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1.
Portions of highland breccia boulder 7 collected during the Apollo 17 mission were studied using UThPb and RbSr systematics. A RbSr internal isochron age of3.89 ± 0.08b.y. with an initial87Sr/86Sr of0.69926 ± 0.00008 was obtained for clast 1 (77135,57) (a troctolitic microbreccia). A troctolitic portion of microbreccia clast 77215,37 yielded a UPb internal isochron of3.8 ± 0.2b.y. and an initial206Pb/207Pb of 0.69. These internal isochron age are interpreted as reflecting metamorphic events, probably related to impacts, which reset RbSr and UPb mineral systems of older rocks.Six portions of boulder 7 were analyzed for U, Th, and Pb as whole rocks. Two chemical groups appear to be defined by the U, Th, and Pb concentration data. Chemical group A is characterized by U, Th, and Pb concentrations and238U/204Pb values which are higher than those of group B. Group A rocks have typical232Th/238U ratios of ～ 3.85, whereas-group B rocks have unusually high Th/U values of ～ 4.1.Whole-rock UPb and PbPb ages are nearly concordant. Two events appear to be reflected in these data — one at ～ 4.4 b.y. and one at ～ 4.5 b.y. The chemical groupings show no correlation with documented ages. The old ages of ～ 4.4 b.y. and ～ 4.5 b.y. may, like the younger ～ 4.0 b.y. ages, be related to basin excavation events.  相似文献   

2.
Sm-Nd data for the Archean granulite and amphibolite facies migmatites of Langøy and Hinnøy in Vesterålen are presented which indicate that their protoliths formed ～2.6 AE ago. Rubidium and U loss during a granulite facies metamorphism at ～1.8 AE caused serious disturbance of total-rock U-Pb and Rb-Sr systems. Therefore these systems do not provide any precise age information for the granulite facies migmatites. For the amphibolite facies migmatites of Vesterålen both SmNd, RbSr and PbPb total-rock systems give model ages of ～2.6 AE. The results on both granulite and amphibolite facies rocks are thus in agreement. Previous interpretations based on PbPb data, which indicated an age of 3.41 AE for the Archean terrane of Vesterålen, are not valid.One SmNd model age from the granulite facies migmatites at Moskenesøy in Lofoten indicates that the protoliths of these migmatites formed ～2.0 AE ago and are thus not related to the Vesterålen migmatites.  相似文献   

3.
KAr,40Ar39Ar and RbSr dates are reported for minerals from the ca. 3700 my-old Amîtsoq and Isua gneisses of the Godthaabsfjord area of West Greenland. KAr dates on biotites and hornblendes range from about 1900 to 3500 my, with hornblendes having a much narrower range (ca. 2250–2750 my) than biotites. One biotite from Isua gives an impossibly high KAr date of 4940 my.40Ar39Ar mineral dates are in close agreement with conventional KAr dates over the entire range of apparent age values. The presence of minor amounts of excess argon is observed in the hornblendes, but radiogenic and excess argon in the biotites are completely homogenised and cannot be differentiated.Rb-Sr measurements on biotites are closely concordant and show that all biotites were completely open to diffusion of radiogenic87Sr at about 1600–1700 my. This is the first proof of a regional thermal event at this time in the Archaean of West Greenland, although similar dates are well known from the Proterozoic belts to the north and south.The evidence suggests that those KAr biotite dates greater than about 2700–2800 my result from excess radiogenic argon incorporated during a thermal event of about this age or, more probably, during the 1600–1700 my Sr isotope homogenisation event. Scatter of mineral dates below about 2700 my could also be due, at least in part, to overprinting by the 1600–1700 my event.KAr mineral dates and an Rb-Sr mineral isochron from a pegmatite associated with the last major rock-forming event in the Godthaabsfjord area, namely the Qo?rqut granite, indicate an age of emplacement of 2580 ± 30 my.  相似文献   

4.
RbSr measurements on the Yatsushiro granite and gneiss, which had been considered stratigraphically to be of possible Precambrian age, are reported. The whole rock isochron for the granite gives an age of 352 ± 8 my with a low initial87Sr/86Sr ratio of 0.7037 ± 0.0006. Data for constituent minerals of the granites are dispersed irregularly around the whole rock isochron (possibly by later tectonic events). For the gneiss, a metamorphic event around 410 my is indicated by the muscovite RbSr ages. The present results do not support the possibility that the Yatsushiro granite and gneiss are Precambrian in age.  相似文献   

