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1.
The times of original fractionation of the Sm and Nd component of clastic sediments from a mantle source (≡ crustal residence age) have been estimated from Sm-Nd model ages calculated relative to a depleted mantle evolution. In this way the provenance and evolution of selected Precambrian and Phanerozoic sediments and metasediments from the British Isles have been examined. Whereas some Archaean and early Proterozoic sediments have Sm-Nd model ages that are close to their stratigraphic age, the Phanerozoic sediments analysed have model ages as much as 2.0 Ga in excess of their stratigraphic age.A more detailed study of Lower Palaeozoic sediments deposited on the northern margin of the Iapetus Ocean provides evidence for a marked change of provenance in the Ordovician after the deposition of the Dalradian Supergroup. A component with comparatively high143Nd/144Nd and Sm/Nd ratio (presumably basaltic) is present in the sediments throughout the accretionary prism. Crustal residence age estimates average about 1.5 Ga for both these Lower Palaeozoic sediments, and modern pelagic clays, and collectively fail to provide any evidence for significant continental growth during the Phanerozoic.  相似文献   

2.
The147Sm143Nd and146Sm142Nd isotope systematics have been investigated in five chondrites and the achondrites Moama and Angra dos Reis (ADOR). The new chondrite data and those we have reported before are all consistent with our previously reported reference values for CHUR (“chondritic uniform reservoir”) of (143Nd/144Nd)CHUR0 = 0.511847 and (147Sm/144Nd)CHUR0 = 0.1967. Most of the bulk chondrites analyzed have 143Nd/144Nd and 147Sm/144Nd within 0.5 ε-units and 0.15% of the CHUR values, respectively. This strongly suggests that the CHUR evolution is now known to within these error limits throughout the history of the solar system. The St. Severin chondrite yields an SmNd internal isochron age of T = 4.55 ± 0.33AE and an initial εNd = 0.11 ? 0.26. Much larger variations in Sm/Nd ratios were measured in mineral separates of the Moama and ADOR achondrites. Thus, very precise ages of 4.46 ± 0.03AE and4.564 ± 0.037AE were obtained for these meteorites, respectively. The initial εNd values obtained for Moama and ADOR are 0.03 ? 0.25and0.14 ? 0.20, respectively. The values obtained on these meteorites are fully consistent with the CHUR evolution curve. Initial εNd data on terrestrial igneous and meta-igneous rocks demonstrates that positive initial εNd values occur throughout the past 4 AE. This confirms our earlier report that a light rare earth element-depleted layer has existed throughout most of the Earth history and is the source of present-day mid-ocean ridge basalts. The inferred shape of the εNd vs. age curve for the depleted mantle suggests profound changes in tectonic regimes with time; in particular, it suggests a much higher rate of recycling of continental materials into the mantle during the Archean as compared to later time periods.146Sm142Nd systematics of ADOR and Moama are supportive of the hypothesis that146Sm was present in the early solar system and suggests a 146Sm/144Sm ratio of about 0.01 for the solar system ～ 4.56 AE ago. This inferred high146Sm abundance cannot be explained as a late injection from a supernova and must be due to galactic nucleo-synthesis.  相似文献   

3.
Nd and Sr isotope determinations on late Precambrian to early Palaeozoic igneous and sedimentary rocks from the Arabian Shield are used to investigate the proportion of reworked “older” crust, and the rate at which new crust was generated during the Pan African event. Eight Rb/Sr whole rock isochrons on igneous suites yield ages in the range 770?590 Ma and initial 87Sr/86Sr ratios of 0.7038?0.7023. These data confirm that magmatism in this area was largely restricted to the period 850-550 Ma, and the initial ratios are sufficiently low to preclude significant contributions from a long-lived upper crustal source. The initial 143Nd/144Nd ratios of a variety of lithologies, including several samples of possible “basement”, are all higher than the contemporaneous values for CHUR (εNd = +1.6 to +6.9), suggesting that many were derived directly from the upper mantle, and that any inferred crustal source regions for the remainder could not have separated from likely LREE-depleted mantle reservoirs before 1200 Ma. The Arabian Shield therefore provides an example of rapid crustal growth during the Late Proterozoic, and contrasts with the Damara intracratonic belt of Namibia where Nd and Sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period.  相似文献   

