More Like this

1. Volatiles and Redox Along the East African Rift

   https://doi.org/10.1029/2024GC011657  

   Brounce, Maryjo; Scoggins, Sara; Fischer, Tobias P; Ford, Heather; Byrnes, Joseph (August 2024, Geochemistry, Geophysics, Geosystems)

   Dixon, Jackie (Ed.)

   Abstract The upper mantle under the Afar Depression in the East African Rift displays some of the slowest seismic wave speeds observed globally. Despite the extreme nature of the geophysical anomaly, lavas that erupted along the East African Rift record modest thermal anomalies. We present measurements of major elements, H₂O, S, and CO₂, and Fe³⁺/ΣFe and S⁶⁺/ΣS in submarine glasses from the Gulf of Aden seafloor spreading center and olivine‐, plagioclase‐, and pyroxene‐hosted melt inclusions from Erta Ale volcano in the Afar Depression. We combine these measurements with literature data to place constraints on the temperature, H₂O, andfO₂of the mantle sources of these lavas as well as the initial and final pressures of melting. The Afar mantle plume is C/FOZO/PHEM in isotopic composition, and we suggest that this mantle component is damp, with 852 ± 167 ppm H₂O, not elevated infO₂compared to the depleted MORB mantle, and has temperatures of ∼1401–1458°C. This is similar infO₂and H₂O to the estimates of C/FOZO/PHEM in other locations. Using the moderate H₂O contents of the mantle together with the moderate thermal anomaly, we find that melting begins at around 93 km depth and ceases at around 63 km depth under the Afar Depression and at around 37 km depth under the Gulf of Aden, and that ∼1%–29% partial melts of the mantle can be generated under these conditions. We speculate that the presence of melt, and not elevated temperatures or high H₂O contents, are the cause for the prominent geophysical anomaly observed in this region. 

   more » « less
2. Pacific Lithosphere Evolution Inferred from Aitutaki Mantle Xenoliths

   https://doi.org/10.1093/petrology/egz047  

   Snortum, Eric; Day, James_M D; Jackson, Matthew G (September 2019, Journal of Petrology)

   Abstract Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), major and trace element abundances, and 187Re–187Os systematics are reported for xenoliths and lavas from Aitutaki (Cook Islands), to investigate the composition of Pacific lithosphere. The xenolith suite comprises spinel-bearing lherzolites, dunite, and harzburgite, along with olivine websterite and pyroxenite. The xenoliths are hosted within nephelinite and alkali basalt volcanic rocks (187Os/188Os ∼0·1363 ± 13; 2SD; ΣHSE = 3–4 ppb). The volcanic host rocks are low-degree (2–5%) partial melts from the garnet stability field and an enriched mantle (EM) source. Pyroxenites have similar HSE abundances and Os isotope compositions (Al2O3 = 5·7–8·3 wt %; ΣHSE = 2–4 ppb; 187Os/187Os = 0·1263–0·1469) to the lavas. The pyroxenite and olivine websterite xenoliths directly formed from—or experienced extensive melt–rock interaction with—melts similar in composition to the volcanic rocks that host the xenoliths. Conversely, the Aitutaki lherzolites, harzburgites and dunites are similar in composition to abyssal peridotites with respect to their 187Os/188Os ratios (0·1264 ± 82), total HSE abundances (ΣHSE = 8–28 ppb) and major element abundances, forsterite contents (Fo89·9±1·2), and estimated extents of melt depletion (<10 to >15%). These peridotites are interpreted to sample relatively shallow Pacific mantle lithosphere that experienced limited melt–rock reaction and melting during ridge processes at ∼90 Ma. A survey of maximum time of rhenium depletion ages of Pacific mantle lithosphere from the Cook (Aitutaki ∼1·5 Ga), Austral (Tubuai’i ∼1·8 Ga), Samoan (Savai’i ∼1·5 Ga) and Hawaiian (Oa’hu ∼2 Ga) island groups shows that Mesoproterozoic to Neoproterozoic depletion ages are preserved in the xenolith suites. The variable timing and extent of mantle depletion preserved by the peridotites is, in some instances, superimposed by extensive and recent melt depletion as well as melt refertilization. Collectively, Pacific Ocean island mantle xenolith suites have similar distributions and variations of 187Os/188Os and HSE abundances to global abyssal peridotites. These observations indicate that Pacific mantle lithosphere is typical of oceanic lithosphere in general, and that this lithosphere is composed of peridotites that have experienced both recent melt depletion at ridges and prior and sometimes extensive melt depletion across several Wilson cycles spanning periods in excess of two billion years. 

