Oxygen isotope characteristics of Karoo picrites with a primitive mantle
affinity
Abstract
Continental flood basalts (CFB) in the Karoo large igneous province have
been divided into the North and South Karoo groups. Picrites from the
Luenha river, Mozambique, have been shown to represent the primitive
mantle-like end-member required to explain the geochemical
characteristics of the North Karoo CFBs, which have elevated ΔNb
compared to the South Karoo CFBs. These picrites exhibit a narrow range
of bulk-rock Nd isotope compositions (εNd180Ma -2.0 to +1.4) but a wide
range of bulk-rock, plagioclase and groundmass Sr isotope compositions
(full range 87Sr/86Sr180Ma 0.704096-0.71061), extending to high values
suggestive of crustal contamination in the origin of these rocks.
Despite this, preliminary O isotope data for olivine from one sample
with elevated 87Sr/86Sr show uniform, mantle-like δ18O values
(4.68±0.38‰ to 5.53±0.37‰). New O isotope data to be acquired in
November 2019 on the NordSIM Cameca IMS 1280 ion microprobe will
determine the O isotope composition of a sample inferred to most closely
represent the parental magma as well as test the intra- and inter-sample
O-isotopic variability of these picrites. Combined with the available
bulk-rock and plagioclase phenocryst compositions, these data allow us
to constrain the progress of crustal contamination and evaluate the
homogeneity of the parental magmas. Most importantly, we aim to
distinguish between the effects upon the samples of crustal
contamination versus mantle source heterogeneity.