4.
The Proterozoic anorogenic magmatic rocks are well developed in the Bayan Obo deposit region. They are composed of trachyte, magnesioarfvedesonite-feldspatite, potash-rhyolite, dacite, rhyolite, quartz porphyry and trachy basalt. A lot of high-K diabase veins (dykes) are also found. These anorogenic magmatic rocks are derived from the mantle. They have lower?Nd(t) (4.52-5.88) with T Nd DM = 1.54-1.92 Ga. Their Nd isotopic compositions and T Nd DM are consistent with those of ores, implying that the ore-forming materials were derived from these anorogenic magmatic rocks. The zircon U-Pb ages of the rocks are 1.8 Ga. Research results indicate that the Bayan Obo Group was replaced by the hydrothermal solution related to the anorogenic magmatic rocks, resulting in the formation of the deposit.  相似文献   

5.
Relict high-pressure granulite-facies rocks have been found in the Ami?tsoq gneisses and inclusions of the older Akilia supracrustal association, on islands south of Godthåb. Only amphibolite-facies assemblages have been found in Ameralik dykes and younger rocks from this area. The Ami?tsoq gneisses are depleted in Rb and U relative to those of Ameralik and Isua. Well-fitted Pb/Pb and Rb-Sr isochrons on Ami?tsoq granulites indicate that this depletion, correlated with the granulite-facies metamorphism, occurred ca. 3600 Ma ago. Textural features suggest that the present cpx + opc + gnt + plag + qtz + hbl assemblages evolved from earlierintermediate-P assemblages (cpx + opx + plag), probably during cooling from the metamorphic peak. Re-equilibrium of olderintermediate-P assemblages in local environments of low ?H2O, during the ca. 2800-Ma metamorphism of the Malene supracrustals, is feasible but is considered unlikely. Either interpretation requires crustal thickness of at least 20 km and geothermal gradients of?30°C/km, ca.3600Ma ago. The higher heat production of early Archaean times was apparently dissipated through oceanic, rather than continental, areas.  相似文献   

6.
U-Pb analyses of zircons from the Godthaabsfjord area in West Greenland yield dates of 2530 ± 30 m.y. for the Qo?rqut granite, 2820 ± 50 m.y. for the Nu?k gneiss, 2600–2900 m.y. for Malene-type supracrustal rocks, and >3800 m.y. for Isua supracrustal rocks. The zircon data are in accord with the stratigraphic sequence except for the Malene supracrustal zircons, which appear to have been variably affected by later metamorphism that did not reset the Nu?k or Amîtsoq zircons in the same area.  相似文献   

7.

The Proterozoic anorogenic magmatic rocks are well developed in the Bayan Obo deposit region. They are composed of trachyte, magnesioarfvedesonite-feldspatite, potash-rhyolite, dacite, rhyolite, quartz porphyry and trachy basalt. A lot of high-K diabase veins (dykes) are also found. These anorogenic magmatic rocks are derived from the mantle. They have lowerɛNd(t) (4.52-5.88) with T Nd DM = 1.54-1.92 Ga. Their Nd isotopic compositions and T Nd DM are consistent with those of ores, implying that the ore-forming materials were derived from these anorogenic magmatic rocks. The zircon U-Pb ages of the rocks are 1.8 Ga. Research results indicate that the Bayan Obo Group was replaced by the hydrothermal solution related to the anorogenic magmatic rocks, resulting in the formation of the deposit.

