Strontium isotope geochemistry of late cretaceous granodiorites,Jamaica and Haiti,greater Antilles

Initial⁸⁷Sr/⁸⁶Sr ratios have been determined for a representative suite of Upper Cretaceous granodiorites and associated rocks from the Above Rocks composite stock in central Jamaica and the Terre-Neuve pluton in northwestern Haiti. The average initial⁸⁷Sr/⁸⁶Sr ratio for severn samples of the Terre-Neuve intrusion is 0.7036, with a range of 0.7026–0.7047. For two samples of the Above Rocks the initial ratios are 0.7033 and 0.7034. A third sample from this intrusive has an initial ratio of 0.7084, which is tenatively attributed to contamination. The initial⁸⁷Sr/⁸⁶Sr ratios indicate that neither ancient sialic crust nor sediments carried down a Benioff zone can be the primary source of the granodioritic magma. K/Rb ratios for these rocks range from 178 to 247, which are much lower than the average values (≥1000) for tholeiitic basalts. It is concluded that the magmas originated primarily by melting of downthrust oceanic crust or adjacent mantle material.     

Newly discovered Late Cretaceous adakites in South Fujian Province: Implications for the late Mesozoic tectonic evolution of Southeast China

The Yongchun pluton is a Late Cretaceous adakitic intrusion in South Fujian Province, Southeast China, with associated metal mineralization. An understanding of the Yongchun pluton is helpful in tectono‐magmatic evolutionary processes, and is important in explaining the origin of related porphyry‐type deposits. Zircons from three samples of the pluton were analyzed by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS), yielding U–Pb ages of 99.50 ±0.87 Ma, 97.74 ±0.59 Ma, and 99.65 ±0.92 Ma. These ages are similar to those of the Sifang, Luoboling, and Sukeng plutons, all of which are related to Cu–Pb–Zn–Mo mineralization within the study area. The Yongchun pluton comprises high‐potassium, calc‐alkaline, metaluminous rocks, with average A/CNK values of 0.91, ⁸⁷Sr/⁸⁶Sr ratios of 0.705 51 to 0.706 83, εNd(t) values of ?4.63 to ?5.90, and two‐stage Nd model (T_2DM) ages of 1.49–1.39 Ga, indicating the magmas were generated by partial melting of Mesoproterozoic continental crust mixed with mantle‐derived magmas. The pluton has geochemical characteristics typical of adakites, such as a high Sr content (average 553 ppm), and low Y (average 15.2 ppm) and Yb (average 1.61 ppm) contents, indicating that the parental magma was formed under high‐pressure conditions. The magmatism was associated with thickening of the lower crust during a change in subduction angle and convergence rate of the paleo‐Pacific Plate at 100 Ma. The compression was limited to South Fujian Province.     

An isotopic mixing model for the origin of granitic rocks in southeastern Australia

A simple mixing process was a significant element in the genesis of many southeastern Australian granitic rocks: major and trace element abundances are near-linearly correlated, Nd and Sr initial isotopic compositions define a simple hyperbolic trend, and many initial ⁸⁷Sr/⁸⁶Sr ratios are a regular function of Rb/Sr ratios. Geological arguments and regression of geochemical variation diagrams place limits on the nature of the end members. The high-Si end member is meta-sedimentary-derived magma comparable in composition to the Cooma Granodiorite, the one pluton in the region surrounded by, and clearly derived from, regional metamorphic rocks. The low-Si end member is basaltic material with similarities to rare gabbros also found in the area. Isotope dilution calculations based upon initial Sr and Nd isotopic compositions deduced for the end members yield realistic hypothetical Sr and Nd concentrations for the end members in a demonstration of the internal consistency of the model. All of the granitic rocks, from hornblende tonalites to cordierite granodiorites, are a single broad family, products of variable mixing between distinct batches of basaltic material and a uniform partial melt of the regional basement.     

