Igneous Crust Research Articles

Central Afar: An analogue for oceanic plateau development

The structure, composition, and evolution of oceanic plateaus are poorly understood and strongly debated. Here, we compared the magmatic history and crustal structure of Afar with the Greenland−Iceland−Faroe Ridge and other oceanic plateaus. Key similarities indicate that Central Afar represents the early stage of development of a specific type of oceanic plateau: a rifted oceanic magmatic plateau (ROMP). These features begin their formation before continental rifting and develop into wide magmatic rift systems capable of isolating slivers of continental crust within the new igneous crust. Importantly, the anomalous magmatism continues through breakup and for several tens of millions of years afterward. The recognition of Central Afar as a precursor of this type of oceanic plateau allows us to better understand their formation. Increased melt production causes early and voluminous magmatism, ultrathick igneous crust, and repeated reorganization of the extension locus during rift/ridge jumps, which delay the onset of oceanization and Penrose-style crustal production. These factors differentiate ROMPs from many magma-rich rifted continental margins and from other types of oceanic plateaus, highlighting that Central Afar and other ROMPs should neither be considered as conventional magma-rich margins nor be considered as normal oceanic crust.

High-precision U[sbnd]Pb geochronology of an ignimbrite flare-up in a silicic large igneous province (Chon Aike, Patagonia)

Silicic large igneous provinces (SLIPs) are periods of particularly voluminous felsic volcanism in the geologic record. Previous work has suggested an overall long lifespan for SLIPs of 20 to 40 Myr, commonly punctuated by shorter-lived ‘flare-ups’ of higher volcanic productivity (1–5 Myr), but detailed studies of individual flare-up events are lacking. The Jurassic Chon Aike SLIP (CASP) is the product of an exceptional geological event wherein voluminous felsic volcanism (ca. 219,000 km3) was generated predominately via crustal anatexis over 45 Myr. We focus on the Late Jurassic El Quemado Complex (EQC), which marked the final stages of felsic volcanism for the CASP. In-situ U-Pb ages for the EQC previously suggested a duration of ∼5 Myr; however, new high-precision CA-ID-TIMS ages indicate the total durations of ignimbrite successions were shorter than 350 kyr. A compilation of zircon U-Pb ages for the eight CASP formations in Patagonia and the Antarctic Peninsula reveals changes in volcanic duration between formations deposited in the intraplate relative to the continental margin, suggesting a spatial control over the magmatic lifespans of geographically restricted systems. We suggest that the rate of magmatism in the CASP was primarily controlled by heterogeneities in the crust, largely between Proterozoic igneous crust and younger, metasedimentary crust that was recently accreted relative to the timing of Jurassic volcanism. The observed duration of volcanism was modulated by these differences in crustal properties from the injection of mafic magmas into the lower crust during extension and mantle upwelling, following rollback of the subducting oceanic slab towards the Paleo-Pacific margin of Gondwana. The short duration in the EQC is the product of unique overlapping conditions that favored crustal melting and resulted in the voluminous ignimbrite flareup (104 km3) of some of the Earth's highest δ18O magmas measured.

An alternative to the igneous crust fluid + sediment melt paradigm for arc lava geochemistry.

A long-standing paradigm of arc geochemistry is that the trace element compositions of arc lavas arise from two compositionally distinct slab components: an aqueous dehydration fluid from the subducting igneous ocean crust that transports "fluid-mobile" elements, such as barium (Ba), and a sediment melt that supplies thorium (Th) and the light rare earth elements. This two-component framework has been widely called upon to explain global geochemical trends as well as geochemical variations within individual arcs, such as the Marianas. Here, we show that this paradigm is inconsistent with mass balance, due to the low Ba contents of igneous ocean crust, and with experimental data, which show that aqueous fluids from the igneous oceanic crust would be too dilute to substantially affect arc compositions. Observations previously attributed to the sediment melt/igneous-crust-fluid hypothesis are better explained by diverse subducting sediment compositions coupled with ambient mantle wedge heterogeneity, both globally and for the Marianas.

