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The South Atlantic Anomaly (SAA) is a region at Earth’s surface where the intensity of the magnetic field is particularly low. Accurate characterization of the SAA is important for both fundamental understanding of core dynamics and the geodynamo as well as societal issues such as the erosion of instruments at surface observatories and onboard spacecrafts. Here, we propose new measures to better characterize the SAA area and center, accounting for surface intensity changes outside the SAA region and shape anisotropy. Applying our characterization to a geomagnetic field model covering the historical era, we find that the SAA area and center are more time dependent, including episodes of steady area, eastward drift and rapid southward drift. We interpret these special events in terms of the secular vari‑ation of relevant large‑scale geomagnetic flux patches on the core–mantle boundary. Our characterization may be used as a constraint on Earth‑like numerical dynamo models.

SUMMARY The Andean Subduction Zone is one of the longest continuous subduction zones on Earth. The relative simplicity of the two-plate system has makes it an ideal natural laboratory to study the dynamics in subduction zones. We measure teleseismic S and SKS traveltime residuals at >1000 seismic stations that have been deployed across South America over the last 30 yr to produce a finite-frequency teleseismic S-wave tomography model of the mantle beneath the Andean Subduction Zone related to the Nazca Plate, spanning from ∼5°N to 45°S and from depths of ∼130 to 1200 km. Within our model, the subducted Nazca slab is imaged as a fast velocity seismic anomaly. The geometry and amplitude of the Nazca slab anomaly varies along the margin while the slab anomaly continues into the lower mantle along the entirety of the subduction margin. Beneath northern Brazil, the Nazca slab appears to stagnate at ∼1000 km depth and extend eastward subhorizontally for >2000 km. South of 25°S the slab anomaly in the lower mantle extends offshore of eastern Argentina, hence we do not image if a similar stagnation occurs. We image several distinct features surrounding the slab including two vertically oriented slow seismic velocity anomalies: one beneath the Peruvian flat slab and the other beneath the Paraná Basin of Brazil. The presence of the latter anomaly directly adjacent to the stagnant Nazca slab suggests that the plume, known as the Paraná Plume, may be a focused upwelling formed in response to slab stagnation in the lower mantle. Additionally, we image a high amplitude fast seismic velocity anomaly beneath the Chile trench at the latitude of the Sierras Pampeanas which extends from ∼400 to ∼1000 km depth. This anomaly may be the remnants of an older, detached slab, however its relationship with the Nazca–South America subduction zone remains enigmatic.

Fil: Portner, Daniel Evan. University of Arizona; Estados Unidos. Carnegie Institution Of Washington; Estados Unidos

The Florianopolis Dyke Swarm is located in Santa Catarina island, comprising also the adjacent continental area, and belog to the Parana Magmatic Province (PMP). the dyke outcrops in the island are 0.1-70.0 m thick and most of them are coast-parallel (NE-SW trending), with subordinate NW-SE trending. the vast majority of the dykes has SiO2 varying from 50 to 55 wt% and relatively high-Ti (TiO2>3.0wt%) contents and these rocks were divided using the criteria commonly used to distinguish the different magma types identified in the volcanic rocks from the PMP. The Urubicy dykes (Sr>550 ppm) are the most abundant and some of them experienced crustal contamination reaching to 10%, as evidenced by low P2O5/K2O (0,30-0,21), high (Rb/Ba)PM (1.0-2.2), and radiogenic Sr and Pb isotopic composition (87Sr/86Sri = 0.70716-back to 125 ma) and 206Pb/204Pbm up to 19.093. The Pitanga (Sr

