Reduction rims in San Carlos olivines: an analog to reduction rims in ureilite olivines -- GeoPeP 2013

4 Discussion

The reduction rims produced in the conducted experiments have a width between ~1 and 79 µm. The heating durations required to produce these rim widths are between 18 and 204 hours at a temperature of 1250°C. Olivine crystals in ureilites generally have reduction rims with a width of 10 to 100 µm. Assuming that the reduction process responsible for these rims occured at a temperature of 1250°C, the minimum time span required to produce a 10 µm rim width in ureilite olivines is between 28 and 32 hours (see Figure 11). The maximum rim width produced in the experiments is ~79 µm equalling a heating duration of 204 hours. Thus, the maximum rim width in ureilite olivines of 100 µm required, at a temperature of 1250°C, a heating duration of more than 204 hours. The rim progression rate of 0.36 +/- 0.03 µm/h, resulting from the slope of the line of best fit in Figure 11, suggests that the heating duration required for a 100 µm reduction rim is about 277 +/- 23.3 hours at a constant temperature of 1250°C, as calculated using Equation 4:
(4) trim  =  (drim)/(vprogress)
with drim = rim thickness in µm and vprogress = 0.36 +/- 0.03 µm/h.
The temperature of the ureilite mantle at the disruption was probably close to 1250°C. The cooling rates of the heated ureilite fragments is a function of their size and, as stated above, controlls the reduction rim widths in the olivines. The observed reduction rim widths in ureilite olivines may thus be a function of the ureilite fragment size, the olivine location within a fragment (rim, mantle, core), or both. The experiments conducted in this work provide the first quantitative estimates on the duration of the formation of ureilite olivine reduction rims.


I thank Dr. T. Di Rocco for his first-class support and help around the clock. Furthermore, I thank Dr. A. Kronz for microprobe analyses and the correction of the data. Special thanks to Prof. Dr. A. Pack for the opportunity to work on this project, the support and inspiring ideas during this work.