Magmatic Budget Research Articles

Magma budget, plutonic growth and lateral spreading at Mt. Etna

AbstractThe quantitative estimation of eruptible magma is essential to assess volcanic hazard. In case of high and frequent volcanic activity, different episodes and cycles can be observed and used to gain insights on magma residence and volcano dynamics. Here, by using surface ground deformation for 26 inflation and 14 deflation phases at Mt. Etna, we inferred two partially overlapping magmatic reservoirs located beneath the summit area in the 4-9 km (inflation sources) and in the 3-6 km (deflating sources) depth ranges. Our geodetic models highlight a continuous magma supply of 10.7 ×106 m3/yr that took place in the last two decades. About 28.5% of this magma (i.e. volume loss inferred by geodetic models) contributed to the effusive activity at the surface, while the remaining 71.5% fed the endogenous volumetric growth of the plutonic crystallized mush and promoted the lateral spreading of Mt. Etna. The consistency of this behavior through time sets strong constraints on the eruptible quantity of magma in forecasting activity during a paroxysm.

DEEP REFLECTION SEISMIC DATA FOR IMPROVED IMAGING OF CRUST STRUCTURE: 2D CASE STUDY OF THE SOUTHWEST SUB-BASIN IN THE SOUTH CHINA SEA

Seismic imaging of crustal structures becomes difficult in the presence of rough basements or complex bathymetry. Here, we present a 900 km deep seismic reflection profile collected across the Southwest Sub-basin (SWSB) of the South China Sea. By analyzing the types and distinctions of noise and effective signals, we employed deep structure migration techniques to improve crustal structure imaging, wide-line processing to predict 3D space multiples, and F-K domain time-space variable adaptive de-ghosting and different offset stacking to enhance the weak signals reflected from deep strucutures. The imaged continental crustal structure in the Penxi Bank exhibits moderate thinning, down to 15 km, and is intersected by continental-ward low-angle normal faults. Within the limitations of the OBS P-wave velocity model, we detected sub-horizontal lower crustal reflections that may be indicative of a weak lower crust. Two small-scale rollover structures along detachment faults rooted and rafted to the top of these weak lower crust. Based on the presence of narrow continent-ocean transitions(COTs), continental-ward detachment faults, and high lithosphere heat flow, we deduced that the mantle lithosphere breakup occurred earlier than the crust in the SWSB. Moreover, the continent-ocean transitions and oceanic crust domains demonstrate rough basements with numerous faults and approximately 20% diffuse or weak Moho reflections. From the southern COT to the initial oceanic domain, the thickness of the crust gradually reduces to only 3-5 km. This suggests a relatively low magmatic budget and protracted tectonic extension from the continental breakup to the onset of seafloor spreading. Within the oceanic crust domain, the crust thickness ranges from approximately 4-6 km, indicating a thinner oceanic crust than normal crust. Lower crustal reflections with a ridge-ward dipping pattern terminate at the Moho reflections and are partly connected to syn-spreading faults, hinting at their possible generation through syn-spreading faulting.

Mode of extension and final architecture of the south Angolan margins controlled by Precambrian lithospheric heterogeneity

