One contribution of 15 to a Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Textures and compositions of minerals can be used to infer the physiochemical conditions present within magmatic systems. Given that plagioclase is an abundant phase in many magmatic systems, understanding the link between texture and process is vital. Here, we present a database of textural and compositional data for > 1800 plagioclase crystals in mid-ocean ridge basalt from the Gakkel Ridge (Arctic Ocean) to investigate the physiochemical conditions and processes that govern the formation of plagioclase textures and compositions. The Gakkel basalts have high modal crystal contents (up to 50%). The crystal cargo is complex, with both individual plagioclase and glomerocrysts showing large variations in crystal habit, zoning and resorption. The most common types of zoning are reverse and patchy; we attribute patchy zoning to infilling following either skeletal growth or resorption. Resorption is abundant, with multiple resorption events commonly present in a single crystal, and results from both magmatic recharge and decompression. Periods of strong undercooling, distinct to quench crystallisation, are indicated by matured skeletal crystals and thin normally zoned melt inclusion-rich bands following resorption. Individual samples often contain diverse textural and compositional plagioclase groups. Furthermore, most plagioclase is not in equilibrium with its host melt. Finally, the porous open structures of some glomerocrysts suggest that they represent pieces of entrained disaggregated mush. We interpret this to indicate that the crystal cargo is not generally phenocrystic in origin. Instead, plagioclase crystals that formed in different parts of a mush-dominated plumbing system were entrained into ascending melts. The textures of individual crystals are a function of their respective histories of (under)cooling, magma mixing and decompression. The morphologies of melt inclusion trapped in the plagioclase crystals are associated with specific host crystal textures, suggesting a link between plagioclase crystallisation processes and melt inclusion entrapment. The database of plagioclase presented herein may serve as a template for the interpretation of plagioclase textures in magmatic systems elsewhere. Electronic supplementary material The online version of this article (10.1007/s00410-019-1587-1) contains supplementary material, which is available to authorized users.

Listvenites (or listwaenites) are produced by CO 2 -metasomatism of mantle-derived ultramafic rocks (Falk & Kelemen, 2015;Halls & Zhao, 1995). They are composed mainly of quartz and carbonate (magnesite and/or dolomite, ±Cr-or Mg-rich micas ± chlorite) and are often associated with serpentinites, ophicarbonates and/ or talc. Since their first description in the literature (Rose, 1837), they have been investigated for one of their main characteristics: the occurrence of mineralizations concentrating economically valuable metals, such as

The global mid-ocean ridge system is the most significant magmatic system on our planet and is the site of 75% of Earth volcanism 1. The vertical extent of mid-ocean ridge magmatic systems has been considered to be restricted: even at the ultraslowspreading Gakkel Ridge, where the lithosphere is thickest, crystallisation depths of magmas feeding eruptions are thought to be <9 km 2. These depths have been determined using the volatile contents of melt inclusions, which are small volumes of magma that become trapped within crystallising minerals. In studies of basaltic magmatic systems, olivine is the mineral of choice for this approach 2-6. However, pressures derived from olivine-hosted melt inclusions are at odds with pressures derived from basalt major element barometers 7 and geophysical measurements of lithospheric thickness 8. Here, we present a comparative study of olivine-and plagioclase-hosted melt inclusions from the ultraslow-spreading Gakkel mid-ocean ridge (Arctic Ocean). We show that the volatile contents of (complexly zoned) plagioclase-hosted melt inclusions correspond to significantly higher crystallisation pressures (mean 270 MPa) than (simply zoned) olivine-hosted melt inclusions (mean 145 MPa). The highest recorded pressure we find equates to 16.4 km depth below the seafloor. These higher depths are consistent with both the thickness of the Gakkel

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