  相似文献   

8.
The Himalayan orogen consists of three major lithologic units that are separated by two major north-dipping faults: the Lesser Himalayan Sequence (LHS) below the Main Central Thrust (MCT), the Greater Himalayan Crystalline Complex (GHC) above the MCT, and the Tethyan Himalayan Sequence (THS) juxtaposed by the South Tibet Detachment fault (STD) over the GHC. Due to widespread meta-morphism and intense deformation, differentiating the above three lithologic units is often difficult. This problem has been overcome by the use of Sm-Nd isotopic analysis. The previous studies suggested that the LHS can be clearly distinguished from the GHC and THS by their Nd isotope compositions. However, the lack of detailed and systematic Sm-Nd isotopic studies of the THS across the Himalaya in general has made differentiation of this unit from the nearby GHC impossible, as the two appear to share overlapping Nd compositions and model ages. To address this problem, we systematically sam-pled and analyzed Nd isotopes of the THS in southeastern Tibet directly north of Bhutan. Our study identifies two distinctive fields in a εNd -TDM plot. The first is defined by the εNd(210 Ma) values of -3.45 to -7.34 and TDM values of 1.15 to 1.29 Ga from a Late Triassic turbidite sequence, which are broadly similar to those obtained from the Lhasa block. The second field is derived from the Early Cretaceous meta-sedimentary rocks with εNd(130 Ma) values from -15.24 to -16.61 and TDM values from 1.63 to 2.00 Ga; these values are similar to those obtained from the Greater Himalayan Crystalline Complex in Bhutan directly south of our sampling traverse, which has εNd(130 Ma) values of -10.89 to -16.32 and Nd model ages (TDM) of 1.73 to 2.20 Ga. From the above observations, we suggest that the Late Triassic strata of the southeast Tibetan THS were derived from the Lhasa block in the north, while the Early Cretaceous strata of the THS were derived from a source similar to the High Himalayan Crystalline Complex or Indian craton in the south. Our interpretation is consistent with the existing palaeocurrent data and provenance analysis of the Late Triassic strata in southeastern Tibet, which indicate the sediments derived from a northern source. Thus, we suggest that the Lhasa terrane and the Indian craton were close to one another in the Late Triassic and were separated by a rift valley across which a large submarine fan was transported southward and deposited on the future northern margin of the Indian continent.  相似文献   

9.
Abstract We present chemical and Sr–Nd–Pb isotopic compositions of three Triassic (226–241 Ma) calc‐alkaline granitoids (the Yeongdeok granite, Yeonghae diorite and Cheongsong granodiorite) and basement rocks in the northern Gyeongsang basin, south‐eastern Korea. These plutons exhibit typical geochemical characteristics of I‐type granitoids generated in a continental magmatic arc. The Yeongdeok and Yeonghae plutons have similar initial Sr, Nd and Pb isotope ratios (87Sr/86Srinitial = 0.7041 ~ 0.7050, ?Nd(t) = 2.3 ~ 4.0, 206Pb/204Pbfeldspar = 18.22 ~ 18.34), but distinct rare earth element patterns, suggesting that the two plutons formed from partial melting of a similar source material at different depths. The Cheongsong pluton has slightly more enriched Sr–Nd–Pb isotopic compositions (87Sr/86Srinitial = 0.7047 ~ 0.7065, ?Nd(t) = 3.9 ~ 2.8, 206Pb/204Pbfeldspar = 18.24 ~ 18.37) than the other two plutons. The Nd model ages of the basement rocks (1.1 ~ 1.4 Ga) are slightly older than those of the plutons (0.6 ~ 1.0 Ga). The initial Sr and Nd isotopic ratios of the plutons can be modeled by the mixing between the mid‐oceanic ridge basalt‐like depleted mantle component and the crustal component represented by basement rocks, which is also supported by Pb isotope data. The Sr and Nd isotope data from granitoids and basement rocks suggest that the Gyeongsang basin, the Hida belt and the inner zone of south‐western Japan share relatively young basement histories (middle Proterozoic), compared with those (early Proterozoic to Archean) of the Gyeonggi and Yeongnam massifs and the Okcheon belt. The Nd isotope data of basement rocks suggest that the Hida belt might be better correlated with the basement of the Gyeongsang basin than the Gyeonggi massif, the Okcheon belt or the Yeongnam massif, although it may represent an older continental margin of East Asia than the Gyeongsang basin considering its slightly older Nd model ages.  相似文献   