We use offshore 2-3D seismic reflection and potential field data coupled with field work to investigate the control of lithospheric heterogeneity and shear zone rejuvenation on the mode of extension and the architectural segmentation of the southern Angola margins. The pre-rift basement is inherited from several Precambrian deformation events ended by the Pan-African/Brasiliano orogen that accreted four different age-dependent lithospheric domains with contrasted thickness, fabrics, thermal structure, and chemical composition. From north to south, the continental crust is composed of Neoproterozoic metapelites, Paleoproterozoic gneisses and migmatites and Archean greenstone belts pervasively intruded by TTG granitoids. The latter are compartmentalized by lithospheric-scale Archean – Paleoproterozoic shear zones with a general sub-vertical fabric. Upon extension, these pre-existing shear zones nucleated extensional deformation, focalized magmatism and accommodated different rates of extension and width of offshore rift domains. In the north, younger Neoproterozoic lithospheric domain deformed with a ductile behavior and multiple crustal detachments promoting lithospheric decoupling and a distributed extension, with late continental break-up. It favored the formation of a wide magma-poor margin where extension was accommodated by low-angle, seaward-dipping extensional faults and mantle-exhumation. To the south, Archean and Paleoproterozoic lithospheric domain with stronger and brittle strength profile promoted a more localized extensional deformation and favored an early lithospheric coupling and continental break-up. This ultimately led to the formation of a short margin with increased magmatic budget, where extension was accommodated by high-angle, landward-dipping extensional faults and mantle-exhumation. We demonstrate that the pre-rift heterogeneity of the lithosphere controlled the mode of extension during the rift and the along-strike variation of margin geometry. Moreover, the rejuvenation of Precambrian shear zones as extensional transverse fault zones also promoted the activation of lithospheric-scale thermal drains facilitating the emplacement of mantle-related melts into the upper crust.

Temporal and spatial evolution of the 2021 eruption in the Tajogaite volcano (Cumbre Vieja rift zone, La Palma, Canary Islands) from geophysical and geodetic parameter analyses

The eruption of the Tajogaite volcano in 2021, in the Cumbre Vieja rift zone (La Palma, Canary Islands), is the most recent but also the most damaging and longest-lasting eruption on the island since historical records began. We analyzed geophysical parameters (seismicity, surface deformation, thermal anomalies, and volcanic activity) with the aim of reconstructing the temporal development of the magma plumbing system. The seismic sequence (> 9000 events, mbLg ≤ 5.0) can be grouped into nine distinct phases, which correspond to well-separated spatial clusters and distinct earthquake regimes. A strong premonitory thermal anomaly was calculated on July 2021 in the South of La Palma. The source of the magma was within the upper mantle (40–20 km depth). A second magma budget (< 15 km depth) indicated a short-term stagnation of alkaline magma within the lower crust, near the Moho. The outline of these earthquakes delineates the magma pathway. Just before and immediately after the eruption (phases 2 and 3), the high-frequency seismicity in the mantle disappeared completely, reappearing ten days after the eruption started. The pattern of deformation began before the eruption with a breakpoint around September 11, after which, deformation accelerated sharply, followed by stabilization in phase 3 and a decrease in the following phases. The level of deformation suggested that a shallow magma intrusion was established. Based on the historical eruptive pattern, it is possible to infer two possible hazard-forecast scenarios: (a) the volcano could enter a prolonged phase of tranquility for more than one century or, (b) a new eruption could occur between ca. 2045 and 2065, with a slightly shorter duration than this recent one. Therefore, if the more pessimistic scenario is assumed, appropriate territorial planning is required.

Upper Crustal Structure of Superfast‐Spread Oceanic Crust Exposed at the Pito Deep Rift: Implications for Seafloor Spreading

AbstractA tectonic window into the upper 2,000 m of oceanic crust generated at the superfast spreading (∼142 mm/yr) southern East Pacific Rise exposes a continuous layered structure of basaltic lavas and sheeted dikes over gabbroic rocks. This relatively simple structure is in accord with expectations for crustal accretion at a very fast spreading rate and high magma budget where magmatic construction keeps pace with plate separation. Detailed observations show that basaltic lava flows dip progressively more steeply inward (toward the spreading axis where they were erupted). Underlying sheeted dikes are faulted and tectonically rotated to dip steeply outward. These structures are interpreted in terms of subsidence beneath the axis of the southern East Pacific Rise during crustal construction that allowed the lava unit to thicken to &gt;400 m without creating comparable relief at the spreading center. Transitional units above and below the sheeted dike complex show that the thickness of upper crustal rock units is modified by tectonic and intrusive processes during accretion. The crustal structure shows that even approaching the superfast spreading end‐member of seafloor spreading, crustal accretion involves dramatic tectonic processes that are not obvious from the surface geology of spreading centers.

