Ocean Floor Basalts Research Articles

Geochemical variations in Japan Sea back‐arc basin basalts formed by high‐temperature adiabatic melting of mantle metasomatized by sediment subduction components

AbstractThe Yamato Basin in the Japan Sea is a back‐arc basin characterized by basaltic oceanic crust that is twice as thick as typical oceanic crust. Two types of ocean floor basalts, formed during the opening of the Japan Sea in the Middle Miocene, were recovered from the Yamato Basin during Ocean Drilling Program Legs 127/128. These can be considered as depleted (D‐type) and enriched (E‐type) basalts based on their incompatible trace element and Sr‐Nd‐Pb‐Hf isotopic compositions. Both types of basalts plot along a common mixing array drawn between depleted mantle and slab sediment represented by a sand‐rich turbidite on the Pacific Plate in the NE Japan fore arc. The depleted nature of the D‐type basalts suggests that the slab sediment component is nil to minor relative to the dominant mantle component, whereas the enrichment of all incompatible elements in the E‐type basalts was likely caused by a large contribution of bulk slab sediment in the source. The results of forward model calculations using adiabatic melting of a hydrous mantle with sediment flux indicate that the melting conditions of the source mantle for the D‐type basalts are deeper and hotter than those for the E‐type basalts, which appear to have formed under conditions hotter than those of normal mid‐oceanic ridge basalts (MORB). These results suggest that the thicker oceanic crust was formed by greater degrees of melting of a hydrous metasomatized mantle source at unusually high mantle potential temperature during the opening of the Japan Sea.

Alteration of ocean floor basalts in the Archean: Silicification and hydrothermal metamorphism

Alteration of ocean floor basalts in the Archean: Silicification and hydrothermal metamorphism

Seafloor basalt alteration and chemical change in the ultra thinly sedimented South Pacific

AbstractDetermining the relationship between ocean floor basalt alteration and sedimentation is fundamental to understanding how oceanic crust evolves with time. Ocean floor basalts recovered at IODP Sites U1365 (∼100 Ma) and U1368 (∼13.5 Ma) in the South Pacific have been subjected to remarkably low sedimentation rates (0.71 and 1.1 m/Myr−1, respectively). We report detailed petrographic and geochemical analysis of basalt cores from these sites in order to investigate what impact sediment insulation has on seafloor alteration beyond 10–15 Myr of ocean crust formation. Both sites exhibit low‐temperature (<150°C) alteration (e.g., iron‐hydroxides, carbonate, and quartz) within a predominantly oxidative regime, albeit with markedly different alteration styles and intensity. Alteration at Site U1365, which is predominantly composed of sheet flows, occurs mainly near sheet flow boundaries and fractures. In contrast, Site U1368 comprises interlayered pillows and thin sheet flows that have been subjected to relatively even levels of alteration. Variation of alteration style and intensity between Sites U1365 and U1368 appear closely tied to lithology and crustal structure. Although alteration‐induced elemental changes at both sites are similar in, e.g., increasing K, Rb, U, Ba, and Fe3+ and decreasing Fe2+, Ca, and Ni, they show distinct differences in Th, which is significantly decreased at Site U1365 but relatively constant at Site U1368. At both sites enrichment of LREEs relative to HREEs is ascribed to alteration. The greater vein abundance and notably higher Fe3+/TiO2, K2O/TiO2, LOI/TiO2, and Rb/TiO2 ratios of representative samples at Site U1365 compared to Site U1368 are attributed to increased alteration intensity. This is mirrored by greater overall chemical change (Fe2O3, FeO, CaO, K2O, Li, Rb, Pb, and U) observed at Site U1365 than those of Site U1368 and other DSDP/ODP sites between 6 and 46 Ma. Since both Sites U1365 and U1368 endured only minimal sedimentation, we attribute the differences in overall chemical change across the two sites to duration of exposure to seawater.

