Showing papers by "Eizo Nakamura published in 2011"

Multistage melt/fluid-peridotite interactions in the refertilized lithospheric mantle beneath the North China Craton: constraints from the Li–Sr–Nd isotopic disequilibrium between minerals of peridotite xenoliths

Abstract: Elemental and Li-Sr-Nd isotopic data of minerals in spinel peridotites hosted by Cenozoic basalts allow us to refine the existing models for Li isotopic frac- tionation in mantle peridotites and constrain the melt/fluid- peridotite interaction in the lithospheric mantle beneath the North China Craton. Highly elevated Li concentrations in cpx (up to 24 ppm) relative to coexisting opx and olivine (\4 ppm) indicate that the peridotites experienced metasomatism by mafic silicate melts and/or fluids. The mineral d 7 Li vary greatly, with olivine (?0.7 to ?5.4%) being isotopically heavier than coexisting opx (-4.4 to -25.9%) and cpx (-3.3 to -21.4%) in most samples. The d 7 Li in pyroxenes are considerably lower than the normal mantle values and show negative correlation with their Li abundances, likely due to recent Li ingress attended by diffusive fractionation of Li isotopes. Two exceptional samples have olivine d 7 Li of -3.0 and -7.9%, indicating the existence of low d 7 Li domains in the mantle, which could be transient and gener- ated by meter-scale diffusion of Li during melt/fluid-peri- dotite interaction. The 143 Nd/ 144 Nd (0.5123-0.5139) and 87 Sr/ 86 Sr (0.7018-0.7062) in the pyroxenes also show a large variation, in which the cpx are apparently lower in 87 Sr/ 86 Sr and slightly higher in 143 Nd/ 144 Nd than coexisting opx, implying an intermineral Sr-Nd isotopic disequilib- rium. This is observed more apparently in peridotites having low 87 Sr/ 86 Sr and high 143 Nd/ 144 Nd ratios than in those with high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd, suggesting that a rela- tively recent interaction existed between an ancient me- tasomatized lithospheric mantle and asthenospheric melt, which transformed the refractory peridotites with highly radiogenic Sr and unradiogenic Nd isotopic compositions to the fertile lherzolites with unradiogenic Sr and radiogenic Nd isotopic compositions. Therefore, we argue that the litho- spheric mantle represented by the peridotites has been het- erogeneously refertilized by multistage melt/fluid-peridotite interactions.

Provenance and reconnaissance study of detrital zircons of the Palaeozoic Cape Supergroup in South Africa: revealing the interaction of the Kalahari and Río de la Plata cratons

Abstract: In order to facilitate the understanding of the geological evolution of the Kalahari Craton and its relation to South America, the provenance of the first large-scale cratonic cover sequence of the craton, namely the Ordovician to Carboniferous Cape Supergroup was studied through geochemical analyses of the siliciclastics, and age determinations of detrital zircon. The Cape Supergroup comprises mainly quartz-arenites and a Hirnantian tillite in the basal Table Mountain Group, subgreywackes and mudrocks in the overlying Bokkeveld Group, while siltstones, interbedded shales and quartz-arenites are typical for the Witteberg Group at the top of the Cape Supergroup. Palaeocurrent analyses indicate transport of sediment mainly from northerly directions, off the interior of the Kalahari Craton with subordinate transport from a westerly source in the southwestern part of the basin near Cape Town. Geochemical provenance data suggest mainly sources from passive to active continental margin settings. The reconnaissance study of detrital zircons reveals a major contribution of Mesoproterozoic sources throughout the basin, reflecting the dominance of the Namaqua-Natal Metamorphic Belt, situated immediately north of the preserved strata of Cape Supergroup, as a source with Archaean-aged zircons being extremely rare. We interpret the Namaqua-Natal Metamorphic Belt to have been a large morphological divide at the time of deposition of the Cape Supergroup that prevented input of detrital zircons from the interior early Archaean Kaapvaal cratonic block of the Kalahari Craton. Neoproterozoic and Cambrian zircons are abundant and reflect the basement geology of the outcrops of Cape strata. Exposures close to Cape Town must have received sediment from a cratonic fragment that was situated off the Kalahari Craton to the west and that has subsequently drifted away. This cratonic fragment predominantly supplied Meso- to Neoproterozoic, and Cambrian-aged zircon grains in addition to minor Silurian to Lower Devonian zircons and very rare Archaean (2.5 Ga) and late Palaeoproterozoic (1.8-2.0 Ga) ones. No Siluro-Devonian source has yet been identified on the Kalahari Craton, but there are indications for such a source in southern Patagonia. Palaeozoic successions in eastern Argentina carry a similar detrital zircon population to that found here, including evidence of a Silurian to Lower Devonian magmatic event. The Kalahari and Rio de la Plata Cratons were thus in all likelihood in close proximity until at least the Carboniferous.

