Chiara M. Petrone

Generation of oceanic-island basalt–type volcanism in the western Trans-Mexican volcanic belt by slab rollback, asthenosphere infiltration, and variable flux melting

Mantle plumes or decompression melting of a heterogeneous mantle have been proposed to explain oceanic-island basalt (OIB) type lavas found in the western Trans-Mexican volcanic belt. We show that mantle plumes cannot account for several geologic observations, whereas decompression melting is unrealistic given the low extension rate in the volcanic arc. On the basis of new geologic and geochemical studies, we propose an alternative model that reconciles geologic data, plate tectonic history, and petrology. Since 8.5 Ma, the volcanic front has migrated trenchward ∼80 km, and OIB began to be emplaced in the rear half of the arc after a period of stalled subduction of the Rivera plate between 7.2 and 4.8 Ma. As a whole, OIB accounts for ∼5% of the total volume of volcanism. Their trace element signatures require an enriched-mantle source, akin to the deep asthenosphere, but also indicate a minor involvement of a subduction component. We propose that during the period of very low convergence at the end of the Miocene, the leading edge of the Rivera slab started to sink into the mantle and continued rolling back until it reached the present dip of 45°. The sinking of the slab may have induced small amounts of asthenosphere to flow laterally into the opening mantle wedge, from the Gulf of California rift on the west and through the subducted Rivera-Cocos plate boundary on the east. This mechanism would produce veins of relatively enriched mantle, which would be able to produce melts with an OIB composition once the flux of fluids released from the subducted slab resumed.

Unusual coexistence of subduction-related and intraplate-type magmatism: Sr, Nd and Pb isotope and trace element data from the magmatism of the San Pedro–Ceboruco graben (Nayarit, Mexico)

Subduction-related and continental intraplate type magmatism coexisted in the San Pedro–Ceboruco (SPC) graben in western Mexico in a rather unusual close association. The magmatic systems belong to the Trans-Mexican Volcanic Belt (TMVB), the active volcanic arc linked to subduction of the Rivera and Cocos plates beneath the North American plate. Three different magmatic series are recognized in the San Pedro–Ceboruco graben: calc-alkaline, Na-alkaline, and transitional. The transitional series can be further subdivided into: Low-Ti; High-Ti and Amado Nervo groups. The different magmatic series are characterized by variable LILE/HFSE and LREE/HFSE ratios. The Na-alkaline series has the most radiogenic Nd compositions and the lowest 87 Sr/ 86 Sr (0.70320–0.70344). The calc-alkaline series shows the lowest eNd and the highest 87 Sr/ 86 Sr (0.70395– 0.70402) overlapping the values for the transitional High-Ti group (0.70390–0.70404). Sr isotopic compositions for transitional Low-Ti (0.70367–0.70394) and transitional Amado Nervo (0.70351–0.70389) groups are intermediate between those of the Na-alkaline and calc-alkaline rocks. All the studied rocks show similar 207 Pb/ 204 Pb (15.58–15.61) and 208 Pb/ 204 Pb (38.41– 38.65), but 206 Pb/ 204 Pb discriminates well the Na-alkaline series (18.90–19.03) from all the others (18.68–18.75). Compositional and isotopic data suggest that the different series derive from distinct parental magmas, which were generated by partial melting of a heterogeneous mantle source characterized by two different components. A depleted mantle (DM) component with low 206 Pb/ 204 Pb and an enriched (EM) component characterized by high 206 Pb/ 204 Pb. These two components

Correlations between silicic volcanic rocks of the St Mary's Islands (southwestern India) and eastern Madagascar: implications for Late Cretaceous India–Madagascar reconstructions