REE Geochemistry and Resource between Co-Rich Crust and Basement in the Pacific Ocean: Based on Magellan Seamount Sample Research

Using Magellan Seamount samples in the Pacific area to carry out the REE chemical analysis and microscopic descript,this paper analyze the content of REE between the basement rock and co-rich crust, distribution characteristics, distribution pattern.The results show that: the three layers samples contain high value, total quantity is about 1000×10-6, compared with normal deep sea sediments, foraminiferal nuclear shell and sea water content of rare earth elements10 ~ 100 times; Basement and co-rich crust have shown consistent in distrubution mode, REE content is perfectly high, co-rich crust of REE is mainly controlled by overlying seawater on the rock points erosion. volcanic eruption, ocean floor basalt weathering and biological chemical deposition on the formation of the crust also has certain influence. REE as a huge resource is superior to exploit, The comprehensive development and utilization of mineral resources provided good premise, it is possible to createREE resource utilization of the new way.

Rehabilitation Idea Begins at Sumers

The Demirci migmatitic quartzofeldspathic gneisses of the Sunnice Massif, exposed in the ‹stanbul Zone north of Bolu, have compositions that show them to be paragneisses, associated with subordinate para- amphibolites. They consist principally of metamorphosed, rather calcareous greywackes, with sediment derived from a continental arc source. They occur in the para-autochthon below a nappe pile (Cele meta-ophiolite), deformed and subjected to amphibolite facies metamorphism before the emplacement of late Neoproterozoic metavolcanic rocks later subjected to greenschist-facies metamorphism (Yellice volcanics). As they are overlain in sequence by nappes comprising sections of the Cele meta-ophiolite with ocean floor basalt and island arc tholeiite compositions respectively, the Demirci paragneisses may represent a section of the ancient continental basement on to which the meta-ophiolite was obducted during the Proterozoic.

The global pattern of trace-element distributions in ocean floor basalts

The magmatic layers of the oceanic crust are created at constructive plate margins by partial melting of the mantle as it wells up. The chemistry of ocean floor basalts, the most accessible product of this magmatism, is studied for the insights it yields into the compositional heterogeneity of the mantle and its thermal structure. However, before eruption, parental magma compositions are modified at crustal pressures by a process that has usually been assumed to be fractional crystallization. Here we show that the global distributions of trace elements in ocean floor basalts describe a systematic pattern that cannot be explained by simple fractional crystallization alone, but is due to cycling of magma through the global ensemble of magma chambers. Variability in both major and incompatible trace-element contents about the average global pattern is due to fluctuations in the magma fluxes into and out of the chambers, and their depth, as well as to differences in the composition of the parental magmas.

Magma chambers on a slow burner

An assessment of crystallization processes occurring in magma chambers in the ocean floor finds an unexpected enrichment in trace elements, reviving an old theory of the cycling of magma in these chambers. See Article p.698 The magmas that produce the ocean-floor basalts have conventionally been considered to have evolved by simple fractional crystallization. But here, Hugh O'Neill and Frances Jenner show that the global distributions of trace elements in ocean-floor basalts describe a systematic pattern that cannot be explained by simple fractional crystallization alone. Rather, trace elements are redistributed by the cycling of magma through the global ensemble of magma chambers at various depths, and are also influenced by the composition of the parental magmas.

Geochemistry and trace element behaviors of eclogite during its exhumation in the Xitieshan terrane, North Qaidam UHP belt, NW China

The Xitieshan terrane, located in the central part of the North Qaidam ultrahigh pressure (UHP) metamorphic belt, China, is mainly composed of orthogneiss and paragneiss and a few intercalated eclogite layers and boudins. Based on their bulk-rock TiO2-contents, the eclogites can be subdivided into a high-Ti group (TiO2>2%) and a low-Ti group (TiO2<2%). Whole-rock major and trace element analyses revealed that the protoliths of the low-Ti eclogites are normal-type mid-ocean ridge basalts (N-MORB), whereas those of the high-Ti eclogites are either enriched-type mid-ocean ridge basalts (E-MORB) or near ridge seamount basalts, respectively. The Sr–Nd isotopes of eclogites of both groups are similar to those of MORB. Those of the low-Ti eclogites are characterized by positive εNd(T) and restricted ISr values and therefore provide further evidence for the formation of the protoliths of the eclogites in an oceanic environment. On the other hand, the Sr–Nd isotopes of high-Ti eclogites show mainly positive but also some negative εNd(T) values and relatively broadly distributed ISr values, indicating minor crustal contamination of the ocean floor basalts. Considering available 750–877Ma protolith ages preserved in zircon cores, it is inferred that some of the eclogites derived from Neoproterozoic protoliths were emplaced onto the crust far ahead of the Paleozoic deep subduction, while the other eclogites originate from a different oceanic crust, e.g., the Paleo-Qilian ocean, indicating multiple orogenies in the geological history of the Xitieshan terrane, China.Whole-rock and in-situ LA-ICP-MS mineral trace element analyses of eclogites revealed two stages of fluid behavior during retrogression that correspond to the two exhumation stages uncovered by phase equilibrium calculations. The mineral scale trace element distributions and trace element inheritance of newly formed amphibole from its precursors indicate that, at the peak metamorphic stage (M1) and at the earlier (eclogite facies) overprint (M2), the fluid was internally controlled by the rock itself. Within a mafic lens, the amount of water-soluble elements (e.g., Rb, Sr, Ba, U, Pb and LREE), observed in the whole-rock compositions as well as in amphiboles, increases from the core (phengite-bearing eclogite) to the rim (amphibolite) and implies an external fluid source for the amphibolite facies retrogression (M3) which should be helpful for the final exhumation of UHP eclogite.

