Ingrid Raczek

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

Case studies on the origin of basalt: III. Petrogenesis of the Mauna Ulu eruption, Kilauea, 1969–1971

During the Mauna Ulu flank eruption on Kilauea, Hawaii, the concentrations in the lavas of the minor elements K, P, Na and Ti, and the incompatible trace elements (analyzed by isotope dilution) K, Rb, Cs, Ba, Sr, and the REE (except Yb) decreased monotonically and linearly with the time (or date) of the eruption. At the same time, the concentrations of the major elements and of Yb, and the ratios of K/Rb, K/Cs, Ba/Rb, 87Sr/86Sr and 143Nd/144Nd remained constant. Most of the scatter in the raw concentration data is removed by a simple correction for olivine (plus chromite) fractionation previously established by Wright et al. (1975). These results are explained by simple equilibrium partial melting of a uniform source. The degree of melting increased by about 20% of the initial value during the course of the eruption. The trace element data are inverted by the method originated by Minster and Allègre (1978) and simplified by Hofmann and Feigenson (1983). The source has the following element (or isotope) ratios: K/Rb=501±7, Ba/Rb=14.0±0.5, Rb/Cs=95±7, Rb/Sr=0.0193 (+0.0045, −0.0090), (Ce/Ba)CN= 1.1±0.1, (Sr/Ba)CN=1.19 (+0.30, −0.19), 87Sr/86Sr=0.703521±0.000016, and 143Nd/144Nd=0.512966±0.000008. The REE pattern of the source has a nearly flat or slightly negative slope (=relative LREE enrichment) between Ce and Dy and a strongly positive slope between Dy and Yb. However, this relative HREE enrichment is poorly constrained by the analytical data, is highly model dependent and may not be a true source feature. The Yb concentration in the source is particularly poorly constrained because it is essentially constant in the melts. On the other hand, this special feature demonstrates that Yb must be buffered by a mineral phase with a high partition coefficient for Yb, namely garnet. The calculated clinopyroxene/garnet ratio in the source is roughly equal to one. In contrast, the source of Kohala volcano had previously been found to contain little or no garnet.

Geochemistry of peridotites and mafic igneous rocks from the Central Dinaric Ophiolite Belt, Yugoslavia

In the Central Dinaric Ophiolite Belt (CDOB) peridotites and associated metamorphic rocks of various grades tectonically overlie an olistostrome melange of middle to late Jurassic age. Peridotites and underlying slices of mafic granulites (partially transformed to gamet amphibolites) are intruded by doleritic dikes which do not occur in the melange. The melange contains blocks of subgreywackes and cherts as well as those of pillow lavas and massive diabase (spilites). CDOB peridotites are in the spinel peridotite facies, but locally spinel-plagioclase peridotites occur as well. All peridotites have lherzolitic compositions showing several significant element correlations: Al2O3, CaO, TiO2, Na2O and Cu are negatively correlated and Ni is positively correlated with MgO. Recent estimates of primitive mantle compositions lie near the low-MgO end point of each correlation trend. Al/Ti and Ca/Al ratios of CDOB lherzolites are for the most part higher than the range observed in chondrites. However, when a few samples with extreme compositions are excluded, Al/Ti and Ca/Al are positively correlated with MgO, and the samples at the low-MgO end have near-chondritic Ca/Al but slightly higher than chondritic Al/Ti ratios. Chondrite-normalized REE patterns of CDOB lherzolites show extreme depletions in LREE providing strong evidence for the absence of any metasomatic renrichment. The lack of correlation between highly incompatible elements (LREE) and moderately incompatible elements (HREE, Ti, Na, Al, Ca) together with the extremely low La/Sm ratios suggest that fractional or very small increment melt removal played a role in the genesis of these lherzolites. Four out of five lherzolites yield and apparent Sm-Nd isochron age of 136±15 Ma with an ɛNb of 6.0±1.1 (bulk rocks and clinopyroxene separates). One sample has an exceptionally high ɛNd of about 23. The mafic igneous rocks scatter around the lower end of the 136 Ma reference isochron allowing, but not proving, a genetic relationship with a mantle having a Nd isotopic composition which is similar to that of CDOB lherzolites. LIL element abundances of spilites and doleritic dike rocks suggest some hydrothermal alteration. In primitive mantle-normalized concentration diagrams none of these mafic igneous rocks shows a significant negative Nb-Ta anomaly. Chondrite-normalized REE patterns of both rock types are essentially flat. Whereas the inferred primary compositions of the spilites compare well with those of E-type MORBs, the doleritic dike rocks show elemental ratios similar to those normally found in back-arc basin tholeiites.

