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San Venanzo intrappenninic volcanic complex, Part I: textural constraints and preliminary definition of volatiles in kamafugitic magmas.

  • April 2013
  • Conference: Basalt 2013
Authors:
Silvia Campagnola at Università Degli Studi Roma Tre
Silvia Campagnola
Federico Lucci at Università degli Studi di Bari Aldo Moro
Federico Lucci
Claudia Romano at Università Degli Studi Roma Tre
Claudia Romano
John White at Eastern Kentucky University
John White
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Abstract

The San Venanzo volcanic complex is located 30 km NNE of the Vulsini Complex on the western edge of Tiber Valley Graben, where the tectonic settings is dominated by active extensional faults (NNW-SSE direction) and trastensional systems (with NE-SW main direction). This complex represents one of the most important and well preserved intrappenninic kamafugitic center and can be subdivided into three major vents: San Venanzo, Pian di Celle and Celli. Pian di Celle represents the major vent (< 2 Kmq) of the San Venanzo complex; it has been classified as an explosive-effusive center since it is possible to recognize a first phase with production of lapilli-ash tuff (surge like stratification), channelized unwelded pyroclastics with bread-crust bombs and abundance of sedimentary lithics (carbonates of Umbria-Marche Succession) and stratified tephra beds. The pyroclastic-ring is overlain, in continuity, by the final production of a kamafugitic/venanzitic (Lct+Ol+Mel+Kls+Phl+Cpx) lava flow.
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Content may be subject to copyright.
San Venanzo intrappenninic volcanic complex, Part I: textural
constraints and preliminary definition of volatiles in
kamafugitic magmas.
S. Campagnola1, F. Lucci1, C. Romano1 & J. White2
1 Geological Science Department, University of Roma Tre, Rome, Italy
2Department of Geography and Geology, Eastern Kentucky University, U.S.A
The San Venanzo volcanic complex is located 30 km NNE of the Vulsini Complex on the western edge of
Tiber Valley Graben, where the tectonic settings is dominated by active extensional faults (NNW-SSE
direction) and trastensional systems (with NE-SW main direction). This complex represents one of the most
important and well preserved intrappenninic kamafugitic center and can be subdivided into three major
vents: San Venanzo, Pian di Celle and Celli.
Pian di Celle represents the major vent (< 2 Kmq) of the San Venanzo complex; it has been classified as an
explosive-effusive center since it is possible to recognize a first phase with production of lapilli-ash tuff
(surge like stratification), channelized unwelded pyroclastics with bread-crust bombs and abundance of
sedimentary lithics (carbonates of Umbria-Marche Succession) and stratified tephra beds. The pyroclastic-
ring is overlain, in continuity, by the final production of a kamafugitic/venanzitic
(Lct+Ol+Mel+Kls+Phl+Cpx) lava flow.
The field analysis has highlighted three major features within this venanzitic lava-flow:
• One first episode, recognized only at the vent (SV3), characterized by a strongly vesiculated
scoriaceous lava-bed.
• A well developed macrocrystalline facies (PEG) with venanzitic mineral association enriched in
Ap+Bt+Kls+Cc, visible at the head of the venanzitic flow.
• Microamygdules (< 3 cm in diameter) with mineralogy similar to the macrocrystalline facies,
observed in the whole lava flow (SV4, SVX).
Given the widespread occurrence of volatiles in these eruptions (first phase characterized by explosive event,
second phase represented by emission of lavas enriched in vesicles and amygdules), we decided to
investigate in greater details volatile distribution and evolution during the eruptive phases via textural
analyses. For this purpose, we selected samples from different phases recognized in the lava flow: the
scoriaceous bed (SV3), the macrocrystalline facies (PEG), a microcrystalline lava with presence of
micrometric amygdules (SVX), and the last emitted portion of venanzite lava at the vent (SV4).
Textural analyses have been performed following the method proposed by Shea et al. (2010).
In order to fully investigate all vesicle sizes, three different magnifications were chosen (20x, 40x, 100x),
increasing statistically the number of images for every magnification step. Different percentages of each
phase (vesicles and amygdules-minerals) were calculated to obtain preliminary constraints on the
relationship between vesicle and crystal distribution.