Cumulate Formation and Melt Extraction from Mush-Dominated Magma Reservoirs: The Melt Flush Process Exemplified at Mid-Ocean Ridges

Abstract Volcanism is the surface expression of extensive magmatic systems, with their intrusive counterpart representing ~80% of the total magma budget. Our knowledge of igneous processes therefore largely relies on our understanding of deep plutonic processes. In continental or oceanic environments, most of the intrusive igneous rocks bear geochemical cumulate signatures (e.g. depletion in incompatible elements and enrichment in compatible ones) that are commonly explained by mineral-melt segregation during differentiation. Deformation-assisted compaction aided by melt buoyancy is usually referred to as the main process involved in melt extraction. However, buoyancy alone is not sufficient, and a number of cumulative rocks are lacking any compaction evidence, opening the potential for the involvement of other processes. In addition, our view of magmatic systems has shifted in the last decades from large melt-rich bodies to crystal-rich magma reservoirs. This paradigm shift challenges some of the long-established first-order igneous concepts like the idea that melt differentiation at depth is mainly governed by (fractional) crystallization; alternatively, the presence of mush potentially favors additional processes such as melt-mush reactions. We propose a novel igneous process for the formation of igneous cumulates, consistent with the mushy nature of oceanic igneous reservoirs, their continuous/cyclic replenishment by primitive melts, and the widespread occurrence of reactive porous flow (RPF) during magma differentiation identified in a growing number of magmatic systems. The melt flush process relies on melt-mush reactions between the primitive recharge melt(s) and crystal mush. Replacement of the more evolved interstitial melt by the primitive recharge melt leading to reactions (dissolution+crystallization) and concomitant extraction of the more evolved melt from the cumulate by buoyancy participate in the acquisition of the final cumulate signature. This process relying on oceanic igneous systems considers for the first time melt inputs and not only melt extraction and matches the petrographic (e.g. mineral dissolution evidence) and geochemical constraints (trace element signatures) brought by natural oceanic samples. We tested various melt-mush reactions likely involved in the early stages of the melt flush process during RPF to investigate their thermodynamic feasibility with the Magma Chamber Simulator. First-order results show that one-step equilibration of primitive melts with primitive to moderately differentiated mush crystals triggers mineral assimilation. Together with the constraints established from the natural rock record, it strengthens the idea that RPF is a potential key process for magma differentiation in magma reservoirs at different evolution stages. The proposed melt flush process eventually adds to other processes involved in cumulate formation like magma compaction or crystal settling and is likely to apply to any other magmatic system from various settings sharing similar reservoir characteristics.

Ignition of the southern Atlantic seafloor spreading machine without hot-mantle booster

The source of massive magma production at volcanic rifted margins remains strongly disputed since the first observations of thick lava piles in the 1980s. However, volumes of extruded and intruded melt products within rifted continental crust are still not accurately resolved using geophysical methods. Here we investigate the magma budget alongside the South Atlantic margins, at the onset of seafloor spreading, using high-quality seismic reflection profiles to accurately estimate the oceanic crustal thickness. We show that, along ~ 75% of the length of the Early-Cretaceous initial spreading centre, the crustal thickness is similar to regular oceanic thickness with an age > 100 Ma away from hot spots. Thus, most of the southernmost Atlantic Ocean opened without anomalously hot mantle, high magma supply being restricted to the Walvis Ridge area. We suggest that alternative explanations other than a hotter mantle should be favoured to explain the thick magmatic layer of seaward dipping reflectors landward of the initial mid-oceanic ridge.

