Quartz-bearing rhyolitic melts in the Earth's mantle

Abstract

The occurrence of rhyolite melts in the mantle has been predicted by high pressure-high temperature experiments but never observed in nature. Here we report natural quartz-bearing rhyolitic melt inclusions and interstitial glass within peridotite xenoliths. The oxygen isotope composition of quartz crystals shows the unequivocal continental crustal derivation of these melts, which approximate the minimum composition in the quartz-albite-orthoclase system. Thermodynamic modelling suggests rhyolite was originated from partial melting of near-anhydrous garnet-bearing metapelites at temperatures ~1000 °C and interacted with peridotite at pressure ~1 GPa. Reaction of rhyolite with olivine converted lherzolite rocks into orthopyroxene-domains and orthopyroxene + plagioclase veins. The recognition of rhyolitic melts in the mantle provides direct evidence for element cycling through earth's reservoirs, accommodated by dehydration and melting of crustal material, brought into the mantle by subduction, chemically modifying the mantle source, and ultimately returning to surface by arc magmatism.

Fig. 1. Texture of rhyolite inclusions.

SEM back scattered electron (BSE) images showing the textures of rhyolite glasses (a–c) found in the different portions of the composite xenolith TL112 (large photo).

Fig. 2. Composition of rhyolite glasses.

SiO₂ vs. Al₂O₃ (a) and vs. alkali (Na₂O + K₂O) (b) covariation diagrams for the three glass types recorded in the TL112a and TL112b composite xenoliths from Tallante, South-East Spain. Compositions of melt inclusions (MI) from Ronda migmatites are also reported. Abbreviations: Qz = quartz.

Fig. 3. Mineralogy and isotopic variations along the xenolith.

Sketch reporting the mineralogical variation observed in the composite xenolith TL112 and the related oxygen isotopic values (expressed as δ¹⁸O ‰, with respect to the SMOW standard). The observed variation allows to schematize within the xenolith three distinct textural domains: peridotite, reaction zone, felsic vein. Emphasis is given to the textural framework in which distinct types of glass inclusions are observed in orthopyroxene crystals of the reaction zone, and to the glassy films recorded around quartz within the vein. Equilibration temperatures inferred by the MELTS modelling are also reported. Abbreviations: Ol olivine; Opx orthopyroxene; Cpx clinopyroxene; S spinel; Pl plagioclase; Ph phlogopite; Qz quartz.

Fig. 4. Ternary quartz-albite-orthoclase diagram illustrating normative compositions of melt inclusions and interstitial glasses (corrected for normative anorthite after).

Triangle apex are: Q quartz; Ab albite; Or orthoclase. The composition of the rhyolite melt-peridotite interaction trend modelled by MELTS (stars and dotted line) and the field of rhyolite melt inclusions in Ronda peridotite are shown for comparison. Eutectic and minimum points of the sub-aluminous haplogranite system at aH₂O = 1.0 (black dots), 0.5 and 0.4 (white dots) are also reported (adapted from the literature^,). Abbreviations: Qz quartz.

Fig. 5. Variation of δ¹⁸O with temperature during the interactions between the rhyolitic melts and the peridotite mantle.

Grey circles and lines represent the theoretical variation of δ¹⁸O values recalculated by mass balance from the MELTS model (Supplementary Table 3). Horizontal lines represent the δ¹⁸O values measured in the quartz crystals (this study) and mineral separates of the different portions of the composite xenolith (orange = reaction zone; blue = vein). The mineralogy predicted by the MELTS model are also reported. Abbreviations: Opx orthopyroxene; Pl plagioclase; Qz quartz; RZ reaction zone; MI melt inclusions. The vertical arrow represents the effect on δ¹⁸O of diffusion assisted re-equilibration of quartz crystals with surrounding Opx in the reaction zone (within type II Melt Inclusions).

Fig. 6. Cartoon showing the subduction and exhumation of continental crust slivers with the formation of rhyolite melts.

Note that in the conceived model the subducted crustal blocks (and the entrained rhyolite melts) are subsequently exhumed to shallower mantle depths during slab roll back processes in relation to their relative buoyancy. Finally, rhyolite melts segregate and escape from the crustal sources and interact with the surrounding peridotite bodies, within a crust-mantle melange, to form a veined mantle similar to by the composite mantle xenoliths reported in this study. Melting of such a metasomatized mantle may produce post-orogenic calc-alkaline to ultrapotassic magmas such as those occurring along the western Mediterranean. SCLM sub continental lithospheric mantle.