Mid-ocean Ridge Basalt Source Research Articles

Identifying volatile mantle trend with the water–fluorine–cerium systematics of basaltic glass

The lithophile elements and isotopic compositions of oceanic basalts suggest they derive from different mantle components. We present new analytical results of un-degassed deep-marine basaltic glasses from various regions, and we find strong linear correlations of R2 ≥ 0.997 between H2O and F as well as H2O and Ce, for which we propose mantle trends. The mantle trends represent global variations of mantle components, ranging from a depleted dry peridotite of mid-ocean ridge basalt (MORB) (DMM, depleted MORB source mantle: H2O = 100 ppm; H2O/Ce = 200; H2O/F = 10) to a hydrous peridotite of Hawaiian ocean island basalt (OIB) of deep mantle origin (FOZO, focal zone: H2O = 750 ppm; H2O/Ce = 200; H2O/F = 18.5). Accordingly, we defined the correlation as a volatile DMM–FOZO trend. Based on our findings, we report novel H2O–F–Ce systematics to discriminate the degree of water depletion in the source mantle and the rehydration of the mantle with recycled surface water through oceanic plate subduction. Using this method, most OIBs are distinguished clearly from the DMM–FOZO trend, and we find that the water in their sources originates from recycled water derived from the hydrated oceanic crust and sediment after various degrees of dehydration (75–95%) in subduction zones.

3D shear wave velocity model of the crust and uppermost mantle beneath the Tyrrhenian basin and margins

The Tyrrhenian basin serves as a natural laboratory for back-arc basin studies in the Mediterranean region. Yet, little is known about the crust-uppermost mantle structure beneath the basin and its margins. Here, we present a new 3D shear-wave velocity model and Moho topography map for the Tyrrhenian basin and its margins using ambient noise cross-correlations. We apply a self-parameterized Bayesian inversion of Rayleigh group and phase velocity dispersions to estimate the lateral variation of shear velocity and its uncertainty as a function of depth (down to 100 km). Results reveal the presence of a broad low velocity zone between 40 and 80 km depth affecting much of the Tyrrhenian basin’s uppermost mantle structure and its extension mimics the paleogeographic reconstruction of the Calabrian arc in time. We interpret the low-velocity structure as the possible source of Mid-Ocean Ridge Basalts- and Ocean Island Basalts- type magmatic rocks found in the southern Tyrrhenian basin. At crustal depths, our results support an exhumed mantle basement rather than an oceanic basement below the Vavilov basin. The 3D crust-uppermost mantle structure supports a present-day geodynamics with a predominant Africa-Eurasia convergence.

Magmatic Response to Subduction Initiation: Part 1. Fore-arc Basalts of the Izu-Bonin Arc From IODP Expedition 352.

The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO2, P2O5, Zr) and Ni compared to MORB, with Na2O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle.

Ultra-magnesian olivine-bearing ultramafic lava blocks within Cenozoic accretionary sediments at Shizuoka, Japan: Implications for young, hot plume activity beneath the western Pacific Plate

Ultra-magnesian igneous olivine is thought to crystallize from ultramafic melts and can therefore provide information about physicochemical conditions and the geochemistry and lithology of the source in the context of extremely hot mantle activity. This study focuses on the petrology and mineralogy of the Shizuoka meimechites, a series of ultra-magnesian olivine-bearing ultramafic lava blocks embedded in middle Eocene–early Miocene accretionary sediments at Shizuoka, Japan. Olivine phenocrysts within the Shizuoka meimechites have Fo compositions of ca. 81–92.8 mol%, where Fo = 100 Mg/(Mg + Fe). These ultra-magnesian olivines have trace-element compositions that are indicative of derivation from ultramafic melts that were in equilibrium with peridotitic mantle olivine. Al-in-olivine geothermometry yielded crystallization temperatures of ~1450 °C for Fo92.5–92.8 olivine, higher than the potential temperature of present-day mid-ocean ridge basalt source mantle material and similar to the potential temperatures of some large igneous provinces and ocean island basalts. Primary melt compositions in equilibrium with Fo92.5–92.8 olivine were estimated using the whole-rock compositions of the ultramafic lavas and are meimechitic, with MgO concentrations of ~21.4–27.0 wt% and SiO2 concentrations of 45.4–44.5 wt%. The potential temperature of the source mantle region for these meimechites (Tp) is estimated to be ~1486 °C–1705 °C, which is consistent with the Al-in-olivine geothermometry when considering adiabatic cooling during magma ascent. The relationship between Tp and Al-in-olivine temperature estimates for these lavas is consistent with global examples of ultra-magnesian olivine-bearing lavas. These lavas provide evidence of hitherto unknown Late Cretaceous–early Miocene deep mantle activity beneath the western Pacific region.

