Plume Component Research Articles

Petrogenesis of flood basalts and shield basanite from Mertolemariam- Abamineos area, northwestern Ethiopian plateau: Assessments for mantle source variations

Petrographic and geochemical data (major and trace elements) are presented for Oligocene flood basalts and Miocene shield lavas from the Mertolemariam-Abamineos area, northwestern Ethiopian Plateau to examine the petrogenesis of the erupted magmas and the nature of mantle source compositions. The Mertolemariam flood basalts have mainly aphyric and plagioclase phyric textures. The Abamineos shield lavas have sparsely clinopyroxene phyric to highly plagioclase-clinopyroxene phyric textures. Geochemical classification shows that the Mertolemariam Oligocene flood basalts are sub-alkaline in composition, whereas the Abamineos Miocene shield lavas are highly alkaline in composition. The Abamineos shield has a unique composition (i.e., basanites) compared with all other shield basalts (transitional to alkaline) in the northwestern Ethiopian Plateau. Major and trace element compositions display two distinct trends between Mertolemariam flood basalts and Abamineos shield basanites. Fractional crystallization, partial melting, and crustal contamination of a homogeneous mantle source cannot explain the compositional variations between the Mertolemariam flood basalts and the Abamineos shield basanites. The trace element composition suggests that the Mertolemariam Oligocene flood basalts were more likely generated from the mixing of OIB (mantle plume) and E-MORB (enriched asthenosphere) mantle components. The Abamineos Miocene shield basanites were derived from the mantle plume (OIB) component. LREE/MREE and LREE/HREE indicate that both groups possibly originated from a mantle source within the stability field of spinel and garnet. In comparison, the Mertolemariam flood basalts were formed by a higher degree of partial melting from relatively shallow depths than the Abamineos shield basanites. We propose a scenario that explains the magmatic genesis in the northwestern Ethiopian Plateau: volcanism initiated by the initial arrival of the mantle plume (OIB-like) beneath the lithosphere, which comes across a geochemically fertile and enriched MORB (E-MORB) mantle component in the upper asthenosphere. The hot mantle plume triggered melting of the fertile and enriched MORB, and then melting occurred in the plume (OIB-like) at depth in the stability field of spinel and garnet. Melts from the mantle plume (OIB-like) and E-MORB components mixed to produce sub-alkaline flood basalts during the Oligocene in the northwestern Ethiopian Plateau Subsequently, melts from the advanced upwelling mantle plume (OIB-like) produced Miocene shield lavas in the northwestern Ethiopian Plateau.

Compositional variations in shield-stage volcanism in Fogo, Cape Verde islands

The intraplate oceanic island of Fogo (Cape Verde islands) is in the shield-stage of its evolution. It experienced a major lateral collapse ca. 70 ka ago, after which a sequence of lavas about 2 km thick infilled the collapse scar, with eruptions continuing to the present day. Nearly 100 lavas from before and after the collapse have been sampled and analysed for bulk rock compositions. These lavas have been divided into five Formations (2 pre-collapse and 3 post-collapse), using stratigraphic principles. There is little difference in the major and trace element compositions of pre- and post-collapse lavas, and the main process that gives rise to their diversity is fractional crystallization of observed mineral phases. Pre-collapse lavas show a narrower range of bulk compositions than the post-collapse lavas, but this may be because the sampling strategy avoided these more altered and poorly exposed lavas. A group of high-P2O5 lavas, erupted both before and after the collapse, may have been extensively contaminated by local seamount-stage intrusive carbonatites. A subset of 30 samples was analysed for Sr, Nd and Pb isotope ratios. They show a plume component which has interacted with two contaminants: (1) an increase in 87Sr/86Sr and decrease in 143Nd/144Nd with decreasing MgO, which indicates varying amounts of assimilation of an enriched lithospheric component coupled to fractionation (AFC), and (2) contamination of the more fractionated magmas, without further fractionation, involving the suspected carbonatite contaminant which originates from a more isotopically depleted source. Overall trends of increasing 87Sr/86Sr, and slight decreases in 143Nd/144Nd and all radiogenic Pb isotope ratios, with decreasing age of eruption, indicate a general increase in the assimilated lithospheric component throughout shield-stage activity. However, there is no clear evidence of any change in isotope compositions across the period of the collapse. All Formations show similar wide and overlapping ranges of isotope compositions, regardless of the timescales represented by each Formation. This implies that much short-term variation in the magma sources was superimposed on the overall trend of increasing lithospheric assimilation.

