Stable Isotope Studies of Some Active Hydrothermal Systems

Citation

Lambert, Steven Judson (1976) Stable Isotope Studies of Some Active Hydrothermal Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/cdjc-wb26. https://resolver.caltech.edu/CaltechTHESIS:12042020-220145357

Abstract

Measurements of ¹⁸O/¹⁶O, ¹³C/¹²C and D/H ratios have been made on rocks and minerals from wells drilled in active hydrothermal systems as an aid to the understanding of various factors which govern the ongoing natural interactions between rocks and fluids, leading to a more thor­ough understanding of rock-fluid interactions that have taken place in the geologic past in such processes as diagenesis, metamorphism, and hydrothermal alteration. Samples from four active hydrothermal systems have been analyzed: The Geysers, western Mayacmas Mountains, California; Valles Caldera, northern Jemez Mountains, New Mexico; Heber Geothermal Anomaly, Imperial Valley, California; the Tiwi area, Luzon, Philippines.

The Geysers, a vapor-dominated hydrothermal system, is developed in Franciscan host rock which contains veins of quartz and calcite whose δ¹⁸O values record the temperatures and isotopic compositions of fluids which prevailed during at least two different episodes of rock-fluid interraction. The first episode took place at about 200°C, during which marine silica and carbonate apparently interacted with ocean water entrapped in the sediments to form veins of quartz and calcite whose δ¹⁸O values were around 19% and 16%, respectively. The calculated water/mineral ratios were less than unity. The water may have pro­foundly influenced the δ¹⁸O values of spilitic basalts during their metamorphism to greenstones. The formation and emplacement of serpen­ tinite bodies were isotopically unrelated to this episode, which was essentially a low-grade burial metamorphism (post-Cretaceous?) of rocks of the Franciscan Group. The second episode, in part recorded by co-genetic vein quartz and calcite δ¹⁸O values 4 to 6% and 1 to 3%, respectively, began with large quantities of meteoric water circulating in fractures in the rock at temperatures of 160 to 180°C in response to the Pliocene Clear Lake magmatism. Isotopic alteration of host rocks was most profound near fractures which carried the hot water. The temperature rose, and with the restricted circulation of fluids the ancestral hot-water system evolved into the presently-active vapor­ dominated system, which, according to the cogenetic vein quartz and calcite δ¹⁸O values, involved temperatures as high as 320°C and fluid/ mineral ratios near unity. The change in the isotopic composition of the host rock during this activity was negligible. The δ¹³C values of vein calcite at The Geysers reflect both a marine carbonate and organic component of carbon.

The other three areas contain hot-water systems, noteworthy of which is the Valles Caldera, an accumulation of previously hydrother­ mally unaltered porous sediments and pumiceous volcanic tuffs. Large volumes of meteoric water have interacted with the rock at various temperatures (100 to 280°C), indicated by changes in calcite, quartz δ¹⁸O values in the Caldera. The shallower, little altered rocks in the Caldera contain calcite and dolomite whose δ¹⁸O values suggest they were deposited in a fresh-water lake.

Poorly-consolidated Pleistocene sand beds, whose highly variable hydrothermal carbonate δ¹⁸O values indicate that they are isolated from one another by impermeable clay beds, form a series of hot-water reservoirs at Heber in the Imperial Valley. The degree of isotopic alteration indicated by the sand and its component quartz is very minor, indicating that this system is very young.

The few data from the Philippines indicate that significant hydro­ thermal alteration has taken place in the Tiwi hot-water system, also. The isotopic changes, however, are less profound than those observed at other areas because the δ¹⁸O of Philippines meteoric water is much closer to the initial δ¹⁸O value of the local host rock than is the case elsewhere.

Measurements of D/H ratios do not seem to be conclusive in char­acterizing degrees of rock-fluid interaction in active hydrothermal systems.

Thesis Files