Peralkaline Rhyolites Research Articles

H2O diffusion in peralkaline to peraluminous rhyolitic melts

The diffusion of water in a peralkaline and a peraluminous rhyolitic melt was investigated at temperatures of 714–1,493 K and pressures of 100 and 500 MPa. At temperatures below 923 K dehydration experiments were performed on glasses containing about 2 wt% H2O t in cold seal pressure vessels. At high temperatures diffusion couples of water-poor (<0.5 wt% H2O t ) and water-rich (~2 wt% H2O t ) melts were run in an internally heated gas pressure vessel. Argon was the pressure medium in both cases. Concentration profiles of hydrous species (OH groups and H2O molecules) were measured along the diffusion direction using near-infrared (NIR) microspectroscopy. The bulk water diffusivity ( $$D_{{{\text{H}}_{2} {\text{O}}_{t} }}^{{}}$$ ) was derived from profiles of total water ( $$C_{{{\text{H}}_{2} {\text{O}}_{t} }}^{{}}$$ ) using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between $$D_{{{\text{H}}_{2} {\text{O}}_{t} }}^{{}}$$ and $$C_{{{\text{H}}_{2} {\text{O}}_{t} }}^{{}}.$$ Both methods consistently indicate that $$D_{{{\text{H}}_{2} {\text{O}}_{t} }}^{{}}$$ is proportional to $$C_{{{\text{H}}_{2} {\text{O}}_{t} }}^{{}}$$ in this range of water contents for both bulk compositions, in agreement with previous work on metaluminous rhyolite. The water diffusivity in the peraluminous melts agrees very well with data for metaluminous rhyolites implying that an excess of Al2O3 with respect to alkalis does not affect water diffusion. On the other hand, water diffusion is faster by roughly a factor of two in the peralkaline melt compared to the metaluminous melt. The following expression for the water diffusivity in the peralkaline rhyolite as a function of temperature and pressure was obtained by least-squares fitting: $$ \log D_{{{\text{H}}_{2} {\text{O}}_{t} }}^{{1\,{\text{wt}}\% }} = ( - 7.09 \pm 0.15) - \frac{(4,\!788 \pm 166) + (0.56 \pm 0.21) \times P}{T} $$ where $$ D_{{{\text{H}}_{2} {\text{O}}_{t} }}^{{1\,{\text{wt}}\% }} $$ is the water diffusivity at 1 wt% H2O t in m2/s, T is the temperature in K and P is the pressure in MPa. The above equation reproduces the experimental data (14 runs in total) with a standard fit error of 0.15 log units. It can be employed to model degassing of peralkaline melts at water contents up to 2 wt%.

The origin of trachyte and pantellerite from Pantelleria, Italy: Insights from major element, trace element, and thermodynamic modelling

Trachyte and peralkaline rhyolite (pantellerite and comendite) frequently comprise the felsic end-member in bimodal suites in continental rift and oceanic island settings. In these settings, the relationship between the mafic (mildly alkaline, or transitional, basalt) and felsic lavas is ambiguous; major- and trace-element models and isotopic data are often consistent with an origin for felsic lavas from either fractional crystallization of transitional basalt or partial melting of alkali gabbro followed by fractional crystallization. In this paper, we present representative mineral analyses and whole-rock analyses from forty samples of a basalt–trachyte–pantellerite suite collected at Pantelleria, Italy, in the Strait of Sicily Rift Zone, and compare the results of major- and trace-element modelling with the results of thermodynamic (MELTS) modelling. From these results we conclude that metaluminous trachyte formed as a result of 70 to 75% low-pressure (0.1 GPa) fractional crystallization of an assemblage of plagioclase, clinopyroxene, olivine, magnetite, and apatite from a hydrous (1.0–1.5 wt.% H 2O) transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite–magnetite–quartz buffer (FMQ−1). The “Daly gap”–a lack of intermediate (~ 49–62 wt.% SiO 2) volcanic rocks–at Pantelleria is concluded to be primarily the result of rapid differentiation through that interval. Relatively rapid crystallization at low pressure may have effected the partial degassing of water-saturated (~ 4 wt.% H 2O) metaluminous trachyte magma. Some metaluminous trachyte lavas have positive Eu anomalies, high K/Rb ratios, high concentrations of Ba, and low concentrations of incompatible trace elements; these are interpreted to be the result of up to 40% accumulation of alkali feldspar. Comenditic trachyte, pantelleritic trachyte, and pantellerite formed after an additional 20 to 80% fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma. The most evolved pantellerite lavas and tuffs are the result of a total of ~ 95% fractional crystallization of transitional basalt, with phenocrysts that equilibrated at low temperatures (< 700 °C), low oxygen fugacities, and high (> 4 wt.%) water contents.

