ISOTOPIC VARIABILITY OF C AND O IN CARBONATITES—PROTEROZOIC TO RECENT

Carbonatites are igneous rocks composed of ≥50% carbonate minerals which can be shown experimentally to be derived from extremely low-degree (<1%) melts of carbonated peridotite. Sr-Nd isotopic compositions indicate they have a source(s) in the deep mantle, and thus carbonatites have the potential to track the geochemical evolution of carbonate components in the mantle. If mantle domains are not mixed, isotopic change with time might be expected as carbonate sediments are recycled into the mantle, particularly after the Archean.

Calcite and dolomite separates of 43 carbonatite samples from 25 centers on five continents have δ¹³C and δ¹⁸O values that show no significant increase or decrease from Proterozoic to Recent time; however, isotopic variability has increased. The post-Archean carbonatites are from distinct periods (~130 Ma to Recent, ~370, 1100-1300 Ma, ~1900, and ~2270 Ma), and their published e_Nd values are greater than 0, indicating mantle sources, but only 40% of the carbonate separates fall into the δ¹⁸O vs. δ¹³C “mantle box.” Carbon in dolomite has a narrower range of isotopic values than carbon in calcite; dolomite has δ¹³C values primarily between -2 and -5 per mil, whereas most of the calcite is -1 to -7 per mil. It is possible that carbon in dolomite represents a more pristine, better preserved sample of mantle compositions, than carbon in calcite. A subset of alkalic igneous rocks and carbonatites from close to the “mantle box” are likely from similar sources, as suggested by their overlapping δ¹³C and δ¹⁸O isotope ratios; δ¹⁸O extends to slightly higher values in associated alkalic igneous rocks, implicating more crustal interaction in the latter.

The range of oxygen isotope values appears to have expanded from Proterozoic to Recent time from δ¹⁸O of +6.3 to +22.2 to δ¹⁸O of +5.8 to +28.2 per mil. The range of carbon isotopic ratios has also expanded over time from δ¹³C of -5.0 to -0.5 to δ¹³C of -8.0 to +1.0 per mil. This increased variability may be a result of increased carbonatite magmatism with time, greater preservation of younger carbonatites, or increased variation of contributions to carbonatite sources, including crustal modification. The variation in oxygen in carbonates either represents an exotic component in the mantle or crustal exchange, or both, but analysis of silicates is necessary to test this.