ALBERT

Description: Olivine textures are potentially important recorders of olivine origin and crystallization conditions. Primary magmatic and xenocrystic origins are commonly considered for olivine from ultramafic to intermediate magmas, whereas secondary olivine origins (i.e. crystals formed by recrystallization or peritectic reaction) are rarely considered in the interpretation of magmatic phenocrysts. The main aim of our study was to determine textures that are characteristic for secondary magmatic olivine and non-characteristic, or at least rare, for primary magmatic olivine. To characterize the textures of the different olivine types, we review previous experimental work and present new textural data for olivine from four melanorite melting and eight basalt crystallization experiments. We qualitatively characterize olivine textures using transmitted light and back-scattered electron microscopy, and semi-quantitatively characterize the 2D surface area of olivine branches and single crystals, their 2D and calculated 3D shapes, and 2D grain boundary segment lengths. Olivine recrystallization yields crystals with dendritic branches, whereas peritectic reaction produces olivine clusters with randomly oriented crystals. In agreement with previous studies, we find that olivine crystal and branch size, grain boundary segmentation, and inclusion relations cannot unequivocally distinguish between olivine of secondary and primary origins. However, recrystallized olivine typically has short prismatic branches, whereas primary magmatic olivine dendrites commonly have elongated branches. The peritectic crystals are closely comparable with glomerocrysts and single polyhedral primary magmatic crystals, but they commonly form small groups of touching crystals and clusters with crystal-poor cores. Spatial association and a comparison with xenocryst textures may further guide the interpretation of their origin, but detailed analyses of core compositions and zoning patterns appear necessary to firmly distinguish peritectic from primary magmatic crystals. Our comparison of experimental and natural samples suggests that dendritic olivine crystals and clusters of polyhedral olivine that commonly occur in mafic and ultramafic igneous rocks should be evaluated for possible primary as well as secondary magmatic origins.

Description: We present the results of phase equilibrium experiments carried out on basanite and phonotephrite lavas from Ross Island, Antarctica. Experiments were designed to reproduce the P–T–X–f O 2 conditions of deep and intermediate magma storage and to place constraints on the differentiation of each of the two predominant lava suites on the island, which are thought to be derived from a common parent melt. The Erebus Lineage (EL) consists of lava erupted from the Erebus summit and the Dry Valley Drilling Project (DVDP) lineage is represented by lavas sampled by drill core on Hut Point Peninsula. Experiments were performed in internally heated pressure vessels over a range of temperatures (1000–1150°C) and pressures (200–400 MPa), under oxidized conditions (NNO to NNO + 3, where NNO is the nickel–nickel oxide buffer), with X H2O of the H 2 O–CO 2 mixture added to the experimental capsule varying between zero and unity. The overall mineralogy and mineral compositions of the natural lavas were reproduced, suggesting oxidizing conditions for the deep magma plumbing system, in marked contrast to the reducing conditions (QFM to QFM – 1, where QFM is the quartz–fayalite–magnetite buffer) in the Erebus lava lake. In basanite, crystallization of spinel is followed by olivine and clinopyroxene; olivine is replaced by kaersutitic amphibole below ~1050°C at intermediate water contents. In phonotephrite, the liquidus phase is kaersutite except in runs with low water content ( X H 2 O fluid 〈 0·2) where it is replaced by clinopyroxene. Experimental kaersutite compositions suggest that the amphibole-bearing DVDP lavas differentiated below 1050°C at 200–400 MPa and NNO + 1·5 to NNO + 2. Olivine- and clinopyroxene-bearing EL lavas are consistent with experiments performed above 1050°C and pressures around 200 MPa. The plagioclase liquidus at 〈1–2 wt % H 2 O suggests extremely dry conditions for both lineages ( X H 2 O fluid approaching zero for EL, ~0·25 for DVDP), probably facilitated by dehydration induced by a CO 2 -rich fluid phase. Our results agree with previous studies that suggest a single plumbing system beneath Ross Island in which DVDP lavas (and probably other peripheral volcanic products) were erupted through radial fractures associated with the ascent of parental magma into the lower crust. The longer travel time of the DVDP lavas through the crust owing to lateral movement along fractures and the lack of a direct, sustained connection to the continuous CO 2 -rich gas flux that characterizes the main central Erebus conduit is probably responsible for the lower temperatures and slightly wetter conditions and hence the change in mineralogy observed.