Abstract: The St Marys Islands (southwestern India) expose silicic volcanic and sub-volcanic rocks (rhyolites and granophyric dacites) emplaced contemporaneously with the Cretaceous igneous province of Madagascar, roughly 88–90 Ma ago. The St Marys Islands rocks have phenocrysts of plagioclase, clinopyroxene, orthopyroxene and opaque oxide, moderate enrichment in the incompatible elements (e.g. Zr = 580–720 ppm, Nb = 43–53 ppm, La/Ybn = 6.9–7.2), relatively low initial 87Sr/86Sr (0.7052–0.7055) and near-chondritic initial 143Nd/144Nd (0.51248–0.51249). They have mineral chemical, whole-rock chemical and isotopic compositions very close to those of rhyolites exposed between Vatomandry–Ilaka and Mananjary in eastern Madagascar, and are distinctly different from rhyolites from other sectors of the Madagascan province. We therefore postulate that the St Marys and the Vatomandry–Ilaka–Mananjary silicic rock outcrops were adjacent before the Late Cretaceous rifting that split Madagascar from India. If so, they provide a valuable tool to check and aid traditional Cretaceous India–Madagascar reconstructions based on palaeomagnetism, matching Precambrian geological features, and geometric fitting of continental shelves. Supplementary material: Mineral analyses, mass-balance calculations and locality information are available at http://www.geolsoc.org.uk/SUP18332.

Mineralogy and magmatic affinity of the Jasra intrusive complex, Shillong Plateau, India

Abstract The rocks of the Jasra intrusive complex (Shillong Plateau, northeastern India) include phlogopite clinopyroxenites (with olivine or perovskite relics), alkali gabbros/monzodiorites, syenites and nepheline syenites. They have a potassic affinity (Na2O/K2O ~1), and their mineralogy is dominated by clinopyroxene with which phlogopite, olivine, amphibole, feldspars, feldspathoids, oxides, orthopyroxenes, perovskite, titanite and other accessory phases are variably associated. The Jasra intrusive rocks are cumulates derived from at least two distinct magmatic liquids. The potassic affinity of the Jasra rocks differs from the nearby Sung Valley ijolitic-carbonatitic complex and from the ultrapotassic lamproitic rocks of the Damodar Valley, which are of approximately the same age. This suggests major variability in the mantle sources of small-volume alkaline volcanism in the Early Cretaceous of northeastern India.

Characterization and genesis of horizontal banding in Brazilian agate: an X-ray diffraction, thermogravimetric and electron microprobe study

Abstract Characterization of Brazilian agates containing a lower horizontally banded section and an upper chamber with bands parallel to the walls shows that these agates formed much later than the 135 Ma Paraná basalt host rock. Age differentiation between the two types of banding shows that the horizontal bands formed between 43 to 63 Ma ago with a final infill of wall-lining bands between 7 and 27 Ma later. The horizontal bands have a higher Al3+concentration and a greater crystallite size than the wall-lining layers; they have a lower mogánite content and defect-site water content. The formation of these agates appears to be the result of a three-stage process. After the separate formation of horizontally banded and wall-lining agate, a silica infill seals the gap between the agate and the cavity wall. The detection of cristobalite in some specimens indicates that genesis of both the horizontally banded and wall-lining deposits in the Brazilian samples proceeds along an amorphous silica → opal-CT → opal-C → chalcedony pathway.

Thin walled pottery from Alife (Northern Campania, Italy)

The ancient town of Allifae (modern Alife) represents one of the most interesting settlements of the Northern Campania area and together with the ancient city of Cales , was a thriving production centre of pottery. Excavations carried out inside the city wall, near the south gate, the so called, Porta Fiume, unearthed a huge dump of thin-walled ware, where the most abundant forms were cups and beakers, decorated with grooves or rouletting. The dump has been dated late Augustan/Tiberian age and the thin-walled vessels found can be identified with similar wares from Allifae, Cubulteria, Caiatia and perhaps Neapolis. Horace in his Sermones (II, 8,39) cited the Allifana beakers (described as fictiles ac subtiles by a Horace scholiast) and they could possibly be identified with the thin-walled wares produced in Allifae. If this the case, then the thin-walled vessels produced in Allifae were known in Rome as early as the end of I century B.C. In order to investigate and characterize the Allifae thin-walled pottery, twenty-one samples were selected and mineralogical-petrographic analyses (OM, XRD, XRF and SEM/EDS) were carried out. The clayey raw material used was a low-CaO alluvial clayey deposit from the Middle Valley of the Volturno River. The potters probably handled the sediment by a levigation process in order to remove the coarser grains, and making the clay suitable to produce such thin walls. Comparison with other regional production of thin-walled pottery allowed us to strictly distinguish the Allifana beakers.

