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

阿蒂特兰、阿马蒂特兰、科阿特佩克和伊洛潘戈（危地马拉、萨尔瓦多萨尔瓦多）是中美洲火山弧北方的四个相邻的破火山口。这些破火山口产生了许多爆炸性喷发的火山岩，这些火山岩区域性地沉积在岸上和离岸，在那里它们形成了整个更新世-全新世地质学和考古学上重要的标志层。他们最近的一次喷发是公元536年左右的伊洛潘戈火山的Tierra 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成分和重叠的沉积分布，在很大程度上阻碍了明确的相关性，这些火山岩和各自的源破火山口。因此，这些火山口的岩浆成分和喷发复发的时间演变，危险性分析的一个重要方面，仍然不完全了解。我们建议应用高空间分辨率分析的锆石提取的中间和远端火山灰与锆石从近端样品进行比较。锆石的化学稳定性是一个主要的优势相比，更常规使用的火山玻璃和较少的保留矿物的分析。锆石的这种倾向性是实现这一建议的三个主要目标的关键：（1）重建伊洛潘戈岩浆系统的时间演化，并确定关键岩浆过程的作用（结晶分离、同化作用和岩浆补给）;（2）补充和改进现有火山灰对比方法，利用尘埃级锆石的年龄和成分作为远源火山灰的指纹;以及（3）将来自天然火山灰沉积物的锆石年龄与那些与火山灰有关的锆石年龄进行比较，这些火山灰被用作晚期古典玛雅陶器的回火剂，目的是澄清这些地质材料在非火山玛雅栖息地中的神秘来源。这些结果对于更好地了解岩浆弧的岩浆起源和储存以及CAVA的地壳结构至关重要。改进后的火山灰年代学将使该地区陆地和海洋沉积环境的地质和地层相互关联成为可能，这可以为其他火山弧建立基准。最后，玛雅陶器中地质材料的来源是重新评估玛雅社会组织现有模型的线索。

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.