10.
Sm-Nd isotopic evolution of chondrites   总被引：8，自引：0，他引：8  
The143Nd/144Nd and147Sm/144Nd ratios have been measured in five chondrites and the Juvinas achondrite. The range in143Nd/144Nd for the analyzed meteorite samples is 5.3 ε-units (0.511673–0.511944) normalized to150Nd/142Nd= 0.2096. This is correlated with the variation of 4.2% in147Sm/144Nd (0.1920–0.2000). Much of this spread is due to small-scale heterogeneities in the chondrites and does not appear to reflect the large-scale volumetric averages. It is shown that none of the samples deviate more than 0.5 ε-units from a 4.6-AE reference isochron and define an initial143Nd/144Nd ratio at 4.6 AE of0.505828 ± 9. Insofar as there is a range of values of147Sm/144Nd there is no unique way of picking solar or average chondritic values. From these data we have selected a new set of self-consistent present-day reference values for CHUR (“chondritic uniform reservoir”) of (143Nd/144Nd)CHUR0 = 0.511836and(147Sm/144Nd)CHUR0 = 0.1967. The new147Sm/144Nd value is 1.6% higher than the previous value assigned to CHUR using the Juvinas data of Lugmair. This will cause a small but significant change in the CHUR evolution curve. Some terrestrial samples of Archean age show clear deviations from the new CHUR curve. If the CHUR curve is representative of undifferentiated mantle then it demonstrates that depleted sources were also tapped early in the Archean. Such a depleted layer may represent the early evolution of the source of present-day mid-ocean ridge basalts. There exists a variety of discrepancies with most earlier meteorite data which includes determination of all Nd isotopes and Sm/Nd ratios. These discrepancies require clarification in order to permit reliable interlaboratory comparisons. The new CHUR curve implies substantial changes in model ages for lunar rocks and thus also in the interpretation of early lunar chronology.  相似文献   

11.
Nd isotopic characteristics of S- and I-type granites   总被引：1，自引：0，他引：1  
The initial Nd and Sr isotopic composition has been determined in S- and I-type granites from the Paleozoic Berridale and Kosciusko Batholiths of southeast Australia. The Nd and Sr isotopic variations form a strongly covariant array with S-types granites having a relatively restricted range inεNd values from ?6.1 to ?9.8 but a large range in initial87Sr86Sr of from 0.7094 to 0.7184. These characteristics are indicative of an～1400-m.y. sedimentary or metasedimentary source for S-types. I-types have variable initial Nd ranging from +0.4 to ?8.9, and a more limited range in initial87Sr86Sr of from 0.70453 to 0.7119. These isotopic characteristics are consistent with a two-component mixing model whereby a depleted mantle-like component (DMC) withεNd = +6 and87Sr86Sr= 0.703, is mixed with a crustal component (CC) havingεNd = ?9 and87Sr86Sr= 0.720. Although this two-component mixing model satisfies the isotopic constraints the source rock chemistry of the I-types is not compatible with the large proportion (up to 50%) of sedimentary material implied by the isotopic data. This indicates that more than two components are required to account for both the isotopic and chemical data. Both the chemical and isotopic data are consistent with I-type granites having been formed from source rocks of predominantly mantle derivation and obtained progressively from the mantle over a period of 1000 m.y. prior to granite formation.  相似文献   