Strontium isotope evidence for crustal contamination of calc-alkaline volcanic rocks from Cerro Galan,northwest Argentina

The Pampean Ranges of northwest Argentina are a basin-and-range tectonic province with a late Precambrian to Paleozoic basement and extensive Miocene-Recent calc-alkaline volcanism. The volcanoes include the large resurgent Cerro Galan caldera, and Recent scoria cones and lava flows. Miocene-Recent volcanic rocks of basalt to dacite composition from the Cerro Galan area exhibit a range of Rb/Sr ratios of 0.043–1.092 and initial⁸⁷Sr/⁸⁶Sr ratios of 0.7057–0.7115 with a clear positive correlation between⁸⁷Sr/⁸⁶Sr and⁸⁷Rb/⁸⁶Sr, indicating an apparent age of ca. 130 Ma. This relationship is interpreted to indicate that the Sr isotope variation in the Cerro Galan volcanic rocks results from mixing of a mantle-derived component with low⁸⁷Sr/⁸⁶Sr (<0.7057) and high Sr (>700 ppm) with a crustal component characterized by higher⁸⁷Sr/⁸⁶Sr (>0.7115) and lower Sr (<240 ppm). It is concluded that the mixing is best explained as a result of a small degree of selective crustal Sr contamination (ca. 10%) of a range of subsequently erupted magmas produced largely by fractional crystallization within the continental crust. We propose that the mantle-derived end-member is derived by partial melting of sub-Andean mantle with an⁸⁷Sr/⁸⁶Sr ratio of ca. 0.704, and that such an Sr isotope ratio characterizes the source region for calc-alkaline volcanic rocks throughout the Andes.     

Evaluation and application of the ion microprobe in the strontium isotope geochemistry of carbonates

A modified AEI-IM20 ion microprobe has been used to measure⁸⁷Sr/⁸⁶Sr ratios in carbonates. A suite of carbonates with varying major elements (Ca, Mg, Fe, Mn) was studied at low and high (M/ΔM ? 3000) mass resolution to determine the types and intensities of molecular species isobaric with Sr peaks; Sr data collected at low mass resolution must be corrected for Ca₂ and CaMgO species. Rb/Sr ratios are extremely low, and correction for⁸⁷Rb is not required (< 0.1‰ of⁸⁷Sr).Usable Sr isotopic data may be obtained from calcite givenSr≥ 400ppm, and for Sr > 5000 ppm a precision of ～ ± 1‰ (± 0.0007) in⁸⁷Sr/⁸⁶Sr (2σ mean) can be achieved under optimum conditions. The corrections for Ca₂ and CaMgO are smaller than the within-run precision in calcite, but in dolomite the correction for CaMgO is + 1.5%. Mass fractionation corrections to⁸⁷Sr/⁸⁶Sr (based on⁸⁶Sr/⁸⁸Sr= 0.1194) are typically +8 to +10‰. Good agreement between ion probe and solid source mass spectrometer results was found for calcites of known Sr isotopic composition: ST4a (～ 400ppm Sr), average ion probe⁸⁷Sr/⁸⁶Sr= 0.7267 ± 0.0015, solid source⁸⁷Sr/⁸⁶Sr= 0.72680 ± 0.00008 [14]; JCG36 (～ 6000ppm Sr), average ion probe⁸⁷Sr/⁸⁶Sr= 0.7056 ± 0.0009, solid source⁸⁷Sr/⁸⁶Sr= 0.70588 ± 0.00009 [16]; JCG44 (～ 6000ppm Sr), average ion probe⁸⁷Sr/⁸⁶Sr= 0.7057 ± 0.0006, solid source⁸⁷Sr/⁸⁶Sr= 0.70540 ± 0.00008 [16]. The ability of the ion microprobe to measure⁸⁷Sr/⁸⁶Sr for 10-μm spots in calcite was used: (1) to measure variation in⁸⁷Sr/⁸⁶Sr of ～ 0.01 on a centimetre scale in a hydrothermally altered basalt from the Isle of Skye, northwestern Scotland; and (2) to determine the Sr isotopic composition of tiny (< 35 μm) calcite grains in a veined mantle lherzolite from Bultfontein, South Africa. Because of calcite's ubiquitous occurrence in many parageneses this technique may offer many opportunities for the measurement of fine scale heterogeneities in⁸⁷Sr/⁸⁶Sr.     

