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Enriched mid‐ocean ridge basalts (E‐MORB) commonly erupt at mid‐ocean ridges (MOR) and seamounts, but their relationship to "depleted" MORB (D‐MORB) and the processes controlling their magmatic evolution at MORs are not fully understood, hence raising more general questions about magma generation in the mantle. We here explore this conundrum through an investigation of the Masirah ophiolite (southeast Oman), a near‐unique "true" MOR ophiolite. Unlike most (e.g., Tethyan) ophiolites, it was not affected by subduction and is therefore potentially able to provide valuable geological insights into the magmatic evolution of a full section of oceanic crust. Previous work has shown that the igneous crust at Masirah was thin (1.5–2.0 km) and constructed from both D‐ and E‐MORB magmas, concluding that it formed at a slow‐spreading ridge at ∼150 Ma followed by an episode of "Nb‐enriched" magmatism with trace‐element enrichments exceeding E‐MORB during intraplate rifting ∼20 Ma later. We reinvestigate the geology of Masirah and present new field observations, geochemical data and high‐precision U‐Pb ages to constrain the magmatic history of seafloor spreading and off‐axis magmatism. We found that D‐MORB and E‐MORB magmatism at Masirah was synchronous and overlapped in both composition and time with the Nb‐enriched magmatism (no older than 135 Ma). Both types of magmatism were therefore integral in the formation of the Masirah ocean crust. The relationship between D‐MORB and E‐MORB magmatism described here may be applicable to modern MORs more broadly, but is especially prominent at Masirah due to reduced magmatism and hence a weaker crustal filter. Plain Language Summary: The oceanic crust forms approximately two‐thirds of the Earth's surface and is continuously being formed at mid‐ocean ridges. Due to the difficulties involved in accessing oceanic crust directly, especially the deeper stratigraphic levels, the magmatic processes involved in forming the oceanic crust remain poorly understood. Ophiolites, fragments of oceanic lithosphere that were emplaced onto the continent, can offer valuable insights to geologists, but since most ophiolites are thought to have originated in marginal oceanic basins or at the initiation of subduction, a direct comparison remains problematic. In this study, we circumvent this problem by investigating the Masirah Ophiolite, a rare example of a "true" mid‐ocean ridge ophiolite. By combining field observations, geochemistry and geochronology at a level of detail not possible for present‐day oceanic crust, we reconstruct the magmatic evolution of the Masirah paleoridge. We present a more accurate formation age for Masirah and find that magmatism was compositionally variable throughout crustal accretion, with later off‐axis melts becoming more enriched in incompatible trace elements. We propose that the large geochemical variations observed at Masirah may be commonplace at modern mid‐ocean ridges, but that these are often masked due to the typically higher volumes of magmatism that enhance mixing and homogenization. Key Points: Two ophiolite nappes exposed on Masirah were formed by ocean crustal accretion at 135 and 131 Ma respectivelyOn‐axis magmatism shows varying degrees of trace element enrichment and depletion, and is followed by Nb‐enriched "near‐axis" magmatismSimilar processes may be common along modern mid‐ocean ridges but masked by the homogenizing effect of an enhanced "crustal filter" [ABSTRACT FROM AUTHOR]

Abstract Enriched mid‐ocean ridge basalts (E‐MORB) commonly erupt at mid‐ocean ridges (MOR) and seamounts, but their relationship to “depleted” MORB (D‐MORB) and the processes controlling their magmatic evolution at MORs are not fully understood, hence raising more general questions about magma generation in the mantle. We here explore this conundrum through an investigation of the Masirah ophiolite (southeast Oman), a near‐unique “true” MOR ophiolite. Unlike most (e.g., Tethyan) ophiolites, it was not affected by subduction and is therefore potentially able to provide valuable geological insights into the magmatic evolution of a full section of oceanic crust. Previous work has shown that the igneous crust at Masirah was thin (1.5–2.0 km) and constructed from both D‐ and E‐MORB magmas, concluding that it formed at a slow‐spreading ridge at ∼150 Ma followed by an episode of “Nb‐enriched” magmatism with trace‐element enrichments exceeding E‐MORB during intraplate rifting ∼20 Ma later. We reinvestigate the geology of Masirah and present new field observations, geochemical data and high‐precision U‐Pb ages to constrain the magmatic history of seafloor spreading and off‐axis magmatism. We found that D‐MORB and E‐MORB magmatism at Masirah was synchronous and overlapped in both composition and time with the Nb‐enriched magmatism (no older than 135 Ma). Both types of magmatism were therefore integral in the formation of the Masirah ocean crust. The relationship between D‐MORB and E‐MORB magmatism described here may be applicable to modern MORs more broadly, but is especially prominent at Masirah due to reduced magmatism and hence a weaker crustal filter.

