Ti diffusion in natural pyroxene

Abstract

Diffusion of Ti has been characterized in natural orthopyroxene and clinopyroxene under buffered conditions (IW, NNO, and QFM) and in air. In all pyroxene compositions studied, titanium diffusion appears relatively insensitive to crystallographic orientation and oxygen fugacity under the range of investigated conditions. For Ti diffusion in a natural enstatite and a Kilbourne Hole orthopyroxene (KBH opx), we obtain the following Arrhenius relations for diffusion over the temperature range 950–1150°C,DEn=1.45×10-11exp(-270±34kJ mol-1/RT)m2s-1,DKBH=3.70×10-11exp(-285±34kJ mol-1/RT)m2s-1.For Ti diffusion in a natural diopside and a chromian diopside at temperatures 900–1250°C we obtain the following Arrhenius relations, respectively,DDi=8.01×10-11exp(-282±20kJ mol-1/RT)m2s-1,DCrDi=2.97×10-11exp(-282±40kJ mol-1/RT)m2s-1.Diffusion rates of Ti in the Cr-diopside and KBH opx are essentially the same, whereas diffusion rates of Ti in the enstatite and diopside are only 1.3–1.8 times and 2.7–2.9 times that in the KBH opx, respectively. Titanium diffusion appears relatively insensitive to pyroxene composition, in contrast to findings for diffusion of some other cations. At magmatic temperatures, the rates of Ti diffusion in pyroxene are comparable to the rates of REE diffusion in opx and the rates of Al and MREE diffusion in diopside, although activation energies of Ti diffusion are considerably lower than those for REE diffusion in the pyroxenes. Finally, diffusion rates of Ti in pyroxenes are 1–3 orders of magnitude smaller than those of Mg and Fe in pyroxenes.Diffusivities of Ti in pyroxenes obtained from this study can be used to understand a range of geochemical mass transfer problems involving diopside and orthopyroxene. The closure temperatures for Ti in orthopyroxene and diopside are very similar to each other for comparable pyroxene grain size. Subsolidus redistribution of Ti between orthopyroxene and diopside results in characteristic reverse Ti zoning in orthopyroxene and normal Ti zoning in diopside. Such zoning patterns in coexisting orthopyroxene and diopside may be used to infer cooling rate of two-pyroxene bearing ultramafic and mafic rocks.