DataSheet2_Pre-Eruptive Magma Configurations and Petrogenetic Relationships of the Rattlesnake Tuff, Oregon–Insights From Spectacularly Banded High-Silica Rhyolite Pumices.XLSX

The 7.1 Ma Rattlesnake Tuff (RST) of eastern Oregon is a widespread and voluminous (>300 km3) ignimbrite composed of 99% crystal poor (≤1%) high-silica rhyolite (HSR) and <1% dacites. Basaltic andesitic to basaltic inclusions within dacites are samples of underpinned mafic magmas. The RST HSR is comprised of five increasingly evolved compositional Groups (E–A), and HSR pumices range from white to dark grey, often co-mingled in spectacular banded pumices. Previously, Groups were interpreted as rhyolites generated by crystal fractionation within a single reservoir, where more evolved rhyolite melts formed from relatively less evolved rhyolite parents. To reassess compositional HSR Groups and their implications for tapping a single or multiple rhyolite reservoirs as well as reevaluating the petrological relationships among groups, we focus on large banded pumices for geochemical analysis. Statistical analysis of existing and new data verified these five compositional Groups and gaps, best characterized by variations in Ba, Eu/Eu*, Eu, FeO*, Hf, and Zr. Wet-liquidus temperatures, storage temperatures, and storage pressures calculated for all HSR Groups indicate similar pre-eruptive conditions (∼6.1–7.5 km depth; storage temperatures of ∼805–895°C). Differentiation trends, trends in storage pressure and temperature, and lack of crystal-rich tuff or country rock corroborate existing models for HSRs that involve a single, density-stratified magma reservoir prior eruption. Density differences are sufficient to prevent convection between layers of HSRs in a single reservoir when water content increases from 2–4 wt% from Groups E–A. However, if HSRs do not represent a liquid line, it is possible to generate HSRs through batch melting of various regional country rock. Yet, HSRs would still accumulate within the same storage zone, where density variations kept HSRs from mixing until eruption when these banded pumices formed. In either scenario, our study underscores the significance of water content and density variations for accumulating rhyolite magmas in a contiguous magma body without mixing. This has implications for other compositionally heterogenous rhyolitic ignimbrites where natural samples do not provide comparable evidence to argue for pre-eruptive confocal storage of different rhyolite magmas as is the case for the Rattlesnake Tuff.

俄勒冈州东部的7.1 Ma（百万年）响尾蛇凝灰岩（Rattlesnake Tuff, 简称RST）是一套分布广泛、体量逾300立方千米的熔结凝灰岩（ignimbrite），其组成包含99%的贫晶（晶含量≤1%）高硅流纹岩（high-silica rhyolite, HSR）与不足1%的英安岩。英安岩中产出的玄武安山质至玄武质包体，代表了下伏镁铁质岩浆的样品。RST高硅流纹岩可划分为五个演化程度逐步升高的组分群（E至A），高硅流纹岩浮石的颜色从白色至深灰色不等，常以壮观的条带状浮石形式相互混熔。此前学界将这些组分群解释为单一岩浆房内结晶分异作用形成的流纹岩，即更演化的流纹质熔体由相对未演化的流纹岩母质演化而来。为重新评估高硅流纹岩组分群的成因及其与单一或多个流纹岩岩浆房的岩浆抽取的关联，并重新厘定各组分群间的岩石学关系，本研究选取大型条带状浮石开展地球化学分析。对已有数据与新增数据的统计分析证实了这五个组分群及其组分间隙，其特征可通过Ba、Eu/Eu*、Eu、FeO*、Hf及Zr的元素变化得到最佳体现。针对所有高硅流纹岩组分群计算得到的湿液相线温度、储集温度与储集压力，显示其喷发前条件高度相似（埋藏深度约6.1~7.5 km；储集温度约805~895 ℃）。分异趋势、储集压力与温度的变化趋势，以及未发现富晶凝灰岩或围岩的证据，均佐证了现有模型：喷发前存在单一密度分层的岩浆房。当组分群E至A的含水量从2 wt%升至4 wt%时，密度差异足以阻止单一岩浆房内不同层位高硅流纹岩之间的对流混合。但若高硅流纹岩并非代表一条液相线序列，则可通过对不同区域围岩的批量熔融生成高硅流纹岩。但此类高硅流纹岩仍会聚集于同一储集带内，密度差异会阻止其相互混合，直至喷发时形成这类条带状浮石。无论上述哪种情景，本研究均强调了含水量与密度差异对于在连续岩浆体内不发生混合的情况下聚集流纹质岩浆的重要意义。这一结论对其他成分不均一的流纹质熔结凝灰岩具有启示意义——这类天然样品无法提供如响尾蛇凝灰岩这般的证据，来证明不同流纹质岩浆在喷发前共储于同一储集空间内。