Feeder Conduits Research Articles

Where are the feeder channels for platinum reefs in the Bushveld Complex?

AbstractThe Bushveld Complex in South Africa hosts the lion’s share of the world’s noble metal resources in platinum reefs – thin layers of silicate/chromite rocks containing platinum-rich sulphides. The reefs are widely attributed to multiple replenishments by ore-forming magmas that have been entering the evolving Bushveld chamber through numerous feeder conduits. The replenishment events are marked by regional and local disconformities/unconformities, significant isotopic shifts, and notable reversals in the whole-rock and mineral compositions. Surprisingly, however, so far no single feeder conduit for platinum reefs has been found despite extensive surface and underground mining for over a century. Feeder conduits appear entirely absent from the Bushveld Complex. This paradox has long been known but has never been specifically addressed. Here, we suggest that the absence of feeder channels is a natural consequence of the magma chamber replenishment through a cumulate pile. The fossilization of the feeder channels in the cumulate pile is likely impeded by two principal factors: (a) a cumulate pile is too hot to enable efficient cooling and crystallization of magma flowing through the channels, and (b) the channels are closed by an adjacent elastically deformable pile immediately after cessation of the magma emplacement. The feeding dykes are thus absent because there is little chance for the conduits to get preserved in a hot and deformable cumulate pile of layered intrusions.

Subsurface structures and nature of seafloor mounds in the northern South China Sea margin: Implications for Mesozoic hydrocarbon exploration

The Chaoshan Depression in the northern South China Sea (SCS) margin hosts thick (ca. 4–9 km) Mesozoic strata including two sets of hydrocarbon source rocks (the Late Triassic-Early Jurassic and Late Jurassic marine facies sequences). It was predicted to be petroliferous but unproven. Numerous submarine mounds are distributed on its seafloor, but whether they are mud volcanoes or magmatic volcanoes remains controversial due to poor knowledge of the subsurface structures. To reveal the nature of these mounds and the Mesozoic hydrocarbon potential in the Chaoshan Depression, multi-channel seismic (MCS) and ocean bottom seismometer (OBS) data along line CS-L1 across three seafloor mounds are processed and analyzed. The MCS profile reveals the existence of fuzzy reflection zones (FRZs) within the Mesozoic strata beneath these mounds. The overlying sedimentary sequences appear uplifted and deformed. The OBS velocity results exhibit that these FRZs feature significantly lower velocities than their host strata with velocity differences ranging from ∼0.25 km/s to 0.71 km/s, likely resulting from vigorous fluid charging rather than magma intrusion. Thus these FRZs are identified as mud-fluid diapirs. Between these diapirs and the overlying seafloor mounds, numerous high-angle faults exist, serving as vertical fluid feeder conduits. Additionally, several bright spots, acting as indicators of gas reservoirs, are observed near these faults. Moreover, bottom simulating reflectors with reverse polarity are imaged within the shallow strata around the seafloor mounds on an adjacent MCS profile, indicating rich gas trapping and/or gas hydrate accumulation. Therefore, we propose that these seafloor mounds are mud volcanoes with fluids coming from the underlying diapirs. As the diapirs extend downward into the Jurassic and Triassic strata, the hydrocarbon gases feeding the mud volcanoes and diapirs are reasoned to originate from the deep Mesozoic source rocks, implying significant Mesozoic hydrocarbon potential in the Chaoshan Depression of the SCS.

Characterising Rhythmic and Episodic Pulsing Behaviour in the Castleton Karst, Derbyshire (UK), Using High Resolution in-Cave Monitoring

The discharge from most karst springs exhibits a consistent and reasonably predictable response to recharge but a few exhibit short-term (‘rhythmic’) changes in flow that are commonly attributed to the geometry of feeder conduits and the action of siphons. This paper investigates water flow in a karst system that exhibits rhythmic and episodic changes in discharge due to variations in flow from two phreatic conduits (Main Rising (MR) and Whirlpool Rising (WR)) that pass through Speedwell Cavern en route to the springs. Water tracing experiments indicate that the conduits receive both allogenic and autogenic recharge. Flow dynamics and conduit behaviour were investigated using high-resolution (2-min) water depth data collected from MR and WR between 2012 and 2015 (when MR was dominant) and between 2021 and 2023 (when WR was dominant). Water depths were also logged in a cave at the upstream end of a conduit draining to both MR and WR and at springs. The short-term temporal variability in water depths at both MR and WR is greater than any documented in previous studies. This is attributed to conduit bedrock geometry and changes in conduit permeability due to sediment accumulation in phreatic loops, which together influence the response to recharge.

