4:00 PM - 4:15 PM
[SCG53-09] ALCHEMI study of cation site preferences in the olivine structure for estimation of cooling rates
Keywords:olivine, cation site distribution, analytical electron microscopy, cooling rate
Olivine has two types of octahedral sites (M1 and M2 sites) in its crystal structure, and Mg and Fe are roughly evenly distributed between the M1 and M2. However, some experimental studies have reported that Fe shows a slight preference for the M1, and the tendency becomes more significant at higher temperatures (e.g., Heinemann et al. 2006). This partitioning behavior of Fe in the olivine has been of interest because it appears to contradict the general trend of cation disordering at high temperatures (e.g., Aikawa et al., 1985). Heinemann et al. (2007) studied this issue using high-temperature X-ray diffraction experiment and formulated the relationship among Fe concentration, temperature, and the M1-M2 site distribution of Fe.
Because the exchange rate of Fe between the M1 and M2 in olivine is relatively faster, the degree of M1-M2 site distribution reached at high-temperatures may be easily modified during cooling. However, the residual degree (frozen degree) at the present time is expected to reflect the past cooling rate (e.g., Redfern et al., 1996). In this study, therefore, we conducted cooling experiment and measurement of cation site distributions in olivine, aiming to develop a method for estimating cooling histories from single olivine grains.
For the analysis of cation site distribution, we adopted micro-sampling using a focus ion beam system (FIB) and Atom-location by channeling-enhanced microanalysis (ALCHEMI, Spence & Taftø, 1983) using transmission electron microscope (TEM). This approach enables us to evaluate specific olivine grains after petrographic observations. Moreover, ALCHEMI is superior in the analysis of impurity and/or multiple elements, which are generally difficult to measure. The investigations of the impurity elements as well as Fe and Mg may progress in understandings of general mechanism of the cation site distribution in olivine structure.
In the experiment, San Carlos olivine was used as the starting material, and ultrathin specimen was prepared from the olivine using a FIB. The ultrathin specimen was placed on a MEMS (Micro Electro Mechanical Systems) chip for the TEM heating holder. The specimen held under vacuum and temperature at 1000°C for 5 hours, and then cooled to room temperature at constant cooling rates (10-3–102 °C/s). In ALCHEMI analysis, characteristic X-ray signals generated under illumination of 200 keV electrons were collected as a function of electron beam direction. The one-dimensional tilting ALCHEMI datasets under the condition exciting the 00l systematic reflection row were obtained (Soeda et al., 2000; Muto & Ohtsuka, 2017; Igami et al., 2018). The Fe, Ca, Ni, Mn distribution coefficients between M1 and M2 were determined by linear regression between the observed ALCHEMI profile and the theoretical profiles calculated using ICSC code (Oxley & Allen, 2003).
The results show that the site preference of each cation shows trends consistent with previous studies: Fe showed a slight preference for the M1 in most cases, Ni strongly concentrated into the M1, and Ca and Mn strongly concentrated into the M2. In addition, it was confirmed that higher cooling rates lead to higher Fe concentration in the M1 site, although no clear relationship was confirmed between the site distribution of Ca, Mn, and Ni and the cooling rate in the experiment. We also measured some natural samples, and results showed that all olivine from volcanic ejecta showed concentration of Fe in the M1, whereas the olivine that have undergone very slow cooling as part of Horoman peridotite body showed Fe concentration in the M2. Though the current analysis introduces errors that cannot be easily ignored for precise estimation of the cooling rate, analyzing multiple olivine in the same petrographic section may be effective approach.
Because the exchange rate of Fe between the M1 and M2 in olivine is relatively faster, the degree of M1-M2 site distribution reached at high-temperatures may be easily modified during cooling. However, the residual degree (frozen degree) at the present time is expected to reflect the past cooling rate (e.g., Redfern et al., 1996). In this study, therefore, we conducted cooling experiment and measurement of cation site distributions in olivine, aiming to develop a method for estimating cooling histories from single olivine grains.
For the analysis of cation site distribution, we adopted micro-sampling using a focus ion beam system (FIB) and Atom-location by channeling-enhanced microanalysis (ALCHEMI, Spence & Taftø, 1983) using transmission electron microscope (TEM). This approach enables us to evaluate specific olivine grains after petrographic observations. Moreover, ALCHEMI is superior in the analysis of impurity and/or multiple elements, which are generally difficult to measure. The investigations of the impurity elements as well as Fe and Mg may progress in understandings of general mechanism of the cation site distribution in olivine structure.
In the experiment, San Carlos olivine was used as the starting material, and ultrathin specimen was prepared from the olivine using a FIB. The ultrathin specimen was placed on a MEMS (Micro Electro Mechanical Systems) chip for the TEM heating holder. The specimen held under vacuum and temperature at 1000°C for 5 hours, and then cooled to room temperature at constant cooling rates (10-3–102 °C/s). In ALCHEMI analysis, characteristic X-ray signals generated under illumination of 200 keV electrons were collected as a function of electron beam direction. The one-dimensional tilting ALCHEMI datasets under the condition exciting the 00l systematic reflection row were obtained (Soeda et al., 2000; Muto & Ohtsuka, 2017; Igami et al., 2018). The Fe, Ca, Ni, Mn distribution coefficients between M1 and M2 were determined by linear regression between the observed ALCHEMI profile and the theoretical profiles calculated using ICSC code (Oxley & Allen, 2003).
The results show that the site preference of each cation shows trends consistent with previous studies: Fe showed a slight preference for the M1 in most cases, Ni strongly concentrated into the M1, and Ca and Mn strongly concentrated into the M2. In addition, it was confirmed that higher cooling rates lead to higher Fe concentration in the M1 site, although no clear relationship was confirmed between the site distribution of Ca, Mn, and Ni and the cooling rate in the experiment. We also measured some natural samples, and results showed that all olivine from volcanic ejecta showed concentration of Fe in the M1, whereas the olivine that have undergone very slow cooling as part of Horoman peridotite body showed Fe concentration in the M2. Though the current analysis introduces errors that cannot be easily ignored for precise estimation of the cooling rate, analyzing multiple olivine in the same petrographic section may be effective approach.