Mid-ocean Ridge Basalt Source Research Articles

The role of calcium phosphates and silicates in the storage and transport of phosphorus at the upper-to-lower mantle transition: An experimental study to 25 GPa in a model peridotitic bulk composition

Calcium (Ca) phosphates are major hosts for phosphorus, halogens and incompatible trace elements in the Earth’s crust and mantle and, by supplying almost all phosphorus to the biosphere, are indispensable for sustaining life on Earth. To better understand the role of Ca-phosphates in the deep silicate Earth portion of the global phosphorus cycle at depths of the transition zone, we performed experiments in the P-T range of 15–25 GPa and 1600–2000 °C using a moderately fertile peridotite doped with 3 % synthetic β-Ca3(PO4)2 and 1 % of a trace element mix containing a range of HFSE, LILE, REEs, Cl and Br. Tuite is stable to at least 25 GPa at which pressure its upper T-stability limit is between 1700 and 1750 °C. At 15 GPa, by comparison, the upper T-stability limit is located between 1750 and 1800 °C, indicating a negative slope of the tuite-out reaction(s). Thus, tuite is stable to at least 700-km depth with a maximum T-stability close to or slightly above the average current mantle adiabat (ACMA) at 20–25 GPa and significantly above the ACMA at 15 GPa. Beyond the P-T stability limit of majoritic garnet, tuite is the principal subsolidus phosphorus carrier in the uppermost lower mantle. Beyond the P-T stability limit of tuite, phosphorus is likely to either (i) enter a Ca3(PO4)2 polymorph with an even higher density, (ii) form an entirely new phosphate phase or (iii) be accommodated in lower mantle silicates in six-fold coordination. Within the P-T range of our experiments, the phases encountered show the following trend of phosphorus contents: Pmelt > Pmajoritic garnet > Polivine ≅ Pringwoodite > Pdavemaoite ≅ Pwadsleyite > Pclinoenstatite ≅ Pbridgmanite ≅ Pferropericlase. Even when coexisting with tuite, bridgmanite and ferropericlase show phosphorus contents < 100 µg/g, and davemaoite has phosphorus contents that do not exceed 390 µg/g, testifying to the very limited phosphorus storage capacity of the lower mantle. Model calculations based on the phosphorus contents of ocean island basalts (OIBs) compared with those of mid ocean ridge basalts (MORBs) indicate that the OIB source is significantly richer in phosphorus than the MORB source and, for a wide range of OIB phosphorus contents, is likely to contain modal apatite. The relative importance of Ca-phosphates and silicates as phosphorus carriers in the peridotitic upper and uppermost lower mantle is strongly dependent upon pressure and the bulk phosphorus content. With increasing P, phosphorus is progressively transferred from Ca-phosphates to silicates with an ensuing decrease in the modal amount of the Ca-phosphates. Dependent upon the bulk phosphorus content, this may cause the Ca-phosphates to disappear before the upper-to-lower mantle boundary is reached, leaving majoritic garnet as the major phosphorus host. Breakdown of garnet will subsequently cause tuite to re-appear in the uppermost lower mantle and to resume its role of the principal subsolidus phosphorus carrier. For sufficiently high bulk phosphorus contents, tuite will remain stable throughout the transition zone, thereby maintaining phosphorus saturation in the coexisting silicates, and will eventually be joined by the tuite newly generated by garnet breakdown.

Coupled Geodynamical‐Geochemical Perspectives on the Generation and Composition of Mid‐Ocean Ridge Basalts

AbstractOwing to their abundance and relative availability on Earth's seafloor, mid‐ocean ridge basalts (MORBs) have a well‐defined chemical element budget, reflected by the low standard deviation associated with typical normal MORB (N‐MORB) composition. However, the exact mechanisms leading to magma differentiation and MORB generation remain debated, which hinders our ability to evaluate MORB parental magma composition. In this study, we leverage the predictive power of the BDD21 numerical framework to obtain a representative trace element budget of parental MORB magma and assess its ability to fractionate into the N‐MORB composition. Utilizing revised parameterizations for mineralogy, melting, and partitioning, we couple BDD21 with numerical simulations of a MOR system driven by seafloor spreading in which we track the evolution of partial melting, mineral modal abundances, and concentrations of incompatible elements. Parental magma compositions are determined once simulations reach a steady state, and magma chamber replenishment models are employed to predict the trace element budget of the erupted liquid. We explore a range of geophysical and geochemical parameters to evaluate their effect on computed trace element concentrations. Previous magma chamber replenishment models are extended to account for multiple crystallization events and melt‐crystal interaction. Modeling outcomes suggest that petrologically constrained fractionation of parental magma compositions obtained through BDD21 yields glass compositions compatible with the N‐MORB budget. Nevertheless, our results show a systematic underestimation of Sr concentration, indicating the presence of recycled oceanic crust in the MORB source region.

Contribution of recycled oceanic crust to the extremely light molybdenum isotopic compositions of mid-ocean ridge basalts from the South China Sea

The contribution of recycled oceanic crust to the mantle source of mid-ocean ridge basalts (MORBs) remains a subject of debate, as radiogenic isotopes alone yield ambiguous insights. Here, we present comprehensive data on whole-rock major element, trace element, and Mo–Nd–Hf isotopic compositions for MORB samples from International Ocean Discovery Program Expedition 349 sites U1431E and U1433B in the eastern (ESB) and southwestern (SWB) subbasins, respectively, of the South China Sea (SCS). The δ98/95Mo values of the SCS MORBs exhibit a significant range (from −0.80‰ to +0.05‰), in contrast to the restricted composition observed in MORBs (δ98/95Mo = −0.19‰ ± 0.01‰). The ESB MORBs display extremely light Mo isotopic compositions (δ98/95Mo = −0.80‰ to −0.24‰) with NdHf isotopic compositions similar to those of Pacific MORB, whereas the SWB MORBs show higher δ98/95Mo values (−0.44‰ to +0.05‰) and depleted NdHf isotopic compositions. The δ98/95Mo values of the SCS MORBs are negatively correlated with Ce/Mo ratios and positively correlated with εHf values, suggesting the incorporation of dehydrated slab components into the source of the SCS MORBs. The subbasin-scale mantle heterogeneity in the SCS can be best explained by varying degrees of interaction between a mantle plume and stagnant slabs in the mantle transition zone, as suggested by mantle tomography. The rigid stagnant slab in the mantle transition zone beneath the SWB largely impeded the upwelling of the mantle plume, whereas the slab beneath the ESB was disrupted by the plume and subsequently transported into the upper mantle. This study, therefore, provides compelling evidence for the involvement of recycled oceanic slabs in generating MORB and the influence of an ascending mantle plume on seafloor spreading in a continental marginal sea. Mantle recycling processes in marginal sea basins exhibit distinct characteristics compared to those occurring in open oceans.

The Origin and Evolution of DMM-Like Lithospheric Mantle Beneath Continents: Mantle Xenoliths from the Oku Volcanic Group in the Cameroon Volcanic Line, West Africa

Abstract The lithospheric mantle as sampled by peridotite xenoliths in some continental settings resembles the source of mid-ocean ridge basalts (MORB). Whether this resemblance is a primary feature or the result of post-formation secondary processes remains controversial. Here, the age, origin and thermochemical evolution of fertile continental mantle are constrained based on the chemical composition of minerals in spinel-facies lherzolite and websterite xenoliths from the Wum maar and Befang cinder cone of the Oku Volcanic Group (Cameroon Volcanic Line, West Africa), combined with in-situ Sr isotope compositions of clinopyroxene and fabric investigation by Electron Backscatter Diffraction (EBSD). The majority of lherzolites (here assigned to Group I) consist of minerals with fertile composition (olivine Fo89, Al-rich pyroxenes, spinel Cr# 0.08–0.10). Clinopyroxene is LREE-depleted and has depleted 87Sr/86Sr (0.7017–0.7020). Crystal-preferred orientation determined by EBSD reveals that clinopyroxene, and sporadically both clino- and orthopyroxene, post-date the olivine framework. Subordinate Group II lherzolites also contain secondary clinopyroxene which is LREE-enriched and has higher 87Sr/86Sr (0.7033). In contrast, the scarce lherzolites of Group III are more refractory: they contain 72–78 vol.% olivine, Al-poor pyroxenes, and spinel with Cr# 0.18. Clinopyroxene (87Sr/86Sr 0.7021) is texturally coeval with olivine and orthopyroxene. Few lherzolites contain amphibole (87Sr/86Sr 0.7031) which post-dates the nominally anhydrous minerals. Most of the websterites (Group A) are aluminous (spinel Cr# 0.04–0.06) with LREE-depleted clinopyroxene having depleted 87Sr/86Sr ratios (0.7017–0.7020) similar to Group I lherzolites. Chemical characteristics of minerals coupled with the crystal-preferred orientation data suggests that Group I lherzolites originated in the spinel stability field by reactive intergranular percolation of an incompatible element-depleted MORB-like melt. Group A websterites likely formed as cumulates from that melt. The Group II lherzolites supposedly occur close to lithosphere-asthenosphere boundary and record interaction with lavas of the Cameroon Volcanic Line, whereas Group III lherzolites occur in the shallow part of the mantle profile and represent the protolith from which the Group I lherzolites were formed. Local crystallization of amphibole and concomitant recrystallization of the host lherzolite were driven by supply of water in an event post-dating the formation of LREE-depleted rejuvenated rocks. Migration of alkaline melts of the CVL apparently did not significantly affect the mineral and chemical composition of the lithospheric mantle, which allowed Group I lherzolites and Group A websterites to retain very low 87Rb/86Sr (average 0.002) and depleted 87Sr/86Sr ratios in clinopyroxene. This not only indicates their formation in the Paleoproterozoic (~2.0–2.25 Ga), possibly during the Eburnean orogeny at the margin of the Congo craton, but also indicates surprisingly little influence of the regionally recognized Pan-African event.