Plate tectonic cycling modulates Earth's 3He/22Ne ratio

The ratio of 3He and 22Ne varies throughout the mantle. This observation is surprising because 3He and 22Ne are not produced in the mantle, are highly incompatible during mantle melting, and are not recycled back into the mantle by subduction of oceanic sediment or basaltic crust. Our new compilation yields average 3He/22Ne ratios of 7.5 ± 1.2 and 3.5 ± 2.4 for mid-ocean ridge basalt (MORB) mantle and ocean island basalt (OIB) mantle sources respectively. The low 3He/22Ne of OIB mantle approaches planetary precursor 3He/22Ne values; ∼1 for chondrites and ∼1.5 for the solar nebula. The high 3He/22Ne of the MORB mantle is not similar to any planetary precursor, requiring a mechanism for fractionating He from Ne in the mantle and suggesting isolation of distinct mantle reservoirs throughout geologic time. New experimental results reported here demonstrate that He and Ne diffuse at rates differing by one or more orders of magnitude at relevant temperatures in mantle materials. We model the formation of a MORB mantle with an elevated 3He/22Ne ratio through kinetically modulated chemical exchange between dunite channel-hosted basaltic liquids and harzburgite wallrock beneath mid-ocean ridges. Over timescales relevant to mantle upwelling beneath spreading centers, He may diffuse tens to hundreds of meters into wallrock while Ne is effectively immobile, producing a mantle lithosphere regassed with respect to He and depleted with respect to Ne, with a net elevated 3He/22Ne. Subduction of high 3He/22Ne mantle lithosphere throughout geologic time would generate a MORB source with high 3He/22Ne. Mixing models suggest that to preserve a high 3He/22Ne reservoir, MORB mantle mixing timescales must be on the order of hundreds of millions of years or longer, that mantle convection has not been layered about the transition zone for most of geologic time, and that Earth's convecting mantle has lost at least 96% of its primordial volatile elements. The most depleted, highest 3He/22Ne mantle may be best preserved in the lower mantle where relatively high viscosities impede mechanical mixing.

Unexpected HIMU-type late-stage volcanism on the Walvis Ridge

Volcanic activity at many oceanic volcanoes, ridges and plateaus often reawakens after hiatuses of up to several million years. Compared to the earlier magmatic phases, this late-stage (rejuvenated/post-erosional) volcanism is commonly characterized by a distinct geochemical composition. Late-stage volcanism raises two hitherto unanswered questions: Why does volcanism restart after an extended hiatus and what is the origin of this volcanism? Here we present the first 40Ar/39Ar age and comprehensive trace element and Sr–Nd–Pb–Hf isotopic data from seamounts located on and adjacent to the Walvis Ridge in the South Atlantic ocean basin. The Walvis Ridge is the oldest submarine part of the Tristan-Gough hotspot track and is famous as the original type locality for the enriched mantle one (EM I) end member. Consistent with the bathymetric data, the age data indicates that most of these seamounts are 20–40 Myr younger than the underlying or nearby Walvis Ridge basement. The trace element and isotope data reveal a distinct compositional range from the EM I-type basement. The composition of the seamounts extend from the St. Helena HIMU (high time-integrated 238U/204Pb mantle with radiogenic Pb isotope ratios) end member to an enriched (E) Mid-Ocean-Ridge Basalt (MORB) type composition, reflecting a two-component mixing trend on all isotope diagrams. The EMORB end member could have been generated through mixing of Walvis Ridge EM I with normal (N) MORB source mantle, reflecting interaction of Tristan-Gough (EM I-type) plume melts with the upper mantle. The long volcanic quiescence and the HIMU-like geochemical signature of the seamounts are unusual for classical hotspot related late-stage volcanism, indicating that these seamounts are not related to the Tristan-Gough hotspot volcanism. Two volcanic arrays in southwestern Africa (Gibeon-Dicker Willem and Western Cape province) display similar ages to the late-stage Walvis seamounts and also have HIMU-like compositions, suggesting a larger-scale event at ∼77–49 Ma. We propose that the EM I-like mantle plumes rise from the edges of the African Large Low Shear Velocity Province (LLSVP; Tristan-Gough, Discovery and Shona hotspot), whereas the HIMU-dominated intraplate lavas (St. Helena, Gibeon-Dicker Willem and Western Cape province) and the late-stage Walvis seamounts tap material from internal portions of the African LLSVP, suggesting possible lateral and/or vertical chemical zonation of the African LLSVP.

Picrite “Intelligence” from the Middle-Late Triassic Stikine arc: Composition of mantle wedge asthenosphere

