QUARTZ-INCLUSION BAROMETRY APPLIED TO MAFIC ECLOGITES IN THE LLANO UPLIFT, CENTRAL TEXAS

Quartz-inclusion barometry based on laser Raman microspectrometry of garnetiferous mafic rocks of the Llano Uplift, central Texas, agrees with conventional barometry, but only when a systematic underestimation of residual pressures by ~0.2 GPa is taken into account.

Enami et al. (2007, Am Min 92:1303) demonstrated that quartz inclusions completely surrounded by garnet preserve a record of pressure at the time of entrapment. Pressures on inclusions are reduced during exhumation, but differences between garnet and quartz in thermal-expansion coefficients, bulk moduli, and shear moduli produce a residual pressure that can be related quantitatively to the entrapment pressure by models of elastic strain. Residual pressures are determined from measurements of the Raman frequencies of quartz inclusions. All values reported here use the most sensitive measure, which is the relative change with pressure in the frequencies for peaks with Raman wavenumbers near 205 cm^-1 and 128 cm^-1.

GRIPS barometry for mafic eclogites in the northern Llano Uplift yields pressures of 1.4±0.1 GPa. Initial results (n=15) from Raman quartz-inclusion barometry yield a median entrapment pressure of 0.7 GPa, computed from residual pressures with a median of 0.2 GPa. The discrepancy with GRIPS barometry suggests that these data systematically underestimate residual pressures. A similar discrepancy was described by Enami et al., whose measured residual pressures were systematically 0.1-0.3 GPa lower than those calculated from known entrapment pressures. If the Llano data for residual pressures are adjusted upward by 0.2 GPa to compensate for the systematic error, the resulting median entrapment pressure is 1.2 GPa (range 0.6-1.9 GPa, discounting two outliers). Values within this range are geologically reasonable: inclusion suites in the garnets record growth during compression, beginning in the amphibolite facies and ending in the eclogite facies; and GRIPS barometry records near-rim conditions and not end-of-growth conditions, so entrapment pressures > 1.4 GPa for some inclusions are also possible.

To use quartz-inclusion barometry confidently as a primary barometer, further work must be done to improve the precision of the technique and to understand and correct for the systematic underestimation of residual pressures.