Sill Intrusions Research Articles

Molecular composition of dissolved organic matter from young organic‐rich hydrothermal deep‐sea sediments

AbstractHydrothermal transformations of dissolved organic matter (DOM) are governed by temperature and sedimentary organic carbon content, resulting in the release of hydrothermal DOM containing bioavailable compounds fueling benthic microbes. However, the temperature‐dependent molecular changes in porewater DOM from organic‐rich hydrothermal sediments, and the extent to which these changes contribute to the marine recalcitrant DOM, remain largely unexplored. Here we investigated the DOM composition of hydrothermal porewater and bottom water samples from the Guaymas Basin, Gulf of California, where basaltic sill intrusions generate hydrothermal petroleum in organic‐rich sediments. Samples containing hydrothermal petroleum with in situ temperatures from 4°C to > 106°C were analyzed using Fourier‐transform ion cyclotron resonance mass spectrometry and parallel factor analysis of excitation‐emission matrices from fluorescent DOM (FDOM). We found that the porewater DOM composition was strongly influenced by temperature and petroleum dissolution, evidenced by the enrichment of hydrothermal DOM with highly unsaturated, oxygen‐depleted aromatic, sulfur‐containing molecular formulae and petroleum‐associated FDOM compared to a cold reference site. In bottom waters, hydrothermal DOM accounted for ~ 26% of the DOM molecular formulae, with 82% exhibiting hydrogen‐to‐carbon ratios < 1.5, indicating their recalcitrance. The remaining ~ 18% of the hydrothermal molecular formulae were aliphatic and saturated, representing the release of bioavailable DOM to the ocean. Our results show that hydrothermal sediments are a source of both bioavailable and recalcitrant DOM, releasing water‐soluble petroleum‐derived compounds to the deep ocean. Our study highlights the need for more quantitative research on the contribution of hydrothermal sediments to deep‐sea DOM cycling.

Diverse intrusion modes during the construction of a high-silica magma reservoir: Evidence from La Obra–Cerro Blanco intrusive suite (central Chile)

Abstract Several conceptual models have been proposed for the amalgamation of granitoid plutons, which range from incremental growth to single-stage emplacement of these systems. This diversity of views has led to intense debate about the thermomechanical state of silicic intrusions and the magma differentiation paths within the crust. In this contribution, we present a comprehensive petrologic, geochronologic, and magnetic fabric data set from the La Obra–Cerro Blanco intrusive suite, which allows us to explore the petrogenesis and magma emplacement processes in the upper crust. This intrusive suite is composed of (1) a vertically zoned granitoid intrusion in spatial association with mafic layers and stocks and (2) a cupola-like high-silica granite. We interpret this intrusive suite as assembled by diverse but coexisting intrusion mechanisms over a time span of ~1.4 m.y. from 21.4 to 19.9 Ma. As indicated by the subhorizontal magnetic lineation, the first stage was dominated by horizontal emplacement of sheet-like intrusions of intermediate compositions, which became increasingly silicic after plagioclase and amphibole fractionation throughout the crustal column. The latest stage was instead dominated by cooling, crystallization, and differentiation of a thickened granitoid body and the formation of a high-silica magma chamber. The steep magnetic lineation and the abundance of aplite and rhyolitic dikes observed in the cupola-like, high-silica granites suggest that this portion acted as an evacuation channel of high-silica magma toward shallower levels, offering a rare opportunity to understand not only silicic magma accumulation and storage in the upper crust, but also the processes connecting the plutonic and volcanic environments.

The timing and duration of large-scale carbon release in the Early Jurassic

Abstract The Toarcian oceanic anoxic event (T-OAE, ca. 183 Ma) in the Early Jurassic was one of the most significant warming events of the Phanerozoic, associated with large-scale carbon emissions, mass extinction, and perturbations to hydrology and ocean chemistry. However, the age and duration of this hyperthermal have long been uncertain, hindering our understanding of the timing and pace of carbon release and the associated environmental and biotic changes. We present high-precision radioisotopic dates bracketing a biostratigraphically constrained record of the T-OAE in Japan. Our geochronology reveals an unexpectedly short T-OAE duration of ∼300 k.y. and a temporal coincidence with Ferrar large igneous province (LIP) magmatism. Our dates refute previous work linking the T-OAE to the earlier Karoo LIP, which was coincident with an earlier carbon cycle perturbation at the Pliensbachian–Toarcian boundary. Our results suggest both events were driven by extremely rapid (perhaps sub-millennial scale) thermogenic carbon degassing during LIP sill intrusion.