Description: A rich history of experimental petrology has revealed the paths by which silicic igneous rocks follow mineral–melt equilibria during differentiation. Subdividing these rocks by ‘molar Al versus Ca + Na + K’ illustrates first-order differences in mineralogy and gives insight into formation mechanisms. Peraluminous magmas, formed by partial melting of sediments, largely owe their attributes and compositions to melting reactions in the protoliths, whereas most metaluminous felsic magmas record both continental and mantle inputs. Peralkaline rhyolites are mainly derived from either protracted crystallization or small degrees of partial melting of basalt, with only a marginal crustal contribution. Most silicic magmas hold 3–7 wt% H 2 O melt , which is inversely correlated with pre-eruptive temperature (700 °C to 〉950 °C) but unrelated to their reduced/oxidized state.

Description: Petrological data have been acquired on natural trachytes from the Chaîne des Puys, French Massif Central, and on experimental products from phase equilibria to (1) constrain the storage conditions of trachytic magmas that lead to explosive eruptions (dome destructions as block-and-ash flows or pumice-and-ash flows) and (2) provide phase relationships and chemical compositions for differentiated alkaline liquids in an intraplate continental context. Phase assemblages, proportions, and compositions have been determined on six trachytes with SiO 2 contents varying from 62 to 69 wt % and alkali contents of 10·5–12·0 wt %. The samples contain up to 30% of phenocrysts, mainly consisting of feldspar (15–17%; plagioclase and/or alkali-feldspar), biotite (2–6%; except in the most SiO 2 -poor sample), Fe–Ti oxides (1–3%) ± amphibole (〈5%), ± clinopyroxene (~1%). All samples have apatite and zircon as minor phases and titanite was found in one sample. Pristine glasses (melt inclusions or residual glasses) in pumice from explosive events are trachytic to rhyolitic (65–73 wt % SiO 2 and 10·5–13·0 wt % alkali). H 2 O dissolved in melt inclusions and the biotite + alkali feldspar + magnetite hygrobarometer both suggest pre-eruptive H 2 O contents up to 8 wt %. These are so far the highest H 2 O contents ever reported for alkaline liquids in an intraplate continental context. Melt inclusions also contain ~3400 ppm chlorine, ~700 ppm fluorine, and ~300 ppm sulphur. Crystallization experiments on the six trachytes were performed between 200 and 400 MPa, 700 and 900°C, at H 2 O saturation, and oxygen fugacity of NNO ± 1. A comparison between the natural and experimental phase assemblage, proportions, and composition suggests magma storage conditions at a pressure of 300–350 MPa (~10–12 km deep), melt H 2 O content ~8 wt % (close to saturation), an oxygen fugacity close to NNO ~0·5, and temperatures increasing from 700 to 825°C with decreasing bulk SiO 2 content of the trachyte. The high H 2 O contents of the trachytes show that wet conditions may prevail during the differentiation of continental alkaline series. Regardless of the size of the magma reservoir assumed to have fed the trachyte eruptions, calculation of thermal relaxation timescales indicates that the tapped magma reservoir(s) are likely to be still partially molten. The four northernmost edifices may correspond to a single large reservoir with a lateral extent of up to 10 km, which could be possibly reactivated in weeks to months if intercepted by new rising mafic magma batches.

