Cooling rates of hyaloclastites: applications of relaxation geospeedometryto undersea volcanic deposits

Glass fragments from three different hyaloclastites have been used to evalu ate the range of cooling rates experienced by undersea volcanic deposits. W e found that the glass fragments retain structures with a range of apparent quench rates from 25 to 0.15 K min(-1). The most rapid cooling rates are i nterpreted to be those resulting from cooling of the lava near the water in terface. Simple conductive cooling models produce a range of quench rates c omparable to those of the more rapidly cooled samples. The very slow appare nt quench rates are unlikely to result from simple linear cooling through t he glass transition, because of the onset of crystallization; instead, they are indicators of a more complex thermal history that involves the anneali ng of glasses at temperatures within the glass transition interval for a dw ell time sufficient to allow the relaxation of the glass to lower temperatu re structures. The thermal history recorded in these samples illustrates th e complexity of eruptive processes and demonstrates that quench rates for n atural glasses retain information relevant to more complex cooling models.