Seafloor Hydrothermal Alteration Research Articles

Metabasic rocks as important nitrogen carriers to forearc depths: Implications for deep nitrogen cycling

Understanding deep nitrogen (N) cycling better requires investigating the delivery of N to subduction systems via various lithologies. Input to subduction zones through mafic rocks is more voluminous and massive as compared to sedimentary rocks which calls for a thorough investigation of the behavior of N in metabasic rocks. Here we estimate the delivery of N to subduction zones at forearc depths by investigating the geochemistry of amphibolites and epidote-blueschists from the Central Qiangtang Metamorphic Belt in Tibet where the metabasic rocks likely represent the transition from oceanic to continental subduction. The rocks contain 21–147 ppm N with δ15N values from +1.8 ‰ to +10.0 ‰, and 147 ppm N is the highest that has been reported in a metabasic rock thus far. Given the N abundances for most of the rocks are much higher than those of altered oceanic crust (i.e. basalts, sheeted dikes and gabbros; 6.0 ± 4.7 ppm), the N is likely neither magmatic nor was introduced in the rocks during hydrothermal alteration prior to subduction. This is confirmed by the K2O/Th versus Ba/Th, Th/U versus Th and Ba/Rb versus K2O plots where these rocks align with the trend of metamorphic fluid alteration rather than seafloor hydrothermal alteration. A two-step process led to N acquisition in the metabasic rocks. In the first step, the metabasic rocks acquired their N from metasediment-derived fluids during metamorphism in the subduction channel. In the second step, some of the metabasic rocks underwent N loss and concomitant enrichment in 15N due to devolatilization within the subduction channel. We modeled the N fluxes at forearc depths in 55 modern-day subduction zones via metasedimentary and metabasic rocks assuming their minimum, median and maximum N concentrations to assess their relative importance in delivery of N to subduction zones. We find that metabasic rocks supply comparable fluxes of N to metasedimentary rocks at forearc depths, even though metasedimentary rocks have at least an order of magnitude higher N abundance than metabasic rocks. The importance of metabasic rocks in N delivery reinforces the need to investigate the behavior of N in them from more locations globally to improve our understanding of deep N cycling.

Petrophysical Facies and Inferences on Permeability at Brothers Volcano, Kermadec Arc, Using Downhole Images and Petrophysical Data

Abstract Downhole data and cores collected during International Ocean Discovery Program (IODP) Expedition 376 at Brothers volcano, Kermadec arc, provide unprecedented, in situ views of volcanic facies and fluid pathways in an actively forming volcanogenic massive sulfide (VMS) ore deposit. Brothers volcano is a submarine caldera with extensive sea-floor hydrothermal alteration. Downhole data were collected in two holes: Hole U1530A at the NW Caldera and Hole U1528D at the Upper Cone. Textural analysis of microresistivity images in Hole U1530A provides a continuous image facies record that greatly improves findings based upon sporadic and partial (18%) core recovery. Between 90 and 214 meters below sea floor (mbsf), the heterogeneous image facies with local pattern variations is consistent with the volcaniclastic facies interpreted from cores. Between 232 and 445 mbsf, a volcanic facies was not recognizable in cores because of overprinting alteration, apart from five intervals of coherent lava flows that were less altered. Based on the fairly constant petrophysical data, Vp-porosity relationship, and presence of five to six coherent image facies intervals on the microresistivity image, we propose that the apparent volcaniclastic textures observed on cores and microresistivity images beneath 232 mbsf are dominantly lava flows. The change from volcaniclastic to dominant lava flow facies occurs over a transition zone (214–232 mbsf) where all petrophysical properties gradually change. In Hole U1528D, cores and petrophysical data show a similar transition from deep coherent lava flows to shallower, largely volcaniclastic sequences at ~270 mbsf. Down to 232 mbsf in Hole U1530A and 360 mbsf in Hole U1528D, the overall first-order downward decrease in porosity is interpreted to be caused by compaction and increased alteration intensity. Volcanic facies and fractures exert a second-order local control on petrophysical properties. Beneath 232 mbsf in Hole U1530A, the prolonged hydrothermal activity is inferred to have diminished local petrophysical property variations within the proposed lava flow-dominated rock package. High downhole fluid temperatures in Hole U1528D contrast with the moderate temperatures in Hole U1530A. Permeable zones show a mix of structural (inferred fault in Hole U1530A) and lithological controls in both holes. Some low-permeability layers and/or lithological interfaces possibly focus fluids laterally in higher-permeability layers, which may act as a trap for metal-rich fluids to form stratabound massive sulfides and deposits. Matrix is likely too low in permeability to conduct fluids but provides perfect conditions for the storage of supersaline brines. In Hole U1530A, located near active vents at the sea floor, the relatively low fluid temperature and the alteration overprint of moderate temperature demonstrate the high spatial and temporal variations at Brothers volcano. The implications of the new stratigraphy and controls on permeability proposed here for Brothers volcano include a better understanding of the following: (1) submarine volcanic eruption sequences, (2) permeability in active submarine volcanoes, and (3) the formation of volcanogenic massive sulfide deposits on (and near) the sea floor.