Episodic Nature of Magmatic Ascent in a Dynamic Conduit System: Evidence From a Late Gabbroic Intrusion Associated with the Eagle Ni-Cu Sulfide Deposit in Northern Michigan, USA

Abstract New exploration studies have revealed a near-vertical, plug-like gabbroic intrusive body emplaced along the feeder conduit of the funnel-shaped Eagle East ultramafic intrusion in the Upper Peninsula of Michigan. U-Pb baddeleyite analysis on this gabbro yielded an age of 1103.4 ± 1.2 Ma. Although a sulfide-deficient gabbro is the principal rock unit, peripheral zones of the intrusion contain feldspathic pyroxenite and melagabbro with minor blebs of chalcopyrite and pyrrhotite. A small pocket-like body of peridotite with significant quantities of sulfide minerals occurs along the edge of the main gabbroic unit. The modes of occurrence of sulfide minerals in the pocket peridotite are similar to the larger Eagle East and Eagle intrusions nearby. The composition of olivine in the main gabbroic rock is Fo24–30, but olivine in pocket peridotite shows a composition of ~Fo74. The Ni-concentration in the olivine of the main gabbro unit is below 100 ppm, but in the Mg-rich olivine of the pocket peridotite, it is between 2,100 and 2,800 ppm. Although δ34S values of sulfide minerals in the pyroxenite rim and in the pocket peridotite range from –0.6 to 7.0‰, the average value is higher than those reported from sulfides in the Eagle East and Eagle intrusions. The origin of the gabbro unit could be explained by a closed-system crystallization of a fractionated late derivative of an original high-MgO magma, while the peridotite pocket is a xenolithic representative of the old peridotitic intrusion dislodged and entrained in a late magmatic pulse.

Fractures, fluid flow and inherited structures in geothermal systems: inputs from the Fe-ore deposits of eastern Elba Island (Northern Apennines, Italy)

AbstractGeothermal systems in terrains affected by polyphase deformation have reservoirs with a 3D geometry that is always difficult to predict. In this paper we describe a fossil exhumed geothermal system exposed in eastern Elba Island that developed in a polyphase folded and faulted setting, which can help us to understand how geothermal fluids circulate in geological bodies with inherited structures. Geothermal circulation at Elba allowed the deposition of Fe-ore deposits (haematite/magnetite and pyrite) and altered rock volumes, which represent tracers of the palaeo-fluid flow. Normal and oblique-slip faults dissected a polyphase folded metasiliciclastic succession and produced a secondary permeability in the range of 5 × 10−13 to 5 × 10−16 m2. From the permeable fault zones acting as feeder conduits, geothermal fluids permeated the hydraulically connected metasiliciclastic rock bodies previously deformed by two generations of folds. Geothermal fluids followed the already defined geometry, thus giving rise to apparent folded mineralized levels. Fluid migration into the metasiliciclastic rocks was possible due to their chemical aggression, which favoured the dissolution and reprecipitation of quartz, and Fe-oxide and sulphide deposition. Renewed fluids maintained their chemical properties (pH value and temperature, mostly). This conclusion provides inputs for reconstructing geothermal conceptual models and evaluating the geothermal potentiality of exploitable areas developing in similar geological settings.

Exploring the role of fluid-solid interactions for modelling volcano deformation

Investigating the temporal development of magma reservoir pressure and associated surface displacements can reveal fundamental aspects of subsurface magmatic processes and aid in eruption forecasting. The limitation with existing volcano deformation models is that they typically ignore magma intrusion dynamics and focus on the response of surrounding rocks to source boundary pressure. Magma fluid dynamics should be incorporated into magmatic modelling to track the temporal development of a system, instead of the widely used kinematic techniques. Here, we compare analytical and numerical solutions for magma intrusion into a shallow reservoir, using two schemes of intrusion boundary condition, inlet pressure and inlet mass flow. Model sensitivity tests are conducted to explore key factors controlling the two-way coupling between solid and fluid components, assuming an incompressible magma for a first-order approach. For intrusions of viscous magma (⪆108 Pa s) or a narrow feeder conduit (5–20 m), applying an inlet pressure causes the resultant pressure and surface deformation to develop at a very slow rate; lower viscosity magmas produce faster deformation rates. The mass flow boundary condition reduces the number of model parameters as it is independent of poorly constrained parameters such as conduit and magma characteristics. For both boundary conditions, reservoir pressurization, and hence spatiotemporal surface deformation, are strongly influenced by reservoir geometry due to geometric compressibility. Our results provide fundamental knowledge to advance to more complex coupled fluid-solid mechanics models in volcano geodesy.