Zinc isotope fractionation during mid-ocean ridge basalt differentiation: Evidence from lavas on the East Pacific Rise at 10°30′N

Zinc (Zn) isotope data for mid-ocean ridge basalts (MORB) are scarce. The possible effects of MORB differentiation and mantle compositional heterogeneity on MORB Zn isotope compositions are poorly understood. In this paper, we report Zn isotope data of MORB lavas from the East Pacific Rise (EPR) at 10°30′N, which are understood to have been derived by a similar extent of melting from a compositionally uniform mantle source but manifest significantly varying extents of magma differentiation (MgO = 7.38–1.76 wt%) to demonstrate unequivocal Zn isotope fractionation during MORB differentiation. These lavas have a small δ66Zn (relative to the JMC-Lyon standard) range of 0.21–0.32‰, with an average of 0.26 ± 0.07‰ (2SD, N = 24), overlapping with literature data. However, δ66Zn values show significant correlations with indices of magma differentiation (e.g., MgO and CaO), with the more differentiated samples having isotopically heavier Zn. We can thus conclude that MORB differentiation can lead to ∼0.1‰ δ66Zn elevation, which is similar to that observed in Hawaiian Kilauea Iki lava lake samples. Fractional crystallization of olivine and clinopyroxene at MgO > ∼3.5 wt% and clinopyroxene and Fe–Ti oxides dominated by ilmenite at MgO < ∼3.5 wt% results in the observed Zn isotope fractionation. The δ66Zn values for primitive N-MORB and MORB source mantle are estimated to be ∼0.22‰ and ∼0.17‰, respectively. Furthermore, we observe a positive correlation between δ66Zn and [La/Sm]N in primitive (MgO = 7.58–9.01 wt%) literature MORB, indicating the existence of possible Zn isotope heterogeneity in the oceanic upper mantle. We thus predict heavy Zn isotope enrichment in mantle sources for E-MORB samples with high [La/Sm]N, which requires future testing.

RETRACTED: Fe3+ partitioning between clinopyroxene and silicate melt at 1–2.5 GPa: Implications for Fe3+ content of MORB and OIB source mantle

Pyroxene is the chief reservoir of Fe 3+ in upper mantle peridotite, but experiments exploring pyroxene/melt Fe 3+ partitioning ( D Fe3+ pyx/liq ) have been restricted to 100 kPa and pyroxene with low alumina. Here we present Fe 3+ partitioning experiments between clinopyroxenes (cpx) and mafic melt at elevated pressures (1–2.5 GPa). Experiments were conducted with f O2 buffered and modulated by Ru + RuO 2 and Fe-Pt alloy capsules, respectively, between ΔQFM −2.7 and +5.1. Fe 3+ /Fe T of both cpx and melt were determined by Fe K -edge X-ray absorption near edge structure spectroscopy. The experimentally synthesized cpx compositions (Al 2 O 3 = 2.36–6.01 wt.%, CaO = 19.33–22.21 wt.%) approximate those expected in basalt source regions. We find that Fe 3+ is moderately incompatible in cpx and D Fe3+ cpx/liq correlates with cpx Al 2 O 3 content, increasing from 0.05 ± 0.09 to 0.81 ± 0.04. D Fe3+ cpx/liq also increases with increasing f O2 . Comparison between experimentally synthesized cpx with those from natural peridotites indicates influences of both temperature and composition on Fe 3+ /Fe T for cpx in spinel and garnet peridotites. The combined effects of decreased pyroxene Al 2 O 3 concentration and pyroxene mode with progressive partial melting of peridotite diminishes the bulk partition coefficients of Fe 3+ , leading to greater Fe 2 O 3 contents in high degree partial melts, and this accounts for an inverse relationship between Na 2 O and Fe 2 O 3 observed in mid-ocean ridge basalts (MORB). Comparison to numerical experiments with pMELTS and the model of Jennings and Holland (2015) show that these models overpredict D Fe3+ cpx/liq for partial melting of the mantle, and so they do not accurately determine the relationship between the f O2 and Fe 2 O 3 of peridotite in basalt source regions. To estimate the Fe 3+ /Fe T ratio of the mantle source of MORB, we modeled liquid Fe 2 O 3 during isentropic batch melting of peridotite at three potential temperatures (1320 °C, 1400 °C, and 1440 °C) for peridotitic sources with Fe 3+ /Fe T ratios between 0.02–0.06. A source with an Fe 3+ /Fe T ratio of 0.038 ± 0.007 matches most of the span of natural MORB. This ratio is similar to that typical of continental lithospheric mantle sampled by xenoliths, but lower than that surmised by several recent experimental and thermodynamic studies. Considering this source Fe 3+ /Fe T but extending the partial melting calculations to higher pressures (2.5–4 GPa) reveals that bulk D Fe3+ perid/liq significantly decreases for garnet peridotite relative to spinel peridotite because the cpx become significantly less aluminous with increasing pressure. This results in high pressure partial melts with greater liquid Fe 3+ /Fe T ratios. Therefore, elevated Fe 3+ /Fe T ratios observed from some oceanic island basalts (OIB), such as those from Hawaii and Iceland, reflect in part the differences in conditions of melting and may not require mantle source regions more oxidized than those that produce MORB.

