Magnetite scavenging and the buoyancy of bubbles in magmas. Part 1: Discovery of a pre-eruptive bubble in Bishop rhyolite |
| |
Authors: | Guilherme A R Gualda Jr" target="_blank">Alfred T AndersonJr |
| |
Institution: | (1) Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637, USA |
| |
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.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|