Preliminary results indicate that:
• the total percentage of vesicles plus amygdules is constant throughout the flow, (16-19% with a
mean value of 17.2%);
• the highest vesicles contents (17%) are obtained for scoriaceous venanzitic bed (in which amygdules
are absents);
• the maximum value of amygdule-like assemblage (15.3%) is related to the macrocrystalline facies
(PEG);
• SVX and SV4 samples, present a vesicle content of approximately 4% and an average of 12% of
amygdules, corresponding to the microcrystalline sample and lava at vent that represents the last
erupted product.
The continuous evolution of the textural features investigated, the similar total percentage of vesicles plus
amygdules throughout the flow, and the absence of structural controls or correlation between fragile
elements and macrocrystalline facies (Lucci et al., 2011), suggest that the macrocrystalline facies cannot be
interpreted as a dykelet intrusion of lava (Stoppa et al., 1997). Alternatively, on the basis of our preliminary
reports on vesicle distribution in the lava body, and in agreement with the models by Melnik (2000), Spieler
et al. (2003) and Mueller et al. (2004), we suggest that the effusive phase of the San Venanzo
kamafugitic/venanzitic lava is originated from a continuous discharge of a fluid-enriched magma. The
activity initiated by the discharge of a bubbly flow in which volatiles were completely exsolved from the
magma, generating the scoriaceous lava present in proximal location. Following this initial event,
macrocrystalline (PEG) and microcrystalline (SVX and SV4) lavas, with the low vesicles percentage (4%),
were erupted, representing the magma, enriched in dissolved volatiles, approaching to the volatile-exsolution
level. Macrocrystalline facies, (PEG), could be interpreted as a macroamygdula itself and represents the first
emission of lava after the scoriaceous event.
Chemical and petrographical analyses of the macrocrystalline amygdules also support this interpretation and
are presented in a second related abstract (Lucci et al, 2013, this volume).
Textural analyses of the crystal distribution along the flow are underway to further investigate the dynamic
and evolution of San Venanzo activity.
Fig.1 caption:
Distribution of intrappenninic kamafugitic centers. Geological map of Pian di Celle ring (San Venanzo
Volcanic Complex) and schematic sketch of Venanzite lava flow (Modified from Lucci et al. 2011).
Fig.2 caption:
Table showing vesicles and amygdules content in venanzite selected samples: for every sample is presented
i) position (from the top to the bottom, in order of emission from the vent) along venanzitic flow (modified
from Lucci et al. 2011), ii) representative binarized image used in texture analysis (black as vesicles, dark
gray as amygdules), iii) schematic plot for cumulative vesicle and amygdule distributions in analyzed
samples.
References:
Lucci F., Cozzupoli D. & Brizzi S. (2011): The “Uncompahgritic Pegmatite” at San Venanzo volcano
(central Italy): an example of late crystallization controlled by fluids carried in the venanzite lava flow. –
Geoitalia 2011, Poster.
Melnik O. (2000): Dynamics of two-phase conduit flow of high viscosity gas-saturated magma: large
varations sustained explosive eruption intensity. – Bulletin of Volcanology, 62: 153-170.
Mueller S., Melnik O., Spieler O., Scheu B. & Dingwell D.B. (2004): Permeability and degassing of dome
lavas undergoing rapid decompression: An experimental determination. – Bulletin of Volcanology, 67(2):
526-538.
Shea T., Houghton B.F., Gurioli L., Cashman K.V., Hammer J.E. & Hobden B.J. (2010): Textural studies of
vesicles in volcanic rocks: An integrated methodology – Journal of Volcanology and Geothermal Research,
190(3-4): 271-289.
Spieler O., Dingwell D.B. & Aldibirov M. (2003): Magma fragmentation speed: An experimental
determination – Journal of Volcanology and Geothermal Research, 129:109-123.
Stoppa F., Sharygin V.V. & Cundari A. (1997): New minerals data from the kamafugite-carbonatite
association: the melilitolite from Pian di Celle, Italy – Mineralogy and petrology, 61: 27-45.
... More recently, Campagnola et al. (2013) suggested that the coarse grain facies should be interpreted as the crystallization product of a melt enriched in dissolved fluids/volatiles. Such fluids would have inhibited nucleation process and favoured the rate of crystal growth. ...
