On the Skaergaard intrusion and forward modeling of its liquid line of descent: A reply to 'Principles of applied experimental igneous petrology' by Morse, 2008, Lithos 105, pp. 395-399

摘要

Forward modeling based on an experimental investigation successfully duplicated main features of the gabbros in the Skaergaard layered series [Thy, P., Lesher, C.E., Nielsen, T.F.D., and Brooks, C.K. 2006. Experimental constraints on the Skaergaard liquid line of descent. Lithos 92,154-180.]. The foundation for the modeling was equilibrium melting experiments that were controlled by temperature and oxygen fugacity at low-pressure conditions. The experimental techniques and methods were chosen to represent a reasonable approximation to the inferred emplacement and crystallization conditions of the Skaergaard intrusion. The dike rocks used as starting materials define a strong differentiation trend that represents the Skaergaard liquid line of descent. This suite of dikes allowed liquidus conditions to be defined for a range of composition and temperature. The initial redox conditions were chosen based on measured and calculated estimates for the gabbros of interest. The melting experiments defined liquidus and subliquidus conditions that were used to understand crystallization of the lower and middle zones of the Skaergaard layered series, and can be extrapolated to the upper zone gabbros assuming perfect fractional crystallization. The forward modeling reproduces the cryptic variation seen in the main gabbro minerals (olivine, augite, plagioclase) well into the upper zone and provides reasonable liquidus temperatures and compositions. It can be shown that based on the assumption of Fe-Ti oxide modes in the middle and upper zones, a range of oxygen fugacity trends can be obtained. We repeat our previous conclusion that iron depletion and strong reduction in oxygen fugacity in the upper zone are only feasible for very high Fe-Ti oxide modes that exceed the experimental evidence as well as the observations from the gabbros. A strong drop in oxygen fugacity in the upper zone requires a significant sink for Fe-Ti oxides that so far has not beea identified. We thus reject Morse's [Morse, S.A., 2008. Principles of applied experimental igneous petrology: a comment on "Experimental Constraints on the Skaergaard liquid line of descent" by Thy, Lesher, Nielsen, and Brooks, 2006, Lithos 92: 154-180. Lithos 105, pp. 395-399.] contention that we violated in our original study established principles of applied experimental igneous petrology. Such principles dictate that experimental and forward models are carefully tested against field observations before petrologic processes can be verified.