The extent to which off-axis hydrothermal activity at slow-spreading ridges relies on proximal magmatism remains unresolved, as exemplified by the debate surrounding the origin of the Lost City Hydrothermal Field (LCHF). The LCHF is hosted by the Atlantis Massif oceanic core complex (30˚N, Mid-Atlantic ridge), which consists of tectonically exhumed peridotites interleaved with gabbroic intrusions. If these gabbros intruded off-axis into cold lithosphere, they may be linked to late-stage magmatism that served as a local heat source for the LCHF. We test this off-axis-emplacement hypothesis by coupling pyroxene- and olivine-based thermometers to constrain the cooling rates of peridotites and gabbros recovered from deep boreholes on the Atlantis Massif. We observe that gabbroic intrusions modify host peridotite chemistry but not peridotite cooling rates. Gabbro cooling rates are consistent with those of adjacent peridotites when adjusted for olivine and pyroxene grain sizes. Similar cooling rates between lithologies across the massif suggest that these gabbros were emplaced deep, on-axis and were then co-exhumed with the peridotites by the detachment fault along a similar P-T pathway. Our results support existing tectonic models that suggest deep (>7 km) crustal growth at the root of the detachment fault, as well as hydrothermal models that invoke deep fluid flow, not magmatic heat, to sustain the LCHF.

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Kuan-Yu Lin is a Staff Scientist at Purdue University and Research Associate at the Smithsonian Institution. His research integrates the geochemistry and petrology of lower crustal and upper mantle lithologies with numerical models to investigate how magmatic processes drive crustal growth, mineralization, and hydrothermal activities along mid-ocean ridges. He has also worked extensively on advancing analytical capabilities to decode the complex chemical signatures preserved in geological materials.