Gareth Fabbro | Nanyang Technological University (original) (raw)

Papers by Gareth Fabbro

Research paper thumbnail of Evolution of the crustal magma plumbing system during the build-up to the 22-ka caldera-forming eruption of Santorini (Greece)

Bulletin of Volcanology, Dec 2013

The formation of shallow, caldera-sized reservoirs of crystal-poor silicic magma requires the gen... more The formation of shallow, caldera-sized reservoirs of crystal-poor silicic magma requires the generation of large volumes of silicic melt, followed by the segregation of that melt and its accumulation in the upper crust. The 21.8 ± 0.4-ka Cape Riva eruption of Santorini discharged >10 km3 of crystal-poor dacitic magma, along with <<1 km3 of hybrid andesite, and collapsed a pre-existing lava shield. We have carried out a field, petrological, chemical, and high-resolution 40Ar/39Ar chronological study of a sequence of lavas discharged prior to the Cape Riva eruption to constrain the crustal residence time of the Cape Riva magma reservoir. The lavas were erupted between 39 and 25 ka, forming a ∼2-km3 complex of dacitic flows, coulées and domes up to 200 m thick (Therasia dome complex). The Therasia dacites show little chemical variation with time, suggesting derivation from one or more thermally buffered reservoirs. Minor pyroclastic layers occur intercalated within the lava succession, particularly near the top. A prominent pumice fall deposit correlates with the 26-ka Y-4 ash layer found in deep-sea sediments SE of Santorini. One of the last Therasia lavas to be discharged was a hybrid andesite formed by the mixing of dacite and basalt. The Cape Riva eruption occurred no more than 2,800 ± 1,400 years after the final Therasia activity. The Cape Riva dacite is similar in major element composition to the Therasia dacites, but is poorer in K and most incompatible trace elements (e.g. Rb, Zr and LREE). The same chemical differences are observed between the Cape Riva and Therasia hybrid andesites, and between the calculated basaltic mixing end-members of each series. The Therasia and Cape Riva dacites are distinct silicic magma batches and are not related by shallow processes of crystal fractionation or assimilation. The Therasia lavas were therefore not simply precursory leaks from the growing Cape Riva magma reservoir. The change 21.8 ky ago from a magma series richer in incompatible elements to one poorer in those elements is one step in the well documented decrease with time of incompatibles in Santorini magmas over the last 530 ky. The two dacitic magma batches are interpreted to have been emplaced sequentially into the upper crust beneath the summit of the volcano, the first (Therasia) then being partially, or wholly, flushed out by the arrival of the second (Cape Riva). This constrains the upper-crustal residence time of the Cape Riva reservoir to less than 2,800 ± 1,400 years, and the associated time-averaged magma accumulation rate to >0.004 km3 year-1. Rapid ascent and accumulation of the Cape Riva dacite may have been caused by an increased flux of mantle-derived basalt into the crust, explaining the occurrence of hybrid andesites (formed by the mixing of olivine basalt and dacite in approximately equal proportions) in the Cape Riva and late Therasia products. Pressurisation of the upper crustal plumbing system by sustained, high-flux injection of dacite and basalt may have triggered the transition from prolonged, largely effusive activity to explosive eruption and caldera collapse.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Melting, Differentiation and Degassing at the Pantelleria Volcano, Italy

Journal of …, Jan 1, 2012

We present the results of the first systematic study of melt compositions at Pantelleria, based o... more We present the results of the first systematic study of melt compositions at Pantelleria, based on both melt inclusions and matrix glasses in pantellerites from 10 eruptions during the last eruptive cycle (<45 kyr). We present major and trace element compositions, as well as data on the volatiles sulphur (S), fluorine (F), chlorine (Cl), water (H2O), carbon dioxide (CO2) and lithium (Li) Rare earth element (REE) compositions were inverted using the program INVMEL to establish the melt fraction vs depth relationship in the Pantellerian mantle source region. Inversion indicates that melts are generated by ∼1·7% melting of a light rare earth element (LREE)-enriched mantle source. The source lies principally within the spinel–garnet transition zone, which, on the basis of trace element ratios, shows some affinity to the source of North African magmatism. Major and trace element data indicate a gap in melt compositions at intermediate compositions, consistent with previously published whole-rock data. This gap rules out the possibility of explaining chemical variability in the Pantelleria lavas merely by changes in the crystal content of the magmas. Principal component analysis of major element glass compositions shows that the liquid line of descent for mafic melt compositions is controlled by clinopyroxene, plagioclase, magnetite and olivine crystallization. Alkali feldspar, clinopyroxene, ilmenite and olivine or aenigmatite crystallization controls the liquid line of descent for the silicic melt compositions, with aenigmatite broadly replacing olivine in the most evolved magmas. Trace element modelling indicates that 96% fractional crystallization is required to generate pantellerites from alkali basalts at Pantelleria (through trachytes, generated after 76% fractional crystallization). We have measured pantellerite volatile concentrations in melt inclusions and in matrix glasses from a variety of eruptions. Melt inclusions, on average, contain 350 ppm S, 3500 ppm F and 9000 ppm Cl. We have measured up to 4·9 wt % H2O and 150 ppm CO2 in melt inclusions. Li–H2O systematics and Cl abundances in melt inclusions are consistent with partitioning of Li and Cl into a subcritical hydrosaline fluid at low pressures. The volatiles H2O and CO2 are used to estimate melt equilibration pressures, which reach a maximum of 1·5 kbar. Temperatures of 800°C are calculated for the most evolved pantellerites, using published feldspar–melt geothermometers, and up to 870°C for the least evolved samples. Low melt viscosities are calculated for the range of pantellerite compositions observed and may account for rapid differentiation by crystal settling. Stable density stratification of the magma chamber is reflected in the eruption of generally progressively more fractionated compositions after the Green Tuff eruption during the last eruptive cycle. Some anomalies in this trend may be explained by variation in the relative rates of eruption vs fractionation. The density stratification is expected to be enhanced and further stabilized by the efficient migration of a fluid phase to the roof of the magma chamber. The sulphur data are used in combination with published experimental partitioning data for peralkaline rhyolites to estimate the sulphur yield to the atmosphere for a large pantelleritic eruption similar to the Green Tuff. This is expected to be markedly higher than for a similar-sized metaluminous rhyolitic or dacitic eruption, mainly owing to the higher bulk sulphur content, lower fluid–melt partition coefficients, and rapid differentiation and vapour phase segregation in the magma chamber.

