CARBONATITE ISOTOPIC COMPOSITIONS: PROTEROZOIC TO RECENT

In 1996, Dan Barker proposed that carbonatite magmas have undergone a process of path-clearing through the lithosphere by intense chemical interaction with wall-rock, so their “survival” rate upon passage is low, explaining their paucity. Using Sr, Nd, and Pb isotopic ratios, Barker (1996) showed that carbonatite sources have become more and more radiogenic from 2700 Ma to the present, purportedly due to interaction with radiogenic lithosphere and from increasingly diverse sources of recycled subducted oceanic slabs deep within the mantle.

Calcite and dolomite separates of 43 carbonatite samples from 25 centers on five continents were analyzed for δ¹³C and δ¹⁸O. No significant increase or decrease was identified from Proterozoic to Recent time in either calcite or dolomite separates. Isotopic variability has increased with time in calcite, but not in dolomite separates. These samples (~130 Ma to Recent, ~370, 1100-1300 Ma, ~1900, and ~2270 Ma) have previously published epsilon-Nd values greater than 0, indicating mantle sources, but only 40% of the carbonate separates fall into the δ¹⁸O vs. δ¹³C “mantle box.” Dolomitic δ¹³C is between -2 and -5 per mil, whereas most of the calcite is -1 to -7 per mil. This might indicate that dolomitic carbon represents better preservation of mantle source material.

Isotopic change with time could be expected in δ¹³C and δ¹⁸O in carbonatites as carbonate sediments are recycled into the mantle. As Barker predicted, there does appear to be a significant increase in diverse sources with time, as indicated by observed increasing isotopic variability in calcite with time. Increased input of subducted slab component in the mantle should play a role in expanded isotopic variability with time, in combination with variable lithospheric and crustal modification. On the other hand, in contrast to published values showing increased radiogenic isotopic component with time, the consistent, nearly uniform values of δ¹³C and δ¹⁸O found in this study from 2.3 Ga to the present, particularly in dolomitic carbonatite, reveal an unchanging source of C and O in the convecting mantle.