| Nuclear geochronometry [1-2] is a new dating method which combines principles of
geochronology with nuclear astrophysics. It is embedded in other scientific fields like
cosmochemistry, cosmology and nuclear theory, which pose tight constraints for nuclear
geochronometry. It is based upon identified Re/Os element ratios ≈ 1, interpreted as the
nuclear production ratio, and ultra-subchondritic initial 187Os/188Os ratios within terrestrial
rocks, suggesting that Earth’s core still contains element ratios and isotopic signatures of at
least two rapid (r) neutron-capture process [3] events. The 13.78 Ga old component,
represented by the isotopic signature of a komatiitic basalt [5085 BasKom] [4] from the
Barberton Greenstone Belt (Onverwacht Group, South Africa), is assigned to the Earth’s
inner core. The other isotopic signatures identified so far within pyroxenites / komatiites
are assigned to its outer core due to at least one gravitational collapse of the old
component, commencing ≈ 3.48 Ga [2] and resulting in one or more additional
r-process event(s). Here I show that 187Re-187Os nuclear geochronometry can also be
successfully applied for dating peridotitic diamond sulphide inclusions by means
of two-point-isochrones (TPI), using a so-called nuclear geochronometer always
as the second data point in a TPI diagram. It turns out that the method may have
a huge potential to constrain the chemical evolution of the SCLM. For example,
TPI ages for Ellendale (Australia) peridotitic diamond sulphide inclusions EL50,
EL23, EL54-1, EL54-3, EL55-1 and EL65 reported in the literature [5] reveal at
least two main fractionation events. The age cluster between 1.4 Ga and 1.5 Ga is
consistent with a previously reported isochrone age [5]. The event ≈ 2.3 ± 0.3 Ga
overlaps the Great Oxidation Event (GOE) between 2.22 Ga and 2.46 Ga. While the ≈
1.4 Ga to 1.5 Ga events lead to fractionation of the 187Re/188Os ratios towards
values typical for mantle peridotite, the latter caused only minor disturbance of the
187Re/188Os nuclear production ratio assigned to the outer core. It cannot be excluded
that a major change in oxygen/sulfur fugacitiy across the core – mantle boundary
(CMB), coincident with the GOE, is responsible for the 187Re/188Os fractionation of
the EL50 sample. Because of its minor degree of fractionation, EL50 can still be
used as a so-called fractionated chronometer for dating those Ellendale peridotitic
diamond sulphide inclusions, which do not show open system behaviour. Whether
the ≈ 1.4 Ga to 1.5 Ga fractionation events are due to an even more pronounced
change in oxygen and/or sulfur fugacities across the CMB, within the mantle or,
alternatively/additionally, reworking of the mantle because of mantle convection and/or
subduction of oceanic crust, remains an open question. This question will be addressed in
future studies.
[1] Roller (2014), GSA Abstr. with Programs, 46, 323. [2] Roller (2014), Abstract
S51B-4444, Fall Meeting, AGU 2014. [3] Burbidge et al. (1957) Revs. Mod. Phys. 29, 547 –
650. [4] Birck et al. (1994), EPSL 124, 139 – 148. [5] Smit et al. (2010) GCA 74, 3292 -
3306. |