Micro-Raman determination of iron redox state in dry natural glasses: Application to peralkaline rhyolites and basalts

The position, scaled intensity and topology of the Raman stretching envelope at ~ 1000 cm − 1 (HF) have been established in complex anhydrous glasses having variable Fe 3+/ΣFe ratios and contrasting compositions of peralkaline rhyolite (pantellerite; FeO T: 7.8 wt.%), iron-poor and iron-rich basalts (FeO T: 7.6 and 10.1 wt.%). Sensitivity of glass structure to iron redox decreases with increasing glass basicity. Raman spectra of pantellerite glasses (Fe 3+/ΣFe: 0.04–0.97) evidence the sharp increase of a band at ~ 970 cm − 1 with the progressive entry of 4-fold Fe 3+ in alkali-bonded Q 3 units. The spectral evolution of basalts with increasing iron oxidation (Fe 3+/ΣFe: 0.05–0.87) is more subtle and is likely determined by formation of less polymerized units. The moderate to low influence of redox state changes on the structure of natural basalt glass, compared with the high sensitivity of their synthetic analogues, can be attributed to strong competition between network-forming cations (Al, Fe) in the absence of an excess of alkalis. We demonstrate that Raman calibration against reference glasses allows precise determination of Fe 2+/Fe 3+ratio but is highly composition dependent. As a consequence of the variable link existing between Fe 2+/Fe 3+ and glass polymerization, specific calibration procedures based on deconvolution, shift or intensity of HF band are required for compositions with high (acid glasses), moderate or poor (basic glasses) sensitivity to iron redox changes, respectively.

Heterogeneous initial Sr isotope compositions of highly evolved volcanic rocks from the Main Ethiopian Rift, Ethiopia

Many individual mineral Sr isotope studies have revealed the complex evolution of highly evolved rocks in upper crustal magmatic systems, casting doubts on the meaning of whole-rock Sr isotopes in such samples. In this paper, whole-rock Sr isotope measurements were replicated (three to 13 times) on six highly evolved peralkaline rhyolites from the Main Ethiopian Rift (MER) to appraise their internal heterogeneity and, thus, the significance of such data. These rocks were all fresh samples of pumice, obsidian and lava. Their maximum Sr contents and ages were 15 μg/g and 1.7 Ma, respectively. Significant small-scale heterogeneities of both Sr isotopes and Rb and Sr contents were observed in most samples, although not necessarily associated with petrological characteristics suggesting possible inheritance processes. Only two, almost crystal-free, obsidian give fairly homogeneous Sr isotope ratios. These results outline the ambiguity of a single whole-rock Sr isotope determination on highly evolved peralkaline rocks, especially when no simultaneous accurate determination of the Rb/Sr is performed. They also suggest that the limitations of Sr whole-rock analyses are not restricted to phenocryst-rich samples as phenocryst-poor obsidian and almost aphyric pumices and lava are also concerned. These data further underscore that unreplicated whole-rock Sr isotope measurements should always be used with great caution in the petrogenetic modeling of highly evolved rocks. However, multiple determinations of whole rock 87Sr/86Sr in the same samples, combined with other geochemical and isotopic data, may provide constraints on the shallow level evolution of these magmas. It is suggested that selective upper crustal contamination and/or interactions with halogen-bearing hydrous fluids, typical of evolved peralkaline magmas, were probably involved in the late magmatic evolution of these MER rhyolites. The more pervasive character of fluid interaction processes would probably better account for the small-scale association of uncontaminated and contaminated signatures in a single sample. Thus, even fresh samples may have their Rb–Sr isotopic system significantly modified by fluid interactions, not as a secondary process but at the late magmatic stage.

The Roles of Fractional Crystallization, Magma Mixing, Crystal Mush Remobilization and Volatile–Melt Interactions in the Genesis of a Young Basalt–Peralkaline Rhyolite Suite, the Greater Olkaria Volcanic Complex, Kenya Rift Valley

The Greater Olkaria Volcanic Complex is a young ( 20 ka) multi-centred lava and dome field dominated by the eruption of peralkaline rhyolites. Basaltic and trachytic magmas have been erupted peripherally to the complex and also form, with mugearites and benmoreites, an extensive suite of magmatic inclusions in the rhyolites. The eruptive rocks commonly represent mixed magmas and the magmatic inclusions are themselves two-, three- or four-component mixes. All rock types may carry xenocrysts of alkali feldspar, and less commonly plagioclase, derived from magma mixing and by remobilization of crystal mushes and/or plutonic rocks. Xenoliths in the range gabbro–syenite are common in the lavas and magmatic inclusions, the more salic varieties sometimes containing silicic glass representing partial melts and ranging in composition from anorthite ± corundum- to acmite-normative. The peralkaline varieties are broadly similar, in major element terms, to the eruptive peralkaline rhyolites. The basalt–trachyte suite formed by a combination of fractional crystallization, magma mixing and resorption of earlier-formed crystals. Matrix glass in metaluminous trachytes has a peralkaline rhyolitic composition, indicating that the eruptive rhyolites may have formed by fractional crystallization of trachyte. Anomalous trace element enrichments (e.g. 2000 ppm Y in a benmoreite) and negative Ce anomalies may have resulted from various Na- and K-enriched fluids evolving from melts of intermediate composition and either being lost from the system or enriched in other parts of the reservoirs. A small group of nepheline-normative, usually peralkaline, magmatic inclusions was formed by fluid transfer between peralkaline rhyolitic and benmoreitic magmas. The plumbing system of the complex consists of several independent reservoirs and conduits, repeatedly recharged by batches of mafic magma, with ubiquitous magma mixing.