The Conrad Rise Revisited: Eocene to Miocene Volcanism and Its Implications for Magma Sources and Tectonic Development

AbstractThe Conrad Rise (CR), located midway between Antarctica and the Southwest Indian Ridge (SWIR), remains one of the least explored submarine large igneous provinces (LIPs) in the Indian Ocean to date. Relying on only seafloor paleomagnetic records, early studies hypothesized that the formation of the CR occurred during the Late Cretaceous. Here, we present new geochemical and geochronological data, including Sr‒Nd‒Pb‒Hf isotopes and 40Ar/39Ar data. Our results indicate that the uppermost part of the CR (Ob and Lena seamounts) unexpectedly formed later than previously predicted, at approximately 40 Ma in an intraplate setting. Another small seamount north of the Ob seamount formed later, at 8.5 Ma. The isotopic composition of lava from the small seamount north of the Ob seamount overlaps with that commonly defined by the Indian plume component. Overall, the isotopic variations defined by the volcanic suite from the CR could be accounted for by a three‐component mixing model involving the common component, lower continental crust, and depleted mantle endmembers. The newly obtained 40Ar/39Ar ages imply that the CR volcanism might have been triggered by major regional plate reorganizations during the middle to late Eocene and the late Miocene, inducing the release of a small upwelling rising from the African large low‐velocity province.

Quantifying BrO and SO2 distributions in volcanic plumes—Recent advances in imaging Fabry-Pérot interferometer correlation spectroscopy

Bromine monoxide (BrO) and sulphur dioxide (SO2) are two gases frequently observed in volcanic plumes by spectroscopic techniques capable of continuous gas monitoring like, e.g., Differential Optical Absorption Spectroscopy (DOAS). The spatio-temporal resolution of DOAS measurements, however, only allows to determine average gas fluxes (minutes to hours resolution). In particular, it is insufficient to record two-dimensional images of SO2 and BrO in real-time (seconds time resolution). Thus, it is impossible to resolve details of chemical conversions of reactive plume constituents. However, these details are vital for further understanding reactive halogen chemistry in volcanic plumes. Therefore, instruments that combine high spatio-temporal resolution and high gas sensitivity and selectivity are required. In addition, these instruments must be robust and compact to be suitable for measurements in harsh and remote volcanic environments. Imaging Fabry-Pérot interferometer (FPI) correlation spectroscopy (IFPICS) is a novel technique for atmospheric trace gas imaging. It allows measuring atmospheric gas column density (CD) distributions with a high spatial and temporal resolution, while at the same time providing selectivity and sensitivity comparable to DOAS measurements. IFPICS uses the periodic transmission spectrum of an FPI, that is matched to the periodic narrowband (vibrational) absorption features of the target trace gas. Recently, IFPICS has been successfully applied to volcanic SO2. Here we demonstrate the applicability of IFPICS to much weaker (about two orders of magnitude) trace gas optical densities, such as that of BrO in volcanic plumes. Due to its high reactivity, BrO is extremely difficult to handle in the laboratory. Thus, based on the similarity of the UV absorption cross sections, we used formaldehyde (HCHO) as a spectral proxy for BrO in instrument characterization measurements. Furthermore, we present recent advances in SO2 IFPICS measurements and simultaneous measurements of SO2 and BrO from a field campaign at Mt Etna in July 2021. We find photon shot-noise limited detection limits of 4.7 × 1017 molec s0.5 cm−2 for SO2 and of 8.9 × 1014 molec s0.5 cm−2 for BrO at a spatial resolution of 512 × 512 pixels and 200 × 200 pixels, respectively. Furthermore, an estimate for the BrO to SO2 ratio (around 10–4) in the volcanic plume is given. The prototype instrument presented here provides spatially resolved measurements of the reactive volcanic plume component BrO. The temporal resolution of our approach allows studies of chemical conversions inside volcanic plumes on their intrinsic timescale.

Heterogeneous Fossil Réunion Plume Component in the Source Region of Enriched MORB Along the Central Indian Ridge Between 12° and 17°S

AbstractMid‐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.

Evaluation of a Revised AERMOD Treatment of Plume Dispersion in the Daytime Elevated Stable Layer

Advanced dispersion models such as AERMOD specifically address the portion of a plume emitted in convective conditions that is sufficiently buoyant to rise into the stable layer above the elevated inversion. This portion of the plume mass is often referred to as the “penetrated plume” because that plume component breaks through the elevated inversion and penetrates into the stable layer aloft. A pre-mature mixing of the penetrated plume to the ground has been identified in the current formulation of AERMOD, which is the USEPA-preferred short-range dispersion model and used in several other countries. This behavior has been observed based on data from field studies where the model is found to overpredict ground-level concentration events due to the penetrated plume component, with the timing of these peak predictions too early in the day. A proposed update to AERMOD to address the penetrated plume issue (referred to as “HBP” for modifications particularly important for “highly buoyant plume”) is documented and evaluated in this manuscript. The revised approach involves a check on the convective mixing height for the current hour as well as the next hour to determine how much of the penetrated plume has been captured by the convective boundary layer by the end of the current hour. The amount of the penetrated plume mass that is allowed to mix to the ground in the HBP modifications depends upon the result of this calculation. The HBP modification has been evaluated as an update to AERMOD for 3 databases along with a sensitivity analysis of the effects of the HBP changes on a variety of stack heights and buoyancy fluxes. The findings of the evaluation indicate that the HBP changes to AERMOD result in reduced overprediction tendencies. “Implications Statement” for the HBP paper being submitted to JAWMA A proposed enhancement to AERMOD to address a pre-mature mixing of penetrated plume material to the ground has been performed by implemented and evaluated by the authors. The enhancement, referred to as the highly buoyant plume (HBP) is based on work developed by Dr. Jeffrey Weil. HBP is designed to better characterize the penetrated plume behavior in the model such that it aligns more closely with observations based on data from field studies.

Chemical variability in volcanic gas plumes and fumaroles along the East African Rift System: New insights from the Western Branch

The origin of magmatic fluids along the East African Rift System (EARS) is a long-lived field of debate in the scientific community. Here, we investigate the chemical composition of the volcanic gas plume and fumaroles at Nyiragongo and Nyamulagira (Democratic Republic of Congo), the only two currently erupting volcanoes set on the Western Branch of the rift. Our results are in line with earlier conceptual models proposing that volcanic gas emissions along the EARS mainly reflect variable contributions of either a Sub-Continental Lithospheric Mantle (SCLM) component or a Depleted Morb Mantle (DMM) component, and deeper fluid. At Nyiragongo and Nyamulagira, our study discards a major contribution of a high 3He/4He mantle plume component in the genesis of volcanic fluids beneath the area. High CO2/3He in fumaroles of both volcanoes is thought to reflect carbonate metasomatism in the lithospheric mantle source. As inferred by previous results obtained on the lava chemistry, this carbonate metasomatism would be more pronounced beneath Nyiragongo. This supports the idea of the presence of distinct metasomes within the lithospheric mantle beneath the Western Branch of the rift.

Радиоизотопные исследования позднекайнозойских вулканических пород Азии и Северной Америки: источники вулканизма глобального, регионального и локального значения

Of global importance are sources of Late Cenozoic volcanic rocks, indicated by fast and slow anomalies of the lower mantle: ASITA, SOPITA, AFITA, and NAITA. ASITA characterizes the LOMU–ELMU protomantle melt anomalies of the lower mantle high velocity skeleton of Asia during the early mantle geodynamic epoch. SOPITA and AFITA are marked by the HIMU plume component, which was generated in the low-velocity lower mantle of the South Pacific and Africa during the middle mantle geodynamic epoch, about 2 Ga. NAITA denotes processes of generation of the North American high-velocity lower-mantle anomaly in the late geodynamic epoch. Of regional importance are sub-lithospheric sources that are characterized by a common geochemical structure of volcanic rocks of large volcanic regions, formed due to the evolution of homogeneous proto-mantle deep reservoirs. An example is sources of the Wulanhada-Hannuoba and Abaga-Dariganga volcanic regions, marking the spatial transition from the LOMU proto-mantle reservoir of the former to the ELMU one of the latter. Local sources of volcanoes are displayed in limited volumes of melts from the heterogeneous lithosphere. An example is sources of the Wudalianchi volcanic field, on which, at first, in the time interval of 2.5–0.8 Ma, the material of the Laoshantou and Gelaqiu sources with a common age of 1.88 Ga occurred, and then, during the spatial-temporal evolution of volcanism in the last 0.6 Ma, the material from these sources was mixed with a younger lithospheric material.