Rifted margin with localized detachment and polyphase magmatism: A new model of the northern South China Sea

Continental passive margins are commonly classified into volcanic (or magma-rich) and non-volcanic (or magma-poor) types based on the magmatic budget. It is difficult, however, to use either of these models to define the northern margin of the South China Sea (SCS). Based on 2-D/3-D time/depth-migrated seismic data and borehole data from the Pearl River Mouth Basin in the northern SCS, we examine the detachment fault systems and syn-rift magmatism in the Baiyun and Liwan Sags. The detachment fault systems and related tilted blocks of the northern SCS established in the Baiyun and Liwan Sags in previous studies may be mainly constrained to the Baiyun Sag. The Liwan Sag, however, acts as a “sag-type” basin accompanied by magmatic domes. Radiometric age of igneous samples from wells, coupled with multi-horizon onlapping terminations within syn-rift layers, reveal four-phase rift-related magmatism in the northern SCS. The crustal thinning in the Baiyun Sag is controlled by a detachment fault, but in the Liwan Sag, it is mainly controlled by a preferential thinning of the lower crust. High thermal conditions and intense syn-rift magmatism weakened the lower crust, which contributes to the depth-dependent thinning in the Liwan Sag.

The heterogeneous mantle massif in south Tibetan ophiolites and its implication for the tectonic evolution of Neo-Tethys

Ophiolitic mantle peridotites show large variations in their geochemical compositions, which provide important information for the histories of mantle melting, melt metasomatism and prior melt depletion in the context of tectonic evolution of ancient oceans. Here we report new petrological and geochemical data for mantle peridotites from the Zhagabu ophiolite in south Tibet, which could represent fossil upper mantle of the Mesozoic Neo-Tethys oceanic lithosphere. The Zhagabu mantle peridotites, including spinel lherzolites and spinel harzburgites, have highly heterogeneous geochemical compositions, with whole-rock Al2O3 of 0.3–2.0 wt% and CaO of 0.3–2.2 wt%, and spinel Cr# [Cr/(Cr + Al)] of 0.21–0.74 and Mg# [Mg/(Mg + Fe2+)] of 0.44–0.74. The clinopyroxenes from the studied samples show extremely depletion in light rare earth element compositions, some of which however are elevated by post-melting melt-rock interactions. Spinel and clinopyroxene compositions collectively suggest that the Zhagabu mantle peridotites have experienced ~9–20% of fractional melting from a depleted mid-ocean ridge basalt mantle source. Trace element modeling of clinopyroxenes further indicates that some of the studied samples follow the refertilization-hydrous melting trends with variable fluid fluxes. Therefore, the heterogeneous compositions of the Zhagabu mantle peridotites are not comparable to those of either abyssal peridotites or forearc peridotites, but most probably record a dynamic transition from the anhydrous condition beneath the Neo-Tethyan mid-ocean ridge to the hydrous condition above the subducted slab. Considering that a forearc spreading is in contradiction with the low magma budget of Yarlung Zangbo ophiolites, the Neo-Tethyan subduction re-initiation was suggested to be achieved along oceanic detachment faults at ultraslow-spreading ridges. The Zhagabu mantle peridotites represent the very heterogeneous mantle massif in south Tibetan ophiolites, and provide an excellent window to investigate the complex mantle dynamics and tectonic evolution of the Neo-Tethys Ocean.

Shallow magma convection evidenced by excess degassing and thermal radiation during the dome-forming Sabancaya eruption (2012\u20132020)

We used a large set of satellite- (visible, infrared, and radar images from Planetscope, MODIS, VIIRS, Sentinel2, Landsat 8, and Sentinel 1) and ground-based data (optical images, SO2 flux, shallow seismicity) to describe and characterize the activity of the Sabancaya volcano during the unrest and eruption phases that occurred between 2012 and 2020. The unrest phase (2012–2016) was characterized by increasing gas and thermal flux, sourced by a convective magma column rising along with the remnants of a buried plug still permeable to fluid flow. Conversely, a new conduit, adjacent to the previous one, fed the eruptive phase (2016–2020) which was instead characterized by a discontinuous extrusive activity, with phases of dome growth (at rates from 0.04 to 0.75 m3 s−1) and collapse. The extrusive activity was accompanied by fluctuating thermal anomalies (0.5–25 MW), by irregular SO2 degassing (700–7000 tons day−1), and by variable explosive activity (4–100 events d−1) producing repeated vulcanian ash plumes (500–5000 m above the crater). Magma budget calculation during the eruptive phase indicates a large excess of degassing, with the volume of degassed magma (0.25–1.28 km3) much higher than the volume of erupted magma (< 0.01 km3). Similarly, the thermal energy radiated by the eruption was much higher than that sourced by the dome itself, an unbalance that, by analogy with the degassing, we define as “excess thermal radiation”. Both of these unbalances are consistent with the presence of shallow magma convection that fed the extrusive and explosive activity of the Sabancaya dome.