Insights into the tectonomagmatic evolution of NW Mexico: Geochronology and geochemistry of the Miocene volcanic rocks from the Pinacate area, Sonora

Miocene volcanic rocks in the Pinacate area, Sonora, record a progressive change in the source of magmatism induced by asthenospheric upwelling and lithospheric thinning. 40 Ar/ 39 Ar age data, mineral chemistry, and major- and trace-element contents allow the identifi cation of two volcanic sequences: an oldest basaltic episode (ca. 20 Ma), and a middle Miocene (12‐15.5 Ma) sequence that consists of mesa basalts with transitional alkali character, calc-alkaline dacites, and high-silica rhyolites evolving toward peralkaline liquids. Sr, Nd, and Pb isotope ratios reveal different sources for the Miocene basalts. The easternmost basalts have signatures indicating a Precambrian lithospheric mantle source, while the westernmost tholeiitic to transitional basalts are related to mixing of lithospheric and asthenospheric mantle. Rhyolites are the result of fractional crystallization of transitional basalt magmas with slight contamination by Precambrian crust. Chemical modeling shows that peralkaline rhyolites are related to slightly higher assimilation during their residence in the upper crust but also to a change in the mantle source of the parent basalt. The evolution of the isotopic signatures in space and time indicates that: (1) the volcanic activity is located over a major lithospheric boundary, i.e., the western limit of the North American Craton; (2) the lithosphere was progressively thinned so that huge volumes of alkalic basalts could access the surface during the Quaternary, building the Pinacate Volcanic Field. Correlation between geochemical signatures and the tectonic evolution of the western margin of the North American Craton shows that a progressive change in the source of magmatism can be related to the development of a slab window during the Miocene.

The Metallogeny of Late Triassic Rifting of the Alexander Terrane in Southeastern Alaska and Northwestern British Columbia

A belt of unusual volcanogenic massive sulfide (VMS) occurrences is located along the eastern margin of the Alexander terrane throughout southeastern Alaska and northwestern British Columbia and exhibits a range of characteristics consistent with a variety of syngenetic to epigenetic deposit types. Deposits within this belt include Greens Creek and Windy Craggy, the economically most significant VMS deposit in Alaska and the largest in North America, respectively. The occurrences are hosted by a discontinuously exposed, 800-km-long belt of rocks that consist of a 200- to 800-m-thick sequence of conglomerate, limestone, marine clastic sedimentary rocks, and tuff intercalated with and overlain by a distinctive unit of mafic pyroclastic rocks and pillowed flows. Faunal data bracket the age of the host rocks between Anisian (Middle Triassic) and late Norian (late Late Triassic). This metallogenic belt is herein referred to as the Alexander Triassic metallogenic belt. The VMS occurrences show systematic differences in degree of structural control, chemistry, and stratigraphic setting along the Alexander Triassic metallogenic belt that suggest important spatial or temporal changes in the tectonic environment of formation. At the southern end of the belt, felsic volcanic rocks overlain by shallow-water limestones characterize the lower part of the sequence. In the southern and middle portion of the belt, a distinctive pebble conglomerate marks the base of the section and is indicative of high-energy deposition in a near slope or basin margin setting. At the northern end of the belt the conglomerates, limestones, and felsic volcanic rocks are absent and the belt is composed of deep-water sedimentary and mafic volcanic rocks. This northward change in depositional environment and lithofacies is accompanied by a northward transition from epithermal-like structurally controlled, discontinuous, vein- and pod-shaped, Pb-Zn-AgBa-(Cu) occurrences with relatively simple mineralogy, to sulfosalt-enriched VMS occurrences exhibiting characteristics of vein, diagenetic replacement, and exhalative styles of mineralization, and finally to Cu-Zn-(Co-Au) occurrences with larger and more clearly stratiform orebody morphologies. Occurrences in the middle of the belt are transitional in nature between structurally controlled types of mineralization that formed in a shallow-water, near-arc setting, to those having a more stratiform appearance, formed in a deeper water, rift-basin setting. The geologic setting in the south is consistent with shallow subaqueous emplacement on the flanks of the Alexander terrane. Northward, the setting changes to an increasingly deeper back- or intra-arc rift basin. Igneous activity in the Alexander Triassic metallogenic belt is characterized by a bimodal suite of volcanic rocks and a previously unrecognized association with mafic-ultramafic hypabyssal intrusions. Immobile trace and rare earth element (REE) geochemical data indicate that felsic rocks in the southern portion of the belt are typical calc-alkaline rhyolites, which give way in the middle of the belt to peralkaline rhyolites. Rhyolites are largely absent in the northern part of the belt. Throughout the belt, the capping basaltic rocks have transitional geochemical signatures. Radiogenic isotope data for these rocks are also transitional (basalts and gabbros: eNd = 4–9 and 87Sr/86Sr initial at 215 Ma = 0.7037–0.7074). Together these data are interpreted to reflect variable assimilation of mature island-arc crust by more primitive melts having the characteristics of either mid-ocean ridge (MORB) or intraplate (within-plate) basalts (WPB). The ore and host-rock geochemistry and the sulfosalt-rich mineralogy of the deposits are strikingly similar to recent descriptions of active sea-floor hydrothermal (white smoker) systems in back arcs of the southwest Pacific Ocean. These data, in concert with existing faunal ages, record the formation of a belt of VMS deposits and occurrences in a propagating intra- to back-arc rift tectonic setting during the Late Triassic. A modern analogue having similar tectonic and metallogenic features is the southward projection of the Lau basin, from the active sea-floor hydrothermal vents of the Valu Fa Ridge to the Taupo volcanic zone of the North Island, New Zealand.

