MAGNETITE SCAVENGING AND THE BUOYANCY OF BUBBLES IN MAGMAS. PART 2: ENERGETICS OF CRYSTAL-BUBBLE ATTACHMENT IN MAGMAS

Abstract

The fate of pre-eruptive bubbles depends largely on their buoyancy, which can be strongly modified by the presence of crystals attached to the bubble–melt interface. We define the attachment energy and attachment force as those resulting from the attachment of a crystal to a bubble. The attachment energy is such that (1) attachment of crystals to bubbles is always favored energetically, and (2) oxide minerals attach to bubbles much more strongly than silicates, because the attachment energy is a strong function of the wetting angle. Attaching crystals to bubbles can cause bubble–crystal pairs to become neutrally buoyant. There is a critical bubble radius below which the attachment force will be strong enough to keep the pair together; we show that crystals as large as 1 mm in diameter can form neutrally buoyant pairs. For early erupted Bishop magma, if all magnetite forms neutrally buoyant pairs with gas bubbles, ca. 0.1–0.2 vol% gas can be stored in the magma; 2–3 vol% of gas can be accounted for if all minerals form neutrally buoyant aggregates. These values are an order of magnitude lower than what is inferred from melt inclusions. Hence, both magnetite-free and magnetite-rich bubbles might have existed, but only a very small fraction of them could have been neutrally buoyant. Importantly, an intrinsic association between magnetite crystals and bubbles is expected. However, most magnetite crystals in the early erupted Bishop are free of bubbles; the puzzling conclusion is that nucleation away from crystals is favored over heterogeneous nucleation on crystal substrates.