Experimental evidence for polybaric differentiation of primitive arc basalt beneath St. Vincent, Lesser Antilles

Equilibrium crystallization experiments have been performed on a primitive high-MgO basalt (HMB) from Soufrière, St. Vincent, with three initial H2O contents (0·6, 2·3 and 4·5 wt %), at pressures of 0·4, 0·7, 1·0 and 1·3 GPa and temperatures from 1350 to 950°C. Redox conditions, as determined by µXANES analysis of Fe3+ in experimental glasses, were 1–4 log units above the nickel–nickel oxide (NNO) buffer. The aim of the study was to explore the differentiation conditions that gave rise to the observed geochemical variation in lavas and plutonic (cumulate) xenoliths from St. Vincent. An experiment with 4·5 wt % initial H2O is multiply saturated close to its liquidus (1180°C and 1·3 GPa) with a spinel lherzolite assemblage, which is consistent with a primary origin for HMB in the mantle wedge. Multiple saturation of HMB with 2·3 wt % H2O was not observed, but is inferred to occur at pressures >1·3 GPa. The experimental results show that initial H2O content has significant influence on differentiation paths of primary HMB magma, with different lava varieties generated under discrete, well-constrained P–T–H2O conditions. Low-magnesian basalts (LMB) can be generated from HMB with 2·3–4·5 wt % H2O at pressures of 1·0–1·3 GPa, corresponding to Moho depths beneath St. Vincent. The CaO contents of LMB are sensitive to differentiation pressure: high-CaO LMB are produced at pressures >0·5 GPa. Basaltic andesites (BA) can be generated at 0·7–1·0 GPa from HMB with 0·6–2·3 wt % H2O. High-alumina basalts (HAB) are produced at mid- to upper-crustal conditions (≤0·4 GPa) by differentiation of HMB with high initial H2O (≥4 wt %) through delay of plagioclase crystallization and dominant fractionation of olivine, clinopyroxene and spinel. St. Vincent andesites could be produced from relatively dry (≤0·6 wt % H2O) HMB only at lower-crustal conditions. This is suggestive of a partial melting origin from precursor HMB that had solidified at depth to produce gabbros with ∼30% hornblende (i.e. ∼0·6 wt % structurally bound H2O). The experimentally determined differentiation conditions are consistent with polybaric differentiation within a hot zone that extends from the Moho and uppermost mantle to the mid- or upper crust. Within the hot zone differentiation occurs by a combination of crystallization of HMB with 2–5 wt % H2O and partial melting of ancestral HMB gabbros. Although the experimental melts provide an excellent match to erupted lava compositions, experimental crystal compositions do not match either phenocrysts or cumulate crystals, as preserved in xenoliths. The failure to reproduce natural crystal compositions suggests that these are formed as differentiated magmas ascend and attain their H2O-saturated liquidi at shallower pressures. Thus there is a disconnect between the high-pressure phase compositions and assemblages that generate liquid compositional diversity and the low-pressure composition and assemblages that occur as phenocrysts and in cumulate xenoliths. This finding lends support to the idea of cryptic fractionation in the generation of arc magmas

Magnetite-bubble aggregates at mixing interfaces in andesite magma bodies

Magnetite is a particularly favourable site for heterogeneous bubble nucleation in magma and yet only very rarely is evidence for this preserved due to the myriad of processes that act to overprint such an association. The possibility of bubble-magnetite aggregates in magmas carries with it interesting implications for the fluid mechanics of magma bodies and for the magma mixing process responsible for the formation of andesites. We use image analysis and statistical methods to illustrate a spatial association between magnetite and bubbles in mafic enclaves. There is a large range in magnetite contents in the enclaves (up to 7.5%) which is related to the porosity of the enclaves, indicating a mechanism of enrichment of the mafic magma in magnetite. In the andesite there is no spatial association between bubbles and magnetite and the magnetite content of the andesite is small. We suggest a mechanism for enclave formation whereby in vapour-saturated magma, bubbles nucleate on magnetite. Upon intrusion into the base of an andesite magma body, these bubble-magnetite aggregates rise and ‘sweep up’ other magnetites, resulting in the accumulation of aggregates at the magma interface. Instabilities lead to the flotation of enclaves, characterized by enrichment in magnetite and bubbles

