The formation of large volumes of crystal-poor silicic magmas is a key question in understanding magmatic system feeding large explosive eruptions. Although the large number of processes involved in the crystal-melt separation (i.e. crystal settling, compaction, filter-press), the efficacy of each process in generating consistent volumes of aphyric eruptible magmas is poorly known. Seeking for an answer, we have experimentally investigated crystallization in presence of a thermal gradient as a possible mechanism for crystal-melt separation, considering both chemical and physical effects acting on a variably crystallized system. Our case study is the phonolitic volcanism of Sabatini Volcanic District (SVD), Central Italy. We used a natural tephri-phonolitic composition as starting material, using a thermal gradient-driven crystallization in order to simulate the crystallization process of a thermally zoned magma chamber. Crystallization degree and melt composition vary along the thermal gradient. In particular, melt composition ranges from the tephri-phonolitic starting composition at the bottom of the charge (hottest and aphyric zone) to phonolitic at the top (cooler and heterogeneously-crystallised zone), reproducing the same liquid line of descent observed in phase equilibria experiments. Backscattered images of experimental products clearly evidence: i) the aphyric tephri-phonolitic melt region at the bottom of the charge; ii) a drop-shaped crystal clustering in the middle zone; and iii) large aphyric belt and pockets (up to 100 µm wide) of phonolitic melt, with large deformed-shaped sanidine occurring at their margin, at the charge top region. Intriguingly, these batches of aphyric phonolitic melt, are separated from the highly crystallized zone by a thick mush of crystals (Cpx+Pl). A mere settling process cannot explain the upward accumulation of the aphyric phonolitic melt, while the high viscosity of the crystalline region would limit the efficacy of compaction and filter-press mechanisms. Alternatively, the brittle behaviour of the crystal framework (Glass < 10% vol.) could have lead to the instability of the crystal mush and the abrupt extrusion of the interstitial glass forming the phonolitic belts and pockets. The occurrence of a fast and intense segregation of the interstitial melt is supported by abundance of deformed crystals of sanidine. Textural features and phase relations observed in the experimentally-reproduced crystal mush are in good agreement with observations from crystalline ejecta emplaced in large phonolitic eruptions of SVD, representative of the crystallizing boundary layer of a phonolitic magma chamber.
Experimental constraints on crystal-melt separation in phonolitic magmas
MASOTTA, MATTEO;
2011-01-01
Abstract
The formation of large volumes of crystal-poor silicic magmas is a key question in understanding magmatic system feeding large explosive eruptions. Although the large number of processes involved in the crystal-melt separation (i.e. crystal settling, compaction, filter-press), the efficacy of each process in generating consistent volumes of aphyric eruptible magmas is poorly known. Seeking for an answer, we have experimentally investigated crystallization in presence of a thermal gradient as a possible mechanism for crystal-melt separation, considering both chemical and physical effects acting on a variably crystallized system. Our case study is the phonolitic volcanism of Sabatini Volcanic District (SVD), Central Italy. We used a natural tephri-phonolitic composition as starting material, using a thermal gradient-driven crystallization in order to simulate the crystallization process of a thermally zoned magma chamber. Crystallization degree and melt composition vary along the thermal gradient. In particular, melt composition ranges from the tephri-phonolitic starting composition at the bottom of the charge (hottest and aphyric zone) to phonolitic at the top (cooler and heterogeneously-crystallised zone), reproducing the same liquid line of descent observed in phase equilibria experiments. Backscattered images of experimental products clearly evidence: i) the aphyric tephri-phonolitic melt region at the bottom of the charge; ii) a drop-shaped crystal clustering in the middle zone; and iii) large aphyric belt and pockets (up to 100 µm wide) of phonolitic melt, with large deformed-shaped sanidine occurring at their margin, at the charge top region. Intriguingly, these batches of aphyric phonolitic melt, are separated from the highly crystallized zone by a thick mush of crystals (Cpx+Pl). A mere settling process cannot explain the upward accumulation of the aphyric phonolitic melt, while the high viscosity of the crystalline region would limit the efficacy of compaction and filter-press mechanisms. Alternatively, the brittle behaviour of the crystal framework (Glass < 10% vol.) could have lead to the instability of the crystal mush and the abrupt extrusion of the interstitial glass forming the phonolitic belts and pockets. The occurrence of a fast and intense segregation of the interstitial melt is supported by abundance of deformed crystals of sanidine. Textural features and phase relations observed in the experimentally-reproduced crystal mush are in good agreement with observations from crystalline ejecta emplaced in large phonolitic eruptions of SVD, representative of the crystallizing boundary layer of a phonolitic magma chamber.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
