Magma Chemistry Research Articles

Arc magmatism in the Delhi Fold Belt: SHRIMP U–Pb zircon ages of granitoids and implications for Neoproterozoic convergent margin tectonics in NW India

The northwestern region of Peninsular India preserves important records of Precambrian plate tectonics and the role of Indian continent within Proterozoic supercontinents. In this study, we report precise SHRIMP zircon U–Pb ages from granitoids from the Sirohi terrane located along the western fringe of the Delhi Fold Belt in Rajasthan, NW India. The data reveal a range of Neoproterozoic ages from plagiogranite of Peshua, foliated granite of Devala, and porphyritic granite of Sai with zircon crystallization from magmas at 1015±4.4Ma, 966.5±3.5 and 808±3.1 respectively. The plagiogranite shows high SiO2, Na2O and extremely low K2O, Rb, Ba, comparable with typical oceanic plagiogranites. These rocks possess low LREE and HREE concentrations and a relatively flat LREE–HREE slope, a well-developed negative Eu-anomaly and conspicuous Nb and Ti anomalies. Compared to the plagiogranite, the foliated Devala granite shows higher SiO2 and moderate Na2O, together with high K2O and comparatively higher Rb, Ba, Sr and REE, with steep REE profiles and a weak positive Eu anomaly. In contrast to the plagiogranite and foliated granite, the porphrytic Sai granite has comparatively lower SiO2 moderately higher Na2O, extremely high Y, Zr, Nb and elevated REE. The geochemical features of the granitoids [HFSE depletion and LILE enrichment, Nb- and Ta-negative anomalies], and their plots in the fields of Volcanic Arc Granites and those from active continental margins in tectonic discrimination diagrams suggest widespread Neoproterozoic arc magmatism with changing magma chemistry in a protracted subduction realm. Our results offer important insights into a long-lived active continental margin in NW India during early and mid Neoproterozoic, consistent with recent similar observations on Cryogenian magmatic arcs widely distributed along the margins of the East African Orogen, and challenge some of the alternate models which link the magmatism to extensional tectonics associated with Rodinia supercontinent breakup.

A New View on the Petrogenesis of the Oman Ophiolite Chromitites from Microanalyses of Chromite-hosted Inclusions

To decipher the petrogenesis of chromitites from the Moho Transition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of ∼500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1–50 μm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr + Al) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, fO2, of ∼3 log units below the quartz–fayalite–magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 950°C at subsolidus conditions, were overgrown by more oxidized host chromite (fO2 ≈ QFM) in association with microcrystal inclusions of silicates (plagioclase An86, clinopyroxene, and pargasite) that were formed between 950 and 1050°C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to ∼0·1°C a−1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional crystallization of chromite from water-saturated MORB. This study suggests that the metamorphic protolith assimilation occurring at the Moho level may dramatically affect MORB magma chemistry and lead to the formation of economic chromium deposits.

Timescales for the growth of sediment diapirs in subduction zones

SUMMARY In this study, we calculate timescales for the growth of gravitational instabilities forming in the sediment layer on the downgoing slab at subduction zones. Subducted metasediments are buoyant with respect to the overlying mantle and may form diapirs that detach from the slab and rise upwards into the mantle wedge. We use a particle-in-cell, finite-difference method to calculate growth rates for instabilities forming within a buoyant, wet-quartz metasediment layer underlying a dense mantle half-space composed of wet olivine. These growth rates are used to determine where sediment diapirs initiate and detach from the slab over a range of subduction zone thermal structures. We find that, given a sufficient layer thickness (200– 800 m, depending on slab-surface and mantle-wedge temperatures), sediment diapirs begin to grow rapidly at depths of ∼80 km and detach from the slab within 1–3 Myr at temperatures ≤900 ◦ C and at depths roughly corresponding to the location of the slab beneath the arc. Diapir growth is most sensitive to absolute slab temperature, however it is also affected by the viscosity ratio between the sediment layer and the mantle wedge and the length-scale over which viscosity decays above the slab. These secondary affects are most pronounced in colder subduction systems with old slabs and faster subduction rates. For a broad range of subduction zone thermal conditions, we find that diapirs can efficiently transport sediments into the mantle wedge, where they would melt and be incorporated into arc magmas. Thus, we conclude that sediment diapirism is a common feature of many subduction zones, providing a potential explanation for the ‘sediment signature’ in the chemistry of arc magmas.