Description: Field observations and petrological studies have recently advanced understanding of the magmatic system of Erebus volcano, renowned for its sustained CO 2 -rich degassing, and long-lived phonolitic lava lake. However, this body of work has highlighted uncertainty in several key parameters, including the magma temperature, redox state and the depth of the reservoir presumed to maintain the lava lake. Here, we use experimentally determined phase equilibria to constrain these unknowns. The experiments ranged in temperature from 900 to 1025°C, in pressure from atmospheric to 300 MPa, in water content from 0 to 8 wt %, and in oxygen fugacity from NNO + 4 (where NNO is nickel–nickel oxide) to QFM – 2 (where QFM is quartz–fayalite–magnetite). The natural system was experimentally reproduced at 950 ± 25°C, a pressure below 200 MPa, redox conditions between QFM and QFM – 1, and remarkably low water contents of less than 0·5 wt %. These findings help in understanding petrological observations, including melt inclusion data, as well as the measured composition of gas emissions from the lava lake. Biotite and amphibole appear in the crystallization sequence at around 925°C, even under very dry conditions (biotite). Both biotite and amphibole are absent in the phonolites erupted over the last 20 kyr at Erebus. The constant abundance of anorthoclase observed in the erupted lavas and bombs indicates that the shallow magmatic system feeding the Erebus lava lake (below pressures of 200 MPa) has been thermally buffered at 950 ± 25°C over this time period, possibly reflecting steady-state connection with the deep feeding system rooted in the mantle. Combined with recent seismological data, our results suggest that if a large phonolitic reservoir exists, then it should lie in the depth range 4–7·5 km. The tight constraints on temperature and redox conditions will be valuable for future thermodynamical and rheological modelling.

Description: The intensive variables of dacitic–rhyodacitic magmas prior to four large Plinian eruptions of Santorini Volcano over the last 200 kyr (Minoan, Cape Riva, Lower Pumice 2 and Lower Pumice 1) were determined by combining crystallization experiments with study of the natural products, including the volatile contents of melt inclusions trapped in phenocrysts. Phase equilibria of the silicic magmas were determined at pressures of 1, 2 and 4 kbar, temperatures of 850–900°C, fluid (H 2 O + CO 2 )-saturation, X H 2 O [= molar H 2 O/(H 2 O + CO 2 )] between 0·6 and 1 (melt H 2 O contents of 2–10 wt %), and redox conditions of FMQ (fayalite–magnetite–quartz buffer) or NNO + 1 (where NNO is Ni–NiO buffer). Experiments were generally successful in reproducing the phenocryst assemblage of the natural products. The phase relationships vary significantly among the investigated compositions, revealing a sensitivity to small variations in whole-rock compositions. Our results show that the pre-eruptive storage conditions of the four silicic magmas were all very similar. The magmas were stored at T = 850–900°C and P ≥ 2 kbar, under moderately reduced conditions (NNO = –0·9 to –0·1), and were poor in fluorine (~500–800 ppm) and sulphur (≤100 ppm), but rich in water and chlorine (5–6 wt % and 2500–3500 ppm, respectively). In all cases, the melts were slightly undersaturated with respect to H 2 O, but most probably saturated with respect to H 2 O + Cl ± CO 2 and a brine. The Santorini magma plumbing system appears to be dominated by a large, long-lived (≥200 kyr) predominantly silicic magma storage region situated at ≥8 km depth, from which crystal-poor melt batches were extracted during the largest caldera-forming eruptions of the volcanic system.

