LOW-TEMPERATURE DIAMONDS IN OCEANIC ROCKS FROM THE WESTERN ALPS

Garnetites from Mn-rich nodules in the Lago di Cignana UHP unit in the Alps contain diamonds. These are the first diamonds known from the Alps, the first from oceanic metasediments, and the lowest-T occurrence (≤600°C) yet reported from UHP rocks. Diamond (Dmd) occurs as faceted octahedral and cuboidal matrix grains in several samples, and as 2-30 micron inclusions within all growth zones of spessartine-rich garnet (Grt). Microdiamonds occur in contact with fluid inclusions and in multiphase solid inclusions (Frezzotti et al., 2010 Fall AGU). Dmd in Grt is associated with quartz and composite inclusions of intergrown magnesite+dolomite surrounded by haloes of Fe³⁺-enriched Grt. This association suggests Dmd growth partly via a reaction such as 2Dol+4Ank+6SiO₂ = 2Mgs+2Andr+3Dmd+9CO₂.

Matrix Dmd is in contact with large, strain-free quartz grains that show no evidence for transformation from coesite. Dmd locally shows thin graphitic overgrowths, but no other signs of reaction. Quartz fills spaces between “puzzle-piece” Grt and Dmd, suggesting that it is texturally late relative to Grt+Dmd growth. The oxygen isotope composition of Qtz (δ¹⁸O=18.1-18.7‰) is out of equilibrium with Grt (δ¹⁸O=17.4-17.9‰), confirming that Qtz grew or recrystallized after formation of Grt and Dmd. The very high Grt δ¹⁸O values are consistent with an inherited low-T seafloor signature, whereas Qtz values indicate an external fluid source, likely from adjacent micaschists.

Previous studies posit that the LdC unit did not penetrate far into the coesite stability field (e.g., Reinecke 1991). However, the presence of Dmd throughout the garnetites supports an extensive excursion to P ≥3.2 GPa at T≤600°C, in agreement with the predictions of pseudosection modeling by Groppo et al. (2009). These P-T conditions require transport to UHP conditions with little concomitant heating. Restriction of the UHP record to a thin tectonic sliver precludes large-scale interpretation of these extreme conditions. However, the data point to extremely rapid subduction of some parts of the downgoing slab, and limited thermal reequilibration during subsequent tectonic imbrication.