Original paper
The identification of high-density gas inclusions by microthermometry
Kerkhof, A. M. van den
Neues Jahrbuch für Mineralogie - Abhandlungen Band 165 Heft 1 (1992), p. 105 - 106
7 references
published: Dec 3, 1992
DOI: 10.1127/njma/165/1992/105
ArtNo. ESP154016501012, Price: 29.00 €
Abstract
The determination of density and composition of fluid inclusions in rocks by the study of phase transitions is a routine study method in order to reveal fluid-rock evolution during geological processes. Of particular interest is the recognition of high-density inclusions of CO2, N2 and CH4, and their mixtures, dominant in lower-crustal and upper-mantle rocks. As a complementary method, Raman micro-spectroscopy is applied for the identification of gas species in fluid inclusions. The merits of Raman analysis in fluid inclusion re search are generally accepted although the accuracy of quantitative gas analysis from Raman spectra, is a matter of discussion (Pasteris et al., 1988). Microthermometry is still the most basic method and requires the understanding of phase theory, in particular for constant-volume systems. Recent improvement of phase diagrams for unary, and ternary systems enables more reliable density determinations (Kerkhof, 1990). Phase transitions other than L-G-homogenization and melting, like partial homogenization, metastable homogenization, sublimation and melting without gas phase, observed in many natural fluid inclusions, appear to be most useful for the interpretation of fluid densities and compositions. A twofold division in constant-volume phase behaviour was made and is based on the final phase transition on warming inclusions after cooling to about - 180 °C: 1) L-G-homogenization (H-type) or 2) S-L- or S-G-homogenization ("sublimation") (S-type). The complete phase sequence for each type is uniquely given by the fluid composition and molar volume. High-density CO2-N2 inclusions typically show S+L+G → S+L → L and are characteristic for several eclogite and granulite facies rocks (Kerkhof, 1988; Andersen et al., 1990; Klemd et al., 1992). Furthermore, the accurate observation of all phase transitions shows the distinction between high-density (> 1.17 g/cm3) CO2 inclusions and lower-density CO2-N2CH4, both showing melting without a gas phase (S-type). In some cases, fluid inclusions earlier recorded as high-density CO2 appeared to be lower-density mixtures. On the other hand, the presence of extremely high-density pure CO2 inclusions (1.21 g/cm3) could be stated in several cases (Olsen & Kerkhof, 1990; Frezzotti et al., 1991).
Keywords
Phase transitions • sublimation • homogenization • fluid-rock evolution