Simultaneous rift-scale inflation of a deep crustal sill network in Afar, East Africa

Decades of studies at divergent plate margins have revealed networks of magmatic sills at the crust-mantle boundary. However, a lack of direct observations of deep magma motion limits our understanding of magma inflow from the mantle into the lower crust and the mechanism of sill formation. Here, satellite geodesy reveals rift-scale deformation caused by magma inflow in the deep crust in the Afar rift (East Africa). Simultaneous inflation of four sills, laterally separated by 10s of km and at depths ranging 9–28 km, caused uplift across a ~ 100-km-wide zone, suggesting the sills are linked to a common mantle source. Our results show the supply of magma into the lower crust is temporally episodic, occurring across a network of sills. This process reflects inherent instability of melt migration through porous mantle flow and may be the fundamental process that builds the thick igneous crust beneath magmatic rifts and rifted margins globally.

Origin of Philippine Sea Basins During Subduction Initiation in the Western Pacific

AbstractUnderstanding the age and dynamics of the overriding plates allows an assessment of competing subduction initiation hypotheses. The Izu‐Bonin‐Mariana margin in the Western Pacific is a key example of initiation and hence it is important to constrain the age and origin of the oldest igneous crust of the supra‐subduction Philippine Sea Plate. We present geochronological and geochemical data of igneous rocks from the oldest ocean basins of the Philippine Sea Plate: the West Philippine and Palau Basins. Basalts from these basins have enriched geochemical characteristics similar to the EM‐2‐like mantle component found in OIB‐like basalts associated with the Oki‐Daito mantle plume. Ages of basalts from the northernmost West Philippine Basin (WPB) and the Palau Basin range from 43.5 to 50.5 Ma, which is similar to the oldest samples associated with the Oki‐Daito mantle plume (48–50 Ma). This implies that the plume contributed to magmatism from the onset of basin formation. It also provides support for the proposition that rifting of the Mesozoic arc terrane and subsequent seafloor spreading of the WPB was triggered by the arrival of the Oki‐Daito mantle plume at the base of the lithosphere. The age of these Philippine Sea Basins implies that only the Mesozoic Daito Ridge Group and the Gagua Ridge existed as Philippine Sea Plate crust before subduction initiation. A major fault activity after 37 Ma in the northernmost WPB demonstrates that careful reconstruction of the Eocene Philippine Sea Plate is critical to understanding plate dynamics during subduction initiation in the Western Pacific.

Relationship Between D‐MORB and E‐MORB Magmatism During Crustal Accretion at Mid‐Ocean Ridges: Evidence From the Masirah Ophiolite (Oman)

AbstractEnriched mid‐ocean ridge basalts (E‐MORB) commonly erupt at mid‐ocean ridges (MOR) and seamounts, but their relationship to “depleted” MORB (D‐MORB) and the processes controlling their magmatic evolution at MORs are not fully understood, hence raising more general questions about magma generation in the mantle. We here explore this conundrum through an investigation of the Masirah ophiolite (southeast Oman), a near‐unique “true” MOR ophiolite. Unlike most (e.g., Tethyan) ophiolites, it was not affected by subduction and is therefore potentially able to provide valuable geological insights into the magmatic evolution of a full section of oceanic crust. Previous work has shown that the igneous crust at Masirah was thin (1.5–2.0 km) and constructed from both D‐ and E‐MORB magmas, concluding that it formed at a slow‐spreading ridge at ∼150 Ma followed by an episode of “Nb‐enriched” magmatism with trace‐element enrichments exceeding E‐MORB during intraplate rifting ∼20 Ma later. We reinvestigate the geology of Masirah and present new field observations, geochemical data and high‐precision U‐Pb ages to constrain the magmatic history of seafloor spreading and off‐axis magmatism. We found that D‐MORB and E‐MORB magmatism at Masirah was synchronous and overlapped in both composition and time with the Nb‐enriched magmatism (no older than 135 Ma). Both types of magmatism were therefore integral in the formation of the Masirah ocean crust. The relationship between D‐MORB and E‐MORB magmatism described here may be applicable to modern MORs more broadly, but is especially prominent at Masirah due to reduced magmatism and hence a weaker crustal filter.

