Active fumarolic solfataric zones represent important
structures of dormant volcanoes, but unlike emitted fluids, their
mineralizations are omitted in the usual monitoring activity. This is the
case of the Campi Flegrei caldera in Italy, among the most hazardous and
best-monitored explosive volcanoes in the world, where the landscape of
Puteoli is characterized by an acid sulfate alteration that has been active at
least since Roman time. This paper provides temperature, mineralogical,
textural, compositional and stable isotope data for those solfataric
terrains sampled at the crater and Pisciarelli slope of the Solfatara
volcano between 2013 and 2019. Temperatures vary between 40 and
95
Active solfataric landscapes are among the most peculiar and fascinating
environments on the Earth that may be considered as planetary analogues
(e.g., White and Hedenquist, 1990; Rye et al., 1992; Lowe et al., 1993;
Zillig et al., 1996; Ciniglia et al., 2005; Rye, 2005; Glamoclija et al.,
2004; Sgavetti et al., 2008). Their peculiarity arises from the stringent
interaction between inorganic (mineral assemblages and geochemistry) and
organic (biota) substances under extreme ambient conditions (pH,
temperature, salinity, oxygen deficiency, etc.) associated with endogenous
degassing (i.e.,
The Solfatara volcano (Campi Flegrei, CF, Italy; Fig. 1a) is perhaps the most famous and hazardous geothermal solfataric setting in the world (e.g., Rittmann, 1950; Rosi and Sbrana, 1987; De Vivo et al., 1989; Barberi et al., 1984; Piochi et al., 2014) with exploration since Greek times up to the medieval period (e.g., Photos-Jones et al., 2016). The generation of new minerals (hereinafter referred to as neogenesis) has received limited discussion in the recent literature (Cortecci et al., 1978; Valentino et al., 1999; Piochi et al., 2015; Russo et al., 2017). In contrast, several studies relate to bradyseism phenomena addressing the various aspects of seismicity, ground deformation and outgassing (e.g., Corrado et al., 1976; Barberi et al., 1984; Chiodini et al., 2016; Cardellini et al., 2017; Moretti et al., 2017), life in these environments (e.g., Zillig et al., 1996; Glamoclija et al., 2004; Sgavetti et al., 2008), and a continuous interest in the use of hydrothermal products as a thermal bath and for medical care (e.g., Photos-Jones et al., 2016; Giacomelli and Scandone, 2012).
This paper focuses on the solfataric mineral assemblages updating our previous research (Piochi et al., 2015) and presenting the result of our progressing work on the CF solfataric volcano. Results derive from temperature determinations contextually to sampling and investigations by optical microscope (OM), X-ray powder diffraction (XRDP), electron diffuse system–back-scattered electron microscopy (EDS-BSEM), diffuse Fourier infrared spectroscopy (DRIFT-FTIR), whole-rock geochemistry (WRG) and stable isotope geochemistry (SIG) of sulfur and oxygen. By merging new and published information (Celico, 1986; Guglielminetti, 1986; Rosi and Sbrana, 1987; Chiodini et al., 1988; Celico et al., 1992; Aiuppa et al., 2006; Caliro et al., 2007; Piochi et al., 2014; Di Giuseppe et al., 2017; Moretti et al., 2017), we reflect on the significance of the sulfate alteration zone and related volcanological implications.
The Solfatara volcano (Fig. 1a, b, c) exhibits impressive and powerful hydrothermal activities with hot fumaroles, thermal springs, mud pools and diffuse outgassing (Allard et al., 1991; Valentino et al., 1999; Chiodini et al., 2001, 2010; Valentino and Stanzione, 2003, 2004; Piochi et al., 2015; Cardellini et al., 2017; and references therein). The hydrothermalism intensely altered the faulted volcano slopes (Rosi and Sbrana, 1987), and the solfataric landscapes (Fig. 1a–e) have locally replaced the original pyroclastic sequences (e.g., Agnano Monte Spina, Astroni and Solfatara tephra) and lavas (Monte Olibano, Solfatara cryptodome) younger than 5 ka (e.g., Di Vito et al., 1999; Piochi et al., 2005).