Seafloor Magnetism Under Hydrothermal Alteration: Insights From Magnetomineralogy and Magnetic Properties of the Southwest Indian Ridge Basalts

AbstractTitanomagnetites in mid‐ocean ridge basalt (MORB) experience variable post crystallization alterations associated with seafloor tectonic and environmental processes. Compared to low‐temperature oxidation, seafloor hydrothermal alteration is thought to be more destructive but its magnetic aftermaths are insufficiently documented. Here we present comprehensive rock magnetic and electron microscopic analyses of fresh and hydrothermally‐altered MORBs dredged from the Longqi and Yuhuang hydrothermal fields, Southwest Indian Ridge. We observe large variations in magnetic properties of fresh MORBs, originated from relative proportions of nano‐scale single‐domain to vortex state and micron‐scale vortex to multi‐domain state dendritic titanomagnetites. Progressive hydrothermal alteration produces secondary magnetite through recrystallization of exsolved and dissolved Fe from primary titanomagnetite. Exsolution is evident by a dual Verwey transition signature and coexisting Ti‐poor titanomagnetites and sphenes in partially chloritized basalts. A schematic model is proposed to explain the variations in magnetomineralogy and magnetic properties with progressive hydrothermal alteration. Intermediate hydrothermal alteration products retain a secondary chemical remanent magnetization (CRM) which is related to the long‐term magnetization variations in oceanic basalts. The established framework allows characterizing MORB hydrothermal alteration and ultimately contributes to resolving the complexity of seafloor magnetism.

Petrological Implications of Seafloor Hydrothermal Alteration of Subducted Mid-Ocean Ridge Basalt

Abstract Determining the mineralogical changes occurring in subducted oceanic crust is key to understanding short- and long-term geochemical cycles. Although numerous studies have explored the mineral assemblages that form in mid-ocean ridge basalt (MORB) at different depths below the Earth’s surface, it is widely recognized that seafloor hydrothermal alteration of the uppermost portion of the oceanic crust can change its composition between a ridge and a trench prior to subduction. In this study, we use petrological modelling to explore the effects of different types of pre-subduction hydrothermal alteration on the phase changes that occur during seafloor alteration of MORB-like compositions during subduction along an average Phanerozoic geotherm. We consider a representative composition of altered oceanic crust, as well as extreme end-member scenarios (pervasive spilitization, chloritization, and epidotization). Our models show that epidotization and chloritization of MORB strongly affects phase equilibria at different depths, whereas spilitization and an average style of alteration produce relatively fewer changes on the mineral assemblage to those expected in a pristine MORB. Devolatilization of MORB during subduction occurs mostly in the forearc region, although the type and extent of alteration strongly control the depth and magnitude of fluid released. Altered compositions carry significantly more H2O to sub- and postarc depths than unaltered compositions; the H2O carrying capacity of unaltered and altered compositions is further enhanced during subduction along colder geotherms. Extremely localized areas affected by epidotization can transport up to 22 times more H2O than unaltered MORB and up to two times more than average altered oceanic crust compositions to depths beyond the arc. Regardless of the extent and style of alteration, the stability of hydrous phases, such as epidote and phengite (important trace element carriers), is expanded to greater pressure and temperature conditions. Thus, hydrothermal alteration of the subducted oceanic slab-top represents a viable, and probably common, mechanism that enhances geochemical recycling between the Earth’s hydrosphere and shallow interior.

Features of seafloor hydrothermal alteration in metabasalts of mid-ocean ridge origin from the Chrystalls Beach Complex

ABSTRACT The Taieri Mouth locale of the Chrystalls Beach Complex (CBC) in the South Island of New Zealand includes well preserved to strongly deformed pillow lavas and flattened veins of epidote, quartz and chlorite intercalated with basalt flows and volcanoclastic breccias. The tectonic affinity for this rare igneous portion of the predominantly sedimentary CBC has not been well established in the context of its regional metamorphic geology. New field, petrographic, geochemical and isotopic evidence suggest a mid-ocean ridge origin for the Taieri metabasalts. Further, paleo-vertical networks of epidote-quartz-chlorite veins and cross-cutting faults provide a record of seafloor fracturing and fluid-flow. Altered pillows and epidote separates have δ18O isotope values ranging from 9.3 to 13.1‰. This indicates slightly enriched δ18O fractionation resulting from seafloor weathering and low-temperature (<250°C) exchange between seawater and hydrothermal fluids in basaltic fractures. Age-corrected 87Sr/86Sr ratios between 0.704135 and 0.70624 show low temperature fluid-rock interactions where the altered pillows and veins did not succumb to major mineralogic changes or isotopic re-equilibration after formation. In contrast, compressed s-fold epidote and coarse quartz veins near metasediments are suggestive of the elevated temperatures and pressures during accretion. We differentiate between episodic seafloor venting and accretional wedge-related alteration recorded within these metabasalts.