   more » « less
3. Effects of H2O–CO2 Fluids, Temperature, and Peridotite Fertility on Partial Melting in Mantle Wedges and Generation of Primary Arc Basalts

   https://doi.org/10.1093/petrology/egad047  

   Lara, Michael; Dasgupta, Rajdeep (July 2023, Journal of Petrology)

   Abstract Many lines of evidence from high P–T experiments, thermodynamic models, and natural observations suggest that slab-derived aqueous fluids, which flux mantle wedges contain variable amounts of dissolved carbon. However, constraints on the effects of H2O–CO2 fluids on mantle melting, particularly at mantle wedge P–T conditions, are limited. Here, we present new piston cylinder experiments on fertile and depleted peridotite compositions with 3.5 wt.% H2O and XCO2 [= molar CO2 / (CO2 + H2O)] of 0.04–0.17. Experiments were performed at 2–3 GPa and 1350°C to assess how temperature, peridotite fertility, and XCO2 of slab-derived fluid affects partial melting in mantle wedges. All experiments produce olivine + orthopyroxene +7 to 41 wt.% partial melt. Our new data, along with previous lower temperature data, show that as mantle wedge temperature increases, primary melts become richer in SiO2, FeO*, and MgO and poorer CaO, Al2O3, and alkalis when influenced by H2O–CO2 fluids. At constant P–T and bulk H2O content, the extent of melting in the mantle wedge is largely controlled by peridotite fertility and XCO2 of slab-fluid. High XCO2 depleted compositions generate ~7 wt.% melt, whereas, at identical P–T, low XCO2 fertile compositions generate ~30 to 40 wt.% melt. Additionally, peridotite fertility and XCO2 have significant effects on peridotite partial melt compositions. At a constant P–T–XCO2, fertile peridotites generate melts richer in CaO and Al2O3 and poorer in SiO2, MgO + FeO, and alkalis. Similar to previous experimental studies, at a constant P–T fertility condition, as XCO2 increases, SiO2 and CaO of melts systematically decrease and increase, respectively. Such distinctive effects of oxidized form of dissolved carbon on peridotite partial melt compositions are not observed if the carbon-bearing fluid is reduced, such as CH4-bearing. Considering the large effect of XCO2 on melt SiO2 and CaO concentrations and the relatively oxidized nature of arc magmas, we compare the SiO2/CaO of our experimental melts and melts from previous peridotite + H2O ± CO2 studies to the SiO2/CaO systematics of primitive arc basalts and ultra-calcic, silica-undersaturated arc melt inclusions. From this comparison, we demonstrate that across most P–T–fertility conditions predicted for mantle wedges, partial melts from bulk compositions with XCO2 ≥ 0.11 have lower SiO2/CaO than all primitive arc melts found globally, even when correcting for olivine fractionation, whereas partial melts from bulk compositions with XCO2 = 0.04 overlap the lower end of the SiO2/CaO field defined by natural data. These results suggest that the upper XCO2 limit of slab-fluids influencing primary arc magma formation is 0.04 < XCO2 < 0.11, and this upper limit is likely to apply globally. Lastly, we show that the anomalous SiO2/CaO and CaO/Al2O3 signatures observed in ultra-calcic arc melt inclusions can be reproduced by partial melting of either CO2-bearing hydrous fertile and depleted peridotites with 0 < XCO2 < 0.11 at 2–3 GPa, or from nominally CO2-free hydrous fertile peridotites at P > 3 GPa. 

   more » « less
4. VapoRock: Thermodynamics of Vaporized Silicate Melts for Modeling Volcanic Outgassing and Magma Ocean Atmospheres

   https://doi.org/10.3847/1538-4357/acbcc7  

   Wolf, Aaron S.; Jäggi, Noah; Sossi, Paolo A.; Bower, Dan J. (April 2023, The Astrophysical Journal)