Distribution of germanium between phenocrysts and melt in peralkaline rhyolites from the Kenya Rift Valley

Abstract Germanium abundances, determined by laser ablation-inductively coupled plasma-mass spectrometry, are presented for phenocrysts and glass matrices from a metaluminous trachyte and four peralkaline rhyolites from the Greater Olkaria Volcanic Complex, Kenya Rift Valley, Africa. Abundances (in ppm) are: sanidine 0.45–0.61; fayalite 4.8–11.7; hedenbergite 5.1–9.0; titanomagnetite 2.7; ilmenite 0.48; amphibole 8.3–8.9; biotite 7.0; chevkinite-(Ce) 309; trachyte glass 3.0; rhyolitic glasses 2.3–3.9. These values are generally greater than those recorded for silicic rocks in the literature, whilst the chevkinite-(Ce) value is the largest yet found in a magmatic mineral. Apparent partition coefficients range from 0.15–0.26 in sanidine to 124 in chevkinite-(Ce). Those for fayalite and hedenbergite increase with whole-rock peralkalinity and Fe content. The possibility of a role for accessory phases in influencing Ge distribution in rock-forming minerals is also raised.

Silicic volcanism at Ljósufjöll, Iceland: Insights into evolution and eruptive history from Ar–Ar dating

Ljósufjöll volcano is the largest outcrop of silicic volcanic material in the volcanic Snaefellsnes flank (non-rifting) zone of Iceland. The silicic eruptives range from trachytes to alkaline and peralkaline (comenditic) rhyolites and show evidence for eruption in both subaerial and subglacial environments. Thirteen silicic eruptive units have been identified and mapped by a combination of field observations and geochemical correlation. The trachytes probably formed by fractional crystallisation of a basaltic magma to form an alkali feldspar-rich trachytic mush, with continued fractionation of the interstitial melt producing low Ba and Sr rhyolite magma. Ar–Ar dating of feldspars and matrix (glass or holocrystalline groundmass) has been carried out on twelve of the units. Many units have a complex Ar-isotopic system, with many samples showing evidence for inherited 40Ar in the form of feldspar xenocrysts. The deduced eruption ages span an age range from < 129ka to > 650ka. The eruptive history of Ljósufjöll probably extends further back than this but the products of previous eruptions have since been removed by erosion or buried. A syenite xenolith was dated at 1.1Ma, indicating that silicic, alkaline magmatism has been occurring at Ljósufjöll for over one million years. Eruptions of silicic material at Ljósufjöll appear to be more common during times of rapid climate change and fluctuating ice volume.

Géochimie et géochronologie des laves felsiques des monts Bamenda (ligne volcanique du Cameroun)

The felsic lavas of the Bamenda Mountains, in the main part of the Cameroon Volcanic Line, are mainly represented by trachytes, with subordinated benmoreites and alkaline to peralkaline rhyolites. New K–Ar geochronological data define two main volcanic episodes, the first one between 18 and 22 Ma, and the second one from 12.5 to 13.5 Ma. Geochemical data indicate that these felsic rocks mainly originated through a fractional crystallization process from mafic magmas. Crustal contamination also occurred during the magma evolution. These new data confirm that there is no time evolution of the volcanism along the Cameroon Volcanic Line and show a similar magma genesis process through space and time.

Physical volcanology and geochemistry of a Late Archaean volcanic arc: Kurnalpi and Gindalbie Terranes, Eastern Goldfields Superterrane, Western Australia

