Chemical determination of the Jorullo volcano in the Michoacán Guanajuato Volcanic Field (MGVF), Mexico

The 1759-1774 eruption of the Jorullo volcano in the Michoacán Guanajuato Volcanic Field (MGVF), Mexico, produced lavas that range in composition from basalt to basaltic andesite. We have conducted new major and trace element and isotopic studies (whole rock Sr-Nd-Pb-Hf-Os, and O isotopes in olivine separates) of the Jorullo lavas and tephras spanning the duration and compositional range of the eruption, to further constrain the potential roles of mantle source heterogeneity, subduction-related metasomatism, and crustal assimilation in the petrogenesis of the Jorullo magmas. This study presents the first Hf, high precision Pb and comprehensive oxygen isotope measurements for Jorullo volcanic rocks. All samples have arc-like trace element patterns with enrichments in large ion lithophile elements (e.g. Ba, Rb, and Pb) and depletions in fluid immobile elements (e.g. Nb, Ta). In addition, the samples show variations in 87Sr/86Sr (0.7038-0.7040), 143Nd/144Nd (0.51280-0.51285), 176Hf/177Hf (0.28297-0.28300), 206Pb/204Pb (18.62-18.66), 207Pb/204Pb (15.57-15.59) and 208Pb/204Pb (38.34-38.43). Osmium isotope signatures are, with one exception, more radiogenic than the depleted and primitive mantle (187Os/188Os = 0.1231-0.1616). Oxygen isotope ratios of olivine phenocrysts (d18.OSMOW = 5.70-6.02 per mil) show limited variation, but are isotopically heavier than normal mantle olivine. The samples define two geochemical groups: high-MgO samples with higher 87Sr/86Sr, lower 143Nd/144Nd and 176Hf/177Hf, and a positive correlation of Sr and Pb isotopes; and low-MgO samples displaying lower 87Sr/86Sr but higher 143Nd/144Nd and 176Hf/177Hf than the former group, and a negative correlation of Sr and Pb isotopes. The high-MgO group comprises most of the early tephra and lavas, whereas the low-MgO group includes most of the late tephra and lavas. These compositional variations are inconsistent with shallow level contamination, but rather are interpreted to reflect mantle source heterogeneity. Trace element and isotopic signatures are consistent with North Mexican Extensional Province (NMEP) mantle metasomatised by subduction components composed of sediment- and oceanic crust-derived hydrous fluid. […]

墨西哥米却肯-瓜纳华托火山场（Michoacán Guanajuato Volcanic Field, MGVF）内的约鲁洛（Jorullo）火山在1759—1774年的喷发活动，产出了成分跨度从玄武岩至玄武安山岩的熔岩。
研究团队针对覆盖该喷发全时段与全成分范围的约鲁洛火山熔岩与火山碎屑，开展了全新的主量元素、微量元素及同位素分析（包括全岩Sr-Nd-Pb-Hf-Os同位素测试，以及橄榄石单矿物分离样的氧同位素分析），旨在进一步限定幔源不均一性、俯冲相关幔交代作用以及地壳混染在约鲁洛岩浆岩石成因中的潜在作用。
本研究首次报道了约鲁洛火山岩的Hf同位素、高精度Pb同位素以及系统性的氧同位素测试数据。
所有样品均呈现弧型微量元素配分模式，表现为大离子亲石元素（large ion lithophile elements, LILE，如Ba、Rb、Pb）富集，而流体不活动元素（如Nb、Ta）亏损。
此外，各样品的同位素比值存在如下变化范围：87Sr/86Sr为0.7038~0.7040，143Nd/144Nd为0.51280~0.51285，176Hf/177Hf为0.28297~0.28300，206Pb/204Pb为18.62~18.66，207Pb/204Pb为15.57~15.59，208Pb/204Pb为38.34~38.43。
除1个例外样品外，所有样品的Os同位素组成均比亏损地幔与原始地幔（187Os/188Os=0.1231~0.1616）更具放射成因特征。
橄榄石斑晶的氧同位素比值（δ¹⁸O_OSMOW=5.70~6.02‰）变化幅度有限，但相较于正常地幔橄榄石更为富集重同位素。
样品可划分为两个地球化学组分：其一为高MgO样品，其87Sr/86Sr比值更高，143Nd/144Nd与176Hf/177Hf比值更低，且Sr与Pb同位素呈正相关关系；其二为低MgO样品，相较于高MgO组分，其87Sr/86Sr比值更低，而143Nd/144Nd与176Hf/177Hf比值更高，且Sr与Pb同位素呈负相关关系。
高MgO组分主要包含早期喷发的火山碎屑与熔岩，而低MgO组分则涵盖了绝大多数晚期喷发的火山碎屑与熔岩。
上述成分变化与浅部地壳混染作用不符，反而被解释为反映了幔源不均一性。
微量元素与同位素特征均与北墨西哥伸展省（North Mexican Extensional Province, NMEP）的地幔被沉积物及洋壳来源的含水流体交代的特征一致。[…]