Primitive, near-primary arc magmas occur as a volumetrically minor ≤100 m thick unit in the Canadian Cordillera of northwestern British Columbia, Canada. These primitive magmas formed an olivine-phyric, picritic tuff near the base of the Middle-Late Triassic Stuhini Group of the Stikine Terrane (Stikinia). A new 40Ar/39Ar age on hornblende from a cross-cutting basaltic dyke constrains the tuff to be older than 221 ± 2 Ma. An 87Sr/86Sr isochron of texturally-unmodified tuff samples yields 212 ± 25 Ma age, which is interpreted to represent syn-depositional equilibration with sea-water. Parental trace element magma composition of the picritic tuff is strongly depleted in most incompatible trace elements relative to MORB and implies a highly depleted ambient arc mantle. High-precision trace element and Hf-Nd-Pb isotopic analyses indicate an origin by mixing of a melt of depleted ambient asthenosphere with ≤2% of subducted sediment melt. Metasomatic addition of non-conservative incompatible elements through melting of subducted Panthalassa Ocean floor sediments accounts for the arc signature of the Stuhini Group picritic tuff, enrichment of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and high field strength elements (HFSE), and anomalous enrichment in Pb.The inferred Panthalassan sediments are similar in composition to the Neogene-Quaternary sediments of the modern northern Cascadia Basin. The initial Hf isotopic composition of the picritic tuff closely approximates that of the ambient Middle-Late Triassic asthenosphere beneath Stikinia and is notably less radiogenic than the age-corrected Hf isotopic composition of the Depleted (MORB) Mantle reservoir (DM or DMM). This suggests that the ambient asthenospheric mantle end-member experienced melt depletion (F ≤ 0.05) a short time before picrite petrogenesis. The mantle end-member in the source of the Stuhini Group picritic tuff is isotopically similar to the mantle source of enriched mid-ocean ridge basalts (E-MORB) erupted today at the southern end of the Explorer Ridge in northeastern Pacific Ocean. The isotopic similarity between the Middle-Late Triassic ambient mantle under Stikinia, and mantle presently tapped at the southern Explorer Ridge suggests that enriched domains in the northeastern Pacific mantle are long-lived (≥222 million years).

New numerical approaches for modeling thermochemical convection in a compositionally stratified fluid

Geochemical observations of mantle-derived rocks favor a nearly homogeneous upper mantle, the source of mid-ocean ridge basalts (MORB), and heterogeneous lower mantle regions. Plumes that generate ocean island basalts are thought to sample the lower mantle regions and exhibit more heterogeneity than MORB. These regions have been associated with lower mantle structures known as large low shear velocity provinces (LLSVPS) below Africa and the South Pacific. The isolation of these regions is attributed to compositional differences and density stratification that, consequently, have been the subject of computational and laboratory modeling designed to determine the parameter regime in which layering is stable and understanding how layering evolves. Mathematical models of persistent compositional interfaces in the Earth’s mantle may be inherently unstable, at least in some regions of the parameter space relevant to the mantle. Computing approximations to solutions of such problems presents severe challenges, even to state-of-the-art numerical methods. Some numerical algorithms for modeling the interface between distinct compositions smear the interface at the boundary between compositions, such as methods that add numerical diffusion or ‘artificial viscosity’ in order to stabilize the algorithm. We present two new algorithms for maintaining high-resolution and sharp computational boundaries in computations of these types of problems: a discontinuous Galerkin method with a bound preserving limiter and a Volume-of-Fluid interface tracking algorithm. We compare these new methods with two approaches widely used for modeling the advection of two distinct thermally driven compositional fields in mantle convection computations: a high-order accurate finite element advection algorithm with entropy viscosity and a particle method that carries a scalar quantity representing the location of each compositional field. All four algorithms are implemented in the open source finite element code ASPECT, which we use to compute the velocity, pressure, and temperature associated with the underlying flow field. We compare the performance of these four algorithms on three problems, including computing an approximation to the solution of an initially compositionally stratified fluid at Ra=105 with buoyancy numbers B that vary from no stratification at B=0 to stratified flow at large B.

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

The behavior of C, H, and S in the solid Earth depends on their oxidation states, which are related to oxygen fugacity (fO2). Volcanic degassing is a source of these elements to Earth's surface; therefore, variations in mantle fO2 may influence the fO2 at Earth's surface. However, degassing can impact magmatic fO2 before or during eruption, potentially obscuring relationships between the fO2 of the solid Earth and of emitted gases and their impact on surface fO2 We show that low-pressure degassing resulted in reduction of the fO2 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 fO2 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 fO2 than modern magmas. Estimates of fO2 of Archean magmas are not this low, arguing for alternative explanations for the oxygenation of the atmosphere.

Submarine Basaltic Glasses from the Galapagos Archipelago: Determining the Volatile Budget of the Mantle Plume

