Did it melt? Reaction microstructures in lower crustal olivine+spinel dolomitic marbles

Goldschmidt 2025 Abstracthttps://doi.org/10.7185/gold2025.26740Did it melt? Reaction microstructuresin lower crustal olivine+spineldolomitic marblesANDREA MARONI1, DANIELE CASTELLI1, VINCENZOFESTA2, CHIARA GROPPO1, FRANCO ROLFO1, RICHARDSPIESS3 AND FABRIZIO TURSI11University of Torino2University of Bari Aldo Moro3University of PadovaIn the last decade, an increasing number of authors havesuggested the possible generation of carbonatitic melts throughpartial melting of carbonatic metasediments (e.g. [1]) in themiddle–lower crust of orogens subjected to high dT/dP ratios [2].The (extremely) high-temperature conditions are not the onlycause for melting of these lithologies, since the presence of anaqueous fluid, either internally derived or externally introduced,can significantly decrease the solidus temperature (e.g. [3]).However, despite the recently emerging interest, melt-producing(and melt-consuming) reactions and related microstructures inmarbles s.l. are still largely unexplored.Here we present evidence of the presence of melt and of melt-residue back-reactions in olivine+spinel dolomitic marbles froma Variscan metasedimentary migmatitic succession exposed inthe Serre and Sila massifs of Calabria (Southern-Italy), whichequilibrated under peak metamorphic conditions up to 800–840°C, at 0.9–1.0 GPa.Calcite melt-pseudomorphs, calcites with peculiar crystal-chemical and microstructural characteristics, and potential two-liquids immiscibility microstructures bear evidence of partialmelting possibly related to chlorite dehydration melting. Melt-residue back-reaction microstructures between the carbonatiticmelt and the silicate fraction have been additionally identified,including coronas and symplectites composed of pargasite,diopside, and dolomite replacing olivine.Finally, coarse-grained calcite crystals developed at the edgesof prograde exsolution-bearing calcite, characterized by high andflat Mg-profiles may represent products of melt crystallizationand subsequent re-equilibration during exhumation.The observed microstructures pave the way for theidentification of partial melting in metacarbonate rocks.Moreover, the obtained results demonstrate that partial meltingcan occur even in relatively pure carbonatic systems, at least on alocal scale, due to the decreased solidus temperatures induced bypresence of aqueous fluids internally released during breakdownof hydrous minerals.[1] Su, Wang, Wang (2024) Lithos, 488-489.[2] Brown & Johnson (2018) American Mineralogist 103(2),181-196.[3] Floess, Baumgartner, Vonlanthen (2015) Earth andPlanetary Science Letters 409, 147-156. (PDF) Did it melt? Reaction microstructures in lower crustal olivine+spinel dolomitic marbles. Available from: https://www.researchgate.net/publication/396879799_Did_it_melt_Reaction_microstructures_in_lower_crustal_olivinespinel_dolomitic_marbles#fullTextFileContent [accessed Oct 30 2025].