Morphological ripening of fluid inclusions and coupled zone-refining in quartz crystals revealed by cathodoluminescence imaging: Implications for CL-petrography, fluid inclusion analysis and trace-element geothermometry

Cathodoluminescence (CL) studies have previously shown that some secondary fluid inclusions in luminescent quartz are surrounded by dark, non-luminescent patches, resulting from fracture-sealing by late, trace-element-poor quartz. This finding has led to the tacit generalization that all dark CL patches indicate influx of low temperature, late-stage fluids. In this study we have examined natural and synthetic hydrothermal quartz crystals using CL imaging supplemented by in-situ elemental analysis. The results lead us to propose that all natural, liquid-water-bearing inclusions in quartz, whether trapped on former crystal growth surfaces (i.e., of primary origin) or in healed fractures (i.e., of pseudosecondary or secondary origin), are surrounded by three-dimensional, non-luminescent patches. Cross-cutting relations show that the patches form after entrapment of the fluid inclusions and therefore they are not diagnostic of the timing of fluid entrapment. Instead, the dark patches reveal the mechanism by which fluid inclusions spontaneously approach morphological equilibrium and purify their host quartz over geological time. Fluid inclusions that contain solvent water perpetually dissolve and reprecipitate their walls, gradually adopting low-energy euhedral and equant shapes. Defects in the host quartz constitute solubility gradients that drive physical migration of the inclusions over distances of tens of μm (commonly) up to several mm (rarely). Inclusions thus sequester from their walls any trace elements (e.g., Li, Al, Na, Ti) present in excess of equilibrium concentrations, thereby chemically purifying their host crystals in a process analogous to industrial zone refining. Non-luminescent patches of quartz are left in their wake. Fluid inclusions that contain no liquid water as solvent (e.g., inclusions of low-density H2O vapor or other non-aqueous volatiles) do not undergo this process and therefore do not migrate, do not modify their shapes with time, and are not associated with dark-CL zone-refined patches. This new understanding has implications for the interpretation of solids within fluid inclusions (e.g., Ti- and Al-minerals) and for the elemental analysis of hydrothermal and metamorphic quartz and its fluid inclusions by microbeam methods such as LA-ICPMS and SIMS. As Ti is a common trace element in quartz, its sequestration by fluid inclusions and its depletion in zone-refined patches impacts on applications of the Ti-in-quartz geothermometer.

Studies on gene expression within germinal centers

During T cell dependent immune responses, the acquisition of B cell memory from naïve cells takes place within a highly specialized microenvironment: The germinal centers (GC) of the secondary lymphoid organs. The GC reaction is a tightly regulated process in which the balance between survival and death is mediated by signals transduced through ligation of critical costimulatory molecules such as CD40 and CD154. While most cognate receptor-ligand interactions occur between T-cells and antigen (Ag)-presenting cells (APC) such as B-cells, evidence of homotypic B cell interactions has emerged. Despite the progress in our understanding of the reaction, several questions remain: (1) What determines the concomitant expression of CD40 and its ligand CD154 by GC B-cells? (2) Which molecules are responsible for inducing GC-B cell survival? and (3) how can cognate T-cell help be recruited into the organized structure of GCs? ^ Because the expression of costimulatory and survival molecules is predominant at distinct Ag-dependent maturation stages, we hypothesized the existence of stage specific gene expression responsible for the regulation of the GC reaction. Our studies reveal several novel genes whose expression may be critical for the GC reaction. The discovery of AKNA reveals the mechanism behind homotypic B cell CD40 and CD40 ligand interactions, which can explain the costimulatory signaling to GC B cells in the absence of T cells. Additionally, the identification of the pro-survival molecule PRELI may provide a novel mechanism for the survival of Ag-selected B cells. We propose that PRELI and its phylogenic homologues represent a novel family of proteins responsible for the protection of cells against caspase-independent apoptosis. Furthermore, we show that GC B cells actively participate in the recruitment of T cells through the secretion of CC and CxC chemokines, thus supporting their mutual involvement in cognate interactions. ^

