**7. Discussion**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

Platy juvenile clasts related to penetration of magma into the host sediment are commonly extensively altered into laumontite and iron oxides (**Figure 11E**). The same assemblage typically replaces irregularly shaped juvenile clasts in peperites

*(A) juvenile clasts (white) altered to laumontite (Lmt), iron oxides (o) and chlorite (Chl). The composition of the mingling siliciclastic sediment (S) remained largely unaltered; (B) a juvenile clast altered to laumontite (Lmt), epidote (Ep) and actinolite (Ac); (C) a clast of fine-grained pyroclastic deposit incorporated in a lava flow has been altered to albite (Ab), quartz (Qtz), chlorite (Chl) and Fe-oxides (o). Some clasts underwent magma assimilation (ma); (D) the mingling fine-grained pyroclastic deposit (S) formed a large oval inclusion in a juvenile clast that has been partially altered into laumontite (Lmt). Smaller inclusions are altered to pumpellyite (Pmp) and quartz, some small spherical inclusions are unaltered (o) and the other replaced by chlorite (Chl). juvenile clasts (jc); (E) an inclusion of fine-grained pyroclastic deposit (s) in a juvenile clast altered to prehnite (Prh) and laumontite (Lmt); (F) inclusions of fine-grained pyroclastic deposit in a juvenile fragment (brownish) altered to pumpellyite (Pmp), epidote (Ep), quartz (Qtz) and albite (Ab). Volcanic* 

The alteration of peperites related to coarse-grained host sediment is characterised by chlorite, interlayered chlorite-smectite with more than 90% of chlorite layers, quartz, and sometimes incipient epidote. Laumontite and other zeolites, and

formed during the emplacement of the Krmarica Sill (**Figure 11F**).

*glass in the juvenile clast is locally replaced by chlorite (Chl).*

pumpellyite, prehnite and epidote are uncommon (**Figure 12A**-**D**).

**316**

**Figure 13.**

The formation of alteration minerals in volcanic-hydrothermal systems is a complex process affected by temperature, pressure, composition of reacting fluids, porosity, permeability and initial composition of the host-rock, duration of hydrothermal activity and superimposed thermal (or hydrothermal) regimes [55, 56]. Nevertheless, there is a general relationship between temperature and the formation of alteration minerals, and some mineral assemblages can be used to interpret temperatures within a geothermal system (**Figure 14**). For heulandite and stilbite, laumontite, yugawaralite, pumpellyite and actinolite widely accepted temperature stability ranges are 100-120°C, 120-220°C, 172-234°C, 200-310°C, 220-310°C and 280-460°C [52, 55–65]. Incipient, fine-grained and poorly crystallised epidote has been encountered in hydrothermal systems of the Philippines [53, 55] and the Nisyros Island [63], respectively, in the temperature range of 180-220°C although epidote generally forms at temperatures higher than 240°C [64, 65]. The temperature stability range of 245-265°C has been reported for mixed-layer R0 and R1 chlorite-smectite from Nesjavellir geothermal field, Iceland [66].

### **Figure 14.**

*Authigenic calcium-aluminosilicate minerals that commonly serve as geothermometers in volcanichydrothermal systems, their chemical formulae and temperature stability ranges, compiled from [52, 55–65].*

The alteration of peperites from the Smrekovec Volcanic Complex indicates close relationship to the rock composition and texture, and therefore, the style of peperite formation. Porosity and permeability of peperites is, in general, lower than that of the host sediment. The types involving fine-grained host sediments have low permeability and low porosity, and in the stratovolcano-hosted hydrothermal system with convective-advective flow regime (**Figure 3**) they must have functioned as aquicludes unable to drain effectively the advective flow of hydrothermal fluids that largely controlled hydrothermal alteration of volcanic deposits [37]. In such hydrothermal system, peperites involving coarse-grained host sediments should be more extensively altered and contained authigenic minerals with higher temperature stability ranges than the types involving fine-grained host-sediments, but that is not the case. On contrary, in this type of peperites significant authigenic calcium-aluminosilicate minerals are often lacking and the most common alteration mineral is chlorite or interlayered chlorite-smectite.

