Processes and timescales of magma evolution prior to the Campanian Ignimbrite eruption (Campi Flegrei, Italy)

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Abstract

The Campi Flegrei caldera collapsed 39 ka in the Neapolitan area (southern Italy) after the Campanian Ignimbrite eruption. This eruption, recognized as the largest and the most cataclysmic volcanic event in the Mediterranean area over the past 200 ka, extruded not less than 300 km3 of trachytic magma. Controversy exists over the timescales required to assemble such large volume of silicic melt and thus whether large magmatic reservoirs can actually persist below active volcanic systems over prolonged periods of time. Uranium-series analyses have been performed on Campanian Ignimbrite whole-rocks, glass matrixes and separated minerals, and the obtained results have been interpreted in combination with data on Sr, Nd, and Pb isotopes from literature. The compositionally most evolved sample which is most radiogenic with respect to Sr isotopes records a reference age of 71 ka. By contrast, U–Th internal isochrones of the three compositionally least evolved samples give identical initial Th isotope ratios and yield consistent ages predating the eruption by up to 6.4 ka. The highest Pb and Nd isotopic ratios and 230Th/232Th activity ratios together with the oldest reference age of the most evolved samples suggest the existence of a resident magma body possibly related to a magmatic system that is known to have fed earlier magmatic activity in the Campi Flegrei area. Conversely, the younger age of the least evolved and least radiogenic magma dates the crystallization/differentiation event of a chemically and isotopically new magma batch entering the reservoir of the resident magma some few thousand years before the cataclysmic eruption. Therefore, the time preceding this large caldera-forming eruption during which the large volume of Campanian Ignimbrite magma assembled and mixed is 6.4 ± 2.1 ka.

Research highlights

► Campanian Ignimbrite eruption is the most cataclysmic event in the Mediterranean area. ► We performed U–Th analyses on Campanian Ignimbrite whole-rocks, glasses and minerals. ► Reference age of the most evolved Campanian Ignimbrite sample is 71 ka. ► Weighted mean age of internal isochrones of the least evolved samples is 45.4 ± 2.1 ka. ► The large Campanian Ignimbrite magma volume assembled 6.4 ± 2.1 ka before eruption.

Introduction

Caldera-forming silicic eruptions are among the most cataclysmic volcanic phenomena documented in the geological record (Smith, 1979). They require generation, accumulation and storage of a large volume of magma in a shallow crustal reservoir for some time prior to eruption. During this time interval magma undergoes crystallization and recharge until eruption begins, likely triggered by arrivals of batches of hotter magma in the reservoir or when the resident magma reaches its level of volatile saturation during crystallization. However, how rapidly a large volume of magma can accumulate, how long it resides prior to eruption, and what the priming time is for an evolved magmatic system to erupt cataclysmically, are still unresolved topics. Magmatic systems may evolve in different ways: some show evidence for prolonged residence time, others for rapid formation or accumulation from pre-existing smaller magma batches.

Short-lived U-series isotopes are an important tool in assessing timescales of magmatic processes (Allegre and Condomines, 1976, Condomines et al., 1988, Condomines and Sigmarsoon, 2003, Cooper and Reid, 2008, Gill and Condomines, 1992, Hawkesworth et al., 2004, Reid, 2003). 238U/230Th isotope systematic can potentially provide age constraints on the growth of mineral phases, reflecting crystallization and differentiation processes at timescales of 10–300 ka. Furthermore, 230Th/232Th activity ratio in volcanic rocks, can be used as an isotopic tracer of the Th/U ratio of the source.

