09:30 〜 09:45
[SMP28-03] Ternary feldspar “geothermospeedometry:” a new temperature-constrained cooling rate meter: development utilizing syenites from the Wadi Dib ring complex, Egypt
キーワード：3成分長石系、地質速度計、地質温度計、累帯構造
Feldspars, one of the most common minerals predominant in the crustal rocks, provide useful information on the formation and evolution of the Earth’s crust. The ternary feldspar system (NaAlSi3O8 (Ab) – KAlSi3O8 (Or) – CaAl2Si2O8 (An)) is characterized by wide two-phase regions with strong temperature dependence even above magmatic conditions (Seck, 1971a, b; Furman and Lindsley, 1988; Elkins and Grove, 1990; Benisek et al., 2004). Because of this, feldspars have been used as geothermometer for igneous and metamorphic rocks (e.g., Barth, 1951; Stormer, 1975; Brown and Parsons, 1981; Kroll et la., 1993; Raas, 1998). However, it is pointed out that equilibrium compositions of feldspars are not preserved in slowly cooled rocks (e.g., Zellmer et al., 1999), which is attributed to the notable differences in mobility of An-Ab (An-Or) and Ab-Or by up to ~8 orders of magnitude at a given temperature (Grove et al., 1984). This issue in application of feldspars as geothermometer has been overcome by estimating equilibrium compositions by adjusting highly mobile alkali components without considering kinetic aspects of the low temperature modification and implications of the estimates (Kroll et al., 1993). The kinetic aspect is represented by diffusivity of elements in feldspar, and many experiments have been conducted to determine coefficients of inter-diffusion of ternary feldspar components (Christoffersen et al., 1983; Grove et al., 1984; Yund, 1986; Schaffer et al., 2014), self-diffusion of Na, K, and Ca (Foland, 1973; Behrens et al., 1990), and tracer diffusion of trace elements, such as Sr, Rb, Li, Mg, Pb (Giletti, 1991; Giletti and Shanahan, 1997; Cherniak and Watson, 1992; Cherniak, 2010; Faak et al., 2013; Van Orman et al., 2014). Based on the diffusivity data, various feldspar geospeedometers have been proposed (Grove et al., 1984; Zellmer et al., 1999; Costa et al., 2003, 2010; Charlier et al., 2012; Faak et al., 2014; Singer et al., 2016). The most extensively developed feldspar geospeedometers are based on diffusional relaxation of trace element heterogeneity. The approach requires many assumptions such as temperature, boundary condition, and initial condition, though the sluggish inter-diffusion of An-Ab or An-Or is exploited to estimate the initial condition. By contrast, geospeedometry utilizing inter-diffusion of three feldspar components (essentially Ab-Or inter-diffusion) has not been paid much attention. It has a distinctive merit in that temperature of diffusive transportation could be constrained because of the strongly temperature-dependent feldspar phase relations, though dependence of closure temperatures (Tc) on grain size, distance from the grain rim (zoning patterns), and other spatial scales can provide temperature information (Ozawa, 1984; Faak et al., 2014). We have constructed temperature-constrained speedometry to estimate cooling rate exploiting the ternary feldspar system as useful thermometer. The method is thus called ternary feldspar “geothermospeedometry,” which emphasize “thermal aspect” (heat or temperature) of speedometry. We developed it by examining feldspars in syenites of the Wadi Dib ring complex (WDRC), Eastern Desert, Egypt. The WDRC is one of the alkaline ring complexes formed intermittently over the Phanerozoic time in the Nubian shield in northeast Africa (Saad et al, 2023). The syenites have various microstructures involving multiple feldspars, such as diverse intergrowth over the scale of microns to millimeters as well as diverse types of zoning of An-Ab-Or and are suited for our purpose. We developed two methods. The first method applicable to neighboring two alkali feldspar grains having distinct An contents with oscillatory zoning. It consists of three steps: (1) determination of Tc at the boundary of neighboring grains with distinct An from the coincidence of chemical potentials of Ab and Or, (2) estimation of Tc profile by the smoothness of the chemical potentials with constraints from (1), and (3) reproduction of the Tc profile by a diffusion model to estimate cooling rates. The principle behind (1) and (2) is temperature records are sequentially and continuously frozen from the core to the rim irrespective of the presence or absence of An zoning. The second method is applicable to neighboring alkali feldspar and plagioclase with notable Or-Ab zoning, which was analyzed with a diffusion modeling. We were successful in estimating temperature-constrained cooling history of the WDRC. This approach may be implemented to ternary solid solutions with large two-phase regions and distinct diffusivity of end components, such as Cr-Al-Fe3+ spinel and ternary pyroxenes.