12.
New data for the direct measurement of the isotopic composition of neodymium in Atlantic Ocean seawater are compared with previous measurements of Pacific Ocean seawater and ferromanganese sediments from major ocean basins. Data for Atlantic seawater are in excellent agreement with Nd isotopic measurements made on Atlantic ferromanganese sediments and are distinctly different from the observed compositions of Pacific samples. These results clearly demonstrate the existence of distinctive differences in the isotopic composition of Nd in the waters of the major ocean basins and are characteristic of the ocean basin sampled. The average εNd(0) values for the major oceans as determined by data from seawater and ferromanganese sediments are as follows: Atlantic Ocean,εNd(0) ? ?12 ± 2; Indian Ocean,εNd(0) ? ?8 ± 2; Pacific Ocean,εNd(0) ? ?3 ± 2. These values are considerably less than εNd(0) value sources with oceanic mantle affinities indicating that the REE in the oceans are dominated by continental sources. The difference in the absolute abundance of143Nd between the Pacific and Atlantic Oceans corresponds to ～106 atoms143Nd per gram of seawater. The correspondence between the143Nd/144Nd in seawater and in the associated sediments suggests the possible application of this approach to paleo-oceanography.Distinctive differences in εNd(0) values are observed in the Atlantic Ocean between deep-ocean water associated with North Atlantic Deep Water and near-surface water. This suggests that North Atlantic Deep Water may be relatively well mixed with respect to Nd isotopic composition whereas near-surface water may be quite heterogeneous, reflecting different sources for surface waters relative to deep water. This suggests that it may be possible to distinguish the sources of water masses within an ocean basin on the basis of Nd isotopic composition.The Nd isotopic variations in seawater are used to relate the residence time of Nd and mixing rates between the oceans.  相似文献   

13.
The samples from the hidden Daqiling muscovite monzonite granite, which has recently been recognized within the Limu Sn-polymetallic ore field, have been analyzed for zircon U-Pb ages and whole rock geochemical and Nd-Hf isotopic compositions to discuss its genesis, source, and tectonic setting. LA-ICP-MS zircon U-Pb dating indicates that the granite crystallized in the late Indosinian (224.8±1.6 Ma). The granite is enriched in SiO2 and K2O and low in CaO and Na2O. It is strongly peraluminous with the A/CNK values of 1.09–1.20 and 1.4 vol%–2.7 vol% normal corundum. Chondrite-normalized REE patterns show slightly right-dipping shape with strongly negative Eu anomalies (δEu =0.08–0.17). All samples show enrichment of LILEs (Cs, Rb and K) and HFSEs (U, Pb, Ce and Hf), but have relatively low contents of Ba, Sr and Ti. The zircon saturation temperatures (T zr) are from 711 to 740°C, which are slightly lower than the average value of typical S-type granite (764°C). The granite has negative ? Nd(t) and ? Hf(t) values, which change from ?9.1 to ?10.1 with the peak values of ?9.2 to ?9.0 and from ?3.7 to ?12.6 with the peak values of ?6 to ?5, respectively. The T DM C (Nd) and T DM C (Hf) values are 1.74–1.82 Ga with the peak values of 1.73–1.75 Ga and 1.49–2.04 Ga with the peak values of 1.5–1.6 Ga, respectively. These characteristics reveal that the source region of the granite is dominantly late Paleoproterozoic to early Mesoproterozoic crustal materials. Seven inherited magmatic zircons are dated at the age of 248.6±4.3 Ma, which suggests the existence of the early Indosinian granite in Limu area. These zircons have the ? Hf(t) values of ?6.7–?2.3, similar to those of the Daqiling granite, implying the involvement of the early Indosinian granite during the formation of the Daqiling granite. Inherited zircon of 945±11 Ma has the ? Hf(t) and T DM(Hf) values of 8.7 and 1.14 Ga, respectively, compatible with those of the Neoproterozoic arc magmatic rocks in the eastern Jiangnan orogenic belt. Therefore we inferred that Neoproterozoic arc magma might have been involved in the formation of the Daqiling granite, and that the Neoproterozoic arc magma belt and continent-arc collision belt between the Yangtze and Cathaysia Blocks might have extended westsouthward to Limu region. It is proposed that the underplating of mantle materials triggered by crustal extension and thinning resulted in partial melting of crustal materials to form the Daqiling granite in the late Indosinian under post-collisional tectonic setting.  相似文献   