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.

Deciphering the upper Famennian Hangenberg Black Shale depositional environments based on multi-proxy record

Presented for the first time in this paper are the results of a detailed multi-proxy investigation conducted on the Hangenberg Black Shale (HBS) from the Polish part of the Laurussian Shelf, which provide details about the environmental conditions in existence during deposition of the shale and the role played by anoxic conditions in the mass extinction events that occurred at the end of the Devonian times. Inorganic and organic redox indicators indicate that bottom water redox conditions changed periodically from being mainly anoxic/euxinic to oxic or being partially depleted in oxygen. U/Th values above 1.25, Ni/Co values above 7 and V/(V+Ni) values above 0.8 recorded from the lower part of the HBS all point to anoxic/euxinic conditions being present, as do high total organic carbon contents (TOC) above 10% and degree of pyritisation (DOP) values around 0.75%. However, the presence of benthic fauna over the lower part of the HBS attests to opportunistic colonisation of the seafloor during oxic episodes. Evidence for similar episodes has also been recognised over the middle part of the HBS, e.g., U/Th values below 1.25, Ni/Co values below 4, V/(V+Ni) values below 0.8, TOC values of 3% to 5.5% and DOP values of 0.4% to 0.75%, plus the common occurrence of benthic fauna. Anoxic conditions returned during the deposition of the upper part of the HSB, though they were not as well developed as when the lower part of the shale was laid down.Of interest is the presence of small pyrite framboids and isorenieratene biomarkers in all the analysed samples, which suggest that euxinic conditions persisted in the photic zone of the water column. A twenty centimetre thick layer of volcanogenic deposits (tuffites) is present in the middle part of the HBS, the geochemical characteristics of which are typical of ocean floor basalts. The occurrence of volcanogenic material below and within the HBS, together with an absence of calcium carbonate over its upper part, the presence of abundant tetrads just above the black shale and a drastic decrease in faunal frequency in the upper part of the HBS imply that volcanism may have caused oceanic acidification (or hypercapnia) that in turn potentially may have influenced the Hangenberg mass extinction event.

The effects of re-homogenization on plagioclase hosted melt inclusions

Melt inclusions trapped in phenocrysts provide a unique picture of magma systems prior to modification by crustal processes. However, post-entrapment crystallization complicates their interpretation. Re-heating the phenocryst to the temperature of entrapment is a commonly applied method to recover the original melt composition. To understand the effects of re-homogenization, we compared the composition of re-heated and naturally quenched melt inclusions and inclusion compositions that had been subjected to over-heating and under-heating to examine the degree to which anomalous compositions were produced. Our results on plagioclase hosted inclusions from ocean floor basalts indicate that the general patterns represented by naturally quenched inclusions are the same as observed for rehomogenized inclusions. Most important, the range of minor elements described for plagioclase hosted inclusions from basalts is found in naturally quenched inclusions, and is therefore not a consequence of the re-homogenization process.

