Serpentinites play a vital role in subduction zone processes: as one of the dominant carriers of water in downgoing plates, they are critical constituents in the global volatile cycle and add complexity to the bulk rheology and density of downgoing and overlying plates, with strong implications for seismicity and mechanical mixing. These rocks can form in a number of tectonic settings where water reacts with peridotite under certain conditions, including at or near mid-ocean ridges and in mantle overlying dehydrating subducted plates. In the HP/LT terrane preserved on the island of New Caledonia, serpentinites outcrop as meter-scale resistant blocks and as highly deformed “matrix” that hosts a range of metasedimentary, metamafic, and meta-ultramafic lenses. The origin of these HP serpentinites has been debated, with competing hypotheses linking protolith to nearby obducted ophiolite or to the subducted oceanic plate. We analyzed 30 serpentinites and associated hybrid rocks from across the HP terrane to discern the tectonic origin of their protoliths and to better understand their reaction history through subduction and exhumation. Whole rock major and trace element and stable isotope geochemistry reveal the existence of at least two distinct types of serpentinites in the HP terrane. Serpentinites in the far NE exhibit elevated HREE’s, δ18O values of 6-10‰, and scatter in major element concentrations. In the SE, a ~1 km2 ultramafic massif contains serpentinites with relatively more depleted REE, δ18O values of 5-7‰, and a more restricted range of major element concentrations, with distinctly higher MgO and lower Al2O3 than samples from the NE. We compare these data to a global geochemical compilation of serpentinites from various tectonic settings. Ongoing Raman spectroscopy work will determine serpentine polymorph(s) and electron probe microanalysis will target isolated relict pyroxene grains and oxide minerals to retrieve additional protolith information. The recognition of multiple types of serpentinites in this HP/LT terrane adds important information to debate about the origin of the ultramafic material, and may speak to complex interactions between the downgoing plate and overlying mantle or to systematic spatial differences in protolith composition or degree of metamorphism or deformation.

Owing to the importance of serpentinites for planetary geochemical and geodynamic processes, there has been much work discerning the origins of their parent rocks, including distinguishing between serpentinites derived from a subducting plate vs. overlying mantle in exhumed subduction complexes. The island of New Caledonia (SW Pacific Ocean) provides a rare window into Cenozoic Pacific subduction processes. The island is unique in exposing both an exceptionally-preserved high-pressure, low-temperature subduction complex and one of the largest supra-subduction zone ophiolites in the world. Previous studies disagree on the origin of serpentinites in the subduction complex. In this study, we analyze twenty-three serpentinites from this subduction complex for whole-rock major and trace element geochemistry and stable isotope (δD, δ18O) compositions. Our data reveal two distinct groups of serpentinites: Group I samples in the northern portion of the complex are pervasively serpentinized, and exhibit enriched heavy rare earth element (REE) compositions and δ18O between +6.7‰ and +10.2‰. In contrast, Group II serpentinites in the south preserve relict orthopyroxene and olivine, and show depleted trace element compositions and comparatively lower δ18O values between +5.1‰ and +8.0‰. We interpret Group I serpentinites to derive from downgoing plate mantle, whereas Group II serpentinites derive from overlying mantle wedge, exhibiting remarkable similarity to the REE geochemistry of the structurally-overlying New Caledonia ophiolite. Our results establish the subduction complex in New Caledonia as an unusual natural record of the entrainment and exhumation of mantle from both the overlying mantle wedge and the downgoing plate in an oceanic subduction zone.