Bookmarks Related papers MentionsView impact

Talks by Gareth Fabbro

Research paper thumbnail of Magmatic processes prior to the 1400-BP, caldera-forming eruption of Rabaul, Papua New Guinea

The 1400BP eruption of Rabaul discharged >11km3 of mainly crystal-poor dacitic magma, and collaps... more The 1400BP eruption of Rabaul discharged >11km3 of mainly crystal-poor dacitic magma, and collapsed a caldera. Following an initial Plinian phase, multiple pyroclastic flows were emplaced. We have sampled throughout the eruptive sequence: the Plinian fall deposit, basal surge unit, the base and top of the massive ignimbrite, and the uppermost ignimbrite veneer deposit. There is no trend in the SiO2 content of the erupted magma with stratigraphic height; all analysed pumices have between 64.9 and 66.3wt% SiO2. Mixing between basaltic and silicic magma is suggested by the presence of mafic xenocrysts (e.g. olivine). There is also evidence for mixing between two dacitic magmas. Whole rock chemical analyses of Rabaul magmas form a single trend on plots of K2O against SiO2, with the exception of a single eruption (the ~7ka Raluan ignimbrite). EMPA analyses of matrix glass from the 1400BP eruption, however, lie on two distinct trends. Most lie on the trend defined by the whole rock chemistry; X-ray maps, however, show that this magma mingled with another, microlite-rich magma with lower K2O. This low K2O glass resembles magma from the Raluan ignimbrite, and may represent remnant magma that was remobilised during the 1400BP eruption. Melt inclusions in plagioclase (An35-55), clinopyroxene (Wo35-45, En40-50) and orthopyroxene (En70-75) phenocrysts also fall on the low K2O trend, as do melt inclusions and matrix glasses from some of the more recent eruptions. LA ICP-MS analyses of these glasses and host minerals provide trace element data that, along with petrological modelling, help pinpoint the sources of these melts. Volatile contents of the melt inclusions have also been measured. Preliminary SIMS data from the 1400BP ignimbrite show that the magma initially held up to 6-7wt% H2O and 1100 ppm CO2. However, most melt inclusions have lower volatile contents and suggest storage at about 200MPa (~7km).

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Magma chamber dynamics prior to the 1400BP eruption of Rabaul, Papua New Guinea

The 1400BP eruption of Rabaul erupted >11km3 of mainly crystal-poor dacitic magma, and lead to ca... more The 1400BP eruption of Rabaul erupted >11km3 of mainly crystal-poor dacitic magma, and lead to caldera collapse. Following an initial Plinian phase, multiple pyroclastic flows were then emplaced. In order to identify any pre-eruptive reservoir stratigraphy, we have sampled throughout the eruptive sequence: the Plinian fall deposit, the basal surge unit, the base and top of the massive ignimbrite, and in the uppermost ignimbrite veneer deposit. The presence of mafic xenocrysts (e.g. olivine) suggests mixing between basaltic and silicic magma, however there is also evidence for mixing between two dacitic magmas. EMPA analyses of matrix glass lie on two distinct linear trends on plots of K2O against SiO2. Most analyses lie on the trend defined by the whole rock chemistry of almost all eruptive products from Rabaul. X-ray maps show that another, microlite-rich magma with low K2O glass is mingled with the high K2O glass. This low K2O glass resembles magma erupted during a previous caldera-forming eruption, and may represent remnant magma that was remobilised during the 1400BP eruption. Melt inclusions in plagioclase (An35-55), clinopyroxene (Wo35-45, En40-50) and orthopyroxene (En70-75) phenocrysts also fall on the low K2O trend. LA ICP-MS analyses of these glasses and the host minerals provide trace element data that, along with petrological modelling, help to pinpoint the sources of these two melts. Volatile contents of the melt inclusions have also been measured. Preliminary SIMS data from the Plinian phase shows that the magma initially held up to 6-7wt% H2O and 1100 ppm CO2. However, most melt inclusions have lower volatile contents and suggest storage at about 200MPa (~7km).