Toward an understanding of disequilibrium dihedral angles in mafic rocks

[1] The median dihedral angle at clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, Θcpp, is generally lower than equilibrium (109° ± 2°). Observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of Θcpp is incorrect in several important respects. First, the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three-grain junctions, and hence Θcpp, is formed during solidification (the igneous process): sub-solidus textural modification in most dolerites and gabbros, previously thought to be the dominant control on Θcpp, is insignificant. Θcpp is governed by mass transport constraints, the inhibiting effects of small pore size on crystallization, and variation in relative growth rates of pyroxene and plagioclase. During rapid cooling, pyroxene preferentially fills wider pores while the narrower pores remain melt-filled, resulting in an initial value of Θcpp of 78°, rather than 60° which would be expected if all melt-filled pores were filled with pyroxene. Lower cooling rates create a higher initial Θcpp due to changes in relative growth rates of the two minerals at the nascent three-grain junction. Low Θcpp (associated with cuspate clinopyroxene grains at triple junctions) can also be diagnostic of infiltration of previously melt-free rocks by late-stage evolved liquids (the metasomatic process). Modification of Θcpp by sub-solidus textural equilibration (the metamorphic process) is only important for fine-grained mafic rocks such as chilled margins and intraplutonic chill zones. In coarse-grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centers of oikocrysts, associated with rounding of chadacrysts

First measurements of OH-C exchange and temperature-dependent partitioning of OH and halogens in the system apatite - silicate melt

We present the first integrated study of carbonate, hydroxyl, fluoride, and chloride ion partitioning in the apatite-melt system. We determined volatile partitioning behavior between apatite and silicate melt for both haplobasaltic andesite and trachyte bulk compositions at 0.5–1 GPa and 1250°C using the piston-cylinder apparatus. All volatile species were analyzed directly in both apatite and glass using secondary ion mass spectrometry (SIMS) and electron probe microanalysis. Distribution coefficients for OH-halogen exchange are similar to those from previous studies, and together with literature data, reveal a significant log-linear relationship with temperature, while the effects of pressure and melt composition are minimal. Meanwhile, halogen-free experiments generate very high C contents (up to 5000 ppm) in apatite. Stoichiometry calculations and infrared spectra indicate that this C is mainly incorporated onto the channel volatile site together with hydroxyl. In halogen-bearing experiments, apatite crystals contain significantly lower C (≤500 ppm), which may be partly incorporated onto the phosphate site while the channel volatile site is filled by OH+F+Cl+C. Our experiments give the first constraints on H2O-CO2 exchange between apatite and silicate melt, with a KD of 0.355 ± 0.05 for the trachyte and 0.629 ± 0.08 for the haplobasaltic andesite. The new constraints on the temperature-dependence of partitioning will enable quantitative modeling of apatite-volatile exchange in igneous systems, while this new partitioning data and method for direct, in situ analysis of C in apatite mark a significant advance that will permit future studies of magmatic C and other volatiles. This has a broad range of potential applications including magmatic differentiation, fractionation, and degassing; quantification of volatile budgets in extraterrestrial and deep earth environments; and mineralization processes

Extensive, water-rich magma reservoir beneath southern Montserrat

South Soufrière Hills and Soufrière Hills volcanoes are 2 km apart at the southern end of the island of Montserrat, West Indies. Their magmas are distinct geochemically, despite these volcanoes having been active contemporaneously at 131–129 ka. We use the water content of pyroxenes and melt inclusion data to reconstruct the bulk water contents of magmas and their depth of storage prior to eruption. Pyroxenes contain up to 281 ppm H2O, with significant variability between crystals and from core to rim in individual crystals. The Al content of the enstatites from Soufrière Hills Volcano (SHV) is used to constrain melt-pyroxene partitioning for H2O. The SHV enstatite cores record melt water contents of 6–9 wt%. Pyroxene and melt inclusion water concentration pairs from South Soufriere Hills basalts independently constrain pyroxene-melt partitioning of water and produces a comparable range in melt water concentrations. Melt inclusions recorded in plagioclase and in pyroxene contain up to 6.3 wt% H2O. When combined with realistic melt CO2 contents, the depth of magma storage for both volcanoes ranges from 5 to 16 km. The data are consistent with a vertically protracted crystal mush in the upper crust beneath the southern part of Montserrat which contains heterogeneous bodies of eruptible magma. The high water contents of the magmas suggest that they contain a high proportion of exsolved fluids, which has implications for the rheology of the mush and timescales for mush reorganisation prior to eruption. A depletion in water in the outer 50–100 μm of a subset of pyroxenes from pumices from a Vulcanian explosion at Soufrière Hills in 2003 is consistent with diffusive loss of hydrogen during magma ascent over 5–13 h. These timescales are similar to the mean time periods between explosions in 1997 and in 2003, raising the possibility that the driving force for this repetitive explosive behaviour lies not in the shallow system, but in the deeper parts of a vertically protracted crustal magma storage system