We report new major, trace and volatile element contents (H_2O, CO_2, F, S, Cl), and new Sr, Nd, Pb and He isotopes on submarine glasses from the Galapagos Archipelago from several dredging expeditions. Four groups are distinguishable on the basis of composition and geographical distribution: the Fernandina group (^3He/^4He > 22 R_A), which is similar to the less degassed primitive mantle; the Sierra Negra group (enriched Pb and Sr isotopes, ^3He/^4He = 8–20 R_A), produced by mixing the Floreana (HIMU-type) and Fernandina end-members; the Pinta group (high Δ7/4, Δ8/4 and Th/La ratios, ^3He/^4He = 6–9 R_A), an enriched mantle (EM)-type mantle indicative of recycled material in the source; and the depleted mantle (DM) group, characterized by an isotopic composition similar to mid-ocean ridge basalts (MORB). Only a single submarine glass with the isotopic composition of the Floreana end-member has been identified in the sample suite. Degassing has significantly lowered the glass CO_2 content with little effect on the H_2O concentration. Volatile data for oceanic basalts reveal that CO_2–H_2O gas–melt equilibration at eruption depth is common in ocean island basalts (OIB) and rare in MORB, suggesting different ratios of melt transport to bubble formation and gas–melt equilibration. The Galapagos glasses range from sulfide saturated to undersaturated, and a subset of samples indicate that S degasses at pressures ≤ 400 bars. Assimilation of hydrothermally altered material affected the volatile contents of a number of samples in the groups. Once shallow-level processes have been accounted for, we evaluate the volatile contents in the different Galapagos mantle sources. Ratios between volatile and refractory elements with similar incompatibilities are used to estimate the volatile budget of the Galapagos mantle plume. Most of the glasses from the Fernandina, Sierra Negra and Pinta groups have high volatile/refractory element ratios, whereas a few pristine DM group lavas have ratios similar to those measured in MORB. The volatile/refractory element ratios are consistent with previous reports for the high ^3He/^4He, HIMU and MORB components. The values measured for the Pinta group, however, are higher than those found in other OIB associated with the presence of recycled material (EM-type). Our data suggest that mixing between the different mantle components is pervasive throughout the archipelago, which acts to normalize the volatile data between the groups. The Fernandina component can be modeled by a 6–20% mixture of the high ^3He/^4He primitive mantle component with the MORB source, assuming a two-layered mantle and using existing estimates of helium concentrations. The resulting estimated volatile content and H/C mass ratio for the high ^3He/^4He primitive mantle are consistent with previous estimates, but calculated C/3He ratios are lower than the canonical ratio. This indicates the following: (1) the estimates require ∼20–50 times higher C or lower ^3He contents, which is difficult to reconcile with the measured volatile/refractory ratios in oceanic basalts; (2) the C/^3He ratio is not constant throughout the mantle; (3) an impact erosion model, rather than a two-layered mantle model, is more consistent with the relatively constant C/3He ratios observed in oceanic basalts, although it is unclear how representative oceanic basalts are of the lower mantle. The high volatile content of the high ^3He/^4He component will affect mantle dynamics and melt migration during plume–ridge interaction as this component would be predicted to be less viscous than the ambient mantle. The lower viscosity material would have an enhanced vertical upwelling, which could explain the buoyancy flux of the Galapagos plume without the need for a temperature anomaly. A lower viscosity, high ^3He/^4He component could also provide an explanation for the lack of high ^3He/^4He in Galapagos Spreading Center lavas erupting in the vicinity of the Galapagos plume.

Effect of water on the fluorine and chlorine partitioning behavior between olivine and silicate melt

Halogens show a range from moderate (F) to highly (Cl, Br, I) volatile and incompatible behavior, which makes them excellent tracers for volatile transport processes in the Earth’s mantle. Experimentally determined fluorine and chlorine partitioning data between mantle minerals and silicate melt enable us to estimate Mid Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB) source region concentrations for these elements. This study investigates the effect of varying small amounts of water on the fluorine and chlorine partitioning behavior at 1280 °C and 0.3 GPa between olivine and silicate melt in the Fe-free CMAS+F–Cl–Br–I–H2O model system. Results show that, within the uncertainty of the analyses, water has no effect on the chlorine partitioning behavior for bulk water contents ranging from 0.03 (2) wt% H2O (DClol/melt = 1.6 ± 0.9 × 10−4) to 0.33 (6) wt% H2O (DClol/melt = 2.2 ± 1.1 × 10−4). Consequently, with the effect of pressure being negligible in the uppermost mantle (Joachim et al. Chem Geol 416:65–78, 2015), temperature is the only parameter that needs to be considered for the determination of chlorine partition coefficients between olivine and melt at least in the simplified iron-free CMAS+F–Cl–Br–I–H2O system. In contrast, the fluorine partition coefficient increases linearly in this range and may be described at 1280 °C and 0.3 GPa with (R2 = 0.99): D_{F}^{text{ol/melt}} = 3.6pm 0.4 times {{10}^{-3}} times {{X}_{{{text{H}}_{text{2}}}text{O}}}left( text{wt }!!%!!text{ } right) + 6 pm 0.4times ,{{10}^{-4}}. The observed fluorine partitioning behavior supports the theory suggested by Crépisson et al. (Earth Planet Sci Lett 390:287–295, 2014) that fluorine and water are incorporated as clumped OH/F defects in the olivine structure. Results of this study further suggest that fluorine concentration estimates in OIB source regions are at least 10% lower than previously expected (Joachim et al. Chem Geol 416:65–78, 2015), implying that consideration of the effect of water on the fluorine partitioning behavior between Earth’s mantle minerals and silicate melt is vital for a correct estimation of fluorine abundances in OIB source regions. Estimates for MORB source fluorine concentrations as well as chlorine abundances in both mantle source regions are within uncertainty not affected by the presence of water.

