TY - JOUR
T1 - Sulfur isotope composition of metasomatised mantle xenoliths from the Bultfontein kimberlite (Kimberley, South Africa): Contribution from subducted sediments and the effect of sulfide alteration on S isotope systematics
AU - Giuliani, A.
AU - Fiorentini, Marco
AU - Martin, Laure
AU - Farquhar, J.
AU - Phillips, D.
AU - Griffin, W.L.
AU - Laflamme, Crystal
PY - 2016/7/1
Y1 - 2016/7/1
N2 - © 2016 Elsevier B.V. Sulfur isotopes are a powerful geochemical tracer in high-temperature processes, but have rarely been applied to the study of mantle metasomatism. In addition, there are very limited S isotope data on sub-continental lithospheric mantle (SCLM) material. For cratonic regions, these data are restricted to sulfide inclusions in diamonds. To provide new constraints on the S isotope composition of the SCLM and on the source(s) of mantle metasomatic fluids beneath the diamondiferous Kimberley region (South Africa), we investigated the S isotope systematics of five metasomatised mantle xenoliths from the Bultfontein kimberlite. Pentlandite and chalcopyrite in these xenoliths were analysed by in situ secondary-ion mass spectrometry (SIMS), with bulk-rock material measured by gas source isotope ratio mass spectrometry techniques. Based on previous studies, the xenoliths experienced different types of metasomatism to one another at distinct times (~180 and ~90-80 Ma). Contained pentlandite grains show variable alteration to heazlewoodite (i.e. Ni sulfide) + magnetite. The in situ S isotope analyses of pentlandite exhibit a relatively restricted range between -5.9 and -1.4‰d34S (compared to VCDT), with no statistically meaningful differences between samples. Chalcopyrite only occurs in one sample and shows d34S values between -5.4 and -1.0‰. The bulk-rock Ssulfide isotope analyses vary between -3.4 and +0.8‰d34S. Importantly, the only sample hosting dominantly fresh sulfides shows a bulk-rock d34S value consistent with the mean value for the sulfides, whereas the other samples exhibit higher bulk 34S/32S ratios. The differences between bulk-rock and average in situ d34S values are directly correlated with the degree of sulfide alteration. This evidence indicates that the elevated 34S/32S ratios in the bulk samples are not due to the introduction of heavy S (commonly as sulfates) and are best explained by isotopic fractionation coupled with the removal of light S during serpentinisation, when pentlandite
AB - © 2016 Elsevier B.V. Sulfur isotopes are a powerful geochemical tracer in high-temperature processes, but have rarely been applied to the study of mantle metasomatism. In addition, there are very limited S isotope data on sub-continental lithospheric mantle (SCLM) material. For cratonic regions, these data are restricted to sulfide inclusions in diamonds. To provide new constraints on the S isotope composition of the SCLM and on the source(s) of mantle metasomatic fluids beneath the diamondiferous Kimberley region (South Africa), we investigated the S isotope systematics of five metasomatised mantle xenoliths from the Bultfontein kimberlite. Pentlandite and chalcopyrite in these xenoliths were analysed by in situ secondary-ion mass spectrometry (SIMS), with bulk-rock material measured by gas source isotope ratio mass spectrometry techniques. Based on previous studies, the xenoliths experienced different types of metasomatism to one another at distinct times (~180 and ~90-80 Ma). Contained pentlandite grains show variable alteration to heazlewoodite (i.e. Ni sulfide) + magnetite. The in situ S isotope analyses of pentlandite exhibit a relatively restricted range between -5.9 and -1.4‰d34S (compared to VCDT), with no statistically meaningful differences between samples. Chalcopyrite only occurs in one sample and shows d34S values between -5.4 and -1.0‰. The bulk-rock Ssulfide isotope analyses vary between -3.4 and +0.8‰d34S. Importantly, the only sample hosting dominantly fresh sulfides shows a bulk-rock d34S value consistent with the mean value for the sulfides, whereas the other samples exhibit higher bulk 34S/32S ratios. The differences between bulk-rock and average in situ d34S values are directly correlated with the degree of sulfide alteration. This evidence indicates that the elevated 34S/32S ratios in the bulk samples are not due to the introduction of heavy S (commonly as sulfates) and are best explained by isotopic fractionation coupled with the removal of light S during serpentinisation, when pentlandite
U2 - 10.1016/j.epsl.2016.04.005
DO - 10.1016/j.epsl.2016.04.005
M3 - Article
SN - 0012-821X
VL - 445
SP - 114
EP - 124
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -