T189 Frontiers in Non-Traditional Stable Isotopes: In Honor of Fang-Zhen Teng, Recipient of the 2014 MSA Award

Tuesday, 21 October 2014
8:00 AM - 12:00 PM
213

COOLING AND CRYSTALLIZATION HISTORIES OF MAGMATIC BODIES FROM IN-SITU MG-FE ISOTOPIC ANALYSES IN OLIVINES: TELLING ZONED FROM ZONED

Paper number: T189-4
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM

Authors

Abstract

In the field of diffusion-based geospeedometry, a challenging task is to identify diffusive transport in chemically zoned minerals. The difficulty comes from the fact that diffusion profiles may not be easily distinguished from concentration gradients inherited during crystal growth. Teng et al. (2011) and Dauphas et al. (2010) recognized that Mg-Fe isotopic analyses could be used to identify diffusive transport in olivine. From analyzing olivine fragments from Kilauea Iki lava lake, HI, Teng et al. (2011) showed that Mg-Fe isotopic compositions in these olivines are negatively correlated. This observation can only be explained by Mg-Fe interdiffusion as light isotopes diffuse faster than their heavier counterparts (Richter et al., 2009). This discovery led Sio et al. (2013) to use micro-analytical techniques to spatially resolve Mg-Fe isotopic profiles in a zoned olivine phenocryst from Kilauea Iki lava lake, showing unambiguous evidence that diffusive transport in olivine is accompanied by large and negatively coupled Mg-Fe isotopic profiles.

Laser ablation MC-ICPMS and SIMS techniques have been developed to analyze Mg-Fe isotopes in chemically zoned olivines, providing spatial resolutions of ~50 microns and precisions of ~0.2 ‰ in δ56Fe and δ26Mg (1 SD). These techniques have been validated by good agreements with Mg-Fe isotopic data obtained using microdrilling followed by MC-ICPMS on the same zoned olivines.

Using these in-situtechniques, diffusive transport has been recognized in martian olivines. Diffusion calculations have been performed to evaluate the effects of melt evolution, crystal growth history, and crystal geometry on Mg-Fe isotopic fractionations. The formulations developed can be used to model chemical-isotopic compositions of zoned olivines to constrain thermal histories.

[1] Teng et al., (2011). EPSL, 308(3), 317-324. [2] Dauphas et al., (2010), 74(11), 3274-3291. [3] Richter et al., (2009) Chemical Geology, 258(1), 92-103. [4] Sio et al., GCA, 123, 302-321.