The volcanism of the Jurassic Chon Aike Silicic LIP influenced by Paleozoic inherited crustal structures in the northeastern Deseado Massif, Patagonia

An area of ∼40 km2 located in the northeastern sector of the Deseado Massif is detailed studied, in which Paleozoic metamorphic and Jurassic volcanic rocks crop out. Two genetically linked and compositionally different Jurassic volcanic events of the Chon Aike Silicic LIP are documented by geological mapping and lithofacies and petrographic analyses. The first event was only represented by andesitic lava flows assignable to the intermediate portion of the Bahía Laura Volcanic Complex (Bajo Pobre Formation). The second was represented by the acidic rocks of the Bahía Laura Volcanic Complex (Chon Aike and La Matilde formations) and it was constituted by low-grade and moderate grade ignimbrites, pyroclastic fall deposits (ash tuffs) and several lava bodies (rhyolitic lava domes and lava flows). Firstly, the effusive eruptions dominated during the intermediate magmatic eruptions, whereas the explosives mechanisms dominated during the first stage of the acidic magma extrusion, to end with effusive eruptions of previously degassed magma. According to the morphology of the NNW-elongated lava domes (feeder conduits plugs) and internal measurements carried out on these rhyolitic bodies, fissure conduits are interpreted, which were directly controlled by the Paleozoic inherited structures during the Jurassic extensional tectonic reactivation ascribed to the initial stages of the Gondwana breakup.

Evolution of magma decompression and discharge during a Plinian event (Late Bronze-Age eruption, Santorini) from multiple eruption-intensity proxies

We have coupled three independent methods to investigate the time evolution of eruptive intensity during the sub-Plinian and Plinian phases of the 3600-year BP Late Bronze-Age eruption of Santorini Volcano: (1) mass eruption rate based on new lithic isopleth maps for multiple layers of the fall deposit, (2) magma decompression rate calculated from vesicle number densities, and (3) magma decompression rate calculated from H2O gradients in melt reentrants, with methods 2 and 3 measured on the same suite of pyroclasts. Mass eruption rate increased by two orders of magnitude, reaching 210 × 106 kg s−1 at the peak of the Plinian phase (plume height 28.4 ± 1.0 km); it then declined in the final stage of fallout emplacement following the first generation of pyroclastic surges. Decompression rates from melt reentrants (0.008 to 0.25 MPa s−1) are two to three orders of magnitude lower than those from vesicle number densities, assuming heterogeneous vesicle nucleation (2 to 19 MPa s−1). Melt reentrants are thought to record slow decompression in the deep feeder conduit, whereas vesicles record much higher rates of decompression in the shallow conduit due to the steep, nonlinear pressure gradients associated with magma vesiculation and fragmentation. Upwardly converging flow from a dike-like, deep conduit to a more cylindrical, shallow conduit may also have played a role in causing upwardly accelerating flow. Variations in deep decompression rate recorded by melt reentrants are decoupled from mass eruption rate, whereas those recorded by vesicles lie in between. Taken with the transition from unsteady to steady Plinian eruption, this may reflect the existence of transient flow conditions in the conduit system due to widening and lengthening of a deep feeder dike as Plinian eruption progressed. As the mass eruption rate rose to its peak value, the fragmentation level fell in the conduit due to increasing rates of magma strain and decompression.

World-Class Fe-Ti-V Oxide Deposits Formed in Feeder Conduits by Removing Cotectic Silicates

AbstractExtremely thick Fe-Ti-V oxide layers are hosted in mafic-ultramafic intrusions of the Emeishan large igneous province (LIP) in the Pan-Xi district of southwest China, accounting for a quarter of the world’s Ti and V resource. It is unclear why these small intrusions contain such huge ore reserves that form world-class Fe-Ti-V oxide deposits. We find that the Hongge intrusion contains 35% Fe-Ti-V oxides, which is twice the typical content in mafic-ultramafic intrusions worldwide and the experimentally determined cotectic proportion in natural ferrobasaltic magma systems. The V content is almost constant in titanomagnetite across the entire Hongge intrusion in the Emeishan LIP, indicating a small (10–20%) proportion of cotectic Fe-Ti-V oxide during fractional crystallization. The bulk composition of the intrusion indicates an open magma system at the time of its formation. Clinopyroxene phenocrysts from overlying basalts contain Fe-Ti-V oxide inclusions, indicating that the phenocrysts crystallized at depth from magma saturated in Fe-Ti-V oxide and were then transported to the surface. We suggest that these intrusions were feeder conduits to the overlying basalts, where the silicates were cotectic with Fe-Ti-V oxides which were then extracted from the underlying intrusion as phenocrysts. Such a fundamental process is key to increase the proportion of oxide minerals in the residual assemblage, thereby upgrading the barren oxide-bearing rocks to world-class Fe-Ti-V oxide deposits in the small intrusions of the Emeishan LIP. A similar process might have occurred in LIPs elsewhere, meaning that intrusions formed as conduit-like open systems to the basalts in LIPs are good exploration targets for giant high-grade Fe-Ti-V oxide deposits.