5.
Comparative UPb dating of zircon, xenotime and monazite from two different samples of the Himalayan “Makalu” granite shows the two U decay series to be in disequilibrium, particularly in monazite. This disequilibrium is due to excess or deficit amounts of radiogenic206Pb which originate from an excess or deficit of230Th, respectively, occurring initially in the mineral. Such an initial disequilibrium is caused by UTh fractionation between the crystallising mineral and the magma. Therefore, the UPb ages of Th-rich minerals such as monazite (and allanite) have to be corrected for excess206Pb due to excess230Th, whereas Th-poor minerals such as zircon and xenotime require a correction for a deficit of206Pb due to deficiency of230Th. The extent of this correction depends on the degree of ThU fractionation and on the age of the rock. For the two monazite populations analysed here, these excess amounts of206Pb were, with reference to the amount of radiogenic206Pb, 8–10% and 15–20% respectively, and less than 1% for zircon and xenotime. The varying degrees of Th enrichment relative to U in monazite show that the ThU partition coefficients for this mineral are not constant within a single granite. Furthermore, for monazite there is evidence for excess amounts of radiogenic207Pb originating from the decay of initial excess231Pa, also enriched during crystal growth.The very low Th/U ratios of 0.196 and 0.167, determined for thetwo whole rocks from which the minerals have been extracted, substantiate the view that granite formation is a fundamental mechanism for ThU fractionation in continental crust.The different ages of 21.9 ± 0.2m.y. and24.0 ± 0.4m.y., obtained by averaging the corrected238U206Pb ages of the monazites, suggest that the apparently homogeneous Makalu granite was generated over a period of at least 2 m.y.  相似文献   

6.
Lower Proterozoic sapphirine-bearing and associated granulites from Central Australia exhibit the greatest range of present-day143Nd/144Nd ratios (∈Nd(O)= ?26.5 to +112.3) yet reported for rocks believed to be cogenetic. The Nd isotopic data and REE abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform CuPbZn sulfide deposits with which they are closely associated. Field relationships, petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite-rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism; calculations employing REE abundances of the sapphirine granulites and associated rocks, combined with bulk solid-fluid distribution coefficient data yield high fluid/rock ratios, consistent with a pre-metamorphic hydrothermal origin for the unusual REE patterns. The SmNd data for these rocks define an age of 1760±75Ma, which is significantly younger than the crust formation age of the terrain ( 2070±125Ma) but indistinguishable from the RbSr whole rock age for granulite facies metamorphism (1790±35Ma). These data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly (perhaps tens of millions of years) before granulite facies metamorphism, followed by redistribution of Nd isotopes or small fractionations of the Sm/Nd ratio during the granulite facies event, and possibly also during intense retrogression which reset RbSr whole rock and UPb zircon and monazite systematics at about 1700 Ma.  相似文献   

7.
Major element, trace element and Sr, Nd, Pb and O isotopic data for a Franciscan Mn-deposit suggest an origin by seafloor hydrothermal circulation. Based onQ-mode factor analysis the cherts and Mn-lenses of the Blue Jay mine formed from a combination of 4 components representing 1 biogenic, 1 hydrothermal, and 2 detrital sources. RbSr, UThPb and O isotopic systematics in the Mn-lenses were affected by input from the hydrothermal circulation of material leached from the underlying basalts. Nd isotopic compositions in both cherts and Mn-lenses are identical and within the range measured for Pacific Ocean water suggesting the REE were not mobilized by hydrothermal activity. Correlation of δ18O with SiO2 and MnO2 in the Mn-lenses implies the lenses formed by simple mixing of hydrothermally derived Mn-oxides with seawater and biogenic silica. δ18O of the cherts is both uniform and depleted relative to DSDP Jurassic cherts but similar to microquartz-bearing cherts of the Monterey Formation: this suggests that diagenetic activity exerted more control on oxygen isotope compositions then hydrothermal alteration or metamorphism. Finally, a well defined RbSr isochron of158 ± 5Myr was obtained for these cherts and opens the possibility of determining absolute radiometric ages for similar cherts throughout the geologic record.  相似文献   

