A 3D VIEW OF VOLCANIC GRANOPHYRE

Granophyre, a fine-grained (<2 mm) intergrowth of quartz and alkali-feldspar, is commonly found in shallow intrusions but can also occur in ejecta of silicic volcanos. While the origin of this texture is typically attributed to rapid simultaneous growth of the two minerals [1], granophyre has also been shown to form under hydrothermal conditions, significantly below the granitic solidus [2]. As recent studies suggest low temperature crystallization of granites [3] and near- or sub-solidus storage of volcanic systems [4], formation of granophyre at low temperatures becomes a valid alternative to rapid growth.

We have gained insight into granophyre formation by characterizing the textural relationships between feldspar and quartz using synchrotron micro-tomography. Thin sections only provide a 2D view of granophyre. Whereas a full 3D view of the texture gives reason into why it forms. Samples of a small eruptive unit of the Fish Canyon Tuff, the Nutras Creek Dacite in CO were chosen as these preserve sanidine phenocrysts that are surrounded by a granophyre rim [5]. Our 3D reconstructions show that the rim of the sanidine phenocryst consists of a ~50-100 μm thick Ba-rich zone. From this Ba-rich zone, granophyre emerges with a texture of granophyric quartz stringers and feldspar that are perpendicular to the edge of the sanidine phenocryst. The texture of the granophyric quartz changes progressively from being more vermicular closer to the sanidine phenocryst to larger and more geometric stringers further away from the phenocryst. Ultimately, these quartz stringers lead to large, euhedral quartz crystals with granophyre-like melt channels inside. These melt channels have a 3D geometric pattern. Thus, the melt channels within the quartz crystals suggests that they formed directly from the granophyre and are closely related to the sanidine. These observations have gone unnoticed in a simple 2D view and offer vital information to decipher whether granophyre formed by rapid growth or under sub-solidus conditions.

[1] Barker (1970) Geol. Soc. Am. Bull. 81, 3339-3350 [2] Schloemer (1964) Geochem. Int. 3, 578-612. [3] Ackerson et al. (2018) Nature. 559, 94-97. [4] Cooper & Kent. (2014) Nature, 506, 480-483. [5] Lipman et al. (1997) Geology 25, 915-918.