The study area is located at Puteoli, the area of maximum ground uplift (in excess of 3 m) and seismicity (more than 16 000 low-magnitude earthquakes), activated during the unrest episodes in 1970/1972 (Corrado et al., 1976) and in 1982/1984, namely “bradyseisms” (Barberi et al., 1984), that are slowly ongoing (e.g., Bodnar et al., 2007; Chiodini et al., 2016; Moretti et al., 2017).
The solfataric area has been exploited for centuries for its alum occurrences (Photos-Jones et al., 2016, and references therein). Intense mining during Roman and medieval times modified their original context (Photos-Jones et al., 2016): the Pisciarelli gorge valley was a quarry, while caving activity exposed the eastern (the Monte Olibano inner wall) and northern flanks of the Solfatara volcano and created rework deposits in the crater floor.
Old pictures and descriptions (Sicardi, 1959) suggest that the most evident manifestations along the SE and NE rims remain roughly the same: (1) the main Bocca Grande fumarole (Fig. 1d) with various exhalative branches northward along the morphological heights; (2) the minor fumarolic vents around the old thermal baths (Sst site; Fig. 1d); and (3) the mud pools (Fig. 1b, c, d, e). Also, the thermal spring in Pisciarelli (Fig. 1a, b, e), known as the “Bulla”, i.e., the bubbling one, has been known at least since medieval times (Photos-Jones et al., 2016). Moreover, the same descriptions indicate the presence of a lake in the Agnano Plain (Fig. 1a). According to Ventriglia (1942), the lake extended up to the slope base of the Solfatara volcano and had a maximum depth of 15 m; drillings recovered related sediments (de Vita et al., 1999). Ventriglia (1942) also indicated high temperatures in the lake preventing fish from living. Today, the area shows several mud pools and thermal springs, while some (“de Pisis” and “Sprudel” springs in the Terme of Agnano; Fig. 1a) has disappeared. Yet, high temperatures can be still detected.
At present, groundwater nearby Solfatara are rich in
Emitted gases include
Sampling was conducted within the Solfatara crater and in the Pisciarelli and Cinofilo areas (Fig. 1a, d, e) with additional sites compared to Piochi et al. (2015); the crater floor, except the pool, was intentionally avoided because of reworking in historical time (Photos-Jones et al., 2016) and thus possible anthropogenic contamination. This study enlarges the dataset on the acid sulfate alteration zone of the Phlegraean area, in order to understand the quiescent dynamics of the volcano. Similar observations and data are also available for Ischia island (Piochi et al., 2019), which belongs to the Phlegraean Volcanic District (Piochi et al., 2005).
Our new collection is, therefore, widening the observation period for the
Puteoli sulfate area that now spans between January 2013 and April 2019
(Table S1 in the Supplement). Selection of sampling sites (hereinafter referred using the abbreviations in Fig. 1d, e) is based on variable macroscopic features including
outgassing “magnitude”, tectonics and fracturing evidence, mineral
occurrences, and exhalative vent locations, as visible in the field and
described in the literature (Allard et al., 1991; Ferrara et al., 1994;
Valentino and Stanzione, 2003; Aiuppa et al., 2013; Bagnato et al., 2014;
Chiodini et al., 2016). A thermocouple digital probe 51/52 II by Fluke with
precision of
Samples were air-dried for several days to 1 week. Subsequently, these were studied under the OM in order to assess their general mineral assemblages. Where possible, the various S-bearing phases (or enriched portions) were handpicked for subsequent isotopic analyses. Figures 2 and 3 show the appearance of the most representative samples.