Sr-Nd isotope geochemistry and petrogenesis of ca. 2.26–2.25 Ga and ca. 2.08 Ga mafic dyke swarms from the Dharwar craton, India: Insights into their mantle sources and geodynamic implications

Whole-rock and Sr-Nd isotopic data on the N-S to NNE-SSW trending ca. 2.26–2.25 Ga Ippaguda-Dhiburahalli and the N-S, NW-SE to NE-SW trending ca. 2.08 Ga Devarabanda mafic dyke swarms of the Dharwar craton have been studied. The ca. 2.26–2.25 Ga dykes are linear in nature and contain markedly high SiO2 (53.27–55.85 wt%) and Mg# (60–67) with low TiO2 (0.56–0.61 wt%) contents. While the ca. 2.08 Ga dykes are emplaced in a radiating fashion and contain relatively low SiO2 (47.34–52.67 wt%) and Mg# (34–53) with high TiO2 (0.67–2.11 wt%) contents. The multi-element patterns for both the dyke swarms reflect enrichment in the large-ion lithophile elements (LILEs) and depletion in the high-field-strength elements (HFSEs) with prominent negative Nb-Ta and Ti anomalies. Furthermore, samples of the ca. 2.26–2.25 Ga swarm are relatively more enriched than the ca. 2.08 Ga samples, which is supported by the Nd-isotopic signature. εNd(t) values in the ca. 2.26–2.25 Ga samples vary between −4.5 and 2.4, whereas in the ca. 2.08 Ga dykes it varies between −0.6 and 0.2. The parental magmas are derived from a MORB-like depleted mantle source for both the dyke swarms. Moreover, the interaction between primary melt and the metasomatized sub-continental lithospheric mantle (SCLM) is suggested for the ca. 2.26–2.25 Ga swarm, whereas, the ca. 2.08 Ga swarm show imprints of crustal contamination. The trace element modelling suggests that the ca. 2.26–2.25 Ga swarm were derived from a relatively low (≤5%) degree of partial mantle melting as compared to the ca. 2.08 Ga swarm with a high (~10–15%) degree of partial mantle melting. A deeper mantle source is suggested for the ca. 2.26–2.25 Ga swarm (Garnet/Spinel >80/20), whereas the ca. 2.08 Ga swarm were derived from a relatively shallower mantle source (Garnet/Spinel ~40/60–80/20). The TDM model age for the ca. 2.26–2.25 Ga swarm is supportive of the Neoarchean convergence-related growth of the Dharwar craton, possibly created an enriched lithospheric mantle reservoir. Absence of OIB like plume component in the ca. 2.08 Ga swarm defines the plume centre not precisely below the cratonic lithosphere of the Dharwar craton but several hundred kilometers away from the cratonic margin.

Early Cretaceous Greater Kerguelen Large Igneous Province and its plumbing systems: A contemplation on concurrent magmatic records of the eastern Indian Shield and adjoining regions

A detailed contemplation is attempted to comprehend the likely connection of a large number of Early Cretaceous mafic, felsic, alkaline (kimberlite, lamproites, ultramafic lamprophyre, etc.), and carbonatite magmatic records of the eastern and north‐eastern region of the Indian Shield and adjoining regions. All these Early Cretaceous magmatic records occupied a large area, covering adjoining regions of south‐eastern Tibet, eastern and north‐eastern India, western Australia, Eastern Indian Ocean, southern Kerguelen Plateau, and north‐eastern Antarctica, are supposed to be part of the Greater Kerguelen Large Igneous Province (LIP) and likely to be related to the break‐up of East Gondwana. Available age results, chiefly ranging from ca. 145 to 100 Ma, support a genetic connection with the Kerguelen mantle plume. However, there are other lines of evidences, particularly radiogenic isotope data that preclude direct involvement of any mantle plume component; likely to be derived from decompression melting related to passive rifting. Two major magmatic phases, ca. 145–130 Ma and ca. 124–100 Ma, have been distinctly identified, which could be related to distinct rifting events. Break‐up of East Gondwana had happened at ca. 137 Ma with separation of Western Australia from Greater India. Later, ca. 124–122 Ma, separation of Antarctica from Eastern India occurred. The later magmatic phases (after ca. 120 Ma) are likely to have an indirect connection with the Greater Kerguelen LIP probably with the mantle plume providing extra heat for melting.