Mid‐ocean ridge basalts (MORB) from the Central Indian Ridge (CIR) between 12° and 17°S show a wide range of geochemical and isotopic variations. Particularly, MORB from a segment between 14° and 15°S are more enriched in incompatible trace elements with more radiogenic Sr and Pb isotope and unradiogenic Nd isotope values than the lavas between 15° and 16°S with geochemical features of normal MORB. However, the causes for the enrichment between 14° and 15°S are poorly constrained. In this study, we re‐examined the CIR MORB from 12° to 17°S with new geochemical data obtained based on high spatial resolution sampling to better understand the nature of the enriched mantle source. Our new geochemical data show that the MORB between 14° and 15°S, with maximum values for (La/Sm)N = 1.95, 87Sr/86Sr = 0.703526 and 206Pb/204Pb = 18.7558, are more enriched than those from the southern segments (16° to 20°S) known to be influenced by the Réunion mantle component. The new trace element and isotopic compositions of MORB suggest that three mantle end‐members are required to explain the geochemical variations shown between 14° and 16°S: the depleted Indian‐type MORB mantle, Réunion Plume (RP), and Seychelles/Madagascar‐like continental crust components. Moreover, our mixing model suggests that the differences in enriched MORB signature from 14° to 20°S are due to variable proportions of continental material previously mixed with the RP. Our study implies that a continental component interacted with the plume into the asthenosphere, possibly beneath Madagascar or below Mauritius Island and the Mascarene plateau. Plain Language Summary: The basaltic lavas erupting on the mid‐ocean ridges are considered a window to understand the geochemical features of the upper mantle because they are produced by partial melting of the underlying mantle. The mid‐ocean ridge basalts (MORB) from the Central Indian Ridge (CIR) between 12° and 17°S show a wide range of geochemical and isotopic variations. Particularly, the MORB from the segment between 14° and 15°S have higher trace element contents with more radiogenic Sr and Pb isotopes and unradiogenic Nd isotopes than those from the neighboring ridges. We investigated the mantle source of the E‐MORB using our new geochemical data of basaltic lava samples collected with high spatial resolution along the ridge between 12° and 17°S and showed that the enriched lavas between 14° and 15°S are not only linked to the Réunion mantle plume, but also to a continental crust component that contaminated the Réunion mantle plume. Moreover, our new results enlarge the isotopic range of the Réunion mantle plume and show the complexity and heterogeneity of the upper mantle underlying the CIR. Key Points: Source of enriched MORB (E‐MORB) from the Central Indian Ridge (12°–17°S) is constrained by Sr‐Nd‐Pb isotope and trace element geochemistryThe E‐MORB source between 14° and 16°S is related to three components: a depleted mantle, Réunion Plume (RP), and continental componentsVariable proportions of continental components mixed with the RP may account for the MORB geochemistry between 14° and 20°S [ABSTRACT FROM AUTHOR]

The Nanpu Sag (NPS), located in northwestern Bohai Bay Basin (BBB), East China, has significant oil discoveries in recent years. The effect of intense faulting and downwraping which generated the intense subsidence of NPS in the Dongying Formation (Fm.) (Ed) (28.5–23.8 Ma) of the Late Oligocene is presented in this paper. We analyzed the geochemical characteristics of the widespread Cenozoic basalts in NPS from drill core samples. Our results show that the magma experienced negligible crustal contamination and slight fractional crystallization, and the features of remarkable positive U, Pb, Sr and Ti anomaly, low Rb/Ba and Rb/Sr values of the NPS basalts were inherited from the source region. The NPS basalts in Ed have distinctive characteristics as sub-alkaline basalts with rare earth elements (REE) patterns parallel to enriched mid-ocean ridge basalts (E-MORB), and were generated from a mixture origin of garnet-lherzolite zone with relatively high degree of partial melting (30%–50%) and garnet + spinel-lherzolite transition zone with partial melting degree ranging from 3% to 15%. In contrast, NPS basalts of other Cenozoic periods or Ed basalts of other areas in eastern China are alkaline with light REE enrichment and oceanic island basalts (OIB)-like-pattern, their magmas originated from garnet + spinel-lherzolite transition zone with partial melting degree lower than 5%. Combined with previous studies of the mantle wedge in the mantle transition zone (MTZ) beneath northeastern Asia, forming around 30 Ma, we infer that the stagnating of the Pacific slab triggered the asthenosphere upwelling and increased the degree of partial melting in mantle source, and relatively weak areas of the North China Craton (NCC) like the Tanlu Fault (TLF) where NPS located on were reactivate and susceptible to be modified. Therefore, the intense faulting and downwarping of NPS in Ed, along with the igneous activities, was a response to the Cenozoic stagnant Pacific slab more than the result of tectonic effects from the Pacific subduction like other areas in northeastern China in Paleogene. • Cenozoic Stagnant Pacific Slab generated mantle wedge beneath Nanpu Sag. • Mantle wedge caused intense fault activities and E-MORB-like basalts in Nanpu Sag. • Heat attenuation triggered intense downwarping after basalts eruption. • Intense faulting and downwarping generate intense subsidence in Nanpu Sag. [ABSTRACT FROM AUTHOR]

