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Showing papers on "Basalt published in 2019"


Journal ArticleDOI
TL;DR: Previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle, which may reflect an influence of the Manus plume during subduction initiation.
Abstract: The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO2, P2O5, Zr) and Ni compared to MORB, with Na2O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle.

99 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used trace element diagrams to distinguish mid-ocean ridge basalts (MORB) from arc-related ones (arc-related basalts) in the 3Tb-Th-2Ta diagram.

95 citations


Journal ArticleDOI
06 Dec 2019-Science
TL;DR: Analysis of the data during the Kīlauea eruption revealed that at least three different sources of magma were feeding the eruption, which allowed forecasting of high-temperature eruptions and improved caldera-collapse models and may help improve real-time hazard responses.
Abstract: INTRODUCTION Fissures sliced through Kīlauea Volcano’s lower east rift zone on 3 May 2018, eventually engulfing hundreds of structures in lava flows and triggering a collapse at the summit. During the eruption, we employed a rapid routine for geochemical analysis of lava, developed over 6 years of monitoring the prior continuous eruption at Kīlauea. The application of this routine elevated lava chemistry to a near real-time data stream in eruption monitoring, similar to seismic and geodetic data. It provided an unparalleled opportunity to understand changes in magma characteristics during a rapidly evolving eruptive crisis. RATIONALE Lava chemistry provides vital information on the underground sources of magma, eruptive conditions, temperature, and physical properties of lava flows. However, analytical techniques are typically slow, leaving chemical analysis of lava as a retrospective tool in the volcano sciences. We developed an analytical procedure to characterize the geochemistry of lava within a few hours of sample collection, allowing us to identify a specific suite of major and trace elements that track lava compositions and estimate lava temperatures through chemical geothermometers. This information was used to inform response teams of shifts in eruptive conditions. RESULTS The initial fissures erupted low volumes of chemically evolved basaltic lavas from 3 to 9 May, which were viscous and cool (~1110°C). On 13 May we detected less-evolved compositions and an increase in inferred lava temperatures (~1130°C). We informed science and response teams that the arrival of more fluid and voluminous lava was likely. Beginning 17 to 18 May, the lava from the primary fissures became increasingly less chemically evolved, hotter, and more fluid. By 28 May, activity focused on a single vent (fissure 8). This vent fed a massive outpouring of hotter (~1145°C) lava that continued for more than 2 months. During this stage, lavas became slightly hotter and lost the cargo of lower-temperature minerals that were initially abundant. The lava carried olivine crystals with unusually high MgO, indicative of the presence of much hotter magma (>1270°C) somewhere in the plumbing system. A second dominant olivine population formed in cooler magma similar to what was being erupted previously at the summit lava lake. We also identified simultaneous, but more explosive, repetitive outbursts of andesite lava. This highly viscous and evolved composition, not previously known from Kīlauea, erupted at low temperatures (1060° to 1090°C) on a fissure offset from the other eruption fissures. The chemical and mineralogical fingerprint of this lava was also detected at other fissures several kilometers from the andesite vent. CONCLUSION Analysis of the data during the eruption revealed that at least three different sources of magma were feeding the eruption. The first two were the chemically evolved basalt of the initial fissures and the highly viscous andesite. Both are volumetrically minor sources that represent distinct pockets of old residual magma from Kīlauea’s east rift zone that evolved for more than 55 years, cooling and crystallizing at depth. The third and volumetrically more substantial source was less-evolved and hotter basalt of fissure 8. This source was similar in composition to the magma erupted at Kīlauea in the years before 2018 and was ultimately derived from the summit region. Draining and collapse of the summit by this voluminous eruption may have stirred up deeper, hotter parts of the summit magma system and sent mixed magma down the rift. By the final 20 days of the eruption, most magma stored within the active rift system had flushed out. Posteruption analyses done by traditional geochemical methods confirmed that the rapid-response routine produced comparable data and validated the models proposed during the active eruption. Our work has demonstrated that geochemical analyses of lava samples in near-real-time can yield critical information that enhances hazard assessments and risk mitigation during an eruption.