The Origin of Late Cenozoic Magmatism in the South China Sea and Southeast Asia

AbstractBasaltic lavas sample recycled crustal materials from their mantle source. Constraining the location and residence time of these recycled materials in the mantle is critical to understand global mantle dynamics. In this study, we present new whole‐rock major and trace element abundances, Sr‐Nd‐Mo‐Os isotopes, water contents and He isotopes of volcanic glasses, U‐Pb ages of zircons, and compositions of melt inclusions, spinels and olivines from the South China Sea (SCS) seamounts lavas. These new data are compared with literature data from intraplate volcanism of similar age from Southeast (SE) Asia. The isotope data of late Cenozoic lavas from the SCS seamounts and SE Asia can be explained by mixing between enriched mantle 2 (EM2) and depleted mid‐ocean ridge basalt mantle components. Our data are consistent with the EM2 signature of late Cenozoic lavas derived from recycled young oceanic crust and sediments. The compositions of olivine phenocrysts indicate an olivine‐dominated (peridotitic) mantle source. There is currently no evidence for a high‐3He/4He mantle plume component beneath the SCS. Our results combined with geophysical data and plate reconstructions suggest that the late Cenozoic magmatism is related to the upwelling of instabilities from the mantle transition zone (MTZ) triggered by a stagnant slab. The SCS seamount lavas sample an enriched MTZ containing young recycled materials, consistent with regional past subduction. Our study provides additional evidence that storage and recycling of crustal materials in or near the MTZ is an important mechanism to develop global mantle heterogeneities sampled by intraplate volcanoes.

Present structure of the Near-Bug area: conditions of formation and history of development

General sequence establishment of geological Precambrian events and associating formations, which were created in them, to the results of isotope age definition, is the task, which has no single valued solution for southwestern part of the Ukrainian Shield. Important is to create a general development model, which will describe the modern geological structure of an area, structural and textural rocks features, accounting PT-conditions in the Earth's crust during the Archean—Paleoproterozoic.
 Isotopic age determination demonstrates, that from the moment of protolith creation (not later than 3.75 billion years ago, up to 1.9 billion years ago), intrusion of mantle melts and partial melting of the lower crustal rocks, occurred many times over. Pobuzhie formation cannot be imagined, as a single process of accumulation, plunge, crumpling into folds and sedimentary strata metamorphism. It is necessary, to take into account, the plume (mantle) component of the general geodynamic process.
 In the structure of the Bug megablock and Golovanevskaya suture zone, two main structural plans are displayed. The main part of the territory displays a region of areal distribution of Archean enderbites (generated 2.8 billion years ago) and Proterozoic granites (generated 2.03 billion years ago). The paper compares the temperature distribution with depth, corresponding to the thermal model of the metamorphic temperatures found in the samples, and the solidus temperatures of the basic rocks. It is shown that at the time of the metamorphism development, 2.0 billion years ago, the rocks were at a depth of more than 20 km, and before that — at an even greater depth. During the Archean and Paleoproterozoic, the center of partial melting was repeatedly renewed here, since the temperatures were higher than the solidus temperature of gabbro. Metamorphic changes (and more often migmatization, partial melting and following crystallization in the granulite facies conditions) happened after the presence of the thermal asthenosphere on the core—mantle border, and were accompanied by bringing the substance from it. Therefore the main part of modern surface is folded by palingenic granites. In Archean and Paleoproterozoic the composition of substances were different.
 After 2.0 billion years ago the level of modern surface was located higher. The second structural plan is presented with vertical structures, building of which often close to concentrically zonal or linear monoclinal. They are confined to fault zones and nodes of their intersections. These structures contain rock complexes, which did not occur until 2.0 billion years ago on any craton in the world.

New geochemical and Sr-Nd-Pb isotope evidence for FOZO and Azores plume components in the sources of DSDP Holes 559 and 561 MORBs

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.

