Cryptoachneliths: Hidden glassy ash in composite spheroidal lapilli

• Published in: Journal of volcanology and geothermal research 2010-09, Vol.196 (1), p.77-90
• Main Authors: Carracedo Sánchez, M., Arostegui, J., Sarrionandia, F., Larrondo, E., Gil Ibarguchi, J.I.
• Format: Article
• Language: English
• Subjects: coalescence; cryptoachneliths; Crystalline rocks; Earth sciences; Earth, ocean, space; electron microscopy; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas; pyroclasts; spheroidal lapilli
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2010.07.009

Cryptoachneliths, perceptible by means of electron microscopy but unresolved under the optical microscope, occur unnoticed inside spheroidal lapilli of ultrabasic composition of the Cabezo Segura volcano (Calatrava volcanic province, Spain). The cryptoachneliths are glassy spherical particles that have compositions of Al-rich silicate with minor amounts of Fe, Ca and other elements. The smallest cryptoachneliths of < 1 μm in diameter (nanoachneliths) joined by coalescence to form microspheres > 1 μm (microachneliths) and homogeneous less regular masses of similar composition. Nano and microachneliths welded each other or to other types of volcanic particles (crystals, crystal fragments, spinning droplets, cognate lithic clasts, etc.) to form spheroidal lapilli and even bomb size clasts within proximal fall deposits of the Cabezo Segura volcano. The welding processes took place inside the eruptive column, previous to the fall of the spheroidal lapilli on top of the volcanic cone. The presence of the cryptoachneliths implies that lapilli and even bomb size tephra within deposits formed during explosive eruptions of low-viscosity basic to ultrabasic magmas should be carefully examined in order to establish key parameters of eruption dynamics, like size, amount and distribution of juvenile fine particles. ►Optical microscopy, SEM and EDX techniques applied to the study of lapilli size (2–64 mm) tephra from the Cabezo Segura volcano in the Calatrava volcanic province of Spain allowed the discovery of nanometer to micrometer in size, welded cryptoachneliths, not previously recognized, that form composite lapilli. Some of the processes that operate within eruptive columns from explosive events (particle collision, coalescence, etc.), whose observation is always problematic due to their opacity and inhomogeneity, might thus be inferred. ►Welding of juvenile pyroclasts inside eruptive columns is the subject of study and debate in recent bibliography. The dispersion and sedimentation of tephra, both in the proximal and distal regions of the volcano, have been modelled using sedimentation models. These simple models, although providing invaluable insights about fundamental processes, do not explicitly predict the dynamics and physical processes that may take place among the various types of particles that compose the eruptive column. The particles there can experience a variety of modifications which are poorly understood. Some of them are revealed by our study impyi