The discovery of chemically and isotopically enriched mid-ocean ridge basalts (E-MORB) has offered substantial insight into the origin, time scales, and length scales of mantle heterogeneity. However, the exact processes involved in producing this E-MORB enrichment are vigorously debated. Additionally, because the ages of E-MORB are not well constrained, the petrogenetic, temporal, and geological relationships between E-MORB and normal (N)-MORB are not known. To investigate these relationships and to explore how melting and melt transport processes contribute to or modify enriched mantle source compositions and generate E-MORB melts beneath mid-ocean ridges, we measured major and trace elements, and Sr, Nd, Hf, Pb, and U–Th–Ra isotopes for a suite of lavas that were collected off-axis, including several E-MORB, at 9–10°N along the East Pacific Rise (EPR). These data show coherent mixing trends among long-lived radiogenic isotopes, U-series nuclides, and incompatible trace elements, implying that mixing of melts from different sources occurs at different depths. Our results are consistent with previous studies that show that melting occurs in a two-porosity melting regime, with high-porosity channels forming deeply in the presence of garnet and transporting enriched melts with large 230Th excesses to the crust, whereas low-porosity channels transport melts more slowly, allowing them to equilibrate at shallow depths and develop large 226Ra excesses at the expense of diminished 230Th excesses. Forward modeling of the trace element data also is consistent with mixing of melts in a two-porosity melting regime. U-series age constraints suggest that E-MORB neither erupt at systematically different times from N-MORB, nor necessarily through different pathways. Previous studies of E-MORB at 9–10°N have suggested that E-MORB compositions could be explained by off-axis eruption. However, when considered in light of previously published magnetic paleointensity and U-series age constraints, recent geological studies, and the most widely accepted contemporary understanding of volcanic construction at 9–10°N EPR, the asymmetric, off-axis distribution of E-MORB at 9–10°N EPR is consistent with, and more simply explained by, a model in which E-MORB erupted within the axial summit trough (AST) and flowed down the ridge flanks (∼0–3 km). These E-MORB subsequently spread away from the AST, and, finally, were preserved on the seafloor through asymmetric construction of the extrusive layer. Taken together, the range of ages of E-MORB at 9–10°N EPR and the geochemical and isotopic mixing trends suggest that enriched melts are continuously supplied to the ridge axis, but because of their small proportions relative to the volumetrically and volcanically dominant N-MORB, E-MORB preservation and exposure is comparatively scarce. [ABSTRACT FROM AUTHOR]

Enriched mid-ocean ridge basalts (E-MORBs) near hotspots are commonly ascribed to be products of mantle plume-ridge interaction, but the genesis of E-MORBs far from hotspots remains controversial. Here we report new major and trace element and Sr-Nd-Pb isotopic data for lavas from Holes 559 and 561 of Deep Sea Drilling Project (DSDP) Leg 82 that are located west of the Mid-Atlantic Ridge (MAR) midway between the Oceanographer and Hayes fracture zones (33–35°N). Our data reveal that E-MORBs from Hole 559 have high (87Sr/86Sr) i (0.70292–0.70294) and (206Pb/204Pb) i (19.504–19.692) and low (143Nd/144Nd) i (0.51297–0.51299) values, similar to the mantle focal zone (FOZO) component, that could be partly coming from the New England mantle plume. We propose that the FOZO component could be widely distributed beneath the northern portion of MAR at ~35 Ma. Hole 561 lavas contain both depleted (D-) and E-MORB. The D-MORBs have depleted Sr-Nd-Pb isotope compositions and were derived from the depleted asthenospheric mantle. Hole 561 E-MORBs have radiogenic (87Sr/86Sr) i (0.70337–0.70341), (143Nd/144Nd) i (0.51296–0.51297) and (206Pb/204Pb) i (19.463–19.525) values that are similar to the proposed common component in the mantle plume sources of lavas in all the Azores islands except for the eastern part of São Miguel. In addition, Hole 561 E-MORBs also display higher Th/La and Nb/La ratios than the most plume-influenced E-MORBs near the Azores hotspot and even the Azores lavas. These suggest that Hole 561 E-MORBs were derived from partial melting of a depleted mantle metasomatized by deep, low-degree melts of the common Azores plume component. We propose that variable amounts of such metasomatized mantle are widely distributed in the asthenospheric mantle beneath the MAR 33–35°N ridge segment since ~19 Ma and their partial melting is also probably responsible for the generation of the modern MAR 33–35°N E-MORBs. [ABSTRACT FROM AUTHOR]