82 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented whole rock major-trace element geochemistry, Sr and Nd isotope, K-feldspar common Pb isotopes, along with in-situ zircon U-Pb ages and Hf isotope data from the granitoids and the greenstone volcanic rocks to characterize their source(s) and to constrain the evolution of the Paleoarchean continental crust.

82 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the ca. 110-104 Ma Gerze lavas (basalts, basaltic andesites, andesite, dacites and rhyolites) in southern Qiangtang.
Abstract: Identification of arc magmatic rock associations in a subduction zone has important implications for specifically revealing the geodynamic evolution of the subduction system. The closure time of the Bangong-Nujiang Tethyan Ocean and the detailed subduction processes have been hotly debated, hindering our understanding of the tectonic evolution of central Tibet. Here, we investigated the ca. 110-104 Ma Gerze lavas (basalts, basaltic andesites, andesites, dacites, and rhyolites) in southern Qiangtang. Fusion of slab fluid-metasomatized mantle wedge could yield the basalts, and such basaltic magmas, if contaminated with ancient basement orthogneisses, could have formed the andesites. The basaltic andesites with high Nb and Nb/La are similar to the Nb-enriched arc basalts and probably originated from slab melt-metasomatized mantle. The dacites were generated by fractional crystallization of the subducted melange-derived intermediate magmas. The rhyolites have geochemical characteristics (high SiO2 and La/Yb; low MgO and Sr/Y) similar to those of Jamaican-type adakites and were possibly sourced from the subducted oceanic plateau at low pressures. The Gerze Jamaican-type adakites and Nb-enriched basalt association could imply intense slab-mantle interactions. The Gerze lava suites show clear arc affinities, indicating that oceanic subduction may have lasted until 100 Ma. Based on previous studies and a noticeable ca. 145-125 Ma magmatic lull in southern Qiangtang, we suggest that the Bangong-Nujiang oceanic subduction geodynamics involved normal subduction (170-145 Ma), flat subduction (145-125 Ma), and slab roll-back (125-101 Ma). Moreover, the flat subduction was most likely caused by subduction of the oceanic plateau. Therefore, we propose, for the first time, that Tethyan oceanic plateau subduction during the Early Cretaceous could explain the tectonic evolution of the Bangong-Nujiang Ocean and distinctive magmatism in southern Qiangtang, central Tibet.

77 citations


Journal ArticleDOI
TL;DR: The authors show that such high pressure signatures can result from melting of mantle sources rather than melting of crust, and they suggest there is a lack of evidence that Earth’s earliest crust melted at depths significantly below 40 km.
Abstract: Much of the present-day volume of Earth’s continental crust had formed by the end of the Archean Eon, 2.5 billion years ago, through the conversion of basaltic (mafic) crust into sodic granite of tonalite, trondhjemite and granodiorite (TTG) composition. Distinctive chemical signatures in a small proportion of these rocks, the so-called high-pressure TTG, are interpreted to indicate partial melting of hydrated crust at pressures above 1.5 GPa (>50 km depth), pressures typically not reached in post-Archean continental crust. These interpretations significantly influence views on early crustal evolution and the onset of plate tectonics. Here we show that high-pressure TTG did not form through melting of crust, but through fractionation of melts derived from metasomatically enriched lithospheric mantle. Although the remaining, and dominant, group of Archean TTG did form through melting of hydrated mafic crust, there is no evidence that this occurred at depths significantly greater than the ~40 km average thickness of modern continental crust. Some of Earth’s earliest continental crust has been previously inferred to have formed from partial melting of hydrated mafic crust at pressures above 1.5 GPa (more than 50 km deep), pressures typically not reached in post-Archean continental crust. Here, the authors show that such high pressure signatures can result from melting of mantle sources rather than melting of crust, and they suggest there is a lack of evidence that Earth’s earliest crust melted at depths significantly below 40 km.

74 citations


Journal ArticleDOI
TL;DR: In this paper, the redox state of glassy crystal-hosted melt inclusions from tephra and quenched lava samples from the Canary and Cape Verde Islands was investigated.

