PETROGENESIS OF JORULLO VOLCANO, MICHOACáN GUANAJUATO VOLCANIC FIELD, MEXICO

The 1759-1774 eruption of Jorullo volcano in the Michoacán Guanajuato Volcanic Field (MGVF), Mexico was one of the most recent monogenetic eruptions in the Trans-Mexican Volcanic Belt. The eruption produced compositionally diverse tephras and lavas, ranging from basalt to basaltic andesite [1]. We have performed elemental and isotopic analyses of 9 lavas and 15 tephras that span the duration of the eruption to investigate the relative roles of crustal assimilation and mantle source heterogeneity in the petrogenesis of the Jorullo magmas.

Our results show pronounced compositional variations (MgO = 4-10 wt%), with both lavas and tephras becoming progressively more evolved throughout the eruption. The samples have calc-alkaline affinities and relative enrichments in large ion lithophile elements and depletions in fluid immobile elements, consistent with a subduction fluid influence. However, variations in Nd isotope ratios and negative correlation of Nd and Sr isotopes could be attributed to crustal assimilation, consistent with the presence of granitic xenoliths in the lavas. Supra-chondritic Os isotope signatures further support the possibility of a role for crustal assimilation. However, the lack of correlation of Nd or Os isotopes with indices of fractionation argues against a significant role for crustal assimilation. In addition, δ¹⁸O signatures in olivine separates from the lavas and tephra are essentially invariant, and lack any correlation with whole rock MgO or olivine Fo contents, further arguing against an important role for crustal assimilation. Instead, the relatively constant but isotopically heavy δ¹⁸O_{Olivine­­ ­­} signatures (δ¹⁸O_SMOW ~5.62-5.97, average =5.78±0.2‰) suggest addition of a slab melt (uppermost altered oceanic crust or sediment) to the mantle wedge, as has been proposed on a regional scale for the MGVF [2]. Pb isotopic compositions of the lavas and tephras are distinct from the local subducting basaltic crust, and vary between the compositions of the subducting pelagic and hemipelagic sediment [3], further consistent with the importance of a sediment-derived component in the mantle wedge beneath Jorullo.

[1] Luhr and Carmichael 1985, Contrib. Mineral. Petrol. 90, 142-161.

[2] Johnson et al. 2009, J. Petrol. 50, 1729-1764.

[3] LaGatta 2003, PhD thesis, Columbia Univ.