Pre-eruptive conditions of the Campanian Ignimbrite eruption: Experimental constraints from phase equilibria and volatile solubility studies and its implication for the Campi Flegrei drilling project
Final Report Abstract
The solubility of H2O- and CO2-bearing fluids in trachytic and trachybasaltic melts, which represent respectively the most and the least evolved compositions of the Campi Flegrei Volcanic District, has been investigated experimentally at 1100 and 1200 °C, respectively, and at 100, 200, 300, 400, and 500 MPa. Phase equilibria experiments were conducted on H2O-CO2 saturated trachytic melts at pressure up to 300 MPa and at temperature between 850°C and 1050°C at relatively oxidizing and reducing conditions. Natural samples from the Campanian Ignimbrite eruption (trachyte) and from the Solchiaro eruption (trachybasalt) were collected around the city of Naples and on Procida Island. It was shown that: Primary magmatic water may be preserved in the erupted material and considered as minimum estimates on volatile concentrations in the melt just prior to the eruption or at the fragmentation event. For the first time deuterium/hydrogen (D/H) ratios were analyzed in natural pumices preheated at different temperatures to remove water adsorbed and/or imprinted by glass alteration processes. It has been determined that heating of the glass to 350 °C efficiently removes most of secondary water and the remaining concentrations represent primary magmatic water. Combining natural and experimental volatile solubility data minimum pressure estimates were provided for the storage and ascent conditions of magmas at Campi Flegrei. The Campanian Ignimbrite magma could have been stored or ponded during its rising path at two different levels: a deeper one corresponding to depth of about 8 to 15 km and a shallower one at about 1 to 8 km. Trachybasalts from Solchiaro were erupted from the deepest level of about 11 km with a storage or ponding level at around 2 to 8 km depth. Although an uncertainty of at least a kilometer has to be considered in estimating storage or ponding depths, these estimates point to significantly deeper magmatic sources for both eruptions as those considered previously. Comparing the paragenesis of the natural samples with those from the phase equilibria experiments, a temperature between 830°C and 900°C was estimated in the pressure range of 100-200 MPa for the magma prior to the CI eruption. Until now few temperature estimations concerning the CI eruption are known in literature. Harker diagrams of experimental and natural samples were used to reconstruct the evolution of the CI melt. It is suggested that the crystal fractionation via differentiation plays a major role during the evolution of the magma prior to the CI eruption. However, other processes such as mixing between several magma chambers and assimilation of host rocks cannot be excluded. Unfortunately, after rejection of the drilling project at the Campi Flegrei Caldera, our project was missing some of the most promising samples. In fact, the drilling should have provided for the first time access to samples of the Campanian Ignimbrite eruption inside of the Campi Flegrei area, giving access to the deposits most close to the reconstructed vent locations. Such materials would have been an ideal completion of the fall out products from larger distance to the Campi Flegrei caldera which has been studied during this project.
Publications
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(2011) A new type of high pressure low-flow metering valve for experiments at continuous decompression: First experimental results for magmatic systems, Am. Mineralogist 96, 1373-1380
Nowak, M., Cichy, S.B., Botcharnikov R.E., Walker, N., and Hurkuck, W.
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(2011) Solubility of H2O and CO2 in shoshonitic melts at 1250°C and pressures from 50 to 400 MPa: Implications for Campi Flegrei magmatic systems. J. Volc. Geotherm. Res. 202, 251–261
Vetere F, Botcharnikov RE, Holtz F, Behrens H, De Rosa R
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(2012) A general viscosity model of Campi Flegrei (Italy) melts. Chem. Geol., 290, 50-59
Misiti V, Vetere F, Freda C, Scarlato P, Behrens H, Mangiacapra A, Dingwell DB
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(2013) Water diffusion in potassium-rich phonolitic and trachytic melts. Chem. Geol. 346, 149-161
Fanara S, Behrens H, Zhang Y
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(2015) Volatiles in magmas related to the Campanian Ignimbrite eruption: Experiments vs. natural findings. Am. Min. 100, 2284–2297
Fanara, S., Botcharnikov, R.E., Palladino, D.M., Adams, F., Buddensieck, J., Mulch, A., Behrens, H.
