Abstract
Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ18OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.
Highlights
Could be thermally decomposed in hot subduction zones to form regional CO2 emissions[8,9]
Evidence for this is found in coesite- and diamond-bearing marbles from orogenic belts that have experienced ultra-high pressure (UHP) metamorphism[10], as well as in high-pressure experiments on carbonated pelite[11] and eclogite[12], which show the persistence of carbonate above the initial melting temperature
In this article we describe carbonatitic xenoliths from basalt which demonstrate that sedimentary limestone can be subducted to at least >120 km, and quickly returned to shallow depths in the lithosphere mantle where they were sampled as carbonatitic melt with compositions little different to the original limestone
Summary
Could be thermally decomposed in hot subduction zones to form regional CO2 emissions[8,9]. In this article we describe carbonatitic xenoliths from basalt which demonstrate that sedimentary limestone can be subducted to at least >120 km, and quickly returned to shallow depths in the lithosphere mantle where they were sampled as carbonatitic melt with compositions little different to the original limestone. The speed of this process leads to the preservation of mineral indicators that allow this path to be described
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