The geology, geochemistry, and petrogenesis of the Huckleberry Cu-Ni-PGE prospect in the Labrador Trough, Canada: Perspectives for regional prospectivity

The Palaeoproterozoic Huckleberry Cu-Ni-(PGE) prospect in the Labrador Trough, northern Québec, represents a ~ 400-m-thick, out-of-sequence sill complex that comprises a ~200-m-thick glomeroporphyritic gabbro, intruded in its centre by a ~200-m-thick differentiated gabbro-peridotite sill and in its gabbroic footwall, several thinner (<30 m) ultramafic sills. Globular sulphides are present at the base of the sill complex, whereas disseminated to net-textured sulphides occur in the ultramafic units (Cu/Ni = 0.1–0.8) as well as their gabbroic footwall (Cu/Ni = 1–3). The glomeroporphyritic gabbro sill stack (MgO ~4 wt%, TiO2 ~0.6 wt%, Na2O + K2O ~2–3%, Eu/Eu* ~ 1.2, Anplg ~70–60) comprises several sills characterised by sharp changes in size and abundance of plagioclase glomerocrysts. We hypothesise that the glomerocrysts represent remobilised crystal mushes that were dislodged from a floatation cumulate in a staging chamber during episodic expulsion of magma. In the central gabbro-peridotite sill, mineral compositions (Foolv ~75–75, Mg#opx ≈ 78–68, Anplg ≈ 78–70) and whole-rock data (MgO ≈ 22 – 27%, TiO2 ≈ 0.4%) suggest that the parent magma was an olivine-saturated basalt containing 8–9 wt% MgO. Whole-rock geochemical data further suggests that the parent magmas did not undergo any significant contamination (La/SmN < 2, S/Se < 4000). The Cu/Pd values of ultramafic units (>10,000) suggest sulphide melt saturation was attained before their final emplacement in magma feeder conduits or staging chambers at relatively low R factors (1000–5000). Downward decreasing concentrations of chalcophile elements in the drill cores suggests that sulphide melt percolated downward from the ultramafic cumulate units into the glomeroporphyritic gabbro footwall. We propose that the footwall ultramafic sills represent downward injections of olivine and sulphide melt from the overlying gabbro-peridotite sill, which mechanically concentrated high volumes of sulphide in narrow sills.

Lateral variations in the Unit 7\u20138 boundary zone of the Rum Eastern Layered Intrusion, NW Scotland: implications for the origin and timing of Cr-spinel seam formation

The Rum Layered Suite, NW Scotland, hosts Cr-spinel seams at the bases of peridotite–troctolite macro-rhythmic units in the eastern portion of the intrusion. Here, we present detailed field observations together with microstructural and mineral chemical analyses for the Unit 7–8 Cr-spinel seam and associated cumulates in the Eastern Layered Intrusion. Detailed mapping and sampling reveal significant lateral variations in the structural characteristics and mineral compositions of the Unit 7–8 boundary zone rocks. Although the Cr-spinel seam is laterally continuous over ~ 3 km, it is absent towards the centre and the margins of the intrusion. The compositional characteristics of Cr-spinel and plagioclase vary systematically along strike, exhibiting a chemical evolution towards more differentiated compositions with increasing distance from the main feeder conduit of the Rum intrusion; the Long Loch Fault. On the basis of our combined datasets, we propose that the upper part of the troctolite, the anorthosite layer underlying the Cr-spinel seam and the seam itself formed during a multi-stage magma replenishment event. The stages can be summarised as follows: (1) peridotite schlieren and anorthosite autoliths formed following melt infiltration and cumulate assimilation in the crystal mush of the Unit 7 troctolite. (2) The anorthosite layer then formed from the Unit 7 troctolite crystal mush by thermal erosion and dissolution due to infiltrating magma. (3) Subsequent dissolution of the anorthosite layer by new replenishing magma led to peritectic in situ crystallisation of the Unit 7–8 Cr-spinel seam, with (4) continued magma input eventually producing the overlying Unit 8 peridotite. In the central part of the Rum Layered Suite, the aforementioned assimilation of the troctolitic footwall formed the anorthosite layer. However, the absence of anorthosite in close proximity to the Long Loch Fault can be explained by enhanced thermochemical erosion close to the feeder zone, and its absence close to the margins of the intrusion, at maximum distance from the Long Loch Fault, may be due to cooling of the magma and loss of erosion potential. In line with other recent studies on PGE-bearing chromitites in layered intrusions, we highlight the importance of multi-stage intrusive magma replenishment to the formation of spatially coupled anorthosite and Cr-spinel seams, as well as the lateral mineral chemical variations observed in the Unit 7–8 boundary zone cumulates.