Seafloor hydrothermal alteration affecting magnetic properties of abyssal basaltic rocks: insights from back-arc lavas of the Okinawa Trough

Seafloor hydrothermal systems in the back-arc region of the Okinawa Trough have been viewed as a modern analogue to the Kuroko-type volcanogenic massive sulfide deposits. Detection of magnetic signatures is widely utilized and assumed to facilitate the understanding of geological controls on hydrothermal system genesis. However, the magnetic properties of seafloor volcanic rocks are still poorly understood because of the difficulties of sample acquisition. Here, we report rock magnetic data along with linked geochemical and petrological data of volcanic rock samples obtained from the Irabu knolls of the southern Okinawa Trough. Both fresh and hydrothermally altered basaltic andesites were successfully obtained from the seafloor via submersible. A fresh sample, with single-domain titanomagnetite grains, is strongly magnetized with NRM intensity of up to 100 A/m. Minute skeletal and dendritic titanomagnetite grains are also observed. A second fresh sample, with multi-domain titanomagnetite grains, contains a greater amount of titanomagnetite grains, but exhibits NRM intensity ~ 10 A/m at most. In contrast to the fresh samples, hydrothermally altered samples show extremely low NRM intensities along with low saturation magnetization and certain contribution of paramagnetic minerals. Grain assemblages of pyrite and chalcopyrite grains appear along cracks in the groundmass. Our results indicated that fine titanomagnetite grains in groundmass within back-arc lava flows are altered due to hydrothermal processes. The recorded primary remanent magnetization of the lava flows is thus partly removed by hydrothermal alteration. Magnetization reduction related to hydrothermal activity produces local crustal magnetization lows and thus enables us to detect hydrothermal alteration zones by utilizing magnetic field measurements in space. In particular, the lavas we examined (via their resultant basaltic andesites) have high Curie temperatures greater than 400 °C, which is significantly higher than those indicated by mid-ocean ridge basalts, suggesting that the thermal effect for crustal magnetization may be less in back-arc settings.

Subduction and atmospheric escape of Earth's seawater constrained by hydrogen isotopes

The hydrogen isotopic (D/H) ratio reflects the global cycling and evolution of water on Earth as it fractionates through planetary processes. We model the water cycle taking seafloor hydrothermal alteration, chemical alteration of continental crust, slab subduction, hydrogen escape from the early Earth, and degassing at mid-ocean ridges, hot spots, and arcs into account. The differences in D/H ratios between present-day oceans, oceanic and continental crust, and mantle are thought to reflect isotopic fractionation through seafloor alteration, chemical alteration, and slab dehydration. However, if the speed of plate tectonics has been nearly constant throughout Earth's history, the degassing and regassing rates are too small to reach the present-day D/H ratios. We show that (a) hydrogen escape from reduced early atmosphere, (b) secular net regassing, or (c) faster plate tectonics on early Earth is needed to reproduce the present-day D/H ratios of the water reservoirs. The low D/H ratio of Archean seawater at 3.8 Ga has previously been interpreted as a signature of (a) hydrogen escape, but we find it can also be explained either by (b) secular net degassing or by (c) faster plate tectonics on early Earth. The rates of hydrogen escape from early Earth and secular regassing on present-day Earth are constrained to be lower than 2.1×1011 kg/yr and 3.9×1011 kg/yr. Consequently, the volume of water in the present-day mantle could result entirely from the regassing through Earth's history. In that case, the volume of initial oceans could be 2 to 3 times larger than that of current Earth. We suggest that, in addition to the D/H ratio of Archean seawater, identifying the D/H ratios of both seawater and mantle throughout Earth's history would allow to distinguish these evolutionary scenarios.