Marine Fe cycling linked to dynamic redox variability, biological activity and post-depositional mineralization in the 1.1 Ga Mesoproterozoic Taoudeni Basin, Mauritania

The concentration of redox sensitive trace metals (RSTEs) and their isotopic composition preserved in Precambrian marine sediments, are critical for the reconstruction of ocean–atmosphere oxygenation history. Particularly, the concentration of Fe, its redox speciation, and isotopic distribution, have gained widespread use for inferring the biogeochemical processes that controlled Fe cycling in Precambrian oceans linked to the reconstruction of Earth surface redox budget. However, questions remain about the biotic and abiotic processes involved in Fe cycling in these ancient oceans, including the impact of post-depositional alterative processes on the reliability of the Fe redox proxy. Here we present a multi-proxy mineralogical and geochemical study of the ∼1.1 Ga Atar and El Mreiti strata of the Taoudeni Basin in Mauritania, to better constrain pathways involved in Fe cycling, linked to Fe mineralogy, redox speciation, isotopic ratios during this time and metamorphism. We compare unmetamorphosed sedimentary deposits with facies metamorphosed by dolerite sill intrusion. The results reveal the occurrence of diagenetic Fe minerals in the basal unmetamorphosed samples associated with light δ56Fe signatures, reflecting dominant anoxic conditions that promoted microbial dissimilatory Fe reduction. Notably, δ56Fe composition of these rocks reveal several fluctuations in evolving seawater redox state from oxic to anoxic/sulfidic conditions associated with changes in sea level stand and periods of full bottom water oxygenation and redox stratification. Overall, Ce anomalies suggest a general up sequence increase in seawater oxygen content. Metamorphosed rocks display heterogeneous δ56Fe distribution, consisting of light and heavy signatures associated with secondary Fe-bearing minerals produced by metamorphic and metasomatic overprinting of carbonated rocks by hot circulating fluids. The results thus indicate metamorphic overprinting of primary seawater δ56Fe promoted by increased mobility of reactive Fe during post-depositional metamorphic transformation. They show that post-depositional metamorphic/metasomatic overprinting complicates direct reconstruction of seawater biogeochemical Fe cycling and redox state using δ56Fe systematics.

Spatial and Temporal Distribution of Igneous Sills in the Central Tarim Basin and Their Geological Implications

Interpretation of the seismic reflection profiles associated with borehole data from the petroleum industry offers a novel way to study sill emplacement in sedimentary basins. This study uses this approach to reveal the intrusive part of the Tarim Large Igneous Province (LIP) within the basin, which has not been systematically reported. A large number of igneous intrusions (sills) are identified in the sedimentary layers of the Central Tarim Basin. The burial depth of the sills is 6–8 km, and they are mainly located within the upper Ordovician strata. According to their seismic facies and drilling data, it is inferred that they are dolerite sills. Based on the uplift of the overlying strata above the intrusions, it is concluded that the sills were mainly formed during the depositional period of the middle Permian Kupukuziman Formation and Kaipailezike Formation (early stage), with a few formed during the depositional period of the upper Permian strata (late stage). It is likely that these two stages of sill intrusion correspond to the main basaltic eruptions within the basin and the mafic dike emplacement in the Bachu area of the Tarim LIP, respectively. The study suggests that that the dolerite sills reported in this study are also an important component of the Permian Tarim LIP.

Microbial anabolic and catabolic utilization of hydrocarbons in deep subseafloor sediments of Guaymas Basin.

