EVIDENCE FROM ULTRA-HIGH-PRESSURE MARBLES FOR RECYCLING OF SEDIMENTS INTO THE MANTLE

ROCKS of crustal origin metamorphosed at ultra-high pressures (P > 2.5 GPa) have been described from several orogenic belts1-5. In the western Alps, for example, ultra-high pressure rocks originally equilibrated at about 750-degrees-C and 3.5 GPa (ref. 1). During decompression, these rocks were cooled considerably and therefore did not pass through the granulite stability field. Here we describe ultra-high-pressure metasediments that have equilibrated at higher temperatures (> 1,100-degrees-C) and followed a different exhumation path. A calcsilicate marble from the Bohemian massif contains clinopyroxenes with potassium-rich feldspar exsolutions. Potassium contents in the original clinopyroxenes indicate crystallization at pressures above 3-4 GPa (refs 6, 7). The high peak pressures and temperatures inferred for this rock, and its association with high-temperature peridotites and high-pressure granulites, suggest that carbonate sediments were subducted into the upper mantle, equilibrated at mantle conditions and were then emplaced in the crust along with mantle rocks. Our observations thus support suggestions based on less direct evidence (such as mass-balance considerations in orogenic belts8, and ocean-island basalt geochemistry9,10) that a limited amount of sediment must be recycled into the mantle.