Constraints from geochemistry and oxygen isotopes for the hydrothermal origin of orthoamphibole mafic gneiss in the New Jersey Highlands, north-central Appalachians, USA

Rare exposures of orthoamphibole mafic (Oam) gneiss of Mesoproterozoic age in the north-central Appalachians are confined to the northwestern New Jersey Highlands where they form thin lens-shaped bodies composed of gedrite and sparse anthophyllite, oligoclase (An13–An20), biotite, magnetite, and local fluorapatite, rutile, and ilmenite. The gneiss is penetratively foliated and has sharp, conformable contacts against enclosing supracrustal paragneiss and marble. Orthoamphibole mafic gneiss is characterized by low SiO2 (48±2.5wt%), CaO (1.9±1.3wt%), and high Al2O3 (18±1.2wt%), Fe2O3 (10.5±1.6wt%), and MgO (12±2.3wt%). Trace element abundances overlap those of unaltered amphibolites in the study area and, coupled with δ18O values of 9.45±0.6‰ (VSMOW) from gedrite separates, support an origin from a basalt protolith.The geochemical and isotopic data are consistent with the formation of Oam gneiss through sea floor hydrothermal alteration of basalt at low temperature of 150–200°C. Mass-balance calculations indicate gains during alteration mainly in MgO and Al2O3 and losses in CaO, Sr, and light rare earth elements. Our results are compatible with the pre-metamorphic alteration of the basalt protoliths through chloritization and plagioclase dissolution that produced a Mg-rich and Ca-poor rock. Subsequent metamorphism of this chlorite-rich rock to the current mineral assemblage of Oam gneiss took place at ca. 1045Ma, during the Ottawan phase of the Grenvillian Orogeny. The close spatial association in the study area of Oam gneiss bodies and sulfide occurrences suggests an affinity to the style of mineralization associated with volcanogenic massive sulfide (VMS)-type deposits.

K isotopes as a tracer of seafloor hydrothermal alteration

At ocean spreading ridges, circulation of seawater through rock at elevated temperatures alters the chemical and isotopic composition of oceanic crust. Samples obtained from drilling into ocean floor and from ophiolites have demonstrated that certain isotope systems, such as 18O/16O and 87Sr/86Sr, are systematically modified in hydrothermally altered oceanic crust. Although K is known to be mobile during hydrothermal alteration, there have not yet been any K-isotope analyses of altered oceanic crustal materials. Moreover, the 41K/39K of seawater was recently found to be significantly higher than that of igneous rocks, so the addition of seawater K to oceanic crust would be expected to generate 41K/39K variations in affected rocks. Here, we report high-precision 41K/39K measurements for samples from the Bay of Islands ophiolite, and we document large variations in 41K/39K, covarying with previous determinations of 87Sr/86Sr. Our data indicate that analytically resolvable 41K/39K effects arise in oceanic crust as a result of hydrothermal alteration. This finding raises the possibility that 41K/39K can be used as an effective tracer of oceanic crust recycled into the mantle, as a diagnostic criterion by which to identify ancient fragments of oceanic crust, and as a constraint on the flux of K between oceanic crust and seawater.

Whole-rock and zircon geochemical distinction between oceanic- and continental-type eclogites in the North Qaidam orogen, northern Tibet

Continental ultrahigh-pressure (UHP) metamorphic belts were tectonically developed from oceanic subduction to continental subduction. Petrological records of the tectonic transition can be preserved if the subducted oceanic crust is exhumed together with dominant crustal rocks of continental origin in collisional orogens. This is illustrated by the coexistence of continental-type and oceanic-type UHP eclogites in the North Qaidam orogen, northern Tibet. An integrated study of whole-rock major-trace elements, Sr-Nd isotopes and mineral O isotopes for eclogites, as well as zircon U-Pb ages and trace elements was carried out for UHP eclogites and gneisses from the North Qaidam orogen. The results allow discriminating between the two types of eclogites in their protolith origin. The continental-type eclogites are dominant there and characterized by Neoproterozoic protolith ages of ca. 850Ma, versatile trace element compositions, mostly positive εNd(t) values, and normal mantle-like δ18O values. Their protoliths are continental mafic igneous rocks that are originated from multiple mantle sources with different compositions. In comparison, the oceanic-type eclogites are minor there and have protolith ages of ca. 500Ma, positive εNd(t) values, and lower δ18O values than normal mantle δ18O values. Their protoliths are a kind of oceanic mafic igneous rocks that experienced high-T seafloor hydrothermal alteration. Both IAB-like and MORB- to OIB-like trace element compositions occur in the oceanic-type eclogites, suggesting their origination from a backarc basin close to the continental margin. Zircon U-Pb dating of the oceanic-type eclogites, continental-type eclogites and gneisses yields similar eclogite-facies metamorphic ages of 427–439Ma, with coesite inclusions in metamorphic zircons from one gneiss. The identical metamorphic ages for the two types of eclogites indicate that the exposed eclogites of oceanic origin were primarily residing in the continental margin and they were entrained by the UHP metamorphic rocks of continental origin for exhumation along the same subduction channel toward the surface. Therefore, identifying the occurrence of oceanic-type eclogites in the continental-type eclogite-gneiss associations is a valid means to decipher the tectonic development from oceanic subduction to continental collision.