186Os–187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys

Abyssal peridotites are oceanic mantle fragments that were recently processed through ridges and represent residues of both modern and ancient melting. To constrain the nature and timing of melt depletion processes, and the composition of the mantle, we report high-precision Os isotope data for abyssal peridotites from three ocean basins, as well as for Os-rich alloys, primarily from Mesozoic ophiolites. These data are complemented by whole-rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), trace- and major-element abundances for the abyssal peridotites, which are from the Southwest Indian (SWIR), Central Indian (CIR), Mid-Atlantic (MAR) and Gakkel Ridges. The results reveal a limited role for melt refertilization or secondary alteration processes in modifying abyssal peridotite HSE compositions. The abyssal peridotites examined have experienced variable melt depletion (2% to >16%), which occurred >0.5Ga ago for some samples. Abyssal peridotites typically exhibit low Pd/Ir and, combined with high-degrees of estimated total melt extraction, imply that they were relatively refractory residues prior to incorporation into their present ridge setting. Recent partial melting processes and mid-ocean ridge basalt (MORB) generation therefore played a limited role in the chemical evolution of their precursor mantle domains. The results confirm that many abyssal peridotites are not simple residues of recent MORB source melting, having a more complex and long-lived depletion history.Peridotites from the Gakkel Ridge, SWIR, CIR and MAR indicate that the depleted MORB mantle has 186Os/188Os of 0.1198356±21 (2SD). The Phanerozoic Os-rich alloys yield an average 186Os/188Os within uncertainty of abyssal peridotites (0.1198361±20). Melt depletion trends defined between Os isotopes and melt extraction indices (e.g., Al2O3) allow an estimate of the primitive mantle (PM) composition, using only abyssal peridotites. This yields 187Os/188Os (0.1292±25), and 186Os/188Os of 0.1198388±29, both of which are within uncertainty of previous primitive mantle estimates. The 186Os/188Os composition of the PM is less radiogenic than for some plume-related lavas, with the latter requiring sources with high long-term time-integrated Pt/Os. Estimates of primitive mantle HSE concentrations using abyssal peridotites define chondritic Pd/Ir, which differs from previous supra-chondritic estimates for Pd/Ir based on peridotites from a range of tectonic settings. By contrast, estimates of PM yield supra-chondritic Ru/Ir. The cause of enhanced Ru in the mantle remains enigmatic, but may reflect variable partitioning behavior of Ru at high pressure and temperature.

The fate of sulfide during decompression melting of peridotite – implications for sulfur inventory of the MORB-source depleted upper mantle

Magmatism at mid ocean ridges is one of the main pathways of S outflux from deep Earth to the surface reservoirs and is a critical step in the global sulfur cycle, yet our understanding of the behavior of sulfide during decompression melting of the upper mantle is incomplete. In order to constrain the sulfur budget of the mantle and reconcile the sulfur and chalcophile element budget of mantle partial melts parental to primitive mid-ocean ridge basalts (MORBs), here we developed a model to describe the behavior of sulfide and Cu during decompression melting by combining the pMELTS thermodynamic model and empirical sulfur contents at sulfide concentration (SCSS) models, taking into account the effect of the presence of Ni and Cu in sulfides on SCSS of mantle-derived melts. Calculation of SCSS along melting adiabat at mantle potential temperature of 1380 °C with variable initial S content in the mantle indicates that the complete consumption or partial survival of sulfide in the melting residue depends on initial S content and degree of melting. Primitive MORBs (Mg#>60) with S and Cu mostly concentrated in 800–1000 ppm and 80–120 ppm are likely mixture of sulfide undersaturated high degree melts and sulfide saturated low degree melts derived from depleted peridotite containing 100–200 ppm S. Model calculations to capture the effects of variable mantle potential temperatures (1280–1420 °C) indicate that for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle owing to the effect of temperature in enhancing sulfide solubility in silicate melt, and higher mantle temperature stabilizing partial melt with higher FeO⁎ and lower SiO2 and Al2O3, all of which generally enhance sulfide solubility. However, sulfide can still be exhausted by ∼10–15% melting with bulk S of 100–150 ppm in the mantle when TP is as low as 1300 °C. We also show that although variation of DCuperidotite/melt and initial Cu in the mantle can all affect the Cu concentration of primitive MORBs, 100–200 ppm S in the MORB source mantle can satisfy both S and Cu geochemistry of partial melts parental to ocean floor basalts.

Spatio-temporal Geochemical Evolution of the SE Australian Upper Mantle Deciphered from the Sr, Nd and Pb Isotope Compositions of Cenozoic Intraplate Volcanic Rocks

