O-Sr isotopic variations in Miocene granitoids from the Aegean: evidence for an origin by combined assimilation and fractional crystallization

Sr, O, and D/H isotopic compositions have been analyzed in Miocene metaluminous to slightly peraluminous (I-type) granitoids of the central Aegean. Individual plutonic complexes display significant variations in their ¹⁸O and initial⁸⁷Sr/⁸⁶Sr compositions.D and ¹⁸O compositions of minerals and whole-rocks are mostly in the magmatic range. Some samples from Naxos and Mykonos/Delos show low D and ¹⁸O values characteristic of meteoric-water-hydrothermal interaction, but as a whole the changes in ¹⁸O and Sr isotopic compositions as a result of hydrothermal alteration were slight, even in instances where marked alteration is petrographically observable. Consequently, the bulk-rock variations of ¹⁸O from 8.1 to 12.0 and of⁸⁷Sr/⁸⁶Sr from 0.70438 to 0.71450 may be regarded as primary and indicative of the conditions of their evolution. Heterogeneous isotopic compositions observed in the individual plutons of Serifos, Ikaria, Samos and Kos may be caused by the multiple intrusion of chemically and isotopically distinct magma pulses, with high viscosities and relatively rapid consolidation in most cases preventing complete homogenization. The granitoids of Serifos, Ikaria and Kos display weak correlations between the initial⁸⁷Sr/⁸⁶Sr and ¹⁸O and 1/Sr. The granitoid province shows a positive correlation between⁸⁷Sr/⁸⁶Sr and ¹⁸O and a non-linear relationship between⁸⁷Sr/⁸⁶Sr and 1/Sr, whereby 1/Sr increases more rapidly than the isotopic ratio as the degree of fractionation of the rocks increases. It is argued that assimilation of older continental material by mantle-derived arc magmas with combined fractionation (AFC) is the most plausible model to explain the chemical and isotopic characteristics of the granitoids and the geological situation in which rock-types trend from granodiorites in the (south)west, near the inferred Oligocene-Miocene suture, to granites in the center and monzonites in the (north)east of the province.     

Rotation effects on measurements of lateral motion

Inertial transducers that measure horizontal movement are affected by rotation of the measurement location. In this paper, the effect of rotation is expressed quantitatively as a fraction of the translational signal amplitude and is shown to depend inversely on the effective radius of rotation of the measurement location and the square of the frequency component of the signal. Correction procedures are presented for various cases that arise in the determination of modal amplitudes and mode shapes for tower structures, suspension bridges and frame building structures. Specific numerical results are included for the CN Tower and the Commerce Court Building in Toronto, Canada, and the Lions' Gate Suspension Bridge in Vancouver, British Columbia. Rotational effects on signals obtained from horizontal transducers can be significant for very low frequency tower structures and suspension bridges, but are not likely to be of importance for frame buildings.     

The influence of cosmic-ray production on extinct nuclide systems

Variations in the atomic abundances of ⁵³Cr, ⁹²Zr, ⁹⁸Ru, ⁹⁹Ru, and ¹⁸²W in meteorites and lunar samples relative to terrestrial values may imply the early decay of radioactive ⁵³Mn, ⁹²Nb, ⁹⁸Tc, ⁹⁹Tc and ¹⁸²Hf, respectively. From this one can deduce nucleosynthetic sites and early solar system timescales. Because these effects are very small, production and consumption of the respective isotopes by cosmic-ray interactions is a concern. It has recently been demonstrated that ¹⁸²W production by neutron capture reactions on ¹⁸¹Ta is crucial for most lunar samples (Leya et al., 2000a). In this study the neutron fluence of each sample was estimated from its nominal cosmic-ray exposure age as deduced from noble gas data. This approach overestimates the true cosmogenic isotopic shift for samples that might have been irradiated very close to the regolith surface. Here we therefore combine our model calculations with the neutron dose proxies ¹⁵⁷Gd/¹⁵⁸Gd and ¹⁴⁹Sm/¹⁵⁰Sm. This allows us to accurately correct the measured W isotopic data for cosmic-ray induced shifts without the explicit knowledge of the exposure age or the shielding depth of the sample simply by measuring ¹⁵⁷Gd/¹⁵⁸Gd and/or ¹⁴⁹Sm/¹⁵⁰Sm in an aliquot. In addition we present new model results for the GCR-induced effects on ⁵³Mn-⁵³Cr, ⁹²Nb-⁹²Zr and ⁹⁸Tc-⁹⁹Tc-⁹⁸Ru-⁹⁹Ru. For each of these systems, except Tc-Ru, a proper cosmic-ray dose proxy is given, permitting the accurate correction of measured isotopic ratios for cosmogenic contributions.     