Identification of mantle peridotite as a possible Iapetan ophiolite sliver in south Shetland, Scottish Caledonides

The Neoproterozoic Dunrossness Spilite Subgroup of south Shetland, Scotland, has been interpreted as a series of komatiitic and mafic lava flows formed in a marginal basin in response to Laurentian continental margin rifting. We show that ultramafic rocks previously identified as komatiites are depleted mantle peridotites that experienced seafloor hydrothermal alteration. The presence of positive Bouguer gravity and aeromagnetic anomalies extending from the Dunrossness Spilite Subgroup northward to the Shetland Ophiolite Complex suggests instead that these rocks may form part of an extensive ophiolite sliver, obducted during Iapetus Ocean closure in a forearc setting. Supplementary material : Supplementary information, including methods, supplementary figures and tabulated data, is available at https://doi.org/10.6084/m9.figshare.c.3469161 .

Petrogenesis and geochemistry of the Late Carboniferous rear-arc (or back-arc) pillow basaltic lava in the Bogda Mountains, Chinese North Tianshan

The tectonic nature of the Chinese Tianshan Orogen during the Late Paleozoic has been long disputed. With aims of providing constraints on this issue, an integrated study of geochronology and geochemistry has been carried out on the Late Carboniferous pillow basaltic lava of the Qijiagou Group from the Bogda Mountains, Chinese North Tianshan. Zircon SHRIMP U–Pb dating of a dacite ignimbrite, which is in conformable contact with the pillow lava, suggests that they were erupted at ~311Ma. The pillow cores and rims show different petrological and geochemical characteristics, suggesting post-magmatic seafloor hydrothermal alteration. Nevertheless, both pillow cores and rims have the MORB-like Sr–Nd–Hf isotopes and arc-like trace element compositions. Clinopyroxene and plagioclase from the pillow lavas are compositionally different from those of the mafic rocks related to the Tarim mantle plume. These observations, together with the tholeiitic index (THI>1) and the Fe/Mn ratios (53–57) of them, indicate that the Bogda pillow lavas may have been generated from a dry and depleted mantle source metasomatized by sediment-derived melts. Compared with basalts of the Izu–Bonin arc-back-arc system, the Bogda Late Carboniferous basaltic lavas show great resemblance to the Izu–Bonin rear-arc basalt (including the arc-like back-arc basalt) in terms of major and trace element and mineral compositions. It suggests that these basalts were likely formed in a rear-arc or back-arc environment.

Tourmaline B-isotopes as tracers of fluid sources in silicified Palaeoarchaean oceanic crust of the Mendon Formation, Barberton greenstone belt, South Africa

Pervasive silicification is a common phenomenon in Early Archaean volcano-sedimentary sequences, reflecting the interaction between Si-rich fluids and the Archaean oceanic crust. In the Palaeoarchaean Barberton greenstone belt, South Africa, the silicified zones locally contain abundant tourmaline. In order to constrain the source of fluids responsible for the alteration, six tourmaline-bearing samples from the 3.3Ga Mendon Formation were investigated by in-situ analyses of mineral chemistry and boron isotopes. The samples comprise a silicified, finely laminated sedimentary chert and five highly altered and silicified komatiites. Electron microprobe analysis (EMPA) indicates that tourmaline is mostly dravitic, with one sample between dravite and Mg-foitite. Tourmaline has a weakly-developed patchy colour- and chemical zonation, with variations in Al, Fe and Mg composition. Some samples show strong zoning in Cr-concentration with Cr enrichment in the core, and locally the high-Cr domains are at contacts with other Cr-rich phases such as magnesiochromite and chromian muscovite (“fuchsite”).Overall, the boron isotope composition (δ11B) ranges from −20.7 to +10.2‰. Two groups of tourmaline are distinguished based on the variation in δ11B ratios within single samples: 1) those with small (<4‰) range in δ11B values (spinifex-textured komatiites and silicified sediment), and 2) those with a large range in δ11B values of up to 18‰ (brecciated and foliated samples). Positive boron isotope values (+6 to +10‰) are found in one spinifex-textured komatiite and are interpreted as being derived from seawater interacting with Archaean oceanic crust in shallow hydrothermal systems. The intermediate boron isotope values (−5 to −8‰) found in all other samples are similar to MORB and may represent boron derived from the Archaean oceanic crust. The rare and exceptionally light boron isotope values (−10 to −21‰) were found in rims of group 2 tourmaline. These values cannot be produced by fractionation alone, and point to the presence of another fluid source. The source of the light boron has yet to be identified, but possible candidates are remobilized boron either derived from granitic crust or from marine evaporites that precipitated from seawater enriched in 10B. In any case, the isotopically light boron appears to have been present during seafloor hydrothermal alteration and deposition of the Mendon Formation at ca. 3.3Ga.