Petrological and geochemical variation during the Soufrière Hills eruption, 1995 to 2010

The andesite lava erupted at the Soufrière Hills Volcano (SHV) is crystal-rich with 33–63% phenocrysts of plagioclase (65%), amphibole (28%), orthopyroxene (7%), and minor Fe–Ti oxide and clinopyroxene microphenocrysts. The andesite hosts mafic enclaves that have similar mineral phases to the andesite. The enclaves are generally crystal-poor but can have up to 27% of inherited phenocrysts from the andesite, the majority of which are plagioclase. The eruption is defined by discrete periods of extrusion called phases, separated by pauses. The enclaves exhibit bulk geochemical trends that are consistent with fractionation. We infer that the intruded mafic liquids of Phases I and II interacted and assimilated plutonic residue remaining from the multiple prior mafic intrusions, while the basaltic liquids from Phases III and V assimilated relatively little material. We also infer a change in the basaltic composition coming from depth. The bulk Fe contents of both magma types are coupled and they both show a systematic interphase variation in Fe content. We interpret the coupled Fe variation to be due to contamination of the andesite from the intruding basalt via diffusion and advection processes, resulting in the erupted andesite products bearing the geochemical imprint of the syn-eruptive enclaves

Banding in the Margins of Basaltic Dykes Indicates Pulsatory Propagation During Emplacement

Basaltic fissure eruptions, which are the most common type of eruption on Earth, are fed by dykes which mediate magma transport through the crust. Dyke propagation processes are important because they determine the geometry of the transport pathway and the nature of any geophysical signals associated with magma ascent. Here, we investigate small-scale (mm–cm wide) banding features at the margins of dykes in the Teno Massif (Tenerife, Spain) and the Columbia River Basalt Province (CRBP) (USA). Similar marginal bands have been reported for dykes in numerous localities around the world. Dyke margins record valuable information about propagation because they are the first material to solidify against the host rock at the propagating dyke tip. We find that the marginal bands are defined by cyclic variations in phenocryst concentration and vesicularity, and we infer that these cyclic variations in texture are a product of cyclic variations in magma flow rates and pressures within the dyke tip. This indicates that dyke emplacement occurs in pulses, with propagation repeatedly hindered by the rapid cooling and solidification of magma in the narrow dyke tip. Using a 1D conduction model, we estimate the time taken for each band to cool and solidify, which provides a timescale of several minutes to tens of minutes for the pulses. The occurrence of similar bands in various volcanic settings suggests that pulsatory propagation is a common, if not ubiquitous, process associated with dyke emplacement

Silicate liquid immiscibility within the crystal mush: Evidence from Ti in plagioclase from the Skaergaard Intrusion.

A key target in the study of layered intrusions is to constrain the liquid line of descent of the magma. However, the evolution of the interstitial liquid is rarely considered, and its liquid line of descent is often assumed to be equivalent to that of the bulk magma. Because of extensive sub-solidus and diffusional changes that occur in slowly cooled rocks, clues to the composition of the interstitial liquid can only be obtained using very slowly diffusing trace elements and components. This study uses the Ti concentrations and anorthite contents of interstitial plagioclase to consider the compositional evolution of the interstitial liquid in the Skaergaard Intrusion. Ti–XAn zoning of interstitial plagioclase does not follow the same cryptic variations that develop in plagioclase primocrysts as a function of stratigraphic height, demonstrating that the bulk and interstitial liquid lines of descent are not equivalent. After Fe–Ti oxides start to crystallize, Ti concentrations decrease in both primocryst and interstitial plagioclase as a result of decreasing melt Ti. However, in the interstitial plagioclase within a single thin section, divergent trends develop adjacent to fine-grained interstitial pockets containing diverse mineral assemblages, which are interpreted to represent the crystallized products of late-stage immiscible liquids. These trends vary systematically as a function of stratigraphic height and spatial location within the intrusion. The distribution and compositions of these plagioclase zoning trends are used to comment on the spatial distribution and differential movement of interstitial immiscible liquids within the intrusion