Mineralogical, Geochemical, and Isotopic Characteristics of the Ejecta from the 5 April 2003 Paroxysm at Stromboli, Italy: Inferences on the Preeruptive Magma Dynamics

The 5 April 2003 explosive eruption at Stromboli emplaced typical basaltic scoria, pumice, and lithic blocks. This paper reports a detailed set of mineralogical, geochemical, and isotopic data on the juvenile ejecta and fresh subvolcanic blocks, including micro-Sr isotope analyses and major and dissolved volatile element contents in olivine-hosted melt inclusions. The juvenile ejecta have compositions similar to those of their analogs from previous paroxysms; the 2003 pumice, however, does not contain stable high-MgO olivine, usually typical of large-scale paroxysms and has lower compatible element contents. Texture, composition, and Sr isotope disequilibrium of crystals in pumice indicate that most of them are inherited from the shallow crystal-rich magma and/or crystal mush. The most primitive magma is recorded as rare melt inclusion in olivine Fo 85―86 . It has a typical S/Cl (1.1) and a total volatile content of 3.1 wt % from which the total fluid pressure was evaluated >240 MPa. Hence, moderate pressure conditions can be envisaged for the mechanism triggering the April 2003 paroxysm. The subvolcanic blocks are shoshonitic basalts with 45―50 vol % of phenocrysts (plagioclase + clinopyroxene + olivine). The late-stage crystallization of the crystal-rich magma lead to the formation ofNa-sanidine with plagioclase An 60―25 + olivine Fo 68―49 + Ti-magnetite ± apatite ± phlogopite ± ilmenite assemblage. Mineralogy, chemistry, and Sr―Nd isotopic signatures of the subvolcanic blocks indicate they represent the slowly cooled equivalents of batches of crystal-rich basaltic magma stored in the uppermost subvolcanic feeding system during the last few years. Cooling might be facilitated by short breaks in the summit crater activity.

Pre-eruptive magmatic processes re-timed using a non-isothermal approach to magma chamber dynamics

Constraining the timescales of pre-eruptive magmatic processes in active volcanic systems is paramount to understand magma chamber dynamics and the triggers for volcanic eruptions. Temporal information of magmatic processes is locked within the chemical zoning profiles of crystals but can be accessed by means of elemental diffusion chronometry. Mineral compositional zoning testifies to the occurrence of substantial temperature differences within magma chambers, which often bias the estimated timescales in the case of multi-stage zoned minerals. Here we propose a new Non-Isothermal Diffusion Incremental Step model to take into account the non-isothermal nature of pre-eruptive processes, deconstructing the main core-rim diffusion profiles of multi-zoned crystals into different isothermal steps. The Non-Isothermal Diffusion Incremental Step model represents a significant improvement in the reconstruction of crystal lifetime histories. Unravelling stepwise timescales at contrasting temperatures provides a novel approach to constraining pre-eruptive magmatic processes and greatly increases our understanding of magma chamber dynamics.

Textural and Compositional Characteristics of Lavas Emitted During the December 2002 to July 2003 Stromboli Eruption (Italy): Inferences on Magma Dynamics

Periodic lava sampling was carried out at the active vents during the entire duration ofthe 28 December 2002 to 22 July 2003 effusive eruption. Major and trace element bulk rock analyses were performed at different laboratories, thereby acquiring four independent sets of analysis. Nd and Sr isotope ratios were obtained on whole rocks and groundmasses, together with micro-Sr isotope analyses on plagioclase and clinopyroxene by microdrilling technique. Crystal size distribution, mineral, and glassy matrix chemistry were analyzed on selected samples. The products show a fairly homogeneous composition, close to that of the crystal-rich scoria that erupted in the previous years. Slight variations of trace elements and isotope ratios between products that erupted before and after the 5 April paroxysm are likely accounted for by limited mixing between the fresh, volatile-rich magma that erupted during the paroxysm and the volatile-poor magma feeding the lava flow. Micro-Sr isotope data show large isotopic disequilibria pointing to the persistence of highly Sr-radiogenic xenocrysts or crystal cores in the shallow magmatic system, probably recycled from the previous activity. Data rule out important changes in the dynamics of the plumbing system shortly before the eruption. A discrete input of deep magma into the lower part of the shallow system some months before the eruption may be at the origin of the increase of the magmatostatic pressure in the conduits, leading to the effusive eruption. An alternative hypothesis considers a nearly steady-state feeding system undergoing gradual, long-term pressure increase in its upper part, eventually leading to periodic lava effusions.