Interpreter's Corner

Basaltic reservoirs have produced hydrocarbon from Yurihara Field in Japan, Quiko Depression in China, and Padra Field of Cambay Basin and Western Offshore Basin in India. The availability of fractured, altered, and vesicular basalts contributes to reservoir development in this stratigraphic unit. This study is conducted in the Kutch-Saurashtra Basin, located at the western continental margin of the Indian subcontinent wherein, the Deccan basalt, with a thickness range of 200–2500 m, overlies Mesozoic sediments. The Jurassic and Cretaceous sediments constitute the main source rock in the area. Several wells have been drilled through the entire basalt section, and some are hydrocarbon bearing in basalt. The entire basalt section in the study area has been classified into four major units using gamma-ray logs. These units have been further subdivided into individual flows and correlated all over the basin. Analysis shows that the base of an individual basalt flow is massive, and the top is differentially altered. Crossplot analysis of P-impedance and VP/VS ratio carried out on logs delineates a zone of moderately weathered/altered basalt, which is due to spheroidal weathering and calibrated with sidewall cores. These moderately altered zones between two successive flows of basalt are the probable reservoir facies for hydrocarbon accumulation, provided that there is an overlying seal in the form of massive or completely altered basalt. Three-dimensional seismic data in the area show an alternating reflection pattern in the basalt section due to the alternation of massive and weathered basalt. The seismic signature of basalt in the area is similar to that of a sedimentary sequence in any given area. Continuity of the identified individual flows in seismic scale has been propagated over the entire seismic, and subsequent inversion has facilitated the deciphering of the probable hydrocarbon-bearing locales within basalt.

A Protolith Reconstruction Model (PRM) for Metabasalt: Quantitative Protolith and Mass Transfer Estimation Based on Machine-learning Approach

Mass transfer in rocks provides a direct record of fluid–rock interaction within the Earth, including metamorphism, metasomatism, and hydrothermal alteration. However, mass transfer analyses are usually limited to local reaction zones where the protoliths are evident in outcrops (1–100 m in scale), from which regional mass transfer can be only loosely constrained due to uncertainty in protolith compositions. In this study, we developed protolith reconstruction models (PRMs) for metabasalt based on a machine learning approach. We constructed PRMs through learning multi-element correlations among basalt compositional datasets, including mid-ocean ridge, ocean island, and island arc basalts. The PRMs were designed to estimate trace-element compositions from inputs of 2–9 selected trace elements, and basalt trace-element compositions (e.g., Rb, Ba, U, K, Pb, Sr, and rare earth elements) were estimated from only four inputs with a reproducibility of ~0.1 log10 units (i.e., ±25%). Using Th, Nb, Zr, and Ti, which are typically immobile during metamorphism, as input elements, the PRM was verified by applying it to seafloor altered basalt with known protoliths. When suitable immobile elements are incorporated, a PRM can yield unbiased and accurate mass transfer analysis of any metabasalt with unknown protolith.

CARACTERIZAÇÃO GEOQUÍMICA DE BASALTO AMIGDALÓIDE ALTERADO PARA USO AGRONÔMICO

Silicate rocks, especially those of volcanic origin, have been studied and applied for rock stonemeal in agricultural soils. Recently, in 2013, the use of rock materials was approved by the Brazil’s fertilizer legislation, receiving the name of remineralizers. Due to the complex nature and mineral variability of the rocks, and the lack of consistent scientific studies, the regulations for this new mineral fertilizer product require that it has characteristics of geochemical composition that provide little information about its fertilizing properties. The present work studies the geochemical characteristics of an altered amygdaloid basalt mined, observing the release of mineral nutrients through leaching tests aiming to evaluated its use with as remineralized. The studied sample comes from mine in Santo Antônio da Patrulha, RS, from an altered basalt amygdaloid horizon, containing adsorbent minerals. Aliquots of mined material and zeolite’s amygdala selected were analyzed to assess their mineralogical, chemical and base composition. Mined material aliquots in two granulometries liable for agronomic application, were subjected to leaching in a solution of citric acid (1.6%) and deionized water, for two solid / liquid ratios (1:1 and 1:4). The results indicated the presence of zeolites and smectites in the mineral matrix, the sum of bases and levels of toxic elements that fit the sample in the category of remineralizers, and the release of mineral nutrients: Ca, Cu, Fe, K, Mg, Mn, Na, P, Si and Zn, with the highest concentration of Si and Ca in solution. The release of a significant amount of nitrogen in the form of nitrate was also observed, with makes this altered rock very interesting to agriculture. Key words: silicate rock, altered amygdaloid basalt, remineralizer, geochemical characterization.

