Experimental alteration of molybdenite: Evaluation of the Re-Os system, infrared spectroscopic profile and polytype

Experiments have been carried out to clarify the effect of alteration on Re -Os system, near infrared (NIR)-infrared (IR) spectroscopic characteristics and polytype of a natural molybdenite mineral (MoS2). The molybdenite samp le was placed in H2O and various media of 0.1 mol/L NaCl, NaHCO3, CaCl2, an d AlCl3 solutions, and heated in a sealed quartz tube at a temperature of 1 80 degrees C for 20 d. The unaltered and altered samples were subsequently used for analysis of Re and Os, NIR microscopic observation. and NIR-IR spe ctroscopy, and microfocus X-ray diffraction (XRD). Molybdenites subjected t o NaCl and NaHCO3 solutions give younger Re-Os ages than that of the origin al unaltered molybdenite. No significant changes in d spacing and width of micro-XRD patterns can be found in these altered molybdenite, indicating th e possibility of Re-Os fractionation without significant structural convers ion of molybdenite mineral. These results strongly suggest that the Re-Os s ystem in molybdenite would be frequently disturbed if it has experienced al teration, because alteration by the low salinity (<1%), low temperature (si milar to 180 degrees C) hydrothermal solution containing NaCl and/or CO2 is commonly found in the natural environment. We maintain, therefore, that on e set of analyses of Re and Os in a sample is not enough to determine wheth er the obtained Re-Os age has been affected by postdepositional alteration, but systematic replicate analyses are indispensable. Additionally, pulveri zing all the collected molybdenite in a sample might give misleading result s because portions which have been altered and experienced Re-Os fractionat ion, may possibly mix into the undisturbed sample and be homogenized. The m olybdenite used for the experiment was originally opaque under NIR light. I nfrared microscopic and spectroscopic profiles show that some parts of the molybdenite subjected to CaCl2 and AlCl3 solutions become transparent to NI R. Increased NIR transmittance is possibly attributed to the removal of the impurity band in molybdenite. It was also found in this study, however, th at change in IR profile does not correlate with the Re-Os fractionation and , therefore, IR measurement solely is not useful to detect disturbance of R e-Os systematics of molybdenite in alteration. Copyright (C) 2000 Elsevier Science Ltd.