8.
Plutons of quartz diorite to adamellite composition give Rb-Sr total-rock and biotite calculated ages of 12 to 176 m.y. along a transect of the Chilean Andes between 45° and 46° S. Miocene biotite dates of quartz diorite from the Puerto Aisen area are interpreted as minimum ages related to probable local structural deformation (faulting). Along a north-south traverse of the Argentine Andes between 40° and 44° S, plutons of similar composition to the Chilean Andes give KAr and RbSr total-rock and mineral dates which range from late Paleozoic to Cretaceous. No systematic unidirectional migration of the radiometric ages is observed.  相似文献   

9.
RbSr and Pb isotopic data are reported for some of the Karroo volcanics from Swaziland. Linear arrays of Karroo data are found in the RbSr and Pb/Pb isochron diagrams. Certain of these linear arrays, if interpreted as isochrons, give ages considerably in excess of the age normally accepted for the Karroo and must be interpreted as “erupted isochrons”. It is argued that these “erupted isochrons” represent mixing lines resulting from contamination of mantle-derived magma by continental crust. It is unlikely that the RbSr “erupted isochron” has any age significance because the Rb/Sr ratios were probably fractionated at the time or eruption, both during the contamination event and during subsequent fractionation. “Erupted isochrons” in the Pb/Pb isochron diagram are much more likely to have time significance because relatively recent U/Pb fractionation events do not significantly affect the Pb isotopic compositions. The Pb/Pb “erupted isochron” has an apparent age of 3260 ± 60 m y. This age is very similar to that of the Kaapvaal craton on which the volcanics rest and it is argued that the “erupted isochron” approximately dates the formation of the Kaapvaal craton.Six Pb isotope analyses of separated feldspar crystals are also reported. Three of these were demonstrably out of isotopic equilibrium with the magma at the time of eruption. This shows that isotopic heterogeneities on the scale of a few millimetres may be preserved between crystals and their host magma.The nature of the crust which contaminated the magma is considered and it seems most likely that the magma was contaminated by heterogeneous crust of uniform age.  相似文献   

10.
UPb analyses of fractions of zircon and monazite (3–8 grains each) and of single zircon grains resolve a lower Ordovician age of 470 ±4m.y. for the Palung granite which occurs in the High Himalayan nappes south of Kathmandu. Its thrusting during the Alpine orogeny under lower greenschist facies conditions did not affect the UPb systems in zircon and monazite. The granite crystallized from a magma which was mainly generated by anatexis of Precambrian continental crust. The magma was heterogeneous with respect to primary ages and/or metamorphic histories of the magma source rocks. This indicates either a derivation from (meta-) sediments or an intense mixing of different crustally derived magmas. The genesis of the Palung granite is possibly related to an orogeny which affected the Indian shield in lower Palaeozoic times. The detected inherited radiogenic lead in the Palung zircons occurs in perfectly homogeneous, transparent crystals; i.e. this radiogenic (“excess”) lead is not related to the presence of old, microscopically visible, overgrown zircon cores. The minimum ages of the inherited lead components range from about 800 to 1700 m.y.  相似文献   

11.
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12.
Mineral and whole-rock REE abundances in garnet lherzolite and megacrystalline nodules from The Thumb display broad correlations with major element compositions. Lherzolites with > 12 modal % clinopyroxene plus garnet (“high-CaAl lherzolites”) have relatively flat chondrite-normalized whole-rock REE patterns. Lherzolites poor in clinopyroxene and garnet (“low-CaAl lherzolites”) have lower HREE in clinopyroxenes and garnets and higher whole-rock LREE/HREE. It is concluded that the low-CaAl lherzolites may have undergone LREE metasomatism after depletion of the major element compositions by partial melting and that much of the garnet now present was originally dissolved in aluminous orthopyroxene. The high-CaAl lherzolites may be interpreted either as primordial mantle samples or as products of equilibration with very LREE-enriched liquids. The “megacrystalline” nodules are medium- to ultracoarse-grained intergrowths and megacrysts with mineral compositions similar to discrete nodule suites in kimberlites. The REE abundances of the megacrystalline minerals are consistent with an origin as cumulates from magma with extremely fractionated REE, similar to minette or kimberlite.The patterns of correlation of REE and major elements in this inclusion suite are similar to the patterns observed in the garnet lherzolite and discrete nodule suites of southern African kimberlites. Both of the subcontinental mantle provinces represented by these suites contain three distinct petrogenetic components: refractory garnet lherzolite enriched in LREE and depleted in HREE, fertile garnet lherzolite with generally chondritic REE abundances, and a suite of ultracoarse minerals precipitated from magma with extremely fractionated REE generally similar to the host magmas.  相似文献   

