Magmatic sill formation during dike opening

The origin of horizontal magma-filled sills is disputed, particularly for extensional settings where the opening of vertical dikes is the predicted mode of magma intrusion. We simulate long-term extension followed by short-term dike opening in a two-dimensional viscoelastic medium representing a plate spreading center. We show that dike opening in extensionally stressed lithosphere can reduce sublithospheric vertical stresses enough for sill opening given three conditions: (1) the Maxwell time of the asthenosphere is <5× the time interval between dike episodes; (2) the average density of the lithosphere is not much greater than the magma density; and (3) the depth of an axial valley is smaller than a few hundred meters. This mechanism explains the presence of sills along much of the axis of faster-spreading ridges and their absence along slower-spreading centers where thick dense lithosphere and/or sizeable axial valleys exist.

Naldrettite (Pd2Sb): A new find in Brazil and comparison with worldwide occurrences

ABSTRACT Naldrettite (Pd2Sb) is a PGM discovered in 2005 in Mesamax Northwest deposit, Ungava region, Quebec, Canada. Before and after its approval, PGM with the naldrettite type composition have been reported from a number of localities worldwide. Most frequently, naldrettite has been documented in magmatic Ni–Cu–PGE sulfide deposits, hydrothermal veins in porphyry coppers of the Cu–Au type, and PGE deposits of Alaskan-type zoned intrusions. Naldrettite has been occasionally found in metasomatic Sb–As sulfide ore, metamorphic Ni–oxide ore, and podiform chromitites, although these occurrences have not been fully constrained by solid chemical analyses or paragenetic reconstruction. In this paper we report the first discovery of naldrettite in Brazil. This new finding occurs in a chromitite sample collected in the Luanga Complex, a Neo-archaean layered intrusion in the Carajás Mineral Province. Paragenetic association with alteration assemblages (ferrianchromite, Fe-hydroxides, chlorite) suggests precipitation of naldrettite from metamorphic hydrothermal fluids. The average composition of the Luanga sample (Pd1.76Pt0.24)Σ2.00(Sb0.57As0.43)Σ1.00 shows major substitution of Pt and As. These elements were derived from the breakdown of primary sperrylite, and were incorporated in naldrettite deposited by percolating fluids, at temperature below 350 °C (maximum temperature registered by the crystallization of associated chlorite). An overview of documented occurrences indicates that naldrettite can form in a variety of igneous rocks (ultramafic, mafic, felsic), even involving minimal concentrations of Pd and Sb. Crystallization of naldrettite generally occurs in the post-magmatic stage due to the activity of hydrothermal fluids containing volatile species Sb, As, Bi, Te, and Pd due to its higher mobility compared with the other PGE. A major issue concerns the origin of fluids that can be: (1) “residual”, after the main crystallization of the host magma, (2) “metamorphic”, during regional metamorphism or serpentinization, and (3) “metasomatic”, emanating from an exotic magma intrusion. The combination of two or three of these factors is the most likely process observed in the naldrettite-bearing complexes.

A petrological and conceptual model of Mayon volcano (Philippines) as an example of an open-vent volcano

Mayon is a basaltic andesitic, open-vent volcano characterized by persistent passive degassing from the summit at 2463 m above sea level. Mid-size (< 0.1 km3) and mildly explosive eruptions and occasional phreatic eruptions have occurred approximately every 10 years for over a hundred years. Mayon’s plumbing system structure, processes, and time scales driving its eruptions are still not well-known, despite being the most active volcano in the Philippines. We investigated the petrology and geochemistry of its crystal-rich lavas (~ 50 vol% phenocrysts) from nine historical eruptions between 1928 and 2009 and propose a conceptual model of the processes and magmatic architecture that led to the eruptions. The whole-rock geochemistry and mineral assemblage (plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxide ± olivine) of the lavas have remained remarkably homogenous (54 wt% SiO2, ~ 4 wt% MgO) from 1928 to 2009. However, electron microscope images and microprobe analyses of the phenocrysts and the existence of three types of glomerocrysts testify to a range of magmatic processes, including long-term magma residence, magma mixing, crystallization, volatile fluxing, and degassing. Multiple mineral-melt geothermobarometers suggest a relatively thermally buffered system at 1050 ± 25 °C, with several magma residence zones, ranging from close to the surface, through reservoirs at ~ 4–5 km, and as deep as ~ 20 km. Diffusion chronometry on > 200 orthopyroxene crystals reveal magma mixing timescales that range from a few days to about 65 years, but the majority are shorter than the decadal inter-eruptive repose period. This implies that magma intrusion at Mayon has been nearly continuous over the studied time period, with limited crystal recycling from one eruption to the next. The variety of plagioclase textures and zoning patterns reflect fluxing of volatiles from depth to shallower melts through which they eventually reach the atmosphere through an open conduit. The crystal-rich nature of the erupted magmas may have developed during each inter-eruptive period. We propose that Mayon has behaved over almost 100 years as a steady state system, with limited variations in eruption frequency, degassing flux, magma composition, and crystal content that are mainly determined by the amount and composition of deep magma and volatile input in the system. We explore how Mayon volcano’s processes and working model can be related to other open-vent mafic and water-rich systems such as Etna, Stromboli, Villarrica, or Llaima. Finally, our understanding of open-vent, persistently active volcanoes is rooted in historical observations, but volcano behavior can evolve over longer time frames. We speculate that these volcanoes produce specific plagioclase textures that can be used to identify similar volcanic behavior in the geologic record.