69 citations


Journal ArticleDOI
26 Sep 2019-Nature
TL;DR: Gl Globally distributed kimberlites have their origins in a single, homogeneous early Earth reservoir that was subsequently perturbed, probably by subduction along the margins of Pangaea, around 200 million years ago, revealing a long-lived and globally extensive mantle reservoir that underwent subsequent disruption.
Abstract: The widely accepted paradigm of Earth's geochemical evolution states that the successive extraction of melts from the mantle over the past 4.5 billion years formed the continental crust, and produced at least one complementary melt-depleted reservoir that is now recognized as the upper-mantle source of mid-ocean-ridge basalts1. However, geochemical modelling and the occurrence of high 3He/4He (that is, primordial) signatures in some volcanic rocks suggest that volumes of relatively undifferentiated mantle may reside in deeper, isolated regions2. Some basalts from large igneous provinces may provide temporally restricted glimpses of the most primitive parts of the mantle3,4, but key questions regarding the longevity of such sources on planetary timescales-and whether any survive today-remain unresolved. Kimberlites, small-volume volcanic rocks that are the source of most diamonds, offer rare insights into aspects of the composition of the Earth's deep mantle. The radiogenic isotope ratios of kimberlites of different ages enable us to map the evolution of this domain through time. Here we show that globally distributed kimberlites originate from a single homogeneous reservoir with an isotopic composition that is indicative of a uniform and pristine mantle source, which evolved in isolation over at least 2.5 billion years of Earth history-to our knowledge, the only such reservoir that has been identified to date. Around 200 million years ago, extensive volumes of the same source were perturbed, probably as a result of contamination by exogenic material. The distribution of affected kimberlites suggests that this event may be related to subduction along the margin of the Pangaea supercontinent. These results reveal a long-lived and globally extensive mantle reservoir that underwent subsequent disruption, possibly heralding a marked change to large-scale mantle-mixing regimes. These processes may explain why uncontaminated primordial mantle is so difficult to identify in recent mantle-derived melts.

60 citations


Journal ArticleDOI
01 Feb 2019-Lithos
TL;DR: The Yudongzi terrane is featured by well-preserved lithologic outcrops of basement rocks, including the tonalite-trondhjemite-granodiorite (TTGs), potassic granites, Neoarchean supracrustal rocks and ~1.85 Ga amphibolites as mentioned in this paper.

59 citations


Journal ArticleDOI
TL;DR: In this article, a clockwise P-T path with nearly isothermal decompression is deduced from mineral zoning and textural relationships characterized by mineral recrystallization and multi-layered coronitic overgrowths of plagioclase and clinopyroxene surrounding garnet porphyroblasts.

58 citations


Journal ArticleDOI
TL;DR: In this article, the authors show that high-Mg eclogite xenoliths, entrained by kimberlites from the mantle lithospheres of ancient continental cores, and rare orogenic eclogs and ophiolites, exhumed or obducted during the closure of palaeo-ocean basins, have elemental and isotopic compositions indicative of protoliths that formed as little-differentiated melts erupted in ancient ocean floors.


Journal ArticleDOI
TL;DR: In this paper, the authors used the MultiCollector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) to analyze 51 geologically, geographically, and geochemically diverse oceanic basalt samples.

Journal ArticleDOI
TL;DR: In this article, the effects of grain size variation on sedimentary unit geochemistry have been taken into account by grouping and analysing geological units according to grain size for identifying the compositional characteristics of Gale crater's sediment source regions.

Journal ArticleDOI
TL;DR: In this paper, the authors measured neodymium isotope ratios in olivine-hosted melt inclusions from lavas of the Azores mantle plume and found that melts from ultra-depleted mantle contribute to the isotopic diversity of erupted lavas.
Abstract: 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.

Journal ArticleDOI
TL;DR: In this paper, the authors measured CO2 concentrations measured in olivine-hosted melt inclusions from Hualalai, Kilauea, Koolau, Loihi, and Mauna Loa to constrain the Hawaiian mantle CO2 content and flux.

