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1. 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. 

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2. Highly siderophile element evidence for mantle plume involvement during opening of the Atlantic Ocean

   https://doi.org/10.1016/j.epsl.2024.118768  

   Day, James MD; Woodland, Sarah J; Nutt, Kimberley L; Stroncik, Nicole; Larsen, Lotte M; Trumbull, Robert B; Pearson, D Graham (September 2024, Earth and Planetary Science Letters)

   Osmium isotope and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data are reported for picrites and basalts from the ∼132 Ma Etendeka large igneous province (LIP) and the ∼60 Ma North Atlantic Igneous Province (NAIP). Picrite dykes of the Etendeka LIP have HSE abundances and 187Os/188Os (0.1276 to 0.1323; γOsi = -0.5 to +3.1) consistent with derivation from high-degree partial melting (>20 %) of a peridotite source with chondritic to modestly supra-chondritic long-term Re/Os. High-3He/4He NAIP picrites from West Greenland represent large-degree partial melts with similarly elevated HSE abundances and 187Os/188Os (0.1273 to 0.1332; γOsi = -0.2 to +3.9). Broadly chondritic Os isotope ratios have also been reported for the ∼132 Ma Paraná LIP and the ∼201 Ma Central Atlantic Magmatic Province (CAMP). Consequently, LIP associated with Atlantic Ocean opening derive, at least in part, from partial melting of peridotite mantle distinct from the depleted mantle associated with mid-ocean ridge basalt volcanism. Modern locations with high-3He/4He (>25RA) include ocean island basalts (OIB) from Ofu (Samoa), Loihi (Hawaii) and Fernandina (Galapagos) in the Pacific Ocean, and from Iceland, which is considered the modern manifestation of NAIP magmatism. Unlike Etendeka and NAIP picrites, these modern OIB have Sr-Nd-Pb-Os isotopes consistent with contributions of recycled oceanic or continental crust. The lower degree of partial melting responsible for modern high-3He/4He OIB gives higher proportions of fusible recycled crustal components to the magmas, with radiogenic 187Os/188Os and low-3He/4He. The high-3He/4He, incompatible trace element-depleted mantle component in both LIP and OIB therefore also has long-term chondritic Re/Os, which is consistent with an early-formed reservoir that experienced late accretion. Atlantic LIP (CAMP; Paraná-Etendeka; NAIP) provide geochemical evidence for a prominent role for mantle plume contributions during continental break-up and formation of the Atlantic Ocean, a feature hitherto unrecognized in other ocean basin-forming events. 

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3. 182W and 187Os constraints on the origin of siderophile isotopic heterogeneity in the mantle

   https://doi.org/10.1016/j.gca.2023.11.003  

   Walker, Richard J.; Mundl-Petermeier, Andrea; Puchtel, Igor S.; Nicklas, Robert W.; Hellmann, Jan L.; Echeverría, Lina M.; Ludwig, Kyle D.; Bermingham, Katherine R.; Gazel, Esteban; Devitre, Charlotte L.; et al (December 2023, Geochimica et Cosmochimica Acta)