UThRa systematics in Kilauea and Mauna Loa basalts, Hawaii

We describe 226Ra230Th238U (dis)equilibria and RbBaThULa concentration relationships in historical and prehistoric lavas from Kilauea and Mauna Loa. (226Ra230Th), (230Th232Th), Th/U, Ba/Rb and Ba/La ratios [but not (Rb,Ba,La)/(Th,U) ratios] are essentially identical in both volcanoes, whereas the absolute concentrations (after correction for olivine crystallization) differ by up to a factor of 2, in response to varying melt fractions. This shows that bulk partition coefficients of these elements are significantly smaller than melt fractions. Very small or absent 230Th238U disequilibrium implies very small or negligible magmatic fractionation between Th and U. 226Ra230Th disequilibria are significantly larger (∼ 20% excess 226Ra on average) but are also independent of melt fraction. The combination of significant RaTh fractionation together with small or absent ThU fractionation provides constraints on recently proposed models to explain U-series disequilibria during partial melting and melt extraction. Instantaneous melt extraction models are rejected: (a) because they are inconsistent with experimentally determined partition coefficients; and (b) more generally because they would require significant covariation of (226Ra230Th) with melt fraction. On the other hand, dynamic melting models involving slow fractional melting or melt infiltration within the garnet stability region, followed by rapid movement through the lithosphere, are consistent with the results and yield melt porosities between 10−3 and 10−2 for plume upwelling velocities of 1 m yr−1. In addition, we tentatively proposed alternative models for creating the Ra excesses in the magma. One such process involves the mobilization of Ra within the volcanic edifice, subsequent advection toward and redeposition within the roof region of the magma chamber, and finally incorporation into the magma itself. Another mechanism for incorporating excess Ra in the magma might be transport of very small amounts of carbonate fluids or carbonatite melts (containing very large excesses of 226Ra) into partially molten regions in the mantle. Given the currently available data and state of knowledge about magma extraction processes, there is no obvious preference for either the purely magmatic models or those involving “extraneous” fluids in the mantle or within the volcanic edifice.

Geochemistry, zircon ages and whole-rock Nd isotopic systematics for Palaeoproterozoic A-type granitoids in the northern part of the Delhi belt, Rajasthan, NW India: implications for late Palaeoproterozoic crustal evolution of the Aravalli craton

Determination of zircon ages as well as geochemical and Sm–Nd isotope systematics of granitoids in the Khetri Copper Belt of the Aravalli mountains, NW India, constrain the late Palaeoproterozoic crustal evolution of the Aravalli craton. The plutons are typical A-type within-plate granites, derived from melts generated in an extensional tectonic environment. They display REE and multi-element patterns characterized by steep LREE-enriched and almost flat HREE profiles and distinct negative anomalies for Sr, P and Ti. Initial e Nd values range from − 1.3 to − 6.2 and correspond to crustal sources with mean crustal residence ages of 2.5 to 2.1 Ga. A lower mafic crustal anatectic origin is envisaged for these granitoids, and the heterogeneous e Nd( t ) values are inferred to have been acquired from the magma source region. Zircon Pb–Pb evaporation and U–Pb ages indicate widespread rift-related A-type magmatism at 1711–1660 Ma in the northern Delhi belt and also suggest a discrete older magmatic event at around 1800 Ma. The emplacement ages of the compositionally distinct A-type granitoid plutons, and virtually coeval granulite metamorphism and exhumation in another segment of the Aravalli mountains, further signify that part of the Aravalli crust evolved during a widespread extensional event in late Palaeoproterozoic time.

