Crystal resorption as a driver for mush maturation: an experimental investigation

The thermal state of a magma reservoir controls its physical and rheological properties: at storage temperatures close to the liquidus, magmas are dominated by melt and therefore mobile, while at lower temperatures, magmas are stored as a rheologically locked crystal network with interstitial melt (crystal mush). Throughout the lifetime of a magmatic system, temperature fluctuations drive transitions between mush-dominated and melt-dominated conditions. For example, magma underplating or recharge into a crystal mush supplies heat, leading to mush disaggregation and an increase in melt fraction via crystal resorption, before subsequent cooling reinstates a crystal mush via crystal accumulation and recrystallisation. Here, we examine the textural effects of such temperature-driven mush reprocessing cycles on the crystal cargo. We conducted high-P-T resorption experiments during which we nucleated, grew, resorbed, and recrystallised plagioclase crystals in a rhyolitic melt, imposing temperature fluctuations typical for plumbing systems in intermediate arc volcanoes (20-40°C). The experiments reproduce common resorption textures and show that plagioclase dissolution irreversibly reduces 3D crystal aspect ratios, leading to more equant shapes. Comparison of our experimental results with morphologies of resorbed and unresorbed plagioclase crystals from Mount St. Helens (USA) reveals a consistent trend in natural rocks: unresorbed plagioclase crystals (found in Mount St. Helens dacite, basalt and quenched magmatic inclusions) have tabular shapes, while plagioclase crystals with one or more resorption horizons (found in Mount St. Helens dacite, quenched magmatic inclusions, and mush inclusions) show more equant shapes. Plagioclase crystals showing pervasive resorption (found in the dacite and mush inclusions) have even lower aspect ratios. We therefore suggest that crystal mush maturation results in progressively more equant crystal shapes: the shapes of plagioclase crystals in a magma reservoir will become less tabular every time they are remobilised and resorbed. This has implications for magma rheology and, ultimately, eruptibility, as crystal shape controls the maximum packing fraction and permeability of a crystal mush. We hypothesise that a mature mush with more equant crystals due to multiple resorption-recrystallisation events will be more readily remobilised than an immature mush comprising unresorbed, tabular crystals. This implies that volcanic behaviour and pre-eruptive magmatic timescales may vary systematically during thermal maturation of a crustal magmatic system, with large eruptions due to rapid wholesale remobilisation of mushy reservoirs being more likely in thermally mature systems

Magmatic processes in developing oceanic crust revealed in a cumulate xenolith collected at the East Pacific Rise, 9°50′N

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 7 (2006): Q12O04, doi:10.1029/2006GC001316.The petrology and geochemistry of a xenolith, a fragment of a melt-bearing cumulate, within a recently erupted mid-ocean ridge (MOR) lava flow provide information on petrogenetic processes occurring within the newly forming oceanic crust beneath the northern East Pacific Rise (NEPR). The xenolith reveals important petrologic information about MOR magmatic systems concerning (1) melt distribution in a crystal-dominated mush; (2) melt-crystal reactions within the mush; (3) the chemistry of melts that have contributed to the cumulate lithology; and (4) the chemistry of axial melts that enter the axial magma system. The xenolith was enclosed within a moderately primitive, normal mid-ocean ridge basalt (NMORB) erupted in 1991 within the neovolcanic zone of the NEPR, at approximately 9°50′N. The sample is a matrix-dominated, cumulate olivine anorthosite, composed of anorthite (An94-90) and bytownite (An89-70), intergranular olivine (Fo86±0.3), minor sulfide and spinel, and intergranular glass. Marginal corrosion of plagioclase, and possibly olivine, and internal remelting of plagioclase indicate syntexis. It is surmised that the pore volume was eviscerated several times with moderately primitive basaltic melts and reduced by intergranular crystallization of forsteritic olivine. The presence of anorthite as a cumulate phase in the xenolith and the observation of anorthite xenocrysts in NMORB lavas, and as a cumulate phase in ophiolite gabbros, indicate that Ca-rich melts that are not a part of the NMORB lineage play an important role in the construction of the oceanic crust.The Mineral Resources Program, USGS, provided support to W.I.R. for this research. Field and laboratory research was supported by NSF grants OCE-9402360, 9403773, and 0138088 to M.R.P. and NSF grants OCE-9819261 and OCE-0525863 to D.J.F

²³⁸U-²³⁰Th model ages versus selected trace element abundances or ratios.

<p>Bumpass sequence (350-190 ka; dark grey) and the Eagle Peak and Twin Lakes sequences (<90 ka; light grey; both from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Clynne3" target="_blank">[37]</a>) are marked on the figure, along with the eruptions from which the zircon were sampled (dashed lines). The eruptive hiatus (190-90 ka) is marked in white. From top to bottom, (a) Hf (ppm) of zircon surfaces and interiors; (B) Eu/Eu* of zircon surfaces and interiors; (c) Th/U of zircon surfaces and interiors.</p

Plagioclase Populations and Zoning in Dacite of the 2004–2005 Mount St. Helens Eruption: Constraints for Magma Origin and Dynamics