The Gramscatho Basin, south Cornwall, UK: Devonian active margin successions

Deep marine deposits of the Gramscatho Basin of south Cornwall reflect two tectonic regimes; Early to Middle Devonian rifting of continental lithosphere with formation of oceanic lithosphere to the south, and Middle Devonian to earliest Carboniferous convergence along its southern margin. Sediments on thinned continental crust to the north and oceanic lithosphere to the south were juxtaposed in the Late Devonian when nappes of deep water flysch and olistostrome were thrust up on to the northern continental margin of the basin. Basin closure was accommodated by forward propagating thrust nappes, accompanied by penecontemporaneous sedimentation. The stratigraphical sequences of major nappes illustrate the progradation of flysch with climactic sedimentation of olistostrome in late Mid- to Late Devonian times. The Lizard Complex, including the Lizard ophiolite, within that nappe stack, constitutes part of one of the GCR sites which are largely in the allochthonous rocks. Many of those sites feature the olistostrome, Roseland Breccia Formation, with its great variety of sedimentary, igneous and metamorphic clasts (up to 1.5 km), and the association of ocean floor basalt and penecontemporaneous acidic volcanics indicative of the coming together of oceanic and continental plates. A site at the top of the parautochthonous continental margin succession displays the erosion products of the youngest nappe as it emerged and advanced across the sediment surface, marking closure of the oceanised Gramscatho Basin and continental collision.

An estimate of 1.9 Ga mantle depletion using the high-field-strength elements and Nd–Pb isotopes of ocean floor basalts, Flin Flon Belt, Canada

The 1.9 Ga Flin Flon Belt is one of the best preserved Paleoproterozoic mafic volcanic provinces, hosting chemically and geologically distinguishable ocean floor and arc basalts. The ocean floor basalts, previously interpreted to have predominantly erupted in a back-arc basin, apparently tapped N-MORB-, E-MORB-, and OIB-like mantle sources. These ocean floor basalts are revisited in this study with a new major element, high-precision trace element, and isotopic (Nd and Pb) data set (∼60 samples). The main impetus behind the study was to combine trace element, particularly Nb/Th, and isotopic compositions to better approximate the contemporaneous depleted mantle, so as to establish more precisely the degree of depletion in the Paleoproterozoic. Through systematic examination of the basalt chemistry, samples influenced by subduction zone processes (low Nb/Th and Ta/W) were isolated first. The Ta/W ratio of the ocean floor basalts is higher than the primitive mantle value and provides strong evidence that the depletion imprint due to the preferential extraction of W in subduction zones was well established by the Paleoproterozoic. Next, a group of unexpectedly high Nb/Ta and high Nb/Th basalts was identified, whose chemical trends (e.g., anti-correlated Nb/Th and LOI) are best explained by post-depositional metamorphic dehydration, resulting in mobility of Th > Ta > Nb. The chemistries of the remaining, least subduction and dehydration modified, samples provide evidence of mixing between long-term (ca. 450 Ma), isotopically distinct mantle reservoirs (Δ + 3 ɛ Nd units) with relatively consistent Nb/Th ratios of 12.5–13.5. The average Nb/Th ratio of 13.0 ± 0.9 for these ocean floor basalts is more precise, and lower, than previous estimates for the 1.9 Ga depleted mantle (∼14.4), which included samples that had suffered preferential Th > Nb loss. The magnitude of mantle depletion required by the new Nb/Th ratio of 13 still justifies significant net crustal growth from the Neoarchean (Nb/Th c. 11.1) to the Paleoproterozoic. When the Archean and Paleoproterozoic Th–Nb evidence is combined with insight from Pb and Nd isotopes, it appears that the mass of the depleted portion of the mantle increased between 2.6 and 2.0 Ga. Namely, the data are best explained with a model in which less depleted (lower?) mantle material was mixed into the quite strongly depleted post-Archean upper asthenosphere. This could possibly indicate the onset and establishment of whole mantle convection.

