Richard Markey

Highly precise Re-Os dating for molybdenite using alkaline fusion and NTIMS

The technique described in this paper represents the modification and combination of two previously existing methods, alkaline fusion and negative thermal ion mass spectrometry (NTIMS). We have used this technique to analyze repeatedly a homogeneous molybdenite powder used as a reference standard in our laboratory. Analyses were made over a period of 18 months, using four different calibrations of two different spike solutions. The age of this standard reproduces at a level of +/-0.13%. Each individual age analysis carries an uncertainty of about 0.4% that includes the uncertainty in the decay constant for (187)Re. This new level of resolution has allowed us to recognize real differences in ages for two grain-size populations of molybdenite from some Archean samples.

Subgrain-scale decoupling of Re and 187Os and assessment of laser ablation ICP-MS spot dating in molybdenite

Abstract Reproducibility of Re-Os molybdenite ages depends on sample size and homogeneity, suggesting that Re and Os are decoupled within individual molybdenite crystals and do not remain spatially linked over time. In order to investigate the Re-Os systematics of molybdenite at the subgrain (micron) scale, we report LA-ICP-MS Re-Os ages for an Archean molybdenite crystal from Aittojarvi, Finland, analyzed in situ in a white aplite matrix. A related Aittojarvi molybdenite (A996D), in the form of a very fine-grained mineral separate, is used as one of our in-house NTIMS standards, and thus its age of 2760 ± 9 Ma is well established. Measurements of ( 187 Re + 187 Os)/ 185 Re on micron scale spots along 200 μm traverses across the crystal yield a wide range of ages demonstrating that, in this case, microsampling of molybdenite does not produce geologically meaningful ages. Experimentation with mineral separations and sample size over a 7-yr period predicted that this would be the outcome. We suggest that 187 Os is more likely to be the mobile species, based on its charge and ionic radius, and that 187 Os becomes decoupled from parent 187 Re with time on the micron and larger scale. Incompatible charge and ionic radius for Os ions formed during reduction of molybdenite-forming fluids may explain the widely observed absence of common (initial) Os in molybdenite. Geologically accurate ages for molybdenite can only be obtained for fully homogenized crystals (or crystal aggregates) so that any post-crystallization 187 Re- 187 Os decoupling is overcome. A growing number of geologically accurate ID-NTIMS 187 Re- 187 Os ages for homogenized molybdenite suggest that postcrystallization mobility of radiogenic 187 Os must be limited to within the molybdenite mineral phase. We suggest that radiogenic 187 Os may be stored in micron scale dislocations, kink bands, and delamination cracks produced by deformation, and that the unusual structure and deformation response of molybdenite results in an increased chemical stability in this mineral. Migration of 187 Os into adjacent silicate phases is highly unlikely, but other contacting sulfides may take in Os. In an example from a Proterozoic skarn deposit at Pitkaranta (western Russia), we demonstrate minor loss of radiogenic 187 Os from molybdenite and a corresponding gain in adjacent chalcopyrite such that the molybdenite age is not perceptibly disturbed, whereas the resulting chalcopyrite ages are impossibly old. Therefore, it is unadvisable to perform Re-Os analytical work on any sulfide in contact or intimate association with molybdenite. In addition to large errors in the age, if the isochron method is employed, initial 187 Os/ 188 Os ratios could be erroneously high, leading to seriously errant genetic interpretations.

A double spike for osmium analysis of highly radiogenic samples

Abstract Geologic samples containing highly radiogenic Os (molybdenites and low-level, highly radiogenic (LLHR) samples) have no internal means by which to correct for mass fractionation during isotopic measurement by mass spectrometry. We describe a double spike for use with highly radiogenic samples, created by combining isotopically enriched 188 Os and 190 Os. Spiking molybdenite and other highly radiogenic minerals with this tracer allows for a fractionation correction, as well as a more reliable determination of common Os relative to analysis using single spikes. The precise isotopic composition of the double spike is determined by a calibration against natural Os, in which two separate measurements are necessary: one each for the pure double spike and the spike–standard mixture. An estimate of the true composition of the spike is obtained by least squares approximation, and the errors are obtained by Monte Carlo methods. Sample analyses are then much more straightforward than the calibration because isotopic compositions of all components are known a priori. Results obtained with a mixed Re-double Os spike demonstrate an improved reproducibility over individual 185 Re and 190 Os spikes. For an Archean in-house molybdenite standard we now observe a reproducibility of 0.08%. The ability to make a fractionation correction is essential for Os measurements made by ion counting. With the double Os spike, young samples and those with low Re contents (i.e., LLHR) can now be accurately analyzed. The 188 Os– 190 Os double spike also allows a determination of the common Os contents of highly radiogenic samples. Common Os is poorly determined for ancient samples with high concentrations of 187 Os, which fortunately are not sensitive to estimates of common Os. Common Os can be reasonably well determined for younger samples and those with low Re contents. We report a common Os concentration of 0.4±0.1 ppb for an 11 Ma molybdenite. Consideration of common Os content is important for age determination of young samples and LLHR samples, and is not possible by other published means of Os analysis.

Re-Os ages for auriferous sulfides from the gold deposits in the Kaczawa Mountains (SW Poland)

A Re-Os six-point isochron age of 317±17 Ma has been obtained for auriferous sulfides from sheeted quartz-veins representing the first stage of ore precipitation at the Radzimowice Au-As-Cu deposit. The age uncertainty is dominated by the fact that sulfides contain very little common Os. Extremely low Re concentrations for gold-bearing sulfides from another gold district, Klecza-Radomice, permitted dating of only one Co-arsenopyrite sample G-4 from Klecza—analyzed as a Low Level Highly Ra diogenic (LLHR) sample. A precise age of 316.6±0.4 Ma was obtained. Re-Os ages indicate gold mineralization in Late Namurian time associated with post-collisional extension and regional uplift in a continental arc setting. Opening of deep-seated structures, marked also by the presence of lamprophyre dykes, may have allowed for the migration of post-magmatic mineralizing fluids from various magmatic sources at about 317 Ma.