13.
A well-defined internal RbSr isochron has been determined for the unbrecciated Nakhla achondrite, defining an age T = (1.24 ± 0.01) AE and an initial 87Sr/86Sr ratio of 0.70254 ± 0.00003. The excellence of fit to the isochron shows that a very thorough Sr isotopic equilibration took place at this time between all phases of the meteorite. Recent petrological observations indicate that the age most probably represents the time of an igneous crystallization event on the parent body of Nakhla. The age shows that we may no longer assume that major chemical and physical differentiation processes took place only in early solar system history. The RbSr systematics are shown not to require late formation of the Nakhla parent body. Geochemical similarities between Nakhla and the earth are discussed.  相似文献   

14.
It is possible to formulate a two-pyroxene (MgFeCa) geothermometer using non-ideal but disordered site models for the pyroxenes. Experimental data on any two of the following three sets are needed: (a) the site occupancy data in orthopyroxene, (b) the site occupancy data in clinopyroxene (both sets of data with varying composition and temperature), and (c) synthetic phase equilibrium data on coexisting pyroxenes. A geothermometer based on site occupancy data is very sensitive to small variations of composition and may be useless for common petrological purposes but immensely useful for studying disequilibrium and kinetic history of the rocks.  相似文献   

15.
Rb and Sr concentrations and Sr isotopic composition have been measured in thermo-mineral waters from the Massif Central in France. Rocks and mineral fractions which are thought to be important Sr contributors were also analyzed. The results suggest the waters to be mixtures of two or three components from different reservoirs, which can be associated with possible source rock types like granite and syenodiorite occurring in the region. The strontium isotopic ratios found in the separated minerals of the syenodiorite stratum indicate no chemical fractionation during the water-rock interaction. A RbSr internal isochron age of(265 ± 10) × 106yr is inferred for the Roche d'Enval syenodiorite.  相似文献   

16.
UPb isotopic measurements on individual zircon crystals combined with morphological analyses permit the identification of three distinct components within the zircon population of the Saedelhorn diorite, a Variscan dike from the western Gotthard (Central Alps, Switzerland): (i) 94% of the grains in the zircon population are elongate crystals with pronounced skeletal morphology indicative of rapid growth from a supercooled melt. (ii) 5% of the population consist of turbid, mostly subhedral zircons frequently showing D-type morphology (classification according to Pupin and Turco [1]) and elevated uranium contents compared to the skeletal variety. Single-crystal and multi-grain UPb isotopic data of group (i) and (ii) zircons define an intrusion age of 293 +5/−4 m.y. for the dike. (iii) Rare, transparent zircon crystals (<1% of the zircon population) yield apparent UPb ages in the range of 370–490 m.y. and display morphological and isotopic characteristics closely resembling those of a Caledonian orthogneiss intruded by the dike. This implies presence of assimilated wall-rock components in the macroscopically homogeneous dike sample.A comparison of the data obtained by conventional analysis of multi-grain zircon fractions and those obtained by grain-by-grain analysis demonstrates that age resolution is considerably improved by single-crystal UPb dating. Furthermore, quantitative identification of zircon components assimilated by the ascending magma along its path to the present level of exposure is feasible by the latter technique. Since it is likely that such zircon grains are common to a broad variety of magma types, valuable information on age and composition of crustal layers not accessible to direct observation may readily become available by application of precise micro-analytical techniques.Low initial143Nd/144Nd(εNd = −2.7) at the time of intrusion of the Saedelhorn dike requires the magma to be derived partially or totally from a crustal source. For this crustal precursor, a model age of 1050 m.y. (TDM) is obtained, indicating that Proterozoic crust was involved in the petrogenesis of the Variscan intrusives of the Gotthard area.  相似文献   

