Ralf Gertisser

Temporal variations in magma composition at Merapi Volcano (Central Java, Indonesia): magmatic cycles during the past 2000 years of explosive activity

Abstract Merapi Volcano (Central Java, Indonesia) has been frequently active during Middle to Late Holocene time producing basalts and basaltic andesites of medium-K composition in earlier stages of activity and high-K magmas from ∼1900 14C yr BP to the present. Radiocarbon dating of pyroclastic deposits indicates an almost continuous activity with periods of high eruption rates alternating with shorter time spans of distinctly reduced eruptive frequency since the first appearance of high-K volcanic rocks. Geochemical data of 28 well-dated, prehistoric pyroclastic flows of the Merapi high-K series indicate systematic cyclic variations. These medium-term compositional variations result from a complex interplay of several magmatic processes, which ultimately control the periodicity and frequency of eruptions at Merapi. Low eruption rates and the absence of new influxes of primitive magma from depth allow the generation of basaltic andesite magma (56–57 wt% SiO2) in a small-volume magma reservoir through fractional crystallisation from parental mafic magma (52–53 wt% SiO2) in periods of low eruptive frequency. Magmas of intermediate composition erupted during these stages provide evidence for periodic withdrawal of magma from a steadily fractionating magma chamber. Subsequent periods are characterised by high eruption rates that coincide with shifts of whole-rock compositions from basaltic andesite to basalt. This compositional variation is interpreted to originate from influxes of primitive magma into a continuously active magma chamber, triggering the eruption of evolved magma after periods of low eruptive frequency. Batches of primitive magma eventually mix with residual magma in the magmatic reservoir to decrease whole-rock SiO2 contents. Supply of primitive magma at Merapi appears to be sufficiently frequent that andesites or more differentiated rock types were not generated during the past ∼2000 years of activity. Cyclic variations also occurred during the recent eruptive period since AD 1883. The most recent eruptive episode of Merapi is characterised by essentially uniform magma compositions that may imply the existence of a continuously active magma reservoir, maintained in a quasi-steady state by magma recharge. The whole-rock compositions at the upper limit of the total SiO2 range of the Merapi suite could also indicate the beginning of another period of high eruption rates and shifts towards more mafic compositions.

Towards reconstruction of the lost Late Bronze Age intra-caldera island of Santorini, Greece

During the Late Bronze Age, the island of Santorini had a semi-closed caldera harbour inherited from the 22 ka Cape Riva Plinian eruption, and a central island referred to as ‘Pre-Kameni’ after the present-day Kameni Islands. Here, the size and age of the intracaldera island prior to the Late Bronze Age (Minoan) eruption are constrained using a photo-statistical method, complemented by granulometry and high-precision K-Ar dating. Furthermore, the topography of Late Bronze Age Santorini is reconstructed by creating a new digital elevation model (DEM). Pre-Kameni and other parts of Santorini were destroyed during the 3.6 ka Minoan eruption, and their fragments were incorporated as lithic clasts in the Minoan pyroclastic deposits. Photo-statistical analysis and granulometry of these lithics, differentiated by lithology, constrain the volume of Pre-Kameni to 2.2–2.5 km3. Applying the Cassignol-Gillot K-Ar dating technique to the most characteristic black glassy andesite lithics, we propose that the island started to grow at 20.2 ± 1.0 ka soon after the Cape Riva eruption. This implies a minimum long-term lava extrusion rate of ~0.13–0.14 km3/ky during the growth of Pre-Kameni.

Incremental Construction of the Unit 10 Peridotite, Rum Eastern Layered Intrusion, NW Scotland

