Many rhyolites contain quartz crystals with relatively Ti-rich rims and Ti-poor cores, with a sharp interface between zones, attributed to partial dissolution followed by overgrowth following a heating event due to mafic recharge of the system. Quartz crystals in the compositionally zoned, high-silica rhyolite Tshirege Member of the Bandelier Tuff erupted at 1.26 Ma from the Valles caldera, New Mexico, show a range in zoning styles with Ti-rich rims becoming more abundant upward in the ignimbrite sheet among progressively less evolved magma compositions. Here we compare times between quartz overgrowth and eruption obtained by applying Ti diffusion coefficients to Ti concentration profiles in Tshirege Member quartz crystals with those from cathodoluminescence (CL) brightness profiles and show that panchromatic CL provides only a crude proxy for Ti in quartz in this unit. Titanium concentrations are measured to detection limits of ~1.2 ppm with small analytical errors (<5%) using MAN backgrounds, blank corrections, and oblique corrected transects to resolve diffusion-relaxed zone boundaries as thin as ~10 μm. Timescales derived from Ti profiles using the widely applied Tiin-quartz diffusion coefficients of Cherniak et al. (2007) range from 60 to 10 000 years, suggesting heating and mobilization events at different times prior to the eruption. However, the use of the newer Ti diffusivities reported by Jollands et al. (2020) yields timescales up to three orders of magnitude longer, including results that are geologically unreasonable for the Bandelier system. We suggest that assumptions commonly made in diffusion modeling, specifically about the form of the Ti zoning profile prior to diffusive relaxation, may be invalid.Melt inclusions in the Ti-poor cores of late-erupted quartz are chemically akin to early erupted melt compositions, while adhering and groundmass glasses more closely reflect the composition of the host pumice. Heating and mobilization events identified from quartz Ti zoning are thus linked to overall compositional zoning of the tuff, which may have been produced by repeated episodes of melting of a crystal cumulate cognate to the early-erupted, evolved rhyolite. Quartz-hosted melt inclusion faceting suggests the development of a crystal mush over a minimum time frame of 1000–10 000 years prior to the recharge events that produced the erupted Tshirege magma at 1.26 Ma.

中文翻译：

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Bandelier Tuff的Tshirege成员的石英中钛的扩散分布以及熔体夹杂化学和形态

许多流纹岩所含的石英晶体具有相对富钛的边沿和贫钛的核，在区域之间具有尖锐的界面，这归因于部分溶解，然后由于加热系统中镁铁质的补给而导致了过度生长。从新墨西哥州的瓦莱斯破火山口以1.26 Ma喷发的带状成分的高硅质流纹岩Tshirege Tshirege成员中的石英晶体显示出一系列的分区样式，富钛边缘在火成岩片中逐渐向上丰富。较少的岩浆成分。在这里，我们比较了通过将T扩散系数应用于Tshirege成员石英晶体中的Ti浓度曲线与来自阴极发光（CL）亮度曲线的Ti扩散曲线所获得的石英过度生长和喷发之间的时间，并表明全色CL仅提供了该单元中石英中Ti的粗略替代物。 。使用MAN背景，空白校正和斜校正样线来解析宽至约10μm的扩散松弛区域边界，将钛浓度测量到检测限为〜1.2 ppm，且分析误差较小（<5％）。利用广泛使用的Cherniak等人的Tiin-Quartz扩散系数从Ti轮廓得出的时标。（2007年）的范围从60到10000年，表明在喷发之前的不同时间发生了加热和动员事件。然而，Jollands等报道的使用较新的Ti扩散系数。（2020年）产生的时间尺度最长可达三个数量级，其中包括对于Bandelier系统而言在地质上不合理的结果。我们建议在扩散模型中通常做出的假设，特别是关于扩散弛豫之前的Ti分区分布的形式的假设可能是无效的。后期喷发石英的贫Ti核中的熔体夹杂物在化学上类似于早期喷出的熔体成分，同时附着和研磨的玻璃更紧密地反映了宿主浮石的成分。因此，从石英Ti分区中识别出的加热和动员事件与凝灰岩的整体组成分区有关，这可能是由于晶状体同源物与早期喷发的，演化的流纹岩的反复熔化而产生的。