BSEM image showing sample texture and occurrences of S-bearing
phases identified by EDS and XRDP analyses at the Puteoli sulfate lands:
Both bulk rocks and separated phases were pulverized in an agate mortar for XRDP, DRIFT-FTIR and WRG. EDS-BSEM and SIG used aliquots of bulk materials and isolated mineral phases. Appendix A provides detailed information about analytical techniques. Details on XRDP and DRIFT-FTIR are in the Supplement together with representative patterns (Figs. S1, S2).
New and previously published (Piochi et al., 2015) mineralogical data for the Solfatara–Pisciarelli area (Tables S1, 1) have provided information on a yearly to monthly basis since 2013 along with measurements of temperature. The mineral assemblage dataset derives from XRDP analyses (Supplement) corroborated by textural and chemical information obtained at the EDS-BSEM. DRIFT-FTIR spectra determined on representative samples display characteristic bands of minerals they include (see below and Supplement) and help in material characterization.
Main hydrothermal minerals detected by XRPD with related ideal chemical formula and sites of occurrence (names as in Fig. 1a, d, e). The complete set of minerals is in Table S1. Refer to the Supplement for details.
Through time, K, Al sulfates (alunite) and native S (Fig. 3) are the main and widely distributed secondary mineral phases associated with surface degassing. Alunogen and pyrite (Fig. 3a, b, d, e, h, i) are second in abundance. All these mineral phases can form single phase concretion or coexist in up to millimeter-sized grains. Alunogen often – if not generally – associates with alunite and occurs in two distinct morphologies (Fig. 3b, d, e). Most commonly, it consists of fibrous tangled masses of white crystals. Where they coexist, alunogen fibers grow from the edges of alunite crystals (e.g., sample L100 zucc in Table 1; Fig. 3d). This appearance seems usual along the fault scarp, north of the pool (L1 site, Fig. 1e). Subordinately, alunogen has thin, platy crystal habits (Fig. 3b, d, e). Many of these crystal groups show rounded to corroded edges suggesting alteration after crystallization (Fig. 3e). Dendritic and/or sometimes bipyramidal crystallites (Figs. 2a, 3) are ubiquitous habits for native sulfur (typically sampled at L1, SMO, some places along ASA, Sst in Fig. 1d, e and Table 1) that mostly cluster within the alunitic surface and the rock voids (Fig. 2c). Along the fracture, sulfur may form a yellow ductile patina (L1 vent, BG, BN in Fig. 1d, e and Table 1). Locally (PINT, PEXT, L19, L20, L60 in Fig. 1e and Table 1), sulfur produces encrustations with a pale yellowish fibrous-like texture (Fig. 2b).
Pyrite (Fig. 2d) occurs as smaller (
Clays have a low relative abundance in the studied samples (Supplement).
They are mostly kaolinite and illite (Tables S1, 1), as derived by the
XRDP traces (see Fig. S1c, d, e) and supported by EDS-BSEM and DRIFT-FTIR
study (see below; Fig. S2 and Supplement). In particular, the infrared
technique is suitable to detecting the kaolinite and the related bands in the
OH region, in agreement with Madejová et al. (2002). Illite usually
occurs in the muds at Pisciarelli (from the geyser and around other emissive
vents) and occasionally at Solfatara (Tables 1, S1). Kaolinite characterizes
the newly formed pool within the Solfatara crater and occurs locally at
Pisciarelli (Fig. 1c, d, e and Table 1). Figure 4 illustrates the platy
particles of kaolinite with typical widths of
BSEM image of kaolinite platy crystals at the new pool of
Solfatara (New P, Fig. 1c, d): the kaolinite plates have a tendency to assembly
Other efflorescent phases (Fig. 3c, i) occur randomly. Rarely, Al and Fe
sulfates (halotrichite) have been identified nearby the Pisciarelli geyser
as crust-like aggregates. Na and
Air-dried evaporation of water sampled at the Pisciarelli pool resulted in the precipitation of mascagnite, tschermigite and letovicite (Figs. 2e, f, S1a and Tables S1, 1). Figure 3c shows the euhedral tschermigite that coexists with native S in the sample L30 eff-blocchetto (Tables S1, 1). Instead, evaporation of Solfatara mud pool water produced alum, as documented already in medieval and Roman times (Photo-Jones et al., 2016). Water from the Stufe di Nerone (west side not shown in figure) crystallized halite.
