Oxygen isotope signature of phosphate (apatite), calcite, and quartz: Applications to metamorphism, igneous processes, surface processes, and early life.

摘要

The phosphate (PO43-) anion is integral to life on earth, found primarily in mineral form as apatite (Ca5(PO 4,CO3)3(OH,C1,F,CO3). At surface temperatures, the phosphate anion is highly resistant to exchange of oxygen with surrounding minerals and fluids in the absence of metabolic activity. Depending upon the degree of phosphate oxygen exchange under conditions of elevated temperature and fluid infiltration, phosphate oxygen isotope signature may prove to be a valuable source tracer and/or biomarker.; In the first part of this dissertation, oxygen isotope analyses of phosphate, calcite, and quartz are used to evaluate the relative resistance of phosphate to oxygen isotope exchange at low to medium grade metamorphism. Metamorphic rock samples analyzed represent temperatures from ∼390°C to ∼560°C (from zeolite to upper amphibolite grades) as well as conditions of chemical alteration of the host rock. Carbonatite samples were also analyzed to determine the effect of igneous and metasomatic processes on phosphate oxygen isotope signature.; In the metamorphic rock samples analyzed, the difference (Delta 18Op-cc) between phosphate oxygen isotope values (delta 18Op) and those of calcite (delta18O cc) exhibits no trend with respect to temperature below ∼530°C, and very poorly defined trend above ∼530°C. In contrast, although the difference (Delta18Op-qtz) between delta 18Op and quartz oxygen isotope values (delta18 Oqtz) exhibits no clear trend with respect to temperature below ∼530°C, above 530°C, Delta18Op-qtz values closely agree with those obtained from equilibrium fractionation equations for corresponding temperatures. These results suggest that unlike calcite and quartz, phosphate is resistant to oxygen isotope exchange with coexisting fluid and mineral phases up to medium grade metamorphism (∼530°C).; The delta18Op values for apatite from igneous carbonatite samples are uniform, irrespective of chemical alteration of [+2.6 to +3.8‰], while [delta18Ocalcite values for the same samples vary widely [+7.6‰ (unaltered igneous) to +16.8‰ (chemically altered)]. These findings suggest that while chemical alteration largely affects calcite delta18O, it does not affect apatite delta 18Op.; Upon having established the extent of phosphate's resistance to oxygen exchange, the second part of this dissertation examines the use of phosphate oxygen isotope signature as a source tracer in two exploratory chapters. The studies detailed in chapters 3 and 4 demonstrate the significant potential of utilizing phosphate oxygen isotope signature as a source tracer and/or biomarker.; Chapter 3 examines the phosphate and carbonate oxygen isotope signature of Tertiary and Quaternary tufas (Mojave, USA) and Archaean (South Africa; Death Valley, USA; and Australia) and modern stromatolites (Sarmiento Lake, Chile). The delta18Op and delta18O c values for Precambrian marine stromatolite samples range from 10.5 to 11.4‰ and from to, respectively. The difference (Delta 18Op-c) between delta18Op values and (delta18Oc) values of the Precambrian marine stromatolites [4.2 to 11.1‰] likely reflects chemical and subsequent isotopic alteration of carbonate over time. The Delta18O p-c values of the modern non-marine Sarmiento Lake stromatolite samples [12.1 to 16.1‰] may reflect to a process heretofore unidentified which yielded a wider range of delta18Op values [10.8 to 16.3‰] than that of the delta18Oc values [28.4 to 28.8‰].; Samples of Tertiary and Quaternary tufas of the Mojave area yielded a narrow range of delta18Op values [14.4 to 18.6 ‰], and a wider range of 8180c values [24.2 to 33.9‰]. The difference (Delta18Op-c) between delta18O p values and (delta18Oc) values [-9.1 to -16.1‰] is linked to disparate temperatures and evaporation/precipitation of water at each locality.; For comparison, one Archaean sample was analyzed from the