Identification of recycled oceanic crust in the mantle source of sulfide-bearing lower crust of the Gangdese magmatic arc, Southern Tibet: Constraints from Sr–Nd–Li isotopes

Post-collisional porphyry copper deposit (PCD) is a new genetic type of PCDs discovered in the Himalayan–Tibetan orogenic belt. It is believed that the formation of ore-forming magmas results from the re-melting of a sulfide-bearing, thickened, juvenile mafic lower crust in a magmatic arc associated with the oceanic plate subduction. However, the material contribution of subduction processes to the formation of this particular arc magma remains unclear. In this paper, we attempted to present a study that includes petrology, mineral geochemistry, whole rock Sr–Nd–Li isotopes, of Late Cretaceous Lilong ultramafic igneous rocks, south Lhasa sub-terrane, Tibet. The target ultramafic igneous rocks are sulfide-bearing cumulates. They show relative depletion in whole rock Nd isotopic compositions, slightly elevated (87Sr/86Sr)i ratios (0.7043–0.7059), and variable δ7Li values (2.8–9.0‰). Simulation results showed that trace element of melts in equilibrium with cumulus hornblende grains exhibit typical arc-like trace element distribution patterns, indicating that these ultramafic igneous rocks crystallized from arc magma. Additionally, the modeled melts showed enrichment in melt-mobile elements such as Ba, Pb, Sr, Th, and light rare earth elements (LREE), and higher Th/Yb ratios, as compared with normal mid-ocean ridge basalt (MORB). These geochemical properties can be explained by the metasomatism of depleted MORB mantle peridotites by hydrous melts derived from subducted seafloor sediments. Whole rock Sr isotopes show correlations with Ba/Nb and Th/La ratios for the modeled melts, showing that fluids from the basaltic igneous crust, in addition to hydrous melts from the seafloor sediments, contribute to their mantle source. In addition, the heterogeneity of whole rock δ7Li values for ultramafic igneous rocks also indicates that their mantle sources have been metasomatized by terrigenous sediment- and altered oceanic crust-derived fluids. Depleted MORB mantle peridotites react with the mixed fluids at sub-arc depths to produce ultramafic metasomatites, and model calculations for trace element contents and Sr–Nd–Li isotopic compositions further validate that the geochemical compositions of the Lilong ultramafic rocks can be clarified by this process. The results show that the deeply subducted oceanic crust may have played an important role in the formation of sulfide-bearing juvenile mafic lower crust of this magmatic arc.

Petrogenesis and tectonic implications of the early Paleozoic granites in the Lincang granitic batholith, southwestern China: Constraints from geochronology, geochemistry and Hf isotopes

This study presents the recently discovered Wulao Shan Ordovician granite within the Lincang granitic batholith, discusses its geochemical affinity, genesis, and tectonic significance, and interprets the proto-Tethys orogenic model in southwest Yunnan. LA-ICP-MS zircon U-Pb analyses indicate a crystalline age of 473.0 ± 2.9 Ma for the Wulaoshan gneissic granite. Geochemical analyses show that the Wulaoshan granite exhibits relatively high SiO2 concentrations (73.4–77.3 wt.%), strong peraluminous properties (ASI > 1.1), strongly Eu-negative anomalies, light and heavy rare earth fractionation, and relative Ba, Nb, Ta, Sr, P, and Ti depletion. SiO2 and P2O5 exhibit a strong negative correlation, whereas Rb and Y-Th with a positive correlation. The magmatic zircons have negative εHf(t) values of -5.47∼2.74. Petrographic observations and whole-rock geochemical and isotopic analyses imply no aluminum-rich minerals such as cordierite, indicating that the Wulaoshan granite is a highly fractionated, high-temperature I-type granite, which may have formed via partial melting of the magnesian-iron igneous crust. Additionally, K-feldspar, plagioclase, and biotite within the granite samples may have undergone intense fractional crystallization. The Wulao Shan gneissic granite is an arc granite associated with subduction, indicating that the Lincang Block underwent proto-Tethys subduction. This finding provides evidence for proto-Tethys subduction down to the Lincang-Simao block and further supports the bidirectional subduction model of the proto-Tethys.