Realgar (sometimes only detected at the EDS-BSEM and not listed in Table S1) and ammonium chloride (Fig. 3g, h) appear as peculiar precipitates at the Bocca Grande and Bocca Nuova sites (Fig. 1d).
Accessory minerals include hematite, quartz and, possibly, Fe hydroxides and phlogopite.
Furthermore, amorphous phases are largely present at various sites (Tables S1, 1), particularly, in muds and in the samples from Bocca Grande and the L1 vent (Fig. 1d, e). The widespread amorphous phases could correspond to material from both the original volcanic rock and alteration. General assumptions (Piochi et al., 2015; Montanaro et al., 2017) indicate amorphous silica, although this merits a more rigorous examination.
Finally, Fe oxide and fresh to variably altered feldspar and biotite are the most common primary volcanic mineral phases.
DRIFT-FTIR spectra collected on selected samples (Fig. S2) produce data
consistent with XRDP results (Fig. S1) and furthermore allow useful details
on structure and eventual minor phases or impurities (Supplement). Table S2
lists the relevant vibration modes of spectra and the proposed mineral
assignments. The crystals formed by evaporation of water in the Pisciarelli
pool (Fig. S2a; Supplement) show a sharp band at 1422–1411 cm
Native S from two different samples (PINT S tozzo 18/10/17 and PINT S
18/1/18 in Table 1; Fig. S2b) is evident in the DRIFT-FTIR spectra at
As expected (Clark et al., 1990), alunite can be determined through its
major band at 3483 cm
Notably, the DRIFT-FTIR spectra of muds from Pisciarelli (Fig. S2d) show a
vibration in the region of 1430 cm
The new pool at Solfatara has peculiar DRIFT-FTIR spectra in
the OH-stretching region (Fig. S2e; note the inset) due to the presence of
kaolinite, in addition to alunite, and minor (or occasional) sulfur,
feldspar, pyrite and amorphous phases. Specifically, these are (i) alunite
(Clark et al., 1990) with a major band at 3483 cm
The four vibration modes of kaolinite in Fig. S2e point to a well-ordered mineral structure (Madejová, 2003; Fitos et al., 2015), giving strong support to the XRDP results (Fig. S1e, Supplement), also in multiphase samples (Madejová et al., 2002).
A new set of
Continued.
Distribution of
The new sulfur isotope results are generally comparable with literature
values for Campi Flegrei (Piochi et al., 2015), although studies earlier
than 2000 (Cortecci et al., 1978; Valentino et al., 1999) also show positive
our new S-isotope data for Pisciarelli include a few positive values (Fig. 5b, c). the new O-isotope values for sulfate are the highest obtained until now
(Fig. 5d). To note, the muds generally have the least heavy oxygen isotopes,
except for samples from 2013–2014 for which O-isotope determinations are
lacking. The diagram also indicates a lowering in the sulfides at Pisciarelli show the different sites display a homogenous range in a likely appearance of a positive correlation between S-isotope results
for pyrite and for sulfate phases coexisting at Solfatara, with two from the
1994 data outside the trend (Fig. 6a). new isotope data for sulfate reveal a difference compared to studies
older than 1990 and the most recent one (Fig. 7).
