the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 May 2024
Revised conceptual model of the Solfatara magmatic-hydrothermal system (Campi Flegrei, Italy), time changes during the last 40 years, and prediction of future scenarios
Abstract. We revised the conceptual model of the Solfatara magmatic-hydrothermal system based on the results of new gas-geoindicators (Marini et al., 2022) and the available geological, volcanological, and geophysical information from surface surveys and deep geothermal wells. Using the new gas-geoindicators, we monitored the temperature and total fluid pressure over a time interval of ~40 years: (i) in the shallow reservoir (0.25–0.45 km depth), where CO equilibrates; (ii) in the intermediate reservoir (2.7–4.0 km depth), where CH4 attains equilibrium; (iii) in the deep reservoir (6.5–7.5 km depth), where H2S achieves equilibrium. From 1983 to 2022, the temperature and total fluid pressure of the shallow reservoir did not depart significantly from ~220 °C and ~25 bar, whereas remarkable, progressive increments in temperature and total fluid pressure occurred in the intermediate and deep reservoirs, with peak values of 590–620 °C and 1200–1400 bar in the intermediate reservoir and 1010–1040 °C and 3000–3200 bar in the deep reservoir, in 2020. The revised conceptual model allowed us to explain the evolution of: (a) pressurization-depressurization in the intermediate reservoir, acting as the “engine” of bradyseism, (b) time changes of total fluid pressure in the deep reservoir, working as the “on-off switch” of magmatic degassing. We also used the revised conceptual model to predict possible future scenarios in the lack of external factors.
- Preprint
(2907 KB) - Metadata XML
-
Supplement
(246 KB) - BibTeX
- EndNote
Status: open (extended)
-
RC1: 'Comment on egusphere-2024-1306', Anonymous Referee #1, 12 Aug 2024
reply
General comments
Marini et al. attempt to integrate new gas geochemical data and existing information into a revised “conceptual model” of the Solfatara magmatic-hydrothermal system at Campi Flegrei. I found the manuscript very difficult to review as a lot of important information, namely regarding the methods, the underlying assumptions, uncertainty assessment, known vs. inferred geology, as well as the exact lines of reasoning from data to interpretation are missing. This renders the work in its current form essentially irreproducible and very difficult to review because essentially almost every paragraph can be challenged. Even some (central) elements of the discussion seem to contradict each other; e.g., the concept of separate “reservoirs” as locations of equilibration of different gas geothermometers is obsolete with the assumption of flow through a vertically connected structure. Also, regarding the core chemistry, the authors (apparently coming from a volcanology background) seem to rely on shallow fumarole gas equilibria and ignore stablished knowledge from the magmatic-hydrothermal systems community for the deeper and higher pressured parts of the system (e.g., H2S generation through the well-known SO2 disproportionation reaction). I think that for these reasons the manuscript should not be published in its current from and requires a complete revise beyond “major revisions”.
In the following I will highlight some key problem areas (and for each one only some facets and ignore many questionable details) of the manuscript but will refrain from a detailed list of all issues.
Specific comments 1: methods and assumptions behind them
The authors rely on three gas equilibria to obtain “distinct equilibration temperatures and related total fluid pressures”. They do not elaborate how these were computed and not even how such a “equilibration temperature” is to be interpreted (e.g.: is it the lowest temperature at which the reaction went to equilibrium along the fluid’s upflow path and which was then preserved due to kinetic limitations at lower temperature?), and how such equilibrium could be demonstrated at all.
It remains completely opaque how the computations were done. I would argue that at least some of the stated computations are currently not meaningfully possible for the lack of adequate thermodynamic data and models, e.g. for a complex gas phase in equilibrium with a 21 or 33.5 wt% aqueous NaCl brine – for the conditions stated there exists neither an adequate solubility model for the gases in such a brine nor are even remotely accurate activity or fugacity coefficients available. How then could temperatures and pressures be derived? I tried to make my way through the main author’s 2022 book (375 pages, the main findings of which the present manuscript is supposed to summarize and explain) but I could not identify a coherent summary of the computations and the underlying assumptions with reasonable effort. This manuscript should deliver a crystal-clear and reproducible description and openly lay out and discuss the major assumptions to become a publishable contribution.
There exists a relatively elaborate assessment of sulfur chemical behavior in the deep vs. shallow parts of magmatic-hydrothermal systems that was established some decades ago in the economic geology community (just to name the works by Einaudi and Hedenquist as an example) including the importance of “rock-buffered” vs. “direct degassing” paths. The very relevant insights presented there go largely ignored by the present authors. Namely, the important SO2 disproportionation reaction to generate H2S (and there are variations of the theme depending on temperature, rock, etc.) is ignored although it is considered most important by many researchers in the field. The reactions (3) and (4) involving calcite also seem odd; most people working on the deeper parts of magmatic-hydrothermal systems would see these reactions going to the left (albeit possibly rather involving SO2 and using plagioclase rather than calcite) to sequester sulfur rather than creating H2S, see, e.g., Henley at al., JVGR 2022. I wonder what “H2S temperatures” would come out if this was properly taken into account of if “H2S temperatures” would have any meaning then.