Evolution of the mafic Villa Senni caldera-forming eruption at Colli Albani volcano, Italy, indicated by textural analysis of juvenile fragments

The Villa Senni Formation (355ka) represents the youngest mafic caldera-forming eruption of the Colli Albani Volcano (Central Italy), and one of the best exposed large mafic ignimbrite successions on Earth. The unusual Si-undersaturated and high-K composition of the magma, and corresponding low magma viscosity, raise important questions about the conditions of magma ascent, eruption, and fragmentation that led to such a large explosive eruption. We examined the juvenile clast textures – that is, the abundance, shape, size and number density of both vesicles and leucite microlites – to reconstruct the fragmentation conditions and to trace major changes in the ascent rate and vesiculation history of the magma associated with caldera collapse. The juvenile textures record two major changes in the eruption dynamics through the stratigraphy. First, the sudden depressurization of the magma chamber and the onset of the first ignimbrite-forming phase are marked by a dramatic increase in the magma ascent velocity inferred by a decrease in vesicle and leucite microlite sizes and increase in their number densities. Second, the progressive restoration of pressure within the magma reservoir as a consequence of caldera collapse is recorded by an inversion of vesicle and leucite microlite textures, which suggest a strong decrease in magma ascent rate. Complex vent conditions in this later phase of the caldera collapse are recorded by the diversity of textural features, variation in magma chemistry and the abrupt changes in the stratigraphic record (including the presence of co-ignimbrite breccias, spatter clasts and lithics).

Nephelinite–melilitite–carbonatite relationships: Evidence from Pleistocene–recent volcanism in northern Tanzania

The younger (<1.2Ma) nephelinite–melilitite–carbonatite-volcanoes in the Gregory Rift Valley of northern Tanzania (termed the Younger Extrusives) comprise one of the most concentrated areas of carbonatite magmatism on Earth. The larger strato-volcanoes, characterised by strongly evolved magmatism, are surrounded by volumetrically much-smaller features (scoria and tuff-cones, explosion craters, lava flows) of carbonate-rich olivine-bearing nephelinites or melilitites that, in their relative primitiveness, are potential parents to the much-largerstrato-volcanoes. The bimodal size distribution is mirrored in bulk magma chemistry, and in their Sr, Nd and Pb isotopes, with the more primitive magmas lying close to a HIMU source; the isotope data for the larger strato-volcanoes cover a wider, linear array interpreted as due to variable degrees of interaction between HIMU magmas and enriched lithosphere, evidence for which is found in Tanzanian mantle xenoliths.An integrated model involves carbonated olivine–nephelinite magma generated in the asthenosphere by melting of carbonate-bearing peridotite. Small amounts of the melt migrate rapidly and are extruded as the olivine–nephelinites and –melilitites. Larger amounts of magma pond in the lithosphere with which they react and, during variable periods of stagnation, varying degrees of olivine–pyroxene–spinel fractionation take place before upward migration of now-differing magma batches. Variable residence time within, and reaction with, variably enriched lithosphere accounts for the array between the HIMU and enriched mantle end-members in the isotope mixing model.

Stress fields acting during lithosphere breakup above a melting mantle: A case example in West Greenland