The Rum Eastern Layered Intrusion (ELI) is the product of part of a ~60 Ma open-system magma chamber. The 16 coupled peridotite/troctolite ± gabbro macro-rhythmic units it contains represent crystallisation of multiple batches of basaltic and picritic magma. Within the ELI, Unit 10 has been considered the type example of batch fractionation of magma on Rum for more than 50 years, successively producing peridotite, troctolite and olivine gabbro. Detailed field observations and logs of the Unit 10 peridotite cumulate are presented here, together with mineralogical and textural analyses of Cr-spinel seams and their peridotite host rocks. Numerous harrisite layers are commonly associated with diffuse, laterally discontinuous platinum-group element (PGE) enriched Cr-spinel seams. Multiple millimetre–centimetre thick Cr-spinel seams occur at the bases, tops and within harrisite layers. These relationships are inconsistent with simple batch fractionation of magma. Critically, the harrisite layers also exhibit centimetre to metre-scale, upward oriented apophyses that point to injection of magma into the overlying cumulate, indicating an intrusive origin for the harrisite. Quantitative textural and chemical analysis suggests that the Cr-spinel seams formed via in situ crystallisation within the crystal mush together with the intrusive peridotites from an assimilation reaction between the replenishing magma and peridotitic crystal mush. Intrusive magma replenishment in Unit 10 caused significant compositional disequilibrium between the crystallising phases in response to the postcumulus migration of reactive liquid, resulting in chemical zoning of intercumulus plagioclase crystals. We propose that the Unit 10 peridotite is intrusive and that repeated small volume magma replenishments are responsible for incremental construction of a large proportion of the peridotite body, similar to recent interpretations of parts of Unit 12 and Unit 14. Moreover, it is suggested that some or all of the injections of magma occurred into the crystal mush, rather than at the magma chamber floor. This new model of intra-mush Cr-spinel formation may have significant economic implications for PGE enrichment in other layered intrusions, such as the peridotite-hosted chromitites of the Stillwater Complex Ultramafic Series (Montana, USA). It is also worth noting that thin platiniferous chromitite seams considered to have formed in situ occur below the Merensky Reef of the Bushveld Complex (South Africa).

On the compositional variability of dalyite, K2ZrSi6O15: a new occurrence from Terceira, Azores

Abstract The rare potassium zirconium silicate dalyite has been identified for the first time on Terceira, Azores, within syenitic ejecta of the Caldeira-Castelinho Ignimbrite Formation. New quantitative analyses of this dalyite were combined with the small number of published analyses from various locations worldwide to evaluate the mineral’s compositional variability. Additionally, solid-state modelling has been applied to assess the site allocations of substituting elements. The new analyses yield the average formula (K1.84Na0.15)Σ=1.99(Zr0.94Ti0.012Hf0.011Fe0.004)Σ=0.967Si6.03O. Model results predict the placement of substituting Hf and Ti in the octahedral site, and highlight the overall complexity in the incorporation of Fe, Mg and Ba. The combined dataset reveals that dalyite found within peralkaline granites and syenites is generally defined by higher Na↔K substitution and lower Ti↔Zr substitution relative to dalyite from highly potassic rocks. The Terceira dalyite exhibits a bimodal variation in the degree of Na↔K substitutionwhich is attributed to a K-enrichment trend induced by late-stage pore wall crystallization and albitization, coupled with the control of pore size upon the degree of supersaturation required to initiate nucleation of dalyite in pores of varying size.

Geochemistry and provenance of Devono-Carboniferous volcano-sedimentary sequences from the Southern Vosges Basin and the geodynamic implications for the western Moldanubian Zone

Abstract The Southern Vosges host a volcano-sedimentary basin subdivided into a Lower Unit of Late Devonian age containing marine greywackes associated with bimodal volcanism, a marine Middle Unit (of Early Carboniferous age) and a terrestrial Upper Unit (of Late Carboniferous age), both accompanied by volcanism of intermediate to evolved composition. Petrographically and geochemically, sediments from the Lower Unit are characterized by mafic volcanic, sedimentary and metamorphic components and a positive Cr-Ni anomaly. A negative Nb anomaly weakens in the Middle and Upper Units. These units are dominated by intermediate to felsic volcanic detritus, and exhibit an enrichment in incompatible trace elements and disappearance of the Cr-Ni anomaly. The facies and composition of the volcano-sedimentary sequences indicate that an extensional basin existed in Late Devonian time (c. 360 Ma). Subduction-related volcanism and successive closure of the basin occurred in the Lower Carboniferous (c. 330 to c. 325 Ma). Sedimentation graded from flysch into molasse in Late Carboniferous time, where terrestrial conditions indicate that continental collision had occurred. The volcanism reported here coincides with arc magmatism in the Mid-German Crystalline High and closure of the Rheno-Hercynian Ocean, which was possibly responsible for the geodynamic evolution of the Southern Vosges Basin.