Genomics discovery of giant fungal viruses from subsurface oceanic crustal fluids

The oceanic igneous crust is a vast reservoir for microbial life, dominated by diverse and active bacteria, archaea, and fungi. Archaeal and bacterial viruses were previously detected in oceanic crustal fluids at the Juan de Fuca Ridge (JdFR). Here we report the discovery of two eukaryotic Nucleocytoviricota genomes from the same crustal fluids by sorting and sequencing single virions. Both genomes have a tRNATyr gene with an intron (20 bps) at the canonical position between nucleotide 37 and 38, a common feature in eukaryotic and archaeal tRNA genes with short introns (<100 bps), and fungal genes acquired through horizontal gene transfer (HGT) events. The dominance of Ascomycota fungi as the main eukaryotes in crustal fluids and the evidence for HGT point to these fungi as the putative hosts, making these the first putative fungi-Nucleocytoviricota specific association. Our study suggests active host-viral dynamics for the only eukaryotic group found in the subsurface oceanic crust and raises important questions about the impact of viral infection on the productivity and biogeochemical cycling in this ecosystem.

Al-Mg and U-Pb chronological records of Erg Chech 002 ungrouped achondrite meteorite

Achondrite meteorites are remnants of the earliest planetary differentiation processes in the Solar System. They have been used to anchor short-lived radiochronometers to absolute ages determined from long-lived radiochronometers. More specifically, when comparing the isotopic systematics of the short-lived 26Al-26Mg chronometer anchored to absolute U-corrected Pb-Pb ages, inferences about the distribution of 26Al (half-life of ∼717 000 yr) in the protoplanetary disk can be evaluated. The ungrouped achondrite Erg Chech (EC) 002 has a distinct mineralogy and more evolved elemental composition compared to basaltic achondrites. In situ and solution 26Al-26Mg chronometry and 53Mn-53Cr chronometry suggest that EC 002 formed within ∼0.7 to 2.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), making it the oldest known sample of igneous crust in the Solar System (Barrat et al., 2021; Anand et al., 2022; Zhu et al., 2022; Fang et al., 2022). Here we present the U-corrected Pb-Pb age and 26Al-26Mg age obtained by MC-ICPMS solution analysis of the same mineral separate samples of EC 002. In addition, six merrillite grains were analyzed by in-situ SIMS to determine their Pb-Pb individual ages.The U isotope composition of EC 002 exhibits internal heterogeneities between leached pyroxene (238U/235U = 137.766 ± 0.027) and the bulk rock (238U/235U = 137.8190 ± 0.0074). The Pb isotope composition of progressively leached pyroxenes are characterized by radiogenic 206Pb/204Pb ratios (ranging from 41 to 23487). Using the U isotope composition of the leached pyroxenes, the resulting age of the 207Pb/206Pb-204Pb/206Pb isochron is 4565.87 ± 0.30 Ma (2σ). The weighted mean of the Pb-Pb ages of seven SIMS analyses of merrillites are 4564.3 ± 5.2 Ma (2σ). These similar ages (within uncertainty) indicate rapid cooling and the absence of significant thermal events after ∼4559 Ma on the parent body of EC 002. The 26Al-26Mg isochron through a bulk rock, pyroxene, fine-grained and four plagioclase fractions defines an initial 26Al/27Al ratio of [8.89 ± 0.79] × 10−6 corresponding to a formation age of 1.83 ± 0.12 Ma after CAIs ([5.23 ± 0.13] × 10−5; Jacobsen et al., 2008). The initial 26Al abundance is consistent with previous MC-ICP-MS 26Al-26Mg reported systematics for EC 002 (Fang et al., 2022), but 0.46 ± 0.13 Myr older than the in situ SIMS 26Al-26Mg age previously reported by Barrat et al. (2021).When anchored to the absolute Pb-Pb age of CV3 CAIs (4567.30 ± 0.16 Ma; Connelly et al., 2012), the Al-Mg model age of EC 002 is 4565.47 ± 0.20 Ma, slightly younger than its U-corrected Pb-Pb age.The concordance of the Pb-Pb and 26Al-26Mg ages of ungrouped CC achondrites when anchored to EC 002 suggest that 26Al was homogeneously distributed between the NC and CC reservoirs at the time of their parent body accretion. Furthermore, the presence of internal U isotope heterogeneities found between mineral and whole-rock samples of EC 002 supports the need of U isotope analysis of meteoritic samples dated using the Pb-Pb chronometer.