We characterize and map the stress fields acting during plate breakup along the West Greenland volcanic margin. The determination of interpolated stress fields is based on an inversion of fault-slip data sets and magma-driven fractures, crosscutting mainly an exposed inner seaward-dipping basaltic wedge (i.e., SDRi: inner Seaward Dipping Reflectors). This SDRi is segmented along-strike, with differently oriented segments. Relative chronology of stress fields is inferred from published age results on oriented dykes. We identify two distinct tectonic episodes (P1 and P2) with a P1–P2 change over at ~54Ma, i.e. during magnetic chron C24R. P1 is syn-magmatic and purely extensional. It is associated with the major crustal stretching event affecting the margin. P1 probably acted as early as the Late Palaeocene. This stress field was first homogeneous with the minimum principal stress σ3 trending ~N060E, defining a P1A stage. During development of the SDRi, σ3 locally reoriented to become orthogonal to each margin segment and, thus, to the continentward-dipping detachment faults bounding the SDRi (P1B). P1 is coeval with lithosphere breakup and is associated with an extension orthogonal to the Labrador–Baffin axis, which is inherited from the Mesozoic. A regional and radical change of σ3 to a ~NS trend takes place during P2, which follows on immediately from P1. P2 is also syn-magmatic. It is associated with only minor extension. σ3 runs parallel to the North American (NAM)/Greenland (GR) kinematic vector from C24R to C13. We establish therefore that the minimum horizontal stress σ3 for P1 and P2 is parallel to the relative displacement of Greenland related to NAM but not to its absolute displacement during the Tertiary. Taking into account those results as well as variations in magma chemistry from P1 to P2, we suggest that tectonic stresses at a volcanic margin could arise from the local dynamics of the melting mantle.

Metamorphic replacement of mineral inclusions in detrital zircon from Jack Hills, Australia: Implications for the Hadean Earth

The Hadean (before 4.0 Ga) crust has long been considered to comprise mainly primitive mafic and ultramafic rocks. However, mineral inclusions in detrital zircons as old as 4.4 Ga from Jack Hills, Australia, have been interpreted to be magmatic and to provide evidence for extensive granitic crust. In situ U-Pb dating of monazite and xenotime inclusions in 4.25–3.35 Ga detrital zircons from Jack Hills shows that these inclusions are not magmatic, but formed during metamorphism at either 2.68 Ga or 0.8 Ga. Monazite-xenotime thermometry of intergrowths in the inclusions and the quartz-muscovite rock matrix constrain temperatures to between 420–475 °C, corresponding with conditions during peak regional metamorphism. Petrography and U-Pb geochronology of zircon inclusions from other localities show that the replacement of primary inclusions may commence in the igneous host rock and continue through weathering, sedimentation, and diagenesis. With increasing metamorphic grade, the inclusion assemblage increasingly reflects the composition of the rock matrix. In Jack Hills, most of the inclusions have the same composition and abundances as the metamorphic matrix, consistent with their formation during metamorphism. The titanium content of quartz inclusions indicates formation temperatures of 350–490 °C, supporting a metamorphic origin. Several lines of evidence indicate that at least some of the muscovite inclusions are also secondary. The lack of apatite inclusions in zircons from Jack Hills, relative to zircon in common granitic rocks, suggests that secondary minerals may have replaced primary apatite. Thus, detrital zircon may not be impermeable to post-depositional fluids, raising doubts about the use of the mineral inclusions they contain to infer initial magma chemistry. These results call for a reassessment of the source melts of the Hadean zircons and the composition of the earliest crust.

B2-15 マグマ化学組成からみた60ka以降の桜島火山・姶良カルデラ地域におけるマグマ供給系の時間変化(マグマ供給系,口頭発表)

B2-15 マグマ化学組成からみた60ka以降の桜島火山・姶良カルデラ地域におけるマグマ供給系の時間変化(マグマ供給系,口頭発表)

Scales of columnar jointing in igneous rocks: field measurements and controlling factors

Columnar jointing is a common feature of solidified lavas, sills and dikes, but the factors controlling the characteristic stoutness of columns remain debated, and quantitative field observations are few in number. In this paper, we provide quantitative measurements on sizing of columnar joint sets and our assessment of the principal factors controlling it. We focus on (1) chemistry, as it is the major determinant of the physical (mechanical and thermal) properties of the lava, and (2) geology, as it influences the style of emplacement and lava geometry, setting boundary conditions for the cooling process and the rate of heat loss. In our analysis, we cover lavas with a broad range of chemical compositions (from basanite to phonolite, for six of which we provide new geochemical analyses) and of geological settings. Our field measurements cover 50 columnar jointing sites in three countries. We provide reliable, manually digitized data on the size of individual columns and focus the mathematical analysis on their geometry (23,889 data on side length, of which 17,312 are from full column sections and 3,033 data on cross-sectional area and order of polygonality). The geometrical observations show that the variation in characteristic size of columns between different sites exceeds one order of magnitude (side length ranging from 8 to 338 cm) and that the column-bounding polygons’ average order is less than 6. The network of fractures is found to be longer than required by a minimum-energy hexagonal configuration, indicating a non-equilibrium, geologically quick process. In terms of the development and characteristic sizing of columnar joint sets, our observations suggest that columns are the result of an interplay between the geological setting of emplacement and magma chemistry. When the geological setting constrains the geometry of the emplaced body, it exerts a stronger control on characteristic column stoutness. At unconstrained geometries (e.g. unconfined lava flows), chemistry plays the major role, resulting in stouter columns in felsic lavas and slenderer columns in mafic lavas.