Olivine chemistry reveals compositional source heterogeneities within a tilted mantle plume beneath Iceland

High-Fo olivine (Fo = Mg/(Mg+Fe) mol%) is an ideal proxy for establishing the compositions of primary melts and their mantle sources. This has been exploited in establishing lithological variations in the mantle source regions of oceanic basalts, including in Iceland. However, previous studies on Icelandic olivine lack spatial and temporal coverage. We present high-precision in-situ major, minor and trace element analyses of Fo-rich olivine from a suite of 53 primitive basalts erupted in the neovolcanic rift and flank zones of Iceland, as well as in older regions of Quaternary and Tertiary crust. Most of these samples have previously been analysed for 3He/4He, which ranges from 6.7 to 47.8 RA, the largest span reported for any oceanic island. By combining trace elemental variability with 3He/4He, we assess the extent of lithological variability in the Icelandic mantle plume. Trace-element ratios that are likely to preserve information about mantle source regions (e.g., Mn/Fe, Ni/(Mg/Fe), Ga/Sc, Zn/Fe and Mn/Zn) suggest a peridotitic mantle source in all rift-related volcanic regions, as well as in the off-rift flank zones of Öræfajökull and Snæfellsnes. However, a signal of a more pyroxenitic mantle lithology is clearly visible in olivine from the South Iceland Volcanic Zone, which represents the southward propagation of the Eastern Rift Zone, while olivine from Tertiary lavas suggests a mixed peridotite-pyroxenite source composition. We are able to identify four components present in the Icelandic mantle: a lithologically heterogeneous plume component with 3He/4He >MORB; a depleted MORB-like peridotite; an isotopically enriched MORB-like peridotite; and a peridotitic component with 3He/4He <MORB, sampled in the off-rift flank zone at Öræfajökull. The spatial distribution of these four components can be explained by a northward tilted mantle plume. This has previously been proposed by geophysical and geochemical studies of both Hawaii and Iceland suggesting that the plume geometry could be a contributing factor in controlling some of the spatial variation in source lithology beneath ocean islands.

Petrogenesis of an alkaline lamprophyre (camptonite) with ocean island basalt (OIB)-affinity at the NW margin of the Cuddapah basin, eastern Dharwar craton, southern India

We report petrology and geochemistry (including Sr and Nd isotopes) of a fresh lamprophyre at Ankiraopalli area at the north-western margin of Paleo-Mesoproterozoic Cuddapah basin, eastern Dharwar craton, southern India. Ankiraopalli samples possess a typical lamprophyre porphyritic-panidiomorphic texture with phenocrysts of kaersutite and diopside set in a plagioclase dominant groundmass. Combined mineralogy and geochemistry classify it as alkaline lampro- phyre in general and camptonite in particular. Contrary to the calc-alkaline and/or shoshonitic orogenic nature portrayed by lamprophyres occurring towards the western margin of the Cuddapah basin, the Ankiraopalli samples display trace element composition revealing striking similarity with those of ocean island basalts, Italian alkaline lamprophyres and highlights an anorogenic character. However, the87 Sr/86 Srinitial (0.710316 to 0.720016) and εNdinitial (– 9.54 to – 9.61) of the Ankiraopalli lamprophyre show derivation from an 'enriched' mantle source showing long term enrichment of incompatible trace elements and contrast from those of (i) OIB, and (ii) nearby Mahbubnagar alkaline mafic dykes of OIB affinity. Combining results of this study and recent advances made, multiple mantle domains are identified in the Eastern Dharwar craton which generated distinct Mesoproterozoic lamprophyre varieties. These include (i) Domain I, involving sub-continental lithospheric mantle source essentially metasomatized by subduction-derived melts/fluids (represented by orogenic calcalkaline and/or shoshonitic lamprophyres at the Mudigubba, the Udiripikonda and the Kadiri); (ii) Domain II, comprising a mixed sub-continental lithospheric and asthenospheric source (represented by orogenic-anorogenic, alkaline to calc-alkaline transitional lamprophyres at the Korakkodu), and (iii) Domain III, representing a sub-continental lithospheric source with a dominant overprint of an asthenospheric (plume) component (represented by essentially alkaline lamprophyres at the Ankiraopalli). Our study highlights the varied mantle source heterogeneities and complexity of geodynamic processes involved in the Neoarchean-Paleo/Mesoproterozoic evolution of the Eastern Dharwar craton.

