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Determination of clinopyroxene-liquid and plagioclase-liquid trace element partition coefficients from the low-Ti Kalkarinji flood basalts and their application to geochemical modelling

L.M. Glass

Partition coefficients are widely used for modelling igneous crystal-melt processes such as partial melting, crystal fractionation and assimilation. In trace element modelling, it is essential to choose an appropriate set of partition coefficients relevant to the pressure-temperature conditions and rock composition of interest. However, partition coefficient data from the literature is of variable quality and often incomplete so that data from different authors using different methods have to be combined. With the advent of laser-ablation ICPMS (Inductively Coupled Plasma Mass Spectrometry) methods, it has become possible to measure a full range of trace elements in natural mineral-melt samples with high sensitivity and precision.The Kalkarinji basalts of northern Australia are characterised by pronounced upper-crust-like geochemical signatures and show well defined geochemical evolution trends on Harker diagrams. It has been suggested that the crustal signatures of low-Ti basalts of this kind are due to shallow-level assimilation-fractional crystallisation (AFC) processes occurring as a result of turbulent flow in magma conduits. In order to test this hypothesis by geochemical modelling it is necessary to have an accurate and appropriate set of mineral-melt partition coefficients.The Kalkarinji basalts are typically aphanitic in texture with rare microphenocrysts of clinopyroxene and plagioclase so that whole-rock compositions effectively represent liquid compositions. Trace element contents of clinopyroxene and plagioclase microphenocrysts were measured directly from thin sections of selected Kalkarinji basalts in order to determine a comprehensive set of partition coefficients. Analyses were undertaken using an UV excimer laser system (193nm wavelength) coupled to an Agilent 7500s quadrupole ICPMS. Only data from inclusion-free crystals showing internal geochemical homogeneity were considered.Table 1: Selected Preferred Mineral Melt Partition Coefficients for Tholeiitic Compositions

Clinopyroxene
Calcic Plagioclase
P
0.039
0.025
Rb
0.003
0.022
Sr
0.069
1.86
Y
0.515
0.007
Zr
0.076
<0.001
Nb
0.010
<0.001
Ba
0.004
0.237
Nd
0.186
0.030
Ta
0.005
0.004
Th
0.005
<0.001

The above values have been used to model FC and AFC geochemical evolution of the Kalkarinji basalts. Applying least-squares regressed data for parent-daughter fractionation steps over a range of 9 to 3wt% MgO, it can be demonstrated (Figure 1) that the observed geochemical evolution trends of the Kalkarinji basalts are consistent with crystal fractionation-only, i.e. no crustal assimilation is required to explain the trends. This result is consistent with Sr and Nd isotopic results obtained last year (Annual Report 2000) and implies that the crustal signature of the basalts was either acquired by crustal assimilation of picritic melts with >10wt% MgO or is a source-related feature.

Figure 1: Trace element (ppm) - MgO (wt%) modelled parent-daughter evolution steps for purely crystal fractionation, calculated using the new partition coefficients. These results demonstrate that assimilation of crustal material is not required to explain the trends in the 9 to 3 wt% MgO range.