O Kimberlito Indaiá, situado 25 km a norte da cidade de Monte Carmelo, na região oeste de Minas Gerais, é intrusivo em granitóides cataclasados associados ao Grupo Araxá, na Faixa de Dobramentos Brasília. O corpo apresenta forma subcircular e diâmetro principal de 220 m na direção NE-SW; em sua porção NE ocorre uma intrusão de kamafugito associada de 120 m de diâmetro. A rocha exibe coloração cinza escura e textura inequigranular bem destacada, sendo constituída por uma matriz afanítica na qual estão dispersos macrocristais de olivina (abundantes), ilmenita, flogopita e piroxênio, além de xenólitos mantélicos e crustais de dimensões variadas. Os xenólitos mantélicos são constituídos, em ordem de abundância, por harzburgitos (41%), lherzolitos (37%), dunitos (14%), mica piroxenitos (6%) e xenólito polimítico (2%). Os harzburgitos, lherzolitos e dunitos exibem, além das fases primárias (olivina, enstatita e diopsídio), teores subordinados de espinélio, cromita, diopsídio, flogopita, ilmenita e raro anfibólio. A textura predominante é grossa (protogranular), ocorrendo subordinadamente as texturas granoblástica e porfiroclástica. Análises químicas por microssonda eletrônica revelaram que as fases silicáticas possuem valores Mg/(Mg+Fe) maiores em harzburgitos e dunitos e menores nos lherzolitos. Os mica piroxenitos (flogopita+enstatita+ilmenita) e os xenólitos polimíticos (olivina+diopsídio+ilmenita+flogopita) apresentam mineralogia mais complexa, possuindo em adição magnetita, perovskita, barita, zircão, badeleíta, pentlandita, galena e uma fase mineral rara ainda não identificada. As razões Mg/(Mg+Fe) das fases silicáticas são mais baixas que as dos xenólitos de peridotitos. As características químicas da flogopita e da ilmenita destes xenólitos indicam similaridades com os xenólitos da suíte MARID. A variedade textural dos xenólitos estudados, de grossa (protogranular) a granoblástica, é uma evidência de processos de recristalização por deformação mecânica. A aplicação de geotermômetros da literatura para os espinélio lherzolitos forneceu temperaturas de equilíbrio entre 655 a 908°C, em concordância com dados de outras intrusões do oeste mineiro. A presença de flogopita e ilmenita sob a forma de bolsões nos xenólitos de peridotitos, bem como nos mica piroxenitos e xenólitos polimíticos, são indicativos da atuação de processos de metassomatismo no manto da região de Monte Carmelo. The Indaiá Kimberlite, located 25 km North of Monte Carmelo in the western Minas Gerais State, is intrusive in cataclased granitoids related to the Araxá Group, part of the Brasilia Fold Belt. The body has a subcircular 220-meter head-diameter shape (NE-SW); its an associated 120-meter diameter intrusion of kamafugite occurs in its NE portion. The rock is dark gray and displays prominent inequigranular texture, consisting of an aphanitic matrix in which macrocrystals of olivine (abundant), ilmenite, pyroxene and phlogopite, as well as mantle and crustal xenoliths of different sizes are dispersed. These mantle xenoliths are composed, in order of abundance, by harzburgites (41%), lherzolite (37%), dunite (14%), mica pyroxenite (6%) and polymictic xenolith (2%). Harzburgites, lherzolites and dunites exhibit, besides the main minerals (olivine, enstatite and diopside), small amounts of spinel, chromite, diopside, phlogopite, ilmenite and rare amphibole. The predominant texture is coarse (protogranular); subordinately, granoblastic and porfiroclastic textures are observed. Chemical analysis by electron microprobe showed that the silicate phases have higher values of Mg/(Mg+Fe) in harzburgites and dunites as compared to lherzolites. Mica pyroxenite (enstatite + phlogopite + ilmenite) and polymictic xenoliths (olivine + diopside + phlogopite + ilmenite) have more complex mineralogical features than the other xenoliths, showing in addition magnetite, perovskite, barite, zircon, baddeleyite, pentlandite, galena and a rare phase not yet identified. The Mg/(Mg+Fe) ratio in silicate phases are lower than in the peridotite xenoliths. The chemical characteristics of phlogopite and ilmenite of these xenoliths indicate similarities with the MARID suite xenoliths. The textural variety of the studied xenoliths, from coarse (protogranular) to granoblastic, is an evidence of crystallization processes by mechanical deformation. T he application of geothermometers described in the literature to spinel lherzolites yielded equilibrium temperatures between 655 and 908 ° C, agreeing with data from other intrusions from western Minas Gerais. The presence of ilmenite and phlogopite in the form of pockets in peridotite xenoliths, as well as in mica pyroxenite and polymictic xenoliths, is indicative of mantle metasomatic processes in the Monte Carmelo region.

