A proposal to study the genesis of Oceanic Island Basalts using Cl isotopes: Graveyard of subducted oceanic crust in the lower mantle?

Subduction of oceanic crust is a crucial part of the Earth crust-mantle mass cycling. Though being studied for decades, the fate of subducted crust remains highly debated today. How deeply down can these crustal materials travel and survive the extreme P-T conditions in the mantle? The lower mantle, which is inferred by Oceanic Island Basalt (OIB), shows evident chemical and isotopic enrichment compared with the upper mantle. These crust-like signatures have led many people to assume that the subducted oceanic crust can traverse the upper mantle and thrust deeply into the lower mantle (e.g. Weaver, 1991 and the references therein). To test this hypothesis, we now propose employing Cl isotopes to trace the subducted crustal materials.

Chlorine has two naturally occurring isotopes: ³⁵Cl (75.8%) and ³⁷Cl (24.2%), both of which are stable isotopes. The large relative mass difference between the two isotopes indicates obvious isotopic fractionation during geological processes. High solubility of Cl makes it highly enriched in seawater, which in turn, results in the elevated Cl abundance in seawater altered basalts and sediments on the seafloor. When the altered oceanic crust and overlying sediments subduct into the mantle, Cl is efficiently extracted and recycled into the mantle wedge, as a result of dehydration (Straub et al., 2003). During this process, the light ³⁵Cl is preferentially lost, leaving the dehydrated oceanic crust and sediments enriched in ³⁷Cl compared with the upper mantle. If these ³⁷Cl-rich materials can be subducted to the lower mantle and contribute to the source of Oceanic Island Basalts, we may expect to see the Cl isotopic composition in Oceanic Island Basalts distinct from Mid-Ocean Ridge Basalts, which represent the upper mantle.

References

Straub, S.M., Layne, G.D., 2003. The systematics of chlorine, fluorine, and water in Izu arc front volcanic rocks: Implications for volatile recycling in subduction zones. Geochimica et Cosmochimica Acta 67, 4179–4203.

Weaver, B.L., 1991. The origin of ocean island basalt end-member compositions: trace element and isotopic constraints. Earth and Planetary Science Letters 104, 381-397.