Late Archaean (2715–2704 Ma) calc-alkaline volcanic complexes in the Kurnalpi Terrane are identical in facies and geochemistry to modern intra-arc volcanic complexes. Preserved volcanic rocks and associated feldspathic sedimentary rocks represent proximal to medial submarine facies of submarine to locally emergent intra-oceanic arc-related volcanoes. The least-altered volcanic rocks are dominantly andesite but range from basalt and basaltic andesite to rare rhyolite, and are the product of magmas similar in composition to those of modern arcs. Intermediate magmas were most likely derived from large ion lithophile element (LILE) enriched mafic magmas by fractional crystallisation (with or without assimilation) processes. The Kurnalpi Terrane thus provides an example of a Late Archaean volcanic arc in which hydrated mantle wedge melting was the main mechanism for magma genesis. TTD-suite volcanism derived from slab melting was either absent or played a subordinate role. The adjacent Gindalbie Terrane contains younger (∼2692–2680 Ma) bimodal (basalt–rhyolite) volcanic complexes, with associated quartz-rich sedimentary rocks, and calc-alkaline intermediate-silicic volcanic complexes. Volcanosedimentary basement to the Gindalbie volcanic sequences is deeply eroded equivalents of the calc-alkaline volcanic complexes, their associated sedimentary rocks and younger basaltic sequences found in the Kurnalpi Terrane. Gindalbie basalt–rhyolite successions locally contain volcanic-hosted massive Cu–Zn mineral deposits. Rhyolites derived by low-pressure fractional crystallisation, or melting, of andesites are more abundant than in the Kurnalpi volcanic complexes. Facies associations and the local occurrence of distinctive incompatible element-enriched, mildly peralkaline (A-type) rhyolites are consistent with rifting of a mature arc system. The two terranes most likely represent volcanic evolution of the same Late Archaean volcanic arc but are different tectonic fragments that were welded together by ∼2660 Ma.

Trachytes, comendites, and pantellerites of the Late Paleozoic bimodal rift association of the Noen and Tost ranges, southern Mongolia: Differentiation and contamination of peralkaline salic melts

The bimodal association of the Noen and Tost ranges is ascribed to the Gobi-Tien Shan rift zone and was formed 318 Ma ago at the continental margin of the North Asian paleocontinent. It is made up of volcanic series of alternating basalts and peralkaline rhyolites with subordinate trachytes, dike belts, and massifs of peralkaline granites. The association also includes a coeval massif of biotite granites. Based on Al2O3 and FeOtot contents, the peralkaline rhyolites are subdivided into comendites (FeOtot 1.5–5.7 wt %, Al2O3 10.5–15.4 wt %) and pantellerites (FeOtot 5.2–7.5 wt %, Al2O3 9.1–10.2 wt %). The peralkaline salic rocks of the bimodal association were formed by the crystallization differentiation of rift basaltic magmas combined with crustal assimilation. The comendites, pantellerites, and peralkaline granites inherited negative Nb and Ta and positive K and Pb anomalies from basalts. They are also similar to basalts in Nd isotope composition (ɛNd(T) = 5.5–7.4) and have nearly mantle oxygen isotope composition (δ18O = 5.9–7.3‰). The most differentiated and least contaminated rocks of the bimodal series of the Noen and Tost ranges are pantellerites. Calculations indicate that the fraction of the residual pantellerite melt was 8% or less of the parental basaltic magma. The comendites were derived from peralkaline salic melts by the assimilation of anatectic crustal melts compositionally similar to biotite granites. The formation of the latter within the Noen and Tost ranges is explained by the specific geodynamic position of the Gobi-Tien Shan rift zone, which was formed near a paleocontinental margin that evolved in an active margin regime shortly before the beginning of rifting.

Crustal thermal state and origin of silicic magma in Iceland: the case of Torfajökull, Ljósufjöll and Snæfellsjökull volcanoes

Pleistocene and Holocene peralkaline rhyolites from Torfajokull (South Iceland Volcanic Zone) and Ljosufjoll central volcanoes and trachytes from Snaefellsjokull (Snaefellsnes Volcanic Zone) allow the assessment of the mechanism for silicic magma genesis as a function of geographical location and crustal geothermal gradient. The low δ18O (2.4‰) and low Sr concentration (12.2 ppm) measured in Torfajokull rhyolites are best explained by partial melting of hydrated metabasaltic crust followed by major fractionation of feldspar. In contrast, very high 87Sr/86Sr (0.70473) and low Ba (8.7 ppm) and Sr (1.2 ppm) concentrations measured in Ljosufjoll silicic lavas are best explained by fractional crystallisation and subsequent 87Rb decay. Snaefellsjokull trachytes are also generated by fractional crystallisation, with less than 10% crustal assimilation, as inferred from their δ18O. The fact that silicic magmas within, or close to, the rift zone are principally generated by crustal melting whereas those from off-rift zones are better explained by fractional crystallisation clearly illustrates the controlling influence of the thermal state of the crust on silicic magma genesis in Iceland.

Pleistocene rhyolitic volcanism at Torfajökull, Iceland: eruption ages, glaciovolcanism and geochemical evolution