In dispersed blocky peperite, matrix composed of siliciclastic host sediment is usually unaltered. The alteration of juvenile clasts often indicates only the reactions of devitrification of volcanic glass, and only juvenile clasts with perlitic cracks can be altered to laumontite and Fe-oxides (**Figure 9A**). The activity of hot fluids originating from heated pore waters can be assumed, but chemical gradients favourable for the formation of laumontite were attained only inside the juvenile clasts with perlitic cracks. Perlitic cracks apparently served as conduits for hot fluids that leached volcanic glass during the flow, and in this manner underwent the changes in chemical composition (e.g. [67]) that finally resulted in crystallisation of laumontite. Far more extensive alteration of blocky peperites involving fine-grained pyroclastic host sediment supports the forementioned explanation. The interaction of heated pore fluids and highly reactive host sediment apparently controlled geochemical evolution of so-formed hydrothermal solutions and the related extensive alteration of all constituents of peperite. Laumontite or mineral assemblages of laumontite, albite, quartz, pumpellyite, incipient epidote and chlorite, or laumontite, prehnite, quartz, chlorite and incipient epidote, or laumontite, analcime and interlayered chlorite-smectite indicate that temperature gradients were prerequisite but not sufficient for alteration to occur and that the main controlling factor were geochemical gradients.

Microglobular peperite has been interpreted as a frozen example of a fuelcoolant interaction (FCI) between magma and fluidised host sediment [5], and the

**319**

*Submarine Stratovolcano Peperite Syn-Formational Alteration - A Case Study of the Oligocene…*

temperatures of alteration reactions in direct contact with magma must have been higher than in blocky peperite. Microglobules of fine-grained pyroclastic sediment underwent alteration, and typically, smaller microglobules are commonly completely altered to authigenic mineral assemblages with higher temperature stability ranges (e.g. pumpellyite, epidote) than the larger ones that remained incompletely altered or altered to authigenic minerals with lower temperature stability ranges (e.g. laumontite) (**Figure 13D**). The relationship indicates that high-temperature

Despite of the complexity of alteration of volcanic deposits in lithofacies associations of the sections Krnes and Smrekovec G34 the assemblages of authigenic minerals in peperites are different than those in the adjacent underlying and overlying autoclastic, pyroclastic or resedimented volcaniclastic deposits irrespectively of their texture and grain-size (**Figures 4** and **5**). Particularly outstanding is the occurrence of pumpellyite, actinolite and epidote. In lavas pumpellyite and actinolite very rarely occur as the replacement of volcanic glass and other primary constituents although the temperatures in cooling lavas could be favourable for

If compared to the section Krnes, the alteration of volcanic deposits in the section Smrekovec G34 has been far more complex owing to the emplacement of the Kramarica Sill. Progressive reactions related to an elevated temperature regime are characterised by the occurrence of prehnite and indicate, at least in the lower half of the section, that the temperatures could have reached over 300°C. Yet, even in such elevated temperature regime the exclusive occurrence of pumpellyite and particularly actinolite in peperites indicates that higher temperatures and specific geochemical conditions related to the formation of peperites must have controlled the crystallisation of pumpellyite and actinolite. The occurrence of the same assemblage in peperites in the section Krnes is particularly important. Here, hydrothermal activity and temperature regime were mainly related to a deep igneous source and the associated convective-advective flow of hydrothermal fluids and elevated geothermal gradients in the area of stratovolcano, and they were not affected, at least significantly, by the emplacement and cooling of the Kramarica Sill. Maximum temperatures have been determined by the temperature stability of laumontite, namely, 234°C which is insufficient for the crystallisation of pumpellyite or actinolite. The alteration of juvenile clasts to laumontite in peperites indicates the presence of hydrothermal conditions although they cannot be conclusively ascribed to specific hydrothermal conditions and geochemical gradients related to