Mantle noble gas abundance ratios inferred from oceanic basalts and model estimates

We investigated mantle noble gas abundance ratios using a global compilation of data from mid-ocean ridge basalts (MORB) and high 3He/4He ocean island basalts (OIB), coupled with calculated abundances based on closed-system evolution models. Assuming that the mantle sources of MORB and high 3He/4He OIB are derived from the same early Earth parental material, variations in their noble gas abundance ratios are useful to ascertain which source is more depleted or degassed, or if the mantle noble gases might be a hybrid of different parental materials. Therefore, noble gas compositions of mantle sources can be a key to elucidating mantle evolution and structure. We calculated the noble gas abundance patterns using a Monte Carlo approach with validated end-members, which provides new estimates for the abundance ratios and their uncertainties. The adequacy of the calculated values was confirmed by existing data. We compiled all published noble gas data extracted by total heating of glassy oceanic basalts, which enables estimation of the pristine abundance patterns of noble gases including Kr for both sources of MORB and OIB. The pristine abundance ratios of the mantle sources for oceanic basalts are estimated by correcting measured values for atmospheric influences and elemental fractionation. The corrected abundance ratios resemble mantle values calculated using the proposed end-members for MORB and high 3He/4He OIB. The estimated relative abundance ratios in the high 3He/4He OIB source are similar to those of C1 chondrites in the range of He to Kr, supporting the hypothesis that a less degassed primitive OIB source can exist in the Earth's mantle. By contrast, the MORB source shows depletion of lighter noble gases, but with high relative helium content compared to the OIB source. The contrast in He/Ne abundance ratios between MORB and OIB, is a key constraint on mantle evolution models, andis expected to reflect several stages of diffusive fractionation during Earth history, or different initial mantle sources, which would require heterogeneous Earth accretion.

Mantle geodynamics and source domain of the East Vietnam Sea opening- induced volcanism in Vietnam and neighboring regions

The spreading of the East Vietnam Sea (EVS, also known as Bien Dong, or the South China Sea), leading to the occurrence of syn-spreading (33-16 Ma) and post-spreading (&lt; 16 to present) volcanism. Syn-spreading magma making up thick layers of tholeiitic basalt with a geochemical composition close to the refractory and depleted mid-ocean ridge basalt (MORB) is mainly distributed inside the EVS basin. The post-spreading magma is widely distributed inside the basin and extended to South and SE China, Hainan island, Southern Laos (Bolaven), Khorat Plateau (Thailand), and Vietnam, showing the typical intraplate geochemistry. Basaltic samples were collected at many places in Indochina countries, Vietnam’s coastal and continental shelf areas, to analyze for eruption age, petrographical, geochemical, and isotopic composition to understand the similarities and differences in the mantle sources between regions. The results reveal that basalts from some areas show geochemical features suggesting they were derived subsequently by spinel peridotite and garnet peridotite melting, forming high-Si, low-Mg, and low-Ti tholeiitic basalt to low-Si, high-Mg, and high-Ti alkaline basalt with the trace element enrichment increasing over time. Other basalts have geochemical and isotopic characteristics unchanged over a long period. The post-spreading basalt’s radiogenic Sr-Nd-Hf-Pb isotopic compositions show different regional basalts distribute in the various fields regardless of eruption age, suggesting that their mantle source feature is space-dependent. The post-EVS spreading basalts expose the regional heterogeneity, reflecting the mixture of at least three components, including a depleted mantle (DM) represented by the syn-EVS spreading source, similar to the DUPAL-bearing Indian MORB source; an enriched mantle type 1 (EM1), and type 2 (EM2). The DM may interact and acquire either EM1 or EM2 in the sub-continental lithospheric mantle; as a result, different eruption at different area acquires distinct isotopic signature, reflecting the heterogeneous nature of the subcontinental lithospheric mantle. The study proposes a suitable mantle dynamic model that explains the EVS spreading kinematics and induced volcanism following the India - Eurasian collision from the Eocene based on the research outcomes.

Molybdenum isotope systematics of lavas from the East Pacific Rise: Constraints on the source of enriched mid-ocean ridge basalt

