PALEOTEMPERATURE OF HYDROTHERMAL ALTERATION FROM CHLORITE GEOTHERMOMETRY AND ITS IMPLICATION FOR AR DATING OF KERGUELEN ARCHIPELAGO BASALTS FROM MT. SENTINELLES

The Kerguelen Archipelago is located in the southern Indian Ocean and is composed mainly of transitional tholeiitic to alkalic basalts and shallow gabbroic intrusions exposed in glacial valleys up to 1 km in relief.  These flood basalts contain numerous alteration minerals, including zeolites and clays.  Many argon dating studies of the archipelago have made the assumption that the alteration has not enhanced Ar diffusive loss from plagioclase or mobility of potassium in the groundmass.  To constrain the paleotemperature of the alteration event, a study was conducted to identify and determine compositions of zeolite and clay minerals.  XRD (X-ray diffraction) analyses of random mounts of the secondary amygdules from three samples of the Sentinelles section identified the zeolite thomsonite and some clay peaks. In oriented mounts, chlorite was the only clay identified in two of the samples. The third sample contained chlorite, minor smectite, minor mica (illite) and possible vermiculite as the clay minerals.  Microbeam (SEM) analysis established that a Mg-rich chlorite and amorphous-SiO₂ were restricted to the outside (rim) on the nodules.  Heulandite was the only confirmed zeolite in the interior of the nodules.  Chlorite geothermometry, based on Si-Al substitution in the tetrahedral site, indicated alteration temperatures are between 234° C and 264° C.  The presence of heulandite also suggests relatively high temperatures occurred during zeolite facies metamorphism.  Using a cooling rate of 1200° C/Ma, modeling of argon closure temperatures (T_c) in plagioclase with an average radius of 30 µm, typical of aphyric Kerguelen basalts, indicates that the T_c is ~ 280° C.  But as grain size decreases so does the T_c of the sample, so basalts with a 10 µm plagioclase radius and the same cooling rate would have a T_c of ~ 238° C.  The relatively high alteration temperatures recorded by chlorite and heulandite indicate that the reliability of K-Ar and ⁴⁰Ar/³⁹Ar dates measured on whole rock and groundmass separates of aphyric basalts from the Kerguelen Archipelago should be re-evaluated.