... Such fluids would have inhibited nucleation process and favoured the rate of crystal growth. Indeed, Campagnola et al. (2013) report a progressive, continue and rapid fabric transition from porphyritic microcrystalline to equigranular macrocrystalline structure. Also Lucci et al. (2011) did not recognize a clear structural controls or correlation between fragile elements and macrocrystalline facies, because the observed joints and fractures are invariably later than the coarse grained batch. ...
... Previous studies (Lucci et al., 2011;Campagnola et al., 2013) have recognised three major facies within Le Selvarelle lava flow: a scoriaceous lava, a melilitolite body in Podere Pantano locality, and a microcrystalline venanzite lava with micrometric amygdales. The identical mineral assemblage shown by the microcrystalline amygdales distributed along the lava flow and within the pegmatoid facies (melilitolites) has been interpreted by Campagnola et al. (2013) as the effect of volatile enrichment that inhibited nucleation, favouring the rate of crystal growth. ...
Strongly SiO2-undersaturated, CaO-rich kamafugitic Pleistocene magmatism in Central Italy (San Venanzo volcanic complex) and the role of shallow depth limestone assimilation
Article
Full-text available
NOTE: Up to October 2 2020 it will be possible to download freely the pdf of this article from this link: https://authors.elsevier.com/a/1bZhH2weQiqYi The Pleistocene (~590–265 ka) San Venanzo volcanic complex belongs to the IAP (Intra-Apennine Province) in central Italy, which comprises at least four small Pleistocene monogenetic volcanoes plus several unrooted pyroclastic deposits with peculiar mineralogical and whole-rock chemical compositions. San Venanzo products are strongly SiO2-undersaturated, CaO- and MgO-rich and show ultrapotassic serial character. The relatively common occurrence of calcite is interpreted in literature as primary mineral. The main rock facies at San Venanzo are calcite-rich scoria and lapilli tuffs, with minor massive lava flows, and a rare pegmatoid variant (melilitolitic pocket). All the San Venanzo rocks are feldspar-free with a typical paragenesis of forsteritic olivine, non-stoichiometric Ca-rich diopside, melilite, leucite, kalsilite, opaque minerals, nepheline, phlogopite, calcite, apatite, cuspidine, wollastonite, kirschsteinite-monticellite s.s. ± glass and other minor and very rare minerals typical of agpaitic melts. Based on petrographic analyses, the studied rocks can be classified as olivine melilitites, olivine leucite melilitites, venanzites (a local variant of kamafugites), calcite leucite melilitolites and Ca-rich olivine leucite melilitite tuffs. Mass balance calculations indicate a direct genetic link between the lava bodies and the pegmatoid melilitolitic pocket through a fractional crystallization process characterized by the removal of ~74% of a melilite-bearing uganditic cumulate made up of melilite, leucite, olivine, kalsilite and chromite. Primitive mantle-normalized patterns of the lavas and tuffs are rather spiked and share negative anomalies for Ba, Nb, Ta, P and Ti resembling typical magmas generated by supra-subduction mantle wedge. These compositions are very different from the only two other kamafugite localities outside Italy (Toro Ankole and Virunga in the East Africa Rift and Alto Paranaiba Igneous Province in SE Brazil). The melilitolite sample is more incompatible element-enriched than the other San Venanzo volcanic rocks, coherently with its evolved liquid composition proposed here. Major and trace element contents indicate a general depletion proportional to the amount of carbonate (exemplified by the CaO content). The negative trends in Harker-type diagrams with CaO as abscissa are compatible with a process of variable interaction between a silicate magma with sedimentary marly carbonates/limestones. The presence of Mg-rich (Fo97–92) and rim-ward CaO-enriched (up to 1.72 wt%) euhedral olivine, as well as the presence of thin kirschsteinite rim around olivine crystals agree with a process of crustal carbonate assimilation by an originally strongly SiO2-undersaturated silicate magma. On the other hand, the lack of feldspars even in the rocks with the highest SiO2, the high CaO content, and the extreme SiO2-undersaturation of San Venanzo products exclude their derivation from a simple peridotitic source. In order to generate these peculiar compositions, the presence of a SiO2-K2O-CaO-rich H2O-bearing component, identified in a carbonated phlogopite peridotite is required. The results of different isotopic systematic (Sr-Nd-Pb-He-Ne-Ar) presented here are compatible with a process of crustal contamination both at mantle source levels (to explain the general N-S isotopic trends recorded in Quaternary volcanic rocks of Italian peninsula and Sicily) and with interaction of ultrabasic melts with limestones at shallow crustal depths.
