TitaniQ recrystallized: experimental confirmation of the original Ti-in-quartz calibrations
TitaniQ recrystallized: experimental confirmation of the original Ti-in-quartz calibrations
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DOI：
10.1007/s00410-015-1120-0
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发表时间：
2015-02
影响因子：
3.5
通讯作者：
Jay B. Thomas;E. Watson;F. Spear;D. Wark
中科院分区：
文献类型：
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作者：
Jay B. Thomas;E. Watson;F. Spear;D. Wark
一些研究报道了钛在石英中的溶解度的P-T依赖性，并且在四个实验校准中的三个之间有密切的一致性。本研究中进行了新的实验，以确定潜在的实验不平衡，并确定哪种石英中钛溶解度校准最准确。石英、金红石和锆石的晶体在初始合成实验中在925 °C和10 kbar下在活塞-圆筒装置中从SiO2-、TiO 2-和ZrSiO 4-饱和的水性流体生长。在本实验中产生了一系列的石英晶体尺寸;通过电子探针分析了大的和小的样品，以确定Ti浓度是否与晶体尺寸相关。阴极射线发光图像和EPMA测量表明，晶间和晶内的Ti浓度的变化是显着的小，无论晶体尺寸。来自合成实验的石英中的平均Ti浓度为392 ± 1 ppmw Ti，其在Wark和沃森（Contrib Mineral Petrol 152：743-754，2006）和托马斯等人（Contrib Mineral Petrol 160：743-759，2010）的10 kbar等压线数据的95%置信区间内。作为对石英中钛校准的交叉检查，我们还测量了来自合成实验的金红石中Zr的浓度。金红石中Zr的平均浓度为4337 ± 32 ppmw Zr，这也在Ferry和沃森的金红石中Zr溶解度校准的95%置信区间内（Contrib Mineral Petrol 154：429-437，2007）。将Ti在石英中的溶解度和Zr在金红石中的溶解度的P-T依赖性作为热压计应用于实验样品。由托马斯等人（Contrib Mineral Petrol 160：743-759，2010）的校准计算的平均石英中的Ti等值线和由Tomkins等人（J Metamorph Geol 25：703-713，2007）的校准计算的平均金红石中的Zr等值线在9.5千巴和920 ℃交叉，这与合成实验的P-T条件非常一致。将来自上述初始合成实验的高Ti石英的分离物用作在20千巴下的后续实验中的起始材料，在该压力下，预期Ti在石英中的溶解度在重结晶石英中显著更低。这些重结晶实验在925 °C的湿和干条件下以及在850 °C的湿条件下进行。湿重结晶和干重结晶实验都产生了多晶石英岩。金红石以石英中的包裹体形式出现，也以分散在沿着石英颗粒边界的单个晶体形式出现。在重结晶实验期间生长的石英具有暗阴极射线发光，表明实质上较低的Ti浓度。来自重结晶实验的石英中的平均Ti浓度在托马斯等人（Contrib Mineral Petrol 160：743-759，2010）的20千巴数据的线性拟合的95%置信区间内。总的来说，合成和重结晶实验的结果证实了用于校准托马斯等人“中的石英中钛溶解度的P-T依赖性的石英中钛浓度。s（Contrib Mineral Petrol 160：743-759，2010）校准表示石英中Ti的平衡浓度。
Several studies have reported theP–Tdependencies of Ti-in-quartz solubility, and there is close agreement among three of the four experimental calibrations. New experiments were conducted in the present study to identify potential experimental disequilibrium, and to determine which Ti-in-quartz solubility calibration is most accurate. Crystals of quartz, rutile and zircon were grown from SiO2-, TiO2-, and ZrSiO4-saturated aqueous fluids in an initial synthesis experiment at 925 °C and 10 kbar in a piston-cylinder apparatus. A range of quartz crystal sizes was produced in this experiment; both large and small examples were analyzed by electron microprobe to determine whether Ti concentrations are correlated with crystal size. Cathodoluminescence images and EPMA measurements show that intercrystalline and intracrystalline variations in Ti concentrations are remarkably small regardless of crystal size. The average Ti-in-quartz concentration from the synthesis experiment is 392 ± 1 ppmw Ti, which is within 95 % confidence interval of data from the 10 kbar isobar of Wark and Watson (Contrib Mineral Petrol 152:743–754, 2006) and Thomas et al. (Contrib Mineral Petrol 160:743–759, 2010). As a cross-check on the Ti-in-quartz calibration, we also measured the concentration of Zr in rutile from the synthesis experiment. The average Zr-in-rutile concentration is 4337 ± 32 ppmw Zr, which is also within the 95 % confidence interval of the Zr-in-rutile solubility calibration of Ferry and Watson (Contrib Mineral Petrol 154:429–437, 2007). TheP–Tdependencies of Ti solubility in quartz and Zr solubility in rutile were applied as a thermobarometer to the experimental sample. The average Ti-in-quartz isopleth calculated from the calibration of Thomas et al. (Contrib Mineral Petrol 160:743–759, 2010) and the average Zr-in-rutile isopleth calculated from the calibration of Tomkins et al. (J Metamorph Geol 25:703–713, 2007) cross at 9.5 kbar and 920 °C, which is in excellent agreement with theP–Tconditions of the synthesis experiment. Separates of the high-Ti quartz from the initial synthesis experiment described above were used as starting material in subsequent experiments at 20 kbar, at which pressure the solubility of Ti in quartz is expected to be significantly lower in the recrystallized quartz. These recrystallization experiments were conducted under wet and dry conditions at 925 °C, and under wet conditions at 850 °C. Both wet and dry recrystallization experiments produced polycrystalline quartzites. Rutile occurs as inclusions in quartz, and as individual crystals dispersed along quartz grain boundaries. Quartz that grew during the recrystallization experiments has dark cathodoluminescence indicating substantially lower Ti concentrations. The average Ti concentrations in quartz from the recrystallization experiments are within the 95 % confidence interval of a linear fit to the 20 kbar data of Thomas et al. (Contrib Mineral Petrol 160:743–759, 2010). Collectively, the results from the synthesis and recrystallization experiments confirm that the Ti-in-quartz concentrations used to calibrate theP–Tdependencies of Ti-in-quartz solubility in Thomas et al.’s (Contrib Mineral Petrol 160:743–759, 2010) calibration represent the equilibrium concentrations of Ti in quartz.