Significance of tridymite distribution during cooling and vapor-phase alteration of ignimbrites
Significance of tridymite distribution during cooling and vapor-phase alteration of ignimbrites
复制标题
鳞石英在熔结岩冷却和气相变化过程中分布的意义
作者：
Yuli Heled;M. C. Rowe;I. Chambefort;C. Wilson
摘要厚层熔结凝灰岩序列包含复杂的侵位和冷却历史，常常掩盖熔结凝灰岩流包之间的接触。在这些序列的矿物学和结构的变化主要是侵位温度和冷却时间的函数。在这里，我们专注于使用的二氧化硅多晶型鳞石英了解气相结晶和失透内熔结凝灰岩流包的关联。与方石英的常见情况相反，我们观察鳞石英的受限域可能为解释侵位后脱玻作用和气相蚀变提供更多相关的限制。本研究考察了通过Whakamaru（新西兰），主教（美国），Grey's Landing（美国）熔结凝灰岩的结构观察与密度测量相结合，基质结晶度，鳞石英的分布和比例方石英。流变灰色的着陆熔结凝灰岩代表一个高温端员的情况下，广泛分布鳞石英（高达20%），导致高岩浆温度和快速脱玻作用在低孔隙度存款。在焊接的Whakamaru和Bishop熔结凝灰岩中，亚稳鳞石英（高达13%）集中在流动包之间的沿着边界。这里鳞石英被解释为在瞬态渗透区中结晶，在压实之前的汽相变化期间形成，其中上部较致密的焊接流动材料用作汽封。我们的研究结果表明，鳞石英可能链接的初始冷却和熔结凝灰岩的历史，气相蚀变和脱玻作用，并可能作为一个潜在的矿物指纹沉积接触，重要的考虑流体在地热储层中的横向运输。
Abstract Thick sequences of silicic ignimbrites contain complex emplacement and cooling histories, often masking contacts between ignimbrite flow packages. Mineralogical and textural variations in these sequences are primarily a function of emplacement temperature and cooling time. Here, we focus on the use of the silica polymorph tridymite to understand the association of vapor-phase crystallization and devitrification within ignimbrite flow packages. As opposed to the common occurrence of cristobalite, the restricted domains in which we observe tridymite may provide more relevant constraints for interpreting post-emplacement devitrification and vapor-phase alteration. This study examines sections through the Whakamaru (New Zealand), Bishop (U.S.A.), and Grey’s Landing (U.S.A.) ignimbrites by combining textural observations with measurements of density, groundmass crystallinity, and the distribution and proportion of tridymite to cristobalite. The rheomorphic Grey’s Landing ignimbrite represents a high-temperature end-member scenario, with widely distributed tridymite (up to 20%) resulting from a high-magmatic temperature and rapid devitrification in a low-porosity deposit. In the welded Whakamaru and Bishop ignimbrites, metastable tridymite (up to 13%) is concentrated along boundaries between flow packages. Here tridymite is interpreted to crystallize in transient permeable zones, forming during vapor-phase alteration prior to compaction, where upper denser-welded flow materials serve as vapor seals. Our results suggest that tridymite may link the initial cooling and welding history of ignimbrites to vapor-phase alteration and devitrification, and may serve as a potential mineralogical fingerprint of depositional contacts, important for consideration of lateral transport of fluids in geothermal reservoirs.