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Processes leading up to the 22 ka silicic caldera-forming eruption of Santorini (Greece): Constraints from crystal trace-element fingerprinting and diffusion chronometry

Caldera-forming eruptions are amongst the most destructive phenomena on this planet. Constraining... more Caldera-forming eruptions are amongst the most destructive phenomena on this planet. Constraining the processes that occur before these eruptions, as well as the timescales of those processes, is vital for forecasting the behaviour of the volcanoes responsible for them. The 21.7 \ensuremath{\pm}0.2~ka caldera-forming Cape Riva eruption of Santorini Volcano discharged at least \ensuremath{\sim}10~km3 of dacitic magma (along with a minor basalt-dacite hybrid andesitic component), and caused caldera-collapse. The eruption was preceded by a \ensuremath{\sim}13~ky period of dacitic effusive and minor explosive volcanism. This effusive activity constructed the 1-2~km3, \textless 200-m-thick Therasia dome complex, centred on the site of the subsequent Cape Riva caldera. The major element geochemistry of the Therasia dacite is very similar to that of the Cape Riva dacite, but its lower incompatible element concentrations (e.g. K, Zr, Rb, La, Ce and Nb) demonstrate that it is a different magma batch. This difference is also apparent in the concentrations of La and Ce in plagioclase crystals from the two magmas. La and Ce diffuse very slowly through plagioclase (\textless 100~\ensuremath{\mu}m in ~107~y at 900\textdegree C), and therefore reflect the concentration of those elements in the parent magma on the timescale of our system (\textless 105~y). The contrasting trace element compositions of the whole rocks and crystals seem to rule out a simple interpretation that the Therasia dacites were leaks from the growing Cape Riva magma chamber. Modelling of diffusion gradients of relatively fast-diffusing elements such as Mg and Sr in plagioclase crystals provides estimates of the residence times of those crystals at magmatic temperatures. Preliminary results also show that plagioclase crystals in the Therasia dacites have maximum residence times of the order of 102-103 years. This is short compared to the total 13~ky duration of effusive volcanism, suggesting that the Therasia lavas may represent the repeated ascent, partial crystallisation and eruption of multiple, small parcels of dacitic magma over a long period of time. Field and 40Ar/39Ar constraints tie down the arrival of the Cape Riva dacite batch in the shallow plumbing system less than 4000~y prior to the Cape Riva eruption. Preliminary diffusion chronometry results for Cape Riva crystals suggest residence times for some plagioclase crystals as short as 10-102 years. This supports the idea that major influx of new, silicic magma occurred shortly before the eruption, and possibly provides more a precise constraint on the timing of this influx.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Precursory leaks and silicic recharge prior to a caldera-forming eruption (Santorini, Greece)

The Cape Riva eruption of Santorini Volcano discharged several km3 of dacitic magma about 22,000 ... more The Cape Riva eruption of Santorini Volcano discharged several km3 of dacitic magma about 22,000 years ago. This eruption was preceded by the extrusion of lava domes and flows (the Therasia dome complex), which have a similar major element composition. The Therasia lavas are interpreted as leaks from the Cape Riva magma body as it was assembled and grew incrementally. They provide information about the evolution of the plumbing system beneath a caldera volcano prior to a large explosive eruption. We have reconstructed the stratigraphy of the eruptive sequence, enabling us to reconstruct the order of eruption of the different leaks. Apart from the first and last lava flows, all the products of the Therasia sequence contain between 64.6–68.7 wt% SiO2: very similar to the major element composition of Cape Riva pumice. Along with this constant chemistry, soils between several of the flows suggest that the Therasia lavas represent a period of stagnation of the plumbing system. 40Ar/39Ar dates show that the period of constructive volcanism lasted 32 kyr, and ended at most 4–0 kyr before the Cape Riva eruption. Although the Cape Riva and Therasia magmas have similar major element concentrations, the Cape Riva magma is depleted in incompatible elements such as K, Zr, Rb and Nb. This might be explained if there was an important influx of silicic magma into the shallow plumbing system in the 1 kyr between the end of the construction of the Therasia dome complex and the onset of the Cape Riva eruption. One implication of this interpretation is that the transition from effusive to explosive activity coincided with this upward transfer. We are at present investigating these processes by studying the zoning patterns in plagioclase crystals. In particular, modelling of trace element diffusion gradients will provide further information about the timescales involved.