Petrogenetic modeling of rock variety in the Khalkhab–Neshveh pluton,NW of Saveh,Iran

The Khalkhab–Neshveh (KN) pluton is a part of Urumieh–Dokhtar Magmatic Arc and was intruded into a covering of basalt and andesite of Eocene to early Miocene age. It is a medium to high‐K, metaluminous and I‐type pluton ranging in composition from quartz monzogabbro, through quartz monzodiorite, granodiorite, and granite. The KN rocks show subtle differentiation trends strongly controlled by clinopyroxene, plagioclase, hornblende, apatite, and titanite, where most major elements (except K₂O) are negatively correlated with SiO₂; and Al₂O₃, Na₂O, Sr, Eu, and Y define curvilinear trends. Considering three processes of magmatic differentiation including mixing and/or mingling between basaltic and dacitic magmas, gravitational fractional crystallization and in situ crystallization revealed that the latter is the most likely process for the evolution of KN magma. This is supported by the occurrence of all rock types at the same level, the lack of mafic enclaves in the granitoid rocks, the curvilinear trends of Na₂O, Sr, and Eu, and the constant ratios of (⁸⁷Sr/⁸⁶Sr)_i from quartz monzodiorite to granite (0.70475 and 0.70471, respectively). In situ crystallization took place via accumulation of plagioclase and clinopyroxene phenocrysts and concentration of these phases in the quartz monzogabbro and quartz monzodiorite at the margins of the intrusion at T ≥ 1050°C, and by filter pressing and fractionation of hornblende, plagioclase, and later biotite in the granitoids at T = ～880°C.     

Origin of Baotoudong syenites in North China Craton: Petrological,mineralogical and geochemical evidence

Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Triassic alkaline magmatic belt along the northern margin of the North China Craton(NCC). Zircon U-Pb age, petrological, mineralogical and geochemical data of the pluton were obtained in this paper, to constrain its origin and mantle source characteristics. The pluton is composed of nepheline-clinopyroxene syenite and alkali-feldspar syenite, with zircon U-Pb age of 214.7±1.1 Ma. Diopside(cores)-aegirine-augite(rims), biotite, orthoclase and nepheline are the major minerals. The Baotoudong syenites have high contents of rare earth elements(REE), and are characterized by enrichment in light rare earth elements(LREE) and large ion lithophile elements(LILE; e.g., Rb, Ba, Sr), depletion in heavy rare earth elements(HREE) and high field strength elements(HFSE). They show enriched Sr-Nd isotopic compositions with initial ~87Sr/~86Sr ranging from 0.7061 to 0.7067 and ε_Nd(t) values from –9.0 to –11.2. Mineralogy, petrology and geochemical studies show that the parental magma of the syenites is SiO_2-undersaturated potassic-ultrapotassic, and is characterized by high contents of Ca O, Fe_2O_3, K_2O, Na_2O and fluid compositions(H_2O), and by high temperature and high oxygen fugacity. The syenites were originated from a phlogopite-rich, enriched lithospheric mantle source in garnet-stable area(80 km). The occurrence of the Baotoudong syenites, together with many other ultrapotassic, alkaline complexes of similar ages on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids from the subducted, altered paleo-Mongolian oceanic crust, and the northern margin of the craton has entered into an extensively extensional regime as a destructive continental margin in Late Triassic.     

Sr fluxes and isotopic compositions of the eleven rivers originating from the Qinghai-Tibet Plateau and their contributions to <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr evolution of seawater

To evaluate influence of chemical weathering of the Qinghai-Tibet Plateau (QTP) on seawater ⁸⁷Sr/⁸⁶Sr variation, river water and sediment samples were collected, and their Sr concentrations and isotopic compositions analyzed, from the seven large rivers that originated from the QTP. By combining these with the data of the Ganges, Brahmaputra, Indus and Irrawaddy originated in the southern QTP, the total Sr flux of the eleven rivers reaches 3.47×10⁹ mol·a⁻¹, which accounts for 10.2% of the total Sr flux transported by the global rivers. The weighted mean ⁸⁷Sr/⁸⁶Sr is 0.71694, higher than the average value of the global rivers. The ⁸⁷Sr_ex (⁸⁷Sr flux in excess of the seawater ⁸⁷Sr/⁸⁶Sr ratio) of the Chinese seven rivers is 1.55×10⁶ mol·a⁻¹, only accounting for about 6% of the value of the eleven rivers originated from QTP, and the Ganges-Brahmaputra system accounts for 86%. We assume that the QTP rivers have no strontium contributions to the oceans before ∼40 Ma and the Sr fluxes of the global rivers, except the QTP eleven rivers, are constant, then a maximum linear increase in Sr fluxes of the QTP rivers from zero to the modern value in response to tectonic uplift can explain ∼69% increase of seawater ⁸⁷Sr/⁸⁶Sr over the past ∼40 Ma and the remainder of 31% is perhaps provided from other factors. Supported by National Natural Science Foundation of China (Grant Nos. 40473009, 40331001, 40873001)     

