Abstract
Little is known about the presence, distribution and size of bubbles in rhyolitic magmas prior to eruption. Using X-ray tomography to study pumice from early-erupted Bishop rhyolite, we discovered a large vesicle with abundant magnetite crystals attached to its walls. Attachment of magnetite crystals to bubble walls under pre-eruptive conditions can explain the cluster of magnetite crystals as a result of bubbles rising and collecting magnetite crystals. Alternatively, bubbles may have nucleated on magnetite crystals and then coalesced to form one large bubble with multiple magnetite crystals attached to it. We argue that the clusters of magnetite crystals could not have formed during or after eruptive decompression, and conclude that this vesicle corresponds to a bubble present prior to eruptive decompression. The inferred presence of pre-eruptive bubbles in the Bishop magma confirms the interpretation that the magma was volatile-saturated prior to eruption. The pre-eruptive size of this bubble is estimated based on three independent approaches: (1) the current size of the vesicle, (2) the total cross-sectional area of the magnetite crystals, and (3) the bubble size required for the aggregate to be neutrally buoyant. These approaches suggest a pre-eruptive bubble 300–850 μm in diameter, with a preferred value of 600–750 μm.
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Acknowledgments
We have benefited from the assistance of Mark Rivers during acquisition of tomographic data. We are indebted to stimulating discussions with Oded Navon at an early stage of this work. Discussions with Mark Ghiorso helped develop the ideas presented here. Reviews by RSJ Sparks and an anonymous reviewer, and editorial handling by Timothy Grove helped improve the manuscript. Reviews of an earlier version of this manuscript by Charles Bacon and Margaret Mangan are greatly appreciated. Portions of this work were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory; GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-0217473), Department of Energy—Geosciences (DE-FG02-94ER14466) and the State of Illinois; use of the APS was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract No. W-31-109-Eng-38. Thanks also to NSF support (EAR-0408707) to A. T. Anderson.
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Communicated by T. L. Grove.
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410_2006_173_MOESM1_ESM.mpg
Video 1. Three-dimensional view of Sample F8-15; glass and vesicles were suppressed for clarity. Magnetite crystals are shaded blue, quartz is shaded green and sanidine is shaded red. A few large magnetite crystals are observed, but most of the crystals are concentrated in an elongated array close to the edge of the sample. Sample is a cylinder with ca. 8 mm in diameter and 7 mm in height. (mpg 13.1 MB)
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Video 2. Detailed three-dimensional view of the large vesicle (shaded yellow) surrounded by an array of magnetite crystals (shaded blue). Magnetite crystals are in contact with the vesicle walls, but most of the crystal volumes are embedded within the glass (suppressed for clarity). Elongated cluster of magnetite crystals is ca. 4.3 x 0.7 mm. (mpg 6.28 MB)
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Video 3. Detailed three-dimensional view of a small region including one magnetite crystal within the cluster shown in Video 2. (a) Only the magnetite crystal is shown (shaded blue). (b) Magnetite crystal and glass (shaded orange) are shown, and it can be seen that the crystal is mostly embedded within the glass. (c) Magnetite crystal and vesicle (shaded yellow) are shown, and reveal that the crystal is in contact with the vesicle as well. (d) Magnetite crystal, and part of the vesicle and glass are shown, and clearly illustrate the contact relations between them. Volume shown is ca. 1 x 0.7 x 0.7 mm. (mpg 33.8 MB)
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Video 4. Detailed three-dimensional view of a small region including a second magnetite crystal within the cluster shown in Video 2. Views are analogous to those in Video 3. Magnetite is shaded blue, glass orange, and vesicle yellow. While the contact between glass and vesicle is more complicated than in Video 2, the same general contact relations are observed, with the magnetite crystal mostly embedded within the glass, but also in contact with the vesicle. Volume shown is ca. 0.9 x 0.6 x 0.5 mm. (mpg 33.9 MB)
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Video 5. Detailed three-dimensional view of a small region including several sanidine crystal fragments within a vesicle. Views are analogous to those in Video 3. Sanidine is shaded red, glass orange, and vesicle yellow. Irregular shapes of the sanidine crystals are indicative that they correspond to crystal fragments, and it can be seen that they are not embedded within the glass and are entirely located within the vesicle. Volume shown is ca. 0.8 x 2 x 3 mm. (mpg 36.5 MB)
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Gualda, G.A.R., Anderson, A.T. Magnetite scavenging and the buoyancy of bubbles in magmas. Part 1: Discovery of a pre-eruptive bubble in Bishop rhyolite. Contrib Mineral Petrol 153, 733–742 (2007). https://doi.org/10.1007/s00410-006-0173-5
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DOI: https://doi.org/10.1007/s00410-006-0173-5