Bookmarks Related papers MentionsView impact

Theses by Gareth Fabbro

Research paper thumbnail of Degassing and Deformation of the Pantelleria Magma Chamber

Fieldwork was undertaken to survey CO2 flux from the flanks of Pantelleria, a quiescent volcano i... more Fieldwork was undertaken to survey CO2 flux from the flanks of Pantelleria, a quiescent volcano in the Sicily Channel. An area of 0.163 km2 was chosen around Favàra Grande, as this was an area of active degassing in previous studies and is at the centre of the deflation seen to occur on the island. Flux measurements were taken using an accumulation chamber method, and samples were also brought back to Cambridge to analyse for δ13C. The total anomalous CO2 flux was found to be 1 300 kg d−1, with an average of 15.6 g m−2 d−1. Precision test were carried out, and the results are considered accurate to 8 %, however there was also an increase of ~70 % over a period of a week, possibly due to meterological conditions. The δ13C of the CO2 was found to be –7‰ , close to the magmatic δ13C.
Pantelleria is currently undergoing a period of deflation, although there was an episode of inflation preceding the 1891 eruption, and others ~500 and ~900 years ago. The amount of gas currently escaping the magma chamber was found to be too little to fully explain the deflation, although the CO2 flux from Favàra Grande may be have been decreasing over the last ten years. Because the magma chamber is large and shallow, small changes in temperature lead to large effects at the surface. A temperature change of 0.001 K yr−1 is large enough to explain the deflation through thermal contraction. The deflation is therefore likely caused by a combination of both degassing and the thermal contraction of the magma chamber.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of The timescales of magmatic processes prior to a caldera-forming eruption

Large, explosive, caldera-forming eruptions are amongst the most destructive phenomena on the pla... more Large, explosive, caldera-forming eruptions are amongst the most destructive phenomena on the planet, but the processes that allow the large bodies of crystal-poor silicic magma that feed them to assemble in the shallow crust are still poorly understood. Of particular interest is the timescales over which these reservoirs exist prior to eruption. Long storage times—up to 10^5 y—have previously been estimated using the repose times between eruptions and radiometric dating of crystals found within the eruptive products. However, more recent work modelling diffusion within single crystals has been used to argue that the reservoirs that feed even the largest eruptions are assembled over much shorter periods—10^1–10^2 y.

In order to address this question, I studied the >10 km3, 22-ka, dacitic Cape Riva eruption of Santorini, Greece. Over the ~18 ky preceding the Cape Riva eruption a series of dacitic lava dome and coulées were erupted, and these lavas are interspersed with occasional dacitic pumice fall deposits (the Therasia dome complex). These dacites have similar major element contents to the dacite that was erupted during the Cape Riva eruption, and have previously been described as “precursory leaks” from the growing Cape Riva magma reservoir. However, the Cape Riva magma is depleted in incompatible elements (such as K, Zr, La, Ce) relative to the Therasia magma, as are the plagioclase crystals in the respective magmas. This difference cannot be explained using shallow processes such as fractional crystallisation or crustal assimilation, which suggests that the Cape Riva and Therasia magmas are separate batches. Furthermore, there is evidence that the Therasia dacites were not fed from a long-lived, melt-dominated reservoir. There are non-systematic variations in melt composition, plagioclase rim compositions, and plagioclase textures throughout the sequence. In addition, high-temperature residence times of plagioclase and orthopyroxene crystals from the Therasia dacites estimated using diffusion chronometry are 10^1–10^2 y. This is short compared to the average time between eruptions (~1,500 y), which suggests the crystals in each lava grew only shortly before eruption. The different incompatible element contents of the Cape Riva and Therasia magmas and plagioclase crystals suggest that a new batch of incompatible-depleted silicic magma arrived in the shallow volcanic plumbing system shortly before the Cape Riva eruption. This influx must have taken place after the last Therasia eruption, which 40Ar/39Ar dates show occurred less than 2,800 ± 1,400 years before the Cape Riva eruption.

The rims of the plagioclase crystals found in the Cape Riva dacite are in equilibrium with a rhyodacite, with a similar composition to the Cape Riva glass. However, the major and trace element zoning patterns of the crystals record variations in the melt composition during their growth. The compositions at the centre of most crystals are the same as the rims; however, these crystals are often partially resorbed and overgrown by more calcic plagioclase. The plagioclase then grades normally back to rim compositions. This cycle is repeated up to three times. The tight relationships between the anorthite, Sr and Ti contents of the different zones suggests that the composition of the plagioclase crystals correlates with the composition of the melt from which they grew. The different plagioclase compositions correspond to dacitic and rhyodacitic melt compositions. The orthopyroxene crystals reveal a similar sequence, although they only record one cycle. These zoning patterns are interpreted to document the assembly of the Cape Riva reservoir in the shallow crust through the amalgamation of multiple batches of compositionally diverse magma. Models of magnesium diffusion in plagioclase and Fe–Mg interdiffusion in orthopyroxene suggest that this amalgamation took place within 10^1–10^2 y of the Cape Riva eruption.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Evolution of the crustal magma plumbing system during the build-up to the 22-ka caldera-forming eruption of Santorini (Greece)

