Showing papers on "Basalt published in 2017"

Earth's first stable continents did not form by subduction

Abstract: Phase equilibria modelling of rocks from Western Australia confirms that the ancient continental crust could have formed by multistage melting of basaltic ‘parents’ along high geothermal gradients—a process incompatible with modern-style subduction. Tim Johnson et al. perform phase equilibria modelling of the Coucal basalts from Western Australia and confirm their suitability as parent rocks of the Archaean continental crust. The authors suggest that these early crustal rocks were produced by 20–30 per cent melting along high geothermal gradients. They conclude that the production and stabilization of the first continents required a protracted, multistage process. When coupled with the high geothermal gradients, this suggests that the continents did not form by subduction. Instead it favours a 'stagnant lid' regime in the early Archaean eon in which a single, rigid plate lay over the mantle. The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 3.5 billion years old) basaltic crust4,5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6,7,8. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9. Thus subduction was not required to produce TTGs in the early Archaean eon.

Earth's first stable continents did not form by subduction

Abstract: Phase equilibria modelling of rocks from Western Australia confirms that the ancient continental crust could have formed by multistage melting of basaltic ‘parents’ along high geothermal gradients—a process incompatible with modern-style subduction Tim Johnson et al perform phase equilibria modelling of the Coucal basalts from Western Australia and confirm their suitability as parent rocks of the Archaean continental crust The authors suggest that these early crustal rocks were produced by 20–30 per cent melting along high geothermal gradients They conclude that the production and stabilization of the first continents required a protracted, multistage process When coupled with the high geothermal gradients, this suggests that the continents did not form by subduction Instead it favours a 'stagnant lid' regime in the early Archaean eon in which a single, rigid plate lay over the mantle The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1 Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 25 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3 In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 35 billion years old) basaltic crust4,5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6,7,8 Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal) We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9 Thus subduction was not required to produce TTGs in the early Archaean eon

Deep carbon cycles constrained by a large-scale mantle Mg isotope anomaly in eastern China

Abstract: Although deep carbon recycling plays an important role in the atmospheric CO2 budget and climate changes through geological time, the precise mechanisms remain poorly understood. Since recycled sedimentary carbonate through plate subduction is the main light-δ26Mg reservoir within deep-Earth, Mg isotope variation in mantle-derived melts provides a novel perspective when investigating deep carbon cycling. Here, we show that the Late Cretaceous and Cenozoic continental basalts from 13 regions covering the whole of eastern China have low δ26Mg isotopic compositions, while the Early Cretaceous basalts from the same area and the island arc basalts from circum-Pacific subduction zones have mantle-like or heavy Mg isotopic characteristics. Thus, a large-scale mantle low δ26Mg anomaly in eastern China has been delineated, suggesting the contribution of sedimentary carbonates recycled into the upper mantle, but limited into the lower mantle. This large-scale spatial and temporal variation of Mg isotopes in the mantle places severe constraints on deep carbon recycling via oceanic subduction.

Tungsten-182 heterogeneity in modern ocean island basalts

Abstract: New tungsten isotope data for modern ocean island basalts (OIB) from Hawaii, Samoa, and Iceland reveal variable 182 W/ 184 W, ranging from that of the ambient upper mantle to ratios as much as 18 parts per million lower. The tungsten isotopic data negatively correlate with 3 He/ 4 He. These data indicate that each OIB system accesses domains within Earth that formed within the first 60 million years of solar system history. Combined isotopic and chemical characteristics projected for these ancient domains indicate that they contain metal and are repositories of noble gases. We suggest that the most likely source candidates are mega–ultralow-velocity zones, which lie beneath Hawaii, Samoa, and Iceland but not beneath hot spots whose OIB yield normal 182 W and homogeneously low 3 He/ 4 He.