New Conceptual Model for the Magma-Hydrothermal-Tectonic System of Krafla, NE Iceland

The complexity of the Krafla volcano and its geothermal system(s) has puzzled geoscientists for decades. New and old geoscientific studies are reviewed in order to shed some light on this complexity. The geological structure and history of the volcano is more complex than hitherto believed. The visible 110 ka caldera hosts, now buried, an 80 ka inner caldera. Both calderas are bisected by an ESE-WNW transverse low-density structure. Resistivity surveys show that geothermal activity has mainly been within the inner caldera but cut through by the ESE-WNW structure. The complexity of the geothermal system in the main drill field can be understood by considering the tectonic history. Isotope composition of the thermal fluids strongly suggests at least three different geothermal systems. Silicic magma encountered in wells K-39 and IDDP-1 indicates a hitherto overlooked heat transport mechanism in evolved volcanos. Basaltic intrusions into subsided hydrothermally altered basalt melt the hydrated parts, producing a buoyant silicic melt which migrates upwards forming sills at shallow crustal levels which are heat sources for the geothermal system above. This can explain the bimodal behavior of evolved volcanos like Krafla and Askja, with occasional silicic, often phreatic, eruptions but purely basaltic in-between. When substantial amounts of silicic intrusions/magma have accumulated, major basalt intrusion(s) may “ignite” them causing a silicic eruption.

Oceanic slab-top melting during subduction: Implications for trace-element recycling and adakite petrogenesis

Arc volcanism and trace-element recycling are controlled by the devolatilization of oceanic crust during subduction. The type of fluid—either aqueous fluids or hydrous melts—released during subduction is controlled by the thermal structure of the subduction zone. Recent thermomechanical models and results from experimental petrology argue that slab melting occurs in almost all subduction zones, although this is not completely supported by the rock record. Here we show via phase equilibrium modeling that melting of either fresh or hydrothermally altered basalt rarely occurs during subduction, even at water-saturated conditions. Melting occurs only along the hottest slab-top geotherms, with aqueous fluids being released in the forearc region and anatexis restricted to subarc depths, leading to high-SiO2 adakitic magmatism. We posit that aqueous fluids and hydrous melts preferentially enhance chemical recycling in “hot” subduction zones. Our models show that subducted hydrothermally altered basalt is more fertile than pristine basaltic crust, enhancing fluid and melt production during subduction and leading to a greater degree of chemical recycling. In this contribution, we put forward a petrological model to explain (the lack of) melting during the subduction of oceanic crust and suggest that many large-scale models of mass transfer between Earth’s surface and interior may require revision.

Origin of Iron Ore Nuggets (“Bohnerze”) through Weathering of Basalt as Documented by Pebbles from the Herbstlabyrinth, Breitscheid-Germany

Namen te študije je bila raziskava izvora kep železa, ki so bile najdene v Herbstlabyrinthu pri Breitscheidu, severni Hessen/Nemčija. Vsega skupaj je bilo zbranih deset prodnikov. Petrografsko prodnike lahko razdelimo v tri skupine: precej svež bazalt, opazno ali popolnoma preperel bazalt (vsebuje železova zrna) in temen peščenjak. Železova zrna izvirajo iz procesa preperevanja (oksidacija in hidratacija) bazaltnih prodnikov. Geokemična študija je pokazala, da so izvorni prodniki bazalta alkalnega tipa in da pripadajo Na-seriji. Magma, iz katere so nastali, je bila nerazvita in je izvirala iz peridotitnega plašča. This study was conducted to investigate the origin of the iron nuggets that have been found in the Herbstlabyrinth at Breitscheid, northern Hessen/Germany. A total of ten pebbles were collected. Petrographically, the pebbles can be divided into three groups: rather fresh basalts, markedly or totally altered basalt (including the iron nuggets) and a dark sandstone. The iron nuggets originate through a progressing alteration (oxidation and hydration) of the basaltic pebbles. The geochemical study shows that the original fresh basaltic pebbles are of alkali type and belong to the Na-series. The magma that gave rise to them was primitive and originated in peridotite mantle source.