Crustal structure and magmatism of the Marvin Spur and northern Alpha Ridge, Arctic Ocean

SUMMARYThe Marvin Spur is a 450-km-long east–west trending escarpment along the northernmost periphery of the Alpha Ridge, starting about 500 km from the coasts of Ellesmere Island and Greenland off the Arctic Ocean margin of North America and running subparallel to the Amerasian margin of the continental Lomonosov Ridge. This region was investigated as part of the Canada–Sweden Polar Expedition in 2016, from which two seismic profiles are presented. The first is a 165-km-long line along the crest of the Marvin Spur. The second is a 221-km-long line extending southwestward from the spur to the northern flank of the Alpha Ridge within the Cretaceous High Arctic Large Igneous Province (HALIP). Multichannel seismic reflection data were acquired along both lines using a 100-m-long streamer, and the airgun shots were also recorded using 16 sonobuoys and 5 stations on the sea ice to calculate a velocity model for the crust from forward modelling of seismic traveltimes. The Marvin Spur profile reveals up to 1100 m of sedimentary rocks on top of a 1-km-thick series of basalts (4.5–5.1 km s−1). Upper and lower crust have velocities of 5.8–5.9 km s−1 and 6.2–6.3 km s−1, respectively, with the upper crust being 1–2 km thick compared to around 13 km for the lower crust. A wide-angle double seismic reflection manifests the top and base of a 6-km-thick lower crustal layer that we interpret as magmatic underplating beneath the continental crust of the Marvin Spur. We correlate a high-amplitude magnetic anomaly on Marvin Spur with a comparable anomaly on Lomonosov Ridge by invoking 110 km of dextral strike-slip motion. Assuming that HALIP-related magmatic deposits generate these anomalies, the strike-slip motion pre-dates the main phase of magmatism (latest Cretaceous, 78 Ma). On the northern Alpha Ridge, sediments are around 1-km-thick and cover a 700 to 1700-m-thick series of basalts with velocities of 4.4–4.8 km s−1. Below is a 3-km-thick layer with intermediate velocities of 5.6 km s−1 and a lower crust with a velocity of 6.8 km s−1. Moho depth is not resolved seismically, but gravity modelling indicates a total thickness of 13 or 18 km for the igneous crust except for the Fedotov Seamount where Moho deepens by about 5 km. Construction of the seamount occurred in multiple magmatic phases, including flow eruptions during deposition of the Cenozoic sedimentary succession post-dating the main HALIP magmatism.

Hydration, melt production and rheological weakening within an intracontinental gneiss dome

The Entia Gneiss Complex (EGC) in central Australia represents a deeply exhumed Paleoproterozoic basement terrane that underwent fluid-catalysed transformation culminating in the formation of a migmatitic gneiss dome during the 450–300 Ma intracontinental Alice Springs Orogeny (ASO). Foremost amongst the changes the EGC experienced during this event was the regional-scale conversion of nominally anhydrous granulite and igneous crust to hydrous amphibolite facies migmatite, together with the emplacement of locally voluminous mica-bearing pegmatite swarms. LA–ICP–MS U–Pb dating of zircon, monazite and xenotime demonstrates that pegmatite emplacement dominantly occurred late in the Alice Springs Orogeny (c. 360–300 Ma). The oldest generation of pegmatites are transposed into the regional Palaeozoic gneissic fabric and formed at c. 1770 Ma, indicating they belong to protolith rocks of the EGC. Three subsequent generations of pegmatite have ages that range between 360 and 300 Ma. The oldest of these is parallel to the migmatitic gneissic fabric and dated at c. 360 Ma, whereas the younger two generations are largely undeformed, discordant and intruded throughout the period c. 350–300 Ma. The capability for a comparatively dry intracratonic orogenic core to repeatedly produce melt over an interval of c. 60 Myr requires explanation. We suggest crustal-scale duplex development during the latter stages of the ASO resulted in the juxtaposition of dry and mechanically strong Paleoproterozoic basement over deeply buried Neoproterozoic–Cambrian sedimentary sequences. Fluid derived from the metamorphic dehydration of these sequences led to hydration, partial melting and rheological weakening of the basement slab. Prior to its hydration at c. 360 Ma, the EGC was metamorphically unresponsive, accounting for its comparatively late timing of deformation and partial melting compared to the overall history of the Alice Springs Orogeny. The development of the crustal-scale duplex system, combined with hydrous melting of the low-density basement slab beneath high-density supracrustal rocks, resulted in a thermomechanically unstable architecture that facilitated the formation of an orogen-scale gneiss dome during the waning stages of intracontinental orogenesis.