Magma residence times have been investigated mostly for caldera-forming eruptions fed by highly differentiated rhyolitic magmas (i.e. Yellowstone and Long Valley calderas — USA; Taupo Volcanic Center — New Zealand; and Olkaria Volcanic Complex — Kenya Rift Valley) (Halliday et al., 1989, Hildreth, 1981, Macdonald et al., 1987, Macdonald et al., 2008, Sutton et al., 2000). For such large (> 500 km3) and oversaturated silicic systems, magma residence times ranging from 10 to more than 100 ka have been inferred (Davies et al., 1994, Halliday et al., 1989, Heumann and Davis, 2002, Turner and Costa, 2007, Vazquez and Reid, 2002). For smaller and more mafic alkaline systems, significantly shorter residence times (10–100 years up to a few thousand years) are reported (Bourdon et al., 1994, Hawkesworth et al., 2004, Pyle, 1992, Scheibner et al., 2008, Schmitt et al., 2010, Sigmarsson, 1996).

The Campanian Ignimbrite (CI) caldera-forming eruption in the Campi Flegrei volcanic area (southern Italy; Fig. 1a) is intermediate in terms of size, composition and alkalinity between large silicic and small highly alkaline magma systems (Bohrson et al., 2006, Civetta et al., 1997, Fedele et al., 2003, Fedele et al., 2008, Fisher et al., 1993, Fowler et al., 2007, Ort et al., 2003, Pappalardo et al., 2002a, Rosi et al., 1999, Signorelli et al., 1999, Signorelli et al., 2001). De Vivo et al. (2001) dated this eruption by 40Ar/39Ar at 39.3 ± 0.1 ka. This age was later confirmed by Fedele et al. (2008) who reported plateau ages between 37.1 and 39.5 ka. The error provided on the age determination by De Vivo et al. (2001) represents the internal error. For this reason and acknowledging this limitation on the precision of the eruption age, for the purpose of this paper we will compare our U–Th mineral ages with a reference age for the CI eruption of 39 ka.

The oldest subaerial volcanic products in the Campi Flegrei area yield ages of about 60 ka (Pappalardo et al., 1999) and are related to volcanism extending beyond the limits of the present caldera (Orsi et al., 1996). However, volcanic rocks dated at about 150 ka and 80 ka at the neighboring islands of Ischia (Cassignol et al., 1982, Vezzoli, 1988) and Procida (Rosi et al., 1988a,b), respectively, document older long-standing evolved magmatic systems within the Phlegraean Volcanic District (Procida and Ischia islands and Campi Flegrei). However, among all the eruption that occurred in the region, the CI was the most cataclysmic, erupting not less than 300 km3 of trachy-phonolitic magma. Pyroclastic products of this eruption were deposited at great distance from the vent as shown by the occurrence of ash in late Pleistocene Mediterranean marine sediments and in several archeological sites of South-eastern and North-eastern Europe (Fedele et al., 2003, Fedele et al., 2007, Thunnel et al., 1979; Fig. 1b). Atmospheric loading by sulfuric aerosol, ash and CO2 from this eruption may have accelerated cooling of Earth's climate (Fedele et al., 2003, Fedele et al., 2007). The CI eruption was accompanied by the collapse of an area of about 230 km2 that was the first expression of the nested Campi Flegrei caldera that evolved further during the past 39 ka (Orsi et al., 1992, Orsi et al., 1996, Acocella et al., 2004).

After more than 20 ka the second largest collapse within the caldera occurred in relation to the Neapolitan Yellow Tuff eruption (NYT; 14.9 ka ± 0.4 ka; Deino et al., 2004, Orsi et al., 1992, Orsi et al., 1995), that extruded > 40 km3 of latitic to trachytic magma. Even though such large caldera-forming eruptions are not among the expected events in case of renewal of volcanism in short- to mid-terms (Orsi et al., 2004, Orsi et al., 2009), the Campi Flegrei caldera is one of the most hazardous volcanic systems on Earth. This is because of its persistent state of activity and the explosive character of volcanism (Costa et al., 2009, Orsi et al., 1999a, Orsi et al., 1999b, Orsi et al., 2004). Due to the high volcanic hazard and the large population (1.5 million people) living within the caldera and its surroundings the volcanic risk of this area is extremely high (Orsi et al., 2004).