Compositional variation in pyrochlores of Amba Dongar carbonatite complex, Gujarat

Pyrochlore is interpreted as a primary magmatic mineral that appeared in early carbonatite phases at Amba Dongar. Later intrusive phases incorporated this early pyrochlore and caused compositional modification, particularly around the rims, in response to changing magma chemistry. Consequently pyrochlore compositions show wide ranges in Nb, Ta, Ca, Ba, Ti and Ce. The final carbonatite phase was ankeritic and rich in hydrothermal fluids, giving rise to extreme compositional zoning and introduction of diverse elements (Si, U, Sr, Th, Fe), in the contained pyrochlore. Enrichment in radioelements such as U lead to metamictization, alteration and A-site vacancy.

SHRIMP U–Pb zircon geochronology and Sr–Nd isotopic systematic of the Neoproterozoic Ghimbi-Nedjo mafic to intermediate intrusions of Western Ethiopia: a record of passive margin magmatism at 855 Ma?

The reworked Pre-Neoproterozoic and juvenile Neoproterozoic terrane of the Western Ethiopian Shield (WES) consists of three N–S trending terranes. These are the western migmatitic gneissic terrane, the central metavolcano sedimentary terrane (CVST) and the eastern migmatitic gneissic terrane. The eastern part of the CVST mostly consists of suture-related ultramafic-metasedimentary complexes, whereas metavolcanics predominate in the western part. Gabbroic to granitic intrusions frequently occur in the CVST and in adjacent areas. New zircon SHRIMP U–Pb ages for two gabbros and three diorites in the Ghimbi-Nedjo region of the WES indicate magmatic crystallization ages. Two pulses of magmatism, at 860–850 and 795–785 Ma, are documented with the former for the first time. The tholeiitic Kemashi diorite and Bikilal-Ghimbi gabbros have oceanic affinities and yield U/Pb zircon ages of 856.3 ± 9.8 and 846.0 ± 7.6 Ma, respectively. The calc-alkaline Gebeya Kemisa pyroxene diorite, and the Senbet Dura hornblende diorite plus the tholeiitic Wayu Meni gabbro, which collectively have arc-back arc characteristics are indistinguishable at ages of 794.3 ± 9.4, 787.7 ± 8.8 and 778.1 ± 6.3 Ma, respectively. Positive eNd (4.5–7.0) and low initial 87Sr/86Sr (0.7029 ± 0.0002) and a mean T DM model age of 0.95 Ga for the Ghimbi-Nedjo region (mean T DM model age of 0.95 Ga for the WES overall) indicate that the magmas were generated from juvenile Neoproterozoic depleted mantle sources, with no discernable involvement of pre-Neoproterozoic continental crust. The occurrence of gabbros and diorites with oceanic tholeiite affinities combined with the new ages suggests that the intrusions were emplaced in the earliest stages of the rifting of Rodinia. This event in the WES led to the development of a passive margin and associated plume-type magmatism at ~855 Ma. The two intrusive groups with differing magma chemistry and ages suggest that the earliest magmatism was tholeiitic and associated with the passive margin system followed by continental breakup to form the Mozambique Ocean. The combination of tholeiitic and calc-alkaline magmatism was related to arc and back-arc basin formation and later terrane accretion (~830–690 Ma).