The Torfajokull central volcano lies in Iceland’s southern flank zone (a non-rifting zone) and last erupted in the 15th century. Peralkaline rhyolites from its pre-Holocene formations have been dated by the Ar-Ar method. Ages from 67±9 ka to 384±20 ka indicate Pleistocene eruptions, with the oldest age (384 ka) also being from the most evolved rhyolite (a pantellerite). The oldest age indicates that a mature and evolved magmatic-volcanic system was well established by the mid-Pleistocene and that the central volcano has been active for at least 400 ka. Good correlation is found between the Ar-Ar ages of sustained rhyolite eruptions into ice sheets (i.e. rhyolite tuya formation) and oxygen isotope stages dominated by cold conditions. This is the first stage of developing a new proxy that uses rhyolitic glaciovolcanic edifices to provide estimates of past Icelandic ice sheet thicknesses. The geochemistry of the dated samples corroborates earlier work showing a simple but enigmatic trend of steadily-decreasing alkalinity towards the present (i.e. older rocks are more evolved). The new ages reveal a hitherto-unrecognised drop in rhyolite alkalinity after 83 ka, which may be linked to the evacuation of c. 16 km3 of rhyolite during a subglacial eruption into the last (Weichselian) ice sheet, for which two new and overlapping Ar-Ar ages of 67±9 ka and 72±7 ka have been obtained. This rhyolite eruption, which is the largest known from Torfajokull, heralded a major change in the magma system as all subsequent eruptions are of small volume (<0.3 km3), dominated by subalkaline compositions, and characterised by interactions with mafic magmas. This change may be linked to lower rhyolite magma replenishment rates and/or to the increasing influence of rift zone volcano-tectonics.

Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions

Many basaltic flood provinces are characterized by the existence of voluminous amounts of silicic magmas, yet the role of the silicic component in sulphur emissions associated with trap activity remains poorly known. We have performed experiments and theoretical calculations to address this issue. The melt sulphur content and fluid/melt partitioning at saturation with either sulphide or sulphate or both have been experimentally determined in three peralkaline rhyolites, which are a major component of some flood provinces. Experiments were performed at 150MPa, 800–900 � C, fO2 in the range NNO – 2 to NNO þ 3 and under water-rich conditions. The sulphur content is strongly dependent on the peralkalinity of the melt, in addition to fO2, and reaches 1000ppm at NNO þ 1 in the most strongly peralkaline composition at 800 � C. At all values of fO2, peralkaline melts can carry 5–20 times more sulphur than their metaluminous equivalents. Mildly peralkaline compositions show little variation in fluid/melt sulphur partitioning with changing fO2 (DS � 270). In the most peralkaline melt, DS rises sharply at fO2 > NNO þ 1 to values of >500. The partition coefficient increases steadily for Sbulk between 1 and 6wt % but remains about constant for Sbulk between 0� 5 and 1wt %. At bulk sulphur contents lower than 4wt %, a temperature increase from 800 to 900 � C decreases DS by � 10%. These results, along with (1) thermodynamic calculations on the behaviour of sulphur during the crystallization of basalt and partial melting of the crust and (2) recent experimental constraints on sulphur solubility in metaluminous rhyolites, show that basalt fractionation can produce rhyolitic magmas having much more sulphur than rhyolites derived from crustal anatexis. In particular, hot and dry metaluminous silicic magmas produced by melting of dehydrated lower crust are virtually devoid of sulphur. In contrast, peralkaline rhyolites formed by crystal fractionation of alkali basalt can concentrate up to 90% of the original sulphur content of the parental magmas, especially when the basalt is CO2-rich. On this basis, we estimate the amounts of sulphur potentially released to the atmosphere by the silicic component of flood eruptive sequences. The peralkaline Ethiopian and Deccan rhyolites could have produced � 10 17 and � 10 18 g of S, respectively, which are comparable amounts to published estimates for the basaltic activity of each province. In contrast, despite similar erupted volumes, the metaluminous Parana´–Etendeka silicic eruptives could have injected only 4� 6 · 10 15 g of S in the atmosphere. Peralkaline flood sequences may thus have greater environmental effects than those of metaluminous affinity, in agreement with evidence available from mass extinctions and oceanic anoxic events.

Quantification of dissolved H 2O in silicate glasses using confocal microRaman spectroscopy

Application of confocal microRaman spectroscopy for quantification of total water content and water speciation was tested for hydrous glasses of various compositions (haplogranite, albite, tonalite, peralkaline rhyolite, dacite, andesite, basalt). Glasses with water contents between 0.5 and 11 wt.% were synthesized in internally heated gas pressure vessels. Total water contents of the glasses were measured by Karl–Fischer titration and IR spectroscopy. To quantify the total water content ( C H 2O t ) by Raman spectroscopy either the integrated intensity of the OH stretching vibration band at 3550 cm − 1 ( A ⁎ 3550) was used directly or A ⁎ 3550 was scaled to low wavenumber Raman bands. A very high reproducibility of A ⁎ 3550 could be achieved with a stand alone Raman microscope LabRam 010. A single regression line reproduces the data of all samples with a 2 σ variation of 1.2 wt.%. However, slightly different trends are observed for polymerized compositions (haplogranite, albite, tonalite), intermediate compositions (dacite, andesite) and depolymerized compositions (basalt) in plots of A ⁎ 3550 vs. water content, implying that the Raman cross section of dissolved hydrous species depends on glass composition. In measurements with a Raman spectrometer T64000, A ⁎ 3550 depends largely on measurement conditions. A good reproducibility could be achieved only by normalizing the OH band intensity with low wavenumber bands. Such an approach has the advantage to eliminate at least partially effects of specific measurement conditions and, hence, opening the possibility to apply the Raman calibration to another spectrometer. For silicic glasses scaling to the area of T–O– T bending band near 500 cm − 1 ( A ⁎ 500) yields the best results. Data for albite, haplogranite and dacite (11 samples, 37 spectra) covering a range of 0.8–7.3 wt.% H 2O are reproduced with a 2 σ standard deviation of 0.16 wt.% H 2O by C H 2 O t ( wt. % ) = 0.28 + 5.135 · A 3550 ⁎ A 500 ⁎ For intermediate and depolymerized compositions the T–O stretching band near 1000 cm − 1 is more intensive than the T–O– T bending band near 500 cm − 1 and, hence, it is more convenient for calibration. Data for andesite and basalt (14 samples, 22 spectra) covering a range of 0.5 to 4.7 wt.% H 2O are reproduced with a 2 σ stand deviation of 0.18 wt.% H 2O by C H 2 O t ( wt. % ) = 3.66 · A 3550 ⁎ A 1000 ⁎ Attempts to determine the relative abundance of OH groups and H 2O molecules in the glasses by decomposing the OH stretching Raman band into 3–5 Gaussians are not consistent with water speciation data derived from near-infrared spectroscopy. We suggest that the intensity growth with water content in the low wavenumber region of the 3550 cm − 1 band reflects only enhanced hydrogen bonding but is not related to the relative abundance of water species.

