Influence of Host Marble Rocks on the Formation of Intrusive Alkaline Rocks and Carbonatites of Sangilen (E. Siberia, Russia)

The study of the O and C isotope composition of calcite from nepheline syenites, ijolites and carbonatites of the Chik intrusion and the intrusions of the Erzin–Tarbagatay group of Sangilen (Eastern Siberia, Russia) showed derivation from alkaline melts enriched with a carbonate component from the host marbleized sedimentary rocks. The calculations showed that about 40% of the initial mass of carbonates involved in the interaction with silicate melts have remained after decarbonation. During the assimilation of the carbonate, an oxygen isotope exchange took place between the residual carbonate material and the silicate phase. Crystallization products of such hybrid magmas are carbonatite veins, calcite-rich nepheline rocks and their pegmatites with a calcite core.

Oxygen isotope exchange between water and carbon dioxide in soils is controlled by pH, nitrate and microbial biomass through links to carbonic anhydrase activity

The oxygen isotope composition of atmospheric carbon dioxide (CO2) is intimately linked to large-scale variations in the cycling of CO2 and water across the Earth's surface. Understanding the role the biosphere plays in modifying the oxygen isotope composition of atmospheric CO2 is particularly important as this isotopic tracer has the potential to constrain estimates of important processes such as gross primary production at large scales. However, constraining the atmospheric mass budget for the oxygen isotope composition of CO2 also requires that we understand better the contribution of soil communities and how they influence the rate of oxygen isotope exchange between soil water and CO2 (kiso) across a wide range of soil types and climatic zones. As the carbonic anhydrases (CAs) group of enzymes enhances the rate of CO2 hydration within the water-filled pore spaces of soils, it is important to develop understanding of how environmental drivers can impact kiso through changes in their activity. Here we estimate kiso and measure associated soil properties in laboratory incubation experiments using 44 soils sampled from sites across western Eurasia and north-eastern Australia. Observed values for kiso always exceeded theoretically derived uncatalysed rates, indicating a significant influence of CAs on the variability of kiso across the soils studied. We identify soil pH as the principal source of variation, with greater kiso under alkaline conditions suggesting that shifts in microbial community composition or intra–extra-cellular dissolved inorganic carbon gradients induce the expression of more or higher activity forms of CAs. We also show for the first time in soils that the presence of nitrate under naturally acidic conditions reduces kiso, potentially reflecting a direct or indirect inhibition of CAs. This effect appears to be supported by a supplementary ammonium nitrate fertilisation experiment conducted on a subset of the soils. Greater microbial biomass also increased kiso under a given set of chemical conditions, highlighting a putative link between CA expression and the abundance of soil microbes. These data provide the most extensive analysis of spatial variations in soil kiso to date and indicate the key soil trait datasets required to predict variations in kiso at large spatial scales, a necessary next step to constrain the important role of soil communities in the atmospheric mass budget of the oxygen isotope composition of CO2.

Application of combined fluid-inclusion and clumped isotope thermometry to biogenic and inorganic carbonates

<p>Biogenic and inorganic carbonates are widely used to reconstruct past temperatures and fluid compositions. For decades, calcification temperatures have been inferred from oxygen isotope composition (δ<sup>18</sup>O) of calcium carbonates (CaCO­<sub>3</sub>) assuming the δ<sup>18</sup>O of the parental fluid and isotopic equilibrium precipitation conditions. The development of the clumped isotope (Δ<sub>47</sub>) thermometer allows for reconstructing equilibrium calcification temperatures without requiring a priori knowledge of the water δ<sup>18</sup>O values.</p><p>Carbonate minerals can also contain several weight percentages of water, which are typically trapped within microscopic pores. These fluid-inclusions may preserve remnants of the parental fluid, which can be analyzed for the δ<sup>18</sup>O as well as hydrogen isotopic (δ<sup>2</sup>H) composition. Subsequently, the δ<sup>18</sup>O of fluid-inclusion and host carbonate may allow for the determination of paleotemperatures by providing constraint on the δ<sup>18</sup>O water value.</p><p>Reasonable equilibrium temperatures can be obtained for speleothem calcites from cave systems. On the contrary, anomalously high temperatures are derived from δ<sup>18</sup>O fluid-inclusion and calcite pairs in soil carbonates possibly suggesting diffusion of trapped water from host CaCO<sub>3</sub>. Deeply-buried and subsequently exhumed (inorganic) calcite veins have yielded discrepant paleotemperature estimates between fluid-inclusion and Δ<sub>47</sub> thermometers. The distinctly lower fluid-inclusion derived temperatures might be attributed to kinetic fraction during initial vein cementation and/or isotopic re-equilibration between fluid-inclusions and CaCO<sub>3</sub> at lower temperatures during uplift.</p><p>Heating experiments demonstrate that the oxygen isotope exchange between fluid inclusions and host carbonate is limited for inorganic calcite and aragonite at high temperatures (175<sup>o</sup>C) for short timescales (90 minutes). In contrast, considerable positive shifts in the δ<sup>18</sup>O of fluid inclusions have been recorded in biogenic aragonites during experimental heating, which coincide with lower carbonate δ<sup>18</sup>O values (albeit to a lesser extent due to the overwhelming amount of oxygen in the CaCO<sub>3</sub>), indicative of re-equilibration between host carbonate and pore fluids. This effect leads to apparently high equilibrium temperatures. In conjunction, the Δ<sub>47</sub> derived temperatures do not change significantly after heating of inorganic aragonite, whereas a considerable higher Δ<sub>47</sub> temperature is derived from aragonitic bivalve samples after heating. The positive shift in both thermometers has interpreted to reflect re-crystallization of CaCO<sub>3</sub> and isotopic re-equilibration between the host carbonate and fluid-inclusions. This exchange might be facilitated by extremely small fluid-inclusions present in biogenic carbonates and/or water associated with organic substances.</p><p>Importantly, these isotopic exchange processes in biogenic aragonites took place in the absence of an external fluid and below the temperature thresholds for solid-state-reordering and the aragonite-to-calcite transition. The novel application of combined fluid-inclusion and clumped isotope thermometry has a proven utility in determining equilibrium precipitation temperatures, monitoring preservation of the primary fluid-inclusions and re-crystallization processes during diagenesis. However, additional experiments and analytical improvements are needed to further constrain the diagenetic behavior of this proxy.</p><p> </p>