Correlating the Campanian Ignimbrite using matrix glass geochemistry and
morphology
Abstract
The Campanian Ignimbrite was emplaced during the 39-ka caldera-forming
eruption of Campi Flegrei near Naples, Italy. This eruption deposited a
trachytic ignimbrite up to 80 km from the caldera and co-ignimbrite
fallout more than 2300 km to the northeast. The pyroclastic density
current was dilute and mobile, overtopping kilometer-high mountains up
to 60 km from the vent. Stratigraphic units within the proximal and
distal deposits of the Campanian Ignimbrite have never been definitively
correlated due to their very dissimilar appearances and the lack of
medial exposure. Correlation of the ignimbrite, specifically which
proximal units formed simultaneously with the majority of the distal
deposits, is necessary to understand the events that formed this dilute
current. The co-ignimbrite lag breccia (Breccia Museo), which represents
the caldera collapse, and an earlier eruptive phase that formed a
high-grade ignimbrite (Piperno) are potential proximal correlates to the
main distal ignimbrite unit. Juvenile matrix material includes pumice
and scoria fragments and bubble-wall shards. A distal outcrop at 970 m
asl, thought to represent deposits from the upper transport system of
the main pyroclastic flow, is dominated by bubble-wall shards. At all
other sites, most juvenile ash shards show textures similar to
associated pumice lapilli. We suggest that much of the ash was from
abrasion of pumice, but the upper current had few pumice clasts and thus
less abrasion-derived ash. Preliminary analyses indicate that early
units have limited chemical compositional ranges (predominantly Na2O
4-7% and K2O 5-9%), whereas the Piperno and the dominant distal unit
both have a broader geochemical range that extends to roughly 9% Na2O
and 2% K2O. The uppermost proximal unit has the most extreme
compositions, approximately 2-6% Na2O and 6-11% K2O. These
observations are consistent with the source of the eruption being either
a large stratified magma chamber that underwent pre-eruptive mingling,
or several semi-isolated magma bodies. Proximal deposits include spatter
from local vents that may have tapped isolated magma bodies. Study of
pyroclasts from the Campanian Ignimbrite allow both correlation of
proximal and distal deposits and a characterization of some aspects of
the pyroclastic current.