Rheological inheritance controls the formation of segmented rifted margins in cratonic lithosphere

Observations from rifted margins reveal that significant structural and crustal variability develops through the process of continental extension and breakup. While a clear link exists between distinct margin structural domains and specific phases of rifting, the origin of strong segmentation along the length of margins remains relatively ambiguous and may reflect multiple competing factors. Given that rifting frequently initiates on heterogenous basements with a complex tectonic history, the role of structural inheritance and shear zone reactivation is frequently examined. However, the link between large-scale variations in lithospheric structure and rheology and 3-D rifted margin geometries remains relatively unconstrained. Here, we use 3-D thermo-mechanical simulations of continental rifting, constrained by observations from the Labrador Sea, to unravel the effects of inherited variable lithospheric properties on margin segmentation. The modelling results demonstrate that variations in the initial crustal and lithospheric thickness, composition, and rheology produce sharp gradients in rifted margin width, the timing of breakup and its magmatic budget, leading to strong margin segmentation.

Magmatism and hotspot trails during and after continental break-up in the South Atlantic

The track of South Atlantic hotspots on oceanic crust is well known. However, the possible hotspot trails of Tristan-Gough (TG) and St. Helena (SH) plume heads on continental crust have not been clearly defined. The occurrence of a large igneous province in Northeast Brazil, previously named EQUAMP, is reviewed, enlarged in time and space, and renamed as the Borborema Large Igneous Province, or BOR-LIP (135-104 Ma). There is strong evidence of an active dual plume system during the South America breakup: The BOR-LIP, in Northeast Brazil, and the Paraná-Etendeka Magmatic Province – PEMP-LIP (135-131 Ma), in SE Brazil and SW Africa. As widely accepted, the post-breakup trajectories of the SH, TG, and Sierra Leone (SL) hotspot trails (on oceanic crust) illustrate how the South American and African plates drifted away from each other. Regarding the pre-breakup trajectories, coherent and robust evidence of the migration path of the TG and SH plume heads on continental crust within the Brazilian side of the margin is presented here. They are remarkable consistent with the northwestward movement of Africa and South America, during Early Cretaceous, when referred to a deeper lithospheric reference frame, not necessarily rooted in the core-mantle boundary. The migration paths on oceanic and continental crust of the SH and TG plumes, are consistent with an intense interplay between deep-sourced mantle plumes and the relative rotation between the South American and African plates, as illustrated by the non-linear TG hotspot trail within the extremely stretched continental crust of the Santos Basin. The expressive magmatism observed in the Santos Basin, stretched during the 130-114 Ma interval, is interpreted as a direct consequence of the change in the magma budget due to the arrival of the TG plume head beneath the basin, when decompression-driven may have released melting of plume material. Large volumes of CO2, sourced by the mantle and found in petroleum accumulations within the Santos Basin, may be also genetically related to the TG plume accelerating the exhumation processes in hyper-extended terrains.

