The compositional variability of the lithospheric mantle at extensional settings is largely caused by thereactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments.The Alpine-Apennine (A-A) ophiolites are predominantly constituted by mantle peridotites and arewidely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition andslow- to ultraslow-spreading settings. Structural and geochemical studies on the A-A mantle peridotiteshave revealed that they preserve significant compositional and isotopic heterogeneity at variable scale,reflecting a long-lived multi-stage melt migration, intrusion and melt-rock interaction history, occurredat different lithospheric depths during progressive uplift. The A-A mantle peridotites thus constitute aunique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreadingenvironments. In this work, we reviewfield, microstructural and chemical-isotopic evidence on themajor stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discusstheir consequences in creating chemical-isotopic heterogeneities at variable scales and enhancingweakening and deformation of the extending mantle. Focus will be on three most important stages: (i)old (pre-Jurassic) pyroxenite emplacement, and the significant isotopic modification induced in the hostmantle by pyroxenite-derived melts, (ii) melt-peridotite interactions during Jurassic mantle exhumation,i.e. the open-system reactive porousflow at spinel facies depths causing bulk depletion (origin of reactiveharzburgites and dunites), and the shallower melt impregnation which originated plagioclase-rich pe-ridotites and an overall mantle refertilization. We infer that migrating melts largely originated asshallow, variably depleted, melt fractions, and acquired Si-rich composition by reactive dissolution ofmantle pyroxenes during upward migration. Such melt-rock reaction processes share significantsimilarities with those documented in modern oceanic peridotites from slow- to ultraslow-spreadingenvironments and track the progressive exhumation of large mantle sectors at shallow depths in oceanicsettings where a thicker thermal boundary layer exists, as a consequence of slow-spreading rate.
Melt migration and melt-rock reaction in the Alpine-Apennine peridotites: Insights on mantle dynamics in extending lithosphere
Rampone Elisabetta;
2020-01-01
Abstract
The compositional variability of the lithospheric mantle at extensional settings is largely caused by thereactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments.The Alpine-Apennine (A-A) ophiolites are predominantly constituted by mantle peridotites and arewidely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition andslow- to ultraslow-spreading settings. Structural and geochemical studies on the A-A mantle peridotiteshave revealed that they preserve significant compositional and isotopic heterogeneity at variable scale,reflecting a long-lived multi-stage melt migration, intrusion and melt-rock interaction history, occurredat different lithospheric depths during progressive uplift. The A-A mantle peridotites thus constitute aunique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreadingenvironments. In this work, we reviewfield, microstructural and chemical-isotopic evidence on themajor stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discusstheir consequences in creating chemical-isotopic heterogeneities at variable scales and enhancingweakening and deformation of the extending mantle. Focus will be on three most important stages: (i)old (pre-Jurassic) pyroxenite emplacement, and the significant isotopic modification induced in the hostmantle by pyroxenite-derived melts, (ii) melt-peridotite interactions during Jurassic mantle exhumation,i.e. the open-system reactive porousflow at spinel facies depths causing bulk depletion (origin of reactiveharzburgites and dunites), and the shallower melt impregnation which originated plagioclase-rich pe-ridotites and an overall mantle refertilization. We infer that migrating melts largely originated asshallow, variably depleted, melt fractions, and acquired Si-rich composition by reactive dissolution ofmantle pyroxenes during upward migration. Such melt-rock reaction processes share significantsimilarities with those documented in modern oceanic peridotites from slow- to ultraslow-spreadingenvironments and track the progressive exhumation of large mantle sectors at shallow depths in oceanicsettings where a thicker thermal boundary layer exists, as a consequence of slow-spreading rate.
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