Guaymas Basin, located in the Gulf of California, is a hydrothermally active marginal basin. Due to steep geothermal gradients and localized heating by sill intrusions, microbial substrates like short-chain fatty acids and hydrocarbons are abiotically produced from sedimentary organic matter at comparatively shallow depths. We analyzed the effect of hydrocarbons on uptake of hydrocarbons by microorganisms via nano-scale secondary ion mass spectrometry (NanoSIMS) and microbial sulfate reduction rates (SRR), using samples from two drill sites sampled by IODP Expedition 385 (U1545C and U1546D). These sites are in close proximity of each other (ca. 1km) and have very similar sedimentology. Site U1546D experienced the intrusion of a sill that has since then thermally equilibrated with the surrounding sediment. Both sites currently have an identical geothermal gradient, despite their different thermal history. The localized heating by the sill led to thermal cracking of sedimentary organic matter and formation of potentially bioavailable organic substrates. There were low levels of hydrocarbon and nitrogen uptake in some samples from both sites, mostly in surficial samples. Hydrocarbon and methane additions stimulated SRR in near-seafloor samples from Site U1545C, while samples from Site U1546D reacted positively only on methane. Our data indicate the potential of microorganisms to metabolize hydrocarbons even in the deep subsurface of Guaymas Basin.

Crustal and uppermost mantle structures of the North American Midcontinent Rift revealed by joint full-waveform inversion of ambient-noise data and teleseismic P waves

The Midcontinent Rift (MCR), hosting several world-class ore deposits, is the fossil remnant of a massive Mesoproterozoic rifting event (1.1 Ga) that did not lead to the formation of an ocean basin. To better understand the lithospheric processes associated with the rifting stage and its subsequent failure, we developed a novel full-waveform joint inversion method using ambient noise data and teleseismic P waves for this seismically inactive region. We apply this approach to three years (2011-2013) of seismic recordings from the Superior Province Rifting EarthScope Experiment (SPREE) (~12 km average station spacing) and the USArray Transportable Array (~70 km average station spacing), and obtain a new 3D high-resolution Vs model down to 100 km depth, as well as Vp and density models down to 60 km depth. The model shows major velocity anomalies in agreement with previous seismic studies for the western arm of the MCR. In particular, we observe high density (2.8-3.0 g/cm3), Vp (6.3-6.5 km/s), and Vs (3.6-3.7 km/s) structures in the shallow upper crust within the rift, likely associated with volcanic rocks. Similar to a previously identified underplated layer, we also observe extensive normal-to-high Vs (3.8-4.2 km/s) along the whole rift axis and Vp (6.8-7.5 km/s) beneath the northern segment of the rift within the lower crust. However, the Vs and Vp values are lower than average for typical underplated materials. We suggest that this underplated layer may represent a combination of different intrusive rock types (e.g., gabbro, anorthosite) developed during magma differentiation processes, or contamination of the mafic magma by surrounding crustal material, or intrusions of sills.

The magma plumbing system of the Northern Carnarvon Basin, offshore Australia: multiscale controls on basin-wide magma emplacement, and implications for petroleum exploration

Abstract The Northern Carnarvon Basin (NCB) located on Australia's North West Shelf hosts an extensive (∼40 000 km 2 ) intrusive igneous complex related to Mesozoic rifting and break-up. Using an extensive suite of modern 3D seismic reflection surveys, we have mapped this intrusive system across the NCB. We identify three predominant intrusion morphologies: stacked sheets of large interconnected sill intrusions (up to ∼170 km long) and smaller (8–30 km long) isolated, strata concordant intrusions, which often interact with normal faults emplaced into deltaic sedimentary rocks, and variably sized (10–40 km long) saucer-shaped intrusions emplaced into marine shales, spread across seven zones (geographically constrained groups of intrusions of a specific morphology). We consider the zones’ margin-parallel orientation, suggesting control by subcrustal extensional processes during rifting, and the variation in intrusion morphology between these zones, suggesting a dominant control by host rock mechanical properties. We integrate previous work with our observations, constraining emplacement to between the Kimmeridgian and Valanginian, coinciding with key phases of margin evolution. Finally, we assess the impact of this intrusive complex on local petroleum systems. There is likely little to no adverse impact on source rock maturation or reservoir contamination by CO 2 , but there is a spatial dissociation between the location of groups of intrusions and the gas fields, particularly in the Exmouth Plateau. This suggests that migrating hydrocarbons may be blocked, baffled and/or redirected by emplaced igneous rocks.