In situ ion-microprobe determination of trace element partition coefficients for hornblende, plagioclase, orthopyroxene, and apatite in equilibrium with natural rhyolitic glass, Little Glass Mountain Rhyolite, California

Abstract: Partially crystalline hornblende gabbro inclusions from the Little Glass Mountain Rhyolite contain euhedral plagioclase, orthopyroxene, hornblende, and apatite crystals in contact with interstitial rhyolitic (71–76% SiO2) glass. Textural and mineral compositional data indicate that the gabbros crystallized sufficiently slowly that surface equilibrium was closely approached at the interface between crystals and the liquid. This rare occurrence represents a natural dynamic crystallization experiment with a “run time” that is not realistically achievable in the laboratory. SIMS analysis of mineral rim-glass pairs have permitted the determination of high-quality, equilibrium trace-element partition coefficients for all four minerals. These data augment the limited partition coefficient database for minerals in high-SiO2 rhyolitic systems. For all minerals, the D values are consistent with those anticipated from crystal-chemical considerations. These data further support a liquid SiO2 control on the REEs (and presumably other elements) partitioning wherein D values systematically increase with increasing liquid SiO2 content.

Simultaneous Determination of Cd, In, Tl and Bi by Isotope Dilution-Internal Standardisation ICP-QMS with Corrections Using Externally Measured MoO+/Mo+ Ratios

Abstract: A method for the simultaneous determination of Cd with In, Tl and Bi by isotope dilution-internal standardisation (ID-IS) ICP-QMS using the same aliquot for rare earth element and other trace element determinations was developed. Samples mixed with an enriched 149Sm spike were decomposed using a HF-HClO4 mixture, which was evaporated and then diluted with HNO3. After determination of Sm by ID-ICP-QMS and Cd, In, Tl and Bi concentrations were determined using the 149Sm intensity as an internal standard. The interference of MoO+ on Cd+ was corrected using the MoO+/Mo+ ratio separately measured using a Mo standard solution, and the validity of the externally determined oxide-forming ratio correction was evaluated. The MoO+/Mo+ ratios measured using the standard solution and samples were ∼ 0.0002 and < 0.002, respectively. Detection limits for Cd, In, Tl and Bi in silicate samples were at levels of < 1 ng g−1 with a total uncertainty of < 7%. Cadmium in the carbonaceous chondrites, Orgueil (CI1), Murchison (CM2) and Allende (CV3) as well as Cd, In, Tl and Bi in the reference materials, JB-2, JB-3, JA-1, JA-2, JA-3, JP-1 (GSJ), BHVO-1, AGV-1, PCC-1 and DTS-1 (USGS) and NIST SRM 610, 612, 614 and 616 were determined to show the applicability of this method. Une methode pour la determination simultanee du Cd avec l’In, le Tl et le Bi par dilution isotopique a standardisation interne (ID-IS) ICP-QMS utilisant le meme aliquot pour la determination des terres rares et d’autres elements traces a ete developpe. Les echantillons melanges avec un ajout (« spike ») enrichi en 149Sm ont ete decomposes en utilisant un melange HF-HClO4, qui a eteevapore et, enfin, dilue avec HNO3. Apres la determination du Sm par ID-ICP-QMS, les concentrations en Cd, In, Tl et Bi ont ete determinees en utilisant l’intensite du 149Sm comme standard interne. L’interference de MoO+ sur Cd+ a ete corrige en utilisant le rapport MoO+/Mo+ mesure separement a l’aide d’une solution de Mo standard, et la validite de cette correction a eteevalue. Les rapports MoO+/Mo+ mesures a l’aide des solutions standards et des echantillons ont ete respectivement d’environ 0.0002 et inferieurs a 0.002. Les limites de detection pour le Cd, l’In, le Tl et le Bi dans les echantillons de silicate ont etea des niveaux inferieurs a 1 ng g−1 avec une incertitude totale inferieure a 7%. Le cadmium dans les chondrites carbonees : Orgueil (CI1), Murchison (CM2) et Allende (CV3) ainsi que le Cd, l’In, le Tl et le Bi dans les materiaux de reference : JB-2, JB-3, JA-1, JA-2, JA-3, JP-1 (GSJ), BHVO-1, AGV-1, PCC-1, DTS-1 (USGS) et NIST SRM 610, 612, 614 et 616 ont ete determines afin de demontrer l’applicabilite de cette methode.