Structure and stratigraphy of the Teplá–Barrandian Neoproterozoic, Bohemian Massif: A new plate-tectonic reinterpretation

The Teplá–Barrandian unit (TBU) of the Bohemian Massif exposes a section across the once extensive Avalonian–Cadomian belt, which bordered the northern active margin of Gondwana during late Neoproterozoic. This paper synthesizes the state-of-the-art knowledge on the Cadomian basement of the TBU to redefine its principal component units, to revise an outdated stratigraphic scheme, and to interpret this scheme in terms of a recent plate-tectonic model for the Cadomian orogeny in the Bohemian Massif. The main emphasis of this paper is on an area between two newly defined fronts of the Variscan pervasive deformation to the NW and SE of the Barrandian Lower Paleozoic overlap successions. This area has escaped the pervasive Variscan (late Devonian to early Carboniferous) ductile reworking and a section through the Cadomian orogen is here superbly preserved. The NW segment of the TBU consists of three juxtaposed allochthonous belts of unknown stratigraphic relation (the Kralovice–Rakovník, Radnice–Kralupy, and Zbiroh–Šárka belts), differing in lithology, complex internal strain patterns, and containing sedimentary and tectonic mélanges with blocks of diverse ocean floor (meta-)basalts. We summarize these three belts under a new term the Blovice complex, which we believe represents a part of an accretionary wedge of the Cadomian orogen. The SE segment of the TBU exposes the narrow Pičín belt, which is probably a continuation of the Blovice complex from beneath the Barrandian Lower Paleozoic, and a volcanic arc sequence (the Davle Group). Their stratigraphic relation is unknown. Flysch units (the Štěchovice Group and Svrchnice Formation) overlay the arc volcanics, and both units contain material derived from volcanic arc. The former was also sourced from the NW segment, whereas the latter contains an increased amount of passive margin continental material. In contrast to the Blovice complex, the flysch experienced only weak Cadomian deformation. The new lithotectonic zonation fits the following tectonic scenario for the Cadomian evolution of the TBU well. The S- to SE-directed Cadomian subduction beneath the TBU led to the involvement of turbidites, chaotic deposits, and 605 ± 39 Ma ocean floor in the accretionary wedge represented by the Blovice complex. The accretionary wedge formation mostly overlapped temporally with the growth of the volcanic arc (the Davle Group) at ~ 620–560 Ma. Upon cessation of the arc igneous activity, the rear of the wedge and some elevated portions of the arc were eroded to supply the deep-water flysch sequences of the Štěchovice Group, whereas the comparable Svrchnice Formation (~ 560 to < 544 Ma) was deposited in a southeasterly remnant basin close to the continental margin. The Cadomian orogeny in the TBU was terminated at ~ 550–540 Ma by slab breakoff, by final attachment of the most outboard ~ 540 Ma oceanic crust, and by intrusion of ~ 544–524 Ma boninite dikes marking the transition from the destructive to transform margin during the early/middle Cambrian.

Geochemistry of garnetiferous Ti–clinohumite rock and talc–kyanite–phengite–almandine schist from the Kokchetav UHP terrane, Kazakhstan: An insight to possible origins of some chemically unusual UHP rocks

Major-element, trace-element and O-isotope analyses were carried out for garnetiferous Ti–clinohumite rocks and eclogites from the Kumdy Kol area, as well as talc–kyanite–phengite–almandine schists (previously named as whiteschists) and eclogites from the Kulet area of the Kokchetav ultrahigh-pressure (UHP) terrane, to explore their possible origins. Both garnetiferous Ti–clinohumite rocks and talc–kyanite–phengite–almandine schists are possibly genetically related to eclogites, as they occur in close association with eclogites enclosed within gneiss/metapelite in these two areas. Most eclogites from the Kumdy Kol and the Kulet areas exhibit chemical characteristics similar to that of ocean floor basalt, although a few show arc affinity with slight Ta and Nb depletions. Garnetiferous Ti–clinohumite rocks from the Kumdy Kol region are characterized by very high Mg, low Si, Ca, Na, and K contents compared with eclogites. They also exhibit LREE depleted patterns with negative Eu anomalies. Their O-isotope compositions are less than + 5.6‰. On the other hand, the rocks show comparable Al, Ni, Cr, Co and HREE contents as eclogites. The Mg enrichment of these garnetiferous Ti–clinohumite rocks is suggested to have originated from their mafic protoliths that had been subjected to mafic rock–ultramafic rock–fluid interactions. Talc–kyanite–phengite–almandine schists from the Kulet area are typically low in Ca and Na, but high in Mg, K, Rb, Ba, Cs, Th, U and O-isotope compositions (+ 6 to + 10‰) relative to eclogites. This peculiar chemical composition can be accounted for if the basaltic protoliths had been subjected to sequential high-temperature (enrichment of Mg) and low-temperature (enrichment of K, Rb, Ba, Cs, Th, U and O-isotope compositions) seawater–basalt interactions. One talc–kyanite–phengite–almandine schist sample KLW 7 is distinctly high in Si and K, but low in Mg and Fe, which can be attributed to more severe low-temperature hydrothermal alterations. However, except for Mg, the enrichment of Rb, Ba, Cs, Th, U and O-isotope compositions in these schist samples could also be enhanced by, or otherwise be interpreted as a result of, fluid influx sourced from enclosing metapelite during subduction.