Postrift buried volcanoes and igneous plumbing systems along a continental ribbon: Insights from the Xisha massif, northwestern margin of the South China Sea

Despite the prevalence of postrift volcanoes and underlying igneous plumbing systems in rifted basins at proximal and hyperextended crustal configurations, well-preserved volcanoes and related intrusions are rare within minimally stretched continental blocks (i.e., continental ribbon). It is now possible to investigate the size, age, and geographical distribution of the buried volcanoes from multibeam, single-, and multichannel seismic data recently collected beneath the South China Sea Xisha massif, which we argue is a continental ribbon. These data made it evident that the Middle Miocene volcanoes frequently generated massive volcanic fields that erupted along the rift fault zones in contrast to the Early Miocene volcanoes, which typically built clusters of small-volume volcanic cones in the rift half-graben. Details include the presence of numerous volcanoes above and to the side of the dome-shaped main edifice that constitutes the middle Miocene volcanic field. Intrusive sills beneath volcanoes are isolated and have a dispersed distribution pattern at different levels, whereas dikes beneath volcanoes are numerous and have vertical zones of disruption that connect to underlying faults or extend through the sediments to the crust. The relationship between the volcanoes and intrusions suggests that shallow igneous plumbing systems within the Xisha massif are most likely dike domains. The Xisha massif has favorable conditions, including a relatively thin sedimentary sequence over a slightly extended continental crust (20–28 km) that might provide sufficient magma pressure for an igneous plumbing system that is primarily fed by dikes. In addition, rifted faults in the upper crust and possibly subvertical foliations in the basement rock mass are thought to be viable routes for magma transport vertically. In conclusion, we emphasize the importance of crustal structure on the continental ribbon in controlling igneous plumbing styles and the distribution of postrift volcanic systems along magma-poor continental margins, including crustal thickness, preexisting faults, heterogeneous basement, and sediments.

Reconstructing the thermal history of the Morondava Basin, Madagascar, after Gondwanan breakup as resolved through detrital zircon (U–Th)/He and U–Pb geochronology

Formation of the Phanerozoic basins of Madagascar coincided with the initial stages of Permian break-up of Gondwana. The largest of these, the Morondava Basin, records the tectonic history of rifting, drifting, and uplift of the Malagasy terrane over the last 300 Myr. Herein, we have applied detrital zircon U–Pb geochronology and (U–Th)/He low-temperature thermochronology to resolve the thermal history of the basin as Madagascar separated from Africa and subsequently India. Coarse-grained siliciclastic samples were taken along two transects parallel to the Morondava River in the central basin, including from the Permo-Triassic failed rift basin and from post-Jurassic passive margin sequences. Detrital zircon U–Pb age distributions are defined by common Neoarchean and Neoproterozoic populations which suggests input from the basement terranes of the Malagasy Central Highlands (Antananarivo domain). Samples also contain Paleo-to Mesoarchean and Cambrian populations likely associated with metamorphic terranes amalgamated during the Pan-African orogeny. Single crystal zircon (U–Th)/He dates are 503 ± 17 Ma to 40 ± 1 Ma with effective uranium (eU) values ranging from 35 to 1760 ppm, which exhibit a strong negative date-eU relationship that indicates partial resetting of the He systematics in zircon throughout the basin. Inverse and forward thermal history models from the data reveal separate thermal histories for strata on either side of basin-dividing Illovo Fault. Under an elevated geothermal gradient, temperatures did not exceed 180 °C either during post-Gondwanan burial of Jurassic rift strata or during Late Cretaceous burial of Early Cretaceous passive margin strata. Intrusion of Turonian-age dikes and sills related to the Marion hot spot likely promoted subtle yet widespread heating. The time of latest cooling during the Paleogene recorded in the basin is poorly constrained by the detrital zircon thermochronology.

How Hydrothermal Cooling and Magmatic Sill Intrusions Control Flip‐Flop Faulting at Ultraslow‐Spreading Mid‐Ocean Ridges