Oceanic crustal structures and temporal variations of magmatic budget during seafloor spreading in the East Sub-basin of the South China Sea

Following magma-poor continental rifting, the South China Sea (SCS) experienced seafloor spreading from ~33 Ma to ~15 Ma with an intervening southward ridge jump at ~25 Ma, generating the oceanic East, Northwest and Southwest Sub-basins. However, the magmatic processes during spreading remain poorly known owing to the lack of oceanic crustal images. Here we present a composite profile comprising three seismic lines that run from the northern continent-ocean transition (COT) to the extinct spreading center of the East Sub-basin to reveal the crustal structures. In the northernmost oceanic portion, the first oceanic crust featuring rough basement with numerous faults, and diffuse and weak Moho reflector, is only 3.9–4.5 km thick, implying relatively low magmatic budget and protracted tectonic extension continued from the magma-poor continental rifting to initial spreading. From the northern COT ocean-ward to the abandoned ridge where the jump event occurred, the basement becomes smooth with subdued faults. The Moho reflector tends to be continuous and strong, and the igneous crust gradually becomes thick (locally up to 8.4 km), suggesting considerably increasing magmatic budget during the early spreading stage. However, in the central portion spanning from the abandoned ridge to the extinct spreading axis, the basement becomes rough with crustal faults again. And the crust gradually becomes thin, to only 3.7 km at the extinct spreading center where a deep median valley is still visible, though severely altered by post-spreading volcanism, implying decreasing magmatic budget and increasing tectonic extension during the late spreading stage. Therefore, the crustal structure variations in the East Sub-basin reflects a prominent increasing to decreasing change of magmatic budget during spreading with the peak around the intervening ridge jump. Thin, tectonized crust and deep median valley around the extinct spreading center further support slow seafloor spreading during the last spreading stage of the SCS which ended at ~15–16 Ma, and also suggest that magma contribution from the Hainan mantle plume might be very limited if any.

The challenge in restoring magma-rich rifted margins: The example of the Mozambique-Antarctica conjugate margins

Using the example of the conjugate Mozambique-Antarctica rifted margins, we investigate how mantle plume magmatism interacts with extensional processes during lithospheric rifting and breakup. The presence of extensive plume related magmatic additions formed during rifting masks the edges of the continental margins, making it difficult to determine a tight conjugate margin restoration and hindering the evaluation of their pre-breakup evolution. Based on rift domain mapping from seismic reflection interpretation, crustal thickness mapping using gravity inversion, lithosphere thinning from subsidence analysis, and the testing of different plate kinematic scenario, we develop a tight initial fit of the conjugate Mozambique-Antarctica margins and describe their evolution by a three stage model. These three stages consist of: i) during early rifting, the filling of accommodation space created by continental crust thinning with Karoo related magmatism resulting in a hybrid crust that maintains bulk crustal thickness producing shallow bathymetry or emergence and allowing SDR formation; ii) the decrease of magmatic budget relative to extension rates leading to bulk crustal thinning and increased bathymetries; and iii) the transition to seafloor spreading with a steady state magmatic budget resulting in normal thickness (6,5 ± 1 km) oceanic crust. This study not only adds new constraints on the regional breakup of Gondwana but also presents a new multidisciplinary methodology for restoring magma rich rifted margins formed in the presence of mantle plumes and for improving the understanding of the nature of such hybrid magma-rich crust.

The Xigaze ophiolite: fossil ultraslow-spreading ocean lithosphere in the Tibetan Plateau

The crust and mantle both in ophiolites (fossil ocean lithosphere) and in modern oceans are enormously diverse. Along-axis morphology and lower crustal accretion at ultraslow-spreading ocean ridges are fundamentally different from those at faster-spreading ridges, and are critical to understanding how crustal accretion varies with spreading rate and magma supply. Ultraslow-spreading ridges provide analogues for ophiolites, to identify those that may have formed under similar conditions. Parallel studies of modern ocean lithosphere and ophiolites therefore can uniquely inform the origin and genesis of both. Here we report the results of structural and petrological studies on the Xigaze ophiolite in the Tibetan Plateau, and compare it with the morphology and deep drilling results at the ultraslow-spreading Southwest Indian Ridge. The Xigaze ophiolite has a complete but laterally discontinuous crust, with discrete diabase dykes and sills cutting both mantle and lower crust. The gabbro units are thin ( c . 350 m) and show upward cyclic chemical variations, supporting for an episodic and intermittent magma supply. These features are comparable with the highly focused magmatism and low magma budget at modern ultraslow-spreading ridges. Thus we suggest that the Xigaze ophiolite represents an on-land analogue of ultraslow-spreading ocean lithosphere. Supplementary material: Single-spot analysis and average compositions for mineral major element compositions (wt.%) of the Jiding gabbro section, Xigaze ophiolite are available at https://doi.org/10.17632/dtzpmwkscp.1

