Fossilized Melts in Mantle Wedge Peridotites

Experimental production of K-rich metasomes through sediment recycling at the slab-mantle interface in the fore-arc

Sediment contribution to the mantle is the key step for the generation of orogenic magmatism to produce its isotopic and geochemical inventory. Even though they are exceptional for the post-collisional settings, there are worldwide examples of arc-related ultrapotassic mafic magmas which require complex multi-stage processes along with sediment melting e.g. in Italy or Pontides of Türkiye. To understand the metasomatism leading mantle to produce ultrapotassic mafic melts, we simulated the reactions of depleted (harzburgite) and fertile (lherzolite) mantle with subducted carbonate-rich sediment at relatively cold (800-850 °C) and shallow (2 GPa, 60-80 km) slab-mantle interfaces. The melting of sediments can trigger the formation of immiscible and conjugate carbonatitic and silicic melts which flux the mantle to develop hydrous minerals and dolomitic melt. The metasomatic growth product is a wehrlite composed of clinopyroxene, phlogopite, carbonate minerals and amphibole, representing a source of choice for Si-undersaturated ultrapotassic lavas. The occurrence of conjugate carbonatitic and silicic melts and their potential physical separation, offer a possibility for fractionation of several canonical trace element ratios such as Th/La, observed in Si-saturated ultrapotassic lavas. The synergy between peridotite-melt interaction and the physical separation of the carbonatitic and extremely K-enriched silicic melts are essential for the compositional evolution of ultrapotassic orogenic magmas and their mantle sources.

Ophiolitic Pyroxenites Record Boninite Percolation in Subduction Zone Mantle

The peridotite section of supra-subduction zone ophiolites is often crosscut by pyroxenite veins, reflecting the variety of melts that percolate through the mantle wedge, react, and eventually crystallize in the shallow lithospheric mantle. Understanding the nature of parental melts and the timing of formation of these pyroxenites provides unique constraints on melt infiltration processes that may occur in active subduction zones. This study deciphers the processes of orthopyroxenite and clinopyroxenite formation in the Josephine ophiolite (USA), using new trace and major element analyses of pyroxenite minerals, closure temperatures, elemental profiles, diffusion modeling, and equilibrium melt calculations. We show that multiple melt percolation events are required to explain the variable chemistry of peridotite-hosted pyroxenite veins, consistent with previous observations in the xenolith record. We argue that the Josephine ophiolite evolved in conditions intermediate between back-arc and sub-arc. Clinopyroxenites formed at an early stage of ophiolite formation from percolation of high-Ca boninites. Several million years later, and shortly before exhumation, orthopyroxenites formed through remelting of the Josephine harzburgites through percolation of ultra-depleted low-Ca boninites. Thus, we support the hypothesis that multiple types of boninites can be created at different stages of arc formation and that ophiolitic pyroxenites uniquely record the timing of boninite percolation in subduction zone mantle.