Abstract“Flip‐flop” detachment mode represents an endmember type of lithosphere‐scale faulting observed at almost amagmatic sections of ultraslow‐spreading mid‐ocean ridges. Recent numerical experiments using an imposed steady temperature structure show that an axial temperature maximum is essential to trigger flip‐flop faults by focusing flexural strain in the footwall of the active fault. However, ridge segments without significant melt budget are more likely to be in a transient thermal state controlled, at least partly, by the faulting dynamics themselves. Therefore, we investigate which processes control the thermal structure of the lithosphere and how feedbacks with the deformation mechanisms can explain observed faulting patterns. We present results of 2‐D thermo‐mechanical numerical modeling including serpentinization reactions and dynamic grain size evolution. The model features a novel form of parametrized hydrothermal cooling along fault zones as well as the thermal and rheological effects of periodic sill intrusions. We find that the interplay of hydrothermal fault zone cooling and periodic sill intrusions in the footwall facilitates the flip‐flop detachment mode. Hydrothermal cooling of the fault zone pushes the temperature maximum into the footwall, while intrusions near the temperature maximum further weaken the rock and promote the formation of new faults with opposite polarity. Our model allows us to put constraints on the magnitude of two processes, and we obtain most reasonable melt budgets and hydrothermal heat fluxes if both are considered. Furthermore, we frequently observe two other faulting modes in our experiments complementing flip‐flop faulting to yield a potentially more robust alternative interpretation for existing observations.

Characterization of a structural trap associated with an intrusive complex: the El Trapial oilfield, Neuquén Basin, Argentina

Abstract Volcanic plumbing systems emplaced in sedimentary basins may exert significant mechanical and thermal effects on petroleum systems. The last decade of research has evidenced that igneous intrusions may enhance thermal maturation of organic matter in source rocks and lead to both small- and large-scale structures that can deeply impact fluid migration or trapping. This contribution describes how the emplacement of a whole intrusive complex generated a dome structure of the overburden, which is the main trapping structure of a large producing oilfield. Our case study is the lower Miocene Cerro Bayo de la Sierra Negra (CBSN) intrusive complex, Neuquén Basin, Argentina, associated with the El Trapial oilfield where the main trapping structure is a large domal antiform centred on the CBSN complex. This study integrates the large subsurface dataset produced during the development of the El Trapial oilfield. More than 1200 vertical wells (producers and injectors) have been drilled in the flanks of CBSN complex. In addition, five 3D seismic cubes have been acquired over the years that have been merged and reprocessed into a single volume. Such a dataset allows a detailed characterization of both the structure affecting the Mesozoic strata and the geometry of the intrusive complex. Igneous rocks have been recognized along the entire stratigraphic section. Sill intrusions appear to concentrate in the shale units and the stacking of them has a direct impact on the doming structure generation. Our study allowed us to establish a direct correlation between the distribution of the intrusions and the extent, amplitude and style of doming, showing that the dome structure results from the emplacement of the intrusive complex. We also show that part of the doming is related to intrusions emplaced in the Mesozoic formations of the Neuquén Basin, whereas the other part of the doming is related to deeper structures not imaged on the geophysical data. We estimate that the amplitude of the doming reaches up to c. 500 m. The voluminous subsurface data, combined with exposed outcrops, makes the CBSN complex a world-class case study for showing how the shallow plumbing system of a volcanic complex may control the growth of large-scale trapping structures for various fluids, such as drinkable water, geothermal fluids and hydrocarbons.

Redbed diagenesis links district-scale mineralising fluids to Cu source rock in the Polaris Zn-Pb(-Cu) district, Arctic Canada

Red sandstone of the Proterozoic Aston Formation (Nunavut, Canada) is exposed in the same geographic area as the Storm Cu showing in the Paleozoic-carbonate-hosted Polaris Zn-Pb district (Arctic Canada). The ∼800-m-thick Aston Formation experienced successive burial and exhumation episodes, with maximum burial (>3 km) at the time of the Devonian Ellesmerian orogeny. A petrographic and in situ SIMS oxygen isotope study identified redbed diagenetic events, associated fluid types and temperatures, and their relative timing. The first hematite coat (Hem1) developed in the Proterozoic. Intrusion of Proterozoic dykes and sills heated local formation water to high temperatures (130–223 °C), resulting in precipitation of patchy Qz1 cement in a shallow-burial environment. The Fe and Si were probably derived through alteration of non-quartz silicate detritus. In the Paleozoic, a bleaching (reducing) event removed most Hem 1; the reduced fluid may have been related to regional Zn mineralisation throughout the Polaris district during the mid-Paleozoic Ellesmerian orogeny. Ensuing pressure-solution during maximum burial was probably also Ellesmerian. A second hematisation episode (Hem 2) encloses the pressure-solved quartz-grain contacts. Coeval with Hem 2 was pervasive Qz2 cementation from a low- to mid-latitude, meteoric-derived, oxidised hydrothermal (140–180 °C) fluid that supplied externally derived Si and Fe, probably in the Early Carboniferous. The nature and timing of the fluid events indicated by the Aston Formation’s diagenetic history closely match the fluid history recorded in both Cu and Zn mineralisation in the Polaris district. This relationship strongly supports the concept that Cu in sedimentary-rock-hosted ore deposits can be sourced from redbeds, even those whose depositional age and environment is much older than, and geodynamically unrelated to, the fluid event(s) that caused diagenetic reddening and Cu transfer from source rock to mineralising site.

