The origin of a zoned ignimbrite: Insights into the Campanian Ignimbrite magma chamber (Campi Flegrei, Italy)

Abstract

Caldera-forming eruptions, during which large volumes of magma are explosively evacuated into the atmosphere from shallow crustal reservoirs, are one of the most hazardous natural events on Earth. The Campanian Ignimbrite (CI; Campi Flegrei, Italy) represents a classical example of such events, producing a voluminous pyroclastic sequence of trachytic to phonolitic magma that covered several thousands of squared kilometers in the south-central Italy around 39 ka ago. The CI deposits are known for their remarkable geochemical gradients, attributed to eruption from a vertically zoned magma chamber. We investigate the relationships between such chemical zoning and the crystallinity variations observed within the CI pyroclastic sequence by combining bulk-rock data with detailed analyses of crystals and matrix glass from well-characterized stratigraphic units. Using geothermometers and hygrometers specifically calibrated for alkaline magmas, we reconstruct the reservoir storage conditions, revealing the presence of gradients in temperature and magma water content. In particular, we observe a decrease in crystallinity and temperature and an increase in magma evolution and water content from the bottom to the top of the magma chamber. We interpret these features as the result of protracted fractional crystallization leading to the formation of a cumulate crystal mush at the base of the eruptible reservoir, from which highly evolved, crystal-poor, water-rich and relatively cold melts were separated. The extracted melts, forming a buoyant, easily eruptible cap at the top of the magma chamber, fed the initial phases of the eruption, until caldera collapse and eruption of the deeper more crystalline part of the system. This late-erupted, crystal-rich material represents remobilized portions of the cumulate crystal mush, partly melted following hotter recharge. Our interpretation is supported by: 1) the positive bulk-rock Eu anomalies and the high Ba and Sr contents observed in the crystal-rich units, implying feldspar accumulation; 2) the positive Eu anomalies in the matrix glass of the crystal-rich units, testifying to the presence of liquid derived from partial melting of low temperature mineral phases within the crystal mush (mostly feldspars); 3) the Ba and Sr-rich rims in the feldspars and positive Eu anomalies in clinopyroxene rims, suggesting late rim growth from a locally enriched melt following cumulate mush remelting and 4) the occurrence of An-rich plagioclase, relict from a more mafic recharge, which acted as a heat source. Our model reconciles many observations made over the years on zoned deposits of such high-magnitude explosive eruptions, and provides a framework to understand magma chamber processes leading up to cataclysmic events.

Long-term magmatic evolution reveals the beginning of a new caldera cycle at Campi Flegrei

“…S3, respectively] and matrix glass compositions with relatively high Ba and Sr contents and positive Eu anomalies (Fig. 2), which suggests complex magma interactions including mixing and partial remelting of cumulate material ( 27 , 28 ).…”

“…1). Note that the fractional crystallization of a feldspar-dominated mineral assemblage drives the residual liquid toward progressive depletion in Ba, Eu/Eu* (dashed arrow), and Sr. A combined effect of fractional crystallization and crystal-liquid separation produces extracted melts with much higher fractionated geochemical signatures compared to those obtained by simple fractional crystallization ( 27 ). This explains why the highly evolved matrix glass compositions (i.e., extracted melts) belong to the crystal-poor units (e.g., pre-CI, early phases of CI, pre-NYT, and Monte Nuovo).…”

A connection between magma chamber processes and eruptive styles revealed at Nisyros-Yali volcano (Greece)

“…The XRD analyses were performed with an AXS D8 Advance spectrometer with a Lynxeye detector (40 kV and 40 mA current, 5-90˚ scans with 0.8 seconds step time). A calibration curve based on glasses containing known vol% crystals (Forni et al, 2016) was used to determine the porphyritic index (Rowe et al, 2012). Between 1 and 3 individual pieces of pumice were analyzed at the XRD for each explosive unit.…”

Gravity modeling finds a large magma body in the deep crust below the Gulf of Naples, Italy

“…For the Campi Flegrei area, this situation seems to be preserved even in the upper crust, e.g. 57 , 58 .…”