Melt inclusion microthermometry: Petrologic constraints from the H2O-saturated haplogranite system

J. J. Student, R. J. Bodnar

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Abstract

A model is presented which predicts the P-V-T-X evolution of crystalline melt inclusions during heating and cooling. In the model, hydrous haplogranite melts (Ab-Or-Qtz-H2O) are trapped as inclusions in quartz, and then allowed to cool and crystallize. Mass and volume relationships for phases (melt-solid-volatile) in the inclusions during cooling and heating are calculated based on available phase equilibria and P-V-T-X data for the haplogranite system. The calculated P-T path followed by the inclusion during heating from room temperature is the reverse of the path followed during cooling following entrapment. As a result of second boiling during crystallization and cooling, the trapped melt releases exsolved H2O into the inclusion cavity. A positive volume change is associated with the release of water from the melt, and during cooling to solidus temperatures, the internal pressure within the inclusion can increase significantly. This overpressure can cause inclusion decrepitation during cooling. The calculated P-T path and the increase in internal pressure is highly dependent upon the values for the partial molar volume of H2Ototal in the melt (V̄H2OtotalM) used in the model, and several values for V̄H2OtotalM have been selected (0.0, 17.0, and 22.0 cm3/mol) which span the range of reported values in the literature. Three different trapping scenarios for haplogranite minimum melt compositions have been described at 500 and 2000 bars total pressure, including: (1) H2O-saturated melt entrapment, (2) immiscible H2O + H2O-saturated melt entrapment (mixed entrapment), and (3) H2O-undersaturated melt entrapment. The pressure at the time of trapping fixes both the minimum melt composition and temperature. Phase relationships during the homogenization process can distinguish each trapping scenario. For the H2O-saturated examples, total homogenization occurs with the simultaneous consumption of quartz, feldspar and vapor by the melt. The entrapment temperature (Tt) is determined by the temperature of complete melting of feldspar and quartz (Tmsil) and the vapor-melt homogenization temperature (Th), which are both equal to Tt. In the mixed scenario, total homogenization occurs by vapor-melt homogenization (Th) at an unreasonably high temperature and does not correspond to (Tt). In the mixed trapping scenario, the temperature of complete melting for feldspar (Tmfeld) corresponds to (Tt), at a temperature lower than vapor-melt homogenization (Th). Initial melt compositions for the H2O-undersaturated scenarios are determined for an activity of water in the melt (aH2OM) equal to 0.5. The minimum entrapment temperature for these H2O-undersaturated melts can be determined by the temperature of complete melting of feldspar and quartz (Tmsil). In this case, Th occurs at a lower temperature than Tmsil and Th defines the water-saturated liquidus temperature.

Original languageEnglish
Pages (from-to)291-306
Number of pages16
JournalPetrology
Volume4
Issue number3
StatePublished - May 1996

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