Melt inclusions at the millennium: toward a deeper understanding of magmatic processes

Abstract Magmatic processes associated with oceanic crustal accretion at slow-spreading mid-oceanic ridges are less well understood compared with those at fast-spreading ridges. Zoned plagioclase in the basalts might record these magmatic processes as a result of the very slow intra-crystal diffusion of CaAl–NaSi. Plagioclase phenocrysts in plagioclase-phyric basalt from Hole U1433B of International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea show complex zoning patterns (e.g. normal, reverse, oscillatory and patchy). These samples provide a rare opportunity to determine the magma dynamics associated with oceanic crustal accretion at slow-spreading ridges through time. Igneous lithological units in Hole U1433B consist of a series of massive lava flows at the bottom and a thick succession of small pillow lava flows at the top. Most of the plagioclase phenocrysts in the massive lava show core–rim zonation with high-An cores (An ∼85%; in mole fraction; Pl-A) in equilibrium with melts that are more primitive than their host. Some high-An cores of Pl-A phenocrysts contain melt inclusions and are depleted in La, Ce, Y and Ti, but enriched in Sr and Eu; this is interpreted as resulting from dissolution–crystallization processes during reaction of hot melt with pre-existing plagioclase cumulates. In the pillow lavas, most of the plagioclase phenocrysts show normal core–mantle–rim zonation (Pl-B) with An contents decreasing gradually from the core to the mantle to the rim, suggesting extensive magma mixing and differentiation. Reversely zoned plagioclases (Pl-C) are sparsely present throughout the basalts, but mostly occur in the lower part of the drill hole. The cores of euhedral Pl-C phenocrysts are compositionally comparable with the mantles of Pl-B phenocrysts, suggesting that the evolved magma was recharged by a relatively primitive magma. Melt inclusion-bearing Pl-A phenocrysts occur mainly in the massive lava, but rarely in the pillow lava, whereas Pl-B phenocrysts are present dominantly in the pillow lava, which reflects reducing melt–rock interaction and enhanced magma mixing, recharging and differentiation from the bottom to the top of the hole. In addition, the extensive magma mixing and differentiation recorded by Pl-B phenocrysts in the pillow lava require the existence of a melt lens beneath the mid-ocean ridge. Consistently, the plagioclase phenocrysts in the pillow lava mostly lack melt inclusions, corresponding to very weak melt–rock reactions, which indicates that the magma was transported through plagioclase cumulates by channel flow and requires a higher magma supply to the magma conduit. Therefore, the textural and compositional variations of plagioclase phenocrysts in the samples reflect the changes in magma dynamics of the mid-ocean ridge basalt through time with respect to oceanic crustal accretion at slow-spreading ridges. Overall, the oceanic crustal accretion process is sensitive to the magma supply. In the period between two episodes of extension, owing to a low melt supply the primitive melt percolates through and interacts with the mush zone by porous flow, which produces melt inclusion-bearing high-An plagioclase through dissolution–crystallization processes. At the initial stage of a new episode of extension, the melt infiltrates the mush zone and entrains crystal cargoes including melt inclusion-bearing high-An plagioclase. During the major stage of extension, owing to a relatively high melt supply the melt penetrates the mush zone by channel flow and can pool as melt lenses somewhere beneath the dikes; this forms intermediate plagioclases and the reverse zoning of plagioclases by magma mixing, recharging and differentiation in the melt lens. Such magmatic processes might occur repeatedly during the episodic extension that accompanies oceanic crustal accretion at slow-spreading ridges, which enhances the lateral structural heterogeneity of the oceanic crust.

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Controls on Eruption Style at La Soufrière de Guadeloupe from Melt Inclusions and Mineral Diffusion Timescales.

10.5194/egusphere-egu21-4545 ◽

2021 ◽

Author(s):

Abigail Metcalfe ◽

Séverine Moune ◽

Jean-Christophe Komorowski

Keyword(s):

Melt Inclusions ◽

Major Element ◽

Linear Trend ◽

Crisis Response ◽

Magma Ascent ◽

Current Phase ◽

Eruption Style ◽

Plinian Eruptions ◽

Volatile Emissions ◽

Magmatic Processes

Signals of volcanic unrest do not usually provide insights into the timing, size and style of future eruptions. However, analysis of past eruptions provides useful information in order to understand the evolution, magma storage and onset of future eruptions, Here, we examine basaltic-andesitic to andesitic eruption deposits from La Soufri&#232;re de Guadeloupe, covering a range of eruption styles, ages and magnitudes. Our work is timely given unrest at this system has increased over the last 25 years, with a potential eruption capable of directly impacting ~70,000 people in Southern Basse-Terre.Here, we study the geochemistry of pre-eruptive magmas and timescales of magmatic processes preceding four explosive eruptions: 1657 Cal. CE (Vulcanian), 1010 Cal. CE (Plinian), ca. 341 Cal. CE (Strombolian) and 5680 Cal. BCE (Plinian). Using diffusion timescale studies of orthopyroxene phenocrysts, we constrain the timing of magma injections into the La Soufri&#232;re de Guadeloupe magmatic reservoir. These range from 35 &#177; 0.37 to 848 &#177; 0.4 days before eruption. Diffusion timescales do not appear to correlate with eruption style/size, but may correlate with other parameters (e.g., magma interactions in the reservoir and/or volatile content of the magma).Major element concentrations in whole rock (WR), groundmass glasses (GM) and melt inclusions (MI) show a strong linear trend. However, this evolution cannot be resolved through fractional crystallisation alone, as there is no clear temporal trend. MIs reveal a relatively homogenous melt composition from the first to the most recent eruptions, ranging from 63.6 &#8211; 78.7 wt% SiO2. Volatiles, including H&#173;2O (2.3-4.4 wt%), CO2 (35-866 ppm) and sulphur (30-202 ppm), are also consistent across the various eruptions. MIs are often more evolved than the GM, indicating they cooled prior to their entrainment. This, along with the different crystal populations observed, suggests a recharge magma has intruded through a mush system and entrained crystals stored there. Crystals in different regions of the mush therefore experience different interactions with magmatic processes.The major element compositional homogeneity across the eruptions indicates that composition does not have a large control on eruption style at this system. However, MI pre-eruptive volatile contents are more concentrated in the larger Plinian eruptions (e.g., CO&#173;2 &#8211; 866 ppm) than the smaller Vulcanian and Strombolian eruptions (e.g., CO&#173;2 &#8211; 674 ppm). Volatile emissions calculated through the petrologic method also differ, with higher total volatile emissions observed in the Plinian eruptions (12 Mt) than the smaller eruptions (0.1 Mt). The Plinian eruptions also have a faster magma ascent rate (0.3-22 m/s) than the vulcanian eruptions (3 m/s) as calculated from mass flux estimates.&#160;Though the composition of the La Soufri&#232;re de Guadeloupe system has remained constant over time, changes in eruption style can result from variations: (i) in the way magma interacts with the mush system, (ii) in the pre-eruptive volatile contents and (iii) in the ascent rates. Understanding the controls on eruption style is important, especially during the current phase of unrest, in order to improve early-warning system efficiency, forecast models, eruption scenario crisis response and long-term risk reduction planning.

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