Bulletin of Volcanology, Dec 2013

The formation of shallow, caldera-sized reservoirs of crystal-poor silicic magma requires the gen... more The formation of shallow, caldera-sized reservoirs of crystal-poor silicic magma requires the generation of large volumes of silicic melt, followed by the segregation of that melt and its accumulation in the upper crust. The 21.8 ± 0.4-ka Cape Riva eruption of Santorini discharged >10 km3 of crystal-poor dacitic magma, along with <<1 km3 of hybrid andesite, and collapsed a pre-existing lava shield. We have carried out a field, petrological, chemical, and high-resolution 40Ar/39Ar chronological study of a sequence of lavas discharged prior to the Cape Riva eruption to constrain the crustal residence time of the Cape Riva magma reservoir. The lavas were erupted between 39 and 25 ka, forming a ∼2-km3 complex of dacitic flows, coulées and domes up to 200 m thick (Therasia dome complex). The Therasia dacites show little chemical variation with time, suggesting derivation from one or more thermally buffered reservoirs. Minor pyroclastic layers occur intercalated within the lava succession, particularly near the top. A prominent pumice fall deposit correlates with the 26-ka Y-4 ash layer found in deep-sea sediments SE of Santorini. One of the last Therasia lavas to be discharged was a hybrid andesite formed by the mixing of dacite and basalt. The Cape Riva eruption occurred no more than 2,800 ± 1,400 years after the final Therasia activity. The Cape Riva dacite is similar in major element composition to the Therasia dacites, but is poorer in K and most incompatible trace elements (e.g. Rb, Zr and LREE). The same chemical differences are observed between the Cape Riva and Therasia hybrid andesites, and between the calculated basaltic mixing end-members of each series. The Therasia and Cape Riva dacites are distinct silicic magma batches and are not related by shallow processes of crystal fractionation or assimilation. The Therasia lavas were therefore not simply precursory leaks from the growing Cape Riva magma reservoir. The change 21.8 ky ago from a magma series richer in incompatible elements to one poorer in those elements is one step in the well documented decrease with time of incompatibles in Santorini magmas over the last 530 ky. The two dacitic magma batches are interpreted to have been emplaced sequentially into the upper crust beneath the summit of the volcano, the first (Therasia) then being partially, or wholly, flushed out by the arrival of the second (Cape Riva). This constrains the upper-crustal residence time of the Cape Riva reservoir to less than 2,800 ± 1,400 years, and the associated time-averaged magma accumulation rate to >0.004 km3 year-1. Rapid ascent and accumulation of the Cape Riva dacite may have been caused by an increased flux of mantle-derived basalt into the crust, explaining the occurrence of hybrid andesites (formed by the mixing of olivine basalt and dacite in approximately equal proportions) in the Cape Riva and late Therasia products. Pressurisation of the upper crustal plumbing system by sustained, high-flux injection of dacite and basalt may have triggered the transition from prolonged, largely effusive activity to explosive eruption and caldera collapse.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Melting, Differentiation and Degassing at the Pantelleria Volcano, Italy