Mid-ocean ridge basalts (MORB) reveal large mantle compositional heterogeneity, whose origin remains debated. Here we present a systematic study of molybdenum isotopes on well-characterized MORB glass samples from the East Pacific Rise (EPR) and near-EPR seamounts. Our analyses show significant Mo isotope variations with δ98/95Mo (relative to NIST SRM3134) ranging from −0.23‰ to −0.06‰. We argue that these Mo isotope variations are not caused by processes of MORB melt generation and evolution but reflect mantle isotopic heterogeneity. Taking together with the literature data, we show that MORB Mo isotope compositions vary systematically with geochemical parameters indicating mantle enrichment. These observations are best explained by two-component mixing between an incompatible element depleted endmember (e.g., low La/Sm, Nb/La, Nb/Zr and Th/Yb, and high Sm/Nd and 143Nd/144Nd) with low δ98/95Mo (∼−0.21‰) and an incompatible element enriched endmember (e.g., high La/Sm, Nb/La, Nb/Zr and Th/Yb, and low Sm/Nd and 143Nd/144Nd) with high δ98/95Mo (∼−0.05‰). The association of heavier Mo isotope compositions with the geochemically more enriched MORB is inconsistent with recycled ocean crust with or without sediment being the enriched endmember. Instead, this is consistent with the enriched endmember being of magmatic origin, most likely lithologies of low-degree melt metasomatic origin dispersed in the more depleted peridotite matrix in the MORB mantle. Thus, with MORB Mo isotope systematics, we confirm that recycled oceanic mantle lithosphere metasomatized by low degree melt plays a key role in the formation of E-MORB source lithologies. Our study also highlights Mo isotopes as an effective tool for studying upper mantle processes.

Geochemical characteristics of back-arc basin lower crust and upper mantle at final spreading stage of Shikoku Basin: an example of Mado Megamullion

This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

The Chicken and Egg Dilemma Linking Dunites and Chromitites in the Mantle–Crust Transition Zone beneath Oceanic Spreading Centres: a Case Study of Chromite-hosted Silicate Inclusions in Dunites Formed at the Top of a Mantle Diapir (Oman Ophiolite)

AbstractThe mantle–crust boundary beneath oceanic spreading centres is a major chemical and thermal interface on Earth. Observations in ophiolites reveal that it is underlined by a dunitic transition zone (DTZ) that can reach a few hundred meters in thickness and host abundant chromitite ore bodies. The dunites have been deciphered as essentially mantle-derived in most ophiolitic massifs; that is, reactional residues of interactions between peridotite and percolating melt(s). Although both dunite and chromitite in ophiolites have been the focus of many studies, the reasons for their systematic association remain unclear. In this study we have explored the inclusion content of the chromite grains disseminated in the dunites from the DTZ exposed in the Maqsad area of the Oman ophiolite where a former asthenospheric diapir is exposed. Similarly to chromite in chromitite ore bodies, disseminated chromite grains in dunites contain a great diversity of silicate inclusions. Based on the major and minor element composition of 1794 single silicate inclusions in chromites from 285 samples of dunite and associated rocks in the DTZ, we infer that the disseminated chromites formed by a similar ‘metallogenic’ process to the chromitites, and that, as a whole, dunites from the DTZ actually represent the low-grade end-member of a single, giant ore body. The nature of the silicate inclusions (amphibole and mica among others) enclosed in chromite grains in dunites from the Maqsad DTZ precludes their crystallization from an anhydrous primitive basaltic melt, and rather calls for a crystallization from a melt hybrid between common mafic melts and more exotic Si-, Na- and volatile-rich fluids. The hybrid parent medium of both dunites and chromitites results from the interaction between an asthenospheric diapir (the mid-ocean ridge basalt source), and a colder, altered lithospheric lid and hydrothermal fluids responsible for this alteration. The excess silica in the hybrid melt is provided by the incongruent dissolution of enstatite from mantle harzburgite and/or from moderate degree of partial melting of the altered gabbroic crust. The chemical composition of the silicate inclusions is more variable when enclosed in the disseminated chromites than in the chromitites, suggesting a greater variability of melt and/or fluid fractions involved in the genesis of dunites than of chromite ores. Finally, the DTZ can be viewed as a metamorphic contact aureole between episodically rising asthenospheric diapirs and formerly accreted axial lithospheric lids. Our conclusion about the chicken and egg dilemma linking dunites and chromitites beneath oceanic spreading centres (i.e. do the chromitites form in response to the formation of dunites or conversely?) is that the mantle dunitization itself is a potential process for the release of Cr and its re-concentration as chromite ores, and that in turn the competition between orthopyroxene (± plagioclase) and chromite fractionation during this fluid–melt–peridotite reaction process is responsible for the great mineralogical and chemical variability of the DTZ dunites.

Timing of Seafloor Spreading Cessation at the Macquarie Ridge Complex (SW Pacific) and Implications for Upper Mantle Heterogeneity

AbstractThe Macquarie Ridge Complex (MRC) on the Australia‐Pacific plate boundary south of New Zealand is an extinct mid‐ocean ridge that has experienced a complex tectonic history and produced highly heterogeneous mid‐ocean ridge basalts (MORBs). When and how seafloor spreading ceased along the proto‐Macquarie mid‐ocean ridge remain elusive, and it is unclear how the mantle source of MORBs is affected by the gradual cessation of seafloor spreading at mid‐ocean ridges. To constrain the tectonic evolution of the MRC, the mantle source variations for MORBs at dying mid‐ocean ridges, and the mechanisms of mantle enrichment and asthenospheric heterogeneities, we report 11 pyroxene, plagioclase, basaltic glass, groundmass, and sericite 40Ar/39Ar and one zircon U‐Pb ages for the MRC MORBs. Our data reveal that basalts from the MRC seamounts were erupted between 25.9 and 1.6 Ma and Macquarie Island at ∼10 Ma. Combined age and plate reconstruction results reveal that the cessation of seafloor spreading at the MRC generally propagated from south to north along the ridge. Basalts produced by the then dying Macquarie mid‐ocean ridge at different times on different seamounts/island show a large variation in isotopic compositions and there is no clear correlation between ages and isotopic ratios. The heterogeneity of mantle source for MORBs from the proto‐Macquarie mid‐ocean ridge suggests that the upper asthenospheric mantle is heterogeneous, and such heterogeneity becomes most obvious at dying mid‐ocean ridges where the degrees of partial melting are low and a large range of melt compositions are produced.