Tectonic control on dyke and sill intrusions of active tectonic margine, case study in Cilacap area, Central Java Indonesia

Tectonic control on dyke and sill intrusions of active tectonic margine, case study in Cilacap area, Central Java Indonesia

Crustal root shapes the plumbing system of a monogenetic volcanic field as revealed by magnetotelluric data

The plumbing systems of subduction-zone volcanoes consist of magma reservoirs with different degrees of evolution at multiple depths. The geometries of plumbing system of intraplate monogenetic volcanic fields are generally characterized by basaltic magma ascending from the mantle and/or a shallower reservoir along dikes. However, the shape and vertical extent of these plumbing systems are largely unknown due to the rareness of robust geophysical data. The possible role of faults and mountain chains in controlling the geometry of magma reservoirs is also poorly understood. To address these limitations, we present a three-dimensional resistivity structure beneath the Holocene Jingpohu Monogenetic Volcanic Field (JMVF) in the Zhangguangcai Range (ZGC), northeastern China. The resistivity model was derived from the magnetotelluric data in a dense net-like array. We identified an inverted cone-shaped high-resistivity (∼10,000 Ω m) anomaly below the ZGC. This anomaly traverses almost the entire crust. We interpret this high-resistivity anomaly to be the stiff basement of the ZGC. At the base of the resistive volume, we resolved a sill-like, low-resistivity anomaly that occurs at the top of the lithospheric mantle. Sheet-like low-resistivity anomalies surrounding the high-resistivity body depart from the lithospheric mantle. We interpret these sill- and sheet-like anomalies as a trans-crustal saucer-shaped magma plumbing system beneath the JMVF. This system involves multi-level magma reservoirs, including basaltic magma at its base, basaltic dikes/sills swarms in the middle and sill intrusions in the upper crust. We propose that the stiff crustal root and the topographic loading of the ZGC work together to deflect the upward magma propagation towards the edges of the ZGC. The magma ascent to the surface is probably controlled by pre-existing, lithospheric faults. Our data show how mountain chains, together with the crustal structure, play a key role in controlling the geometry of multi-level plumbing systems of intraplate monogenetic volcanic fields.

Lithospheric Sill Intrusions and Present‐Day Ground Deformation at Rhenish Massif, Central Europe

AbstractThe Rhenish Massif in Central Europe, which includes the Eifel Volcanic Fields, has shown ongoing ground deformation and signs of possible unrest. A buoyant plume exerting uplift forces at the bottom of the lithosphere was proposed to explain such deformation; the hypothesis of (possibly concurrent) melt accumulation in the crust/lithospheric mantle has not been explored yet. Here, we test deformation models in an elastic half‐space considering sources of varying aspect ratio, size and depth. We explore the effects of data coverage, noise and uncertainty on the inferred source parameters. We find that the observed deformation would require melt accumulation in sub‐horizontal sill‐like structures expanding at the rate of up to ∼0.045 km3/yr. We discuss feasibility, limitations and possible interpretations of our resulting models and elaborate on further observations which may help constrain the structure of the Rhenish Massif magmatic system.