Controls on the disseminated Ni-Cu-PGE sulfide mineralization at the Tubane section, northern Molopo Farms Complex, Botswana: Implications for the formation of conduit style magmatic sulfide ores

The Molopo Farms Complex (MFC) is located near the western margin of the Archean Kaapvaal Craton, in southern Botswana. The complex hosts low grade disseminated Ni-Cu and platinum group elements (PGE) mineralization restricted to the lower ultramafic cumulates, notably in the feldspathic orthopyroxenites. The mineralized feldspathic orthopyroxenites contain up to 0.47 ppm Pt + Pd + Au manifested in the form of platinum group mineral (PGM) assemblages dominated by PGE bismuthotellurides, asernides and minor Au and Ag alloys. The PGM encountered are froodite (PdBi2), moncheite (PtTe2), michenerite (PdBiTe), sperrylite (PtAs2) largely associated with altered silicates and sulfide minerals. The association of PGM with sulfides suggests that they were originally concentrated by sulfide melt, while late stage reaction of sulfides with hydrothermal fluids resulted in the observed spatial proximity of PGM with altered silicates. The Cu/Pd ratios of most of the rocks fall within the primitive mantle range. However, the Cu/Pd ratios of the feldspathic orthopyroxenites are more variable, with few samples falling above and below the primitive mantle range suggesting the prior sulfide extraction from the magmas and the presence of cumulus sulfides. This is supported by the Cu/Zr ratios of these rocks which are also variable above and below unity. Trace element ratios on the (Th/Yb) PM versus (Nb/Th) PM plot indicate strong crustal contamination across all units including those not in proximity to the country rocks. The δ34S of the magmatic rocks are variable between −6.3‰ and −11.4‰. In contrast, the δ34S of dolomitic siltstones of the Transvaal Supergroup which constitute the immediate country rocks vary from −4.1‰ to +0.6‰. These data indicate that crustal contamination largely occurred prior to magma emplacement. It is inferred that the most important contaminant and source of crustal S were the Ramotswa and Ditlhojana Formations that include sulfidic shales at depth. We consider that magnesian basaltic magmas of B1 Bushveld magma composition underwent crustal contamination, assimilation of crustal S and segregation of sulfide melt in the feeder conduit or staging-chamber prior to final magma emplacement. The feeder conduit or staging-chamber was subsequently invaded by further B1 magma. This magma entrained some of the sulfides lodged in the plumbing system and flushed them to the final MFC chamber studied here. After the staging-chamber had already partially solidified, it was invaded by yet another influx of magma, possibly even more primitive than the B1 Bushveld magma. Being a crystal mush, this magma assimilated and entrained some of the previously formed orthopyroxenites manifested by fragments of orthopyroxenite in the harzburgites of the final MFC chamber. The presence of centimeter-sized globular sulfides and fragments of orthopyroxenite within harzburgites suggests that the complex was emplaced as separate pulses of magmas. Therefore, it is considered that sulfide accumulations may remain undiscovered in the feeder zones or the staging-chamber.

In Situ Multiple Sulfur Isotope and S/Se Composition of Magmatic Sulfide Occurrences in the Labrador Trough, Northern Quebec