Coevolution of sedimentary and strongly peraluminous granite phosphorus records

Phosphorus plays an important role in both surface biological cycles as a critical macronutrient and in magmatic systems as the backbone for trace element-rich phosphate minerals. Phosphorus cycling between igneous and sedimentary systems throughout Earth history has had potentially profound effects on biogeochemical cycles. Previous studies have focused on the impacts of enhanced contribution of phosphorus from igneous rocks on marine phosphorus budgets. In contrast, the effect of changing sedimentary phosphorus abundances on the phosphorus budget of magmas has not been investigated. Here I present a compilation of phosphorus concentrations in strongly peraluminous granites (SPGs), which are derived through the partial melting of sedimentary rocks, spanning the past ∼3.5 billion years. Maximum phosphorus concentrations of SPGs increase abruptly in the Phanerozoic, closely following a similar, previously documented increase in phosphorus concentrations in marine siliciclastic sediments deposited after 720 Ma. The effect of metamorphic, magmatic, and subsolidus processes on SPG bulk-rock phosphorus concentrations are considered in turn. Lower average bulk-rock phosphorus concentrations in SPGs derived from metasedimentary rocks deposited prior to 720 Ma likely reflect lower phosphorus concentrations of their metasedimentary sources. The SPG compilation both confirms previous observations of phosphorus abundances in the marine sedimentary record and provides an alternative archive in that it integrates large volumes of continental shelf to slope sedimentary rocks. A clear shift in the crustal phosphorus cycle is documented with the initiation of enhanced recycling of sedimentary phosphorus into the igneous crust, where it can be again weathered and returned to the ocean to fuel further photosynthesis. Although phosphorus concentrations in zircon have been used to distinguish between derivation from strongly peraluminous “S-type” from metaluminous “I-type” granites, the results presented here suggest that phosphorus concentrations in SPGs have a time dependency and that phosphorus concentrations in zircon from SPGs may vary similarly in time.

Paleomagnetism and paleolatitude of igneous drill core samples from the Izu-Bonin forearc and implications for Philippine Sea plate motion

The Philippine Sea plate is an integral part of the east Asian plate mosaic but its past motions can only be reconstructed from paleomagnetic data. Moreover, because the plate is largely submarine, few fully-oriented paleomagnetic data are available. International Ocean Discovery Program (IODP) Expedition 352 cored at four sites on the Izu-Bonin forearc, penetrating ∼1170 m of forearc igneous crust radiometrically dated at ∼51 Ma. Paleomagnetic measurements on 361 samples produced paleo-inclination data that were used to estimate paleolatitude. Twenty five independent magnetic units give a paleocolatitude of 90.1° ±4.4° (2σ uncertainty), implying a paleolatitude exactly on the equator. Unit variance agrees with global paleosecular variation models, implying that these variations have been properly sampled and averaged. The paleolatitude implies northward drift of ∼28°, which agrees with other Izu-Bonin forearc sites of similar age. A slight difference in northward drift with southern Philippine Sea plate sites is consistent with plate rotation. Certain parts of the cored sections yield samples that were remagnetized to the present geocentric axial dipole inclination after ∼30 Ma, implying a tectonic event of unclear origin.