Description: Experiments have been performed both on a peraluminous leucogranitic (DK89) and a F-, Li-, P-rich pegmatitic (B2) melt to constrain the stability of micas in evolved crustal silicic magmas and refine mica-melt partition coefficients for F, Li, and Be. The experiments were conducted in parallel in two f O 2 ranges, "oxidizing" (NNO +1 to +3) and "reducing" (NNO –1.6 to –1.4). One two-stage reducing-oxidizing experiment was conducted in a vessel fitted with a H 2 -permeable Shaw-type membrane. The approach toward equilibrium was tested by imposing long experimental durations and combining mica crystallization experiments with mica dissolution experiments using mica seeds. Experimental micas and melts were analyzed for major elements by electron microprobe and for light elements by nuclear microprobe. At 3 kbar, 620 °C, and under oxidizing conditions, B2 crystallized only muscovite, the biotite seeds reacting to form a new mica intermediate between phengite and Li-rich phengite. Under reducing conditions, biotite (siderophyllite composition) appeared as the stable mica. The two-stage experiment yielded a composite mica assemblage with siderophyllite cores mantled by muscovite rims. At 3.5–3.8 kbar, 720 °C, and under oxidizing conditions, DK89 crystallized only aluminous biotite and muscovite seeds reacted to form biotite-bearing assemblages; muscovite appeared together with biotite at 700 °C. Under reducing conditions, Al-rich biotite is also the stable mica at 720 °C. Partition coefficients show that F and Li are preferentially incorporated in biotite rather than in muscovite, the opposite as for Be. Biotite fractionation buffers the F and increases the Li and Be contents of the residual melt. Muscovite increases the Li content of the melt and has little influence on F and Be concentrations. Our experiments reproduce mica assemblages and compositions typical of Variscan pegmatites and leucogranites, yet very Li-rich micas ( e.g ., lepidolites) were not obtained. The results stress the differential influence of f O 2 on mica stability in moderately and highly fractionated crustal melts. Mica crystallization in leucogranites does not appear to be strongly dependent on f O 2 . In contrast, a very strong influence of f O 2 on stable mica assemblages is demonstrated for the pegmatitic melt. The reducing experiments emphasize the existence of a stability field for biotite in melts poor in Fe, Mg, and Ti. If f O 2 is reducing, biotite must crystallize in moderately to highly evolved peraluminous crustal melts. In contrast, the crystallization of muscovite as the sole mica in evolved crustal melts constitutes an indicator of oxidizing f O 2 . Such an oxidizing evolution that deviates from classical buffered T -log f O 2 trajectories is the consequence of a mechanism of magma "self-oxidation" that is proposed to result from dissociation of H 2 O in the melt.

Description: Andesitic arc volcanism is the most common type of subduction-related magmatism on Earth. How these melts are generated and under which conditions they evolve towards silica-rich liquids is still a matter of discussion. We have performed crystallization experiments on a representative andesite sample from the Upper Scoriae 1 (USC-1) eruption of Santorini (Greece) with the aim of understanding such processes. Experiments were performed between 1000 and 900 °C, in the pressure range 100–400 MPa, at f O 2 from QFM (quartz–fayalite–magnetite) to NNO (nickel–nickel oxide) + 1·5, with H 2 O melt contents varying from saturation to nominally dry conditions. The results show that the USC-1 andesitic magma was generated at 1000 °C and 12–15 km depth (400 MPa), migrated to shallower levels (8 km; 200 MPa) and intruded into a partially crystallized dacitic magma body. The magma cooled to 975 °C and generated the phenocryst assemblage and compositional zonations that characterize the products of this eruption. An injection of basaltic magma problably subsequently triggered the eruption. In addition to providing the pre-eruptive conditions of the USC-1 magma, our experiments also shed light on the generation conditions of silica-rich magmas at Santorini. Experimental of runs performed at f O 2 ~ NNO + 1 (± 0·5) closely mimic the compositional evolution of magmas at Santorini whereas those at reduced conditions (QFM) do not. Glasses from runs at 1000–975 °C encompass the magma compositions of intermediate-dominated eruptions, whereas those at 950–900 °C reproduce the silicic-dominated eruptions. Altogether, the comparison between our experimental results and natural data for major recent eruptions from Santorini shows that different magma reservoirs, located at different levels, were involved during highly energetic events. Our results suggest that fractionation in deep reservoirs may give rise to magma series with a tholeiitic signature whereas at shallow levels calc-alkaline trends are produced.