Acid compositions in a veined-lower mantle, as indicated by inclusions of (K,Na)-Hollandite + SiO2 in diamonds

Abstract Diamonds from Juína area, Brazil, are known by their superdeep origin related to recycling of oceanic crust, basalts and pelagic sediments in the lower mantle. Chemical and structural data on mineral inclusions support this hypothesis. We have studied the inclusions of five diamonds from Juininha alluvial deposits, part of the Juína province, and we have found for the first time in terrestrial rocks, a paragenetic assemblage only observed in high-pressure experiments and/or extraterrestrial rocks. The results suggest the presence of a veined-type lower mantle, where an assemblage of hydrous basaltic origin (MgFeAl-perovskite) is crosscut by pockets and/or veins of intermediate to acid compositions, dominantly constituted by SiO2 + (K,Na)-Hollandite. The silicate paragenesis (KNa-Hollandite and SiO2) can be produced by melting of a pelagic metasedimentary source subducted together with the oceanic slab down to the uppermost lower mantle. Such a veined-type lower mantle puts in evidence a derived source modified by melting/dehydration of multicomponent from a subducted slab.