The rheological evolution of alkaline Vesuvius magmas and comparison with alkaline series from the Phlegrean Fields, Etna, Stromboli and Teide

Somma-Vesuvius is considered one of the highest-risk volcanic systems in the world due to its high population density and record of highly destructive explosive activity. Eruptive style at Vesuvius varies greatly, alternating between effusive and explosive activities, and is likely strongly controlled by the evolution of the physical and chemical properties of the magma. Nevertheless, with the exception of the 1631 eruption, the rheological properties of Vesuvius magmas remain largely unconstrained. Here, we investigate the Newtonian shear viscosity ( η) of dry and hydrous melts from the Mercato (plinian) and 1906 (violent strombolian) eruptions. These eruptions differ in size, eruptive style and magma chemistry (from phonolite to phono-tephrite). To evaluate the dry liquid viscosity variation covered by the eruptive products of the recent activity at Vesuvius, we measured the melt viscosities of bulk rock compositions and, for highly crystalline samples, of the separated groundmasses of tephras from the Pollena and 1906 eruptions. Hydrated samples with up to 4.24 wt% dissolved water were synthesised in a piston cylinder apparatus at confining pressure up to 10 kbar. The dry high temperature and the dry and hydrous low-temperature viscosities were obtained by combining the concentric cylinder and micropenetration techniques. The measured viscosities were parameterized by a modified Vogel–Fulcher–Tammann equation, accounting for the effect of water content, and were compared with previous measurements and models. At magmatic temperatures, the viscosities of Mercato samples are about four orders of magnitude higher than that of the least viscous investigated products from the 1906 eruption. Complex numerical models to forecast eruptive scenarios and their environmental impact are extremely sensitive to the accuracy of the input parameters and constitutive equations of magma properties. As a consequence, the numerical expressions obtained here are of particular relevance in the context of hazard assessment related to the different possible eruptive scenarios at Vesuvius through numerical simulation tools. The effect of composition on the liquid viscosities is compared to other high-Na (e.g., samples from Teide and Etna) and high-K (e.g., samples from Stromboli and Phlegrean Fields) alkaline magmas.

Morphology and growth trends of accessory zircons from various granitoids of the South-western Bohemian Massif (Moldanubicum, Austria)

As the main objective of the present study, the morphological development of accessory zircons from four granitoids (pearl gneiss, fine-grained granite, coarse-grained gneiss, and Weinsberg granite) of the South-western Bohemian Massif was described in detail. On the one hand, this was realized by the classical approach, including a statistical evaluation of external zircon morphologies with the typology scheme. On the other hand, direct insight into the growth of single crystals was established by the production of crystal sections parallel and perpendicular to the crystallographic c-axis and by their subsequent imaging with the electron microprobe. Regarding the second method, eventual morphological trends were represented as a function of the growth rates of single crystal forms. Except for the coarse-grained gneiss, zircon crystals of the investigated granitoids show similar morphological trends according to both methods, starting with a dominant {1 0 0} prism and equally sized pyramids. Final crystal habit, however, is marked by the predominance of {1 1 0} and {2 1 1}. Zircon crystals from the coarse-grained gneiss run through a completely different development with a change of the prism habit from {1 1 0} to {1 0 0} and a more or less static growth of {1 0 1} with only slight modifications in size. Comparison of the results with data from the literature underlines the role of magma chemistry, magma temperature, and cooling rate as the main factors responsible for growth trends of accessory zircon.

Late Cretaceous porphyry Cu and epithermal Cu–Au association in the Southern Panagyurishte District, Bulgaria: the paired Vlaykov Vruh and Elshitsa deposits