Sodic Pyroxene and Sodic Amphibole as Potential Reference Materials for In Situ Lithium Isotope Determinations by SIMS

Two large pegmatitic crystals of sodic pyroxene (aegirine) and sodic amphibole (arfvedsonite) from the agpaitic igneous Ilímaussaq Complex, south Greenland were found to be suitable as reference materials for in situ Li isotope determinations. Lithium concentrations determined by SIMS and micro‐drilled material analysed by MC‐ICP‐MS generally agreed within analytical uncertainty. The arfvedsonite crystal was homogeneous with [Li] = 639 ± 51 μg g^?1 (2s, n = 69, MC‐ICP‐MS and SIMS results). The aegirine crystal shows strongly developed sector zoning, which is a common feature of aegirines. Using qualitative element mapping techniques (EPMA), the homogeneous core of the crystal was easily distinguished from the outermost sectors of the crystals. The core had a mean [Li] of 47.6 ± 3.6 μg g^?1 (2s, n = 33) as determined by SIMS. The seven micro‐drilled regions measured by solution MC‐ICP‐MS returned slightly lower concentrations (41–46 μg g^?1), but still overlap with the SIMS data within uncertainty. Based on MC‐ICP‐MS and SIMS analyses, the variation in δ⁷Li was about 1‰ in each of the two crystals, which is smaller than that in widely used glass reference materials, making these two samples suitable to serve as reference materials. There was, however, a significant offset between the results of MC‐ICP‐MS and SIMS. The latter deviated from the MC‐ICP‐MS results by ?6.0 ± 1.9‰ (2s) for the amphibole and by ?3.9 ± 1.9‰ (2s) for the aegirine. This indicates the presence of a significant matrix effect in SIMS determinations of Li isotopes for amphibole and pyroxene relative to the basalt glasses used for calibration. Based on the MC‐ICP‐MS results, mean δ⁷Li values of +0.7 ± 1.2‰ (2s, n = 10) for the arfvedsonite crystal and of ?3.7 ± 1.2‰ (2s, n = 7) for the core of the aegirine crystal were calculated. Adopting these values, SIMS users can correct for the specific IMF (instrumental mass fractionation) of the ion probe used. We propose that these two crystals serve as reference materials for in situ Li isotope determinations by SIMS and pieces of these two crystals are available from the first author upon request.     

The enhanced element enrichment in the supercritical states of granite–pegmatite systems

In this paper, we show that supercritical fluids have a greater significance in the generation of pegmatites,and for ore-forming processes related to granites than is usually assumed. We show that the supercritical melt or fluid is a silicate phase in which volatiles; principally H_2O are completely miscible in all proportions at magmatic temperatures and pressures. This phase evolves from felsic melts and changes into hydrothermal fluids, and its unique properties are particularly important in sequestering and concentrating low abundance elements, such as metals. In our past research, we have focused on processes observed at upper crustal levels, however extensive work by us and other researchers have demonstrated that supercritical melt/fluids should be abundant in melting zones at deep-crustal levels too. We propose that these fluids may provide a connecting link between lower and upper crustal magmas,and a highly efficient transport mechanism for usually melt incompatible elements. In this paper, we explore the unique features of this fluid which allow the partitioning of variouselements and compounds, potentially up to extreme levels,and may explain various features both of mineralization and the magmas that produced them.     

东印度洋近20 ka以来的生物硅记录及其古生产力意义

通过对东印度洋钻孔CJ01-185的生物硅来探讨东印度洋古生产力的变化和古气候演化的响应。CJ01-185钻孔的生物硅含量在末次冰期最低,为0.86%;而到了全新世晚期生物硅含量达到1.89%。全新世晚期生物硅的堆积速率明显大于末次冰期。随着全新世海平面的快速上升,巽他海峡贯通,来自爪哇海的陆源物质输入到东印度洋,导致全新世的生物硅含量和生物硅的堆积速率增加。研究表明：巽他海峡贯通前,研究区的古生产力主要受东南季风变化和上升流的活动影响;而巽他海峡贯通后,古生产力很明显受到陆源物质输入的控制,东南季风变化和上升流的影响较弱。