In addition, the
Distribution of
Covariation of
Table S3 reports the whole-rock geochemical compositions of selected
samples. As expected, samples are highly hydrated and sulfur-rich, due to the
OH group and/or S in the crystalline network (see ideal formula in Table 1)
and/or the presence of native S in the analyzed sample. LOI (loss on ignition) can be up to 80 wt % (sample L20 camino 18.9.17), although it is most commonly at 20 wt %–30 wt %,
and the S content is up to more than 50 wt %. Carbon is generally low
(
With respect to the local volcanic rock substrate (Table S3; Fig. 8a), some
lithophile elements (Si, Al, P, Sc, Ti, V, Zr, Ba, Yb, Th, Hf) are
comparable or depleted (for examples, Be at
Lithophile
Throughout the years, the various Solfatara and Pisciarelli sampling sites have displayed a nearly constant mineral alteration assemblage (Table 1). Commonly, the mineral neogenesis variably develops on sub-millimeter to decimeter to meter scales, in relation to the outgassing dynamics, runoff, weather conditions, outcropping substrate and anthropogenic activity.
However, the various sites further display reproducible rock geochemistry and stable isotope compositions at the timescale of the survey and with respect to the oldest data (e.g., Valentino et al., 1999) as well; they can be considered reference points for future investigations.
Based on the presented dataset, we propose the existence of major alteration sub-zones, in which some (minor or peculiar) mineral phases appear or disappear, in response to changing physical–chemical boundary conditions mainly associated with weather circumstances, i.e., mostly humidity and water abundance. These sub-zones are discriminated by their dominant and repetitive mineralogy, rock chemistry and isotopic compositions and characterized by temperature variations in a narrow range. Such a constancy is revealed when comparing results reported by Sicardi (1959) (see Geological setting in Sect. 2.1) with the present results, corroborating the existence of “stationary” sub-zones that are presented in the following. The only exception is the mud pool in the crater.
The Pisciarelli and Solfatara pools (Fig. 1d, e) are the two major and
distinct sub-zones. They display persistent differences in dynamics,
temperature and mud (solute plus water) mineralogy. The main pool at
Pisciarelli shows vigorous boiling (Fig. 1b, e), with temperatures ranging
from 63.9 to 94.3
The PINT-PEXT sub-zone (Fig. 1e) – an isolated morphological height – is
composed of an alunitic-rich low-cohesive reddish terrain with a temperature
of around 95
A hole up to 2–3 m deep represents a distinct sub-zone that we emphasize
because it opened 180 m north of the main pool within the crater in May
2017, by surface collapse. A gray viscously boiling mud fills the hole (Fig. 1c, d), with a minimum temperature of 70
Cross plots of trace elements in solfataric samples. Fields
envelop the various genetic settings, following Ercan et al. (2016) and
based on the
Finally, a rather broad sub-zone includes the other various sampling sites
that are characterized by encrustations of alunite with a well-defined,
although relatively large, range of
Widespread alunite formation reflects the potassium and feldspar-rich rock substrate on which they develop (see Piochi et al., 2014, 2015, and references therein).
Vapor effluents around the various geysers and vents at Pisciarelli are the
most important factors affecting the mineral neogenesis at the
alunite-dominated sub-zones. Pisciarelli is a decameters-deep incision on
the NE Solfatara slope, and the degassing vents are constrained in a
gorge-like morphology. This setting favors the stagnation of the
hydrothermal steam that impregnates the rock substratum and supplies
elements to the formation of a variety of Na and
Vapor emission outflow and the conditions of hydrothermal steam stagnation are dependent on atmospheric pressure and wind conditions.
It is thus likely that the meteoric weather is the main cause for the appearance and disappearance (and vice versa) of some phases.
This is also particularly evident for the PINT, PEXT, L19 and L20 (Fig. 1e) and
the SMO, ASA and SSt (Fig. 1d) sub-zones that may typically present bipyramid
and/or fine dendritic sulfur crystallites (Fig. 2a). Their crystallization
seems to be favored by relatively strong exhalations and porous terrain
(PINT, PEXT, L19, L20; Fig. 1e) or conditions where gases remain briefly
trapped (SMO, some places along ASA, Sst; Fig. 1d). Respective conditions
also prevail in close proximity (
However, native S disappears during runoff, and we have macroscopically determined at several places that re-crystallization needs 1–2 months, if not longer (i.e., sample L20 camino; Fig. 1e).