Records of geomagnetism, climate, and tectonics across a Paleoarchean erosion surface

Paleomagnetism has provided key constraints on the evolution of Earth's climate, geomagnetic field, and continental geography through Phanerozoic and Proterozoic time. Extending these constraints into the Archean eon has been particularly challenging due to the paucity of the ancient rock record. Here we report paleomagnetic measurements on the NASA Astrobiology Drilling Project (ABDP)-8 core drilled through one of the world's least deformed and least metamorphosed Paleoarchean [3200–3600 million year old (Ma)] rock successions located in the East Strelley Belt of the eastern Pilbara Craton, Australia. Our results show that the ∼3350 Ma Euro Basalt preserves a shallow magnetic inclination that appears to have formed as a result of early seafloor hydrothermal alteration, suggesting that the evaporitic carbonate platform of the conformably underlying Strelley Pool Formation was deposited in a near-equatorial location. This is consistent with (although does not require) late Paleoarchean climatic zoning, low orbital obliquity, and a geocentric axial dipole (GAD) field geometry similar to that of the Phanerozoic. The Euro Basalt paleopole overlaps with previously published Paleoarchean poles from the East Pilbara craton and with time-equivalent poles reported from the Barberton Greenstone Belt of the Kaapvaal craton, supporting the existence of a Paleoarchean Vaalbara continental aggregation.

Mapping of Seafloor Hydrothermally Altered Rocks Using Geophysical Methods: Marsili and Palinuro Seamounts, Southern Tyrrhenian Sea

Research Article| December 01, 2014 Mapping of Seafloor Hydrothermally Altered Rocks Using Geophysical Methods: Marsili and Palinuro Seamounts, Southern Tyrrhenian Sea Marco Ligi; Marco Ligi † 1Istituto di Scienze Marine, CNR, Via Gobetti 101, 40129, Bologna, Italy †Corresponding author: e-mail, marco.ligi@bo.ismar.cnr.it Search for other works by this author on: GSW Google Scholar Luca Cocchi; Luca Cocchi 2Istituto Nazionale di Geofisica e Vulcanologia, Roma 2, Via Pezzino Basso 2, 19025, Fezzano (La Spezia), Italy Search for other works by this author on: GSW Google Scholar Giovanni Bortoluzzi; Giovanni Bortoluzzi 1Istituto di Scienze Marine, CNR, Via Gobetti 101, 40129, Bologna, Italy Search for other works by this author on: GSW Google Scholar Filippo D’Oriano; Filippo D’Oriano 1Istituto di Scienze Marine, CNR, Via Gobetti 101, 40129, Bologna, Italy Search for other works by this author on: GSW Google Scholar Filippo Muccini; Filippo Muccini 2Istituto Nazionale di Geofisica e Vulcanologia, Roma 2, Via Pezzino Basso 2, 19025, Fezzano (La Spezia), Italy Search for other works by this author on: GSW Google Scholar Fabio Caratori Tontini; Fabio Caratori Tontini 3GNS Science, Ocean Exploration Section, 1 Fairway Dr., Avalon, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar Cornel E. J. de Ronde; Cornel E. J. de Ronde 3GNS Science, Ocean Exploration Section, 1 Fairway Dr., Avalon, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar Cosmo Carmisciano Cosmo Carmisciano 2Istituto Nazionale di Geofisica e Vulcanologia, Roma 2, Via Pezzino Basso 2, 19025, Fezzano (La Spezia), Italy Search for other works by this author on: GSW Google Scholar Economic Geology (2014) 109 (8): 2103–2117. https://doi.org/10.2113/econgeo.109.8.2103 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Marco Ligi, Luca Cocchi, Giovanni Bortoluzzi, Filippo D’Oriano, Filippo Muccini, Fabio Caratori Tontini, Cornel E. J. de Ronde, Cosmo Carmisciano; Mapping of Seafloor Hydrothermally Altered Rocks Using Geophysical Methods: Marsili and Palinuro Seamounts, Southern Tyrrhenian Sea. Economic Geology 2014;; 109 (8): 2103–2117. doi: https://doi.org/10.2113/econgeo.109.8.2103 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract Hydrothermal alteration processes involve mineralogical, chemical, and textural changes as a result of hot aqueous fluid-rock interaction under evolving boundary conditions. These changes affect the physico-chemical properties of the rocks, enabling high-resolution geophysical prospecting to be an important tool in the detection of seafloor hydrothermal alteration. Here we present the results of recent geophysical investigations of the Marsili and Palinuro volcanic complexes, southern Tyrrhenian Sea, during the 2010 TIR10 and 2011 MAVA2011 cruises by the R/V Urania. The new dataset includes a dense grid of multibeam bathymetry; seafloor reflectivity, magnetic and gravity lines; and high-resolution single (CHIRP) and multichannel seismic profiles. The surveys were focused on areas known to host intense hydrothermal alteration in order to provide a more detailed description of the Marsili and Palinuro hydrothermal systems. Ground-truthing was based on earlier discoveries of hydrothermal vents and their associated deposits, and on direct observations made by ROV dives. High-resolution morpho-bathymetry, sonar reflectivity, rock magnetization, and density distribution together enabled us to assess the extent of seafloor hydrothermal alteration and its relationship to local volcanic and tectonic structures. Hydrothermal alteration associated with the Marsili seamount is largely distributed along primary volcano-tectonic structures at the ridge crest. By contrast, at Palinuro hydrothermal alteration is mostly associated with secondary volcanic structures such as collapsed calderas and volcanism reactivation along ring faults. In particular, evidence for intense hydrothermal activity occurs at Palinuro where volcanotectonic features interact with regional tectonic structures. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Decrease of seawater CO2 concentration in the Late Archean: An implication from 2.6 Ga seafloor hydrothermal alteration