   Major and trace element abundances, including highly siderophile elements, and 187Os and 182W isotopic compositions were determined for ca. 89 Ma mafic and ultramafic rocks from the islands of Gorgona (Colombia) and Curaçao (Dutch Caribbean). The volcanic systems of both islands were likely associated with a mantle plume that generated the Caribbean Large Igneous Provence. The major and lithophile trace element characteristics of the rocks examined are consistent with the results of prior studies, and indicate derivation from both a chemically highly-depleted mantle component, and an enriched, or less highly-depleted mantle component. Highly siderophile element abundances for these rocks are generally similar to rocks with comparable MgO globally, indicating that the major source components were not substantially enriched or depleted in these elements. Rhenium-Os isotopic systematics of most rocks of both islands indicate derivation from a mantle source with an initial 187Os/188Os ratio between that of the contemporaneous average depleted mid-ocean ridge mantle and bulk silicate Earth. The composition may reflect either an average lower mantle signature, or global-scale Os isotopic heterogeneity in the upper mantle. Some of the basalts, as well as two of the komatiites, are characterized by calculated initial 187Os/188Os ratios 10-15% higher than the chondritic reference. These more radiogenic Os isotopic compositions do not correlate with major or trace element systematics, and indicate a mantle source component that was most likely produced by either sulfide metasomatism or ancient Re/Os fractionation. Tungsten-182 isotopic compositions measured for rocks from both islands are characterized by variable 182W values ranging from modern bulk silicate Earth-like to strongly negative values. The 182W values do not correlate with major/trace element abundances or initial 187Os/188Os compositions. As with some modern ocean island basalt systems, however, the lowest 182W value (-53) measured, for a Gorgona olivine gabbro, corresponds with the highest 3He/4He previously measured from the suite (15.8 R/RA). Given the lack of correlation with other chemical/isotopic compositions, the mantle component characterized by negative 182W and possibly high 3He/4He is most parsimoniously explained to have formed as a result of isotopic equilibration between the mantle and core at the core-mantle boundary. 

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4. Ancient Melt-Depletion in Fresh to Strongly Serpentinized Tonga Trench Peridotites

   https://doi.org/10.1093/petrology/egab088  

   Day, James_M D; Brown, Diana B (December 2021, Journal of Petrology)

   ABSTRACT Peridotites from the Tonga Trench are some of the deepest-derived and freshest ever obtained from the seafloor. This study reports new bulk-rock major-, trace-, highly siderophile-element (HSE) abundance and 187Os/188Os data, as well as major- and trace-element abundances of mineral phases for NOVA88D dredge peridotites. The samples are harzburgites that experienced varying degrees of serpentinization, recorded in their loss on ignition (LOI) values, from zero to 16.7%. Degree of serpentinization in samples is correlated with Na, B, K, Sr, Ca, Rb and U, and weakly correlated with W, Fe, Pb, Cs and Li abundances, but is uncorrelated with other lithophile elements, most especially the rare earth elements (REE). Serpentinization had no systematic effect on the HSE abundances or 187Os/188Os compositions in the harzburgites. NOVA88D harzburgites record >18% melt depletion which has resulted in heterogenous distribution of the HSE within the rocks, likely due to retention of these elements within sub-micron sized alloy or sulphide phases. Time of rhenium depletion (TRD) ages, recorded by Os isotopes, average ~ 0.7 ± 0.4 Ga and can be as ancient as 1.5 Ga. Some harzburgite compositions are consistent with minor melt infiltration processes modifying incompatible trace element compositions and Re abundances, with a possible melt infiltration event at ~120 Ma based on 187Re-188Os, prior to the inception of subduction at the Tonga Trench at ~52 Ma. Evidence for ancient melt depletion, combined with limited melt processing since inception of subduction suggests that NOVA88D harzburgites represent melt residues incorporated into the Tonga arc, rather than their geochemical signatures being produced beneath the recent arc. Estimates of fO2 (~ − 0.4 ± 0.4 ΔFMQ) and olivine-spinel equilibration temperatures for the Tonga Trench samples (830 ± 120 ̊C) are similar to abyssal peridotites and some Izu-Mariana-Bonin peridotites. These values are unlikely to relate directly to recorded degrees of melt depletion and melt depletion ages in the rocks. Refractory residues from prior melt depletion events are probably common in the convecting mantle, and those with high degrees of melt depletion (>18%) and relatively ancient melt depletion ages (<2 Ga) are likely to have been formed during prior melting processes rather than melting processes within their current tectonic setting. These refractory peridotites can be incorporated into a range of tectonic settings, including into mid-ocean ridges, succeeding arcs, or within the continental lithospheric mantle, where they may play a limited role in melt generation processes. 

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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. 

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