Origin of minor and trace element compositional diversity in anorthitic feldspar phenocrysts and melt inclusions from the Juan de Fuca Ridge

Üstünışık

Lange

et al. 2020

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Major, trace, and isotopic compositions of plagioclase megacrysts from plagioclase ultraphyric basalts (PUB) from multiple mid‐ocean ridges were determined to characterize the liquid compositions from which they were derived. Large (up to 2 cm) plagioclase megacrysts exhibit homogeneous major element compositions, with variable degrees of trace element and isotopic heterogeneity. The pattern and degree of heterogeneity is itself variable between megacrysts, with some being homogenous and others exhibiting a wide range of trace element and isotopic zoning. Liquids calculated to be in equilibrium with the plagioclase megacrysts exhibit a range equivalent to lavas erupted from entire ridge segments. Many of the megacrysts may be considered to be cognate xenocrysts in that they are related to the same mantle material as the host lavas in which they were carried to the surface. The observed trace element patterns together with the isotopic diversity suggest a complex set of processes are active, that include mixing of distinct primitive melts, fractional crystallization, and diffusive reequilibration. The degree to which each process influences the megacrysts varies between individual crystals in each sample. Equally important is the range of composition we see within individual crystals. For example, Ba concentrations vary by as much as a factor of 5 but not systematically from core to rim.

Section: Discussionsupporting

confidence: 85%

Section: Introductionmentioning

confidence: 99%

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

Üstünışık

Lange

et al. 2020

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“…This class of MORB is characterized by >15 volume % plagioclase, with individual crystals sometimes >1 cm in diameter. Previous work on these crystals (Adams et al, 2011;Lange et al, 2013;Nielsen, 2011;Nielsen et al, 1995Nielsen et al, , 2017Sours-Page et al, 1999) has documented their high anorthitic character and the large variability of their trace element and isotopic compositions. In addition, new information based on the CO 2 content of plagioclase-hosted MI suggests that many of these megacrysts crystallized in the upper mantle (~3-5 kbars), as deep as or deeper than olivine phenocrysts found in MORBs (Drignon et al, 2018).…”

Section: Statement Of the Problemmentioning

confidence: 99%

Reequilibration Processes Occurring in Plagioclase‐Hosted Melt Inclusions From Plagioclase Ultraphyric Basalts

Drignon

Tepley

et al. 2019

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Plagioclase‐hosted melt inclusions are infrequently used to investigate magmatic processes owing to the perception that they are less robust than olivine‐hosted inclusions. Based on a set of time series experiments ranging from 30 min to 4 days, we demonstrate that plagioclase‐hosted melt inclusions can preserve the original (primitive) magmatic signal if steps are taken to correct for post entrapment crystallization. Diffusion within plagioclase‐hosted melt inclusions is sufficiently fast that the melt inclusions can be homogenized within 30 min through heating experiments. As heating time increases, the composition of melt inclusions drifts. We attribute this longer‐term phenomenon to plastic deformation of the host plagioclase (crystal relaxation) inducing a decrease in the internal pressure in the melt inclusion combined with diffusion of elements across the host/melt inclusion interface. The rate of chemical reequilibration within melt inclusions is limited by the much slower rate of diffusion within the solid host. Indeed, the host/melt inclusion partition coefficient for MgO decreases by 25% from the 30‐min to the 4‐day experiments. Our results suggest that the primary character of plagioclase‐hosted melt inclusions can be recovered if one recognizes and corrects for the effects of the complex physical and chemical processes that occur during melt inclusion homogenization.

“…Aluminum‐rich spinel is present both as a groundmass phase and as inclusions in plagioclase. Plagioclase phenocrysts are characterized by abundant melt inclusions [ Danyushevsky et al ., ; Kohut and Nielsen , ; Adams et al ., ], and typically exhibit a slightly elongate morphology ranging in size from 0.2 to 20 mm, with most falling between 1 and 10 mm (Figure ). Plagioclase crystals exhibit resorption textures that suggest a complicated transport history, and are consistent with the observed chemical disequilibria between the plagioclase and their host lava [ Meyer and Shibata , ; Nielsen et al ., ; Cordier et al ., ].…”

Section: Characteristics Of Plagioclase Ultraphyric Basaltsmentioning

confidence: 99%

“…Understanding the petrogenesis of PUB lavas is important because the plagioclase phenocrysts within these magmas contain a wealth of unique information about crustal processes, as well as the characteristics of the primitive magma array present in the lower oceanic crust. Plagioclase crystals and plagioclase‐hosted melt inclusions can extend to compositions that have not been sampled as part of the array of MORB glasses [ Sours‐Page et al ., ; Adams et al ., ; Lange et al ., ] and can often record more primitive compositions than those erupted at the surface [ Sinton et al ., ; Kohut and Nielsen , ]. Thus, understanding how PUB lavas form provides the context for interpreting the compositions recorded in these phenocrysts.…”

Section: Introductionmentioning

confidence: 97%

The petrogenesis of plagioclase‐phyric basalts at mid‐ocean ridges

Lange

Tepley

et al. 2013

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[1] Plagioclase ultraphyric basalts (PUBs) have been sampled along most mid-ocean ridges with ultraslow to intermediate spreading rates. Over the past 40 years, the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation. However, when the global population of PUB lavas is examined, this hypothesis becomes less compelling. PUB host lavas demonstrate a large range of compositions and densities, similar to aphyric glasses from the same ridge segments. Most importantly, the majority of PUB host liquids are less dense than their phenocryst cargo, meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment. Furthermore, PUB lavas have never been sampled on axis at fast-spreading centers or from locations with noted contemporaneous axial magma chambers, where PUBs should be abundant if plagioclase is buoyant in mid-ocean ridge basalt (MORB). Instead, we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust, possibly followed by loss of olivine during magma ascent. PUBs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts, which precludes long crustal residence times for these magmas. In addition to being a proxy for lower spreading rates, our findings also suggest that PUB eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber.