The partitioning of trace elements between clinopyroxene and trachybasaltic melt during rapid cooling and crystal growth

We present the variation in trace element partition
coefficients measured at the interface between rapidly
cooled clinopyroxene crystals and co-existing melts.
Results indicate that, as the cooling rate is increased,
clinopyroxene crystals are progressively depleted in Si, Ca
and Mg counterbalanced by enrichments in Al (mainly
tetrahedral Aliv), Na and Ti. Partition coefficients (Ds) for
rare earth elements (REE), high field strength elements
(HFSE) and transition elements (TE) increase with
increasing cooling rate, in response to clinopyroxene
compositional variations. The entry of REE into the M2
site is facilitated by a coupled substitution where either Na
substitutes for Ca on the M2 site or Aliv substitutes for Si in
the tetrahedral site. The latter substitution reflects an
increased ease of locally balancing the excess charge at M2
as the number of surrounding Aliv atoms increases. Due to
the lower concentration of Ca in rapidly cooled clinopyroxenes,
divalent large ion lithophile elements (LILE) on
M2 decrease at the expense of monovalent cations. Conversely,
higher concentrations of HFSE and TE on the M1
site are facilitated as the average charge on this site
increases with the replacement of divalent-charged cations
by Alvi. Although crystallization kinetics modify clinopyroxene
composition, deviations from equilibrium partitioning
are insufficient to change the tendency of a trace
element to be compatible or incompatible. Consequently,
there are regular relationships between ionic radius,
valence of the trace element and D. At both equilibrium
and cooling rate conditions, Ds for isovalent cations define
parabola-like curves when plotted against ionic radius,
consistent with the lattice strain model, demonstrating that
the partitioning of trace elements is driven by charge balance
mechanisms; cation substitution reactions can be
treated in terms of the energetics of the various chargeimbalanced
configurations.