Effects of diabase intrusions on surrounding clastic rock reservoir in early diagenetic stages: a case study of Early Permian Fencheng formation in the dabasong uplift, junggar basin

A stage of mafic magmatic activity occurred in Early Permian in the Dabasong Uplift of Junggar Basin, part of the magma intruded into the normal sedimentary and shallow buried fine sandstone to form diabase, and part of the magma erupted to form basalt. The surrounding fine sandstone just entered in the early diagenetic stage A when the magma intruded. The compaction of the surrounding clastic rock and rupture of a small number of clastic grains were caused by the extrusion of the magma intrusion. The presence of chemically deposited alkaline minerals such as calcite, dolomite, shortite, natural alkali, and northupite indicates an alkali lake sedimentary environment for the Fengcheng Formation. Primary alkaline minerals dissolved from the surrounding rocks were subsequently transported and precipitated to form cements. The formation of the calcite cements and calcite metasomatism resulted in considerable densification of the surrounding rock during early diagenesis and destruction of the reservoir quality. The mafic magma had abundant Fe2+ and Mg2+ ions and was deficient in K+ ions, resulting in large amounts of chlorite and iron precipitation in the surrounding rock mainly composed of clay. We have analyzed the influence of an ultrashallow intrusion on the surrounding clastic rock during the early diagenetic period, which provided a typical reference for establishing a systematic mechanistic model of how magmatic intrusions affect the surrounding rock.

Caldera’s Breathing: Poroelastic Ground Deformation at Campi Flegrei (Italy)

Ground deformation at Campi Flegrei has fuelled a long-term scientific debate about its driving mechanism and its significance in hazard assessment. In an active volcanic system hosting a wide hydrothermal circulation, both magmatic and hydrothermal fluids could be responsible, to variable degrees, for the observed ground displacement. Fast and large uplifts are commonly interpreted in terms of pressure or volume changes associated with magma intrusion, while minor, slower displacement can be related to shallower sources. This work focuses on the deformation history of the last 35 years and shows that ground deformation measured at Campi Flegrei since 1985 is consistent with a poroelastic response of a shallow hydrothermal system to changes in pore pressure and fluid content. The extensive literature available for Campi Flegrei allows constraining system geometry, properties, and conditions. Changes in pore pressure and fluid content necessary to cause the observed deformation can then be calculated based on the linear theory of poroelasticity. The predicted pore pressure evolution and fluid fluxes are plausible and consistent with available measurements and independent estimates.

The stratigraphic record of continental breakup, offshore NW Australia

Continental breakup involves a transition from rapid, fault-controlled syn-rift subsidence to relatively slow, post-breakup subsidence induced lithospheric cooling. Yet the stratigraphic record of many rifted margins contain syn-breakup unconformities, indicating episodes of uplift and erosion interrupt this transition. This uplift has been linked to mantle upwelling, depth-dependent extension, and/or isostatic rebound. Deciphering the breakup processes recorded by these unconformities and their related rock record is difficult because associated erosion commonly removes the strata that help constrain the onset and duration of uplift. We examine three major breakup-related unconformities and intervening rock record in the Lower Cretaceous succession of the Gascoyne and Cuvier margins, offshore NW Australia, using seismic reflection and borehole data. These data show the breakup unconformities are disconformable (non-erosive) in places and angular (erosive) in others. Our recalibration of palynomorph ages from rocks underlying and overlying the unconformities shows: (i) the lowermost unconformity developed between 134.98–133.74 Ma (Intra-Valanginian), probably during the localisation of magma intrusion within continental crust and consequent formation of continent-ocean transition zones (COTZ); (2) the middle unconformity formed between ~134–133 Ma (Top Valanginian), possibly coincident with breakup of continental crust and generation of new magmatic (but not oceanic) crust within the COTZs; and (iii) the uppermost unconformity likely developed between ~132.5–131 Ma (i.e. Intra-Hauterivian), coincident with full breakup of continental lithosphere and the onset of seafloor spreading. During unconformity formation, uplift was focused along the continental rift flanks, likely reflecting landward flow of lower crustal and/or lithospheric mantle from beneath areas of localised extension towards the continent (i.e. depth-dependent extension). Our work supports the growing consensus that the ‘breakup unconformity’ is not always a single stratigraphic surface marking the onset of seafloor spreading; multiple unconformities may form and reflect a complex history of uplift and subsidence during the development of continent-ocean transition.

Repeating earthquakes and ground deformation reveal the structure and triggering mechanisms of the Pernicana fault, Mt. Etna

Structure and dynamics of fault systems can be investigated using repeating earthquakes as repeatable seismic sources, alongside ground deformation measurements. Here we utilise a dataset of repeating earthquakes which occurred between 2000 and 2019 along the transtensive Pernicana fault system on the northeast flank of Mount Etna, Italy, to investigate the fault structure, as well as the triggering mechanisms of the seismicity. By grouping the repeating earthquakes into families and integrating the seismic data with GPS measurements of ground deformation, we identify four distinct portions of the fault. Each portion shows a different behaviour in terms of seismicity, repeating earthquakes and ground deformation, which we attribute to structural differences including a segmentation of the fault plane at depth. The recurrence intervals of repeating earthquake families display a low degree of regularity which suggests an episodic triggering mechanism, such as magma intrusion, rather than displacement under a constant stress.