AbstractThe interaction between mafic-ultramafic magma and crustal sulfide is considered a key process in the formation of magmatic Ni-Cu-platinum group element (PGE) sulfide deposits. Integrated S/Se and multiple sulfur isotope studies are the most robust in constraining the role of crustal sulfur during ore genesis. In the present study, we report the first integrated S/Se and multiple sulfur isotope study of magmatic sulfide occurrences in the Labrador Trough, namely, on the recently discovered Idefix PGE-Cu and Huckleberry Cu-Ni-(PGE) prospects.Whole-rock and in situ S/Se values (~810–3115) of magmatic sulfides and their host rocks are consistent with S loss during postmagmatic hydrothermal alteration, negating their use in interpreting the origin of S. Values of ∆33S ~0 indicate no record of the assimilation of Archaean sulfur. Disseminated (–0.5 to +2.5‰) and globular (3.0–4.5‰) sulfides at Idefix as well as globular sulfides (2.1–9.6‰) at Huckleberry have δ34S values greater than the accepted mantle range, suggesting that crustal S played a role in the formation of these sulfides. In contrast, disseminated and net-textured sulfides at Huckleberry have variable δ34S values (–4.6 to +3.2‰) that are mostly within the accepted mantle range, excluding one anomalous sample that records relatively higher δ34S values (11.9–15.0‰). It is proposed that sulfide melt segregated in response to the addition of small proportions of crustal S prior to the final emplacement of the host intrusions, i.e., in a feeder conduit or staging chamber. Isotopic exchange between the sulfide melt and silicate magma has diluted and, in places, eradicated a crustal δ34S signature.

Origin of non-cotectic cumulates: A novel approach

Abstract Plutonic mafic complexes are composed of cumulates in which minerals mostly occur in cotectic proportions. This is consistent with a concept that basaltic magma chambers predominantly crystallize in situ from margins inward. However, cumulates with two (or more) minerals in proportions that are at odds with those expected from liquidus phase equilibria also locally occur in these complexes. Such non-cotectic cumulates are commonly attributed to either mechanical separation of minerals crystallizing from the same parental magma or mechanical mixing of minerals originating from different parental magmas. Here we introduce a novel concept that does not require any of these processes to produce non-cotectic cumulates. The model involves melts that start crystallizing upon their cooling, while ascending along feeder conduits from deep staging reservoirs toward the Earth’s surface. Depending on the degree of cooling, the melts become successively saturated in one, two, and more liquidus phases. Given that most crystals are kept in suspension, the resulting magmas would contain a cargo of equilibrium phenocrysts in notably non-cotectic proportions. The replenishment of basaltic chambers developing through in situ crystallization by such magmas is likely responsible for the occasional formation of non-cotectic cumulates in plutonic mafic complexes.

A review of producing fields inferred to have upslope stratigraphically trapped turbidite reservoirs: Trapping styles (pure and combined), pinch-out formation, and depositional setting

Siliciclastic turbidite systems that pinch out updip toward their proximal margin are prime targets for hydrocarbon exploration, especially in deep-water basins. Such “upslope stratigraphic traps” potentially offer large-volume discoveries but have significant geological risks, notably because of ineffective closure or containment. In the published literature, at least 20 fields from 11 basins globally with 6–7 billion BOE of cumulative discovered reserves have been inferred to be reliant on upslope pinchout traps. These fields are reviewed in terms of their interpreted trapping styles, pinch-out formation process, and depositional-tectonic setting. Reservoirs display a range of upslope trapping styles, including pure (depositional and erosional) stratigraphic pinch-outs and combined stratigraphic-structural traps. In one-third of cases, faulting appears intimately linked to updip trapping, either through offsetting slope feeder conduits or assisting pinch-out development, and in some cases, faulting may be the most important updip trapping element. Sediment bypass and erosion in proximal areas is the most common inferred pinch-out formation mechanism. Some reservoirs also demonstrate the ability of erosional truncation by mud-prone channels and mass transport deposits to form viable stratigraphic traps and seals. Encouragingly for exploration, robust pinchout traps occur in various tectonic settings on a variety of different slope types and positions along the slope profile. Most large-volume discoveries to date, however, are restricted to the toe-of-slope environment in graded passive margins or out-of-grade rift and transform margin settings. Insights into the nature and occurrence of upslope stratigraphic traps are important for future exploration, especially for evaluating new license areas and risking prospects.

3D morphology and timing of the giant fossil pockmark of Beauvoisin, SE Basin of France