Recent enrichment in the source region of arc magmas from Luzon island, Philippines: Sr and Nd isotopic evidence

Luzon, the Philippines, is wedged between east- and west-directed subducting slabs. Westward subduction has resulted in pre-Miocene igneous rocks from northern Luzon characterized by 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios typical of intra-oceanic arcs where sediment involvement is minor. In addition, Pliocene to Holocene volcanic eruptions of Sr and Nd isotopic signatures similar to those of the pre-Miocene rocks have developed along the Bicol arc in southern Luzon. These rocks are also associated with the westward subduction. In contrast, Pliocene to Holocene volcanoes have been generated by eastward subduction along the western coast of central Luzon (the Bataan arc in the north and the Mindoro arc in the south) and are significantly higher in radiogenic Sr and lower in radiogenic Nd than the rocks discussed above. The pre-Miocene and Bicol arc isotopic characteristics appear to reflect the mixing of small amounts of products from the Philippine Sea crust with the mantle wedge. Magmas erupted along the Bataan and Mindoro arcs may reflect that larger amounts of sediment are available for subduction in the South China Sea. Rocks containing the highest radiogenic Sr and lowest radiogenic Nd values so far noted in Philippine igneous rocks occur in the Mindoro arc and may have been contaminated by slivers of the continental Mindoro-North Palawan terrane, which apparently had been carried down to mantle depths after the collision of this terrane and the arc in the Miocene. These interpretations imply that Nd (and, by inference, other rare-earth elements) can migrate from the subducted slab to the mantle wedge. Thus, variable Nd isotopic ratios in island-arc magmas (e.g., Sunda and Lesser Antilles arcs) may not be a primary feature of the mantle wedge, but a consequence of variations in the release of Nd from the subducted slab.

Rb-Sr systematics of Permian volcanites in the Schwarzwald (SW-Germany): Part II: Age of eruption and the mechanism of Rb-Sr whole rock age distortions

AbstractFrom the quartz porphyry occurrences of Haigerach and Geisberg (Central Schwarzwald), which were dated by minerals and whole-rock samples, the whole-rock samples generally yielded lower ages than the minerals. This result agrees with other studies on apparently rejuvinated whole-rock ages of acid volcanic rocks (compare McKerrow et al. 1980). Petrographic and geochemical observations show that two processes disturbed the Rb-Sr relations:1)fast alterations immediately after extrusion, such as sericitization during autohydrothermal bleaching in the pipe area of the volcanoes, gain of Rb, addition of common Sr.2)loss of radiogenic 87Sr due to long-lasting exchange processes with groundwaters. The combined result of the two processes is a kind of isochron rotation simulating younger whole-rock ages and higher initial Sr isotope ratios. Our results on the two quartz porphyries indicate that whole-rock dating of such acid volcanics is not succesful.

Petrogenesis of the magmatic complex at Mount Ascutney, Vermont, USA

The Ascutney Mountain complex of eastern Vermont, USA, is a composite epizonal pluton of genetically related gabbro to granite intrusives. Nd isotopic data are reported for mafic rocks, granites, and nearby country rock. The parental mafic magma producing the complex 122 m.y. ago had 87Sr/86Sr=0.7039, 143Nd/144Nd=0.512678 (ɛNd=+3.8) and δ18O=6.1‰, indicating a mantle source with time-integrated lithophile element depletion. Uniform initial radiogenic isotope ratios for granites, which are undistinguishable from those for the most primitive gabbro, suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in δ18O, to about 7.8‰, is the result of fractional crystallization. Mafic rocks show a large range in initial 143Nd/144Nd ratio, from about 0.51267 to 0.51236 (ɛNd= +3.7 to −2.5), which is correlated with elevated 87Sr/86Sr ratios and δ18O. These data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50% crust. The local country rocks include gneiss and schist and assimilation involved representatives of both rock types. The isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement.The Precambrian gneisses of the Chester dome and overlying lower Paleozoic schists have essentially identical Nd isotope systematics which suggest a crustal formation age of about 1.6. b.y. The parental sediments for the schists were apparently derived from a protolith similar to the gneissic basement without appreciable Sm/Nd fractionation.