Oxygen Isotopic Constraints on the Origin of N O D U L a R Silica-apatite from the Har Peres Pyroclastics, Golan Heights, Israel

-Oxygen isotope composition of three types of unique nodules which consist of amorphous silica-apatite, cristobalite-apatite and tridymite-apatite associations interspersed amidst basaltic pyroclastics from the Hat Peres volcano, Golan Heights, Israel is reported. Unusual isotopic temperature (75~ estimated from oxygen isotope fractionation between cristobalite (6'80 = +25.5%0-apatite (6'80 = + 12.90/~) pair suggests that the nodule was not formed by present-day pedogenesis as has been previously proposed, but was a xenolith incorporated probably from the underlying siliceous phosphorites at a higher temperature. An observed negative oxygen isotopic fractionation (6180 = -5.1% 0 between tridymite (6'80 = +9.9%~) and associated apatite ( 6 1 8 0 = -~ 15.0%0) is indicative of the nodular formation under disequilibrium conditions. A plausible mechanism of formation of the apatite (and calcite) associated with tridymite is an epitaxial overgrowth on template tridymite ofmagmatic origin under the current weathering regime. Oxygen isotopic evidence indicates a complicated origin for the nodules. Key Words--Apatite, Har Peres pyroclastics, Oxygen isotope. I N T R O D U C T I O N Singer et al (1991) studied nodular minerals composed of silica-apatite which are interspersed amidst late Pleistocene basaltic pyroclastic strata in volcanic cones of the Golan Heights, Israel. The dominant mineral phase of silica is tridymite, with occasional amorphous silica, cristobalite and quartz. From field occurrence, solution chemistry of pore waters squeezed out from the pyroclastics and surface morphology as revealed by scanning electron microscopy, they proposed authigenic formation, where the minerals are precipitated from solutions draining pyroclastics that were undergoing meteoric weathering at normal temperature and pressure conditions. The temperature dependence of oxygen isotope fractionations among silica-water and phosphate-water can be approximated by experimental, empirical and theoretical equations (Kyser 1987). The oxygen isotope fractionation between a co-existing mineral pair provides the equilibrium temperature of formation. Then the oxygen isotopic composition of ambient waters from which the minerals have precipitated can be estimated. Secondary modification due to oxygen isotopic exchanges with ambient waters under normal temperatures is limited as long as the original structure is retained (Juillet Leclerc and Labeyrie 1987; Shemesh et al 1988). The objective of the present study is conducted to use oxygen isotope geothermometry to elucidate the genesis of the nodules consisting of silica-apatite association in pyroclastics from Golan Heights, Israel. Copyright 9 1994, The Clay Minerals Society MATERIALS AND METHODS