The resolution of data acquired over modern seafloors does not allow imaging of the inner features of a fluid seep structure, particularly in the shallow subsurface. In the SE Basin of France (Drôme), fossil cold seep structures comprising fossil-rich carbonate lenses were identified about 30 years ago within the Oxfordian (Late Jurassic) Terres Noires Formation. These structures were first interpreted as pseudo-bioherms related to hydrothermal activity, but comparison with active seep sites on modern margins, together with isotopic analyses, led to a reinterpretation involving cold fluids instead. To date, all seep sites have generally been studied individually without considering any link to neighbouring or more distant sites. Based on 23 high-resolution stratigraphic logs within the structure coupled to mosaicked aerial photographs from a drone survey, 19 fluid seep events were correlated in the area, including two new sites exposed as a result of weathering. We have shown that each identified sub-site is composed of subvertically stacked fossil-rich carbonate lenses interbedded with marls, which developed in smooth, 4–6 m deep depressions beneath the local seabed. The nodules present within the depressions are of primary importance as they mark the area of active seeping. This general organization is very similar to the modern Regab giant pockmark in the Lower Congo Basin where only a few sub-sites are active at the same time. A spatio-temporal 3D reconstruction of the position of these sub-sites shows that the carbonate lenses are organized into clusters comprising up to seven sub-sites grouped together in the same stratigraphic interval and the same geographical zone. A sandbox experiment where gas is injected at constant flow rate at the base of a box filled with a matrix of water-saturated grains displays a pattern consisting of disturbed sediments inside a parabolic-shaped area. This parabolic shape was also identified on a seismic profile across the Regab giant pockmark, suggesting that the processes are similar for the Beauvoisin and Regab seep areas. The laboratory experiments also show that the seeping conduit is stable during a given period of time and suddenly shifts laterally. This is mainly as a result of the collapse of the conduit, the lateral migration and the reopening at a new position. The general log obtained in the Beauvoisin seep area suggests a similar pattern with periods of seeping alternating with periods of quiescence, each of which is c . 200 kyr. Even if a pockmark seems to have been inactive for a long period of time, this could be due to the lateral shift of the feeder conduit, meaning that the sub-seafloor is still charged with gas. This is of primary importance for risk assessment, hydrocarbon exploration and general understanding of geobiology at seafloor seeps.

Investigating the Magmatic Plumbing System of a Monogenetic Scoria Cone: A Field and Laboratory Study of the Cienega Scoria Cone, Cerros del Rio Volcanic Field, New Mexico

AbstractData on the evolution of magmatic plumbing systems are essential for the understanding of magma motion in monogenetic volcanoes and associated volcanic hazard. Combined field (relationships and petrography) and laboratory (rock magnetism, anisotropy of magnetic susceptibility, and paleomagnetism) analysis of a quarried scoria cone offers a unique opportunity to study its magmatic plumbing system. In this study magma emplacement processes were constrained using these methods at two vents, Cerro Pequeño and Cerro Grande, of the composite Cienega scoria cone volcano, in New Mexico, USA. The dikes and lava flows associated with Cerro Pequeño were emplaced in two stages within a short period of time relative to secular variation. In the first stage, the magma was injected into the main conduit of the Cerro Pequeño and then channeled into secondary conduits (e.g., the exposed dike system) moving away from the vent. In the late stage, magma from the northern Cerro Grande was injected into the Cerro Pequeño dike system toward its main vent. The dynamic evolution of the magmatic system observed at the Cienega volcano is similar to that observed in other larger volcanic edifices. As in this study, the investigation of the magma plumbing systems of many scoria cones reveals that the subsurface magmatic system involves multiple feeder conduits that evolve during the life of the volcanic system. This study provides significant insight for the characterization of magma propagation at the shallow level beneath the volcanic edifice and highlights the related hazards associated with small active volcanic systems worldwide.

Continuous Gravity and Tilt Reveal Anomalous Pressure and Density Changes Associated With Gas Pistoning Within the Summit Lava Lake of Kīlauea Volcano, Hawai‘i

AbstractGas piston events within the summit eruptive vent of Kīlauea Volcano, Hawai‘i, are characterized by increases in lava level and by decreases in seismic energy release, spattering, and degassing. During 2010–2011, gas piston events were especially well manifested, with lava level rises of tens of meters over the course of several hours, followed by a sudden drop to preevent levels. The changes in lava level were accompanied by directly proportional changes in gravity, but ground deformation determined from tilt was anticorrelative. The small magnitude of the gravity changes, compared to the large changes in volume within the vent during gas pistons, suggests that pistoning involves the accumulation of a very low‐density (100–200 kg/m3) foam at the top of the lava column. Co‐event ground tilt indicates that rise in lava level is paradoxically associated with deflation (the opposite is usually true), which can be modeled as an increase in the gas content of the magma column between the source reservoir and the surface. Gas pistoning behavior is therefore associated with not only accumulation of a shallow magmatic foam but also more bubbles within the feeder conduit, probably due to the overall decrease in gas emissions from the lava lake during piston events.

