DECODING SHALLOW FRAGMENTATION PATHWAYS OF MAGMA THROUGH JUVENILE ASH COLOR AND 3D MICROTEXTURES AT LOS ESPINOS VOLCANO
Explosive eruptions in monogenetic fields can shift rapidly between magmatic and water-influenced fragmentation, but the extent to which juvenile-ash color reliably tracks these changes is seldom tested against quantitative microtextures.
We study the adakitic deposits of Los Espinos volcano (Michoacán–Guanajuato Volcanic Field, Mexico) to evaluate whether color-defined juvenile populations, when integrated with SEM surface textures and X-ray microtomography, can constrain shallow fragmentation processes and edifice genetics.
Six tephra beds were analyzed using grain-componentry and morphometrics, complemented by SEM imaging and micro-CT of representative clasts. Four recurrent juvenile types were identified: (A) black, dense tachylite-like clasts with very low vesicularity; (B) dark-brown intermediates; and (C–D) green to Chetwode-green, glassy sideromelane-like clasts with high vesicularity and fluidal morphologies.
Morphometric and microtextural contrasts separate compact Type A grains from more elongate, less compact C–D grains, while micro-CT characteristics show abundant vesicles but limited development of connected vesicle networks. Lithics are most abundant in lower units, whereas vesicular green juveniles dominate upward. These observations are inconsistent with sustained, purely phreatomagmatic comminution and instead indicate a hybrid regime in which bubble-driven vesiculation supplied most fragmentation energy while intermittent magma–water interaction enhanced fragmentation efficiency and rapidly quenched clasts. The inferred shallow fragmentation depth and the shallow lithic assemblage support the classification of Los Espinos as a shallow-rooted tuff cone modulated by local groundwater. More generally, the results demonstrate that juvenile-ash color can serve as a robust process proxy when explicitly cross-validated with internal microtextural features, providing a workflow transferable to other small-volume hydrovolcanic eruptions.