14.
Petrographic and chemical evidence suggests that boulders from a conglomeratic unit in the Isua supracrustal succession were derived by the erosion of an acid volcanogenic sediment. Six samples of the boulders and surrounding matrix yield a Rb-Sr whole rock isochron with a slope corresponding to an age of 3860 ± 240 m.y. (2 sigma error), but consideration of the initial87Sr/86Sr ratio constrains the possible age of formation of 3710 ± 900 m.y. This is in general agreement with a published Pb/Pb age of 3760 ± 70 m.y. on Isua banded ironstones.Pb isotope compositions as well as highly fractionated, heavy element depleted, rare earth element abundance patterns for the boulders suggest that their igneous precursors were derived from a source region with a similar geochemical history to that of some components of the 3700–3800 m.y. old Ami?tsoq gneisses, involving fractionation of garnet during their evolution.A Pb/Pb whole-rock isochron for Ami?tsoq gneisses from Isua yields an age of 3800 ± 120 m.y. (2σ), in good agreement with previously published Rb-Sr age data on the same rocks. The rock leads are highly unradiogenic and demonstrate substantial U depletion at least 3800 ± 120 m.y. ago. A two-stage model for the U-Pb system yields an average238U/204Pb (μ1) value of 9.3 ± 0.2 for the source region, which is significantly different from the published value of 9.9 ± 0.1 for the Isua iron formation. This indicates the existence of U-Pb heterogeneities between the source regions of plutonic and supracrustal rocks by about 3700–3800 m.y. ago. Attempts to apply U-Pb whole-rock dating to the Ami?tsoq gneisses were unsuccessful because of geologically recent U loss, possibly due to groundwater leaching.A Rb-Sr whole-rock isochron on a suite of Ami?tsoq gneiss samples from a different locality in the Isua region has yielded an age of 3780 ± 130 m.y.In contrast to the Godthaab area, there is no geochronological evidence at Isua for major rock-producing or tectonothermal events after about 3700 m.y. ago. The entire gneiss-supracrustal system developed within the approximate interval 3900–3700 m.y. ago.  相似文献   

15.
Sm-Nd and Rb-Sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of target materials which were impact melted to form these objects. Tektites have large negative εNd(0) values and are uniform within each tektite group while the εSr(0) are large positive values and show considerable variation within each group. Chemical, trace element, and isotopic compositions of tektites are consistent with production by melting of sediments derived from old terrestrial continental crust. Each tektite group is characterized by a uniform Nd model age,TCHURNd, interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites: (1) ～1.15 AE for Australasian tektites; (2) ～1.91 AE for Ivory Coast tektites; (3) ～0.9 AE for moldavites; (4) ～0.65 AE for North American tektites, and (5) ～0.9 AE for high-Si irghizites. Sr model ages,TURSr, are variable within each group reflecting Rb-Sr fractionation and in the favorable limit of very high Rb/Sr ratios, approach the time of sedimentation of the parent material which melted to form the tektites. Australasian tektites are derived from ～0.25 AE sediments, moldavites from ～0.0 AE sediments, Ivory Coast tektites from ～0.95 AE sediments. Possible parent sediments of other tektite groups have poorly constrained ages. Our data on moldavites and Ivory Coast tektites are consistent with derivation from the Ries and Bosumtwi craters, respectively. Irghizites are isotopically distinct from Australasian tektites and are probably not related. Sanidine spherules from a Cretaceous-Tertiary boundary clay have initial εNd ～ +2; εSr ～ +5 and are not derived from old continental crust or meteoritic feldspar. They may represent a mixture of basaltic oceanic crust and sediments, implying an oceanic impact. These isotopic results are also consistent with a volcanic origin for the spherules.  相似文献   