Unusual δ26Mg values in oceanic crust basalts from the South China Sea

Abstract Whether or not oceanic crust basalts are affected by plate subduction is a hot topic of debate. The South China Sea is one of the largest marginal basins in the western Pacific Ocean and has been surrounded by subduction of the Pacific plate and Indian plate, yet, to date, no study has clearly shown evidence of subduction in the geochemistry of volcanism in the basin due to a lack of sampling of igneous crust basalts on the seafloor. The International Ocean Discovery Program Expedition 349 cored seafloor basalts near the fossil spreading ridges of the eastern (Site U1431) and southwestern (Site U1433 and U1434) subbasins in the South China Sea. The recovered basalt samples indicated a pyroxenite-bearing peridotite mantle source. Here, we report Mg isotopic data from 14 of these oceanic crust basalt samples. The δ26Mg values of most basalts from the three drill holes were higher (up to −0.10‰) than that of the average mantle (−0.25‰). The lack of correlations of δ26Mg with geochemical indices of magmatic processes (e.g., MgO, CaO/Al2O3, La/Sm, Nb/Zr) suggests that crystal fractionation and partial melting had insignificant effects on the Mg isotopic compositions of the South China Sea basalts. Thus, the variations in Mg isotopes were inherited from their mantle sources. Considering the highly varied Ce/Pb ratios and elevated 87Sr/86Sr values but mantle-like 143Nd/144Nd values, we propose that the varied δ26Mg values were likely caused by metasomatism of subduction-released fluids. The coupling of Mg and Sr-Fe isotopes provides robust evidence that the high-δ26Mg values of the South China Sea basalts resulted from mixing among pyroxenite-bearing peridotite mantle, the nearby Hainan plume materials, and subducting serpentinite-released fluids. Therefore, these Mg isotopes suggest that the mantle source of the South China Sea basalts was influenced by subducted materials, providing further evidence of the initial expansion, formation, and evolution of the South China Sea during plate subduction.

Shallow Depth, Substantial Change: Fluid-Metasomatism Causes Major Compositional Modifications of Subducted Volcanics (Mariana Forearc)

Mass transfer at shallow subduction levels and its ramifications for deeper processes remain incompletely constrained. New insights are provided by ocean island basalt (OIB) clasts from the Mariana forearc that experienced subduction to up to ∼25–30 km depth and up to blueschist-facies metamorphism; thereafter, the clasts were recycled to the forearc seafloor via serpentinite mud volcanism. We demonstrate that the rocks were, in addition, strongly metasomatized: they exhibit K2O contents (median = 4.6 wt%) and loss on ignition (median = 5.3 wt%, as a proxy for H2O) much higher than OIB situated on the Pacific Plate, implying that these were added during subduction. This interpretation is consistent with abundant phengite in the samples. Mass balance calculations further reveal variable gains in SiO2 for all samples, and increased MgO and Na2O at one but losses of MgO and Fe2O3* at the other study site. Elevated Cs and Rb concentrations suggest an uptake whereas low Ba and Sr contents indicate the removal of trace elements throughout all clasts. The metasomatism was likely induced by the OIBs’ interaction with K-rich fluids in the subduction channel. Our thermodynamic models imply that such fluids are released from subducted sediments and altered igneous crust at 5 kbar and even below 200°C. Equilibrium assemblage diagrams show that the stability field of phengite significantly increases with the metasomatism and that, relative to not-metasomatized OIB, up to four times as much phengite may form in the metasomatized rocks. Phengite in turn is considered as an important carrier for K2O, H2O, and fluid-mobile elements to sub-arc depths. These findings demonstrate that mass transfer from the subducting lithosphere starts at low P/T conditions. The liberation of solute-rich fluids can evoke far-reaching compositional and mineralogical changes in rocks that interact with these fluids. Processes at shallow depths (&amp;lt;30 km) thereby contribute to controlling which components as well as in which state (i.e., bound in which minerals) these components ultimately reach greater depths where they may or may not contribute to arc magmatism. For a holistic understanding of deep geochemical cycling, metasomatism and rock transformation need to be acknowledged from shallow depths on.

Methodology for Evaluating the North Atlantic Igneous Province Reveals Multiple Hotspots at Breakup

AbstractThe North Atlantic Igneous Province (NAIP) has been the focus of numerous studies. However, a systematic integrated methodology to assess the NAIP, has so far been lacking. For this purpose, a synclinal oceanic rift approach is developed from the stratigraphic record. Mechanisms are addressed combining trace element and seismic velocity data as a proxy for excess mantle potential temperature (ΔTp), using simple mantle melting models. Unconformities play a major role in this assessment, providing evidence for a drop in ΔTp to ambient levels (≤50°C). Three phases can be delineated in the stratigraphic record: a (a) Pre‐; (b) Syn‐; and (c) Post‐breakup phase. The Syn‐breakup phase is bound at the base by the Breakup Unconformity and at the top by the Magmatic Unconformity. At breakup, magmatic crustal thickness (H) can be estimated from addition of basalt‐ and Lower Igneous Crust isopachs. These seismic estimations can be further constrained by observations on volcanic transport directions. This allows differentiation between hotspots and Volcanic Passive Margins (VPMs), where hotspots are characterized by continuous ΔTp amplitude variations, whereas VPMs only display declining or absent fluctuations post breakup. On Iceland and the Greenland‐Iceland‐Faroe Ridge, H is primary controlled by ΔTp fluctuations promoting ridge jumps and second, associated fluctuating active upwelling ratio (χ ∼ 2–4). Results suggest that there were at least three hotspots active at breakup. The West Greenland and proto‐Jan Mayen hotspots lost importance in the Oligocene, while Iceland remained and is currently the main hotspot in the North Atlantic.

