. The rocks are characterized by enriched large-ion lithophile elemental concentrations than that of mid-oceanic ridge basalts. Their high-field strength elements and heavy rare-earth elemental concentrations, however, are as that of mid-oceanic ridge basalts, a feature which is also reflected in the ratios of their large-ion lithophile elements against high-field strength elements and heavy rare-earth elements, wherein we find these ratios are higher than N-type MORB. Further, the rocks show negative Nb anomaly, high Th/Ta ratio and low La/Nb ratio, which are also characteristics of subduction-related magmatism.The isotopic studies carried out on these samples show that, the Sm-Nd and Rb-Sr dating did not yield much spread, but suggested a Sm-Nd metamorphic age of ∼960 Ma. Rb-Sr dating gave ages ∼886 Ma, suggesting the reworking of the Rb and Sr elements during subsequent tectonothermal overprinting. The Nd model ages (TDMNd) of these rocks show a relatively restricted range of 1120 to 1357 Ma, suggesting mafic magmatism ∼1200 Ma. The positive eNd values (+4.22 to +6.07) shown by these rocks, represent a juvenile crustal fragment derived from melting of mantle precursors, without significant reworking of older crustal material. It is proposed that these rocks were produced by partial melting of a mantle source, characterized by LILE enrichment, related principally to dehydration of subducted oceanic crust.  相似文献   

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