K/U of the MORB Source and Silicate Earth

AbstractPotassium (K) informs on the radiogenic heat production, atmospheric composition, and volatile element depletion of the Earth and other planetary systems. Constraints on the abundance of K in the Earth, Moon, and other rocky bodies have historically hinged on K/U values measured in planetary materials, particularly comparisons of the continental crust and mid‐ocean ridge basalts (MORBs), for developing compositional models of the bulk silicate Earth (BSE). However, a consensus on the most representative K/U value for global MORB remains elusive despite numerous studies. Here, we statistically analyze a critical compilation of MORB data to determine the K/U value of the MORB source. Covariations in the log‐normal abundances of K and U establish that K is 3–7 times less incompatible than U during melting and/or crystallization processes, enabling inverse modeling to infer the K/U of the MORB source region. These comprehensive data have a mean K/U for global MORB = 13,900 ± 200 (2σm; n = 4,646), and define a MORB source region with a K/U between 14,000 and 15,500, depending on the modeled melting regime. However, this range represents strictly a lower limit due to the undefined role of fractional crystallization in these samples and challenges preserving the signatures of depleted components in the MORB mantle source. This MORB source model, when combined with recent metadata analyses of ocean island basalt (OIB) and continental crust, suggests that the BSE has a K/U value &gt;12,100 and contains &gt;260 × 10−6 kg/kg K, resulting in a global production of ∼3.5 TW of radiogenic heat today and 1.5 × 1017 kg of 40Ar over the lifetime of the planet.

Predominantly recycled carbon in Earth's upper mantle revealed by He-CO2-Ba systematics in ultradepleted ocean ridge basalts

Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical components in Earth's upper mantle, and provide constraints on the proportions of mantle carbon that are juvenile (primordial) vs. tectonically recycled. Here we present new results for submarine basalt glasses, erupted within the Garrett Transform Fault (GTF) and to its north and south along the East Pacific Rise (EPR). Analyses of 3He/4He, and He and CO2 concentrations in vesicles and glass were performed, through a series of crushing and melting experiments plus FTIR spectroscopy. Trace elements were analyzed by laser ablation ICP-MS.The GTF basalts provide further tests for the origin of volatile-undersaturated basalts using CO2/Ba and CO2/Nb systematics. CO2 is highly correlated with Ba and Nb in basalts erupted in the transform domain (n = 13, including 5 undersaturated basalts) and does not show the variability expected from mixing between undegassed and variably degassed melts. Rather, the melts appear to originate from a heterogeneous mantle source that was variably depleted through partial melting, and limited mixing of melts is involved in their generation. The CO2/Ba and CO2/Nb weight ratios of 106 ± 8 and 308 ± 27, respectively, are similar to values determined previously for a global suite of undersaturated mid-ocean ridge basalts (MORBs).The ridge and transform domains have distinct 3He/4He ratios. Along the nearby EPR, 3He/4He = 8.5 - 9.1 RA, while within the Garrett Transform Fault 3He/4He = 9.2 - 10.1 RA. These two basalt populations are also distinct in their Pb-Sr-Nd isotope compositions based on earlier regional studies. The distinct populations result from partial melting of two different mantle source compositions. Melting of depleted mantle containing a small amount (∼1 to 5%) of enriched, ancient heterogeneities occurs beneath the EPR. Melting of ultradepleted mantle (in which the heterogeneities have been removed by earlier melting beneath the EPR) occurs beneath the GTF. This explains the distinction between intra-transform and spreading ridge domains for 3He/4He, if the heterogeneities were enriched in U, Th and He and had low 3He/4He as would be found in tectonically recycled material. The enriched mantle component sampled by the EPR basalts has molar CO2/3He =2×109, and it dominates the CO2/3He ratio generally ascribed to the upper mantle source for mid-ocean ridge basalts. In contrast, the ultradepleted MORB mantle component sampled by the GTF basalts has CO2/3He =3×108 or less. This indicates that most of the carbon in Earth's upper mantle originates from tectonic recycling.