Magmatism at Passive Margins: Effects of Depth‐Dependent Wide Rifting and Lithospheric Counterflow

AbstractRifted passive margins exhibit a large variety in the timing, distribution, and amount of magmatism. The factors controlling magmatism during rifted margin formation, remain, however, incompletely understood, partly owing to the complex rifting styles. In this study, we use 2‐Dimensional numerical models to investigate the effects of depth‐dependent wide rifting and lithospheric counterflow on magmatism during rifted margin formation. Results show that a strong crust promotes narrow margins with a sharp transition to normal thickness oceanic crust whereas a weak crust promotes depth‐dependent wide rifting, with preferential removal of mantle lithosphere, leading to formation of wide margins with over‐thickened (&gt;18 km) igneous crust in the distal margin. Counterflow of depleted lithospheric mantle, in contrast, may delay syn‐rift magmatism, and result in exhumed a‐magmatic continental mantle at narrow margins. The combination of wide rifting and lithospheric counterflow results in magma‐poor wide margins, with in some cases a transition to excess magmatic activity at breakup. Our models provide an explanation for the contrasting magmatic productivity at narrow and wide rifted margins, such as observed in the Lofoten‐Vesterålen, Newfoundland, Kwanza Basin, and Orange Basin margins.

Seismic Formation Fluid Pressure Observations Reveal High Anisotropy of Oceanic Crust

AbstractFractures and faults in igneous oceanic crust affect hydrothermal circulation and hydration of the oceanic plates. Seismic observations have provided information about lithospheric fabrics, but direct measurements of oceanic crust’s elastic properties were not made. We report determinations of compressibility of the upper igneous crust (of 3.6 Ma) of the Juan de Fuca plate, using observed formation fluid pressure oscillations in sealed boreholes associated with passing surface waves from distant earthquakes. We determine an azimuthal variation of formation‐matrix compressibility by a factor of ∼5, with the crust being most compressible in the plate‐spreading direction across the structural fabric inherited from crustal creation. This is equivalent to a seismic compressional wave speed anisotropy of ∼50%–60%, much greater than that of standard seismic measurements (typically &lt;20%). This likely reflects a previously unresolved degree of fracturing of the uppermost oceanic crust, consistent with existing observations that suggest a high degree of hydraulic permeability anisotropy.

70 million years of seafloor spreading and magmatism in the South Atlantic

A basic tenet of seafloor spreading is the need to spatially accommodate the magnitude of lateral spreading. As new spreading proceeds and crust is transported away from the spreading axis, the space created is filled by both the magmatic accretion of new oceanic crust and the brittle stretching of that crust by extensional faults. The ratio between the magmatism and faulting, referred to as M, is dynamically related to the rate of spreading and the topographic roughness of the generated crust. Here we use a multichannel seismic dataset recently collected in the South Atlantic Ocean to examine how the relationships between spreading rate, M, and topographic roughness have changed over 70 million years of crustal ages all created at one spreading segment. We frequency filter the topographic profile of the top of the igneous crust to remove long wavelength signal and determine the best window length for analysis. We then use a moving window to quantify how M and the root mean square of topographic roughness vary across the age transect. Qualitatively and quantitatively, we find spreading rate and M are positively correlated while M and spreading rate are negatively correlated with the topographic roughness of the crust. We are able to expand the age and length scale of an analysis of M by an order of magnitude compared to previous studies. Our analysis also finds that the spreading center in the South Atlantic has likely alternated between an axial high and axial valley twice in the past.