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.

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Oxygen isotope composition of the Phanerozoic ocean and a possible solution to the dolomite problem

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1719681115 ◽

2018 ◽

Vol 115 (26) ◽

pp. 6602-6607 ◽

Cited By ~ 28

Author(s):

Uri Ryb ◽

John M. Eiler

Keyword(s):

Oxygen Isotope ◽

Isotope Exchange ◽

Isotope Composition ◽

Temporal Variations ◽

Oxygen Isotope Exchange ◽

Diagenetic Alteration ◽

Isotope Studies ◽

Dolomite Problem ◽

Clumped Isotope

The18O/16O of calcite fossils increased by ∼8‰ between the Cambrian and present. It has long been controversial whether this change reflects evolution in the δ18O of seawater, or a decrease in ocean temperatures, or greater extents of diagenesis of older strata. Here, we present measurements of the oxygen and ‟clumped” isotope compositions of Phanerozoic dolomites and compare these data with published oxygen isotope studies of carbonate rocks. We show that the δ18O values of dolomites and calcite fossils of similar age overlap one another, suggesting they are controlled by similar processes. Clumped isotope measurements of Cambrian to Pleistocene dolomites imply crystallization temperatures of 15–158 °C and parent waters having δ18OVSMOWvalues from −2 to +12‰. These data are consistent with dolomitization through sediment/rock reaction with seawater and diagenetically modified seawater, over timescales of 100 My, and suggest that, like dolomite, temporal variations of the calcite fossil δ18O record are largely driven by diagenetic alteration. We find no evidence that Phanerozoic seawater was significantly lower in δ18O than preglacial Cenozoic seawater. Thus, the fluxes of oxygen–isotope exchange associated with weathering and hydrothermal alteration reactions have remained stable throughout the Phanerozoic, despite major tectonic, climatic and biologic perturbations. This stability implies that a long-term feedback exists between the global rates of seafloor spreading and weathering. We note that massive dolomites have crystallized in pre-Cenozoic units at temperatures >40 °C. Since Cenozoic platforms generally have not reached such conditions, their thermal immaturity could explain their paucity of dolomites.

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

SOIL ◽

10.5194/soil-7-145-2021 ◽

2021 ◽

Vol 7 (1) ◽

pp. 145-159

Author(s):

Sam P. Jones ◽

Aurore Kaisermann ◽

Jérôme Ogée ◽

Steven Wohl ◽

Alexander W. Cheesman ◽

...

Keyword(s):

Carbon Dioxide ◽

Microbial Biomass ◽

Oxygen Isotope ◽

Isotope Exchange ◽

Isotope Composition ◽

Oxygen Isotope Composition ◽

Mass Budget ◽

Oxygen Isotope Exchange ◽

Soil Communities ◽

Atmospheric Mass

Abstract. 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.

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