We investigated plagioclase phenocrysts in dacite of the 2004–5 eruption of Mount St. Helens to gain insights into the magmatic processes of the current eruption, which is char­acterized by prolonged, nearly solid-state extrusion, low gas emission, and shallow seismicity. In addition, we investigated plagioclase of 1980–86 dacite. Light and Nomarski microscopy were used to texturally characterize plagioclase crystals. Electron microprobe analy­ses measured their compositions. We systematically mapped and categorized all plagioclase phenocrysts in a preselected area according to the following criteria: (1) occurrence of zones of acicular orthopyroxene inclusions, (2) presence of dissolution surface(s), and (3) spatial association of 1 and 2. Phenocrysts fall into three main categories; one category con­tains four subcategories. The range of anorthite (An) content in 2004–5 plagio­clase is about An57–35 during the last 30–40 percent crystal­lization of plagioclase phenocrysts. Select microphenocrysts (10–50 μm) range from An30 to An42. Anorthite content is lowest near outermost rims of phenocrysts, but zonation pat­terns between interior and rim indicate variable trends that correlate with textural features. Crystals without dissolution surfaces (about 14 percent of total) show steadily decreas­ing An content outward to the crystal rim (outer ~80 μm). All other crystals are banded as a consequence of dissolu­tion; dissolution surfaces are band boundaries. Such crystals display normal outward An zoning within a single band that, following dissolution, is then overgrown abruptly by high-An material of the next band. Swarms of acicular orthopyroxene inclusions in plagioclase are characteristic of 2004–5 dacite. They occur mostly inward of dissolution surfaces, where band composition reaches lowest An content. The relative propor­tions of the three crystal types are distinctly different between 2004–5 dacite and 1980s dome dacite. We propose that crystals with no dissolution surfaces are those that were supplied last to the shallow reservoir, whereas plagioclase with increasingly more complex zoning patterns (that is, the number of zoned bands bounded by dissolution surfaces) result from prolonged residency and evolution in the reservoir. We propose that banding and An zoning across multiple bands are primarily a response to thermally induced fluctuations in crystallinity of the magma in combination with recharge; a lesser role is ascribed to cycling crystals through pressure gradients. Crystals without dissolution surfaces, in contrast, could have grown only in response to steady(?) decompression. Some heating-cooling cycles probably postdate the final eruption in 1986. They resulted from small recharge events that supplied new crystals that then experi­enced resorption-growth cycles. We suggest that magmatic events shortly prior to the current eruption, recorded in the outermost zones of plagioclase phenocrysts, began with the incorporation of acicular orthopyroxene, followed by last resorption, and concluded with crystallization of euhedral rims. Finally, we propose that 2004–5 dacite is composed mostly of dacite magma that remained after 1986 and under­went subsequent magmatic evolution but, more importantly, contains a component of new dacite from deeper in the mag­matic system, which may have triggered the new eruption

Geochemical and petrological diversity of mafic magmas from Mount St. Helens

ISSN:0010-7999ISSN:1432-096

Zircon ²³⁸U-²³⁰Th model age distribution histograms of zircon from the three eruptive units analyzed from the LVC: ∼27 ka dacite of Lassen Peak (top), ∼1.1 ka rhyodacite of Chaos Crags (top middle) and 1915 dacite of Lassen Peak (bottom middle).

<p>The bottom histogram compiles all the zircon ages from across all three units. Parts of the histograms in black represent the surface (rim) analyses of zircon, while white are polished interior analyses. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157.s001" target="_blank">Appendix S1</a> for full ages and errors.</p

Hf versus Eu/Eu* for LVC zircon, color-coded for Ti-in-Zr temperatures.

<p>At temperatures greater than approximately 730°C (Hf <9,500 ppm), Eu/Eu* varies across a much wider range (0.4–0.75). This represents the zircon growth during the down-temperature path after a rejuvenation event while those with Eu/Eu* between 0.2–0.4 and Hf >9,000 ppm represent growth in the crystallizing mush in baseline zircon storage conditions.</p

Yb/Gd versus Th/U for LVC zircon.

<p>Crystal mush “cold” storage conditions are marked with a dashed box while the dashed line with the arrows marks the path to warmer conditions during rejuvenation events. Th/U should increase while Yb/Gd decreases as greater basaltic input heats the crystal mush <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Klemetti2" target="_blank">[41]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Barth1" target="_blank">[51]</a>.</p

Localized Rejuvenation of a Crystal Mush Recorded in Zircon Temporal and Compositional Variation at the Lassen Volcanic Center, Northern California

<div><p>Zircon ages and trace element compositions from recent silicic eruptions in the Lassen Volcanic Center (LVC) allow for an evaluation of the timing and conditions of rejuvenation (reheating and mobilization of crystals) within the LVC magmatic system. The LVC is the southernmost active Cascade volcano and, prior to the 1980 eruption of Mount St. Helens, was the site of the only eruption in the Cascade arc during the last century. The three most recent silicic eruptions from the LVC were very small to moderate-sized lava flows and domes of dacite (1915 and 27 ka eruptions of Lassen Peak) and rhyodacite (1.1 ka eruption of Chaos Crags). These eruptions produced mixed and mingled lavas that contain a diverse crystal cargo, including zircon. ²³⁸U-²³⁰Th model ages from interior and surface analyses of zircon reveal ages from ∼17 ka to secular equilibrium (>350 ka), with most zircon crystallizing during a period between ∼60–200 ka. These data support a model for localized rejuvenation of crystal mush beneath the LVC. This crystal mush evidently is the remnant of magmatism that ended ∼190 ka. Most zircon are thought to have been captured from “cold storage” in the crystal mush (670–725°C, Hf >10,000 ppm, Eu/Eu* 0.25–0.4) locally remobilized by intrusion of mafic magma. A smaller population of zircon (>730°C, Hf <10,000 ppm, Eu/Eu* >0.4) grew in, and are captured from, rejuvenation zones. These data suggest the dominant method to produce eruptible melt within the LVC is small-scale, local rejuvenation of the crystal mush accompanied by magma mixing and mingling. Based on zircon stability, the time required to heat, erupt and then cool to background conditions is relatively short, lasting a maximum of 10 s–1000 s years. Rejuvenation events in the LVC are ephemeral and permit eruption within an otherwise waning and cooling magmatic body.</p></div