Discovery of paleozoic Fe-Mg carpholite in Motalafjella, Svalbard Caledonides: A milestone for subduction-zone gradients

Paleozoic blueschist facies rocks are relatively scarce on Earth due to warmer geothermal gradients at that time and/or later reequilibration. Ferro-magnesiocarpholite (Fe-Mg carpholite), the typical low-temperature blueschist facies index mineral in metapelites, was discovered 30 yr ago and is known only in Tethyan belts metamorphosed < 80 m.y. ago. Herein we report the discovery of Paleozoic Fe-Mg carpholite in the ca. 470 Ma blueschists of Motalafjella, Svalbard Caledonides, the oldest known occurrence on Earth. The carpholite-bearing rocks reached pressure-temperature (P-T) conditions of 15-16 kbar and 380-400 degrees C and followed a nearly isothermal exhumation path. In the cooling Earth perspective, these P-T estimates for Motalafjella blueschists demonstrate the existence of cold subduction-zone gradients (similar to 7 degrees C/km) from the middle Paleozoic onward.