Multiple element diffusivities in natural olivine xenocryst from high-Mg diorite

Jörg Hermann, Qian Qing, Hugh St.C. O'Neill
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Microphotograph (X Nicols) of an olivine xenocrist embedded in a diorite matrix consisting of plagioclase, clinopyroxene, amphibole, biotite and quartz. The olivine grain is surrounded by a small corona of orthopyroxene. The change in interference colors from core to rim is related with an increase in fayalite component in olivine.

Despite solid-state diffusion being central to many geological processes, little is known about the factors controlling the rates of diffusion of different species in silicate minerals. Theoretical modeling of diffusivities in silicates suffers from a general lack of empirical data against which the modeling can be tested.

Olivine xenocrysts (0.5-3 mm in diameter, up to 20% in volume) were found in the chilled margin of one of the plutons of high-Mg diorite from Handan-Xingtai, central North China block, which was formed at an intracontinental setting. These hybridized high-Mg dioritic rocks formed during cooling from ~ 1000°C. One crystal with favorable dimensions and orientation, and despite some dissolution still retaining a crystal face, indicating minimal dissolution (Fig. 1), was selected for detailed study. Concentration profiles of Mg, Fe, Mn and Ni were determined by electron microprobe. The olivine was normally zoned in Mg/Fe, with Fo# [100*Mg/(Mg + FeT)] decreasing from core (89.1-93.2) to rim (73.2-81.4). Element mapping with the electron microprobe showed a gradual change of Fe, Mg, and Mn contents (Fig. 2). Concentration profiles of trace elements (Li, Na, Al, P, Ca, Sc, Ti, V, Cr, Mn, Co, Ni and Y) were then determined along the same or similar profiles by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). The profiles were acquired using different spot sizes as well as continuing scans using a 7 x 70 µm slit (Fig. 2).

The obtained data (Fig. 3) allow trace diffusion coefficients to be evaluated relative to Mg-Fe interdiffusion under conditions that cannot be accessed in the laboratory. The effective diffusion coefficients of many trace elements (Li, Ca, Sc, Mn, Co, Ni and Y) fall within a factor of three of each other and of the mean Mg-Fe interdiffusion coefficient, in agreement with results from laboratory experiments at higher temperatures (Spandler et al. 2007). By contrast, the profiles for Na, Ti and V imply much faster diffusion rates, while P shows no discernible diffusion. The Al and Cr profiles, which are well correlated with each other, are highly complex and variable on a small length scale. These data show that the diffusion coefficients of cations in olivine are not simple functions of either ionic charge or ionic radius. Using published Mg-Fe interdiffusion coefficients (Dohmen and Chakraborty, 2007), the characteristic residence time of the olivine xenocryst is modeled to be about 102 to 103 years.

Profiles through the olivine grain. B) Major element data for the forsterite content and NiO and MnO in wt.% from electron microprobe analysis. The red line refers to modeled diffusion profiles indicating that the olivine had a ~1000 year residence time at 950-1000°C. C), D) and E) Trace element profiles in olivine determined with LA-ICP-MS analyses (all values in ppm). Continuous lines were measured in scan mode whereas symbols refer to single spot analyses. Note the similar diffusion behavior of ions with different charges such as Li (1+), Mn (2+) and Y (3+).

Mg distribution X-ray map of the investigated olivine. Mg continuously decreases from core to rim. Also shown are the different ablation pits (180 µm, 70 µm spot size) and the tracks from the LA-ICP-MS analyses.

Dohmen R, Chakraborty S (2007) Fe-Mg diffusion in olivine ||: point defect chemistry, change of diffusion mechanisms and a model for calculation of diffusion coefficients in natural olivine. Physics and Chemistry of Minerals 34, 409-430, doi: 10.1007/s00269-007-0158-6

Spandler C, O'Neill, HStC. Kamenetsky V.S. (2007) Survival times of anomalous melt inclusions from element diffusion in olivine and chromite. Nature 447, 303-306. doi: 10.1038/nature05759.