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）北部的四个邻近的火山口。这些破火山口产生了许多爆炸性喷发的火山，这些火山区域性地沉积在海岸上和海岸外，在更新世-全新世期间形成了重要的地质和考古标志层。他们最近的一次喷发，大约在公元536年的伊洛潘戈火山喷发，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，通常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火山与其各自的源火山口之间明确的相关性。因此，这些破火山口岩浆成分的时间演化和喷发的重现，这是危害分析的一个重要方面，仍然不完全清楚。我们建议应用高空间分辨率分析从内侧和远端提取的锆石，并与近端样品的锆石进行比较。锆石的化学稳定性与传统的火山玻璃分析方法和留存性较差的矿物分析方法相比，具有很大的优势。锆石的这种倾向性是实现本研究三个主要目标的关键：(1)以高时间分辨率重建Ilopango岩浆系统的时间演化，识别关键岩浆过程（分离结晶、同化和岩浆补给）的作用；(2)利用尘粒锆石的年龄和成分作为远端岩屑的指纹图谱，补充和完善现有的岩屑对比方法；(3)比较天然火山灰沉积物中的锆石与晚期古典玛雅陶器中用作回火剂的火山灰的锆石，目的是澄清这些地质物质在非火山玛雅栖息地的神秘来源。预期的结果对于更好地了解岩浆弧中的岩浆起源和储存以及CAVA的地壳结构至关重要。改进的地层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为重新评估玛雅社会组织的现有模型提供了线索。

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.