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.
more »
« less
Heavy iron in large gem diamonds traces deep subduction of serpentinized ocean floor
Subducting tectonic plates carry water and other surficial components into Earth’s interior. Previous studies suggest that serpentinized peridotite is a key part of deep recycling, but this geochemical pathway has not been directly traced. Here, we report Fe-Ni–rich metallic inclusions in sublithospheric diamonds from a depth of 360 to 750 km with isotopically heavy iron (δ 56 Fe = 0.79 to 0.90‰) and unradiogenic osmium ( 187 Os/ 188 Os = 0.111). These iron values lie outside the range of known mantle compositions or expected reaction products at depth. This signature represents subducted iron from magnetite and/or Fe-Ni alloys precipitated during serpentinization of oceanic peridotite, a lithology known to carry unradiogenic osmium inherited from prior convection and melt depletion. These diamond-hosted inclusions trace serpentinite subduction into the mantle transition zone. We propose that iron-rich phases from serpentinite contribute a labile heavy iron component to the heterogeneous convecting mantle eventually sampled by oceanic basalts.
more »
« less
- Award ID(s):
- 2025779
- PAR ID:
- 10222052
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 7
- Issue:
- 14
- ISSN:
- 2375-2548
- Page Range / eLocation ID:
- eabe9773
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract Nitrogen is considered to be transported from Earth′s surface to the top of the lower mantle through subduction. However, little is known on the transportation and fate of subducted nitrogen to the Earth′s interior during slab‐mantle interactions. In this study, the stability of subducted sedimentary nitrogen in the reduced mantle was investigated to 35 GPa and 1600 K by laser‐heated diamond anvil cell experiments and first‐principles calculations. Our results showed that subducted nitrogen‐bearing silicates and fluids could not coexist with the metallic iron or iron‐rich alloys, and reacted with them to form different products at high pressure‐temperature conditions. Combining our results with previous data, we re‐determined the relative stability of iron‐light element binary compounds to 35 GPa and 1600 K to be Fe‐O > Fe‐N > Fe‐S > Fe‐C. This stability sequence contributes to explaining the observation that iron nitrides are trapped as inclusions in sulfur‐depleted lower‐mantle diamonds and are absent in sulfur‐rich ones. The recycling efficiency of subducted sedimentary nitrogen is strongly related to the availability of the metallic iron of the reduced mantle. Hydration of the metallic iron limits the storage of nitrogen in it and contributes to recycling nitrogen to Earth′s surface. Therefore, unlike subducted continental sediments, subducted marine sediments are unlikely to transport a large amount of surficial nitrogen to the metallic iron of the reduced mantle in which nitrogen could reside over long geologic periods.
-
The mantle section of the Late Neoproterozoic Tays ophiolite in the Arabian Shield consists principally of thoroughly serpentinized peridotite with characteristics typical of depleted mantle protoliths from a fore-arc environment. The serpentinite is altered along shear zones and thrust planes to gold-bearing listvenite bodies of various sizes. These bodies are divided into carbonate listvenite and silica‐carbonate listvenite; they may be dyke-like or lenticular in form, and are yellowish-brown, reddish-brown, or greyish in outcrop. Carbonate list- venite expresses schistose deformation fabrics concordant to fabric in the host serpentinite, whereas silica‐car- bonate listvenite is undeformed at field scale and contains a generation of undeformed minerals at thin-section scale. Silica‐carbonate listvenite contains Cr-rich muscovite (fuchsite) and base-metal sulfides and is enriched in Zn, Pb, Cu, Ag, and Au along with SiO2. The transformation of serpentinite along shear zones to different types of listvenite reflects successive episodes of fluid-mediated metasomatism. Carbonate listvenite develops first, driven by infiltration of CO2–bearing fluids during serpentinization of the original fore-arc peridotite. Silica‐carbonate listvenite marks a later episode associated with infiltration of K-bearing, SiO2-saturated fluids released during emplacement of the ophiolite. Listvenitization in the Tays serpentinite concentrated gold in sub-economic to economic extents, with concentrations increasing from host serpentinite (2–4 ng/g) to carbonate listvenite (267–937 ng/g) to silica‐carbonate listvenite (1717–3324 ng/g).more » « less
-
more » « less
Abstract Continental formation models invoke subduction or plume‐related processes to create the buoyant, refractory character of continental lithospheric mantle (CLM). From similarities in melt depletion, major element composition, modal clinopyroxene, and Os isotope systematics it has been proposed that oceanic mantle lithosphere is the likely protolith to non‐cratonic CLM, however, a direct link between the two has been difficult to ascertain. Using dredged mantle peridotite xenoliths from the Ferrel Seamount, off the west coast of Baja California, Mexico, we show that tectonic isolation of an oceanic plate may lead to formation of non‐cratonic CLM. Ferrel xenoliths are coarse‐grained spinel lherzolite, or rare harzburgite. Bulk‐rock and clinopyroxene trace element compositions reveal two‐stages of melt refertilization following melt depletion, with infiltration by mid‐ocean ridge basalt‐type melts, followed by melt addition from host alkali basalt. Melt depletion correlations with187Os/188Os and highly siderophile element abundances indicate preserved melt depletion and refertilization processes are ancient. From these observations, the Ferrel xenoliths represent lithosphere from the abandoned Pacific‐Farallon ridge. The history of melt depletion, followed by MORB‐melt refertilization is consistent with the peridotites representing oceanic mantle lithosphere that was subsequently incorporated into the Baja‐Guadalupe microplate during “ridge jump.” These peridotites demonstrate that isolation of oceanic lithosphere that is rafted onto a continental margin provides a viable means for producing non‐cratonic CLM. We suggest that continuation of late‐stage, low degree melt refertilization may provide a link between oceanic lithosphere and non‐cratonic CLM and propose a tectonic model to preserve and facilitate this continued evolution.
-
The conditions of methane (CH 4 ) formation in olivine-hosted secondary fluid inclusions and their prevalence in peridotite and gabbroic rocks from a wide range of geological settings were assessed using confocal Raman spectroscopy, optical and scanning electron microscopy, electron microprobe analysis, and thermodynamic modeling. Detailed examination of 160 samples from ultraslow- to fast-spreading midocean ridges, subduction zones, and ophiolites revealed that hydrogen (H 2 ) and CH 4 formation linked to serpentinization within olivine-hosted secondary fluid inclusions is a widespread process. Fluid inclusion contents are dominated by serpentine, brucite, and magnetite, as well as CH 4( g ) and H 2( g ) in varying proportions, consistent with serpentinization under strongly reducing, closed-system conditions. Thermodynamic constraints indicate that aqueous fluids entering the upper mantle or lower oceanic crust are trapped in olivine as secondary fluid inclusions at temperatures higher than ∼400 °C. When temperatures decrease below ∼340 °C, serpentinization of olivine lining the walls of the fluid inclusions leads to a near-quantitative consumption of trapped liquid H 2 O. The generation of molecular H 2 through precipitation of Fe(III)-rich daughter minerals results in conditions that are conducive to the reduction of inorganic carbon and the formation of CH 4 . Once formed, CH 4( g ) and H 2( g ) can be stored over geological timescales until extracted by dissolution or fracturing of the olivine host. Fluid inclusions represent a widespread and significant source of abiotic CH 4 and H 2 in submarine and subaerial vent systems on Earth, and possibly elsewhere in the solar system.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/sciadv.abe9773