Fluorite solubility in hydrous haplogranitic melts at 100 MPa

Fluorite is the most common fluoride mineral in magmatic silicic systems and its crystallization can moderate or buffer fluorine concentrations in these settings. We have experimentally determined fluorite solubility and speciation mechanisms in haplogranitic melts at 800–950 °C, 100 MPa and aqueous-fluid saturation. The starting haplogranite compositions: peraluminous (alumina saturation index, ASI = 1.2), subaluminous (ASI = 1.0) and peralkaline (ASI = 0.8) were variably doped with CaO or F 2O −1 in the form of stoichiometric mineral or glass mixtures. The solubility of fluorite along the fluorite–hydrous haplogranite binaries is low: 1.054 ± 0.085 wt.% CaF 2 (peralkaline), 0.822 ± 0.076 wt.% (subaluminous) and 1.92 ± 0.15 wt.% (peraluminous) at 800 °C, 100 MPa and 10 wt.% H 2O, and exhibits a minimum at ASI ∼ 1. Fluorite saturation isotherms are strongly hyperbolic in the CaO–F 2O −1 space, suggesting that fluorite saturation is controlled by the activity product of CaO and F 2O −1, i.e., these components are partially decoupled in the melt structure. The form of fluorite liquidus isotherms implies distinct roles of fluorite crystallization: in Ca-dominant systems, fluorite crystallization is controlled by the fluorine concentration in the melt only and remains nearly independent of calcium contents; in F-rich systems, the crystallization of fluorite is determined by CaO contents and it does not buffer fluorine concentration in the melt. The apparent equilibrium constant, K, for the equilibrium CaO + cF 2O −1 = CaF 2 (+ associates) is log K= − (2.449 ± 0.085)·Al 2O 3 exc + (4.902 ± 0.066); the reaction-stoichiometry parameter varies as follows: c= − (0.92 ± 0.11)·Al 2O 3 exc + (1.042 ± 0.084) at 800 °C, 100 MPa and fluid saturation where Al 2O 3 exc are molar percent alumina in excess over alkali oxides. The reaction stoichiometry, c, changes at subaluminous composition: in peralkaline melts, competition of other network modifiers for excess fluorine anions leads to the preferential alkali–F short-range order, whereas in peraluminous compositions, excess alumina associates with calcium cations to form calcioaluminate tetrahedra. The temperature dependence of fluorite solubility is described by the binary symmetric Margules parameter, W = 36.0 ± 1.4 kJ (peralkaline), 39.7 ± 0.5 kJ (subaluminous) and 32.8 ± 0.7 kJ (peraluminous). The strong positive deviations from ideal mixing imply the occurrence of CaF 2–granite liquid–liquid immiscibility at temperatures above 1258 °C, which is consistent with previous experimental data. These experimental results suggest very low solubilities of fluorite in Ca-rich melts, consistent with the lack of fluorine enrichment in peralkaline rhyolites and calc-alkaline batholiths. On the other hand, high CaO concentrations necessary to crystallize fluorite in F-rich peraluminous melts are not observed in nature and thus magmatic crystallization of fluorite in topaz-bearing silicic suites is suppressed. A procedure for calculating fluorite solubility and the liquidus isotherms for a whole-rock composition and temperature of interest is provided.