GEOCHEMISTRY AND U-PB ZIRCON AGE OF THE INTERNAL OSSA-MORENA ZONE OPHIOLITE SEQUENCES: A REMNANT OF RHEIC OCEAN IN SW IBERIA

The Early Paleozoic geodynamic evolution in SW Iberia is believed to have been dominated by the opening of the Rheic Ocean. The Rheic Ocean is generally accepted to have resulted from the drift of peri-Gondwanan terranes such as Avalonia from the northern margin of Gondwana during Late CambrianEarly Ordovician times. The closure of the Rheic Ocean was the final result of a continent-continent collision between Gondwana and Laurussia that produced the Variscan orogen. The Ossa-Morena Zone is a peri-Gondwana terrane, which preserves spread fragments of ophiolites - the Internal Ossa-Morena Zones Ophiolite Sequences (IOMZOS). The final patchwork of the IOMZOS shows a complete oceanic lithospheric sequence with geochemical characteristics similar to the ocean-floor basalts, without any orogenic fingerprint and/or crustal contamination. The IOMZOS were obducted and imbricated with high pressure lithologies. Based on structural, petrological and whole-rock geochemical data, the authors argue that the IOMZOS represent fragments of the oceanic lithosphere from the Rheic Ocean. Zircon SHRIMP U-Pb geochronological data on metagabbros point to an age of ca. 480 Ma for IOMZOS, providing evidence of a well-developed ocean in SW Iberia during this period, reinforcing the interpretation of the Rheic Ocean as a wide ocean among the peri-Gondwanan terranes during Early Ordovician times.

Geochemical characterization of oceanic basalts using Artificial Neural Network

The geochemical discriminate diagrams help to distinguish the volcanics recovered from different tectonic settings but these diagrams tend to group the ocean floor basalts (OFB) under one class i.e., as mid-oceanic ridge basalts (MORB). Hence, a method is specifically needed to identify the OFB as normal (N-MORB), enriched (E-MORB) and ocean island basalts (OIB).We have applied Artificial Neural Network (ANN) technique as a supervised Learning Vector Quantisation (LVQ) to identify the inherent geochemical signatures present in the Central Indian Ocean Basin (CIOB) basalts. A range of N-MORB, E-MORB and OIB dataset was used for training and testing of the network. Although the identification of the characters as N-MORB, E-MORB and OIB is completely dependent upon the training data set for the LVQ, but to a significant extent this method is found to be successful in identifying the characters within the CIOB basalts. The study helped to geochemically delineate the CIOB basalts as N-MORB with perceptible imprints of E-MORB and OIB characteristics in the form of moderately enriched rare earth and incompatible elements. Apart from the fact that the magmatic processes are difficult to be deciphered, the architecture performs satisfactorily.

Oceanic spreading center–generated basaltic crust and associated sulfidic and carbonate-rich hydrothermal deposits in the Archean (ca. 3 Ga), North Spirit Lake greenstone belt, Ontario, Canada

