TRACING THE DEEP CARBON CYCLE: BORON ISOTOPE INVESTIGATION OF MANTLE-DERIVED CARBONATES

Boron and its isotopes have become a viable and reliable non-traditional stable isotope system for tracking geological processes due to recent advances in analytical techniques and instrument sensitivity over the last few decades. Boron is a highly mobile, volatile element that has been proven to be a powerful isotopic tracer, especially in fluid-mediated processes. The large natural variation in δ¹¹B (~100‰), due to the relatively high mass difference between ¹⁰B and ¹¹B, and its incompatible nature, leads to a distinct depletion in the asthenospheric mantle (-7 ± 1‰) and strong enrichment in crustal sources (up to +26‰). Moreover, boron is widely used as a proxy for carbon in various fields, such as deciphering paleoclimates. Thus, boron’s link to the carbon cycle makes it a valuable tool in investigating the source region of carbonate-rich, mantle-derived rocks, such as carbonatites and their associated silicate rocks.

Boron’s presence in the deep mantle, evidenced by boron-bearing blue diamonds, provides motivation for its application in carbonatites. The abundances of boron for a majority of carbonatites investigated to date are <1 ppm, which are similar to those for fresh mid-ocean ridge basalts (MORBs) and oceanic island basalts (OIBs). The isotopic signature of boron has yet to be well-established for carbonatites, but preliminary data has revealed a temporal trend that spans the past ~2.1 billion years of Earth history. Boron isotopic signatures for older carbonatites (>500 Ma) have asthenospheric mantle values, while younger carbonatites (<500 Ma) show more variation and exhibit positive values (-4 to +5‰). The latter indicates the presence of recycled boron-enriched material into the carbonatite source magma over time offering insight into the recycling mechanism of subducted material within the mantle. The innovative use of boron isotopes in carbonatites will further the understanding of both the petrogenesis of carbonatites worldwide and the temporal evolution of Earth’s upper mantle.