In situ strontium and sulfur isotope investigation of the Ni-Cu-(PGE) sulfide ore-bearing Kevitsa intrusion, northern Finland

The ~ 2.06-Ga Kevitsa mafic-ultramafic intrusion in northern Finland hosts a large disseminated Ni-Cu-PGE deposit. The deposit occurs in the ultramafic olivine-pyroxene cumulates and shows a range in Ni tenors varying from 4–7 wt% (regular ore) to > 10 wt% (Ni-PGE ore). There are also a metal-poor sulfide mineralization (false ore) and contact mineralization that are uneconomic (Ni tenor 0.7045) in comparison to the estimated depleted mantle Sr isotope ratio of ~ 0.702 at 2.06 Ga. The sulfur δ34S values are generally higher than + 2‰, which together with the Sr isotope data imply involvement of crustal material in the genesis of the Kevitsa intrusion and its ores. The 87Sr/86Sr(i) values obtained from the ore-bearing domain of the intrusion show stratigraphic variation and exceed 0.7050, with the maximum value reaching up to 0.7109. In contrast, in rocks around the ore domain, the initial Sr isotope compositions remain more or less constant (0.7047–0.7060) throughout the intrusive stratigraphy. The isotope data suggest that the ore-bearing domain of the intrusion represents a dynamic site with multiple injections of variably contaminated magma, whereas the surrounding part of the intrusion experienced a less vigorous emplacement history. No correlation is observed between the strontium and sulfur isotope compositions. This is explained by bulk assimilation of the silicate magma in a deeper staging magma chamber and variable assimilation of sulfur during magma transport into the Kevitsa magma chamber. The low level of metals in false ore and the Ni-depleted nature of its olivine suggest that some sulfides may have precipitated and deposited in the feeder conduit during the initial stage of magma emplacement. Cannibalization of early-formed sulfides by later magma injections may have been important in the formation of the economic ore deposit.

Multiple reinjections and crystal mush compaction in the solidification evolution of the Karaj Dam basement Sill, Northern Iran

The Karaj Dam basement Sill is a 460m–thick saucer shaped sill, situated in the Alborz Magmatic Belt, Northern Iran. The results of geochemical, textural and field relations reveal characteristics of a sill with a well-developed S-shaped compositional profile which could be subdivided into distinct parts and suggest that the sill was repeatedly split and reinjected with fresh magma in the upper half of the previous emplacements (over- to partly intra-accretion). Whole rock and mineral compositional profiles have recorded five to six discrete injections of magma, each of which individually show an upward increase in terms of primitivity which represents partial crystallization in feeder conduits. The first three small successive pulses of magma, emplaced in the basal 150m of the floor sequence, were followed by voluminous fourth and fifth pulses in the upper portion of the sill. During final two pulses the system acts as a closed system for each independently, and evolves through compositional convection or compaction aided in-situ crystallization. Theoretical models for convection and compaction shows the significance of both processes. Considering the final porosity of 0.1<φ<0.45, calculated convective velocities (0.2<Vc<227m/year) are higher than the calculated crystal accumulation rate of the basal 150m and upper portion of the sill (Va=1 and 0.1m/year), when the crystal mush varies between 1 to 0.01m with the liquid viscosity ranging from 85 to 15Pa·s. Our calculations further indicate that compaction driven velocity of liquid expulsion (ω−w) hardly exceeds the Va in the basal 150m of the floor sequence. The highest velocity is reached (ω−w=~1m/year) only if the crystal mush thickness is no less than 240m with the porosity of 0.6 and the liquid viscosity of no more than 15Pa·s. On the other hand, compaction is highly effective in the upper portion of the sill. Transfer of residual liquid from the compacting lower solidification front to the dilating upper solidification front resulted in characteristic chemical and mineralogical effects, such as the depletion of the lower half of the sill and the enrichment of the sandwich zone in incompatible elements and modal granophyre. Crystallization of the fourth pulse of magma produced a peak in incompatible clement concentrations at a sandwich horizon located at the 410m level, where the floor and roof of the sill appeared to converge at this stage. Subsequent to the time of this enriched zone crystallizing, the fifth pulse of magma was emplaced near this level and inflated the chamber vertically for ~200m. Cumulus material containing interstitial melt and subsequent buoyancy driven upward transport of interstitial melt in this pulse concentrated incompatible elements at ~47m below the main peak, at the stratigraphic height of 360–370m. The boundaries between the successive pulses were cryptic and represent a gradational contact in terms of grain size, chemical composition and crystallization sequence. This implies a short time interval between the emplacements of the magma. This sill is comparable with Antarctica Beacon Sill and Hudson River Palisades Sill, and the recognition of evidence for reinjection and compaction in these macroscopically uniform sills, as well as the Karaj Dam basement Sill, suggests that these processes may be common in the construction of sills with a thickness of more than 100m.