Journal Article

Kiglapait Mineralogy III: Olivine Compositions and Rayleigh Fractionation Models<sup>*</sup>

S. A. MORSE

in Journal of Petrology

Volume 37, issue 5, pages 1037-1061
Published in print October 1996 | ISSN: 0022-3530
e-ISSN: 1460-2415 | DOI: http://dx.doi.org/10.1093/petrology/37.5.1037
Kiglapait Mineralogy III: Olivine Compositions and Rayleigh Fractionation Models*

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Olivines, present throughout the layered cumulates of the Kiglapait Intrusion, record extreme iron enrichment during fractional crystallization. Mn is less compatible than Fe; the exchange coefficient KD for Mn/Fe (OL-LIQ) has values near 0.95 but they drop to 0.7 near the Lower Zone-Upper Zone boundary. Ca is depleted by more than a factor of 30 relative to experimental values at 5 kbar pressure. Attempts to find a sink for Ca in the Lower Zone without invoking liquid as a reactant are unconvincing, and questions of metastability arise. Cumulus olivines vary systematically to lower Fo contents with stratigraphic height. Using equilibrium values for the exchange coefficient KD (Fe/Mg, 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 Feenrichment 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.

Keywords: olivine; Rayleigh fractionation; Kiglapait Intrusion; cumulative theory

Journal Article.  0 words. 

Subjects: Petrology

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