The Tasmanian dolerites, part of the Ferrar Province of Australia and Antarctica, have some trace-element and isotopic compositions that suggest continental contamination of mantle-derived magmas. The debate has centred on whether the contamination occurred during intrusion into the crust, or if the mantle source itself was contaminated. The behaviour of Sr and O isotopes suggests that the mantle source had a δ18O composition of +6‰ and an initial 87Sr/86Sr ratio of 0.709, which supports the latter contention. Recently published Re-Os data likewise dismiss upper crustal contamination: Re-Os isotopic compositions of magnetite-rich mineral separates from seven Tasmanian dolerites yield an isochron that gives the same age, within uncertainties, as other dating techniques, namely 175 ± 5 Ma. Moreover, Re-Os data from a study of Antarctic Ferrar Province samples lie on the same isochron and the data together give an age of 177.3 ± 3.5 Ma. The initial 187Os/188Os of 0.125 ± 0.033 is the calculated mantle composition at the time. These results support previous models that attribute chemical features of the Ferrar magmas to re-enrichment of a depleted mantle source region rather than processes involving assimilation of crust by basaltic magma.
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