Intraplate basaltic volcanic rocks ranging in age from Late Cretaceous to Holocene are distributed across southeastern Australia in Victoria and eastern South Australia. They comprise four prov- inces differentiated on the basis of age and spatial distribution. The youngest of these (<4􏱭6 Ma) is the Newer Volcanic Province (NVP), which incorporates lava flows, scoria cones and maars, distrib- uted across western and central Victoria into South Australia. The oldest eruptive rocks belong to the 95–19 Ma Older Volcanic Province, which comprises basaltic lava flows and shallow intrusions distributed across eastern and central Victoria. When examined within the broader framework of geochemical data available for Cretaceous to Cenozoic intraplate volcanism in southeastern Australia, new major, minor and trace element and Sr, Nd and Pb isotope analyses of volcanic rocks from the NVP suggest that their parental magmas originated from a distinctively different mantle source compared with that of the Older Volcanics. We propose that the magmas repre- sented by the Older Volcanics originated from low degrees of partial melting of a mixed source of Indian mid-ocean ridge basalt (MORB)-source mantle and calcio-carbonatite metasomatized sub- continental lithospheric mantle (SCLM), followed by up to 20% fractional crystallization. The mag- mas of the youngest (<500 ka) suite of the NVP (the Newer Cones) were generated by up to 13% partial melting of a garnet-rich source, followed by similar degrees of fractional crystallization. We also suggest that the temporally intermediate Euroa Volcanics (􏰂7 Ma) reflect chemical evolution from the source of the Older Volcanics to that of the Newer Cones. Furthermore, energy- constrained recharge, assimilation and fractional crystallization (EC-RAxFC) modelling suggests that the Sr isotope signature of the 􏰂4􏱭6–1Ma Newer Plains component of the NVP can be ex- plained by up to 5% upper crustal assimilation. On the basis of these results and data from the lit- erature for mantle xenoliths, we propose a geodynamic model involving decompression melting of metasomatized veins at the base of the SCLM generating the Older Volcanics and modifying the ambient asthenosphere of Indian MORB isotope character. This was followed by thermal erosion and entrainment of the resulting depleted SCLM into the modified Indian MORB-source astheno- spheric mantle, generating the Newer Cones. Such a model is in agreement with recent geophys- ical observations in the area suggesting edge-driven convection with shear-driven upwelling as a potential geodynamic model resulting in temporal upwelling in the region.

Noble gas composition of subcontinental lithospheric mantle: An extensively degassed reservoir beneath Southern Patagonia

Patagonia, in the Southern Andes, is one of the few locations where interactions between the oceanic and continental lithosphere can be studied due to subduction of an active spreading ridge beneath the continent. In order to characterize the noble gas composition of Patagonian subcontinental lithospheric mantle (SCLM), we present the first noble gas data alongside new lithophile (Sr–Nd–Pb) isotopic data for mantle xenoliths from Pali-Aike Volcanic Field and Gobernador Gregores, Southern Patagonia. Based on noble gas isotopic compositions, Pali-Aike mantle xenoliths represent intrinsic SCLM with higher (U+Th+K)/(3He, 22Ne, 36Ar) ratios than the mid-ocean ridge basalt (MORB) source. This reservoir shows slightly radiogenic helium (3He/4He = 6.84–6.90 RA), coupled with a strongly nucleogenic neon signature (mantle source 21Ne/22Ne = 0.085–0.094). The 40Ar/36Ar ratios vary from a near-atmospheric ratio of 510 up to 17700, with mantle source 40Ar/36Ar between 31100−6800+9400 and 54000−9600+14200. In addition, the 3He/22Ne ratios for the local SCLM endmember, at 12.03±0.15 to 13.66±0.37, are higher than depleted MORBs, at 3He/22Ne = 8.31–9.75. Although asthenospheric mantle upwelling through the Patagonian slab window would result in a MORB-like metasomatism after collision of the South Chile Ridge with the Chile trench ca. 14 Ma, this mantle reservoir could have remained unhomogenized after rapid passage and northward migration of the Chile Triple Junction. The mantle endmember xenon isotopic ratios of Pali-Aike mantle xenoliths, which is first defined for any SCLM-derived samples, show values indistinguishable from the MORB source (129Xe/132Xe=1.0833−0.0053+0.0216 and 136Xe/132Xe=0.3761−0.0034+0.0246).The noble gas component observed in Gobernador Gregores mantle xenoliths is characterized by isotopic compositions in the MORB range in terms of helium (3He/4He = 7.17–7.37 RA), but with slightly nucleogenic neon (mantle source 21Ne/22Ne = 0.065–0.079). We suggest that this MORB-like metasomatism was capable of overprinting the noble gas composition of Gobernador Gregores due to recent metasomatism of the SCLM because of asthenospheric mantle upwelling in response to opening of the Patagonian slab window. The 40Ar/36Ar ratios vary from a near-atmospheric ratio of 380 up to 6560, with mantle source 40Ar/36Ar between 8100−700+1400 and 17700−3100+4400. The lower 40Ar/36Ar ratio of the Gobernador Gregores mantle source, compared with that of Pali-Aike, attests that the Patagonia SCLM was affected significantly by atmospheric contamination associated with the recycled oceanic lithosphere.