Furthermore, periods of intense rainfall determine the timing areal extent and depth of the mud pools, as well as the generation of secondary mud vents and the erosion in Pisciarelli and its periodic water puddle. Sicardi (1959) already noted the occurrence of mud vents and black mud pools following rainy periods. Notably, pools at Pisciarelli are supported by anthropogenic embankment.
Meteoric and surface waters can dilute the aggressive endogenous fluids determining alteration degree conditions low enough for the generation of illite, or other clays (Pirajno, 2008) at Pisciarelli. Further studies need to be performed in order to better characterize clays as they can bear information useful to further constrain the hydrothermal setting.
Al and Fe sulfates (halotrichite) have been rarely found nearby the Pisciarelli geyser (see G in Fig. 1e; Tables 1, S1).
The distribution of sulfates appears irregular, and this should be an subject of future investigations.
The style of mineralization (Arribas, 1995; Sillitoe, 1993; Pirajno, 2008; Ercan et al., 2016) and the stable isotope results (Rye et al., 1992) allow the classification of alteration and differentiation of genetic environments. Table 3 summarizes characteristic mineralogical, lithological and isotopic features of these environments, in comparison to observations made at the study sites. Several contrasting interpretations can result from the data.
Summary of the mineralogical and isotopical features at the acid sulfate area following Rye et al. (1992) and Hedenquist and Lowerstern (1994).
Alunite plus kaolinite form in steam-heated environments at 100 to 160
Nevertheless, alunite shows grain sizes in the range of 50 to 100
Only the alunite coexisting with kaolinite in the new hole pool exhibits
the finest grain size. Accordingly, the XRDP and DRIFT-FTIR analyses of CF
samples point to slightly ordered kaolinite forms that usually occur at
temperatures
However, when considering litho-geochemical parameters, schematic diagrams
further produce contrasting visions. For example, following Ercan et al. (2016), the clay-bearing muds can be ascribed to a variable supergene to
hypogene alteration field in the binary diagram of immobile Zr vs.
The stable isotope geochemistry of minerals supports an interpretation of
steam-heated to supergene environments (Fig. 7). S-isotope equilibrium
occurs between sulfides and sulfates, with reliable re-calculated
temperatures in high-sulfidation environments (Arribas, 1995). In contrast,
this equilibrium cannot be accounted for at Campi Flegrei, and any reliable
temperatures result from the S-isotope fractionation between sulfates and
Actually, Campi Flegrei lacks the occurrence of enargite and luzonite, both diagnostic for high-sulfidation environments. Instead, it shows minor occurrences of realgar (AsS) as well as cinnabar (HgS) (Tables 1, S1), and orpiment has also been described (Russo et al., 2017).
Significantly lower
Measured vs. theoretical fractionation values. Theoretical values
based on temperature measurements were calculated following Ohmoto and Rye (1979) and Rye et al. (1992). Fields for steam-heated (white) and supergene
(gray) environments are from Rye et al. (1992): dashed envelop for
alunite–pyrite (circle) or alunite–
Furthermore,
Finally, we are not able to directly identify any microbial sulfur cycling,
although FTIR and rock geochemistry corroborate the absence of or limited
biota contribution. The analyzed samples do not exhibit bands attributable
to C
Merging all available information, it appears that observations concerning both the apparent “stationarity” at sub-zones and a seemingly contradictory classification environment reflect the evolving conditions that have followed the last magma intrusion and eruption and that probably are overlapping through time.
The solfataric alteration zone has a strongly limited extent within the central sector of the Campi Flegrei caldera. It coincides with the area of eruptive vents (e.g., Mt. Olibano, Accademia, Solfatara; Fig. 1a) and uplift of the most recent period of volcanism (Di Vito et al., 1999). The zone appears to be limited under the later Fossa Lupara and Astroni vents, while outgassing and thermal aquifers occur within the caldera. However, there is an indication for their discrete, more than their continuous distribution, both across the caldera and through depth (Guglielminetti, 1986).
