Abstract

Intermediate to silicic volcanic eruptions often emit more S than predicted by petrological models; this is known as the excess S problem. While most common minerals in these magmas are poor in volatile elements, the occurrence of large phenocrysts of S-rich haüyne (with as much as ∼13 wt% SO3) in phonolites holds much promise for better constraining volcanic volatile budgets in differentiated alkaline magmatic systems. We examined textural zonation patterns in haüyne separates from Tenerife (Spain), using mineral oil to enhance grain transparency. Included phases were characterized by energy dispersive spectroscopy, X-ray mapping, and Raman spectroscopy. Slow growth of phenocrystic haüyne, inferred from zones with few inclusions, probably represents cooling-induced crystallization from S-rich melt during storage in the upper crust. By contrast, rapid growth of haüyne, generating wispy zones containing Fe-rich haüyne laths and zones rich in melt inclusions, fluid inclusions, and Fe sulfide inclusions, is associated with magma recharge and/or upward percolation of a low-density fluid phase (i.e., gas sparging). Both processes can bring new pulses of S from deep within the magmatic system into the shallow reservoir. Zones containing thousands of fluid inclusions provide direct physical evidence that the melt was fluid saturated during periods of rapid haüyne growth. The transfer of S-rich fluid should occur in all volatile-rich magmatic systems, including dacitic-rhyolitic arc systems with large S excesses, but is difficult to document in subduction zone magmas devoid of a large S-rich mineral phase like haüyne.

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