Subduction initiation and ophiolite crust: new insights from IODP drilling

Abstract: International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

The global systematics of primitive arc melts

Abstract: We extracted all volcanic arc rock analyses calculated to be in equilibrium with mantle olivine from the global georoc database. This results in 938 primitive melt compositions from 30 arcs. Based on geochemical criteria six principal types of primitive arc melts can be distinguished: calc-alkaline basalts and andesites, tholeiitic basalts, highly depleted tholeiitic andesites, shoshonites and low-Si basalts. Their major element systematics indicates that last mantle equilibration occurred mostly at 1.0-2.5 GPa, 1220-1350°C for tholeiitic and calc-alkaline basalts, at 0.5-1.2 GPa and ∼1200°C for depleted tholeiitic andesites, and at 0.7-1.2 GPa, 1050-1150°C for calc-alkaline andesites. Quantitative treatment of major and trace elements suggests that the different melt types can be explained by a combination of variable mantle wedge preconditioning (degree of depletion prior to slab component addition, metasomatism in the lithosphere), variation in the amount and nature of the slab component added, and - for primitive calc-alkaline andesites - reactive fractionation in the lithospheric top of the mantle wedge. The different slab components are best characterized by high Na2O, TiO2, Zr and Th for slab melts; high K2O/Na2O and more pronounced Nb, Sr, and Pb anomalies for fluids; and high K2O at high K2O/Na2O for supercritical liquids. A slab component that is dominantly a slab melt is common in continental but rare in intra-oceanic arcs, consistent with comparatively cooler slabs in intra-oceanic subduction zones. A majority of the arcs has more than one melt type, testifying for heterogeneity in the mantle wedge and added slab component.

Remobilization of crustal carbon may dominate volcanic arc emissions.

Abstract: The flux of carbon into and out of Earth's surface environment has implications for Earth's climate and habitability. We compiled a global data set for carbon and helium isotopes from volcanic arcs and demonstrated that the carbon isotope composition of mean global volcanic gas is considerably heavier, at -3.8 to -4.6 per mil (‰), than the canonical mid-ocean ridge basalt value of -6.0‰. The largest volcanic emitters outgas carbon with higher δ13C and are located in mature continental arcs that have accreted carbonate platforms, indicating that reworking of crustal limestone is an important source of volcanic carbon. The fractional burial of organic carbon is lower than traditionally determined from a global carbon isotope mass balance and may have varied over geological time, modulated by supercontinent formation and breakup.

The roles of pyroxenite and peridotite in the mantle sources of oceanic basalts

Abstract: Subduction of oceanic crust generates chemical and lithological heterogeneities in the mantle. An outstanding question is the extent to which these heterogeneities contribute to subsequent magmas generated by mantle melting, but the answer differs depending on the geochemical behaviour of the elements under investigation: analyses of incompatible elements (those that preferentially concentrate into silicate melts) suggest that recycled oceanic crust is an important contributor, whereas analyses of compatible elements (those that concentrate in crystalline residues) generally suggest it is not. Recently, however, the concentrations of Mn and Ni—two elements of varying compatibility—in early-crystallizing olivines, have been used to infer that erupted magmas are mixtures of partial melts of olivine-rich mantle rocks (that is, peridotite) and of metasomatic pyroxene-rich mantle rocks (that is, pyroxenite) formed by interaction between partial melts of recycled oceanic crust and peridotite. Here, we test whether melting of peridotite alone can explain the observed trend in olivine compositions by combining new experimental data on the partitioning of Mn between olivine and silicate melt under conditions relevant to basalt petrogenesis with earlier results on Ni partitioning. We show that the observed olivine compositions are consistent with melts of fertile peridotite at various pressures—importantly, melts from metasomatic pyroxenites are not required. Thus, although recycled materials may well be present in the mantle source regions of some basalts, the Mn and Ni data can be explained without such a contribution. Furthermore, the success of modelling the Mn–Ni contents of olivine phenocrysts as low-pressure crystallization products of partial melts of peridotite over a range of pressures implies a simple new approach for constraining depths of mantle melting.

Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth’s oxygenation

Abstract: The behavior of C, H, and S in the solid Earth depends on their oxidation states, which are related to oxygen fugacity (fO_2). Volcanic degassing is a source of these elements to Earth’s surface; therefore, variations in mantle fO_2 may influence the fO2 at Earth’s surface. However, degassing can impact magmatic fO_2 before or during eruption, potentially obscuring relationships between the fO_2 of the solid Earth and of emitted gases and their impact on surface fO_2. We show that low-pressure degassing resulted in reduction of the fO_2 of Mauna Kea magmas by more than an order of magnitude. The least degassed magmas from Mauna Kea are more oxidized than midocean ridge basalt (MORB) magmas, suggesting that the upper mantle sources of Hawaiian magmas have higher fO_2 than MORB sources. One explanation for this difference is recycling of material from the oxidized surface to the deep mantle, which is then returned to the surface as a component of buoyant plumes. It has been proposed that a decreasing pressure of volcanic eruptions led to the oxygenation of the atmosphere. Extension of our findings via modeling of degassing trends suggests that a decrease in eruption pressure would not produce this effect. If degassing of basalts were responsible for the rise in oxygen, it requires that Archean magmas had at least two orders of magnitude lower fO_2 than modern magmas. Estimates of fO_2 of Archean magmas are not this low, arguing for alternative explanations for the oxygenation of the atmosphere.

Heterogeneity in mantle carbon content from CO 2 -undersaturated basalts.

Abstract: The amount of carbon present in Earth's mantle affects the dynamics of melting, volcanic eruption style and the evolution of Earth's atmosphere via planetary outgassing. Mantle carbon concentrations are difficult to quantify because most magmas are strongly degassed upon eruption. Here we report undegassed carbon concentrations from a new set of olivine-hosted melt inclusions from the Mid-Atlantic Ridge. We use the correlations of CO2 with trace elements to define an average carbon abundance for the upper mantle. Our results indicate that the upper mantle carbon content is highly heterogeneous, varying by almost two orders of magnitude globally, with the potential to produce large geographic variations in melt fraction below the volatile-free solidus. Such heterogeneity will manifest as variations in the depths at which melt becomes interconnected and detectable, the CO2 fluxes at mid-ocean ridges, the depth of the lithosphere-asthenosphere boundary, and mantle conductivity.

Primordial helium entrained by the hottest mantle plumes

Abstract: Helium isotopes provide an important tool for tracing early-Earth, primordial reservoirs that have survived in the planet's interior. Volcanic hotspot lavas, like those erupted at Hawaii and Iceland, can host rare, high 3He/4He isotopic ratios (up to 50 times the present atmospheric ratio, Ra) compared to the lower 3He/4He ratios identified in mid-ocean-ridge basalts that form by melting the upper mantle (about 8Ra; ref. 5). A long-standing hypothesis maintains that the high-3He/4He domain resides in the deep mantle, beneath the upper mantle sampled by mid-ocean-ridge basalts, and that buoyantly upwelling plumes from the deep mantle transport high-3He/4He material to the shallow mantle beneath plume-fed hotspots. One problem with this hypothesis is that, while some hotspots have 3He/4He values ranging from low to high, other hotspots exhibit only low 3He/4He ratios. Here we show that, among hotspots suggested to overlie mantle plumes, those with the highest maximum 3He/4He ratios have high hotspot buoyancy fluxes and overlie regions with seismic low-velocity anomalies in the upper mantle, unlike plume-fed hotspots with only low maximum 3He/4He ratios. We interpret the relationships between 3He/4He values, hotspot buoyancy flux, and upper-mantle shear wave velocity to mean that hot plumes-which exhibit seismic low-velocity anomalies at depths of 200 kilometres-are more buoyant and entrain both high-3He/4He and low-3He/4He material. In contrast, cooler, less buoyant plumes do not entrain this high-3He/4He material. This can be explained if the high-3He/4He domain is denser than low-3He/4He mantle components hosted in plumes, and if high-3He/4He material is entrained from the deep mantle only by the hottest, most buoyant plumes. Such a dense, deep-mantle high-3He/4He domain could remain isolated from the convecting mantle, which may help to explain the preservation of early Hadean (>4.5 billion years ago) geochemical anomalies in lavas sampling this reservoir.

