Geology:Mantle chlorine isotope signature and melt−rock interaction revealed by apatite at the Southwest Indian Ridge

Naixiao Xu^1,2, Hao Cheng¹, Geoffrey D. Bromiley², Henry J.B. Dick¹, and Lingmin Zhang¹

1 State Key Laboratory of Marine Geology, Tongji University, Shanghai, China

2 School of GeoSciences, Grant Institute, University of Edinburgh, Edinburgh EH9 3FE, UK

Abstract: The extent of chlorine isotope heterogeneity within Earth’s mantle remains a subject of active debate. To contribute to this discussion, we present the first in situ chlorine isotope measurements of apatite from Integrated Ocean Drilling Program Hole U1473A (Southwest Indian Ridge) gabbros. The apatites record a pristine mantle δ³⁷Cl value of -0.51‰ ± 0.04‰, consistent with global mid-ocean ridge basalt averages. Systematic negative covariations between δ³⁷Cl and Cl content across lithological boundaries cannot be explained by fractional crystallization alone. Compared to altered gabbros from Ocean Drilling Program Hole 735B (Atlantis Bank), apatites from U1473A show lower δ³⁷Cl values. The preservation of euhedral textures and constant OH concentrations in these apatites, combined with amphibole geochemistry, rules out lower-temperature seawater alteration. Instead, these trends reflect high-temperature processes, including melt-rock interaction, brine assimilation, or mantle heterogeneity. Our findings demonstrate that δ³⁷Cl variability in mantle-derived apatites records both crustal recycling and intrinsic mantle heterogeneity, confirming their utility as reliable tracers of deep Earth volatile cycling.

Full article:https://doi.org/10.1130/G53563.1