Mabel Norma Costas Ulbrich

Petrology, geochemistry and Sr–Nd isotopes of the Trindade and Martin Vaz volcanic rocks (Southern Atlantic Ocean)

Abstract Geochemical, mineralogical and Sr–Nd isotopic analyses were performed on the strongly undersaturated volcanic rocks of the Trindade and Martin Vaz islands, located in the South Atlantic Ocean (Brazil). The magmatic activity at Trindade was essentially bimodal, generating various ultrabasic rocks and mesocratic to leucocratic phonolites. The youngest volcanic events are made up exclusively of ultrabasic rocks, whereas the older episodes are characterized by abundant phonolites. Basic rocks (45

The volcanic-subvolcanic rocks of the fernando de noronha archipelago, southern atlantic ocean: Mineral chemistry

Fernando de Noronha archipelago presents an older Remédios Formation with subvolcanic intrusions, belonging to two different alkaline series, the sodic (undersaturated: basanites, tephrites, essexites, tephriphonolites, phonolites), and potassic ones (mildly undersaturated to silicic, with alkali basalts, basaltic trachyandesites, trachyandesites, trachytes), and lamprophyres. The upper Quixaba Formation presents nephelinite flows and basanites. A third minor unit, São José, is constituted by basanites carrying mantle xenoliths. Magnesian olivines occur in the Remédios basanites and alkali basalts, and in nephelinites. Melilites are present as groundmass grains in melilite melanephelinites (MEM). Clinopyroxenes (cpx) are mostly salites to titaniferous salites (Remédios sodic series), grading into aegirines in the differentiated aphyric phonolites. Cpx in the lamprophyres show disequilibrium textures. In the Quixaba flows, cpx are salites, enriched in Mg (especially in MEM). Amphiboles, remarkably, are common in tephriphonolites and phonolites and in basaltic trachyandesites, sometimes with disequilibrum zoning textures, and a conspicuous phase in lamprophyres. Dark micas are present as groundmass plates in MEM, OLM and PYM (olivine and pyroxene melanephelinites), with compositional variety (enriched in Ti, Ba, Sr) depending on the composition of the parent rock; BaO can be as high as 16–19%. Feldspars crystallize as calcic plagioclases, sanidines and anorthoclases, depending on the rock types, as phenocrysts and in groundmass, both in Quixaba and Remédios rocks; they are absent in nephelinites. Nephelines are found in Remédios sodic series types and Quixaba rocks. Haüyne and noseane are rarely observed in Remédios rocks.

Concentration of Inorganic Elements Content in Benthic Seaweeds of Fernando de Noronha Archipelago by Synchrotron Radiation Total Reflection X-Ray Fluorescence Analysis (SRTXRF).

SRTXRF was used to determine As, Ba, Br, Ca, Co, Cr, Cs, Cu, Dy, Fe, K, Mn, Mo, Ni, Pb, Rb, Sr, Ti, V, and Zn in eleven seaweed species commonly found in Fernando de Noronha: Caulerpa verticillata (J. Agardh) (Chlorophyta), Asparagopsis taxiformis (Delile), Dictyurus occidentalis (J. Agardh), Galaxaura rugosa (J. Ellis & Solander) J. V. Lamouroux, G. obtusata (J. Ellis & Solander) J. V. Lamouroux, G. marginata (J. Ellis & Solander) J. V. Lamouroux (Rhodophyta), Dictyota cervicornis (Kützing), Dictyopteris justii (J. V. Lamouroux), Dictyopteris plagiogramma (Montagne) Vickers, Padina gymnospora (Kützing) Sonder, and a Sargassum sp. (Phaeophyta). Data obtained were compared to those from the analysis of other parts of the world seaweeds using different analytical techniques and were found to be in general agreement in terms of major and minor elemental components. Results provide baseline information about the absorption and accumulation of these elements by macroalgae in the area.

Geochemistry of the alkaline volcanicsubvolcanic rocks of the Fernando de Noronha Archipelago, southern Atlantic Ocean

O Arquipelago de Fernando de Noronha mostra na sua ilha principal, na parte central, a Formacao Remedios (idade em torno de 8 - 12 Ma) constituida por rochas piroclasticas basais invadidas por diques, plugs e domos de variadas rochas igneas, capeadas por derrames e piroclasticas das rochas maficas a ultramaficas da Formacao Quixaba (idade de 1 a 3 Ma), separada da unidade inferior por uma superficie de erosao extensa e irregular. Uma serie predominante sodica de Remedios (basanitos, tefritos, tefrifonolitos, essexito, fonolitos) pode ser diferenciada da serie moderadamente potassica (alcali basaltos, traquiandesitos, traquitos), pertencente a mesma unidade. Ambas as series mostram tendencias geoquimicas geralmente continuas em diagramas de variacao de elementos maiores e tracos, indicando origem por cristalizacao fracionada (principalmente, separacao de minerais maficos, incluindo apatitas e titanitas). Sao encontradas evidencias mineralogicas e texturais apontando para origens hibridas em algumas rochas intermediarias (e.g., fenocristais reabsorbidos de piroxenio em traquiandesitos basalticos e alguns lamprofiros). As rochas primitivas do evento Quixaba, tambem alcalinas, sao mormente melanefelinitos fortemente insaturados e basanitos. A geologia (superficie de erosao), estratigrafia (dois eventos magmaticos distintos separados por periodo prolongado), petrografia (evento Remedios petrograficamente variado, o Quixaba mais uniforme) e geoquimica indicam que as ilhas sao o resultado de prolongado vulcanismo, alimentado por fundidos intermitentes de manto enriquecido, nao relacionado com atividade de pluma astenoferica.

Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications

The Pocos de Caldas Alkaline Massif, located in southeast Brazil, is composed of tinguaites, as well as phonolites and nepheline syenites (NeS), together with assorted pyroclastic rocks. The latter constitute deposits within the Vale do Quartel, and show fragments of alkaline rocks (NeS, tinguaites), basaltic and ultramafic volcanics/subvolvanics, sandstones, and quartz-feldspathic rocks from the crystalline basement. NeS fragments with melanite, previously unknown, and cm-sized fragments of phlogopite megacrysts from related conduit breccia were chosen for detailed studies. The NeS fragments are medium-to coarse grained and contain K-feldspar (Or 95-83), secondary albite (Ab 97), altered nepheline, clinopyroxene (diopside, minor hedenbergite) and partially altered, zoned, idiomorphic melanite. These NeS fragments are typically metaluminous, and point to the existence of similar intrusions hidden at depth, an important input to the massifs petrology. The phlogopite megacrysts are compositionally homogeneous, with high Ti, Mg and Al contents; Ba and F are minor components. Two phlogopite aliquots were dated and yielded identical Ar-40/Ar-39 plateau ages of 87.1(+/- 0.5) Ma and 86.6(+/- 0.5) Ma, which represent minimum and maximum values for phlogopite crystallization and breccia emplacement, respectively. Previously obtained ages for the Pocos de Caldas rocks are concentrated in the 75-86 Ma range interval.