Vlaykov Vruh–Elshitsa represents the best example of paired porphyry Cu and epithermal Cu–Au deposits within the Late Cretaceous Apuseni–Banat–Timok–Srednogorie magmatic and metallogenic belt of Eastern Europe. The two deposits are part of the NW trending Panagyurishte magmato-tectonic corridor of central Bulgaria. The deposits were formed along the SW flank of the Elshitsa volcano-intrusive complex and are spatially associated with N110-120-trending hypabyssal and subvolcanic bodies of granodioritic composition. At Elshitsa, more than ten lenticular to columnar massive ore bodies are discordant with respect to the host rock and are structurally controlled. A particular feature of the mineralization is the overprinting of an early stage high-sulfidation mineral assemblage (pyrite ± enargite ± covellite ± goldfieldite) by an intermediate-sulfidation paragenesis with a characteristic Cu–Bi–Te–Pb–Zn signature forming the main economic parts of the ore bodies. The two stages of mineralization produced two compositionally different types of ores—massive pyrite and copper–pyrite bodies. Vlaykov Vruh shares features with typical porphyry Cu systems. Their common geological and structural setting, ore-forming processes, and paragenesis, as well as the observed alteration and geochemical lateral and vertical zonation, allow us to interpret the Elshitsa and Vlaykov Vruh deposits as the deep part of a high-sulfidation epithermal system and its spatially and genetically related porphyry Cu counterpart, respectively. The magmatic–hydrothermal system at Vlaykov Vruh–Elshitsa produced much smaller deposits than similar complexes in the northern part of the Panagyurishte district (Chelopech, Elatsite, Assarel). Magma chemistry and isotopic signature are some of the main differences between the northern and southern parts of the district. Major and trace element geochemistry of the Elshitsa magmatic complex are indicative for the medium- to high-K calc-alkaline character of the magmas. 87Sr/86Sr(i) ratios of igneous rocks in the range of 0.70464 to 0.70612 and 143Nd/144Nd(i) ratios in the range of 0.51241 to 0.51255 indicate mixed crustal–mantle components of the magmas dominated by mantellic signatures. The epsilon Hf composition of magmatic zircons (+6.2 to +9.6) also suggests mixed mantellic–crustal sources of the magmas. However, Pb isotopic signatures of whole rocks (206Pb/204Pb = 18.13–18.64, 207Pb/204Pb = 15.58–15.64, and 208Pb/204Pb = 37.69–38.56) along with common inheritance component detected in magmatic zircons also imply assimilation processes of pre-Variscan and Variscan basement at various scales. U–Pb zircon and rutile dating allowed determination of the timing of porphyry ore formation at Vlaykov Vruh (85.6 ± 0.9 Ma), which immediately followed the crystallization of the subvolcanic dacitic bodies at Elshitsa (86.11 ± 0.23 Ma) and the Elshitsa granite (86.62 ± 0.02 Ma). Strontium isotope analyses of hydrothermal sulfates and carbonates (87Sr/86Sr = 0.70581–0.70729) suggest large-scale interaction between mineralizing fluids and basement lithologies at Elshitsa–Vlaykov Vruh. Lead isotope compositions of hydrothermal sulfides (206Pb/204Pb = 18.432–18.534, 207Pb/204Pb = 15.608–15.647, and 208Pb/204Pb = 37.497–38.630) allow attribution of ore-formation in the porphyry and epithermal deposits in the Southern Panagyurishte district to a single metallogenic event with a common source of metals.

High-Ti amphibole as a petrogenetic indicator of magma chemistry: evidence for mildly alkalic-hybrid melts during evolution of Variscan basic–ultrabasic magmatism of Central Iberia

Central Iberian Variscan granite batholiths and anatectic complexes are punctuated by coeval stocks of hydrous, high-K calc-alkaline, ultrabasic to intermediate rock series. Despite their overall calc-alkaline affinity, the mafic–ultramafic members contain high-Ti amphibole oikocrysts rimmed by lower-Ti amphibole ± cummingtonite and high-Ti amphibole replacing early phlogopite. To understand the factors controlling the saturation of high-Ti amphibole in the parental magmas, clinopyroxene-melt, phlogopite-melt and amphibole-melt relationships are reviewed. This analysis reveals that for melts with intermediate compositions, the affinity of TiO2 for amphibole rises in alkalic magmas. Accordingly, mildly alkalic trachytoid to subalkaline medium- to high-K andesite and dacite compositions are estimated for interstitial high-Ti amphibole-saturated melts. Amphibole Ce/Pb ratios reveal a mantle–crust hybrid nature for interstitial melts with subalkaline trachytoid compositions. The hydrous character of the Variscan basic magmas favoured an overall magmatic evolutionary trend with a low rate of variation of Na2O with respect to silica during amphibole crystallization.

