Cumulus olivines vary systematically to lower Fo contents with stratigraphic height. Using equilibrium values for the exchange coefficient KD (FeMg -1, OL-LIQ), and the observed mode of olivine and residual porosity, a Rayleigh fractionation calculation reproduces the observed Lower Zone trend without recourse to multiple injections of magma. An anomalously Fe-rich region above the Main Ore Band in the Upper Zone is ascribed to ponding of Fe-rich magma at the floor of the intrusion until later, buoyant residual magma entrained the Fe-rich residua. Such Fe-rich ponds probably account for the abnormal Fe enrichment shown by some other Fenner-trend layered intrusions.
Summation over the observed crystal compositions yields values of XMg that do not represent the equilibrium liquid, but that reduce to it by a simple multiplier p = 0.78. This result means that if KD is known, the high temperature, liquidus olivine compositions can be retrieved, or vice-versa. Because the observed olivine compositions are supported by realistic forward models, there is no need to invoke evolved boundary-layer liquids to explain the in situ crystallization of Kiglapait cumulates. Classical cumulate theory suffices.
Current Contents
................
J. Petrology Editor's Page