| 187Re - 187Os nuclear geochronometry is a newly developed dating method especially (but
not only) for PGE hosting magmatic ore deposits. It combines ideas of nuclear astrophysics
with geochronology. For this, the concept of sudden nucleosynthesis [1-3] is used to calculate
so-called nucleogeochronometric Rhenium-Osmium two-point-isochrone (TPI) ages. Here,
the method is applied to the Sudbury Igneous Complex (SIC) and the Stillwater Complex
(SC), using a set of two nuclear geochronometers. They are named the BARBERTON (
Re/Os = 0.849, 187Os/186Os = 10.04 ± 0.015 [4]) and the IVREA (Re/Os = 0.951,
187Os/186Os = 1.9360 ± 0.0015 [5]) nuclear geochronometer. Calculated TPI ages are
consistent with results from Sm-Nd geochronology, a previously published Re-Os
Molybdenum age of 2740 ± 80 Ma for the G-chromitite of the SC [6] and a Re-Os
isochrone age of 1689 ± 160 Ma for the Strathcona ores of the SIC [7]. This leads to an
alternative explanation of the peculiar and enigmatic 187Os/186Osi isotopic signatures
reported from both ore deposits. For example, for a TPI age of 2717 ± 100 Ma the
Ultramafic Series of the SC contains both extremely low (subchrondritic) 187Os/186Osi
ratios (187Os/186Osi = 0.125 ± 0.067) and extremely radiogenic isotopic signatures
(187Os/186Osi = 6.55 ± 1.7, [6]) in mineral separates (chromites) and whole rock samples,
respectively. Within the Strathcona ores of the SIC, even more pronounced radiogenic
187Os/186Os initial ratios can be calculated for TPI ages between 1586 ± 63 Ma
(187Os/186Osi = 8.998 ± 0.045) and 1733 ± 84 Ma (187Os/186Osi = 8.901 ± 0.059). These
results are in line with the recalculated Re-Os isochrone age of 1689 ± 160 Ma
(187Os/186Osi = 8.8 ± 2.3 [7]). In the light of nuclear geochronometry, the occurrence of
such peculiar isotopic 187Os/186Osi signatures within one and the same lithological
horizon are plausible if explained by mingling of the two nucleogeochronometric
(BARBERTON and IVREA) reservoirs containing very old rapid (r) neutron-capture
process signatures from (at least) two different events. In this scenario, intermediate
187Os/186Osi ratios are due to mixing, resulting from the interaction of the two ancient
components. Since there is a striking agreement between the nucleogeochronometric
TPI ages and the conventional isochrone and mineral ages for the SC and SIC,
respectively, nuclear geochronometry may offer an enormous potential for exploration
purposes in mining industry, especially if additional geochemical and petrologic
cross-correlations are taken into account. Furthermore, there might be now a new and
promising opportunity to understand the link between magmatic ore forming processes and
global geodynamics.
[1] Burbidge et al. (1957) Revs. Mod. Phys. 29, 547 – 650. [2] Cameron (1957), CRL-41,
Atomic Energy of Canada Limited, Chalk River, Ontario. [3] Hoyle et al. (1960) ApJ 132,
565 – 590. [4] Birck et al. (1994) EPSL 124, 139-148. [5] Roller (1997), PhD Thesis, RKP
N+T, Munich. [6] Marcantonio et al. (1993), GCA 57, 4029 – 4037. [7] Walker et al. (1991)
EPSL 105, 416 – 429. |