Trace element partitioning in lamproitic magmas—the Gaussberg olivine leucitite

Abstract

Partition coefficients for compatible and incompatible trace elements for use in modelling the origin and evolution of lamproitic and other potassic rocks are reported. These are based on values obtained by in situ determination using laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) on mineral/glass pairs in examples of the Gaussberg olivine leucitite, a classic leucite lamproite. Leucite/melt partition coefficients for 32 elements show values above 1 only for Cs and Rb, whereas values for Ni, Ga and Co are 0.4–0.1. D Lc/Lq for most other elements are in the range 0.07–0.0005, and delineate a gently sloping pattern to lower values for elements generally considered to be “incompatible”. Olivine/melt partition coefficients show higher values for the compatible elements Ni, Co, Mn and Cr than in all basaltic systems investigated experimentally to date. D Ol/Lq for heavy rare earth elements (HREE) are slightly lower than in basalts, whereas most other elements give similar values. Clinopyroxene/melt partition coefficients appear to vary widely as a function of the Al 2O 3 content of the rock. Values obtained here for the Gaussberg rock consistently show appreciably higher D Cpx/Lq than in basaltic systems for the incompatible elements Cs, Rb, Ba, Th, U, Nb, Ta, La and Ce, which is attributed to a larger M2 site. Early salitic cores have patterns with lower D-values for these elements and higher values for HREE, similar to those known from alumina-rich potassic rocks. Thus, the values obtained here should be applied only with caution to nonlamproitic rocks. Other notable characteristics are high D U/ D Th and low D Zr and D Hf, the latter of which is attributed to structural characteristics of peralkaline melts.