Stable oxygen isotopes of calcite veins in ODP Site 146-892

In situ secondary ionization mass spectrometry (SIMS) analyses of oxygen isotopes in authigenic calcite veins were obtained from an active thrust fault system drilled at Ocean Drilling Program (ODP) Site 892 (44°40.4'N, 125°07.1'W) along the Cascadia subduction margin. The average d18OPDB value of all samples is -9.9 per mil and the values are the lowest of any measured in active accretionary prisms. Ranges in individual veins can be as much as 19.6 per mil. There is an isotopic stratigraphy related to the structural stratigraphy. Mean isotope values in the hanging wall, thrust, and footwall are -14.4 per mil, -9.5 per mil, and -5.2 per mil, respectively. Several veins and crosscutting vein sequences show a general trend from lower to higher d18O values over time. Isotopic and textural data indicate several veins formed by a crack-seal mechanism and growth into open fractures. The best explanation for the strong 18O depletions is periodic rapid flow from 2-3 km deeper in the prism. Relatively narrow isotopic ranges for most veins suggest that fluids were derived from a similar source depth for each episode of fluid pulse and calcite crystallization. Structural and mass balance considerations are consistent with a record preserved in the veins of ten to hundreds of thousands of years. The fluid pulses may relate to periodic large earthquake events such as those recognized in the paleoseismicity records from the Cascadia margin.

(Table 2) Chemical composition of quenched glasses from the Mohns and Knipovich Ridges

The aim of this project was a petrogeochemical study of igneous rocks in the areas of the Mohns and Knipovich Ridges, both being the northern extensions of the Mid-Atlantic Ridge (MAR), using data available for quenching glass samples collected during Cruises 36 and 38 of R/V Akademic Mstislav Keldysh and during Cruise 15 of R/V Professor Logachev. Results of igneous rock studying from the Mohns and Knipovich Ridges at the background of evolution of the total North Atlantic Province, which had been identified earlier from tectonic and geophysical data, showed that igneous rocks of the Knipovich Ridge can be ranked as shallow tholeiites, primary melts of which were relatively rich in Na and Si and poor in Fe. This type of magma is characteristic of colder regions of the oceanic lithosphere. Its occurrence in the Knipovich Ridge and its potential propagation up to the Gakkel Ridge suggest that igneous rocks of this region originated under conditions of passive spreading in contrast to the MAR region in vicinity of Iceland and Azores, where substantial contribution of hotter material of a rising plume contributed to formation of the oceanic crust. The North Atlantic Ocean is the youngest province in terms of ocean-floor opening. Geologically and geophysically it is one of well studied regions of the World Ocean. Nevertheless some basic key items of its origin still remain to be clarified. In 1975 Scatler et al. proved specifics of this region manifested in growth of the gravity field, and also in relative height of the ocean floor in the region of 33-70°N, which was associated by them with rise of the hotter mantle, as compared with common regions of the Mid-Atlantic Ridge. Later this view was confirmed by character of magmatism, which differed in depth of generation and by melting degree of the resulting primary magma. Uniqueness of the North Atlantic region was also proved by the fact that this region was marked by extensive geochemical anomalies associated with Azores, Iceland, and Jan Mayen. All of these data allow to consider the northern part of the MAR (north of 33°N) as an united global geotectonic province. The Mohns and Knipovich Ridges located north of Iceland locate at the northern end of this province. This is the least known region. Therefore, new data for ridge areas of 73-77°N are needed for more complete geologic history of the Arctic Basin. The aim of this study was to carry out a complex comparison of magmatism at the Mohns and Knipovich Ridges with magmatism at large segments of the MAR northern province and to reconstruct mechanisms of primary magma formation, as well as conditions of their fractionation. This paper was based on results of studying quenched glasses, which reflect evolution of melt in the course of its formation.

