Abstract
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.
Highlights
Multiple lines of petrological evidence support the occurrence of magma mixing: (1) mixed chemical and textural plagioclase populations (Dungan et al 1979; Meyer and Shibata 1990; Pan and Batiza 2003); (2) plagioclase morphologies indicative of disequilibrium (Kuo and Kirkpatrick 1982); (3) the presence of plagioclase that is too anorthitic to be in equilibrium with its host melt (e.g. Dungan et al 1979); and reverse zoning (e.g. Dungan et al 1978; Hellevang and Pedersen 2008)
We have shown that plagioclase crystal cargo from the Gakkel Ridge is complex and records a range of textures and compositions
A first-order observation that can be taken from the presented plagioclase compositions is that skeletal crystals and skeletal cores of matured skeletal crystals have lower core An contents compared to tabular and resorbed habits (Fig. 5a)
Summary
Textures and compositions of minerals record the physiochemical conditions and processes occurring within magmatic systems (Vance 1965; Meyer and Shibata 1990; Ginibre et al 2002; Pan and Batiza 2002; Pietranik et al 2006; Ridley et al 2006; Ginibre and Wörner 2007; Hellevang and Pedersen 2008; Viccaro et al 2010; Cashman and Blundy 2013; Neave et al 2014; Coote and Shane 2016; Bouvet de Maisonneuve et al 2016). Nielsen et al 1994; Lange et al 2013; Drignon et al 2018); layered intrusions (Maaløe 1976), the slow inter-diffusion of CaAl–NaSi prevents equilibration of adjacent compositional zones (Morse 1984; Grove et al 1984) This slow diffusion preserves textures over long timescales, providing petrologists with an observable record of the processes occurring within a magmatic system. Mush zone disaggregation is attributed to the presence of anorthitic plagioclase xenocrysts (Ridley et al 2006) and open-structured crystal networks (Pan and Batiza 2003) in MORB, as well as the formation of plagioclase ultraphyric basalts (PUBs) commonly sampled at ultraslow- to intermediate-spreading ridges (Lange et al 2013). The importance of interactions between percolating melt and pre-existing crystal frameworks within mush zones is demonstrated by textures of individual plagioclase (Coumans et al 2015) and plagioclase in cumulate xenoliths (Ridley et al 2006)
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