Abstract
New geochronological, geochemical, and Sr-isotopic data on volcanics
erupted before the Campanian Ignimbrite (CI, 37 ka) and the Neapolitan
Yellow Tuff (NYT, 12 ka) caldera-forming eruptions at Campi Flegrei
(CF) have allowed us to investigate the behavior and temporal evolution
of the Phlegraean magmatic system. The most prominent feature of
the CF magmatic system was the existence of a large, trachytic magma
chamber, episodically recharged, which fed eruptions for tens of
thousands years before the CI and NYT eruptions. During the pre-CI
caldera activity, magmas were episodically erupted from vents located
outside the present caldera structure. These magmas ranged in composition
from trachyte to alkali-trachyte, with Sr-isotope ratios increasing
through time, and becoming identical to that of the CI magma, at
about 44 ka ago. This suggests that the Phlegraean magmatic system
before the CI eruption was acting as an open system. It was being
progressively replenished by new batches of magma that mixed with
the resident less radiogenic, fractionating trachytic magmas and
was periodically tapped. The magma chamber evolution culminated in
the catastrophic eruption of the voluminous (150 km3 DRE), chemically
and isotopically zoned CI trachytic magmas, and in the resultant
CI caldera formation. Subsequent to the CI eruption, during a period
of moderate subaereal volcanic activity of about 20 ka duration,
magmas predominantly trachytic to alkali-trachytic in composition
and isotopically similar to the last emitted CI magma were erupted
from vents located inside the CI caldera. The temporal trend shown
by Sr-isotope ratios provides evidence for a new input of alkali-trachytic
magma, at ca. 15 ka, with 87Sr/86Sr ratio identical to that of the
alkali-trachytic magma feeding the first phase of the NYT eruption.
These data testify to the arrival in a short time span of a new trachytic
to alkali-trachytic magma in the system, isotopically distinct from
the CI magma, that gave rise about 3 ka later to eruption of the
NYT (40 km3 DRE).
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