   Abstract Silicate vapors play a key role in planetary evolution, especially dominating early stages of rocky planet formation through outgassed magma ocean atmospheres. Our open-source thermodynamic modeling software “VapoRock” combines the MELTS liquid model with gas-species properties from multiple thermochemistry tables. VapoRock calculates the partial pressures of 34 gaseous species in equilibrium with magmatic liquid in the system Si–Mg–Fe–Al–Ca–Na–K–Ti–Cr–O at desired temperatures and oxygen fugacities (fO₂, or partial pressure of O₂). Comparison with experiments shows that pressures and melt-oxide activities (which vary over many orders of magnitude) are reproduced to within a factor of ∼3, consistent with measurement uncertainties. We also benchmark the model against a wide selection of igneous rock compositions including bulk silicate Earth, predicting elemental vapor abundances that are comparable to (Na, Ca, and Al) or more realistic than (K, Si, Mg, Fe, and Ti) those of the closed-source MAGMA code (with maximum deviations by factors of 10–300 for K and Si). Vapor abundances depend critically on the activities of liquid components. The MELTS model underpinning VapoRock was calibrated and extensively tested on natural igneous liquids. In contrast, MAGMA’s liquid model assumes ideal mixtures of a limited set of chemically simplified pseudospecies, which only roughly approximates the nonideal compositional interactions typical of many-component natural silicate melts. Finally, we explore how relative abundances of SiO and SiO₂provide a spectroscopically measurable proxy for oxygen fugacity in devolatilized exoplanetary atmospheres, potentially constrainingfO₂in outgassed exoplanetary mantles. 

   more » « less
5. Preserved Nd-Hf-Os isotope variability in replacive channels from the Lanzo ophiolite: Traces of incomplete melt aggregation in the shallow mantle

   A. Sanfilippo; C.Z. Liu; V. Salters; A. Mosconi; A. Zanetti; R. Tribuzio (December 2023, Chemical geology)

   N/A (Ed.)

   Long-lived radiogenic isotopes of abyssal peridotites, residues of MORB extraction, show that the asthenosphere is intrinsically heterogeneous, which is inherited from ancient melting events and crustal recycling during Earth's history. Yet, Mid Ocean Ridge Basalts (MORB) have a rather uniform average composition, suggesting that the variability of their mantle source is concealed during their ascent. Here we document that mantle heterogeneity is exceptionally well preserved in high permeability mantle conduits from the Lanzo South mantle massif, Western Italian Alps. Nd-Hf-Os isotopes of decametre-scale replacive bodies provide evidence for the existence of two generations of mantle channels. The first generation consists of dunites concordant to the main foliation of host peridotites. The replacive dunites include clinopyroxene with MORB-like incompatible element signature and initial (160 Ma) ƐNd and ƐHf ranging from +4 to +7 and from +10 to +15, respectively. The second generation, made up of pyroxene-poor harzburgites discordant to the main foliation, is geochemically depleted in incompatible elements and its clinopyroxene displays highly radiogenic Hf isotopes (initial ƐHf up to +202). The mantle channel heterogeneity is confirmed by Resingle bondOs isotopes and platinum-groups elements. The MORB-type dunites have high Pt, Pd and, locally, Re, and have 187Os/188Os ratios similar to the host peridotite (0.122–0.128). On the other hand, the depleted bodies have lower Pt, Pd and Re, and 187Os/188Os ratios ranging from those of host peridotites (0.124) to highly unradiogenic values (0.118) in the most refractory sample. The preserved heterogeneity in trace elements, PGE, and Nd-Hf-Os isotopes highlights infiltration of melts from a highly heterogeneous mantle, still partially preserved within these mantle bodies. If applied to present-day Mid Ocean Ridges, our model indicates that the isotopic variability of melts migrating through replacive mantle conduits is by far larger than magmas erupted on the seafloor, which implies that diverse mantle components are mainly delivered and homogenised above the crust-mantle boundary. 

   more » « less