Isotopic geochemistry of granitoids in the Jinshajiang suture zone, SW China

The Jinshajiang suture zone, located in the eastern part of the Tethyan tectonic domain, is noticeable for a large-scale distribution of Late Jurassic to Triassic granitoids. These granitoids were genetically related to the evolution of the Paleo-Tethys Ocean. The Beiwu, Linong and Lunong granitoids occur in the middle zone of the Jinshajiang Suture Zone, and possess similar geochemical features, indicating they share a common magma source. SIMS zircon U-Pb dating reveals the Beiwu, Linong and Lunong granitic intrusions were emplaced at 233.9±1.4 Ma (2 sigma), 233.1 ±1.4 Ma (2 sigma) and 231.0±1.6 Ma (2 sigma), respectively. All of these granitoids are enriched in abundances of Si (SiO2 =65.2-73.5 wt.%), and large-ion-lithophile-elements (LILEs), but depleted in high-field-strength-elements contents (HFSEs, e.g., Nb, Ta, Ti). In addition, they have low P2O5 contents (0.06-0.11 wt.%), A/CNK values ([molecular Al2O3/(CaO+Na2O+K2O)], mostly<1.1) and 10000Ga/Al ratios (1.7-2.2), consistent with the characteristics of I-type granites. In terms of isotopic compositions, these granitoids have high initial 87Sr/86Sr ratios (0.7078-0.7148), Pb isotopic compositions [(206Pb/204Pb)t=18.213-18.598, (207Pb/204Pb)t=15.637-15.730 and (208Pb/204Pb)t=38.323-38.791], zircon d18O values (7. per mil-9.3 per mil) and negative eNd(t) values (-5.1 to -6.7), suggesting they were predominantly derived from the continental crust. Their Nb/Ta ratios (average value=8.6) are consistent with those of the lower continental crust (LCC). However, variable ?Hf(t) values (-8.6 to +2.8) and the occurrences of mafic microgranular enclaves (MMEs) suggest that mantle-derived melts and lower crustal magmas were involved in the generation of these granitoids. Moreover, the high Pb isotopic ratios and elevated zircon d18O values of these rocks indicate a significant contribution of the upper crustal composition. We propose a model in which the Beiwu, Linong and Lunong granitoids were generated under a late collisional or post-collisional setting. It is possible that this collision was completed before Late Triassic. Decompression induced mantle-derived magmas underplated and provided the heat for the anatexis of the crust. Hybrid melts including mantle-derived and the lower crustal magmas were then generated. The hybrid melts thereafter ascended to a shallow depth and resulted in some degree of sedimentary rocks assimilation. Such three-component mixing magmas source and subsequent fractional crystallization could be responsible for the formation of the Beiwu, Linong and Lunong granitoids.

Chemical composition of clinopyroxenes and melt inclusions in clinopyroxenes from rocks of the Malaita Island, Ontong Java Plateau (Pacific Ocean)

The paper is based on new results of melt inclusion studies in minerals. Physicochemical and geochemical parameters of plateau basalt magmatic systems of the Siberian Platform and Ontong Java Plateau (Pacific Ocean) have been established. The studied melts are enriched in Fe. That differs them from magmatic melts of mid-ocean ridges (MOR). A comparative analysis of data on inclusions has shown a similarity of continental and oceanic plateau basalt magmatic systems. They considerably differ from those of MOR and intraplate oceanic islands. Crystallization of oceanic plateau basalts took place at lower temperatures and pressures as compared with similar rocks of the Siberian Platform. The data on inclusions evidence that the melts of the Siberian Platform and the Malaita Island underwent a serious evolution in contrast to magmas of the Nauru Basin that have more stable geochemical parameters. The most fractionated low-temperature high-Fe magmas with elevated contents of trace and rare-earth elements occur in the Malaita Island (Ontong Java Plateau) magmatic system.