Journal of …, Jan 1, 2012

We present the results of the first systematic study of melt compositions at Pantelleria, based o... more We present the results of the first systematic study of melt compositions at Pantelleria, based on both melt inclusions and matrix glasses in pantellerites from 10 eruptions during the last eruptive cycle (<45 kyr). We present major and trace element compositions, as well as data on the volatiles sulphur (S), fluorine (F), chlorine (Cl), water (H2O), carbon dioxide (CO2) and lithium (Li) Rare earth element (REE) compositions were inverted using the program INVMEL to establish the melt fraction vs depth relationship in the Pantellerian mantle source region. Inversion indicates that melts are generated by ∼1·7% melting of a light rare earth element (LREE)-enriched mantle source. The source lies principally within the spinel–garnet transition zone, which, on the basis of trace element ratios, shows some affinity to the source of North African magmatism. Major and trace element data indicate a gap in melt compositions at intermediate compositions, consistent with previously published whole-rock data. This gap rules out the possibility of explaining chemical variability in the Pantelleria lavas merely by changes in the crystal content of the magmas. Principal component analysis of major element glass compositions shows that the liquid line of descent for mafic melt compositions is controlled by clinopyroxene, plagioclase, magnetite and olivine crystallization. Alkali feldspar, clinopyroxene, ilmenite and olivine or aenigmatite crystallization controls the liquid line of descent for the silicic melt compositions, with aenigmatite broadly replacing olivine in the most evolved magmas. Trace element modelling indicates that 96% fractional crystallization is required to generate pantellerites from alkali basalts at Pantelleria (through trachytes, generated after 76% fractional crystallization). We have measured pantellerite volatile concentrations in melt inclusions and in matrix glasses from a variety of eruptions. Melt inclusions, on average, contain 350 ppm S, 3500 ppm F and 9000 ppm Cl. We have measured up to 4·9 wt % H2O and 150 ppm CO2 in melt inclusions. Li–H2O systematics and Cl abundances in melt inclusions are consistent with partitioning of Li and Cl into a subcritical hydrosaline fluid at low pressures. The volatiles H2O and CO2 are used to estimate melt equilibration pressures, which reach a maximum of 1·5 kbar. Temperatures of 800°C are calculated for the most evolved pantellerites, using published feldspar–melt geothermometers, and up to 870°C for the least evolved samples. Low melt viscosities are calculated for the range of pantellerite compositions observed and may account for rapid differentiation by crystal settling. Stable density stratification of the magma chamber is reflected in the eruption of generally progressively more fractionated compositions after the Green Tuff eruption during the last eruptive cycle. Some anomalies in this trend may be explained by variation in the relative rates of eruption vs fractionation. The density stratification is expected to be enhanced and further stabilized by the efficient migration of a fluid phase to the roof of the magma chamber. The sulphur data are used in combination with published experimental partitioning data for peralkaline rhyolites to estimate the sulphur yield to the atmosphere for a large pantelleritic eruption similar to the Green Tuff. This is expected to be markedly higher than for a similar-sized metaluminous rhyolitic or dacitic eruption, mainly owing to the higher bulk sulphur content, lower fluid–melt partition coefficients, and rapid differentiation and vapour phase segregation in the magma chamber.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Magmatic processes prior to the 1400-BP, caldera-forming eruption of Rabaul, Papua New Guinea

The 1400BP eruption of Rabaul discharged >11km3 of mainly crystal-poor dacitic magma, and collaps... more The 1400BP eruption of Rabaul discharged >11km3 of mainly crystal-poor dacitic magma, and collapsed a caldera. Following an initial Plinian phase, multiple pyroclastic flows were emplaced. We have sampled throughout the eruptive sequence: the Plinian fall deposit, basal surge unit, the base and top of the massive ignimbrite, and the uppermost ignimbrite veneer deposit. There is no trend in the SiO2 content of the erupted magma with stratigraphic height; all analysed pumices have between 64.9 and 66.3wt% SiO2. Mixing between basaltic and silicic magma is suggested by the presence of mafic xenocrysts (e.g. olivine). There is also evidence for mixing between two dacitic magmas. Whole rock chemical analyses of Rabaul magmas form a single trend on plots of K2O against SiO2, with the exception of a single eruption (the ~7ka Raluan ignimbrite). EMPA analyses of matrix glass from the 1400BP eruption, however, lie on two distinct trends. Most lie on the trend defined by the whole rock chemistry; X-ray maps, however, show that this magma mingled with another, microlite-rich magma with lower K2O. This low K2O glass resembles magma from the Raluan ignimbrite, and may represent remnant magma that was remobilised during the 1400BP eruption. Melt inclusions in plagioclase (An35-55), clinopyroxene (Wo35-45, En40-50) and orthopyroxene (En70-75) phenocrysts also fall on the low K2O trend, as do melt inclusions and matrix glasses from some of the more recent eruptions. LA ICP-MS analyses of these glasses and host minerals provide trace element data that, along with petrological modelling, help pinpoint the sources of these melts. Volatile contents of the melt inclusions have also been measured. Preliminary SIMS data from the 1400BP ignimbrite show that the magma initially held up to 6-7wt% H2O and 1100 ppm CO2. However, most melt inclusions have lower volatile contents and suggest storage at about 200MPa (~7km).

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Magma chamber dynamics prior to the 1400BP eruption of Rabaul, Papua New Guinea

The 1400BP eruption of Rabaul erupted >11km3 of mainly crystal-poor dacitic magma, and lead to ca... more The 1400BP eruption of Rabaul erupted >11km3 of mainly crystal-poor dacitic magma, and lead to caldera collapse. Following an initial Plinian phase, multiple pyroclastic flows were then emplaced. In order to identify any pre-eruptive reservoir stratigraphy, we have sampled throughout the eruptive sequence: the Plinian fall deposit, the basal surge unit, the base and top of the massive ignimbrite, and in the uppermost ignimbrite veneer deposit. The presence of mafic xenocrysts (e.g. olivine) suggests mixing between basaltic and silicic magma, however there is also evidence for mixing between two dacitic magmas. EMPA analyses of matrix glass lie on two distinct linear trends on plots of K2O against SiO2. Most analyses lie on the trend defined by the whole rock chemistry of almost all eruptive products from Rabaul. X-ray maps show that another, microlite-rich magma with low K2O glass is mingled with the high K2O glass. This low K2O glass resembles magma erupted during a previous caldera-forming eruption, and may represent remnant magma that was remobilised during the 1400BP eruption. Melt inclusions in plagioclase (An35-55), clinopyroxene (Wo35-45, En40-50) and orthopyroxene (En70-75) phenocrysts also fall on the low K2O trend. LA ICP-MS analyses of these glasses and the host minerals provide trace element data that, along with petrological modelling, help to pinpoint the sources of these two melts. Volatile contents of the melt inclusions have also been measured. Preliminary SIMS data from the Plinian phase shows that the magma initially held up to 6-7wt% H2O and 1100 ppm CO2. However, most melt inclusions have lower volatile contents and suggest storage at about 200MPa (~7km).