Crystal/melt partitioning of water and other volatiles during the near-solidus melting of mantle peridotite: Comparisons with non-volatile incompatible elements and implications for the generation of intraplate magmatism

Concentrations of H 2 O, F, Cl, C, P, and S have been measured by secondary ion mass spectrometry (SIMS) in experimentally produced peridotite phases (including clinopyroxene, orthopyroxene, olivine, garnet, amphibole, and mica) and coexisting basanitic glasses. Because only two experiments produced glasses on quenching (with the melt phase in others reverting to felt-like crystallite masses) H 2 O concentrations in melts were also separately determined from mass-balance relationships and by assuming constant H 2 O/La in melts and starting materials. The resulting values were used to calculate mineral/melt partition coefficients ( D values) for H 2 O [where D H 2 O crystal/melt = (mass fraction of H 2 O in crystal)/(mass fraction of H 2 O in melt)] for conditions of 1025–1190 °C and 1.0–3.5 GPa. These gave 0.0064–0.0164 for clinopyroxene, 0.0046–0.0142 for orthopyroxene, 0.0015–0.0016 for olivine, and 0.0016–0.0022 for garnet. Although less information was obtained for the other volatiles, F was found to be significantly more compatible than H 2 O during peridotite melting, whereas Cl is significantly less compatible. S also has small but appreciable solubilities in amphiboles and micas, but not in pyroxenes or olivine. The solubility of C in silicate minerals appears to be negligible, although C was present in coexisting melts (~0.5 wt% as CO 2 ) and as residual graphite during experiments. The D values for H 2 O in clinopyroxene and orthopyroxene are positively correlated with iv Al but negatively correlated with the H 2 O concentrations of melts (when considered as wt%). These relationships are consistent with the broad trends of previously published partitioning data. Although some of the concentration dependence can be related to cross-correlation between iv Al in pyroxenes and H 2 O concentrations in melts (via the latter’s control of liquidus temperatures) this relationship is too inconsistent to be a complete explanation. A concentration dependence for D H 2 O mineral/melt can also be independently predicted from speciation models for H 2 O in silicate melts. Thus it is likely that D H 2 O pyx/melt is influenced by both iv Al and the absolute concentration of H 2 O in melts. D H 2 O / D Ce for clinopyroxene is inversely correlated with M2 site radii. Because the latter decrease with increasing pressure and temperature, relatively hot and/or deeply derived melts should be enriched in Ce relative to H 2 O when compared to melts from cooler and shallower mantle sources. Conversely, melts from H 2 O-rich settings (e.g., subduction zones) should have higher H 2 O/Ce than their source rocks. When combined with previously obtained partitioning data for non-volatile elements (from the same experiments), our data are consistent with the enrichment of intraplate basalt sources in both volatile and non-volatile incompatible elements by small-degree melts derived from local mid-ocean ridge basalt sources. In this way, volatiles can be seen to play an active role (via their promotion of partial-melting and metasomatic processes) in the auto-regulation of incompatible element concentrations in the depleted upper mantle.

Plate tectonics and mantle plumes as a basis of deep-seated Earth’s tectonic activity for the last 2 Ga

The onset of the modern tectonic style that combines plate-related and plume-related mechanisms has been discussed. Such a tectonic style could have started on the Earth when all layers of our planet had formed. Analysis of available geological data shows that the solid inner core crystallized by 2.7 Ga. Typical geologic complexes appeared on the continents as a result of plate tectonics processes at that time. The layer D″ that accumulates lithospheric slabs, which do not remain at the upper-lower mantle interface but can go down to the core-mantle boundary, was finally produced by 2 Ga. At pressures and temperatures of the D″ layer perovskite, a principal mineral of the lower mantle transforms into postperovskite phase. The isotope data suggest the existence of mantle (EM-I and EM-II) domains, being the sources for oceanic island basalts (OIB) and the depleted upper mantle (asthenosphere), the source for mid-ocean ridge basalts (MORB), from at least 2 Ga. It is accepted that the recent tectonic style started on the Earth at that period. Deep-seated processes suggest the involvement of all Earth’s layers. The asthenosphere–lithosphere interaction is responsible for different types of large surface structures that develop as fold mountains, oceanic spaces, and subduction zones. The descending lithospheric slabs (cold mantle material) and ascending mantle material in hot mantle provinces or the so-called low shear velocity provinces are responsible for the lower mantle convection. The plume is produced in the layer D″, which accumulates the descending lithosphere slabs as well as light elements released from the outer liquid core, which are oxidized in this layer, thus resulting in thermochemical plume formation. At the same time the molten ferric iron penetrates the core. So, we see the interaction of all Earth’s layers. The article also considers the evolutionary history of the Siberian continent over 700 Myr as early as the breakdown of Rodinia and the formation of mountain folds and rifting structures and the associated metallogeny.