Detailed mapping of a portion of the North Spirit Lake greenstone belt reveals a basal basaltic volcanic unit (ca. 3 Ga) with a komatiitic component and strong major and trace element geochemical affinity with typical mid-ocean ridge basalts (e.g., N-MORB). The overlying transitional unit includes basaltic tuffs, which have close geochemical affinity to the underlying basalts, and to N-MORB. Interbedded with and overlying the tuffs is a thin, laterally continuous sulfide-rich unit with chert interbeds, which is in turn overlain by interbedded carbonate, chert, and iron-rich sedimentary rocks. The chemical sedimentary rocks of the transitional unit display rare-earth element (REE) patterns and positive Eu anomalies consistent with derivation from fluids associated with submarine hydrothermal vent fields. The sulfide unit is interpreted to represent proximal hydrothermal vent deposits derived from black smoker activity near a spreading center. The overlying carbonate and chert mounds and beds are interpreted as hydrothermal deposits formed at lower temperatures in a more distal setting relative to the spreading center. The carbonate deposits may have formed in an environment analogous to that of the Lost City hydrothermal field (Kelley et al., 2001), where abundant chimneys, dendrites, and beds of carbonate are produced ∼15 km from the mid-Atlantic rift axis. The basal volcanic unit consequently is interpreted as ocean-floor basalts formed at an Archean spreading center, with the basaltic tuffs representing the last volcanic emanations, and with the chemical sediments representing hydrothermal-vents deposits both proximal and distal to the spreading center. The capping siliciclastic unit likely is part of an accretionary wedge, possibly representing ocean closure after a protracted seafloor spreading episode. The sedimentary rocks were derived, in part, from a weathered tonalite-trondhjemite-granodiorite source. The supracrustal rocks of the North Spirit Lake greenstone belt provide an unusually clear window into some tectonic activity on the primitive Earth, including evidence for the existence of ocean-floor basalts, proximal and distal hydrothermal activity, and possibly a protracted cycle of ocean opening and destruction.

Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle

The origin of komatiites, volcanic rocks formed by unusually extensive melting of mantle rocks, mainly during the Archaean, is the subject of much debate. They are thought to have been produced either by anhydrous melting of anomalously hot mantle, or by hydrous melting at temperatures only a little higher than those of today. Berry et al. have now determined the oxidation state of iron in pristine samples of a 2.7 billion-year-old komatiitic magma from Belingwe, Zimbabwe. Their findings are consistent with near-anhydrous melting of a source with an oxidation state similar to that of present-day ocean-floor basalt. The results suggest that the Belingwe melt was a product of high mantle temperatures of about 1,700 °C, rather than melting under hydrous conditions, confirming the existence of anomalously hot mantle in the Archaean. The original iron isotope ratios of a 2.7 billion-year-old komatiitic magma from Belingwe, Zimbabwe have been determined. These measurements are consistent with near-anhydrous melting of a source with similar oxidation state to that of present-day ocean-floor basalt. The results support the identification of the Belingwe komatiite as a product of high mantle temperatures of ∼ 1,700 °C, rather than melting under hydrous conditions, confirming the existence of anomalously hot mantle in the Archean. Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle1,2,3 or hydrous melting at temperatures only modestly greater than those found today4,5. Here we determine the original Fe3+/ΣFe ratio of 2.7-Gyr-old komatiitic magma from Belingwe, Zimbabwe6, preserved as melt inclusions in olivine, to be 0.10 ± 0.02, using iron K-edge X-ray absorption near-edge structure spectroscopy. This value is consistent with near-anhydrous melting of a source with a similar oxidation state to the source of present-day mid-ocean-ridge basalt. Furthermore, this low Fe3+/ΣFe value, together with a water content of only 0.2–0.3 wt% (ref. 7), excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H2 and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron (that is, the Fe3+/ΣFe ratio would be ∼1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures (∼1,700 °C), rather than melting under hydrous conditions (3–5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.

Deducing a reducing mantle

Increasingly sophisticated techniques are being used to persuade ancient rocks to yield information about conditions on and in the early Earth — for instance, about the oxidation state of the mantle. The origin of komatiites, volcanic rocks formed by unusually extensive melting of mantle rocks, mainly during the Archaean, is the subject of much debate. They are thought to have been produced either by anhydrous melting of anomalously hot mantle, or by hydrous melting at temperatures only a little higher than those of today. Berry et al. have now determined the oxidation state of iron in pristine samples of a 2.7 billion-year-old komatiitic magma from Belingwe, Zimbabwe. Their findings are consistent with near-anhydrous melting of a source with an oxidation state similar to that of present-day ocean-floor basalt. The results suggest that the Belingwe melt was a product of high mantle temperatures of about 1,700 °C, rather than melting under hydrous conditions, confirming the existence of anomalously hot mantle in the Archaean.