Hydrothermal alteration of Tertiary igneous rocks from the Isle of Skye,northwest Scotland

Hydrothermal alteration of Tertiary gabbros from Skye involved the reaction of igneous olivine, augite, hypersthene, plagioclase, magnetite, and ilmenite with aqueous fluid primarily to combinations of talc, chlorite, montmorillonite, calcic amphibole, biotite, and secondary magnetite. Lesser amounts of calcite, epidote, quartz, sphene, prehnite, and garnet also developed. During mineralogical alteration of gabbro there was a net addition to rock of K, Na, Sr, and H₂O and a net loss of Mg. Gabbro was oxidized early in the hydrothermal event and later reduced. Iron and silicon were probably initially lost and later added. There is no evidence for significant change in the Al or Ca content of the gabbros. Hydrothermal alteration of Skye gabbro involved not only large-scale migration of ¹⁸O, ¹⁶O, D and H but also of K, Na, Sr, Mg, and probably Fe and Si.Mineral thermometry indicates that pyroxenes in the gabbros crystallized at 1000° C–1150° C and were very resistent chemically as well as isotopically to later hydrothermal alteration. Hypothetical equilibrium between primary and secondary mafic silicates suggests that mineralogical alteration of gabbro occurred at 450°–550° C. The lack of correlation between mineralogical and isotopic alteration of gabbro requires that much isotopic alteration occurred at temepratures above those at which the secondary minerals developed, 550°–1000° C. The chemical alteration of gabbro is correlated with its mineralogical alteration and therefore occurred at 450°–550° C.Measured progress of the mineral-fluid reactions was used to estimate the amount of H₂O fluid that infiltrated the gabbro as primary olivine was converted to talc+magnetite at 525°–550° C. Calculated fluid-rock ratios are in the range 0.2–6 (volume basis) and are smaller than values estimated from isotopic data (fluid/rock 1–10, volume basis). Both isotopic and petrologic data point to pervasive flow of fluid through crystalline rock at elevated temperatures of 500°–1000° C. Isotopic fluid-rock ratios are larger than petrologic fluid-rock ratios because isotopic alteration of cooling gabbro began earlier and at higher temperatures than did the mineralogical alteration.