Recycling and “re-hydration” of degassed magma inducing transient dissolution/crystallization events at Stromboli (Italy)

Intrusive degassing and recycling of degassed and dense magma at depth have been proposed for a long time at Stromboli. The brief explosive event that occurred at the summit craters on 9 January 2005 threw out bombs and lapilli that could be good candidates to illustrate recycling of shallow degassed magma at depth. We present an extensive data set on both the textures and the mineral, bulk rock and glassy matrix chemistry of the “9 Jan” products. The latter have the common shoshonitic–basaltic bulk composition of lavas and scoriae issued from typical strombolian activity. In contrast they differ by the heterogeneous chemistry of their matrix glasses and their crystal textures that testify to crystal dissolution event(s) just prior magma crystallization upon ascent and eruption. Comparison between mineral paragenesis of the natural products and experimental phase equilibria suggest water-induced magma re-equilibration. We propose that mineral dissolution is related to water enrichment of the recycled degassed magma, via differential gas bubble transfer and to some extents its physical mixing with volatile-rich magma blobs. However, all these features illustrate transient processes. Even though evidence of mineral dissolution is ubiquitous at Stromboli, its effect on the bulk magma chemistry is minor because of the subtle interplay between mineral dissolution and crystallization in magmas having comparable bulk chemistry.

A critical evaluation of recent models for Lau–Tonga arc–backarc basin magmatic evolution

New trace element, Sr-, Nd-, Pb- and Hf isotope data provide insights into the evolution of the Tonga–Lau Basin subduction system. The involvement of two separate mantle domains, namely Pacific MORB mantle in the pre-rift and early stages of back-arc basin formation, and Indian MORB mantle in the later stages, is confirmed by these results. Contrary to models proposed in recent studies on the basis of Pb isotope and other compositional data, this change in mantle wedge character best explains the shift in the isotopic composition, particularly 143Nd/ 144Nd ratios, of modern Tofua Arc magmas relative to all other arc products from this region. Nevertheless, significant changes in the slab-derived flux during the evolution of the arc system are also required to explain second order variations in magma chemistry. In this region, the slab-derived flux is dominated by fluid; however, these fluids carry Pb with sediment-influenced isotopic signatures, indicating that their source is not restricted to the subducting altered mafic oceanic crust. This has been the case from the earliest magmatic activity in the arc (Eocene) until the present time, with the exception of two periods of magmatic activity recorded in samples from the Lau Islands. Both the Lau Volcanic Group, and Korobasaga Volcanic Group lavas preserve trace element and isotope evidence for a contribution from subducted sediment that was not transported as a fluid, but possibly in the form of a melt. This component shares similarities with that influencing the chemistry of the northern Tofua Arc magmas, suggesting some caution may be required in the adoption of constraints for the latter dependent upon the involvement of sediments from the Louisville Ridge. A key outcome of this study is to demonstrate that the models proposed to explain subduction zone magmatism cannot afford to ignore the small but important contributions made by the mantle wedge to the incompatible trace element inventory of arc magmas.

Petrology of the Nohi Rhyolite and its related granitoids: A Late Cretaceous large silicic igneous field in central Japan

The Nohi Rhyolite (5000–7000 km 3) is one of the largest Late Cretaceous ash-flow fields, related to the Kula/Pacific Plate subduction, in central Japan. It consists mainly of thick, welded ash-flow sheets (a few 100 to > 1000 km 3) infilling cauldrons. Ash-flow tuffs of the Nohi Rhyolite are characterized as crystal-rich rhyodacite to rhyolite, fiammes of which have 30–45 vol.% phenocrysts (quartz, feldspars and minor mafic minerals). The crystal-rich rhyolitic magma sampled by some units of the Nohi Rhyolite formed mainly by fractional crystallization (plagioclase, pyroxene and ilmenite) from a parental crystal-rich rhyodacite. The parental magma of the largest Gero Ash-Flow Sheet (> 2200 km 3) is considered to have had a lower original water content than typical intermediate to evolved magmas produced in subduction zones. The Nohi Rhyolite has similar general petrological properties to the hypabyssal granitoid porphyries and the batholithic Naegi-Agematsu Granite, which is high-K, I-type, ilmenite-series dacitic to rhyolitic in composition and lacking mafic rocks. This petrological similarity, together with a close spatial and temporal relationship indicates that they formed as a series of large silicic magmatic events on the eastern margin of the Eurasian Continent during the Late Cretaceous. There are slight differences in trace element composition within subdivisions of the Nohi Rhyolite and granitoids, which probably relate to the region of magma emplacement rather than a temporal change in magma chemistry.