Neoarchean granite-greenstone belts and related ore mineralization in the North China Craton: An overview

Abstract: Tectonic processes involving amalgamations of microblocks along zones of ocean closure represented by granite-greenstone belts (GGB) were fundamental in building the Earth's early continents. The crustal growth and cratonization of the North China Craton (NCC) are correlated to the amalgamation of microblocks welded by 2.75–2.6 Ga and ∼2.5 Ga GGBs. The lithological assemblages in the GGBs are broadly represented by volcano-sedimentary sequences, subduction-collision related granitoids and bimodal volcanic rocks (basalt and dacite) interlayered with minor komatiites and calc-alkalic volcanic rocks (basalt, andesite and felsic rock). The geochemical features of meta-basalts in the major GGBs of the NCC display affinity with N-MORB, E-MORB, OIB and calc-alkaline basalt, suggesting that the microblocks were separated by oceanic realm. The granitoid rocks display arc signature with enrichment of LILE (K, Rb, Sr, Ba) and LREE, and depletion of HFSE (Nb, Ta, Th, U, Ti) and HREE, and fall in the VAG field. The major mineralization includes Neoarchean BIF-type iron and VMS-type Cu-Zb deposits and these, together with the associated supracrustal rocks possibly formed in back-arc basins or arc-related oceanic slab subduction setting with or without input from mantle plumes. The 2.75–2.60 Ga TTG rocks, komatiites, meta-basalts and metasedimentary rocks in the Yanlingguan GGB are correlated to the upwelling mantle plume with eruption close to the continental margin within an ocean basin. The volcano-sedimentary rocks and granitoid rocks in the late Neoarchean GGBs display formation ages of 2.60–2.48 Ga, followed by metamorphism at 2.52–2.47 Ga, corresponding to a typical modern-style subduction-collision system operating at the dawn of Proterozoic. The late Neoarchean komatiite (Dongwufenzi GGB), sanukitoid (Dongwufenzi GGB and Western Shandong GGB), BIF (Zunhua GGB) and VMS deposit (Hongtoushan-Qingyuan-Helong GGB) have closer connection to a combined process of oceanic slab subduction and mantle plume. The Neoarchean cratonization of the NCC appears to have involved two stages of tectonic process along the 2.75–2.6 Ga GGB and ∼2.5 Ga GGBs, the former involve plume–arc interaction process, and the latter involving oceanic lithospheric subduction, with or without arc-plume interaction.

Gallium isotopic evidence for extensive volatile loss from the Moon during its formation.

Abstract: The distribution and isotopic composition of volatile elements in planetary materials holds a key to the characterization of the early solar system and the Moon's formation. The Moon and Earth are chemically and isotopically very similar. However, the Moon is highly depleted in volatile elements and the origin of this depletion is still debated. We present gallium isotopic and elemental measurements in a large set of lunar samples to constrain the origin of this volatile depletion. We show that while Ga has a geochemical behavior different from zinc, both elements show a systematic enrichment in the heavier isotopes in lunar mare basalts and Mg-suite rocks compared to the silicate Earth, pointing to a global-scale depletion event. On the other hand, the ferroan anorthosites are isotopically heterogeneous, suggesting a secondary distribution of Ga at the surface of the Moon by volatilization and condensation. The isotopic difference of Ga between Earth and the Moon and the isotopic heterogeneity of the crustal ferroan anorthosites suggest that the volatile depletion occurred following the giant impact and during the lunar magma ocean phase. These results point toward a Moon that has lost its volatile elements during a whole-scale evaporation event and that is now relatively dry compared to Earth.