Journal ArticleDOI
TL;DR: Newly compiled Fe3+/ΣFe and V/Sc ratios presented here indicate that island arc rocks became more oxidized 800–400 Ma, which provides evidence that the oxidized nature of modern island arc magmas is due to an oxidized mantle source and represents a rare example of a change in the surface biogeochemistry influencing the igneous rock record.
Abstract: A rise in atmospheric O2 levels between 800 and 400 Ma is thought to have oxygenated the deep oceans, ushered in modern biogeochemical cycles, and led to the diversification of animals. Over the same time interval, marine sulfate concentrations are also thought to have increased to near-modern levels. We present compiled data that indicate Phanerozoic island arc igneous rocks are more oxidized (Fe3+/ΣFe ratios are elevated by 0.12) vs. Precambrian equivalents. We propose this elevation is due to increases in deep-ocean O2 and marine sulfate concentrations between 800 and 400 Ma, which oxidized oceanic crust on the seafloor. Once subducted, this material oxidized the subarc mantle, increasing the redox state of island arc parental melts, and thus igneous island arc rocks. We test this using independently compiled V/Sc ratios, which are also an igneous oxybarometer. Average V/Sc ratios of Phanerozoic island arc rocks are elevated (by +1.1) compared with Precambrian equivalents, consistent with our proposal for an increase in the redox state of the subarc mantle between 800 and 400 Ma based on Fe3+/ΣFe ratios. This work provides evidence that the more oxidized nature of island arc vs. midocean-ridge basalts is related to the subduction of material oxidized at the Earth's surface to the subarc mantle. It also indicates that the rise of atmospheric O2 and marine sulfate to near-modern levels by the late Paleozoic influenced not only surface biogeochemical cycles and animal diversification but also influenced the redox state of island arc rocks, which are building blocks of continental crust.

Journal ArticleDOI
TL;DR: In this article, the authors presented a new geochemical study of 26 dolerite samples from 6 deep boreholes in the Brazilian basins, including wholerock major and trace elements, whole-rock Sr-Nd isotopes and detailed biotite mineral chemistry.

Journal ArticleDOI
TL;DR: The buoyancy of a plume is studied by investigating basalts from the Changbaishan volcano, beneath which a mantle plume from the hydrous MTZ is observed via seismology, and suggests that the mantle plumes did not have a significant excess heat, and that the plume upwelled because of buoyancy resulting from water supplied from the Pacific slab in the MTZ.
Abstract: Magmatism at some intraplate volcanoes and large igneous provinces (LIPs) in continental areas may originate from hydrous mantle upwelling (i.e. a plume) from the mantle transition zone (MTZ) at 410–660 km depths in the Earth’s deep interior. However, the ultimate origin of the magmatism, i.e. why mantle plumes could have been generated at the MTZ, remains unclear. Here, we study the buoyancy of a plume by investigating basalts from the Changbaishan volcano, beneath which a mantle plume from the hydrous MTZ is observed via seismology. Based on carefully determined water contents of the basalts, the potential temperature of the source mantle is estimated to be 1310–1400 °C, which is within the range of the normal upper mantle temperature. This observation suggests that the mantle plume did not have a significant excess heat, and that the plume upwelled because of buoyancy resulting from water supplied from the Pacific slab in the MTZ. Such a hydrous mantle plume can account for the formation of extremely hydrous LIP magmatism. The water was originally sourced from a stagnant slab and stored in the MTZ, and then upwelled irrespective of the presence or absence of a deep thermal plume.

Journal ArticleDOI
Gaoxue Yang1, Yongjun Li1, Lili Tong1, Zuopeng Wang1, Fenghao Duan1, Qian Xu1, Hai Li1 
TL;DR: In this paper, the authors provide an overview of the salient features of twelve OIB-hosting accretionary complexes with ages from Late Neoproterozoic to Early Carboniferous in western the Central Asian Orogenic Belt (CAOB), and distinguish rock association of typical seamounts from accretional complexes and shear zones, including pillow basalt, volcanic breccia, limestone, olistostromes and terrestrial clastic rocks.