Before continents attained a critical aerial dimension on the early Earth, hydrothermal carbonation of subseafloor crust is considered to have played the dominant role in fixing CO2 from the CO2-rich ocean. However, it is uncertain how and when the seawater CO2 level decreased and the strong carbonation of oceanic crust ceased. Here we report the depth profiles of the volume concentration and the carbon isotopes of calcites in the Late Archean/Paleoproterozoic volcanic rocks (Fortescue and Hamersley groups), exposed in the southwestern Pilbara Craton, Western Australia. The depth profiles indicate that 2.6Ga seafloor hydrothermal carbonation is well preserved in the study area and that the CO2 content of subseafloor crust per seafloor unit area is estimated to be clearly lower than those in the Early and Middle Archean and similar to the Phanerozoic equivalents. This suggests that the CO2 concentration in seawater decreased from the Middle Archean to the Late Archean. This period broadly corresponds to the time of the first appearance of supercontinent on Earth. The amalgamation of continents has the potential to decrease seawater CO2 concentration due to the removal of platform carbonate to continental interior. Subsequent fragmentation of supercontinent likely cause the carbonate deposition around newly created continental shelves. It is therefore implied that seawater CO2 concentration in the early Earth was lowered by not only the hydrothermal carbonation of subseafloor crust but also through the formation and breakup of supercontinent in the Late Archean.

Geochemistry of Neoarchean mafic volcanic rocks and late mafic dikes in the Zanhuang Complex, Central Orogenic Belt, North China Craton: Implications for geodynamic setting

An Archean mélange belt has been recognized in the Zanhuang Complex, Central Orogenic Belt (COB), North China Craton (NCC). All units of the mélange belt are intruded by a ca. 2.5Ga granite pluton and cross-cut by undeformed 2.5Ga pegmatites, which constrains the Archean formation age of the mélange. Some exotic mafic–ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the mélange and mainly consist of metabasaltic rocks, deformed pillow lavas with epidosite cores, gabbros, and ultramafic rocks. Both metabasalts and epidosites are characterized by enrichment in light rare earth elements (LREE) and pronounced negative Nb and Zr anomalies on chondrite- and primitive mantle- normalized diagrams. These geochemical characteristics are consistent with supra-subduction zone geochemical signatures. Based on the pillow structures and seafloor hydrothermal alteration, combined with the geochemical characteristics, an intra-oceanic arc–forearc setting is proposed for the formation of these exotic blocks. Furthermore, a west-dipping intra-oceanic subduction zone is proposed to have formed in an ocean between the Eastern Block and an arc terrane prior to 2.5Ga. The collision between the arc and the Eastern Block of the NCC happened at ca. 2.5Ga, resulting in the formation of the Archean mélange belt in the Zanhuang Complex. All of the units of the mélange belt were intruded by late mafic dikes, which are present as boudinaged or lenticular shapes in the mélange and surrounding gneisses and are intruded by younger ca. 2.5Ga pegmatites. Geochemical characteristics of the late mafic dikes are consistent with an arc-related mantle source region, rather than an OIB-like source. Accordingly, pending determination of their precise age, an arc-polarity reversal event is proposed to have occurred following the arc–continent collision, resulting in east-dipping subduction beneath the newly accreted arc and Eastern Block, forming the swarm of arc-affinity mafic dikes. The new subduction zone further resulted in ponded magma below the accreted arc/Block, which induced the partial melting of thickened crust and the intrusion of ca. 2.5Ga granitic and pegmatitic melts into the mélange and the mafic dikes. The mafic dikes were then deformed and metamorphosed when the remaining open part of the ocean between the arc-collision modified margin of the Eastern Block collided with the Western Block of the NCC.

