On the origin of Tasmanian dolerites

The Tasmanian dolerites, part of the Ferrar Province of Australia and Antar ctica, have some trace-element and isotopic compositions that suggest conti nental contamination of mantle-derived magmas. The debate has centred on wh ether 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 delta O-18 composition of +6 parts pe r thousand and an initial Sr-87/Sr-86 ratio of 0.709, which supports the la tter contention. Recently published Re-Os data likewise dismiss upper crust al contamination: Re-Os isotopic compositions of magnetite-rich mineral sep arates 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 li e on the same isochron and the data together give an age of 177.3 +/- 3.5 M a. The initial Os-187/Os-188 of 0.125 +/- 0.033 is the calculated mantle co mposition at the time. These results support previous models that attribute chemical features of the Ferrar magmas to re-enrichment of a depleted mant le source region rather than processes involving assimilation of crust by b asaltic magma.