The Tennis Ball Marker in the south-eastern Bushveld Complex: comparison with the Boulder Bed of the Western Bushveld Complex and proposed genesis by disaggregation of intrusive sills in processes akin to those involved in the formation of peperites and mafic enclave swarms

Abstract The Tennis Ball Marker (TBM) is a distinctive lithology that is particularly well developed near the base of the Rustenburg Layered Suite (RLS) on the farm Middelkraal 221 JS, approximately 20 km south of the town of Roossenekal in the Eastern Limb of the Bushveld Complex. The name refers to a texture in which approximately tennis ball-sized spheroidal aggregates of feldspathic orthopyroxenite or melanorite occur within a lighter-coloured gabbronorite host rock. We have identified two well-defined layers in which pyroxenitic spheroids are densely packed, with spheroids more sparsely distributed elsewhere in the host gabbronorite. The TBM at Middelkraal has previously been described as a contact phenomenon where the Main Zone (MZ) has been contaminated by footwall lithologies that include basaltic lavas of the Dullstroom Formation. Our geochemical data, in tandem with new geological mapping, suggest that the TBM is in fact hosted by the Marginal Sill Phase (MSP), and not by the MZ. The MSP is a regional feature that separates the overlying units of the RLS from its floor rocks throughout this region. Sills that make up the MSP were injected on a regional plane of weakness in the primary stratigraphy prior to the intrusion of the remainder of the RLS. The MSP does not represent the chilled carapace of a magma chamber, as implied in previous studies, and there is no evidence of a genetic relationship with the MZ. Our geochemical data indicate, furthermore, that the gabbronorite constituting the dominant lithology of the MSP that hosts the TBM at Middelkraal formed from a sequence of several magma influxes. The TBM is ascribed to one or more subsequent intrusions of pyroxenitic magma into the still hot gabbronoritic host sequence. There are no comparable lithologies reported from other layered intrusions, and the closest analogy to the TBM is the Boulder Bed in the Upper Critical Zone (UCZ) of the Western Bushveld Complex, which has received considerably more attention in the literature than has the TBM. The Boulder Bed has been ascribed variously to in-situ agglomeration of clusters of orthopyroxene resulting from liquid immiscibility, in situ breakup of a pre-existing pyroxenite layer, possibly due to seismic events, or the disaggregation of a late-stage pyroxenitic sill intruding into the magma chamber. We subscribe to the latter mechanism for both the TBM and the Boulder Bed, drawing on similarities with mafic magmatic enclaves, where mafic sills have been shown to have disaggregated on intrusion into earlier bodies of felsic to intermediate igneous rock, or peperites, where magma intruded fluidised sediments.

Contact metamorphic reactions and fluid–rock interactions related to magmatic sill intrusion in the Guaymas Basin

Abstract. Igneous basaltic intrusions into young organic-rich sedimentary basins have a major impact not only on the carbon cycle but also on major and trace element transfers between deep and superficial geological reservoirs. The actively rifting Guaymas Basin in the Gulf of California, which was drilled by the International Ocean Discovery Program during Expedition 385, represents the nascent stage of an ocean characterized by siliceous organic-rich sediments (diatom ooze) intruded by a very dense network of basaltic sills. This study focuses on Site U1546 where the relatively high geothermal gradient (over 200 ∘C km−1) induces early diagenetic transformations in both pore waters and sediments, involving sulfide, carbonate and silica. Geochemical and mineralogical characterizations of the sediment at sill contacts indicate that sulfides and silica polymorphs are the main phases impacted by contact metamorphism, being evident by a transition from opal-CT to quartz and pyrite to pyrrhotite, respectively. Mass balance calculations have been used to estimate mass transfers in metamorphic aureoles. In the top contact aureole, predominantly isochemical metamorphism is reflected by the presence of authigenic quartz and disseminated 20–50 µm sized pyrrhotite crystals, filling primary interstitial space, and partial dissolution of detrital feldspar grains. In the bottom contact aureole, quartz and euhedral pyrrhotite crystals occur, which are up to 4 times larger than those at the top contact. Significant metamorphism of sediments is observed in the lower contact aureole, where plagioclase recrystallizes around the detrital feldspars and locally euhedral pyroxenes are included in patches of carbonate cement; this suggests precipitation from carbon-rich fluids at temperatures (T) higher than 300 ∘C. The lower contact aureole also is more enriched in CaO, Na2O, Fe2O3 and trace elements (Cu, As, Zn, etc.) compared to the upper contact. Based on these petrological investigations, a conceptual model of magma–sediment–fluid interaction is proposed distinguishing top and bottom contact processes. Initial contact metamorphism due to sill emplacement is characterized by dehydration reactions in sediments and crystallization of new minerals. It was followed by carbonate precipitation from the released fluids. At a final stage, the temperature re-equilibrated with the geothermal gradient and the rocks were further altered by hydrothermal fluids.