The alteration of peperites can be regarded as syn-formational hydrothermal, although it is local, specific and ephemeral lasting until thermal gradients persisted. Authigenic mineral assemblages developed in peperites from the Smrekovec Volcanic Complex are rare on a worldwide scale and have not been identified in such context yet. Geochemical evolution of heated pore fluids circulating in the vicinity of the source of heat controlled the formation of authigenic mineral assemblages and the presence of unstable, reactive volcanic material was crucial for

Peperites are commonly developed in submarine environments with contemporaneous volcanic activity and sedimentation. And although the occurrence and complex processes of formation have been studied and explained in many modern and ancient geological settings worldwide [4, 10–14], and particularly in the

*DOI: http://dx.doi.org/10.5772/intechopen.95480*

their formation.

the formation of peperites.

their formation and diversity.

**8. Conclusion**

conditions could not persist for a long period of time.

### *Submarine Stratovolcano Peperite Syn-Formational Alteration - A Case Study of the Oligocene… DOI: http://dx.doi.org/10.5772/intechopen.95480*

temperatures of alteration reactions in direct contact with magma must have been higher than in blocky peperite. Microglobules of fine-grained pyroclastic sediment underwent alteration, and typically, smaller microglobules are commonly completely altered to authigenic mineral assemblages with higher temperature stability ranges (e.g. pumpellyite, epidote) than the larger ones that remained incompletely altered or altered to authigenic minerals with lower temperature stability ranges (e.g. laumontite) (**Figure 13D**). The relationship indicates that high-temperature conditions could not persist for a long period of time.

Despite of the complexity of alteration of volcanic deposits in lithofacies associations of the sections Krnes and Smrekovec G34 the assemblages of authigenic minerals in peperites are different than those in the adjacent underlying and overlying autoclastic, pyroclastic or resedimented volcaniclastic deposits irrespectively of their texture and grain-size (**Figures 4** and **5**). Particularly outstanding is the occurrence of pumpellyite, actinolite and epidote. In lavas pumpellyite and actinolite very rarely occur as the replacement of volcanic glass and other primary constituents although the temperatures in cooling lavas could be favourable for their formation.

If compared to the section Krnes, the alteration of volcanic deposits in the section Smrekovec G34 has been far more complex owing to the emplacement of the Kramarica Sill. Progressive reactions related to an elevated temperature regime are characterised by the occurrence of prehnite and indicate, at least in the lower half of the section, that the temperatures could have reached over 300°C. Yet, even in such elevated temperature regime the exclusive occurrence of pumpellyite and particularly actinolite in peperites indicates that higher temperatures and specific geochemical conditions related to the formation of peperites must have controlled the crystallisation of pumpellyite and actinolite. The occurrence of the same assemblage in peperites in the section Krnes is particularly important. Here, hydrothermal activity and temperature regime were mainly related to a deep igneous source and the associated convective-advective flow of hydrothermal fluids and elevated geothermal gradients in the area of stratovolcano, and they were not affected, at least significantly, by the emplacement and cooling of the Kramarica Sill. Maximum temperatures have been determined by the temperature stability of laumontite, namely, 234°C which is insufficient for the crystallisation of pumpellyite or actinolite. The alteration of juvenile clasts to laumontite in peperites indicates the presence of hydrothermal conditions although they cannot be conclusively ascribed to specific hydrothermal conditions and geochemical gradients related to the formation of peperites.

The alteration of peperites can be regarded as syn-formational hydrothermal, although it is local, specific and ephemeral lasting until thermal gradients persisted. Authigenic mineral assemblages developed in peperites from the Smrekovec Volcanic Complex are rare on a worldwide scale and have not been identified in such context yet. Geochemical evolution of heated pore fluids circulating in the vicinity of the source of heat controlled the formation of authigenic mineral assemblages and the presence of unstable, reactive volcanic material was crucial for their formation and diversity.
