3D Quantitative Confocal Laser Microscopy of Ilmenite Volume Distribution in Alpe Arami Olivine

The deep origin of the Alpe Arami garnet lherzolite massif in the Swiss Alps proposed by Dobrzhinetskaya et al. (Science, 1996) has been a focus of heated debate. One of the lines of evidence supporting an exhumation from more than 200 km depth includes the abundance, distribution, and orientation of magnesian ilmenite rods in the oldest generation of olivine. This argument has been disputed in terms of the abundance of ilmenite and consequently the maximum TiO₂ content in the discussed olivine. In order to address this issue, we have directly measured the volume fraction of ilmenite of the oldest generation of olivine by applying confocal laser scanning microscopy (CLSM). CLSM is a method which allows for three-dimensional imaging and quantitative volume determination by optical sectioning of the objects. The images for 3D reconstruction and measurements were acquired from petrographic thin sections in reflected laser light with 488 nm wavelength. Measurements of more than 80 olivine grains in six thin sections of our material yielded an average volume fraction of 0.31% ilmenite in the oldest generation of olivine from Alpe Arami. This translates into 0.23 wt.% TiO₂ in olivine with error in determination of ±0.097 wt.%, a value significantly different from that of 0.02 to 0.03 wt.% TiO₂ determined by Hacker et al. (Science, 1997) by a broad-beam microanalysis technique. During the complex geological history of the Alpe Arami massif, several events of metamorphism are recorded which all could have caused increased mobility of the mineral components. Evidence for loss of TiO₂ from olivine is the tendency for high densities of ilmenite to be restricted to cores of old grains, the complete absence of ilmenite inclusions from the younger, recrystallized, generation of olivine, and reduction in ilmenite size and abundance in more serpentinized specimens. These observations suggest that only olivine grains with the highest concentrations of ilmenite are close to the original amount of TiO₂ incorporated in the olivine. Our measurements show maximum volume fraction of ilmenite of 1.21%, corresponding to 0.9±0.38 wt.% of TiO₂. Even the most conservative scenario reveals concentration of TiO₂ in olivine of more than 0.5 wt.%, a value comparable to that reported by Dobrzhinetskaya et al. (1996) and an order of magnitude greater than any previous measurement of TiO₂ in olivine. Experiments by Dobrzhinetskaya et al. (Chem. Geol, 2000) found that such high solubility can occur, but only at P>10GPa at mantle temperatures.