Spinel insights into mantle source characteristics and petrogenetic processes in the western Hellenic Volcanic Arc

Mafic enclaves are usually considered so as to indirectly investigate primitive melts in subduction settings. Spinel, among the earliest crystallizing phases in primitive magmas, provides key constraints on melt composition and, in turn, on mantle source characteristics. Despite being one of the oldest active subduction systems, the Aegean subduction zone remains highly complex, with its petrogenetic processes and evolutionary history still often under debate. Mafic enclaves are relatively common along the present–day Hellenic Volcanic Arc, particularly at its margins, with representative occurrences on Nisyros Island and the Methana Peninsula at the eawstern and western margins, respectively. In contrast, basaltic lavas are generally scarce, with the most primitive ones identified in the submarine Pausanias Volcanic Field at the western margin of the arc. This study focuses on the western margin of the Hellenic Volcanic Arc and presents a comparative investigation of spinel major element chemistry, obtained by EPMA, between mafic enclaves from the Methana Peninsula as well as mafic enclaves and lavas from both Methana Peninsula and its adjacent submarine Pausanias Volcanic Field. Mafic enclaves examined in this study reveal a previously unrecognized spinel population at Methana, characterized by distinct mineral chemistry. These spinels are notably Al–rich and Cr–poor (Cr# < 60) compared to previously reported spinels from Methana enclaves and lavas (Cr# > 60), and closely resemble spinels from Pausanias lavas. Variations in spinel chemistry can reflect either differences in primary melt compositions, resulting from mantle source heterogeneity or related to variable degrees of partial melting of a common mantle source, or else the evolution of a primary melt through subsequent petrogenetic processes. However, the compositional similarity between spinels from Methana mafic enclaves and Pausanias lavas suggests that the submarine Pausanias Volcanic Field may represent eccentric volcanism related to the Methana Volcanic Field rather than an independent system. This interpretation is supported by the close spatial association of the two volcanic fields and their near–contemporaneous development. These findings highlight the need for further integrated studies to better constrain mantle source characteristics, primitive melt, and petrogenetic processes in the western margin of the Hellenic Volcanic Arc that will shed new light and enhance our understanding about the mechanisms and the dynamics of the Aegean subduction zone.

Acknowledgments

This work is part of the first author's Ph.D. research, which is financially supported by the «Andreas Mentzelopoulos Foundation».