Crystallization and Degassing in the Basement Sill, McMurdo Dry Valleys, Antarctica

The Basement Sill is part of the Ferrar Large Igneous Province exposed in the McMurdo Dry Valleys, Antarctica. The sill is � 330 m thick in the Bull Pass area and 450 þ m thick in the Dais area, � 12 km to the west, and is characterized by phenocryst-free lower and upper margins and an orthopyroxene-rich central ‘tongue’ (opx 1^5 mm in size). Halogen variations in apatite from a suite of samples collected along vertical transects through the sill were examined to evaluate the process of crystallization-induced degassing (i.e. second boiling) and its effects on magma chemistry. Apatite grains from any given sample are generally unzoned with respect to Cl and F concentrations, but may vary by 20^30 mol% in the halogen site between grains. Overall average Cl/F mass ratios increase with height from the lower margin to the center of the sill, and then decrease to near zero towards the top margin where the rocks are relatively oxide-rich. The Cl/F trend parallels those of bulk MgO and grain size. The upper margin contains abundant mafic pegmatoids and the apatite in these segregations has lower Cl/F ratios compared with that in the host-rocks, although REE show no measurable difference. Numerical modeling illustrates that a cooling and crystallizing sill initially develops two separate vapor-saturated zones at the lower and upper margins owing to the irreversible heat loss to the cooler country rock. Vapor separating from the lower zone migrates upward into hotter silicate liquid, where it is resorbed, thus increasing the Cl/F mass ratio of the liquid. This process leads to saturation and precipitation of apatite from the liquid with a higher Cl/F ratio than would otherwise occur. Volatile enrichment can also aid compaction and grain growth in the central part of the sill. In contrast, the relatively Fe-rich, Cl-poor nature of the upper zone rocks suggests that these rocks may have crystallized from more evolved, degassed silicate liquid, possibly compacted out of the underlying crystal mush. In addition, as vapor sourced from the lower and central parts of the sill ascends into the cooler upper zone of the sill, the vapor may be localized (along with late interstitial silicate liquid) to form pegmatoids at temperatures at which Cl is less favored in apatite and can be leached from existing apatite by the ascending vapor, the latter causing the observed decrease in the Cl/F mass ratio of apatite in the (evolved) pegmatoids.

Modeling the sharp compositional interface in the Pu`u `Ō`ō magma reservoir, Kīlauea volcano, Hawai`i

Lavas from the early episodes of the Pu`u `Ō`ō eruption (1983–1985) of Kīlauea Volcano on the island of Hawai`i display rapid compositional variation over short periods for some episodes, especially the well‐sampled episode 30 with ∼2 wt% MgO variation in &lt;4 hours. Little chemical variation is observed within the episode 30 lavas before or after this abrupt change, suggesting a sharp compositional interface within the Pu`u `Ō`ō dike‐like shallow reservoir. Cooling‐induced crystal fractionation in this reservoir is thought to be the main control on intraepisode compositional variation. Potential explanations for a sharp interface, such as changing reservoir width and wall rock thermal properties, are evaluated using a simple thermal model of a dike‐like body surrounded by wall rock with spatially variable thermal conductivity. The model that best reproduces the compositional data involves a change in wall rock thermal conductivity from 2.7 to 9 W m−1 C−1, which is consistent with deep drill hole data in the east rift zone. The change in thermal conductivity may indicate that fluid flow in the east rift zone is restricted to shallow depths possibly by increasing numbers of dikes acting as aquicludes and/or decreasing pore space due to formation of secondary minerals. Results suggest that wall rock thermal gradients can strongly influence magma chemistry in shallow reservoirs.