Temperatures and Cooling Rates Recorded by the New Caledonia Ophiolite: Implications for Cooling Mechanisms in Young Forearc Sequences. (18th January 2022)

• Record Type: Journal Article
• Authors: Secchiari, Arianna; Montanini, Alessandra; Cluzel, Dominique
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 23:Number 1(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-01-18
• Subjects: subduction zones -- forearc mantle -- New Caledonia -- ophiolites -- thermal history -- obduction; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2021GC009859
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: To unveil how forearc lithosphere cools and re‐equilibrates, we carried out a comprehensive geothermometric investigation of the New Caledonia ophiolite, which represents a rare example of proto‐arc section generated during subduction infancy. A large data set, including more than 80 samples (peridotites and mafic‐ultramafic intrusives), was considered. Closure temperatures calculated for the lherzolites using slow (TREE‐Y ) and fast diffusing (TCa‐in‐Opx, TBKN, TCa‐in‐Ol, TOl‐Sp ) geothermometers provide some of the highest values ever documented for ophiolitic peridotites, akin to modern sub‐oceanic mantle. Cooling rates deduced from TREE‐Y and TBKN yield values of ≈10 −3 °C/y, similar to those obtained with TCa‐in‐Ol . These features are consistent with a post‐melting history of emplacement, possibly along a transform fault, and thermal re‐equilibration via conduction. Cpx‐free harzburgites register a high‐T evolution, followed by quenching and obduction. The relatively high TCa‐in‐Ol, TOl‐Sp and cooling rates computed from TCa‐in‐Ol (≈10 −3 °C/y) are atypical for this geodynamic setting, mirroring the development of an ephemeral subduction system, uplift and emplacement of the Peridotite Nappe. Temperature profiles across the crust‐mantle transect point to high closure temperatures, with limited variations with depth. These results are indicative of injection and crystallization of non‐cogenetic magma batches in the forearc lithosphere, followed by thermal re‐equilibration at rates of ≈10 −4 –10 −3 °C/y. Our study shows that the thermal conditions recorded by forearc sequences are intimately related to specific areal processes and previous lithospheric evolution. Thus, detailed sampling and exhaustive knowledge of the geological background are critical to unravel the cooling mechanisms in this geodynamic setting. Plain Language Summary: The New Caledonia ophiolite represents a rare example of a complete forearc section, extending from the lower crust to the mantle, originated at the beginning of the Eocene subduction. Our study aims to shed light on the thermal evolution of this ophiolite, which constitutes one of the largest ophiolite complexes worldwide, gaining‐at the same time ‐ insights into the mechanisms that govern cooling of forearc lithosphere. For this purpose, different geothermometers were applied to mantle peridotite and mafic‐ultramafic intrusive samples. Geothermometers are equations based on temperature‐dependent major element and/or trace element exchange reactions among mineral phases, which allow calculating equilibrium temperatures starting from mineral chemical compositions. Equilibrium temperatures obtained with various thermometers can be linked to the cooling evolution and heat extraction rates of the studied samples. With this study, we were able to track the post‐melting evolution and re‐equilibration at lower temperature conditions for all the investigated rock types. A comprehensive investigation of the Massif du Sud sequence, from the lower crust to the upper mantle, allowed us to build thermal profiles across the forearc lithosphere. Overall, our results contribute to unravel the cooling mechanisms of the crust‐mantle sequences in this poorly explored tectonic setting. Key Points: A comprehensive geothermometric investigation of the New Caledonia ophiolite has been carried out The computed temperatures and cooling rates (≈10 −4 –10 −3 °C/y) contrast with previous results on ophiolitic and forearc peridotites The thermal conditions recorded by forearc sequences can be linked to specific areal processes and previous lithosphere evolution