The studied deposits are young and nearly coeval (
The alteration zone locally presents high Ti, Ba, Au, As, Hg, Tl and S concentrations relative to the above parent basement lithology (Fig. 8).
The zone also appears anomalous in terms of ammonium content. Therefore, we
here adopt ammonium as a possible tracer, but we have no information yet
about the various contributing sources for the N species and the cycling of
nitrogen at the local scale. The presence of
The concentration of some metals and metalloids requires sources different
from the parent basement. Anthropogenic contributions are obviously possible
(Alloway, 2013), e.g., when considering that
High concentrations (20–100 g L
In summary, we propose an environmental setting that merges all collected
information (Fig. 11). Fluid outflows from discrete aquifers hosted in
sediments – and bearing organic imprints – feed the Pisciarelli site
giving its ammonium peculiarity. Our purpose does not exclude the possible
biological contribution that has been ascertained in the studied sites
(e.g., Ciniglia et al., 2005; Glamoclija et al., 2004). However, marine
strata and a volcano-clastic sequence intercepted by deep drillings (San
Vito1, Mofete and CF23 wells; Rosi and Sbrana, 1987; Piochi et al., 2014)
are considered as the key sediments for the
Sketch of the acid sulfate alteration zone at the Campi Flegrei
caldera (Fig. 1a). The subsurface is constrained by borehole (deep from Rosi and
Sbrana, 1987; Piochi et al., 2014, and shallow from de Vita et al., 1999) and
geophysics (Di Giuseppe et al., 2017) information. The presence of
Shallow and deeper aquifers are interconnected via a network of “communicating vessels” through a fault system, allowing deeper and shallower water to mix and be expelled at Pisciarelli. This justifies an apparent persistence of thermal springs around the Agnano Plain also in the presence of the desiccating lake described by Ventriglia (1942). It also supports the depth of the water table, being at a higher topographic position in the Solfatara area with respect to the surroundings (Bruno et al., 2007).
In the model, we further speculate that the acid sulfate alteration zone at the Campi Flegrei is actually evidence of a paleo-conduit. This is based on field observations showing that alteration deposits locally underlie the most recent eruptive units (e.g., Astroni) that are unaltered moving away from the acid sulfate zone. Therefore, the texture of the mineral assemblage, the enrichment in some metals and the litho-geochemical parameters are a relict of a “high-sulfidation system”. The evolutionary dynamics within the conduit and, in particular, the water overflows from the aquifers alternating with runoff processes explain the contradictory mineral environments with superimposed intermediate and advanced argillic alteration.
At present, a steam-heated (or low-sulfidation) environment (as derived by
most isotope data on alunites; see previous section) is developing in
relation to the presence of aquifers and their chemical compositions. This
is in agreement with previous studies (e.g., Aiuppa et al., 2006; Piochi et
al., 2015; Gresse et al., 2017). Following Hedenquist and Lowenstern (1994),
this is also in agreement with the shift in
Furthermore, the presence of
The acid sulfate alteration zone at Pisciarelli and Solfatara is located in the sector of the Campi Flegrei caldera that has been the most volcanically active area in the last 5 kyr. The alteration zone includes discrete sub-zones with very constant mineralogy, temperature and chemistry, considering the studied time interval. Outgassing dynamics, weather conditions and runoff are the most important factors affecting the generation of new mineral phases at the sub-millimeter to decimeter to meter scales.
The new minerals include alunite, alunogen, native sulfur, pyrite, kaolinite and subordinately mascagnite.
The limited areal extent of the alteration zone underlying the most recent
unaltered volcanic units, its mineralization texture and style, the
Based on presently available data, several key aspects await further investigations.
