DIRECT MEASUREMENTS OF LATENT-HEAT DURING CRYSTALLIZATION AND MELTINGOF A UGANDITE AND AN OLIVINE BASALT

Step-scanning calorimetric measurements using a Setaram HT1500 calorim eter were performed between 800 and 1400 degrees C on two natural samp les: a ugandite from the East African rift and an olivine basalt from the western Mexican are. Our measurements provide the first in-situ qu antitative assessment of enthalpy during melting of initially crystall ine natural samples. The distribution of latent heat across the liquid us-solidus intervals of the two samples is distinctly different, refle cting significant variation in the sequence and abundance of mineral p hases during melting (clinopyroxene and leucite in the ugandite; olivi ne, clinopyroxene, and plagioclase in the basalt). Our data further in dicate that the common assumption of a uniform distribution of latent heat across the liquidus-solidus interval of a magma is a reasonable a pproximation for the olivine basalt, but is grossly in error for the u gandite. This is due to cotectic precipitation of leucite and clinopyr oxene, leading to a large, disproportionate release of latent heat ear ly in the crystallization sequence. The implication for the thermal hi story of a crystallizing ugandite magma is that the rate of heat loss during conductive cooling will initially be more rapid than the averag e rate. The net result will be to produce lower magmatic temperatures after a given cooling interval relative to models assuming a uniform r elease of latent heat. An additional series of scanning calorimetric e xperiments were performed at variable rates (1, 2, and 3 degrees/min) to evaluate the role of kinetics on the distribution of enthalpy durin g both melting and crystallization of the ugandite and olivine basalt. The results indicate that clinopyroxene is the most important mineral phase in controlling the shapes of the enthalpy profiles during cooli ng; this is due to its large enthalpy of fusion and its tendency for s luggish nucleation, followed by rapid crystallization at temperatures that vary with cooling rate. The resolution of the calorimeter (in ter ms of heat detected per unit time) is also important in determining th e shapes of the observed enthalpy profiles during these rapid scans. E stimates based on the observed calorimetric signal associated with mel ting of olivine, and the lack of a calorimetric signal during melting of leucite, combined with known enthalpies of fusion for the two phase s, indicate detection limits of approximately 0.6-1.2 kJ per 5 min inc rements.