Probing the Earth’s Deep Interior Through Geochemistry

Basaltic volcanism provides a window into the Earth’s mantle. Seeing through this window requires seeing through the processes of magma genesis and evolution that distort the view. Radiogenic isotope ratios, and to a lesser extent incompatible elements and stable isotope ratios, allow us to see through these distortions and infer the nature and evolution of various mantle magma sources. Geochemical studies of mid-ocean ridge basalts (MORB) reveal that the upper mantle was long ago depleted in incompatible elements through previous episodes of melt extraction, likely associated with crust formation. Oceanic island basalts (OIB) are the products of mantle plumes and hence provide us with a view of the deepest parts of the Earth’s mantle. These plumes can be divided into several genera, EMI, EM II, and HIMU, most of which contain several species. Little or no mixing is observed between these genera, but all mix with a prevalent mantle component, PREMA, that contributes to many if not all plumes. Although the mantle comprising plumes also show evidence of previous melt extraction events, the more prominent chemical signature is of material anciently recycled from the Earth’s surface through subduction of oceanic crust and sediment, subduction erosion, and foundering of lower continental crust. This is most unambiguously manifest in variations in stable isotope ratios that could only have been produced at or near the surface of the Earth. In addition to a recycled component, many plumes also contain a quite primitive component, manifested primarily in their noble gas isotope ratios. The primitive component seems most closely associated with PREMA. In contrast to hot spot and mid-ocean ridge volcanism, subduction-related volcanism allows an assessment of the flux into the Earth’s mantle. It was apparent from early Pb isotope data that sediment was subducted along with oceanic crust, an observation later dramatically confirmed by the discovery of cosmogenic 10Be in subduction zone magmas. Because of this sediment, the flux into the mantle at subduction zones is more incompatible-element enriched than MORB. Although some fraction of this material is extracted into subduction zone magmas, a variety of approaches reveal that the flux into the deep mantle remains incompatible-element enriched. Other studies, particularly geophysical ones, indicate that this subduction flux is substantially augmented by subduction erosion of the overriding plate. Over Earth’s history these processes together with continental crustal founding may have delivered a volume of material equal to the present volume of the continents. The discovery that all modern terrestrial rocks have an excess of 142Nd, which is the decay product of the extinct radionuclide 146Sm, compared to chondrites, reopens the question of the Earth’s composition. One important implication is that the concentrations of heat producing elements, U, Th, and K, may be 25 % or more lower than previously thought. Another implication is that the depleted source of MORB may constitute a very large fraction of the mantle, perhaps nearly all of it. On the other hand, the heat flow from the core may be substantially greater than previously thought. This implies that the lowermost mantle and the core-mantle boundary layer may play a dominant role in driving mantle convention, and ultimately all geologic processes. Geophysical evidence is revealing that the lowermost mantle is indeed a dynamic and interesting region. The challenge for mantle geochemists is to integrate their observations with those of geophysicists and progress is slowly being made in this respect.

Depleted components in the source of hotspot magmas: Evidence from the Ninetyeast Ridge (Kerguelen)

Although most ocean island basalts (OIB) are enriched in incompatible elements relative to mid-ocean ridge basalts, OIB depleted in these elements also occur on some islands. The Ninetyeast Ridge (NER) in the eastern Indian Ocean is a 5000 km long hotspot track defined by submarine basaltic volcanoes that were islands when they formed from 43 to 77 Ma. A subset of NER basalts, described as depleted, has high abundances of Sc, Y and Lu, which are relatively compatible in clinopyroxene and especially in garnet. It is unusual for magmas to have the trace element characteristics of a mineral. A likely explanation is that the depleted NER basalts were derived from a source that was created as a garnet- and clinopyroxene-bearing residue during partial melting. When this residue formed, the extent of melting must have been low as not all of the garnet and clinopyroxene was melted. To provide sufficient time for the relatively high Lu/Hf of the residue to develop the high 176Hf/177Hf that is characteristic of depleted NER basalts, this melting event must have been ancient. In the second much younger melting event that formed the NER, the extent of melting was sufficiently high to eliminate garnet and clinopyroxene from the ancient residue. Basalts erupted on a segment of the Mid-Atlantic Ridge near the Azores were also derived from an ancient garnet-bearing residue. Residues from ancient partial melting events involving low extents of melting are the dominant source of mid-ocean ridge basalts and depleted magmas associated with the Kerguelen and Azores hotspots. In contrast, a very different process has been inferred for creating the source of depleted Icelandic basalts. Their source was gabbro containing cumulate plagioclase and clinopyroxene. Such gabbros are common in the lower oceanic crust, and if recycled into the Icelandic hotspot they are a source of depleted Icelandic basalts.

Xenon isotopes in the MORB source, not distinctive of early global degassing

AbstractAlthough fissiogenic Xe in mid‐ocean ridge basalt (MORB) has long been used inferred to infer an early (&gt;4 Ga) catastrophic mantle degassing, decomposition of the analyzed gas in terms of its constituents requires several assumptions. The canonical interpretation has not been tested using the full spectrum of possible initial components and radiogenic inputs. We use a Markov chain Monte Carlo approach that examines a broad range of Xe isotopic components present during Earth formation and evolution. Our best fit simulations are consistent with the preservation of much higher Pu/U in MORB source Xe than previously recognized but are equally supportive of both limited and early catastrophic loss. We show that an initially Xe depleted upper mantle that becomes progressively ingassed through both radiogenic ingrowth and subduction of evolving atmospheric Xe is equally consistent with all evidence, underscoring the need for improvements to our knowledge of fission Xe spectra, MORB Xe measurements, and Pu geochemistry.