Reappraising the eruptive history of the Alchichica Maar Volcano (Mexico) based on
Sr–Nd isotopes: Understanding the role of the magma source region on the growth of
small-volume volcanoes
Alchichica Maar is in the eastern sector of the Trans-Mexican Volcanic Belt, where
volcanics range from basalts to rhyolites, with 87Sr/86Sr and 143Nd/144Nd values between
0.70293 to 0.70447 (εSr = −21.1 to −4.5) and 0.512733 to 0.512949 (εNd = +3.4 to +6.2),
respectively. In this study, Nd and Sr isotopic data and new major and trace elements
compositions are provided for the lava flow, the scoria cone, and the phreatomagmatic
deposits composing Alchichica Maar Volcano. These results are used in combination
with published data to reexamine its eruptive history, though to be formed by more
than one eruptive event. The cogeneticy of the magmas that fed the eruptions of these
deposits is examined to infer the role of the deep magma source region and post-magmatic
alterations on the construction of the Alchichica Maar magmatic system. Alchichica
samples present 87Sr/86Sr values between 0.70387 and 0.70447 (εSr = −9.0 to −0.4)
and 143Nd/144Nd values from 0.512749 to 0.512773 (εNd = +2.0 to +2.8). While the 87Sr/86Sr
values suggest a slight increment in radiogenic 87Sr from the pre-maar lower lava
flow to the upper maar eruptions indicative of magma contamination within the upper
crust and by hydrodynamic mingling during the phreatomagmatic phases, the slight variation
in Nd isotopes suggest an overall homogeneous source where small batches of magmas
formed episodically to feed monogenetic eruptions, which subsequently yielded the
pre-maar lava flow, the pre-maar surge, the scoria cone, and the maar units. This
study reinforces the idea that an already “tapped” source region of a monogenetic
volcano may become fertile again, and new eruptions can occur from near or old vent
sites. It also highlights the hazardous potential of small-volume volcanoes.