Rb–Sr isochron ages of the Cretaceous granitoids in the Ryoke belt, Kinki district, Southwest Japan

Abstract Granitoids are widely distributed in the Ryoke belt and have been divided into four main igneous stages based on their field setting. In this paper, we present Rb–Sr isochron ages for the younger Ryoke granitoids (second stage to fourth stage) in the Kinki district. The Yagyu granite (second stage) gave a Rb–Sr whole‐rock isochron age of 74.6 ± 10.9 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70938 ± 0.00016, and a Rb–Sr mineral isochron age of 71.8 ± 0.1 Ma. The Narukawa granite (second stage) yielded a Rb–Sr mineral isochron age of 79.5 ± 0.4 Ma. A Rb–Sr whole‐rock isochron age of 78.3 ± 3.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70764 ± 0.00014 was obtained for the Takijiri adamellite (third stage). The Katsuragi quartzdiorite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 85.1 ± 18.3 Ma (initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70728 ± 0.00006), and mineral isochron ages of 76.9 ± 0.5 Ma and 74.8 ± 0.5 Ma. The Minamikawachi granite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 72.8 ± 2.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70891 ± 0.00021. These age data indicate that the igneous activity in younger Ryoke granitoids of Kinki district occurred between 80 and 70 Ma, except for the Katsuragi quartz diorite. The isotopic data on the various igneous stages in Kinki district correspond with the relative timing from field observations. Based on the initial ⁸⁷Sr/⁸⁶Sr ratios, the granitoids of the Ryoke belt in Kinki district are spatially divided into two groups. One is granitoids with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.707–0.708, distributed in the southern part of the Ryoke belt. The other is granitoids with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.708–0.710 distributed in the northern part of the Ryoke belt. The initial ⁸⁷Sr/⁸⁶Sr ratios of granitoids increase with decreasing (becoming younger) Rb–Sr whole‐rock isochron ages.     

The strontium isotopic composition of seawater,and seawater-oceanic crust interaction

The⁸⁷Sr/⁸⁶Sr ratio of seawater strontium (0.7091) is less than the⁸⁷Sr/⁸⁶Sr ratio of dissolved strontium delivered to the oceans by continental run-off (～0.716). Isotope exchange with strontium isotopically lighter oceanic crust during hydrothermal convection within spreading oceanic ridges can explain this observation. In quantitative terms, the current⁸⁷Sr/⁸⁶Sr ratio of seawater (0.7091) may be maintained by balancing the continental run-off flux of strontium (0.59 × 10¹² g/yr) against a hydrothermal recirculation flux of 3.6 × 10¹² g/yr, during which the⁸⁷Sr/⁸⁶Sr ratio of seawater drops by 0.0011. A concomitant mean increase in the⁸⁷Sr/⁸⁶Sr ratio of the upper 4.5 km of oceanic crust of 0.0010 (0.7029–0.7039) should be produced. This required⁸⁷Sr enrichment has been observed in hydrothermally metamorphosed ophiolitic rocks from the Troodos Massif, Cyprus.The post-Upper Cretaceous increase in the strontium isotopic composition of seawater(～0.7075–0.7091) covaries smoothly with inferred increase in land area. This suggests that during this period the main factor which has caused variability in the⁸⁷Sr/⁸⁶Sr ratio of seawater strontium could have been variation in the magnitude of the continental run-off flux caused by variation in land area. Variations in land area may themselves have been partly a consequence of variations in global mean sea-floor spreading rate.     