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... More recently, Campagnola et al. (2013) suggested that the coarse grain facies should be interpreted as the crystallization product of a melt enriched in dissolved fluids/volatiles. Such fluids would have inhibited nucleation process and favoured the rate of crystal growth. ...
... Such fluids would have inhibited nucleation process and favoured the rate of crystal growth. Indeed, Campagnola et al. (2013) report a progressive, continue and rapid fabric transition from porphyritic microcrystalline to equigranular macrocrystalline structure. Also Lucci et al., 2011 did not recognize a clear structural controls or correlation between fragile elements and macrocrystalline facies, because the observed joints and fractures are invariably later than the coarse grained batch. ...
... Previous studies (Lucci et al., 2011;Campagnola et al., 2013) have recognised three major facies within Le Selvarelle lava flow: a scoriaceous lava, a melilitolite body in Podere Pantano locality, and a microcrystalline venanzite lava with micrometric amygdales. The identical mineral assemblage shown by the microcrystalline amygdales distributed along the lava flow and within the pegmatoid facies (melilitolites) has been interpreted by Campagnola et al. (2013) as the effect of volatile enrichment that inhibited nucleation, favouring the rate of crystal growth. ...
Earth-Science Reviews Strongly SiO 2 -undersaturated, CaO-rich kamafugitic Pleistocene magmatism in central Italy (San Venanzo volcanic complex) and the role of shallow depth limestone assimilation
Article
  • Jun 2020
The Pleistocene (~590-265 ka) San Venanzo volcanic complex belongs to the IAP (Intra-Apennine Province) in central Italy, which comprises at least four small Pleistocene monogenetic volcanoes plus several unrooted pyroclastic deposits with peculiar mineralogical and whole-rock chemical compositions. San Venanzo products are strongly SiO2-undersaturated, CaO- and MgO-rich and show ultrapotassic serial character. The relatively common occurrence of calcite is interpreted in literature as primary mineral. The main rock facies at San Venanzo are calcite-rich scoria and lapilli tuffs, with minor massive lava flows, and a rare pegmatoid variant (melilitolitic pocket). All the San Venanzo rocks are feldspar-free with a typical paragenesis of forsteritic olivine, non-stoichiometric Ca-rich diopside, melilite, leucite, kalsilite, opaque minerals, nepheline, phlogopite, calcite, apatite, cuspidine, wollastonite, kirschsteinite-monticellite s.s. ± glass and other minor and very rare minerals typical of agpaitic melts. Based on petrographic analyses, the studied rocks can be classified as olivine melilitites, olivine leucite melilitites, venanzites (a local variant of kamafugites), calcite leucite melilitolites and Ca-rich olivine leucite melilitite tuffs. Mass balance calculations indicate a direct genetic link between the lava bodies and the pegmatoid melilitolitic pocket through a fractional crystallization process characterized by the removal of ~74% of a melilite-bearing uganditic cumulate made up of melilite, leucite, olivine, kalsilite and chromite. Primitive mantle-normalized patterns of the lavas and tuffs are rather spiked and share negative anomalies for Ba, Nb, Ta, P and Ti resembling typical magmas generated by supra-subduction mantle wedge. These compositions are very different from the only two other kamafugite localities outside Italy (Toro Ankole and Virunga in the East Africa Rift and Alto Paranaiba Igneous Province in SE Brazil). The melilitolite sample is more incompatible element-enriched than the other San Venanzo volcanic rocks, coherently with its evolved liquid composition proposed here. Major and trace element contents indicate a general depletion proportional to the amount of carbonate (exemplified by the CaO content). The negative trends in Harker-type diagrams with CaO as abscissa are compatible with a process of variable interaction between a silicate magma with sedimentary marly carbonates/limestones. The presence of Mg-rich (Fo 97-92) and rim-ward CaO-enriched (up to 1.72 wt%) euhedral olivine, as well as the presence of thin kirschsteinite rim around olivine crystals agree with a process of crustal carbonate assimilation by an originally strongly SiO2-undersaturated silicate magma. On the other hand, the lack of feldspars even in the rocks with the highest SiO2 , the high CaO content, and the extreme SiO2-undersaturation of San Venanzo products exclude their derivation from a simple peridotitic source. In order to generate these peculiar compositions, the presence of a SiO2-K2O-CaO-rich H2O-bearing component, identified in a carbonated phlogopite peridotite is required. The results of different isotopic systematic (Sr-Nd-Pb-He-NeAr) presented here are compatible with a process of crustal contamination both at mantle source levels (to explain the general N-S isotopic trends recorded in Quaternary volcanic rocks of Italian peninsula and Sicily) and with interaction of ultrabasic melts with limestones at shallow crustal depths.