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Processes leading up to the 22 ka silicic caldera-forming eruption of Santorini (Greece): Constraints from crystal trace-element fingerprinting and diffusion chronometry

Caldera-forming eruptions are amongst the most destructive phenomena on this planet. Constraining... more Caldera-forming eruptions are amongst the most destructive phenomena on this planet. Constraining the processes that occur before these eruptions, as well as the timescales of those processes, is vital for forecasting the behaviour of the volcanoes responsible for them. The 21.7 \ensuremath{\pm}0.2~ka caldera-forming Cape Riva eruption of Santorini Volcano discharged at least \ensuremath{\sim}10~km3 of dacitic magma (along with a minor basalt-dacite hybrid andesitic component), and caused caldera-collapse. The eruption was preceded by a \ensuremath{\sim}13~ky period of dacitic effusive and minor explosive volcanism. This effusive activity constructed the 1-2~km3, \textless 200-m-thick Therasia dome complex, centred on the site of the subsequent Cape Riva caldera. The major element geochemistry of the Therasia dacite is very similar to that of the Cape Riva dacite, but its lower incompatible element concentrations (e.g. K, Zr, Rb, La, Ce and Nb) demonstrate that it is a different magma batch. This difference is also apparent in the concentrations of La and Ce in plagioclase crystals from the two magmas. La and Ce diffuse very slowly through plagioclase (\textless 100~\ensuremath{\mu}m in ~107~y at 900\textdegree C), and therefore reflect the concentration of those elements in the parent magma on the timescale of our system (\textless 105~y). The contrasting trace element compositions of the whole rocks and crystals seem to rule out a simple interpretation that the Therasia dacites were leaks from the growing Cape Riva magma chamber. Modelling of diffusion gradients of relatively fast-diffusing elements such as Mg and Sr in plagioclase crystals provides estimates of the residence times of those crystals at magmatic temperatures. Preliminary results also show that plagioclase crystals in the Therasia dacites have maximum residence times of the order of 102-103 years. This is short compared to the total 13~ky duration of effusive volcanism, suggesting that the Therasia lavas may represent the repeated ascent, partial crystallisation and eruption of multiple, small parcels of dacitic magma over a long period of time. Field and 40Ar/39Ar constraints tie down the arrival of the Cape Riva dacite batch in the shallow plumbing system less than 4000~y prior to the Cape Riva eruption. Preliminary diffusion chronometry results for Cape Riva crystals suggest residence times for some plagioclase crystals as short as 10-102 years. This supports the idea that major influx of new, silicic magma occurred shortly before the eruption, and possibly provides more a precise constraint on the timing of this influx.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Precursory leaks and silicic recharge prior to a caldera-forming eruption (Santorini, Greece)

The Cape Riva eruption of Santorini Volcano discharged several km3 of dacitic magma about 22,000 ... more The Cape Riva eruption of Santorini Volcano discharged several km3 of dacitic magma about 22,000 years ago. This eruption was preceded by the extrusion of lava domes and flows (the Therasia dome complex), which have a similar major element composition. The Therasia lavas are interpreted as leaks from the Cape Riva magma body as it was assembled and grew incrementally. They provide information about the evolution of the plumbing system beneath a caldera volcano prior to a large explosive eruption. We have reconstructed the stratigraphy of the eruptive sequence, enabling us to reconstruct the order of eruption of the different leaks. Apart from the first and last lava flows, all the products of the Therasia sequence contain between 64.6–68.7 wt% SiO2: very similar to the major element composition of Cape Riva pumice. Along with this constant chemistry, soils between several of the flows suggest that the Therasia lavas represent a period of stagnation of the plumbing system. 40Ar/39Ar dates show that the period of constructive volcanism lasted 32 kyr, and ended at most 4–0 kyr before the Cape Riva eruption. Although the Cape Riva and Therasia magmas have similar major element concentrations, the Cape Riva magma is depleted in incompatible elements such as K, Zr, Rb and Nb. This might be explained if there was an important influx of silicic magma into the shallow plumbing system in the 1 kyr between the end of the construction of the Therasia dome complex and the onset of the Cape Riva eruption. One implication of this interpretation is that the transition from effusive to explosive activity coincided with this upward transfer. We are at present investigating these processes by studying the zoning patterns in plagioclase crystals. In particular, modelling of trace element diffusion gradients will provide further information about the timescales involved.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Degassing and Deformation of the Pantelleria Magma Chamber