Trace Element and Isotopic Evidence for Recycled Lithosphere from Basalts from 48 to 53°E, Southwest Indian Ridge

AbstractMantle source heterogeneity and magmatic processes have been widely studied beneath most parts of the Southwest Indian Ridge (SWIR). But less is known from the newly recovered mid-ocean ridge basalts from the Dragon Bone Amagmatic Segment (53°E, SWIR) and the adjacent magmatically robust Dragon Flag Segment. Fresh basalt glasses from the Dragon Bone Segment are clearly more enriched in isotopic composition than the adjacent Dragon Flag basalts, but the trace element ratios of the Dragon Flag basalts are more extreme compared with average mid-ocean ridge basalts (MORB) than the Dragon Bone basalts. Their geochemical differences can be explained only by source differences rather than by variations in degree of melting of a roughly similar source. The Dragon Flag basalts are influenced by an arc-like mantle component as evidenced by enrichment in fluid-mobile over fluid-immobile elements. However, the sub-ridge mantle at the Dragon Flag Segment is depleted in melt component compared with a normal MORB source owing to previous melting in the subarc. This fluid-metasomatized, subarc depleted mantle is entrained beneath the Dragon Flag Segment. In comparison, for the Dragon Bone axial basalts, their Pb isotopic compositions and their slight enrichment in Ba, Nb, Ta, K, La, Sr and Zr and depletion in Pb and Ti concentrations show resemblance to the Ejeda–Bekily dikes of Madagascar. Also, Dragon Bone Sr and Nd isotopic compositions together with the Ce/Pb, La/Nb and La/Th ratios can be modeled by mixing the most isotopically depleted Dragon Flag basalts with a composition within the range of the Ejeda–Bekily dikes. It is therefore proposed that the Dragon Bone axial basalts, similar to the Ejeda–Bekily dikes, are sourced from subcontinental lithospheric Archean mantle beneath Gondwana, pulled from beneath the Madagascar Plateau. The recycling of the residual subarc mantle and the subcontinental lithospheric mantle could be related to either the breakup of Gondwana or the formation and accretion of Neoproterozoic island arc terranes during the collapse of the Mozambique Ocean, and is now present beneath the ridge.

Identification of chondritic krypton and xenon in Yellowstone gases and the timing of terrestrial volatile accretion

Identifying the origin of noble gases in Earth's mantle can provide crucial constraints on the source and timing of volatile (C, N, H2O, noble gases, etc.) delivery to Earth. It remains unclear whether the early Earth was able to directly capture and retain volatiles throughout accretion or whether it accreted anhydrously and subsequently acquired volatiles through later additions of chondritic material. Here, we report high-precision noble gas isotopic data from volcanic gases emanating from, in and around, the Yellowstone caldera (Wyoming, United States). We show that the He and Ne isotopic and elemental signatures of the Yellowstone gas requires an input from an undegassed mantle plume. Coupled with the distinct ratio of 129Xe to primordial Xe isotopes in Yellowstone compared with mid-ocean ridge basalt (MORB) samples, this confirms that the deep plume and shallow MORB mantles have remained distinct from one another for the majority of Earth's history. Krypton and xenon isotopes in the Yellowstone mantle plume are found to be chondritic in origin, similar to the MORB source mantle. This is in contrast with the origin of neon in the mantle, which exhibits an isotopic dichotomy between solar plume and chondritic MORB mantle sources. The co-occurrence of solar and chondritic noble gases in the deep mantle is thought to reflect the heterogeneous nature of Earth's volatile accretion during the lifetime of the protosolar nebula. It notably implies that the Earth was able to retain its chondritic volatiles since its earliest stages of accretion, and not only through late additions.

The bromine and chlorine isotopic composition of the mantle as revealed by deep geothermal fluids

Chlorine (Cl) and bromine (Br) are rare elements when considering the whole Earth. However, being highly volatile elements, their delivery and retention processes during planetary accretion and chemical differentiation provide important clues about the formation of the Earth. Variations in Cl and Br isotopic systems (37Cl/35Cl or δ37Cl and 81Br/79Br or δ81Br) among terrestrial reservoirs could trace these processes. While the Br isotopic value of the mantle remains entirely unknown, the measurement of mantle Cl isotopic values is a controversial subject, with measured δ37Cl values ranging from −3 to 0‰ in mid-ocean-ridge basalts (MORBs) and from −2 to +3‰ in oceanic island basalts (OIBs). Here, we report newly-determined δ81Br and δ37Cl values, together with noble gas 3He/4He (R) ratios, measured in geothermal fluids from production wells of three Mexican fields: Cerro Prieto, Las Tres Vírgenes, and Los Azufres. Relationships between 3He/4He ratios and both δ37Cl and δ81Br suggest that geothermal fluid volatiles have three distinct sources: (1) a local crustal source, enriched in radiogenic 4He (R = 1.7–1.9Ra, where Ra is the atmospheric 3He/4He ratio), and halogens from brines with δ37Cl and δ81Br of +0.1 and +0.3‰ respectively; (2) the mantle wedge, with 3He/4He ratios of 6–6.5Ra, typical of arc volcanism, and δ37Cl and δ81Br of −0.4 and −1.0‰ respectively, typical (for Cl) of fluids derived from the dehydration of serpentinite in the subducting slab; and (3) a mantle source, with 3He/4He ratios of 7.7–8.2Ra, typical of MORBs, and δ37Cl and δ81Br of +0.9 and +0.7‰ respectively. These results suggest that the primitive mantle Cl isotopic composition was positive – possibly ≥+3‰, as measured in some OIBs – and inherited during the Moon forming impact. The progressive subduction of isotopically lighter halogens over the last 2–3 Ga could have progressively lowered this initial value to those currently measured in the depleted mantle beneath Mexico. It is speculated that the different isotopic values measured in mantle rocks and fluids could reflect the heterogeneous regassing of subducted halogens and the inefficient homogenization of recycled material in the MORB source, as suggested in other studies by the heterogeneous isotopic compositions of the heavier Ar and Xe of the convective mantle.