17.
This work reports the preliminary results of a study of the relationship between tectonic environment and major element chemistry of volcanic rocks. Using a file of 8400 analyses of rocks of varying ages and geographic distribution it has been found that a simple ternary plot of MgOFeO(total)Al2O3 may be used to distinguish five tectonic environments: ocean ridge and floor, ocean island, continental, orogenic, and spreading center island. Subalkaline “basaltic-andesites” (51–56% SiO2 calculated anhydrous) are used in this study. Alkaline rocks do not generally show the simple patterns of tholeiitic or subalkaline rocks. The ocean island suite is bimodal with one mode tholeiitic and the other alkaline. A bimodal age/chemistry relationship in the continental suite is tentatively related to different conditions during and after active breaking up of continents (rifting and drifting stages).Most of the Archean rocks in our file generally do not fall in the orogenic field. It is unlikely that modern island arc models can be generally applied to Archean greenstone belts. Locally, however, there may be Archean rocks analogous in some respect to modern calc-alkali rocks.  相似文献   

18.
The distribution of Ca and Mg among coexisting olivine, clinopyroxene and orthopyroxene has been studied in a piston-cylinder apparatus in the temperature range 1100–1300°C and pressure range 9–41 kbar. Ca in olivine decreases with increasing pressure and decreasing temperature. The pressure effect is the result of Ca going into the higher-coordination M2 site in clinopyroxene as pressure is increased. For the CaMg exchange reaction between olivine and clinopyroxene, ΔV°=0.249J bar?1 mole?1; this is sufficient for pressure estimates accurate to ±3kbar if temperatures of equilibration are independently known. CaMg exchange between olivine and orthopyroxene is not sufficiently pressure dependent to be used as a geobarometer.Application of the olivine-clinopyroxene geobarometer to coarse garnet lherzolites from southern Africa gives P-T results consistent with a continental geotherm. For spinel lherzolites from southwestern United States, the geotherm appears to be displaced to higher temperatures indicating oceanic affinities. Application of the geobarometer to natural systems requires assumptions about activity relationships in clinopyroxene which should be checked by experiment.  相似文献   

19.
A 250-μm fragment in the Luna 20 fines has a very fine-grained “igneous” texture and has the composition (wt.%): SiO2, 41.1; TiO2, 0.35; Al2O3, 27.2; Cr2O3, 0.14; FeO, 4.2; MnO, 0.06; MgO, 8.5; CaO, 17.8; Na2O, 0.05; and K2O < 0.02. It contains ～ 65% plagioclase An99–100, ～ 15% olivine Fo90, ～ 2% Mg-Al spinel and the remainder an unusual interstitial phase with composition SiO2, 34.8; TiO2, 1.78; Al2O3, 18.3; Cr2O3, 0.04; FeO, 14.1; MnO, 0.22; MgO, 5.0; CaO, 24.1; Na2O, 0.34; K2O < 0.02. This fragment probably represents a portion of a normal highland rock (anorthositic norite) which was heated to a very high temperature by impact, lost volatiles including SiO2, and then partially crystallized. The observed phases and their inferred crystallization sequence are consistent with experimental results in the system CaOMgOAl2O3SiO2 (Schairer and Yoder, 1969), assuming the unusual phase to be a residual glass. This type of internal fractionation, leading to silica depletion in the residuum, is different from that normally observed in lunar rocks and is attributed to slightly lower bulk SiO2 resulting from vapor fractionation due to impact (which also results in lower Na2O and other volatiles). Because differentiation of the type shown by this fragment is rare in lunar materials, we infer that such major-element vapor fractionation is uncommon on the surface of the moon. The experimental CaOMgOAl2O3SiO2 phase relations also have a bearing on the lunar model proposed by D.L. Anderson in 1973: his “refractory” original lunar composition would differentiate to produce silica deficient liquids, like the unusual phase in our fragment, rather than the normal lunar crustal rocks.  相似文献   

20.
Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A SmNd mineral isochron for a plagioclase lherzolite yields an age,T = 427 ± 32 Ma and initialεNd = + 10.4 ? 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzoliteP-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The SmNd model age of the plagioclase lherzolite totalrock isTCHURNd = 3.4 AE. This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a SmNd mineral isochron age ofT = 435 ± 21 Ma andεNd = + 6.7 ? 0.3. A pyroxenite dike yields an initialεNd = + 7.3 ± 0.4. The initialεNd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite—demonstrating that they are not genetically related. RbSr data do not give any coherent pattern. However, some bounds can be put on initial Sr values ofεSr ? ?21 for the plagioclase lherzolite andεSr ? ?8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and SmNd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source ～ 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt (～ 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle ～ 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude.  相似文献   

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