Evidence for mantle metasomatism by hydrous silicic melts derived from subducted oceanic crust

Abstract

The low concentrations of niobium, tantalum and titanium observed in island-arc basalts are thought to result from modification of the sub-arc mantle by a metasomatic agent, deficient in these elements, that originates from within the subducted oceanic crust. Whether this agent is an hydrous fluid or a silica-rich melt has been discussed using mainly a trace-element approach and related to variable thermal regimes of subduction zones. Melting of basalt in the absence of fluid both requires high temperatures and yields melt compositions unlike those found in most modern or Mesozoic island arcs. Thus, metasomatism by fluids has been thought to be the most common situation. Here, however, we show that the melting of basalt under both H2O-added and low-temperature conditions can yield extremely alkali-rich silicic liquids, the alkali content of which increases with pressure. These liquids are deficient in titanium and in the elements niobium and tantalum and are virtually identical to glasses preserved in mantle xenoliths found in subduction zones and to veins found in exhumed metamorphic terranes of fossil convergent zones. We also found that the interaction between such liquids and mantle olivine produces modal mineralogies that are identical to those observed in metasomatized Alpine-type peridotites. We therefore suggest that mantle metasomatism by slab-derived melt is a more common process than previously thought.

Slab melting and slab melt metasomatism in the Northern Andean Volcanic Zone : adakites and high-Mg andesites from Pichincha volcano (Ecuador)

“…It was shown experimentally that during this kind of interaction, hydrous minerals (pargasitic amphibole and phlogopite) crystallize in the mantle, while olivine and clinopyroxene are consumed [Sen and Dunn, 1995;Rapp et al, 1999 ;Prouteau et al, 2001]. As a result, the ensuing interacting magma is notably enriched in MgO and depleted in SiO 2.…”

Slab melting versus slab dehydration in subduction-zone magmatism

“…There is a long-standing debate about whether the fluids released from a subducting slab are aqueous fluid, hydrous silicate melt, supercritical fluid (SCF), or a combination of these (2,4,(8)(9)(10)(11)(12). Whether the subducting slab melts or dehydrates depends on the thermal structure and the phase relation of slab materials under hydrous conditions.…”

Sr–Nd isotope geochemistry and tectonomagmatic setting of the Dehsalm Cu–Mo porphyry mineralizing intrusives from Lut Block, eastern Iran

“…Thus, a strong participation of phlogopite decomposition (but not necessarily its complete melting) in the generation of the parental magmas would account for the potassium-rich nature and high Rb/Sr ratios displayed by the Dehsalm intrusives. Metasomatism of mantle peridotite by slab melts produces orthopyroxene, clinopyroxene, garnet, phlogopite, and richterite or pargasite (Sen and Dunn 1994;Rapp et al 1999;Prouteau et al 2001).…”