Ubiquitous ultra-depleted domains in Earth’s mantle

Partial melting of Earth’s mantle generates oceanic crust and leaves behind a chemically depleted residual mantle. The time-integrated composition of this chemically depleted mantle is generally inferred from basalts produced at mid-ocean ridges. However, isotopic differences between oceanic mantle rocks and mid-ocean ridge basalts suggest that mantle and basalt composition could differ. Here we measure neodymium isotope ratios in olivine-hosted melt inclusions from lavas of the Azores mantle plume. We find neodymium isotope ratios that include the highest values measured in basalts, and suggest that melts from ultra-depleted mantle contribute to the isotopic diversity of the erupted lavas. Ultra-depleted melts have exceedingly low preservation potential during magma extraction and evolution due to progressive mixing with melts that are enriched in incompatible elements. A notable contribution of ultra-depleted melts to the Azores mantle plume therefore implies that variably depleted mantle is the volumetrically dominant component of the Azores plume. We argue that variably depleted mantle, sometimes ranging to ultra-depleted compositions, may be a ubiquitous part of most ocean island and mid-ocean ridge basalt sources. If so, Earth’s mantle may be more depleted than previously thought, which has important implications for the rate of mass exchange between crust and mantle, plume dynamics and compositional stratification of Earth’s mantle. Depleted mantle is a volumetrically dominant component of the Azores plume and possibly of oceanic basalt sources more generally, according to neodymium isotope compositions of olivine-hosted melt inclusions from lavas of the Azores mantle plume.

Oxygen fugacity at the base of the Talkeetna arc, Alaska

The origin of the more oxidized nature of arc magmas as compared to that of mid-ocean ridge basalts (MORB) is debated, considered to be either a feature of their mantle source, or produced during crustal transit and eruption. Fe3+/FeT ratios (Fe3+/[Fe3+ + Fe2+]) in arc volcanic rocks and glasses and thermodynamic oxybarometry on mantle xenoliths from arc lavas indicate elevated magmatic oxygen fugacity ( $$f_{{{\text{O}}_{ 2} }}$$ ), whereas, redox-sensitive trace elements ratios and abundances in arc volcanic rocks have been used to suggest that arcs have source regions with $$f_{{{\text{O}}_{ 2} }}$$ similar to the MORB source. Here, we take an alternative approach by calculating the $$f_{{{\text{O}}_{ 2} }}$$ of the uppermost mantle and lowermost ultramafic cumulates from the accreted Jurassic Talkeetna arc (Alaska). This approach allows us to quantify the $$f_{{{\text{O}}_{ 2} }}$$ of the sub-arc mantle and of primary arc magmas crystallizing at the base of an island arc, which have not been affected by processes during crustal transit and eruption which could affect their $$f_{{{\text{O}}_{ 2} }}$$ . Implementing olivine–spinel oxybarometry, we find that the upper mantle (harzburgites and lherzolites) and ultramafic cumulates (clinopyroxenites and dunites) crystallized between + 0.4 and + 2.3 log units above the fayalite-magnetite-quartz buffer, consistent with previous studies suggesting that the sub-arc mantle is oxidized relative to that of MORB. In addition, the Talkeetna paleo-arc allows us to examine coeval lavas and their redox-sensitive trace element ratios (e.g., V/Sc). The average V/Sc ratios of high MgO (> 6 wt%) lavas are 6.7 ± 1.6 (2σ), similar to that of MORB. However, V/Sc ratios must be interpretted in terms the degree of partial melting, as well as, the initial V/Sc ratio of the mantle source in order to derive information about $$f_{{{\text{O}}_{ 2} }}$$ of their mantle source. The V/Sc ratios of Talkeetna lavas are consistent with the elevated $$f_{{{\text{O}}_{ 2} }}$$ recorded in the sub-arc mantle and primitive cumulates (olivine Mg# [Mg/(Mg + Fe)] × 100 > 82) if a depleted mantle source underwent 15–20% melting. Our results suggest that the arc mantle is, on average, more oxidized than the MORB source and that V/Sc ratios must be interpreted in the context of a partial melting model where all model parameters are appropriate for arc magma genesis. This study reconciles V/Sc ratios in arc volcanic rocks with $$f_{{{\text{O}}_{ 2} }}$$ of primary arc basalts and the sub-arc mantle from the same locality.

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.