Spherulite formation in obsidian lavas in the Aeolian Islands, Italy

The authors wish to gratefully acknowledge Andy Tindle (The Open University) for assistance with EMP analyses, and Richard Darton and David Evans (Keele University) for assistance with XRD and Prof Alun Vaughan and Nicola Freebody (University of Southampton) with Raman analyses. LAB is grateful to Sophie Blanchard for support with MATLAB. The authors acknowledge support from Keele University, and grants from the Mineralogical Society (UK and Ireland) and Volcanic and Magmatic Studies Group. The authors thank Silvio Mollo and Francesca Forni for their detailed and helpful comments.

Petrogenesis of the peralkaline ignimbrites of Terceira, Azores

The recent (< 100 ka) volcanic stratigraphy of Terceira, Azores, includes at least seven peralkaline trachytic ignimbrite formations, attesting to a history of explosive eruptions. In this study, the petrogenesis and pre-eruptive storage conditions of the ignimbrite-forming magmas are investigated via whole-rock major and trace element geochemistry, melt inclusion and groundmass glass major element and volatile compositions, mineral chemistry, thermobarometric models, and petrogenetic modelling. Our primary aims are to develop a model for the magmatic plumbing system from which the ignimbrite-forming trachytes of Terceira were produced by evaluating various petrogenetic processes and constraining pre-eruptive magma storage conditions. We also place the ignimbriteforming magmas into the context of the Terceira suite and discuss the potential implications of preeruptive magma conditions for eruptive behaviour. Results indicate that ignimbrite-forming, comenditic trachytes are generated predominantly by extended fractional crystallization of basaltic parental magmas at redox conditions around 1 log unit below the fayalite–magnetite–quartz buffer. This is achieved via a polybaric fractionation pathway, in which mantle-derived basalts stall and fractionate to hawaiitic compositions at lower crustal depths ( 15 km), before ascending to a shallow crustal magma storage zone ( 2–4 km) and fractionating towards comenditic trachytic compositions. The most evolved pantelleritic magmas of Terceira (not represented by the ignimbrites) are plausibly generated by continued fractionation from the comenditic trachytes. Syenite autoliths represent portions of peralkaline trachytic melt that crystallized in situ at the margins of a silicic reservoir. Trachytic enclaves hosted within syenitic autoliths provide direct evidence for a two-stage mingling process, in which ascending hawaiites are mixed with trachytic magmas in the shallow crustal magma storage zone. The resulting hybridized trachytes then ascend further and mix with the more evolved peralkaline trachytes in the uppermost eruptible cap of the system, passing first through a syenitic crystal mush. The reduced viscosities of the peralkaline silicic magmas of this study in comparison with their metaluminous counterparts facilitate rapid crystal–melt segregation via crystal settling, generating compositionally zoned magma bodies and, in some instances, relatively crystalpoor erupted magmas. Reduced viscosity may also inhibit highly explosive activity (e.g. formation of a sustained eruption column), and limit the majority of explosive eruptions to low pyroclastic fountaining or ‘boil-over’ eruption styles. The formation of intermediate composition magmas within the system is considered to be limited to episodic mixing between mafic and silicic magmas.

Linking In Situ Crystallization and Magma Replenishment via Sill Intrusion in the Rum Western Layered Intrusion, NW Scotland