Specific comments 2: the core of the conceptual model
When looking closer one has a hard time to understand reasoning behind the “conceptual model”. On the one hand, three “reservoirs” at different depth, separated by aquicludes, are postulated; on the other hand – to make the fluids migrate from a magma at >8km depth to the surface – connected flow through a vertically extensive “structure” needs to be invoked.
Now, first and foremost, I had a hard time understanding why the different gas equilibration temperatures should be specifically connected to the individual “reservoirs” and how the latter were inferred in the first place. Is there some circular reasoning? Or does actual, drilled geology with porosity and permeability measurements come into play? There is some mentioning of wells etc. but the exact reasoning remained blurry to me, at best. So, one cannot even tell if the depth of “reservoirs” and “aquicludes” is well-constrained or just a guess.
Then, if there is a vertically connecting structure – what is the meaning of the temperatures then? “Equilibration” will happen (and be “frozen in” if my above speculation about the meaning of “equilibration temperature” is correct) somewhere along the flow path and why should that be connected to the depth of any of the “reservoirs” then? Wouldn’t it make more sense that the increase in apparent temperatures and pressures (if correct) reflects rather a change in the hydraulic regime (or the degassing rate) such that the chemical signal of deeper fluids gets better preserved rather than a specific “reservoir” getting hotter and stronger pressurized (see, e.g., the overpressure waves in a magmatic-hydrothermal systems self-developing in the simulations of Weis et al., 2012; later suggested also by Lupi et al. for Campi Flegrei). What would speak for and against such different possibilities, why aren’t they considered and also tested against the data?
In the whole conceptual model discussion, speculative ideas (such as Fournier’s “self-sealing” quartz layer for which later studies found little evidence), inferred vs. drilled geology, geochemical data with different possible interpretations etc. are just mingled without testing for plausibility etc. Personally, I think that this is quite far away from best practices; the different ideas should be formulated as hypotheses and then tested to the degree possible.
Specific comments 3: drawing straight lines without reasoning
The temperature and pressure profiles in Figs. 5 and 6 hinge on the – untested – assumption that the gas equilibration temperatures are representative for three “reservoirs” at different depth. In Fig. 5, temperature in each reservoir is taken to be constant. Unless I missed something important no reason is given why this should be the case. Rather, it is taken as granted (out of nothing) and then it is postulated that convection inside the reservoir homogenizes the temperature. Between the reservoirs – again: unless I overlooked something important – straight lines are drawn without reasoning and then it is stated that the “the heat transfer appears to be controlled by conduction”. I think this is quite poor scientific practice to just draw a straight line in the absence of data and then to assume it is correct and make such a conclusion.
For fluid pressure (Fig. 6) the “reservoirs” are also drawn to have constant pressure even if more than a km high. This is obviously unphysical as there would have to be a hydrostatic pressure gradient (not necessarily linear as density of the fluid may vary with depth).
Some other specific comments
I only list a few, which I think are important:
- 1: I think that a Google Earth snapshot is a no-go and contains much less info than, e.g., a simplified line art map showing the main geologic structures and the main geographic locations. Add a square that show the location of Fig 1b. In 1b show a scale rather than an unexplained coordinate system.
- Avoid self-celebrating statements in the introduction.
- “standard deviation” in line 98 includes what? Just the effect of variable gas analysis? What’s the uncertainty of the thermodynamic analysis to obtain T and P? Possibly much larger?
- I can’t follow the reasoning in 4.3; if you assume that (1) and (2) work well separately, than (5) should work as well as it is simply a linear combination of the two, right?
Citation: https://doi.org/10.5194/egusphere-2024-1306-RC1 -
AC1: 'Reply on RC1', Matteo Lelli, 25 Aug 2024
reply
Dear Editor,
please see the attached file as a reply to the RC1. The revised manuscript is ready to be submitted, please advise us how to proceed.
Best Regards,
Matteo Lelli
-
RC2: 'Reply on AC1', Anonymous Referee #1, 06 Sep 2024
reply
Please find my reply as attached pdf
-
RC2: 'Reply on AC1', Anonymous Referee #1, 06 Sep 2024
reply
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 396 | 127 | 24 | 547 | 20 | 14 | 17 |
- HTML: 396
- PDF: 127
- XML: 24
- Total: 547
- Supplement: 20
- BibTeX: 14
- EndNote: 17
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1