Nanoscale variations in (OS)-O-187 isotopic composition and HSE systematics in a Bultfontein peridotite

Research areas:
  • A. N. Wainwright
  • A. Luguet
  • A. Schreiber
  • R. O. C. Fonseca
  • G. M. Nowell
  • Jean-Pierre Lorand
  • R. Wirth
  • P. E. Janney
Understanding the mineralogical controls on radiogenic chronometers is a
fundamental aspect of all geochronological tools. As with other common
dating tools, it has become increasingly clear that the Re-Os system can
be impacted by multiple mineral formation events. The accessory and
micrometric nature of the Re-Os-bearing minerals has made assessing this
influence complex. This is especially evident in cratonic peridotites,
where long residence times and multiple metasomatic events have created
a complex melting and re-enrichment history. Here we investigate a
harzburgitic peridotite from the Bultfontein kimberlite (South Africa)
which contains sub-micron Pt-Fe-alloy inclusions within base metal
sulphides (BMS). Through the combination of the focused ion beam
lift-out technique and low blank mass spectrometry we were able to
remove and analyse the Pt-Fe-alloy inclusions for their Re-Os
composition and highly siderophile element (HSE) systematics.
Six repeats of the whole-rock yield Os-187/Os-188 compositions of
0.10893-0.10965, which correspond to Re depletion model ages (T-RD) of
2.69-2.79 Ga. The Os, Ir and Pt concentrations are slightly variable
across the different digestions, whilst Pd and Re remain constant. The
resulting HSE pattern is typical of cratonic peridotites displaying
depleted Pt and Pd. The Pt-Fe-alloys have PUM-like Os-187/Os-188
compositions of 0.1294 +/- 24 (2-s.d.) and 0.1342 +/- 38, and exhibit a
saw-tooth HSE pattern with enriched Re and Pt. In contrast, their BMS
hosts have unradiogenic Os-187/Os-188 of 0.1084 +/- 6 and 0.1066 +/- 3,
with TRD ages of 2.86 and 3.09 Ga, similar to the whole-rock
The metasomatic origin of the BMS is supported by (i) the highly
depleted nature of the mantle peridotite and (ii) their Ni-rich sulphide
assemblage. Occurrence of Pt-Fe-alloys as inclusions within BMS grains
demonstrates the genetic link between the BMS and Pt-Fe-alloys and
argues for formation during a single but continuous event of silicate
melt percolation. While the high solubility of HSE within sulphide
mattes rules out early formation of the alloys from a S-undersaturated
silicate melt and subsequent scavenging in a sulphide matte, the
alignment of the Pt-Fe-alloy inclusions attests that they are
exsolutions formed during the sub-solidus re-equilibration of the high
temperature sulphide phases.
The significant difference in Os-187/Os-188 composition between the
included Pt-Fe-alloys and their BMS host can only be accounted for by
different Re/Os. This suggests that the formation of Pt-Fe-alloy
inclusions within a BMS can result in the fractionation of Re from Os. A
survey experiment examining the partitioning of Re and Os confirmed this
observation, with the Re/Os of the Pt-Fe-alloy inclusion up to ten times
higher than the co-existing BMS. This fractionation implies that, when
Re is present in the sulphide melt, the TRD ages of BMS containing alloy
inclusions do not date the loss of Re due to partial melting, but rather
its fractionation into the Pt-Fe-alloys. As such, BMS ages should be
used with caution when dating ancient partial melting events. (C) 2016
Elsevier B.V. All rights reserved.