Re os dating sulfides
Sample preparation time is, in general, low compared to the amount of chemical purification necessary for a TIMS analysis, and is limited to the amount of time necessary to cut and polish a thick (ca.
1) play a crucial role in the distribution of Re, Os, and Pb in mantle rocks and are thus fundamental to obtaining absolute ages by direct geochronology using the Re–Os and Pb–Pb isotope systems on mantle samples. The focus on basalts is not only because of a lack of mantle rocks exposed at the Earth’s surface. Moreover, in the last few years, combined Pb–Pb and Re–Os isotope studies of peridotite-hosted sulfides have shed new light on mantle geochronology (Burton et al. Whole-rock Re–Os analyses of mantle-derived peridotites almost inevitably reflect the mixing of different generations of BMS because they have experienced multiple generations of processes.
Significant sample preparation time is also saved as a single polished section may contain several 10s of grains that would otherwise have to be prepared separately for analysis by TIMS.
However, care must be taken in the preparation of sulfides for laser ablation analysis as it is possible to inadvertently pluck some phases from the BMS grains.
Mantle samples exist as hundreds of exposures of peridotites, pyroxenites and diamonds, either brought to the surface as accidental xenoliths and xenocrysts during kimberlitic or alkali basaltic volcanism (for comprehensive reviews, see Pearson et al. 2016, this volume; Luguet and Reisberg 2016, this volume), or as orogenic, ophiolitic and abyssal peridotite obducted at convergent margins and drilled / dredged from oceanic basins (e.g., Bodinier and Godard 2014; Becker and Dale 2016, this volume). Problems arise in analyzing mantle samples because of the mobility during melting of the incompatible trace elements upon which early geochronological measurements relied (e.g., Rb–Sr, Sm–Nd, U–Pb, Th–Pb, Lu–Hf isotope systems). The movement of Os during melting, recrystallization, or metasomatism may occur independently of Re and there can be separate addition of Re (e.g., Alard et al. 2002, 2004), making the interpretation of bulk-rock Re–Os isotope data ambiguous in some samples, in the context of the timing of melt depletion (see reviews in Reisberg et al. For example, in the study of BMS inclusions in the Western Gneiss region of Norway, Beyer et al.
This chapter reviews the occurrence of BMS in mantle samples and the role that they play in controlling the Re–Os and Pb isotope systematics of the mantle. Mantle rocks exposed at the Earth’s surface typically undergo melting (i) as a result of adiabatic decompression, such as beneath a mid ocean ridge, continental rift, or in a mantle plume, and/or (ii) by lowering the solidus with CO or water such as when fluid fluxes into the mantle wedge at convergent margins, or carbonated peridotite generates carbonatite and kimberlite. (2004) identified at least two generations of BMS with model ages that ranged from 4.0 to Os in peridotitic residues often can be adequately explained by ancient melt depletion, using bulk-rock Re–Os isotope systematics indiscriminately can have its drawbacks (Reisberg et al. Like the standard caveats in other radiogenic isotopic systems, the Re–Os geochronology of bulk-rock peridotites has two built-in weaknesses: (i) mixing ages can be generated that are not geologically meaningful model ages, and (ii) potential parent (Re)–daughter (Os) mobility either directly or in sulfide liquids at deep-mantle temperatures lead to problems in interpreting Re–Os isotopic composition of sulfides as an absolute age.