16.
Published data showing a linear correlation between initial Nd and Sr isotope compositions in young basalts indicate the existence of a spectrum of isotopically distinct reservoirs in the mantle which represent either (1) mixtures of two homogeneous endmember reservoirs, one of which may be undifferentiated material or (2) fractionated reservoirs all derived from a homogeneous initial reservoir with the same ratio of enrichment factors for Sm/Nd and Rb/Sr. The slope of the correlation, which can be described approximately by (87Sr/86Sr) = ?3.74114 (143Nd/144Nd) + 2.61935orεNd = ?2.7 εSr, places constraints on the origin of these reservoirs and hence on the chemical evolution of the crust-mantle system. The reservoirs could be residual regions of the mantle left after ancient partial melting events. If so, the requirement of constant relative fractionation of Sm/Nd and Rb/Sr in refractory residues is a strong constraint on partial melting models. Calculations suggest that batch melting models are more compatible with this constraint than are fractional melting models, but models incorporating currently accepted distribution coefficients and residual phase assemblages cannot reproduce the observed isotope effects except under highly specific conditions. The slope of the correlation is not consistent with the hypotheses that chemical structure in the mantle is due to accretional heterogeneity or variable loss of elements to the core. If the mantle reservoirs are complementary in composition to the continental crust, and if the crust + mantle has εNd = 0andεSr = 0 and chondritic Sr/Nd, then Rb/Sr in the crust is calculated to be less than 0.10, suggesting that the crust may be more mafic in composition and contain a smaller proportion of the earth's Rb and heat-producing elements than previously estimated.  相似文献   

17.
Up to now the age of granulite gneisses intruded by the Zabargad mantle diapir has been an unsolved problem. These gneisses may represent either a part of the adjacent continental crust primarily differentiated during the Pan African orogeny, or new crust composed of Miocene clastic sediments deposited in a developing rift, crosscut by a diabase dike swarm and gabbroic intrusions, and finally metamorphosed and deformed by the mantle diapir. Previous geochronological results obtained on Zabargad island and Al Lith and Tihama-Asir complexes (Saudi Arabia) suggest an Early Miocene age of emplacement for the Zabargad mantle diapir during the early opening of the Red Sea rift. In contrast, SmNd and RbSr internal isochrons yield Pan African dates for felsic and basic granulites collected 500–600 m from the contact zone with the peridotites. Devoid of evidence for retrograde metamorphic, minerals from a felsic granulite provide well-defined RbSr and SmNd dates of 655 ± 8 and 699 ± 34 Ma for the HP-HT metamorphic event (10 kbar, 850°C). The thermal event related to the diapir emplacement is not recorded in the SmNd and RbSr systems of the studied gneisses; in contrast, the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the RbSr isotopic system of the mafic granulite. The initial143Nd/144Nd ratio of the felsic granulite is higher than the contemporaneous value for CHUR and is in agreement with other Nd isotopic data for samples of upper crust from the Arabian shield. This result suggests that source rocks of the felsic granulite were derived at 1.0 to 1.2 Ga from either an average MORB-type mantle or a local 2.2 Ga LREE-depleted mantle. Zabargad gneisses represent a part of the disrupted lower continental crust of the Pan African Afro-Arabian shield. During early stages of the Red Sea rifting in the Miocene, these Precambrian granulites were intruded and dragged upwards by a rising peridotite diapir.  相似文献   

