Flow-by-flow chemical stratigraphy and evolution of thirteen Serra Geral Group basalt flows from Vista Alegre, southernmost Brazil

The geochemical characterization of thirteen Serra Geral Group flows in the Vista Alegre region (RS-SC), southern Brazil, displays the homogeneous basaltic composition near 50 wt.% SiO2. Each of the five basal flows (Pitanga-type, high-Ti/Y ~600, TiO2 > 3 wt.%) and eight upper flows (Paranapanema-type, medium Ti/Y ~400, TiO2 > 2 wt.%) can be identified from their chemical composition; sets of flows have parallel variation in chemical composition. The flow-by-flowcorrelation in four sections shows the horizontal position of the flows in three profiles and an approximately 200-m downdrop of the Itapiranga block with respect to the Frederico Westphalen block. The world-class amethyst geode mineralization and the systematic presence of native copper in the basalts make the correlation of great geological and economic significance.<br>A caracterização geoquímica de treze derrames do Grupo Serra Geral na região de Vista Alegre (RS e SC), sul do Brasil, exibe uma composição basáltica homogênea próxima a 50% de SiO2. Os cinco derrames basais são classificados quimicamente como tipo Pitanga (alto Ti/Y ~600 e TiO2 > 3 em peso percentual), os demais oito derrames possuem médio Ti/Y ~400 com TiO2 ~2.5 em peso percentual, sendo classificados como magma tipo Paranapanema. Cada derrame pode ser identificado através de sua composição química e correlacionado, com variação paralela entre os perfis estudados. A correlação derrame a derrame nos quatro perfis demonstra uma posição horizontal em três perfis e um rejeito vertical de aproximadamente 200 m do bloco Itapiranga em relação ao bloco Frederico Westphalen. A presença de jazidas de ametista em geodos e a sistemática ocorrência de cobre nativo nos basaltos da região tornam a correlação de grande significado geológico e econômico

The Prinsen af Wales Bjerge formation lavas, East Greenland: The transition from tholeiitic to alkalic magmatism during Palaeogene continental break-up

We present elemental and isotopic (Sr-Nd-Pb-Hf-Os-He) data on primitive alkalic lavas from the Prinsen af Wales Bjeige, East Greenland. Stratigraphical, compositional and Ar-40-Ar-39 data indicate that this inland alkalic activity was contemporaneous with the upper parts of the main tholeiitic plateau basalts and also postdated them. The alkalic rocks show a marked crustal influence, indicating establishment of new magmatic plumbing systems distinct from the long-lived coastal systems that fed the relatively uncontaminated plateau basalts. The least contaminated lavas have high He-3/He-4 isotope ratios (R/R-A 12.4-18.5), sub-chondritic Os-187/(OSi)-O-88 (0.120-0.126), low epsilonNd(i) (similar to + 4) and epsilonHf(i) (similar to+ 6) that plot below the 'Nd-Hf mantle array', and trace element characteristics similar to HIMU ocean. island basalt. (OIB). The uncontaminated magma is inferred to have more radiogenic Pb-206/Pb-204 values (>19.2) than the plateau basalts and Icelandic basalts, and thus represents a possible 'enriched' component to explain the compositional variations within the plateau basalts. One model to explain these compositional features is preferential melting of recycled material within the plume upwelling beneath the thick lithospheric cap, with He-3 contributed from volatile-rich fluids from elsewhere in the Icelandic plume. The exact nature of the recycled component is not yet resolved, although Hf isotope compositions rule out any significant role for recycled pelagic sediment, and the low Os-187/Os-188 limits the participation of recycled basaltic material and argues instead for a contribution from the mantle section of the recycled slab

The petrogenesis of sodic island arc magmas at Savo volcano, Solomon Islands

Savo, Solomon Islands, is a historically active volcano dominated by sodic, alkaline lavas, and pyroclastic rocks with up to 7.5 wt% Na2O, and high Sr, arc-like trace element chemistry. The suite is dominated by mugearites (plagioclase–clinopyroxene–magnetite ± amphibole ± olivine) and trachytes (plagioclase–amphibole–magnetite ± biotite). The presence of hydrous minerals (amphibole, biotite) indicates relatively wet magmas. In such melts, plagioclase is relatively unstable relative to iron oxides and ferromagnesian silicates; it is the latter minerals (particularly hornblende) that dominate cumulate nodules at Savo and drive the chemical differentiation of the suite, with a limited role for plagioclase. This is potentially occurring in a crustal “hot zone”, with major chemical differentiation occurring at depth. Batches of magma ascend periodically, where they are subject to decompression, water saturation and further cooling, resulting in closed-system crystallisation of plagioclase, and ultimately the production of sodic, crystal and feldspar-rich, high-Sr rocks. The sodic and hydrous nature of the parental magmas is interpreted to be the result of partial melting of metasomatised mantle, but radiogenic isotope data (Pb, Sr, Nd) cannot uniquely identify the source of the metasomatic agent. Electronic supplementary material The online version of this article (doi:10.1007/s00410-009-0410-9) contains supplementary material, which is available to authorized users

210Pb-226Ra disequilibria in young gas-laden magmas

We present new U- Th- Ra- Pb and supporting data for young lavas from southwest Pacific island arcs, Eyjafjallajökull, Iceland, and Terceira, Azores. The arc lavas have significant U and Ra excesses, whereas those from the ocean islands have moderate Th and Ra excesses, reflecting mantle melting in the presence of a water-rich fluid in the former and mantle melting by decompression in the latter. Differentiation to erupted compositions in both settings appears to have taken no longer than a few millennia. Variations in the (Pb/Ra) 0 values in all settings largely result from degassing processes rather than mineral-melt partitioning. Like most other ocean island basalts, the Terceira basalt has a 210 Pb deficit, which we attribute to ∼8.5 years of steady Rn loss to a CO-rich volatile phase while it traversed the crust. Lavas erupted from water-laden magma systems, including those investigated here, commonly have near equilibrium (Pb/Ra) 0 values. Maintaining these equilibrium values requires minimal persistent loss or accumulation of Rn in a gas phase. We infer that degassing during decompression of water-saturated magmas either causes these magmas to crystallize and stall in reservoirs where they reside under conditions of near stasis, or to quickly rise towards the surface and erupt