Experimental constraints on the damp peridotite solidus and oceanic mantle potential temperature

Abstract: Decompression of hot mantle rock upwelling beneath oceanic spreading centers causes it to exceed the melting point (solidus), producing magmas that ascend to form basaltic crust ~6 to 7 kilometers thick. The oceanic upper mantle contains ~50 to 200 micrograms per gram of water (H2O) dissolved in nominally anhydrous minerals, which—relative to its low concentration—has a disproportionate effect on the solidus that has not been quantified experimentally. Here, we present results from an experimental determination of the peridotite solidus containing known amounts of dissolved hydrogen. Our data reveal that the H2O-undersaturated peridotite solidus is hotter than previously thought. Reconciling geophysical observations of the melting regime beneath the East Pacific Rise with our experimental results requires that existing estimates for the oceanic upper mantle potential temperature be adjusted upward by about 60°C.

Shallow melting of MORB‐like mantle under hot continental lithosphere, Central Anatolia

Abstract: Widespread mafic volcanism, elevated crustal temperatures, and plateau-type topography in Central Anatolia, Turkey, could collectively be the result of lithospheric delamination, mantle upwelling, and tectonic escape. We use results from 40Ar/39Ar geochronology, basalt geochemistry, and a passive-source broadband seismic experiment obtained in a collaborative international effort (Continental Dynamics-Central Anatolia Tectonics) to investigate the upper mantle structure and evolution of melting conditions over an ∼2400 km2 area south and west of Hasan volcano. New 40Ar/39Ar dates for the basalts mostly cluster between 0.2 and 0.6 Ma, but some scoria cones are as old as 2.5 Ma. Basalts are dominantly Mg-rich (Mg# = 62–71), moderately alkaline (normative Ne < 5 wt %), and, based on major and trace element signatures, derived from a peridotitic source. Covariations between radiogenic isotope and trace element signatures reveal contributions from a subduction-related component and intraplate-like mantle asthenosphere, as well as from ambient upper mantle. Central Anatolian basalts reflect maximum mantle potential temperatures of <1350°C and an average pressure of melt equilibration of 1.4 GPa, which are cooler and shallower than for basalts from Eastern and Western Anatolia. When considered in light of regionally slow upper mantle shear wave velocities, the mantle lithosphere may be thin and infiltrated by melts, or largely absent. An absence of secular changes in melting conditions suggests little to no lithospheric thinning over the past ∼1 Ma, despite evidence for lithospheric extension. Hasan basalts appear to be generated by decompression melting in response to the rollback of the Cyprean slab.

The end of Midcontinent Rift magmatism and the paleogeography of Laurentia

Abstract: Paleomagnetism of the North American Midcontinent Rift provides a robust paleogeographic record of Laurentia (cratonic North America) from ca. 1110 to 1070 Ma, revealing rapid equatorward motion of the continent throughout rift magmatism. Existing age and paleomagnetic constraints on the youngest rift volcanic and sedimentary rocks have been interpreted to record a slowdown of this motion as rifting waned. We present new paleomagnetic and geochronologic data from the ca. 1090–1083 Ma “late-stage” rift volcanic rocks exposed as the Lake Shore Traps (Michigan), the Schroeder-Lutsen basalts (Minnesota), and the Michipicoten Island Formation (Ontario). The paleomagnetic data allow for the development of paleomagnetic poles for the Schroeder-Lutsen basalts (187.8°E, 27.1°N; A 95 = 3.0°, N = 50) and the Michipicoten Island Formation (174.7°E, 17.0°N; A 95 = 4.4°, N = 23). Temporal constraints on late-stage paleomagnetic poles are provided by high-precision, 206 Pb- 238 U zircon dates from a Lake Shore Traps andesite (1085.57 ± 0.25 Ma; 2s internal errors), a Michipicoten Island Formation tuff (1084.35 ± 0.20 Ma) and rhyolite (1083.52 ± 0.23 Ma), and a Silver Bay aplitic dike from the Beaver Bay Complex (1091.61 ± 0.14 Ma), which is overlain by the Schroeder-Lutsen basalt flows. These Michipicoten Island Formation dates are the youngest yet obtained from Midcontinent Rift volcanic rocks and indicate that rift magmatism was active for at least 25 m.y. The addition of these late-stage paleomagnetic poles to the Laurentian apparent polar wander path suggests that rapid motion of Laurentia continued throughout the entirety of rift volcanism.