Hyperquenched volcanic glass from Loihi Seamount, Hawaii

Explosive submarine basaltic eruptions, occurring in water depths of several kilometres, commonly result in the formation of layered volcaniclastic deposits. In order to quantify the cooling of such volcaniclastics, we have identified and separated two types of glassy fragments from these deposits on Loihi seamount: 1) fine (30-80 μm) limu o Pele bubble wall fragments and 2) coarser (0.8-1.2 mm) dense angular fragments. These pristine basaltic glasses have been subjected to differential scanning calorimetry (DSC) from room temperature up to temperatures above their glass transition. Heat capacity (c_p, in J g^{- 1} K^{- 1}) data reveal glassy and liquid regimes separated by clear hysteresis behaviour within the glass transition. The transient c_p in the glass transition interval exhibits a deep trough before the peak, indicating very high cooling rates. It is a classic feature of so-called "hyper-quenching", having been observed in DSC experiments on glasses produced using the splat-quench [D.B. Dingwell, P. Courtial, D. Giordano and A.R.L. Nichols, Viscosity of peridotite liquid, Earth and Planetary Science Letters 226(1-2), 127-138, 2004.], and cascade fibre-spinning [Y.Z. Yue, J.D. Christiansen and S.L. Jensen, Determination of the fictive temperature for a hyperquenched glass, Chemical Physics Letters 357(1-2), 20-24, 2002.] techniques. The trough is deepest for the limu o Pele fragments. Energy matching methods, developed for the estimation of cooling rates for such glasses, yield a rate of 10^5.31 K s^{- 1} for the cooling of the fine sieve fraction of limu glass on the sea floor at Loihi. Thus, limu o Pele volcaniclastics, believed to be formed during mild submarine pyroclastic eruptions, have experienced the fastest cooling of any natural volcanic glass measured to date. Such extreme cooling rates are likely to impact on many chemical and physical aspects of the stability of these glasses on the sea floor, as well as their use as proxies for field variables of the Earth's physical state and as monitors of the efficiency of chemical lithosphere-hydrosphere exchange.