In particular, a detailed survey of the distribution of aquifers in the
subsurface will foster our understanding of caldera dynamics and contribute
to the debate existing between a “hydrothermal” (Moretti et al., 2017) vs.
a “magmatic” (Cardellini et al., 2017) unrest. Assessing the composition
and spatial extent of aquifers – also including the contribution from rainfall – is crucial in solving the non-magmatic role in processes at the
surface. Soluble acid components (
What causes the presence of
Finally, the Pisciarelli site appears suitable for studies related to biota
and the origin and evolution of life. Here, the water dominance, nitrogen
richness,
Our dataset is in Tables 2 and S1, S2 and S3 in the Supplement.
XRDP and DRIFT-FTIR patterns were acquired at the Osservatorio Vesuviano (Istituto Nazionale di Geofisica e Vulcanologia, Naples, Italy).
The XRDP instrument was a PANalytical X'Pert equipped with a high speed
PIXcel detector (Mormone et al., 2014). The configuration includes an
Ni filter, CuK
DRIFT was mounted on a Nicolet 670 NexusTM, both by ThermoFisher Scientific
S.p.a. (Società per Azioni). The FTIR comprises a heated ceramic (Globar) source, a 670 laser
unit, a KBr beam splitter and an MCT (mercury, cadmium, telluride) detector, constantly purged from a
high-pressure Nitrox dry air and
The appearance, morphology and chemical composition of minerals were determined on selected samples prepared as opaque mounts coated by cord and rod graphite, in JEOL and ZEISS electron microscope (EDS-BSEM) facilities. The JEOL-JSM 5310, equipped with a Link EDS and Inca 4.08 software (CISAG Laboratory University of Napoli Federico II), has operating conditions of 15 kV accelerating voltage, 50–100 mA filament current, variable spot size and 50 s net acquisition time. The ZEISS instrument is a SIGMA field emission scanning electron microscope (Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Naples, Italy), equipped with an XMAN micro-analysis system by Oxford, controlled by SMARTSEM and AZTEC softwares. Operating conditions for SIGMA were 15 kV accelerating voltage, 50–100 mA filament current, 5–10 nm spot size and variable acquisition time (several to tens of seconds). The ZEISS microscope allowed acquiring images in Fig. 3.
WRG was carried out at Bureau Laboratories Ltd.
(Vancouver, Canada). Major elements were analyzed by an inductively coupled
plasma emission spectrometer (ICP-ES) using
Sulfur and oxygen isotope measurements were performed directly on pure
mineral separates without and with further chemical preparation in the
stable isotope laboratory at the Institut für Geologie und
Paläontologie (University of Münster). Chemical preparation was
different depending on sample type: i.e., sulfates
The supplement related to this article is available online at:
MP and AM conducted sampling campaigns and prepared samples for analyses. GB participated in some of the sampling campaigns. AM conducted the XRPD analyses and interpreted the patterns. MP acquired, elaborated on and interpreted the DRIFT-FTIR spectra and, in collaboration with AM and GB, performed the EDS-BSEM investigations. HS determined the stable isotope values and contributed to data elaboration; MP did data representations and stable isotope data modeling. MP prepared the paper. All authors contributed to the final paper.
The authors declare that they have no conflict of interest.
Osservatorio Vesuviano (Istituto Nazionale di Geofisica e Vulcanologia) funded analyses of whole-rock geochemistry; we are therefore grateful to the directors, namely Giuseppe De Natale and Francesca Bianco. We are also grateful to colleagues at the Osservatorio Vesuviano: Rosario Avino is kindly thanked for BG/BN sample collections in 2018. Enrica Marotta and Pasquale Belviso provided the thermos-probe. Giorgio Angarano, the Tennis Hotel and Stufe di Nerone allowed the free access at the sampling sites. We appreciated comments and suggestions from Franco Pirajno and an anonymous reviewer that improved the data presentation and discussion. We would also like to thank the Editor Kei Ogata and editorial staff for managing this paper.
This research has been supported by the INGV (grant no. FIRS 08-6-5-056 to Monica Piochi).
This paper was edited by Kei Ogata and reviewed by Franco Pirajno and one anonymous referee.