Zircon in tephra as a novel tool to decrypt geologic and archaeological archives: a case study from Central America

Atitlan、Amatitlan、Coatepeque和Ilopango(危地马拉、萨尔瓦多)是中美洲火山弧(CAVA)北部的四个相邻的火山口。这些火山口产生了许多爆炸性喷发的冰盖岩层，这些冰盖岩层区域性地沉积在海上和近海，在整个更新世-全新世形成了具有重要地质和考古意义的标志层。他们最近的一次喷发是公元前536年左右的伊洛潘戈火山的Tiera Blanca Joven事件，对当地的玛雅文明造成了灾难性的影响，并对全球气候产生了切实的影响。然而，往往难以区分的化学和矿物学成分以及重叠的沉积分布，在很大程度上阻碍了这些冰盖与其各自来源的火山口之间的明确对比。因此，作为危险分析的一个重要方面，这些火山口的岩浆成分和喷发复发的时间演变仍然不完全清楚。我们建议应用高空间分辨率分析从中端和远端样品中提取的锆石，并与从近端样品中提取的锆石进行比较。与更传统的火山玻璃和较少保留矿物的分析相比，锆石的化学稳定性是一个主要优势。锆石的这种倾向性是实现这一提议的三个主要目标的关键：(1)重建Ilopango岩浆系统的时间演化，并在高时间分辨率上确定关键岩浆过程(分结晶、同化和岩浆补给)的作用；(2)通过使用尘埃大小的锆石的年龄和成分作为远端冰盖的指纹，补充和改进现有的Tephra对比方法；以及(3)将天然Tephra矿床的锆石年龄与与火山灰有关的年龄进行比较，以澄清这些地质物质在非火山玛雅生境中的神秘来源。预期的结果对于更好地了解岩浆起源和岩浆弧中的储存以及洞穴的地壳结构至关重要。改进的岩石学将允许在该地区的陆地和海洋沉积环境中进行地质和地层对比，这可以为其他火山弧设定基准。最后，玛雅陶器中岩土材料的来源是重新评估玛雅社会组织现有模式的线索。

Atitlan, Amatitlan, Coatepeque and Ilopango (Guatemala, El Salvador) are four neighboring calderas in the northern part of the Central American Volcanic Arc (CAVA). These calderas have produced numerous explosively erupted tephras which are regionally deposited on- and off-shore where they form geologically and archaeologically important marker horizons throughout the Pleistocene-Holocene. Their most recent eruption, the Tierra Blanca Joven event of Ilopango volcano at around 536 CE, was disastrous for the local Maya civilization and had tangible effects on global climate. Often indistinguishable chemical and mineralogical compositions and overlapping depositional distributions, however, have largely hampered unambiguous correlations between these tephras and their respective source calderas. In consequence, the temporal evolution of magma compositions and eruptive recurrence for these calderas, an important aspect for hazard analysis, remain incompletely understood. We propose to apply high spatial resolution analysis of zircon extracted from medial and distal tephra in comparison with zircon from proximal samples. The chemical stability of zircon is a major advantage compared to the more conventionally used analysis of volcanic glasses and less retentive minerals. This propensity of zircon is the key to meet the three major objectives of this proposal: (1) to reconstruct the temporal evolution of the Ilopango magma system and identify the roles of key magmatic processes (fractional crystallization, assimilation, and magma recharge) at high temporal resolution; (2) to complement and improve existing methods for tephra correlation by using the ages and compositions of dust-sized zircon as a fingerprint for distal tephras; and (3) to compare zircon ages from natural tephra deposits with those associated with volcanic ash used as a temper in late-classic Maya pottery with the goal to clarify the enigmatic provenance of these geomaterials in non-volcanic Maya habitats. The anticipated results are critical for better understanding magma origins and storage in magmatic arcs, as well as the crustal structure of the CAVA. Improved tephrochonology will permit geologic and stratigraphic correlations in terrestrial and marine depositional environments in the region, which can set a benchmark for other volcanic arcs. Lastly, the provenance of geomaterials in Maya pottery is a clue to re-evaluate existing models for Maya societal organization.