View
New mineral data from the kamafugite-carbonatite association: The melilitolite from Pian di Celle, Italy
Article
Full-text available
  • Mar 1997
A detailed mineralogical investigation of a Plan di Celle sill rock (San Venanzo, Italy), classified as melilitolite and associated with venanzite and carbonatitic pyroclasts, revealed new and rare mineral parageneses, considered as characteristic of the kamafugite-carbonatite association. These are formed by several accessory minerals, including minerals of the cuspidine family, gotzenite, khibinskite, minerals of the rhodesite- delhayelite- macdonaldite family, pyrrhotite, bartonite and (Fe, Ni, Co) monoarsenide, mostly optically and chemically identified also in fluid inclusions. The chemical composition of these minerals and their probable crystallisation succession, deduced from textural relationships, demonstrates extensive atomic substitutions, notably for Ca, Ti, Mg and alkali, essentially reflecting high concentrations of REE, Sr, Ba, Nb and Zr, which significantly varied during crystallisation. Molecular alkali excess over Al and high Ca content in (H2O, F CO2)-rich, Si-undersaturated liquid(s) are considered the dominant factors in controlling the stability of disilicate-type minerals. Separation of the carbonatite liquid from the silicate magma, constrained by textural and fluid inclusion data, was fundamental in moving the residuum onto a strongly peralkaline trend which stabilised the sulphides under changed redox conditions.
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Dynamics of two-phase conduit flow of high-viscosity gas-saturated magma: Large variations of sustained explosive eruption intensity
Article
Full-text available
  • Aug 2000
 The ascent of gas-saturated magma in a conduit can lead to the transition from the laminar flow of a bubble-rich melt to the turbulent flow of particle-rich hot gas in upper part of the conduit. This process is investigated with a help of a disequilibrium two-phase flow model for steady and unsteady conditions. For the description of different gas-magma flow regimes in the conduit separated sets of equations are used. The main difference from previous conduit flow models is the consideration of the pressure difference between a growing bubble and the surrounding melt. For a bubbly liquid to evolve into a gas-particle flow a critical overpressure value must be exceeded. This transition region is simulated by means of a discontinuity, namely by a fragmentation wave. The magma flow calculations are carried out for a given conduit length and overpressure between the magma chamber and the atmosphere. The steady solution of the boundary problem is not unique and there are up to five distinct steady regimes corresponding to fixed eruption conditions. Within the framework of the quasi-steady approach (governing parameters being varied monotonically) the transition from one regime to another can take place suddenly and is accompanied by the fundamental restructuring of the conduit flow, resulting in rapid or abrupt changes in the intensity of explosive eruptions. Abrupt intensification of an explosive eruption occurs when the chamber pressure becomes sufficiently less than saturation pressure, and therefore corresponds to the case of shallow-depth magma chamber and high initial water content. A regime with minimum flow rate may relate to the growth of a lava dome following the explosive phase of eruption. These models can explain geological observations that imply large and sudden changes of discharge rate in large-magnitude explosive eruptions, particularly at the transition between plinian phase and ignimbrite formation.