Syn-rift magmatic characteristics and evolution at a sediment-rich margin: Insights from high-resolution seismic data from the South China Sea

The interaction between magmatic and extensional processes related to the formation of rifted margins has been and still is highly debated. The interpretation of magmatic additions, timing of onset and budget of magma during rifting remains controversial and poorly constrained. In this study we explore the emplacement of magmatic additions in a rift system with high sedimentation rates and a well-established stratigraphic framework and discuss the perspectives toward resolving some of these problems.In this paper, we present new high-resolution seismic profiles imaging the distal margin and transition to oceanic crust in the northern South China Sea (SCS). Based on the observation of magma-related structures and the interrelationship with the sedimentary sequence, as well as the drilling results of IODP Expeditions 367&368, we define characteristics and timing of magmatic additions. The result shows that magmatic activity is expressed mainly by laccoliths and sills in the sediment-rich distal margin. Their emplacement initiated during the rifting stage, propagated seaward and continued until the early post-rift. Breakup was triggered by the formation of ENE striking cone-shaped volcanoes.In this study we show that the amount of magmatic additions progressively increases during rifting and lasted until 23.8 Ma, i.e. after the cessation of rifting. First order estimates of the amount of magmatic additions provide a first step toward understanding the magmatic budget of the northern SCS during rifting, which, despite its importance, is still lacking from most present-day rifted margins.

Melt inclusion vapour bubbles: the hidden reservoir for major and volatile elements

Olivine-hosted melt inclusions (MIs) provide samples of magmatic liquids and their dissolved volatiles from deep within the plumbing system. Inevitable post-entrapment modifications can lead to significant compositional changes in the glass and/or any contained bubbles. Re-heating is a common technique to reverse MI crystallisation; however, its effect on volatile contents has been assumed to be minor. We test this assumption using crystallised and glassy basaltic MIs, combined with Raman spectroscopy and 3D imaging, to investigate the changes in fluid and solid phases in the bubbles before and after re-heating. Before re-heating, the bubble contains CO2 gas and anhydrite (CaSO4) crystallites. The rapid diffusion of major and volatile elements from the melt during re-heating creates new phases within the bubble: SO2, gypsum, Fe-sulphides. Vapour bubbles hosted in naturally glassy MIs similarly contain a plethora of solid phases (carbonates, sulphates, and sulphides) that account for up to 84% of the total MI sulphur, 80% of CO2, and 14% of FeO. In both re-heated and naturally glassy MIs, bubbles sequester major and volatile elements that are components of the total magmatic budget and represent a “loss” from the glass. Analyses of the glass alone significantly underestimates the original magma composition and storage parameters.

Extension modes and breakup processes of the southeast China-Northwest Palawan conjugate rifted margins