Protogranular spinel-peridotite mantle xenoliths and their host sodic alkaline lavas of Cretaceous to Paleogene age occur at the same latitude ≈26°S in central eastern Paraguay and Andes. Na- alkaline lavas from both regions display similar geochemical features, differing mainly by higher Rb content of the Paraguayan samples. Sr, Nd, and Pb isotope ratios are also similar with predominant trends from depleted to enriched mantle components. The mantle xenoliths are divided into two main suites, i.e. relatively low in potassium and incompatible elements, and high in potassium and incompatible elements. The suite high in potassium occurs only in Paraguay. Compositions of both suites range from lherzolite to dunite indicating variable “melt extraction”. Clinopyroxenes from the xenoliths display variable trace element enrichment/depletion patterns compared with the pattern of average primitive mantle. Enrichment in LREE and Sr coupled with depletion of Nb, Ti and Zr in xenoliths from both areas are attributed to asthenospheric metasomatic fluids affecting the lithospheric mantle. Metasomatism is apparent in the sieve textures and glassy drops in clinopyroxenes, by glassy patches with associated primary carbonates in Paraguayan xenoliths. Trace element geochemistry and thermobarometric data indicate lack of interaction between xenoliths and host lavas, due to their rapid ascent. Sr and Nd isotope signatures of the Andean and Paraguayan xenoliths and host volcanic rocks plot mainly into the field of depleted mantle and show some compositional overlap. The Andean samples indicate a generally slightly more depleted mantle lithosphere. Pb isotope signatures in xenoliths and host volcanic rocks indicate the existence of a radiogenic Pb source (high U/Pb component in the source) in both areas. In spite of the distinct tectonic settings, generally compressive in the Central Andes (but extensional in a back-arc environment), and extensional in Eastern Paraguay (rifting environment in an intercratonic area), lavas and host xenoliths from both regions are similar in terms of geochemical and isotopic characteristics

Chemical data of heavy minerals from Lages diatremes in southern Brazil have been studied with the aim of characterizing the sample source(s). Three groups of minerals are recognized: I) aluminian-chromian pyroxene, pyrope garnet and chromian spinel, which represent disaggregated fragments of spinel, spinel+garnet and garnet facies peridotite; II) low-Cr aluminian pyroxene that occurs as megacrysts are high pressure phases (7-12 kb) being crystallized from an alkaline-like evolving magma; III) low-Cr aluminian diopside of crustal origin. Evidence of carbonatitic cryptic metasomatic enrichment is shown by clinopyroxenes of Groups I and II. The data do not support a kimberlitic affinity as it has been suggested for the diatremes. Rather, they are interpreted as vents related to the alkaline magmatism affecting the area in Late Cretaceous. The alkaline parental magma of the pyroxene megacrysts was generated from a metasomatized mantle at garnet facies that incorporated fragments of the surrounding still fertile mantle. Presumably at spinel-facies level the magma began to fractionate the megacrysts, whose crystallization proceeded over a large range of falling pressure and temperature. The chemical similarities between Group III clinopyroxenes and those from the differentiated lithotypes indicate that the magma carried this mineral phase on its evolution, at crustal conditions, towards a more evolved alkaline composition. Still, a non-cognate origin for the Group III clinopyroxenes cannot be discarded.Dados químicos de minerais pesados dos diatremas de Lages no sul do Brasil foram estudados com o propósito de caracterizar as fontes das rochas. Três grupos de minerais são reconhecidos: I) piroxênio aluminoso-cromífero, granada piropo e espinélio cromífero, representando fragmentos desagregados de espinélio, espinélio+granada e granada da fácies peridotito; II) piroxênio aluminoso com baixo Cr, correspondendo a megacristais, com as fases de alta pressão (7-12 kb) cristalizadas a partir de magma alcalino em evolução; III) diopsídio aluminoso com baixo Cr e origem crustal. Clinopiroxênios dos Grupos I e II mostram evidências de enriquecimento metassomático críptico de natureza carbonatítica. Os dados não confirmam a afinidade kimberlítica sugerida para esses diatremas. Ao contrário, eles são interpretados como condutos relacionados ao magmatismo alcalino que afetou a área no Cretáceo Superior. O magma parental alcalino dos megacristais de piroxênio foi originado a partir de um manto metassomatizado na fácies granada que aprisionou fragmentos do ainda fértil manto adjacente. Presumivelmente na fácies espinélio teve início o fracionamento dos megacristais, cuja cristalização se deu em condições de pressão e temperatura decrescentes. As similaridades entre os clinopiroxênios do Grupo III e aqueles dos litotipos mais diferenciados sugere que essa fase mineral foi transportada pelo magma no curso de sua evolução, em condições crustais, para uma composição alcalina mais evoluída. Ainda, uma formação não-cogenética para os clinopiroxênios do Grupo III não pode ser descartada.