Ancient lithospheric lherzolite xenolith in alkali basalt from Baja California

K, Rb, Sr contents and Sr isotopic composition are reported for (1) the coexisting silicate minerals of a spinel lherzolite xenolith, (2) the whole rock xenolith, and (3) the host alkali basalt from the Pleistocene-Recent San Quintin volcanic field in Baja California. The data also include major element chemistry of the four mineral phases of the xenolith. The olivine-spinel and pyroxene thermometers indicate that the last temperature of equilibration of the xenolith in the upper mantle was about 1100°C.K, Rb, and Sr abundances are extremely low in the minerals of the xenolith, in contrast with the general enrichment of Ca, Al, Fe and Na in the whole xenolith. Furthermore, the abundance levels and the⁸⁷Sr/⁸⁶Sr ratios of the minerals are greatly lowered by surface washing of the minerals in2N cold HCl for three minutes. It is suggested that this is due to grain boundary and surface contamination of the minerals which took place in the upper mantle by a vapor phase deposition, prior to the inclusion of the xenolith in the basalt. The source of the vapor phase must have a⁸⁷Sr/⁸⁶Sr ratio greater than 0.7070, the highest measured ratio in the unwashed orthopyroxene. Sr in the host alkali basalt has a⁸⁷Sr/⁸⁶Sr ratio of 0.7031, unrelated to the grain boundary material.The acid-washed minerals, the unwashed minerals and the acid-washed whole rock xenolith show a scatter on a⁸⁷Rb/⁸⁶Sr versus⁸⁷Sr/⁸⁶Sr diagram. However, the surface-washed minerals and the whole rock alone define a straight-line relationship with a positive slope, which corresponds to an age3.4 ± 0.3AE(2σ) for the rock and an initial⁸⁷Sr/⁸⁶Sr of0.70057 ± 0.0004 (2σ). The age of 3.4 AE for the lherzolite is interpreted as its last involvement in a small degree of partial melting and the consequent extreme depletion of the large ion lithophile elements from the constituent minerals.     

87Rb87Sr chronology of H chondrites: Constraint and speculations on the early evolution of their parent body

A precise⁸⁷Rb-⁸⁷Sr whole-rock isochron for H chondrites and an internal isochron for Tieschitz (H3) have been determined. The age and⁸⁷Sr/⁸⁶Sr initial ratio of the whole rocks are4.52 ± 0.05 b.y. and0.69876 ± 0.00040(λ(⁸⁷Rb) = 1.42 × 10^?11yr^?1). For Tieschitz, whereas handpicked separates plot on a well-defined line, heavy liquid separates scatter in the⁸⁷Rb/⁸⁶Sr vs.⁸⁷Sr/⁸⁶Sr diagram. Leaching experiments by heavy liquids indicate that they might have a sizeable effect on Tieschitz minerals. The age and⁸⁷Sr/⁸⁶Sr initial ratio as determined by handpicked separates are4.53 ± 0.06 b.y. and0.69880 ± 0.00020, indistinguishable from the whole-rock isochron.These results are interpreted as “primitive isochrons” dating the condensation of chondrites from the solar nebula. The best value of this event is given by joining both isochrons together at4.518 ± 0.026 b.y. and⁸⁷Sr/⁸⁶Sr= 0.69881 ± 0.00016. The near identity of this initial ratio with the one of Allende white inclusions argues in favor of a sharp isochronism of condensation from a⁸⁷Sr/⁸⁶Sr homogeneous nebula. Data from Guaren?a [11] and Richardton [48] are interpreted as secondary internal isochrons, 100 m.y. after the condensation of the whole rocks.The data are then used to constrain a thermal evolution model of the H chondrite parent body. This body might have a 150–175 km radius, and might have been heated by²⁶Al. An²⁶Al/²⁷Al ratio of 4–6 × 10^?6 is enough for heating such a body. Further tests for this model are proposed.     

Marine87Sr/86Sr ratios from the Jurassic to Pleistocene: evidence from groundwaters in Israel

Oceanic⁸⁷Sr/⁸⁶Sr ratios during Jurassic to Pleistocene have been determined by analysing fresh waters from marine limestone and dolomite aquifers. The results are in good agreement with published data from well preserved fossil material. The⁸⁷Sr/⁸⁶Sr ratios obtained are 0.7070 for Lower to Middle Jurassic, 0.7075 for Late Cretaceous, 0.7080 for Lower to Middle Eocene and 0.7087 for Pleistocene aquifer waters. The value of⁸⁷Sr/⁸⁶Sr for the Eimer and Amend isotopic standard was 0.7082.     

