Testing the hypothesis that simulated melting can identify evolving liquid compositions and crystallization models at the Stillwater Complex, Montana, USA

Layered intrusions present a number of problems in understanding how they crystallized, including identification of parental magma and the effect of trapped liquid on modifying the original mineral assemblage. This work explores how simulated remelting of rocks from layered intrusions using the MELTS program can potentially allow one to retrieve the original liquid composition (both major and trace elements) as well as the original mineral compositions. Although it can ideally distinguish between conventional factional crystallization and in situ crystallization models, model calculations demonstrate that equilibrium crystallization of interstitial liquids trend normal to parent liquid fractionation trends on AFM plots and are thus sensitive to errors in estimates of initial liquid fraction. Although major elements of retrieved interstitial liquids are only modestly affected by the late loss of a small but incompatible-element-rich liquid late in the crystallization of the rock, it is the concentration of the incompatible element in this late liquid fraction that are commonly used to estimate the original liquid fraction. This technique is applied to the Stillwater Complex, Montana, where modeled major-element interstitial liquid trends are broadly normal to possible major-element Rayleigh or in situ fractionation trends of the parent liquid. Although modeled incompatible trace-element ratios show more consistent trends, relying on incompatible elements to estimate the proportion of residual liquid can underestimate the trapped liquid shift effect on mafic minerals. A potentially better estimate can be found by simulated melting to reproduce primocryst plagioclase core compositions, which do not re-equilibrate easily in the absence of wholesale recrystallization. It is concluded that modeled Stillwater major-element liquid compositions are strongly affected by open-system behavior, particularly late-stage liquid migration and loss during solidification.

Duke Scholars

Author Alan E. Boudreau Earth and Climate Sciences