Sea-floor metamorphism recorded in epidosites from the ca. 1.0 Ga Miaowan ophiolite, Huangling anticline, China

The epidosites are interpreted to form in upflow zones at the base of ore-forming oceanic hydrothermal systems that vent as black smokers on the sea floor. This study presents new field, major and trace element, and oxygen isotope data for the recently discovered epidosites in the ca. 1.0 Ga Miaowan (庙湾) ophiolite located near the northern margin of the Yangtze craton. The epidosites occur mainly in the cores of strongly deformed, lensoidal amphibolites. Field observations, major and trace elements and oxygen isotopes suggest that the epidosites were formed by metasomatism of ocean floor basalts, diabase dykes, and gabbros during seafloor hydrothermal alteration.

Trace element and isotopic fingerprints in HP–LT metamorphic rocks as a result of fluid–rock interactions (Ile de Groix, France)

Whole rock trace element and isotopic compositions of different HP–LT metamorphic rocks of the Ile de Groix were analysed to characterise geochemical fingerprints during subduction and exhumation in a late Palaeozoic HP metamorphic terrain. Massive metabasites of hydrothermally altered enriched mid-ocean ridge basalt (E-MORB) origin are in association with banded metabasic rocks of volcano-sedimentary origin and metapelites. Fluid-rock interactions that likely occurred during seafloor hydrothermal alteration and early subduction metasomatism increased δ18O values, as well as K2O, Na2O, MgO, and LILE contents and decreased CaO contents of metabasites. Most metabasites have retained their early-subduction and pre-HP trace element and isotopic composition, even for rocks metamorphosed to lower eclogite-facies P–T conditions. Micaschists also preserved apparent pelitic protolith trace element values and oxygen isotopic compositions. During retrograde metamorphism related to the exhumation, metabasites were rehydrated by fluids in equilibrium with the host rock compositions, which were likely derived from the basic rocks. This style of fluid–rock interaction formed a greenschist facies mineral assemblage. Metabasites that underwent pervasive alteration by seafloor hydrothermal and metasomatism processes prior to peak metamorphism, show greater effects of retrogression and albitisation, probably because they were richer in H2O and Na2O. The variety of metamorphic assemblages on the Ile de Groix is thus directly related to the pre-HP rock composition. The extent of retrogression in the western part of the Ile de Groix primarily reflects stronger metasomatic intensities than in the eastern part.

Extreme element mobility during transformation of Neoarchean (ca. 2.7 Ga) pillow basalts to a Paleoproterozoic (ca. 1.9 Ga) paleosol, Schreiber Beach, Ontario, Canada

This study reports petrographic, major and trace element, and oxygen isotopic data for a subaerial weathering profile at Schreiber Beach, Ontario, Canada. The weathering profile developed on Neoarchean (ca. 2.7Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (ca. 1.88Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation. Rocks at Schreiber Beach have been classified as pillow cores, pillow rims, hyaloclastites, weathered red basalts, and weathered brown to green basalts. There are gradual textural, mineralogical, and geochemical transitions from unweathered basalts to intensely weathered hematite-bearing basalts with stratigraphic height.Rocks at Schreiber Beach underwent seafloor hydrothermal alteration and greenschist facies regional metamorphism in the Neoarchean. Following the subaerial weathering, they were buried and re-metamorphosed to greenschist facies in the Paleoproterozoic. Given the preservation of primary igneous textures and trace element patterns in pillow cores, they are used as a protolith to estimate element mobility during seafloor alteration, metamorphism, and subaerial weathering. Chloritized pillow rims and hyaloclastites are characterized by moderate losses or gains of major (Si, Ca, Na, Fe, Ti, K) and trace (LILE, REE, HFSE, and transition metals) elements during Neoarchean seafloor alteration. Alteration of these rocks at relatively low temperatures is indicated by enrichment in 18O (δ18O=+10.4 to +12.7‰) relative to fresh basalt (~+5.5‰).Subaerial weathering resulted in significant textural, mineralogical (e.g., hematite precipitation), and geochemical variations in metamorphosed basalts, causing additional redistribution of most major and trace elements and further enrichment in 18O. The highest δ18O values (+16.1 to +17.1‰) occur at the silica-rich base of the weathering profile. Many elements, including LILE, REE and HFSE, are strongly enriched (>100%) or depleted (up to 96%) relative to their original abundances. Alkaline metasomatism, which is a characteristic of Precambrian paleosols, is also evident at Schreiber Beach, where extreme enrichment in K (280 to 1000%), Rb (390 to 660%), and Cs (1160 to 2560%) occurs in the weathering profile.