Journal ArticleDOI
TL;DR: In this article, the authors investigate the tectonomagmatic evolution of the northwest Atlantic where breakup-related igneous rocks were emplaced during several Paleogene events associated with lithospheric stretching, continental breakup, and the formation of new oceanic basins.

Journal ArticleDOI
TL;DR: In this article, geochronological and geochemical determinations are analyzed to identify the Albian-Cenomanian continental-margin orogenic belt and simultaneous igneous province of Pacific Asia.
Abstract: Geological data and about 300 precision geochronological and geochemical determinations are analyzed to identify the Albian–Cenomanian continental-margin orogenic belt and simultaneous igneous province of Pacific Asia. The orogenic belt represents a newly formed region of continental lithosphere that resulted from the deformation of mainly Jurassic–Early Cretaceous epioceanic terranes. The igneous province is made up of volcanic and intrusive complexes of mostly Albian age, which are syn-orogenic within the belt and post-orogenic beyond it. The igneous rocks include A-, I-, and S-type granitoids; adakites; and VAB- and OIB-type basalts and their intrusive analogues. Both the orogenic belt and the igneous province were formed in a tectonic setting of a transform continental margin between 110 and 95 Ma. The interval of 103–97 Ma became the peak of orogenic and magmatic activity caused by upwelling of the hot asthenospheric mantle through destructed stagnant slabs of the pre-Albian subduction. The Albian–Cenomanian age of the large gold, copper, tungsten, tin, and other ore deposits argues in favor of recognition of the corresponding metallogenic province. The first age data (100–97 Ma) are reported for the granite from the Malmyzh deposit within the Sikhote-Alin Ridge.

Journal ArticleDOI
01 Jan 2019-Lithos
TL;DR: Based on detailed field mapping, Zhang et al. as mentioned in this paper identified three rock assemblages in the Hebukesaier ophiolitic melange: ophiola, seamount, and rhyolite porphyry, which constrain the formation time of the oceanic crust.

Journal ArticleDOI
TL;DR: In this paper, the authors present silicon isotopic constraints on the formation of representative trondhjemitic and granitic plutons from the Kaapvaal craton that range in age from 3.51-2.69 billion years ago.
Abstract: The Archaean continental crust comprises two major groups of silicon-rich granitoids: the tonalite–trondhjemite–granodiorite and granite–monzonite–syenite suites, which differ in their sodium-to-potassium ratios. How these felsic granitoids evolved from their mafic precursors remains elusive and the subject of great debate. Here, we present silicon isotopic constraints on the formation of representative trondhjemitic and granitic plutons from the Kaapvaal craton that range in age from 3.51–2.69 billion years ago. We identified very consistent silicon isotopic signatures, all uniformly 0.1–0.2‰ heavier than rocks of the modern continental crust. This unusual composition is explained by the melting of a mafic source that included significant proportions (15–35 wt%) of silicified basalts, which were common supracrustal rocks before 3 billion years ago. Before the melting event that formed the granitoid magmas at depth, portions of the mafic source rocks were enriched in silica by interaction with silica-saturated seawater. The addition of silica depresses the stability of amphibole at similar water activity, allowing trondhjemitic and granitic melt production at lower temperatures from protoliths with contrasting silica contents: 52–57 and ≥60 wt%, respectively. This explains why granitoids were able to form very early in Earth history but did not emerge in significant amounts on other rocky planets. Granitic continental crust in the Archaean formed from a basaltic source that was enriched in silica due to interaction with the early oceans before melting, according to silicon isotope analyses on rocks from the Kaapvaal craton.

Journal ArticleDOI
TL;DR: In this paper, the Greenland-Iceland plume system was studied and the authors reported the existence of primordial domains in Earth's present-day mantle, and they also reported Pb isotopic compositions, including the variations in 182W/184W of a sample from a laboratory reference standard in parts per million, and 3He/4He, as well as highly siderophile element concentrations and Re-Os isotopic systematics of basaltic samples erupted at different times during the ~60 Ma history of the Greenland Iceland plume.