Anabolic and Catabolic Microbial Activity in Hydrocarbon-rich Sediments of Guaymas Basin

Guaymas Basin, located in the Gulf of California, Mexico, is a young marginal ocean basin with high sedimentation rates of >1 mm/year, active seafloor spreading, and steep geothermal gradients in its sediment. It hosts a unique microbial subseafloor biosphere as these conditions lead to thermal cracking of sedimentary organic matter and the production of bioavailable organic carbon compounds and hydrocarbons already at shallow depths. The abundance and diversity of potential microbial substrates raise the question of which substrates are being used for catabolic and anabolic microbial metabolism. We thus analyzed the microbial uptake of hydrocarbons and inorganic nitrogen using nanoscale secondary ion mass spectrometry (NanoSIMS) analysis after incubation with stable-isotope labeled substrates. Incubations were carried out with samples from two International Ocean Discovery Program (IODP) Exp. 385 drill sites. Site U1545 is characterized by undisturbed sedimentary strata and a temperature gradient of 225°C/km, whereas Site U1546 has experienced a sill intrusion at greater depth, below the cored interval. The intrusion led to temporary heating of the sediment, but a temperature gradient of 221°C/km indicates thermal equilibration with the surrounding sediment since sill emplacement. Incubations were carried out with 13C-benzene + 2H-hexadecane + 15NH4Cl or 13C-methane + 15NH4Cl at in-situ temperature (4-62°C) and pressure (25 MPa) for 42 days. Additionally, sulfate reduction rates (SRR) were measured by incubating the samples with four aliphatic hydrocarbons + four aromatic hydrocarbons or methane and radioisotope-labeled 35SO42- for 10 days, also at in-situ temperature and pressure. The NanoSIMS analyses reveal that a few samples showed detectable microbial assimilation of hydrocarbons. Nitrogen was significantly assimilated in some samples incubated with methane. The assimilation mostly occurred in samples from near the seafloor (2 and 44 meter below seafloor (mbsf)). Our results indicate that anaerobic microorganisms in Guaymas Basin take up measurable amounts of hydrocarbons and inorganic nitrogen even in the relatively short incubation time of 42 days. The results of the SRR measurements indicate that a mixture of hydrocarbons and methane increases the SRR in samples from near the seafloor (2 mbsf) and around the sulfate-methane transition zone (44 and 55 mbsf) but not in samples from greater depths. Our results show that anaerobic microorganisms in Guaymas Basin can use hydrocarbons for anabolic and catabolic metabolism in this extreme environment. Given the high abundance of various carbon compounds, nitrogen appears to be a limiting factor for cellular growth.

NEW DATA ON THE GEOLOGICAL STRUCTURE AND PRECAMBRIAN EVOLUTION OF THE DENMAN GLACIER WEST SIDE MOUNTAIN FRAME (EAST ANTARCTICA): FIRST PALEOARCHEAN AGE FOR PLAGIOGNEISES

Mountain frame of the Denman Glacier is a little explored and at the same time an most essential region of East Antarctica in the context of studying the Precambrian geological history and geodynamic evolution of the Archean protocratons of East Antarctica. Present paper provides original U–Pb isotope geochronology data from zircons of metamorphic and intrusive rocks sampled from the Denman Glacier western flank outcrops. Geodynamic interpretation of geochronology data are also presented. For the first time, for such East Antarctic location, crystallization time (3355 ± 5.4 Ma) for plagiogneisses magmatic protolith was obtained. The Davis Paleoarchean protocraton in the Archean time interval of evolution was subject to multistage polymetamorphism in the intervals ~3100–3000, 2900–2800 Ma ago. The late stage is associated with crustal extension, which is marked by syntectonic intrusion of ultramafic dikes and pyroxenite sills (2827 ± 6 Ma). The formation time of granite veins and subalkaline granitoid pluthons corresponds to the time of tectono-thermal activisation in the interval of 550–510 Ma traced also along significant part of East Antarctic (the Pan-African tectono-thermal event). The Davis terrane shows a significant similarity in the time of formation and evolution of geodynamic processes with the Paleo-Mesoarchean protocratons of East Antarctica, as well as India and Australia.