Ti Partition Coefficients Research Articles

Origin of minor and trace element compositional diversity in anorthitic feldspar phenocrysts and melt inclusions from the Juan de Fuca Ridge

[1] Melt inclusions trapped in phenocryst phases are important primarily due to their potential of preserving a significant proportion of the diversity of magma composition prior to modification of the parent magma array during transport through the crust. The goal of this investigation was to evaluate the impact of formational and post-entrapment processes on the composition of melt inclusions hosted in high anorthite plagioclase in MORB. Our observations from three plagioclase ultra-phyric lavas from the Endeavor Segment of the Juan de Fuca Ridge document a narrow range of major elements and a dramatically greater range of minor and trace elements within most host plagioclase crystals. Observed host/inclusion partition coefficients for Ti are consistent with experimental determinations. In addition, observed values of DTi are independent of inclusion size and inclusion TiO2 content of the melt inclusion. These observations preclude significant effects from the re-homogenization process, entrapment of incompatible element boundary layers or dissolution/precipitation. The observed wide range of TiO2 contents in the host feldspar, and between bands of melt inclusions within individual crystals rule out modification of TiO2 contents by diffusion, either pre-eruption or due to re-homogenization. However, we do observe comparatively small ranges for values of K2O and Sr compared to P2O5 and TiO2 in both inclusions and crystals that can be attributed to diffusive processes that occurred prior to eruption.

Clinopyroxene–melt trace element partitioning and the development of a predictive model for HFSE and Sc

Clinopyroxene–melt trace element partitioning experiments were carried out in the system Na2O–CaO–MgO–Al2O3–SiO2 at pressures of 1, 2.3 and 3 GPa and temperatures of 1508 to 1811 K, to investigate the effects of temperature (T), pressure (P) and composition (X) on partition coefficients. Of particular interest were elements entering the octahedral M1-site. Ion probe analyses of run products produced crystal–melt partition coefficients (D) for 16 elements (Na, Ca, Al, Cl, Sc, Ti, Fe, Zr, In, La, Ce, Nd, Sm, Ho, Yb and Hf). With the exception of D Na, partition coefficients for all elements studied decrease with increased P and T, despite the concomitant increase in the Al content of the T-site. Fitting partition coefficients for isovalent series of cations to the elastic strain model of Blundy and Wood (1994) produced values for the site radius (r 0), effective elastic modulus (E) and strain-free partition coefficient (D 0). At each pressure, E values for the M1 and M2-sites increase with increasing Al concentration in the T-site $$ \left( {X_{\text{Al}}^{T} } \right) $$ . For a given bulk composition, E values decrease with increased T. The decrease in E with increasing T accounts for the remarkable constancy of the degree of fractionation between chemically similar elements, e.g. $$ D\left( {{\frac{\text{Zr}}{\text{Hf}}}} \right) $$ , over the range of pressures studied here. $$ E_{{{\text{M}}1}}^{4 + } $$ for our experiments is found to be higher than predicted by the Hazen and Finger (1979) relationship between elastic moduli and interatomic distance. This is explained by deformation of the M1-site polyhedron leading to relative displacement and kinking of the clinopyroxene T-site chains. We developed expressions for $$ E_{{{\text{M}}1}}^{4 + } $$ , $$ r_{{0,{\text{M}}1}}^{4 + } $$ , D Sc and D Ti as functions of P, T and composition. We show the feasibility of using calculated D Ti values in the prediction of D Zr and D Hf. Scandium and Ti partition coefficients were modelled based on the thermodynamic description for the crystal–melt exchange reaction and in terms of the energetics of the different charge-imbalanced configurations produced by insertion of a heterovalent trace cation. The resulting equations produce values of D Sc and D Ti that are within a factor of 2 of other experimentally determined values. Fits of the equations along the lherzolite solidus show that D Sc remains compatible in clinopyroxene at high pressure and that ratios of Zr/Hf and Ti/Eu should vary subtly with the pressure at which melting occurred.

Ti-V plots and the petrogenesis of modern and ophiolitic lavas

Plots of Ti vs. V for many modern volcanic rock associations are diagnostic of tectonic setting and can be used to determine possible tectonic settings of ophiolites as well. The basis of this plot is the variation in the crystal/liquid partition coefficients for vanadium, which range with increasing oxygen fugacity from > 1 to ≪ 1. Since the partition coefficients for Ti are almost always ≪ 1, the depletion of V relative to Ti is a function of the fO2 of the magma and its source, the degree of partial melting, and subsequent fractional crystallization. Volcanic rocksfrom modern island arcs have Ti/V ratios of ⩽20, except for calc-alkaline volcanics which show the effects of magnetite fractionation. MORB and continental flood basalts have Ti/V ratios of about 20–50 and alkaline rocks have Ti/V generally >50. Back-arc basinbasalts may have either arc-like or MORB-like Ti/V ratios, and sample suites from single back-arc basins may have Ti/V ratios ranging from 10 to 50. This range in Ti/V ratios in samples from a restricted geographical area may be diagnostic of the back-arc setting. The Ti/V plot is applied here to published data on ophiolites from a variety of postulated settings and in general supports the conclusions of previous investigators. Ophiolites from the western Mediterranean (Corsica, northern Apennines) and the “lower” Karmoy volcanics have Ti/V trends similar to MORB; the “upper” Karmo'y volcanics have alkaline Ti/V ratios. Lavas and tonalites in the Papuan ultramafic belt, the high-Mg andesites of Cape Vogel, and the upper pillow lavas at Troodos all have Ti/V ratios < 20, consistent with formation in an island arc setting. More specific evaluation of the tectonic setting of these and otherophiolites requires application of detailed geologic and petrologic data as well as geochemistry. The Ti/V discrimination diagram, however, is a potentially powerful adjunct to these techniques.