Abstract Our understanding of continent-ocean transition structures and magmatism in the absence of excessive magmatic additions has been guided by the observations and models developed at the magma-poor Iberia-Newfoundland conjugate margins. Recently these models have been challenged in the South China Sea in light of new IODP Expeditions 367-368-368X. We have used an integrated analysis of high quality seismic reflection and gravity anomaly data, calibrated against recent deep sea drilling results, to investigate margin structure and tectono-magmatic interplay during continental breakup and early seafloor spreading between the SE China-NW Palawan conjugate margins. The Eocene-Oligocene South China Sea rifting initiates in a heterogeneous and likely thermally un-equilibrated lithosphere formed by the Mesozoic Yenshanian orogeny. Gravity and joint inversion methods confirm lateral variation of basement densities across the conjugate margins. Lithospheric and basement heterogeneities induced a rifting style characterized by a series of highly thinned rift basins associated with extensional faulting soling out at various crustal levels. Final rifting in late Eocene triggered decompression melting forming mid-ocean ridge type magmatism, which emplaced within thinned continental crust as deep intrusions and shallow extrusive rocks. This initial magmatic activity was concomitant with continued deformation of continental crust by extensional faulting. Integrated analysis of seismic reflection profiles and gravity anomaly data combined with deep-sea boreholes accurately locate the continent-ocean boundary. We show that the initial igneous crust, continentward of oceanic magnetic anomaly C10n, is asymmetric in width and in morphology for the conjugate margins. The wider and faulted newly accreted domain on the SE China side indicates that magmatic accretion is associated with tectonic faulting during the formation of initial oceanic lithosphere. We suggest that deformation was not symmetrically distributed between the conjugate margins during the initiation of seafloor spreading but evolved asymmetrically until the stabilisation of the spreading ocean ridge around C10n. The analysis of the South China Sea breakup reveals a transient interplay between faulting, magmatic budget and extension rates during the formation of the continent-ocean transition and initial emplacement of igneous crust.

The Role of Inherited Lithospheric Heterogeneities in Defining the Crustal Architecture of Rifted Margins and the Magmatic Budget During Continental Breakup

AbstractDuring the final stage of continental rifting, stretching localizes in the future distal domain where lithospheric necking occurs resulting in continental breakup. In magma‐poor margins, the lithospheric necking is accompanied by crustal hyperextension, serpentinization, and exhumation of mantle lithosphere in the continent‐ocean transition domain. In magma‐rich margins, the necking is accomplished by the emplacement of large amounts of volcanics in the continental‐ocean transition, in the form of seaward dipping wedges of flood basalts (seaward dipping reflections). This study examines the factors controlling the final crustal architecture observed in rifted margins and the magmatic budget during continental breakup, using observations from the Labrador Sea. The latter shows magma‐rich breakup with seaward dipping reflections documented in the north and magma‐poor breakup with a wide domain of exhumed serpentinized mantle recorded in the south. The pre‐rift strength of the lithosphere, defined by the inherited thermal structure, composition, and thickness of the lithospheric layers, controls the structural evolution during rifting. While variations in the magmatic budget associated with breakup are controlled primarily by the interaction between the pre‐rift inheritance, the timing and the degree of mantle melting, in relation to lithospheric thinning and mantle hydration.

Thermal Effects of Magma Intrusion on Ocean Temperatures Through Ice Ages, with Particular Reference to Interglacial Events

Recent observations are used to crudely quantify and scale the direct heating of the global ocean surface by submarine volcanism. These suggest a current submarine magma budget that is substantially greater than the prevailing consensus. The current ocean heating effect is estimated at c. 0.7W/m2. Interglacial events are geologically sudden and predictably cyclical, overtly caused by the combined effect of Milankovitch cycles. Several possible causes for interglacial warmings arising from these cycles have been proposed, and rejected as inadequate. These are summarily reviewed. It is concluded, in the absence of other capable causes, and in particular given the evidence of continued ocean warming throughout the Dryas cooling events, that ice age interglacial events are probably caused by a sustained increase in the rate of submarine volcanic heating of the oceans. The primary cause of increasing emissions is suggested as the rising levels of solid gravitational tides caused by the combined orbital forcing of all three main Milankovitch cycles. These tides are at their largest during interglacial warmings. How the dominant climate control system of oceanic feedback responds to this internal perturbation change is also discussed. In particular how the upper equilibrium temperature of an interglacial period is imposed by this control.