Fieldwork was undertaken to survey CO2 flux from the flanks of Pantelleria, a quiescent volcano i... more Fieldwork was undertaken to survey CO2 flux from the flanks of Pantelleria, a quiescent volcano in the Sicily Channel. An area of 0.163 km2 was chosen around Favàra Grande, as this was an area of active degassing in previous studies and is at the centre of the deflation seen to occur on the island. Flux measurements were taken using an accumulation chamber method, and samples were also brought back to Cambridge to analyse for δ13C. The total anomalous CO2 flux was found to be 1 300 kg d−1, with an average of 15.6 g m−2 d−1. Precision test were carried out, and the results are considered accurate to 8 %, however there was also an increase of ~70 % over a period of a week, possibly due to meterological conditions. The δ13C of the CO2 was found to be –7‰ , close to the magmatic δ13C.
Pantelleria is currently undergoing a period of deflation, although there was an episode of inflation preceding the 1891 eruption, and others ~500 and ~900 years ago. The amount of gas currently escaping the magma chamber was found to be too little to fully explain the deflation, although the CO2 flux from Favàra Grande may be have been decreasing over the last ten years. Because the magma chamber is large and shallow, small changes in temperature lead to large effects at the surface. A temperature change of 0.001 K yr−1 is large enough to explain the deflation through thermal contraction. The deflation is therefore likely caused by a combination of both degassing and the thermal contraction of the magma chamber.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of The timescales of magmatic processes prior to a caldera-forming eruption

Large, explosive, caldera-forming eruptions are amongst the most destructive phenomena on the pla... more Large, explosive, caldera-forming eruptions are amongst the most destructive phenomena on the planet, but the processes that allow the large bodies of crystal-poor silicic magma that feed them to assemble in the shallow crust are still poorly understood. Of particular interest is the timescales over which these reservoirs exist prior to eruption. Long storage times—up to 10^5 y—have previously been estimated using the repose times between eruptions and radiometric dating of crystals found within the eruptive products. However, more recent work modelling diffusion within single crystals has been used to argue that the reservoirs that feed even the largest eruptions are assembled over much shorter periods—10^1–10^2 y.

In order to address this question, I studied the >10 km3, 22-ka, dacitic Cape Riva eruption of Santorini, Greece. Over the ~18 ky preceding the Cape Riva eruption a series of dacitic lava dome and coulées were erupted, and these lavas are interspersed with occasional dacitic pumice fall deposits (the Therasia dome complex). These dacites have similar major element contents to the dacite that was erupted during the Cape Riva eruption, and have previously been described as “precursory leaks” from the growing Cape Riva magma reservoir. However, the Cape Riva magma is depleted in incompatible elements (such as K, Zr, La, Ce) relative to the Therasia magma, as are the plagioclase crystals in the respective magmas. This difference cannot be explained using shallow processes such as fractional crystallisation or crustal assimilation, which suggests that the Cape Riva and Therasia magmas are separate batches. Furthermore, there is evidence that the Therasia dacites were not fed from a long-lived, melt-dominated reservoir. There are non-systematic variations in melt composition, plagioclase rim compositions, and plagioclase textures throughout the sequence. In addition, high-temperature residence times of plagioclase and orthopyroxene crystals from the Therasia dacites estimated using diffusion chronometry are 10^1–10^2 y. This is short compared to the average time between eruptions (~1,500 y), which suggests the crystals in each lava grew only shortly before eruption. The different incompatible element contents of the Cape Riva and Therasia magmas and plagioclase crystals suggest that a new batch of incompatible-depleted silicic magma arrived in the shallow volcanic plumbing system shortly before the Cape Riva eruption. This influx must have taken place after the last Therasia eruption, which 40Ar/39Ar dates show occurred less than 2,800 ± 1,400 years before the Cape Riva eruption.

The rims of the plagioclase crystals found in the Cape Riva dacite are in equilibrium with a rhyodacite, with a similar composition to the Cape Riva glass. However, the major and trace element zoning patterns of the crystals record variations in the melt composition during their growth. The compositions at the centre of most crystals are the same as the rims; however, these crystals are often partially resorbed and overgrown by more calcic plagioclase. The plagioclase then grades normally back to rim compositions. This cycle is repeated up to three times. The tight relationships between the anorthite, Sr and Ti contents of the different zones suggests that the composition of the plagioclase crystals correlates with the composition of the melt from which they grew. The different plagioclase compositions correspond to dacitic and rhyodacitic melt compositions. The orthopyroxene crystals reveal a similar sequence, although they only record one cycle. These zoning patterns are interpreted to document the assembly of the Cape Riva reservoir in the shallow crust through the amalgamation of multiple batches of compositionally diverse magma. Models of magnesium diffusion in plagioclase and Fe–Mg interdiffusion in orthopyroxene suggest that this amalgamation took place within 10^1–10^2 y of the Cape Riva eruption.

Bookmarks Related papers MentionsView impact