Guided by the previous studies we measured U–Th isotopes by Thermal Ionization Mass Spectrometry (TIMS) on whole rock, glass and phenocrysts from pumice fragments of the CI eruption. The main aims of our study were i) to better constrain the processes leading to the formation of the CI magma chamber, ii) to further characterize distinct magmas involved in the eruption and iii) to place temporal constraints on the pre-eruptive ages and residence times of magmas accumulated prior to this eruption. The new U–Th isotope data are discussed together with our published Sr and Nd isotope data on the same rocks and Sr, Nd and Pb isotopes from literature, in order to constrain the relationships between processes of crystallization/differentiation and mixing, magma storage and eruption through time. This temporal aspect of magma evolution is extremely important in order to assess the history and behavior of such a hazardous volcanic system.

Section snippets

The CI magmatic feeding system

The superposition of two compositionally different CI units was only observed in one exposure at Mondragone. However, a core drilled in the northern part of the city of Naples (Pappalardo et al., 2002a) offered the only opportunity to study in a continuous stratigraphic sequence the variable pyroclastic currents deposits emplaced during the CI eruption. The information yielded by the drilled core, together with that from exposed CI deposits, allowed Pappalardo et al. (2002a) to reconstruct the

Sampling strategy

For the purpose of this study and according to the chemostratigraphic reconstruction of Civetta et al., 1997, Pappalardo et al., 2002a, samples representative of each of the different magmas erupted during the CI eruption were collected. MT 1 and the intermediate composition hybrid magma were sampled at Mondragone, along the South-eastern slopes of Monte Massico, 38 km from the vent area (Fig. 1c). The composite pumice sample Mondragone 152a2 (Mond152a2), representative of MT 1, was collected

Analytical methods

Pumice samples were washed several times in de-ionized water, then dried, crushed to particles < 1 cm and powdered in an agate mortar. The outer part of each pumice fragment was removed before preparation, by using a dentist drill. Whole rocks were analyzed for major and trace elements (see Arienzo et al., 2009 for details) at the Georg August Universität, Geowissenschaftliches Zentrun Göttingen (GZG), Göttingen (Germany) by X-ray fluorescence spectrometry (XRF) and Inductively Coupled Plasma

U–Th disequilibria

Measured 238U/234U activity ratios (Table 1) on CI whole rocks, glasses and phenocrysts of selected samples range from 0.996 ± 0.011 to 1.017 ± 0.020, suggesting that the analyzed samples have not been affected by secondary alteration. Positive linear relationships exist between degree of differentiation and Th and U concentrations as well as 230Th/232Th activity ratios, as well as Sr and Nd isotope ratios (Fig. 3). U–Th disequilibria for whole rocks and glasses are shown in the equiline diagram of

Magma source characteristics

The isotopic and chemical variations through time of the Campi Flegrei magmas (Arienzo et al., 2009, D'Antonio et al., 2007, Di Renzo et al., 2011, Pabst et al., 2008, Pappalardo et al., 1999, Tonarini et al., 2004), in particular those erupted in the 60–10 ka time interval, provide useful information on both processes and magma components within the magmatic system feeding Campi Flegrei volcanism. In the following, we will combine our U–Th isotope data and mineral ages with Sr, Nd, and Pb

Conclusions

U-series data presented here provide (a) absolute age information on a crystallization/differentiation event involving the least differentiated and least radiogenic CI MT 2, hence providing a minimum residence time for this melt, and (b) allow us to calculate a reference age for the most radiogenic and differentiated CI MT 1. Furthermore, based on Sr, Nd, and Pb isotope compositions in combination with (230Th/232Th) ratios measured on samples from pre-CI, CI and post-CI eruptions, we suggest

Acknowledgements

The authors warmly thank G. Mengel for her technical support in the clean laboratory, A. Carandente and P. Belviso for their support during samples preparation. The authors thank S. de Vita for his support during field work, M. D'Antonio, B. Scheibner and S. Pabst for discussion and suggestion during this work. We are grateful to the Editor T.M. Harrison and two anonymous reviewers for their useful criticism and suggestions. We gratefully acknowledge the support by the Abt. Isotopengeochemie at

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