Geochronology and Petrogenesis of the Cretaceous Antampombato–Ambatovy Complex and Associated Dyke Swarm, Madagascar

The Antampombato–Ambatovy complex is the largest intrusion in the central–eastern part of the Cretaceous flood basalt province of Madagascar, with an exposed surface area of about 80 km2. It has an 40Ar/39Ar incremental heating age of 89·9 ± 0·4 Ma and a U–Pb age of 90 ± 2 Ma. The outcropping plutonic rocks range from dunite and wehrlite, through clinopyroxenite and gabbro, to sodic syenite. A dyke swarm cross-cutting some of the above lithologies (and the nearby Precambrian basement rocks) is formed of picritic basalts, alkali to transitional basalts, benmoreites and rhyolites; some of the latter are peralkaline. A few basaltic dykes have cumulate olivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereas others have characteristics more akin to those of primitive liquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200 ppm Ni). These basalts have relatively high TiO2 (2·2 wt %) and total iron (14 wt % as Fe2O3), and moderate contents of Nb (10–11 ppm) and Zr (c. 100 ppm). Initial (at 90 Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basalt dykes are 0·7030–0·7037 and 0·51290–0·51283, respectively. Syenites and peralkaline rhyolites have Sr- and Nd-isotope ratios of 0·7037–0·7039 and 0·51271–0·51274, respectively. The data suggest derivation of the parental magmas from a time-integrated depleted mantle source, combined with small amounts of crustal contamination in the petrogenesis of the more evolved magmas. The isotopic compositions of the mafic–ultramafic rocks are most similar to those of the mid-ocean ridge basalt (MORB)-like igneous rocks of eastern Madagascar, and suggest the existence of an isotopically ‘depleted’ component in the source of the entire Madagascar province, even though the Antampombato basalts are chemically unlike the lavas and dykes with the same depleted isotopic signature found in western Madagascar. If this depleted component is plume-related, this suggests that the plume has a broadly MORB-source mantle composition. The existence of isotopically more enriched magma types in the Madagascan province has several possible petrogenetic explanations, one of which could be the interaction of plume-related melts with the deep lithospheric mantle beneath the island.

Relationships between Mafic and Peralkaline Silicic Magmatism in Continental Rift Settings: a Petrological, Geochemical and Isotopic Study of the Gedemsa Volcano, Central Ethiopian Rift

Petrological and geochemical data are reported for basalts and silicic peralkaline rocks from the Quaternary Gedemsa volcano, northern Ethiopian rift, with the aim of discussing the petrogenesis of peralkaline magmas and the significance of the Daly Gap occurring at local and regional scales. Incompatible element vs incompatible element diagrams display smooth positive trends; the isotope ratios of the silicic rocks (Sr/Sr ˆ 0 70406---0 70719; Nd/Nd ˆ 0 51274---0 51279) encompass those of the mafic rocks. These data suggest a genetic link between rhyolites and basalts, but are not definitive in establishing whether silicic rocks are related to basalts through fractional crystallization or partial melting. Geochemical modelling of incompatible vs compatible elements excludes the possibility that peralkaline rhyolites are generated by melting of basaltic rocks, and indicates a derivation by fractional crystallization plus moderate assimilation of wall rocks (AFC) starting from trachytes; the latter have exceedingly low contents of compatible elements, which precludes a derivation by basalt melting. Continuous AFC from basalt to rhyolite, with small rates of crustal assimilation, best explains the geochemical data. This process generated a zoned magma chamber whose silicic upper part acted as a density filter for mafic magmas and was preferentially tapped; mafic magmas, ponding at the bottom, were erupted only during post-caldera stages, intensively mingled with silicic melts. The large number of caldera depressions found in the northern Ethiopian rift and their coincidence with zones of positive gravity anomalies suggest the occurrence of numerous magma chambers where evolutionary processes generated silicic peralkaline melts starting from mafic parental magmas. This suggests that the petrological and volcanological model proposed for Gedemsa may have regional significance, thus furnishing an explanation for the large-volume peralkaline ignimbrites in the Ethiopian rift.

Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley

Crystallization experiments on three comendites provide evidence for the genetic relationships between peralkaline rhyolites in the central Kenya rift valley. The crystallization of calcic clinopyroxene in slightly peralkaline rhyolites inhibits increase in peralkalinity by counteracting the effects of feldspar. Fractionation under high fO2 conditions produces residual liquids that are less, or only slightly more, peralkaline than the bulk composition. In contrast, crystallization under reduced conditions (<FMQ, where FMQ is the fayalite–magnetite–quartz buffer) and at high fF2 inhibits calcic clinopyroxene and yields residual liquids that are more peralkaline than coexisting alkali feldspar, whose subsequent crystallization increases the peralkalinity of the liquid. A marginally peralkaline rhyolite [molar (Na2O + K2O)/Al2O3 (NK/A) = 1·05] can yield a more typically comenditic rhyolite (NK/A = 1·28) after 95 wt % of crystallization. This comendite yields pantelleritic derivatives (NK/A >1·4) after 25 wt % crystallization. Upon further crystallization, extreme peralkaline compositions (NK/A ≤2·5) are obtained, with relatively low SiO2 (66 wt %) and Al2O3 (7·4 wt %), and high FeO (10·2 wt %) and Na2O (8·4 wt %) contents. In the absence of crystallization of sodic phases such as arfvedsonite or aegirine, fractionation may yield even more extreme compositions. Pantelleritic rhyolites can be produced at temperatures below 800°C, at low fO2, high fF2, by either extreme fractional crystallization or near-solidus melting of less peralkaline, but more silicic, sources.