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Permeability and degassing of dome lavas undergoing rapid decompression: An experimental determination
Article
Full-text available
  • Jul 2005
The gas permeability of volcanic rocks may influence various eruptive processes. The transition from a quiescent degassing dome to rock failure (fragmentation) may, for example, be controlled by the rocks permeability, in as much as it affects the speed by which a gas overpressure in vesicles is reduced in response to decompression. Using a modified shock-tube-based fragmentation bomb (Alidibirov and Dingwell 1996a,b; Spieler et al. 2003a), we have measured unsteady-state permeability at a high initial pressure differential. Following sudden decompression above the rock cylinder, pressurized gas flows through the sample. Two pressure transducers record the pressure signals above and below the sample. A transient 1D filtration code has been developed to calculate permeability using the experimental decay curve of the lower pressure transducer. Additionally an analytical steady-state method to achieve permeability is presented as an alternative to swiftly predict the sample permeability in a sufficiently precise manner. Over 100 permeability measurements have been performed on samples covering a wide range of porosity. The results show a general positive relationship between porosity and permeability with a high data scatter. Our preferred interpretation of the results is a combination of two different, but overlapping effects. We propose that at low porosities, gas escape occurs predominantly through microcracks or elongated micropores and therefore could be described by simplified forms of Kozeny–Carman relations (Carman 1956) and fracture flow models. At higher porosities, the influence of vesicles becomes progressively stronger as they form an increasingly connected network. Therefore, a model based on the percolation theory of fully penetrable spheres is used, as a first approximation, to describe the permeability-porosity trend. In the data acquired to date it is evident, that in addition to the porosity control, the samples bubble size, shape and distribution strongly influence the permeability. This leads to a range of permeability values up to 2.5 orders of magnitude at a given porosity.
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Magma Fragmentation Speed: an experimental determination
Article
The propagation speed of a fragmentation front, combined with the ascent velocity of magma is, in all likelihood, a controlling factor in the dynamics of explosive volcanic eruptions. Direct measurement of the ‘fragmentation speed’ in natural systems appears to be impossible at present. Fortunately, laboratory experiments can provide information on the propagation speed of the fragmentation front. Here we present the results of fragmentation speed determinations using a so-called ‘fragmentation bomb’. These are, to the best of our knowledge, the first in situ fragmentation speed determinations performed on magma. Natural magma samples (Merapi basaltic andesite, Mount St. Helens dacite and Unzen dacite) have been investigated in the temperature range of 20–950°C and at pressures up to 25 MPa. Two techniques have been employed. Firstly, in experiments at 20°C, dynamic pressure transducers were placed above and below the magma samples and the fragmentation speed of the magma sample was derived from an analysis of the decompression curves. Secondly, at elevated temperatures, an alternative technique was introduced and successfully employed. This involved the severing via fragmentation of conducting wires placed within the samples at various heights. Fragmentation speeds are very low, falling in the range of 2–70 m/s and increasing with an increase in the magnitude of the decompression step responsible for the fragmentation. The first high-temperature determination seems consistent with low-temperature results. Implications for explosive volcanism are discussed briefly.
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Textural studies of vesicles in volcanic rocks: An integrated methodology
Article
Vesicles in volcanic rocks are frozen records of degassing processes in magmas. For this reason, their sizes, spatial arrangements, numbers and shapes can be linked to physical processes that drive magma ascent and eruption. Although numerous techniques have been derived to describe vesicle textures, there is no standard approach for collecting, analyzing, and interpreting vesicular samples. Here we describe a methodology for techniques that encompass the entire data acquisition process, from sample collection to quantitative analysis of vesicle size and number. Carefully chosen samples from the lower, mean and higher density/vesicularity endmembers are characterized using image nesting strategies. We show that the texture of even microvesicular samples can be fully described using less than 20 images acquired at several magnifications to cover efficiently the range of existing vesicle sizes. A new program (FOAMS) was designed to perform the quantification stage, from vesicle measurement to distribution plots. Altogether, this approach allows substantial reduction of image acquisition and processing time, while preserving enough user control to ensure the validity of obtained results. We present three cameo investigations — on basaltic lava flows, scoria deposits and pumice layers — to show that this methodology can be used to quantify a wide range of vesicle textures, which preserve information on a wide range of eruptive conditions.
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The "Uncompahgritic Pegmatite" at San Venanzo volcano (central Italy): an example of late crystallization controlled by fluids carried in the venanzite lava flow
  • Jan 2011
  • F Lucci
  • D Cozzupoli
  • S Brizzi
Lucci F., Cozzupoli D. & Brizzi S. (2011): The "Uncompahgritic Pegmatite" at San Venanzo volcano (central Italy): an example of late crystallization controlled by fluids carried in the venanzite lava flow.Geoitalia 2011, Poster.