Early Cretaceous I‐type granites in the southwest Fujian Province: new constraints on the late Mesozoic tectonic evolution of southeast China

Zircons from two samples of the Sukeng pluton in the southwest Fujian Province, China, were analyzed by LA–ICP–MS with the aim of determining the timing of formation. The zircons from the two samples yield similar U–Pb ages of 100.47 ± 0.42 and 102.46 ± 0.69 Ma, indicating that the Sufeng pluton is contemporaneous with the Sifang and Luoboling plutons, all of which are also related to Cu–Au–Pb–Zn–Mo mineralization within the study area. All three plutons have geochemical features of I‐type granites, are high‐ to mid‐K calc‐alkaline metaluminous rocks, and have average molar Al₂O₃/ (CaO+Na₂O+K₂O) values of 0.95, initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70465–0.70841, εNd(t) values at 101 Ma from –1.72 to –7.26, and two‐stage Nd model ages (T_2DM) from 1.16 to 1.60 Ga. Zircons within these plutons have εHf(t) values at 101 Ma from –3.5 to 6.25 and T_2DM ages from 0.74 to 1.46 Ga, suggesting these I‐type granites formed from magmas generated by partial melting of Mesoproterozoic to Neoproterozoic continental crust that mixed with mantle‐derived magmas. The magmatism was associated with thickening of the lower crust caused by collisions between microcontinents in the Cathaysian Block, which were driven by Early Cretaceous subduction of the Pacific Plate.     

Geochemical and Sr–Nd–Pb isotopic investigation of Triassic granitoids and basement rocks in the northern Gyeongsang Basin, Korea: Implications for the young basement in the East Asian continental margin

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/⁸⁶Sr_initial = 0.7041 ~ 0.7050, ?_Nd(t) = 2.3 ~ 4.0, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 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/⁸⁶Sr_initial = 0.7047 ~ 0.7065, ?_Nd(t) = 3.9 ~ 2.8, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 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.     

Numerical models for diagenesis and the Neogene Sr isotopic evolution of seawater from DSDP Site 590B

A numerical model for the diagenetic exchange of Sr between carbonates and their pore fluids during sedimentation and compaction has been developed. The model has been applied to data from DSDP Site 590B in order to assess the accuracy with which the Sr isotope record in the carbonate sediment reflects that of seawater. The most important process affecting the Sr in the solid carbonate is exchange with the pore fluid due to solution-reprecipitation, but the concentration or isotopic composition of Sr in the solid itself gives little or no information as to the magnitude of this exchange. The key to determining the rate of exchange is the pore fluid, where the variations of Sr²⁺ and⁸⁷Sr/⁸⁶Sr with depth are very sensitive indicators. The logical structure of applying the model to data from DSDP 590B is to find by successive iteration an ocean history (i.e., the initial⁸⁷Sr/⁸⁶Sr and Sr concentration of each increment of carbonate deposited) and a rate of Sr exchange between pore water and solid carbonate such that the model matches the present Sr concentration and⁸⁷Sr/⁸⁶Sr of both pore water and solid carbonate.Once all the data are matched, the model provides an estimate of the rate of Sr exchange due to solution-reprecipitation and the evolution of⁸⁷Sr/⁸⁶Sr in seawater over the past 20 million years. For DSDP 590B we find that solution-reprecipitation decreases rapidly with depth, from a near surface value of about 10% per million years to about 1% per million years below 200 m. This rate of exchange of Sr results in the carbonates of DSDP 590B preserving an accurate record of the Sr isotopic evolution of the ocean over the past 5 million years, but for ages greater than 5 million years the⁸⁷Sr/⁸⁶Sr ratio of the carbonate is systematically displaced from that of the seawater in which it was deposited. The maximum difference is of order 5 × 10⁻⁵.     

Chemical and isotopic study of extraterrestrial particles from the ocean floor

We report chemical, mineralogic and Rb-Sr data on deep-sea spherules and on particles from an Antarctic Ocean core in which an excess Ir content has been identified.⁸⁷Sr/⁸⁶Sr compositions in the deep-sea spherules are determined to 1–2‰ and are in the range 0.730–0.757. The⁸⁷Sr/⁸⁶Sr compositions and the Sr concentrations are in the range observed for the majority of chondritic meteorites.⁸⁴Sr/⁸⁸Sr ratios are normal to within 1%. Extreme depletion of Rb relative to the chondritic abundance is found in the deep-sea spherules. These data support the inference based on chemical composition and mineralogy that the deep-sea spherules are produced by the ablation or heating of meteoroids in the Earth's atmosphere with substantial loss of Rb by volatilization. Most terrestrial sources for the deep-sea spherules can be excluded, based on the chemical composition and on the Sr isotopic composition. The results on vesicular, Ir-rich particles from the Antarctic Ocean core give⁸⁷Sr/⁸⁶Sr in the range 0.703–0.705 and within the range observed for ocean island basalts but significantly above mid-ocean ridge basalts (MORB). A crystalline basaltic particle from this core shows non-radiogenic⁸⁷Sr/⁸⁶Sr= 0.701 ± 0.001, in the range observed for MORB and basaltic achondrites. The Sr data on the vesicular particles do not provide positive support for an extraterrestrial provenance for these materials. The basaltic particles cannot reasonably be the primary source of the high Ir concentration and some other lithic component remains to be identified.     

