Disturbance of the Re-Os chronometer of molybdenites from the late-Caledonian Galway Granite, Ireland, by hydrothermal fluid circulation.

Abstract

Re-Os ages were determined for molybdenites from the late-Caledonian Galway Granite in the west of Ireland. Molybdenites from three localities within the batholith, Mace Head, Murvey, and Travore, yield respective Re-Os ages of 425.2 ± 3.1 Ma, 383.2 ± 8.1 and 426.1 ± 5.5 Ma, and 338.9 ± 5.6 and 464 ± 28 Ma. The molybdenites from the three localities are believed to be contemporaneous. However, the Re-Os ages obtained for each locality are not only in disagreement with the ages of the samples from the other localities but also are inconsistent within a single locality. Replicate analyses of a molybdenite standard (HLP-5) characterized by AIRIE using our method (Suzuki et al., 1992) yields reproducible Re-Os ages, which are in agreement with the results of Markey et al. (1998). Therefore, the age discrepancies in Galway molybdenites in this study should not be attributed to analytical errors. Suzuki et al. (2000) revealed experimentally that the Re-Os system in molybdenite can behave as an open-system when immersed in warm aqueous NaCl or NaHCO 3 bearing fluid. Detailed fluid inclusion studies of molybdenite vein quartz and granite quartz (Gallagher et al., 1992;O'Reilly et al., 1997) have shown that the Galway Granite was the locus for repeated fluid events, i.e., 1. Expulsion of aqueous carbonic magmatic fluids during the final stages of crystallization, 2. Meteoric fluid convection probably driven by waning magmatic heat and 3. Incursion of basinal brines associated with fluorite mineralization at least 100 Ma after granite consolidation. We suggest that younger hydrothermal fluids perhaps associated with fluorite mineralization may have perturbed the Re-Os systematics of molybdenites from the Galway Granite.

Accurate and precise Re-Os molybdenite dates from the Galway Granite, Ireland. Critical comment on “Disturbance of the Re-Os chronometer of molybdenites from the late-Caledonian Galway Granite, Ireland, by hydrothermal fluid circulation” by Suzuki et al., Geochemical Journal, 35, 29-35, 2001

“…These dates are in agreement with those reported by Stein et al (1997) (221.5 ± 0.3 Ma) and Markey et al (1998) (221.3 ± 0.2 Ma), which were obtained through the different techniques of alkaline fusion and NTIMS techniques. The HLP-5 Re-Os age reported by Suzuki et al (2001) is also in agreement with an average Re-Os age of HLP-5 of 220.5 ± 0.5 Ma reported by Selby et al (2003), obtained by Carius tube methods and NTIMS isotopic analysis. It is important to note that the HLP-5 "control" sample is a 2 kg powder concentrate (~44 µm) obtained from separating ~0.13 to 0.2 mm molybdenite (Du Andao, pers.…”

“…molybdenite from this study contrast with those reported by Suzuki et al (2001) (Table 1). Firstly, our Re and 187 Os contents are highly reproducible for replicate analyses of the same sample.…”

“…Although this interpretation supports conclusions from two other studies (McCandless et al, 1993;Suzuki et al, 2000), it is at discord with many other studies, which demonstrate the highly robust nature of the Re-Os molybdenite chronometer (e.g., Stein et al, 1997Stein et al, , 1998aStein et al, , 1998bFrei et al, 1998;Watanabe et al, 1999;Watanabe and Stein, 2000;Raith and Stein, 2000;Selby and Creaser, 2001a, b;Selby et al, 2002;Stein and Bingen, 2002;Bingen and Stein, 2003). The disparity between the conclusion drawn by Suzuki et al (2001) of disturbance to the Re-Os molybdenite system, and the conclusions of the many studies noted above, raises some concern for the perceived reliability of the molybdenite Re-Os chronometer in the geological community. In response, we present here accurate and reproducible Re-Os molybdenite dates from Galway Murvey and Galway Mace of the Galway granite, Ireland, previously investigated by Suzuki et al (2001), and suggest that inaccurate and irreproducible Re-Os molybdenite dates of Suzuki et al (2001), relate to inadequate sample preparation methods.…”

“…The disparity between the conclusion drawn by Suzuki et al (2001) of disturbance to the Re-Os molybdenite system, and the conclusions of the many studies noted above, raises some concern for the perceived reliability of the molybdenite Re-Os chronometer in the geological community. In response, we present here accurate and reproducible Re-Os molybdenite dates from Galway Murvey and Galway Mace of the Galway granite, Ireland, previously investigated by Suzuki et al (2001), and suggest that inaccurate and irreproducible Re-Os molybdenite dates of Suzuki et al (2001), relate to inadequate sample preparation methods.…”

“…Secondly and importantly, the derived Re-Os dates are highly reproducible, and are indistinguishable within uncertainty of the U-Pb zircon age of the host Galway granite (412 ± 15 Ma, Pidgeon, 1969 (recalculated using the U decay constants of Jaffey et al, 1971)). Suzuki et al (2001) propose that discrepancies observed in their Re-Os dates are not the result of analytical protocols (microwave digestion, Re and Os measurements by ICP-MS), because replicate analyses of the HLP-5 "control" sample used by the AIRIE group, Colorado State University, yielded an age of 220.0 ± 2.1 Ma (n = 4). These dates are in agreement with those reported by Stein et al (1997) (221.5 ± 0.3 Ma) and Markey et al (1998) (221.3 ± 0.2 Ma), which were obtained through the different techniques of alkaline fusion and NTIMS techniques.…”