Journal ArticleDOI
TL;DR: The first high precision U-Pb zircon geochronology from two alkaline ultramafic-felsic layered intrusive complexes from Taimyr is reported, showing synchronicity between these and the main Siberian Traps (sub)volcanic pulse, and the presence of a second Dinerian-Smithian pulse.
Abstract: Emplacement of large volumes of (sub)volcanic rocks during the main pulse of the Siberian Traps occurred within <1 m.y., coinciding with the end-Permian mass extinction. Volcanics from outside the main Siberian Traps, e.g. Taimyr and West Siberia, have since long been correlated, but existing geochronological data cannot resolve at a precision better than ~5 m.y. whether (sub)volcanic activity in these areas actually occurred during the main pulse or later. We report the first high precision U-Pb zircon geochronology from two alkaline ultramafic-felsic layered intrusive complexes from Taimyr, showing synchronicity between these and the main Siberian Traps (sub)volcanic pulse, and the presence of a second Dinerian-Smithian pulse. This is the first documentation of felsic intrusive magmatism occurring during the main pulse, testifying to the Siberian Trap’s compositional diversity. Furthermore, the intrusions cut basal basalts of the Taimyr lava stratigraphy hence providing a minimum age of these basalts of 251.64 ± 0.11 Ma. Synchronicity of (sub)volcanic activity between Taimyr and the Siberian Traps imply that the total area of the Siberian Traps main pulse should include a ~300 000 km2 area north of Norilsk. The vast aerial extent of the (sub)volcanic activity during the Siberian Traps main pulse may explain the severe environmental consequences.

Journal ArticleDOI
TL;DR: In this article, a melting-assimilation-fractional crystallization (FFL) model was used to show that komatiitic basalts were generated from Al-depleted and -undepleted KOMatiites by assimilation and fractional crystallisation processes.

Journal ArticleDOI
TL;DR: In this paper, the authors report trace element and isotope data on a fossil mantle section exposed in the ophiolitic Lanzo South massif, Italy, and show the existence of clinopyroxenes in geochemically depleted melt-infiltrated harzburgites with an extreme radiogenic Hf-isotopic composition but MORB-like Nd isotopic compositions.

Journal ArticleDOI
01 Apr 2019-Geology
TL;DR: Using a numerical model of recharge, mixing, and diffusional reequilibration, this article showed that this type of crustal processing results in elevated Ni and lower Ca contents of forsterite-rich olivine, causing erroneously high estimates of the proportion of pyroxenite-derived melt.
Abstract: Ocean-island basalts (OIBs) provide a unique insight into the extent of lithological heterogeneity (peridotite vs. pyroxenite) in Earth’s convecting mantle. However, crustal processing of these mantle melts significantly influences minor-element concentrations in olivine phenocrysts, challenging the suitability of this widely used approach to identify lithological variations in their mantle source. Using a numerical model of magma recharge, mixing, and diffusional reequilibration, we show that this type of crustal processing—which is widely observed in OIBs—results in elevated Ni and lower Ca contents of forsterite-rich olivine, causing erroneously high estimates of the proportion of pyroxenite-derived melt. We applied our model of magma recharge and mixing to several OIBs, including the Galapagos Islands, Canary Islands, and La Reunion. In particular, we critically examined olivine compositional variations in basalts from the eastern Galapagos, which display Sr- and Pb-isotope ratios similar to normal mid-ocean-ridge basalts. While previous interpretations (based on olivine chemistry) argued for a significant contribution from pyroxenite-derived melt, our results indicate that the postulated presence of pyroxenite in the eastern Galapagos mantle is an artifact of processing of magmas and their olivine cargo as they transition through the crust, consistent with major-element and isotopic evidence for a dominantly peridotitic source in this region. This new model for magma recharge and mixing may have important implications for our understanding of lithological heterogeneity beneath OIBs globally, and it highlights the importance of considering crustal processes when attempting to interpret olivine compositions with regard to mantle heterogeneity.

Journal ArticleDOI
TL;DR: The E'min ophiolitic melange as discussed by the authors has a geochemical make-up similar to those of suprasubduction-zone (SSZ)-type Ophiolites formed in a forearc setting and may represent the initial subduction in northern West Junggar.