“Mantle” Rb/Sr and 87Sr/86Sr ratios for clinopyroxenes from Norwegian garnet peridotites and pyroxenites

Clinopyroxenes separated from garnetiferous ultramafic rocks in the core zone of the Norwegian Caledonides have rubidium concentrations of 0.008 to 0.064 ppm, strontium concentrations of 23.5 to 421 ppm, and ⁸⁷Sr/⁸⁶Sr ratios of 0.7011 to 0.7029. The very low Rb/Sr ratios of the clinopyroxenes (less than 0.0004) suggest that their ⁸⁷Sr/⁸⁶Sr values have not varied significantly over geologic time and may approximate the initial ⁸⁷Sr/⁸⁶Sr of the eclogite-facies ultramafic mineral assemblages at their time of formation. The ultramafic rocks occur in a basement complex that yields Rb-Sr whole-rock and U-Pb zircon ages of about 1800 m.y. Garnetiferous ultramafic rocks are apparently lacking in younger (Sveconorwegian or Caledonian) sialic sequences, raising the possibility that the eclogite-facies metamorphism may have occurred at least 1800 m.y. ago. The Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios of the clinopyroxenes are as predicted for the ancient upper mantle under most evolutionary models. However, the data do not preclude the possibility that the eclogite-facies metamorphism occurred in the crust. The garnetiferous ultramafic rocks are generally enclosed by large volumes of dunite which could have shielded the eclogite-facies assemblages from contamination by fluids from the country rock during metamorphism.     

Strontium and neodymium isotopic evidence for the heterogeneous nature and development of the mantle beneath Afar (Ethiopia)

Neodymium isotope and REE analyses of recent volcanic rocks and spinel lherzolite nodules from the Afar area are reported. The¹⁴³Nd/¹⁴⁴Nd ratios of the volcanic rocks range from 0.51286 to 0.51304, similar to the range recorded from Iceland. However, the⁸⁷Sr/⁸⁶Sr ratios display a distinctly greater range (0.70328–0.70410) than those reported from the primitive rocks of Iceland. Whole rock samples and mineral separates from the spinel lherzolite nodules exhibit uniform¹⁴³Nd/¹⁴⁴Nd ratios (ca. 0.5129) but varied⁸⁷Sr/⁸⁶Sr ratios in the range 0.70427–0.70528.The SrNd isotope variations suggest that the volcanic rocks may have been produced by mixing between two reservoirs with distinct isotopic compositions. Two possible magma reservoirs in this area are the source which produced the “MORB-type” volcanics in the Red Sea and Gulf of Aden and the anomalous source represented by the nodule suite. The isotopic composition of the volcanics is compatible with mixing between these two reservoirs.It is shown that the anomalous source with a high⁸⁷Sr/⁸⁶Sr ratio cannot have been produced by simple processes of partial melting and mixing within normal mantle. Instead the high⁸⁷Sr/⁸⁶Sr is equated with a fluid phase. A primitive cognate fluid, subducted seawater or altered oceanic lithosphere may have been responsible for the generation of the source with a high⁸⁷Sr/⁸⁶Sr ratio.     

87Sr/86Sr ratios for basalt from Loihi Seamount,Hawaii

⁸⁷Sr/⁸⁶Sr ratios of 15 samples of basalt dredged from Loihi Seamount range from 0.70334 to 0.70368. The basalt types range from tholeiite to basanite in composition and can be divided into six groups on the basis of abundances of K₂O, Na₂O, Rb and Sr and ⁸⁷Sr/⁸⁶Sr ratio. The isotopic data require that the various basalt types be derived from source regions differing in Sr isotopic composition. The Loihi basalts may be produced by mixing of isotopically distinct sources, but the tholeiites and alkalic basalts from Loihi do not show a well-developed inverse trend between Rb/Sr and ⁸⁷Sr/⁸⁶Sr that is characteristic of the later stages of Hawaiian volcanoes such as Haleakala and Koolau.