Everton Marques Bongiolo

The missing half of the subduction factory: shipboard results from the Izu rear arc, IODP Expedition 350

ABSTRACT IODP Expedition 350 was the first to be drilled in the rear part of the Izu-Bonin, although several sites had been drilled in the arc axis to fore-arc region; the scientific objective was to understand the evolution of the Izu rear arc, by drilling a deep-water volcaniclastic section with a long temporal record (Site U1437). The Izu rear arc is dominated by a series of basaltic to dacitic seamount chains up to ~100-km long roughly perpendicular to the arc front. Dredge samples from these are geochemically distinct from arc front rocks, and drilling was undertaken to understand this arc asymmetry. Site U1437 lies in an ~20-km-wide basin between two rear arc seamount chains, ~90-km west of the arc front, and was drilled to 1804 m below the sea floor (mbsf) with excellent recovery. We expected to drill a volcaniclastic apron, but the section is much more mud-rich than expected (~60%), and the remaining fraction of the section is much finer-grained than predicted from its position within the Izu arc, composed half of ashes/tuffs, and half of lapilli tuffs of fine grain size (clasts <3 cm). Volcanic blocks (>6.4 cm) are only sparsely scattered through the lowermost 25% of the section, and only one igneous unit was encountered, a rhyolite peperite intrusion at ~1390 mbsf. The lowest biostratigaphic datum is at 867 mbsf (~6.5 Ma), the lowest palaeomagnetic datum is at ~1300 mbsf (~9 Ma), and the rhyolite peperite at ~1390 mbsf has yielded a U–Pb zircon concordia intercept age of (13.6 + 1.6/−1.7) Ma. Both arc front and rear arc sources contributed to the fine-grained (distal) tephras of the upper 1320 m, but the coarse-grained (proximal) volcaniclastics in the lowest 25% of the section are geochemically similar to the arc front, suggesting arc asymmetry is not recorded in rocks older than ~13 Ma.

Caracterização petrográfica e mineralógica de brechas magmático-hidrotermais no complexo alcalino de Itatiaia, estado do Rio de Janeiro: ocorrências de fluorita e minerais de ETR

The meso-cenozoic alkaline magmatism in Rio de Janeiro is represented by several plutonic/volcanic bodies, which occurred on W-E fracture and lineaments zones associated with the development of the Southeastern Brazilian Rift. One of main occurrences of those rocks is the Itatiaia Alkaline Complex (IAC), which consists of nepheline-syentite, dykes of trachyte/phonolite and magmatic-hydrothermal breccia. This paper shows petrographic and mineralogical caracterization (optical petrography, SEM-EDS and XRD analysis) of breccia. The breccia comprises lithoclasts of trachyte and cristaloclasts of biotite and K-feldspar immersed in microcrystalline groundmass. As hydrothermal mineral it were observed albite, carbonate, sanidine, clinochlore, pyrite (also in lithoclasts), biotite, apatite, fluorite, tourmaline, sericite, synchysite (A parisite), Nb-rutilo, sphalerite, epidote and orthoclase (adularia?). Minerals associated to weathering are gibbsite, melanterita and cancrinite. The new occurrences of fluorcarbonates (synchysite and parisite) and Nb-rutile at the IAC, are rare minerals commonly associated with hydrothermal alteration in alkaline-carbonatite complexes. The hydrothermal paragenesis, as well as comparison with data from other alkaline associations of Rio de Janeiro, suggests that the study rocks consist in late-magmatic/hydrothermal autobreccia, associated to epithermal systems of low sulphidation subtype and generated during interaction with low-temperature alkalic fluids. The description of fluorite and REE-minerals at IAC opens the possibility to find those minerals in other alkaline complex of Rio de Janeiro.

Pleistocene alkaline rocks of Martin Vaz volcano, South Atlantic: low-degree partial melts of a CO2-metasomatized mantle plume

ABSTRACT Several alkaline massifs on inland southeastern Brazil extend offshore, roughly parallel to ~20° S, through a seamount chain of the Vitoria-Trindade ridge. This paper presents the first extensive work on the Martin Vaz volcano through whole-rock and Sr and Nd isotopic composition of volcanic and subvolcanic lithotypes from the Martin Vaz Island, located at the easternmost of this volcanic chain. These alkaline rocks were generated during the Plio-Pleistocene (~0.47 My, 40Ar/39Ar dating in whole-rock) and represent the crystallization of sodic magmas of nephelinitic composition that evolved through fractional crystallization towards phonolites. Calculations from P–TLiquidus using PELE software show temperatures of 1045°C and 818°C, viscosity of 2.47 log Poise and 5.02 log Poise, and densities of 2.57 g/cm3 and 2.26 g/cm3 for nephelinite and phonolite, respectively. Like in Trindade Island, the nephelinitic volcanism in Martin Vaz may represent a Strombolian and/or Hawaii-type eruption due to low viscosity magma according to its physical properties whereas phonolitic intrusions present higher viscosity characteristics forming lava domes. The 87Sr/86Sr (~ 0.703800) and 143Nd/144Nd (~ 0.512750) ratios of lavas from the seamounts and Martin Vaz do not vary significantly, pointing to partial melting process from a homogeneous mantle source showing isotope signature close to HIMU. Beside the restrict variation on these isotopic ratios, a conspicuous enrichment in incompatible trace elements, mainly LREE, indicates that metasomatism is a recent process and not a long-term source characteristic. Non-modal partial melting models (fractional melting and batch melting) suggest that the source of the Martin Vaz magmatism is consistent with the garnet-lherzolite mantle stability field (>90 km depth; Tb/Yb >0.7), generated about 3.0 GPa by very small degree of partial melting of an enriched wet mantle source (F = 0.03–0.04) with 2.5 wt. % of CO2.