18.
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   

19.
The distinctly different, εNd(0) values of the Atlantic, Indian, and Pacific Oceans requires that the residence time of Nd in the ocean (i.e., τNd) be on the order of, or less than, the ocean mixing time of ∼ 500–1500 yr. However, estimates of τNd, based on river influxes, range from 4000 to 15,000 yr, thus exceeding the ocean mixing time. In order to reconcile the oceanic Nd budget and lower the residence time by roughly a factor of 10, an additional, as yet unidentified, and hence “missing Nd flux” to the ocean is necessary. Dissolution of materials deposited on continental margins has previously been proposed as a source of the missing flux. In this contribution, submarine groundwater discharge (SGD) is examined as a possible source of the missing Nd flux. Neodymium concentrations (n = 730) and εNd(0) values (n = 58) for groundwaters were obtained from the literature in order to establish representative groundwater values. Mean groundwater Nd concentrations and εNd(0) values were used along with recent estimates of the terrestrial (freshwater) component of SGD (6% of river discharge on a global basis) to test whether groundwater discharge to the coastal oceans could account for the missing flux. Employing mean Nd concentrations of the compiled data base (i.e., 31.8 nmol/kg for all 730 analyses and 11.3 nmol/kg for 141 groundwater samples from a coastal aquifer), the global, terrestrial-derived SGD flux of Nd is estimated to range between 2.9 × 107 and 8.1 × 107 mol/yr. These estimates are of the same order of magnitude, and within a factor of 2, of the missing Nd flux (i.e., 5.4 × 107 mol/yr). Applying the SGD Nd flux estimates, the global average εNd(0) of SGD is predicted to be − 9.1, which is similar to our estimate for the missing Nd flux (− 9.2), and in agreement with the mean (± S.D.) εNd(0) measured in groundwaters (i.e., εNd(0) = −8.9 ± 4.2). The similarities in the estimated SGD Nd flux and corresponding εNd(0) values to the magnitude and isotope composition of the missing Nd flux are compelling, and suggest that discharge of groundwater to the oceans could account for the missing Nd flux. Future investigations should focus on quantifying the Nd concentrations and isotope compositions of groundwater from coastal aquifers from a variety of coastal settings, as well as the important geochemical reactions that effect Nd concentrations in subterranean estuaries in order to better constrain contributions of SGD to the oceanic Nd budget.  相似文献   

20.
We report trace element, samarium (Sm)–neodymium (Nd) and lead (Pb) isotopic data for individual micro-and mesobands of the Earth's oldest Banded Iron Formation (BIF) from the ∼ 3.7–3.8 Ga Isua Greenstone Belt (IGB, West Greenland) in an attempt to contribute to the characterization of the depositional environment and to the understanding of depositional mechanisms of these earliest chemical sediments. Rare earth element (REE)-yttrium (Y) patterns of the individual mesobands show features of modern seawater with diagnostic cerium (Ce/Ce⁎), presodymium (Pr/Pr⁎) and Y/holmium (Ho) anomalies. Very low high field strength elements (HFSE) concentrations indicate essentially detritus-free precipitation. Uranogenic Pb isotope data define a correlation line with a slope of 3691 ± 41 Ma, indicating that the uranium (U)–lead (Pb) system remained closed after the formation of this BIF. High 207Pb/204Pb relative to 206Pb/204Pb ratios compared to average mantle growth evolution models are a feature shared by BIF, penecontemporaneous basalts and clastic volcanogenic metasediments and are indicative of the ultimate high-μ (238U/204Pb) character of the source region, an essentially mafic Hadean protocrust. Sm–Nd isotopic relations on a layer-by-layer basis point to two REE sources controlling the back-arc basin depositional environment of the BIF, one being seafloor-vented hydrothermal fluids (εNd (3.7 Ga) ∼ + 3.1), the other being ambient surface seawater which reached its composition by erosion of parts of the protocrustal landmass (εNd(3.7 Ga) ∼ + 1.6). The validity of two different and periodically interacting water masses (an essentially two-component mixing system) in the deposition of alternating iron- and silica-rich layers is also reflected by systematic trends in germanium (Ge)/silicon (Si) ratios. These suggest that significant amounts of silica were derived from unexposed and/or destroyed mafic Hadean landmass, unlike iron which probably originated from oceanic crust following hydrothermal alteration by deep percolating seawater. Ge/Si distributional patterns in the early Archean Isua BIF are similar to those reported from the Paleoproterozoic Hamersley (Western Australia) BIF, but overall Ge concentrations are about one order of magnitude higher in the Archean BIF. This seems consistent with other lines of evidence that the ambient Archean seawater was enriched with iron relative to Proterozoic and recent seawater.  相似文献   

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