The significance of plagioclase textures in mid-ocean ridge basalt (Gakkel Ridge, Arctic Ocean)

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. Electronic supplementary material The online version of this article (10.1007/s00410-019-1587-1) contains supplementary material, which is available to authorized users.

Trace Element and Isotopic Characteristics of Plagioclase Megacrysts in Plagioclase Ultraphyric Basalts (PUB)

“…MIs are common in plagioclase megacrysts in most samples, ranging in morphology from small inclusions in bands (e.g., Figure 1a) to large irregular inclusions (most common in South west Indian Ridge (SWIR) megacrysts). In the samples studied here, we see many of the textures described by Bennett et al (2019) for samples from the Gakkel Ridge, which are indicative of the complex processes described. However, in general, plagioclase megacrysts from PUB are typically more anorthitic, homogeneous, and simple in terms of zoning (e.g., Figure 1) compared to those samples described by Bennett et al (2019).…”

“…In the samples studied here, we see many of the textures described by Bennett et al (2019) for samples from the Gakkel Ridge, which are indicative of the complex processes described. However, in general, plagioclase megacrysts from PUB are typically more anorthitic, homogeneous, and simple in terms of zoning (e.g., Figure 1) compared to those samples described by Bennett et al (2019). Further examples of the crystal textures and the morphology and distribution of MIs are presented in the supplements to Nielsen et al (2017) Olivine phenocrysts are always present but are typically <2% by volume and, depending on the sample, can display either subhedral or more resorbed disequilibrium textures (Burleigh, 2012).…”

Consequences of a crystal mush-dominated magma plumbing system: a mid-ocean ridge perspective

“…One sample contains clinopyroxene only as plagioclase-clinopyroxene glomerocrysts. A detailed textural study of these glomerocrysts indicates that they represent fragments of crystal mush that were entrained in the basalt prior to eruption [111].…”

Reaction Between Mid‐Ocean Ridge Basalt and Lower Oceanic Crust: An Experimental Study

“…Phenocrysts and xenoliths in MORB samples provide further evidence of magma replenishment and subsequent reaction. Glassy MORB samples are widely known to host olivine and plagioclase crystals, which are commonly not in equilibrium with their surrounding glass (Bennett et al, ; Moore et al, ; Pan & Batiza, , ). Some of these samples contain phenocrysts proportions as high as 50% (Bennett et al, ; Hellevang & Pedersen, ; Lange et al, ), suggesting physical mixing and/or chemical reaction between a phenocryst‐poor magma with a crystal‐bearing mush.…”

“…Glassy MORB samples are widely known to host olivine and plagioclase crystals, which are commonly not in equilibrium with their surrounding glass (Bennett et al, ; Moore et al, ; Pan & Batiza, , ). Some of these samples contain phenocrysts proportions as high as 50% (Bennett et al, ; Hellevang & Pedersen, ; Lange et al, ), suggesting physical mixing and/or chemical reaction between a phenocryst‐poor magma with a crystal‐bearing mush. Melt‐bearing cumulate xenoliths in MORB from the East Pacific Rise and Iceland provide strong evidence for complex melt‐mush reaction history in the lower oceanic crust, including the passage of multiple MORB magmas through the network of intergranular pores, and disequilibrium‐induced dissolution and subsequent growth of crystals in the mush (Gurenko & Sobolev, ; Ridley et al, ).…”