Ar diffusion and solubility measurements in plagioclases using the ultra-violet laser depth-profiling technique

We describe the first direct measurements of Ar diffusion and solubility in plagioclases using ultra-violet (UV) laser ablation depth-profiling and noble gas mass spectrometer analyses of experimentally treated (599–1000 °C, 50–200 MPa of Ar) crystal fragments of labradorite and oligoclase. Labradorite ⁴⁰Ar gain diffusion profiles were measured, yielding an activation energy of 26.72±4.58 kcal mol⁻¹ (118.0±19.16 kJ mol⁻¹) and a frequency factor of 9.77×10⁻⁹ (+8.79×10⁻⁸, −8.79×10⁻⁹) cm² s⁻¹ (95% confidence). The Ar solubility in labradorite was measured yielding a value of <0.2 ppb bar⁻¹, which is similar to or lower than many rock forming minerals. The labradorite diffusion parameters indicate Ar closure temperatures of 211 °C for a spherical diffusion geometry, and 243 °C for a planar diffusion geometry (for 100 µm-diameter grains, with cooling rates of 10 °C Ma⁻¹). The data indicate that labradorite is less Ar retentive than K-feldspar at low temperatures, but more Ar retentive than K-feldspar at high temperatures, corroborating previous work on plagioclase. The relatively slow Ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems.

本研究采用紫外（UV）激光剥蚀深度剖面法，结合对经实验处理（599~1000 ℃、50~200 MPa氩气环境）的拉长石与更长石晶体碎片开展的稀有气体质谱仪分析，首次直接测定了氩气在斜长石中的扩散行为与溶解度。
研究测定了拉长石的⁴⁰Ar过剩扩散剖面，得到95%置信度下的活化能为26.72±4.58 kcal mol⁻¹（118.0±19.16 kJ mol⁻¹），频率因子为9.77×10⁻⁹ cm² s⁻¹，误差范围为+8.79×10⁻⁸至−8.79×10⁻⁹。
同时测定了氩气在拉长石中的溶解度，结果为<0.2 ppb·bar⁻¹，该值与多数造岩矿物的溶解度相当甚至更低。
基于拉长石的扩散参数，可推算得到：球形扩散几何条件下氩气的封闭温度为211 ℃，平面扩散几何条件下为243 ℃（针对直径100 μm的颗粒，冷却速率为10 ℃·Ma⁻¹）。
实验数据表明，低温环境下拉长石的氩气保留能力弱于钾长石，而高温环境下则更强，这一结果与此前针对斜长石的研究结论一致。
拉长石在岩浆温度下相对缓慢的氩气扩散速率，或可解释酸性火山岩系中大颗粒斜长石常表现出较老年龄的普遍观测现象。