Tuffaceous Mud is a Volumetrically Important Volcaniclastic Facies of Submarine Arc Volcanism and Record of Climate Change

The inorganic portion of tuffaceous mud and mudstone in an oceanic island arc can be mostly volcanic in origin. Consequently, a large volume of submarine volcaniclastic material is as extremely finegrained as products of subaerial eruptions ( 75% of the mud is volcanic, and that most of it was derived from proximal rear arc volcanic sources. It faithfully preserves integrated igneous geochemical information about arc evolution in much the same way that terrigenous shales track the evolution of continental crust. In addition, their high sedimentation rate enables high resolution study of climate cycles, including the effects of Pleistocene glaciation on the behavior of the Kuroshio Current in the Shikoku Basin south of Japan.

Characterization of Nd Radiogenic Isotope Signatures in Sediments From the Southwestern Atlantic Margin

Based on radiogenic isotope systems of neodymium (Epsilon Neodymium-ɛNd(0), 143Nd/144Nd, 147Sm/144Nd and Sm-Nd Depleted Mantle Model ages-TDM) from current and previous data from current and previous data of continental rocks and associated sediments, this work provides the provenance isotope signatures of Holocene sediments from the Southwestern Atlantic Margininof Core 7616 and Core 7620. The isotopic variability reported along the cores 7616 and 7620 are related to paleoceanographic and paleoclimatic changes. Further, we display the first Nd radiogenic isotope system distribution map of the study area. , this work provides provenance distribution of bulk sediment deposited those proxies inin southeastern South American upper margin, from ~7000 to 900 cal. yr BP. Here The map is the result of the principal component and clustering analyses of data compiled from the Ribeira belt, Luis Alves craton and Parana basin rocks. Differences in we present a A Nd isotope isotopic signatures allowed the distinction between the sediment sources of the cores.a conspicuous large range of The variabilitydifference between them. Core 7616 exhibits ɛNd(0) average value of -10.5, which is similar to, TDM =1.4 Ga and 143Nd/144Nd =0.512103, while the Core 7620 shows ɛNd(0) average value of -17.5, TDM =1.8 Ga and 143Nd/144Nd =0.51177. The relative more radiogenic Nd ratios from the Core 7616 are associated to the contribution of sediments from the Parana basin, nevertheless less radiogenic values are observed along the Core 7616 between ~2000 and 1800 cal. yr BP. We have attributed the lower Nd ratios, recorded during this interval, to the decreasing influence from the Rio de la Plata estuary in the Core 7616. Remarkable less radiogenic Nd ratios are also recorded in the Core 7620 during the late Holocene. The intensification of the NE winds and the South America Summer Monsoon (SASM) enhanced the terrigenous input from the Paraiba do Sul River and southwards sediment transport by the Brazil Current (BC), providing higher contribution of less radiogenic Data from Core metasediments from the Paraiba do Sul geotectonic domain to the Core 7620.

Geoconservação da Ilha da Trindade: Principais Aspectos e Potencial de Uso

The Trindade Island is located near to the parallel of Vitoria (ES), 1167 km from the Brazilian coast. It is considered as the most recent manifestation of alkaline volcanism of Brazil (<3.6 Ma). It represents the eastern end of an EW chain of guyots (Vitoria-Trindade) generated along a fracture zone associated to mantle plume activity beneath the South American plate. The island is composed by alkaline olivine-nephelinitic and phonolitic volcanic cycles, which evolves through fractional crystallization process. The lithostratigraphic units of the island (Trindade Complex, Desejado Sequence, Morro Vermelho Formation, Valado Formation and Vulcao do Paredao) show important volcanic structures, easily recognizable through the geomorphology of great landscape value. Structures like necks and radial dikes (eroded volcanic systems), vast plateaus formed by lava flows and/or pyroclastic deposits and even part of a large cinder cone, are among the structures observed in the island. Trindade justify its importance of geoconservation and maintenance its geosites beyond the use of its heritage in a sustainable manner due to the values: geological (petrological/petrogenetic and volcanological), geomorphological and historic (which refers to the time of the voyages of discovery). However, Trindade does not have potential for the development of geotourism or any activity comprising large numbers of people because it is distant and inaccessible. Its great value is scientific and didactic value, being an important field school for teaching, not only geology, but how different areas of the natural sciences.