| 187Re -187Os nuclear geochronometry is a new age dating method combining principles of
geochronology with nuclear astrophysics. It has been successfully applied to PGE hosting
magmatic ore deposits like the Late Archean Stillwater Complex (SC), Montana, USA [1].
The pronounced isotopic dichotomy of the SC has been interpreted as being due to the
interaction of two magmatic components with extremely different 187Os/188Osi but
Re/Os ≈ 1. A Mercury-like planetary contraction with Fermi-pressure controlled
core collapse at about 3.48 Ga producing heavy nuclides has been suggested [2] to
explain the ultra-subchondritic 187Os/188Osi of the SC, consistent with the observed
increase in PGE abundances within komatiites [3] and in magnetic field strength
between 3.6 Ga and 3 Ga [4]. It contradicts a partial melting event of primitive
mantle or a chondritic late veneer PGE addition [3] to the Earth. Besides, rocks
and plagioclase from the SC show uniform Th/U ≈ 4 [5], consistent with Th/U
= 4.1 ± 0.3 as derived from 12 Barberton komatiites [6]. For the Earth’s core, a
high Th/U > 4.3 has recently been proposed [7]. This seems to contradict global
and average MORB Th/U [8]. However, assuming that the r-process nuclides were
produced in the same nucleosynthetic event(s), mixing of the two reservoirs could
explain the decreasing Th/U ratios observed in oceanic basalts since 3.5 Ga [7, 8]. To
test this hypothesis, two nucleogeochronometric 232Th/238U evolution lines are
plotted versus time, starting with an r-process production ratio 232Th/238U ≈ 0.96
[9] at 13.781 Ga and 3.48 Ga, respectively. It turns out that the model explains
successfully global MORB 232Th/238U between 1.45 and 4.3 [8] by mixing of
the two 232Th/238U components. Hence, it can be shown for the first time that a
high Th/U ≈ 4.3 in the core is consistent with global and average MORB Th/U
ratios.
[1] Roller (2015) Geophys. Res. Abstr. 17, EGU2015-17. [2] Roller (2015), T13A-2982,
AGU Fall Meeting. [3] Maier et al. (2009) Nature 460, 621-623. [4] Tarduno et al. (2015),
Science 349, 521-524. [5] Wooden et al. (1991), Contrib. Mineral. Petrol. 107, 80-93. [6]
Brévart et al. (1986), EPSL 77, 293-303. [7] Wohlers et al. (2015), Nature 520, 337-340. [8]
Arevalo et al. (2010), Chem. Geol. 271, 70-85. [9] Roller (2015), 78th Ann. Meeting Met.
Soc., Abstr. #5041. |