Quartz grains in recrystallized chert nodules from contact-metamorphosed
limestones of the Christmas Mountains, Texas, contain clusters of fluid
inclusions of < 1-3 mum, except in a zone near the intrusive contact. At
70 m from the contact, these inclusions form uniform clouds in the cores
of the grains, surrounded by narrow inclusion-free rims. At 50 m, the
inclusion clouds are found only in some of the grains and occupy < 2-3
vol% at the core. Nodules close to the contact (< 12 m) have no clouds.
The inclusions in the clouds contain mainly CO2 and H2O, as determined
by microthermometric, Raman, and micro-FTIR analyses. The mode of
occurrence suggests that these inclusions were the earliest to form and
contain fluids internally generated, most likely by the breakdown of
organic matter in chert. This is thought to have occurred before peak
metamorphism. The presence of larger inclusions, also containing mainly
CO2-H2O, on the periphery of some clouds suggests that some small
inclusions coalesced into larger ones. This coalescing process may have
formed some of the inclusions present in the higher-grade nodules, which
appear to be not directly associated with a cloud, although most were
probably formed later. Inclusion-free rims in the lower-grade quartz
suggest that some of the fluids from early inclusions were freed by
dissolution and reprecipitation and by recrystallization of quartz and
were lost from the rock. The absence of inclusion clouds at the highest
grade is attributed to this loss. The preservation in inclusions of
fluids generated during a prograde process (before peak metamorphism)
does not necessarily rule out the peak metamorphic influx of fluids.
However, stable isotope analyses also indicate the absence of a
pervasive flow of external fluids through the nodules during peak
metamorphism.