The construction of layered mafic-ultramafic intrusions has traditionally been attributed to gravitydriven accumulation, involving the mechanical settling of crystals onto the magma chamber floor, at the interface between the crystal mush at the base and overlying replenishing magma, such that the layered sequence of cumulates (i.e., the crystal mush) at the floor aggrades upwards. The Rum Western Layered Intrusion (WLI) is a ~250 m sequence of layered peridotite cumulates comprising the structurally lowest portion of the Rum Layered Suite (RLS). As such, it is taken to represent the oldest sequence in the RLS and has been assumed to young upwards. The WLI hosts the largest proportion of harrisite, a cumulate composed of skeletal olivine that formed by in situ crystallisation, in the Rum layered intrusion. Harrisite layers in the WLI ubiquitously exhibit extremely irregular upward-oriented apophyses, up to several metres high and metres across, alongside laterally extensive dome-like structures; features consistent with intrusive, sill-like emplacement of harrisite. The distribution and abundance of harrisite therefore points to chaotic sill-like emplacement of the magmas that produced at least half of the WLI cumulate. This probably occurred various ambient crystal mush temperatures and punctuated intervals during cumulate formation. The harrisite layers are associated with numerous Cr-spinel seams occurring along the tops, bases, and interiors of these layers, suggesting they formed in situ alongside harrisite sills within the crystal mush. Detailed quantitative textural and mineral chemical analysis of Cr-spinel seams support a simple in situ crystallisation process for their formation. It is suggested the Cr-spinel seams form within melt channels that develop along the same hot tears that allowed the harrisite parental melts to enter the crystal mush. The chemistry and texture of Cr-spinel is controlled by the volume of through-flow of melt through the melt channel. Where melt flux through channels was high, sulphide and platinumgroup minerals are more abundant, highlighting the key economic implications of this model for the platinum-group element enrichment of chromitite horizons in layered intrusions. We also highlight the role of infiltration metasomatism at multiple levels of the WLI, where porous percolation of interstitial melt and reactive liquid flow played a key role in cumulate formation, supporting the notion of layered intrusion growth by incremental sill emplacement.

Peralkaline felsic magmatism of the Atlantic islands

The oceanic-island magmatic systems of the Atlantic Ocean exhibit significant diversity in their respective sizes, ages, and the compositional ranges of the magmas they have produced. Nevertheless, almost all of the major Atlantic islands and island groups have produced peralkaline felsic magmas, implying that similar petrogenetic regimes may be operating throughout the Atlantic Ocean, and elsewhere. The origins of peralkaline magmas are frequently linked to low-degree partial melting of enriched mantle, followed by protracted differentiation in the shallow crust. Nevertheless, additional petrogenetic processes such as magma mixing, crustal melting, and contamination have been identified at numerous peralkaline centres. The onset of peralkalinity leads to magma viscosities lower than those typical for metaluminous felsic magmas, which has profound implications for processes such as crystal settling. A literature compilation demonstrates trends which suggest that the peralkaline magmas of the Atlantic Ocean islands are generated primarily via extended (up to ~ 95 %), open system fractional crystallisation of mantle-derived mafic magmas. Crustal assimilation is likely to become more significant as the system matures and low-solidus material accumulates in the crust. Magma mixing occurs between mafic and felsic end-members, and also between variably-evolved felsic magmas, and is often recognised via hybridised intermediate magmas. The peralkaline magmas are hydrous, and are frequently zoned in composition, temperature, and/or water content. They are typically stored in shallow crustal magma reservoirs (~ 2 to 5 km), which are maintained by mafic replenishment. Low melt viscosities (1 × 101.77 to 1 × 104.77 Pa s) facilitate two-phase flow, facilitating the formation of alkali-feldspar crystal mush. This mush may then contribute melt to an overlying melt lens via filter pressing or partial melting. We utilise a three stage model to account for the establishment, development, and termination of peralkaline magmatism in the ocean island magmatic systems of the Atlantic. We suggest that the overall control on peralkaline magmatism in the Atlantic is magma flux rate, which controls the stability of upper crustal magma reservoirs. The abundance of peralkaline magmas in the Atlantic suggests that their development must be a common, but not inevitable, stage in the evolution of ocean islands.

G UEST , J., C OLE , P., D UNCAN , A. & C HESTER , D. 2003. Volcanoes of Southern Italy. Earth in View Series. ix + 284 pp. London, Bath: Geological Society of London. Price £65.00 (paperback). ISBN 1 86239 138 6

Southern Italy is a wonderful place to study volcanic processes and volcanic environments. There are well-known active volcanoes such as Vesuvius, Stromboli, Vulcano and Etna, and other, perhaps less familiar, ones such as those of the Campanian Province, several of the Aeolian Islands and a few volcanic islands in the Strait of Sicily. The volcanoes of southern Italy are most notable for the long historical record of their activity, the ad 79 eruption of Vesuvius being the first volcanic eruption ever to be described in detail. Their presence has always had a direct effect on the cultural and social life of the region. Famous archaeological sites provide insight into the human history of those living around the volcanoes, which has been closely entwined with volcanic activity and, in many places, has been severely influenced by devastating eruptions. Because of their …