Variable Crustal Production Originating From Mantle Source Heterogeneity Beneath the South East Indian Ridge and Amsterdam‐St. Paul Plateau

AbstractThe Amsterdam‐St. Paul (ASP) Plateau formed by interaction between the South East Indian Ridge (SEIR) and the ASP mantle plume during the last 10 Myr. The combined bathymetry and gravity‐derived crustal thickness anomalies along the present and paleoaxes of the SEIR atop the plateau indicate: (1) a thicker crust and shallower water depth along the southern part of segment I2 during much of the last 10 Myr; (2) an earlier decrease (~1.4 Ma) in crustal thickness along the southern part of I2 compared to the northern part (~0.9 Ma) during the most recent period of reduced magmatism; (3) a topographic transition at ~0.7 Ma and during the last 0.1 Myr; and (4) an approximately uniform crustal thickness (8 km) along the entire I2 segment today. These observations require spatial and temporal variations in magma production during construction of the ASP Plateau over the last 3 Myr. We propose that during periods of weaker plume magma flux, spatial variations in upper mantle temperature and composition are small, and lead to small variations in crustal thickness along‐axis. In contrast, during periods of stronger plume magma flux, spatial contrasts in upper mantle temperature and composition (fertility) are large, leading to significant variations in crustal thickness. Along‐axis variations of 3He/4He, Δ8/4Pb, K/Ti, and Na8 in “zero‐age” basalts indicate that there is a gradient in the underlying mantle material, from a “common” mantle plume component (Δ8/4Pb~60) stronger in the north to a DUPAL component (Δ8/4Pb~110) dominating in the south.

Zircon U-Pb geochronology and geochemistry of Late Devonian–Carboniferous granitoids in NW Iran: implications for the opening of Paleo-Tethys

ABSTRACT We present zircon U-Pb crystallization ages combined with bulk rock major and trace element geochemistry and Sr-Nd-Pb and zircon in-situ Hf isotopic compositions of the Amand and Moro granitoid intrusions in northwest Iran. The Amand and Moro plutons include granite and syeno-diorite with LA-ICP-MS U-Pb zircon ages of 367 ± 6.8 Ma and 351 ± 1.3 Ma, respectively, representative of Late Devonian-Early Carboniferous magmatic activity in NW Iran. Geochemical characteristics such as typical enrichments in alkalis, Nb, Zr, Ga and Y, depletion in P and Sr and fractionated REE patterns with high Ga/Al ratios and Eu negative anomalies are consistent with A-type magmatic signatures. The granitoids are classified as A2-type and within-plate granitoids. The bulk rock geochemistry (enrichments in Th, Nb and, high Th/Yb, Zr/Y ratios) along with low variation of 143Nd/144Nd(i) and 87Sr/86Sr(i) ratios and positive zircon εHf(t) support the role of a mantle plume component for the evolution of the Amand and Moro A-type granitoids in an extensional tectonic environment. In fitting with wider regional knowledge, this magmatism occurred during Paleo-Tethys opening in northern Gondwana.

Geochemical and geochronological evidence for a Middle Permian oceanic plateau fragment in the Paleo-Tethyan suture zone of NE Iran

The mafic–ultramafic Fariman complex in northeastern Iran has been interpreted as a Paleo-Tethyan ophiolitic fragment with subduction- and plume-related characteristics as well as a basin deposit on an active continental margin. Contributing to this issue, we present geochemical, geochronological, and mineralogical data for transitional and tholeiitic basalts. Thermodynamic modeling suggests picritic parental magmas with 16–21 wt% MgO formed at plume-like mantle potential temperatures of ca. 1460–1600 °C. Rare pyroxene spinifex textures and skeletal to feather-like clinopyroxene attest to crystallization from undercooled magma and high cooling rates. Chromium numbers and TiO2 concentrations in spinel are similar to those in intraplate basalts. 40Ar–39Ar dating of magmatic hornblende yielded a plateau age of 276 ± 4 Ma (2σ). Transitional basalt with OIB-like trace element characteristics is the predominant rock-type; less frequent are tholeiitic basalts with mildly LREE depleted patterns and picrites with intermediate trace element characteristics. All samples show MORB-OIB like Pb/Ce, Th/La, and Th/Nb ratios which preclude subduction-modified mantle sources and felsic crustal material. Tholeiitic basalts and related olivine cumulate rocks show MORB-like initial eNd values of + 9.4 to + 6.2 which define a mixing line with the data for the transitional basalts (eNd ca. + 2.6). Initial 187Os/188Os ratios of 0.124–0.293 support mixed sources with a high proportion of recycled mafic crust in the transitional basalts. High concentrations of highly siderophile elements are in agreement with the high mantle potential temperatures and inferred high-melting degrees. It is argued that the Fariman complex originated by melting of a mantle plume component as represented by the OIB-like transitional basalt and entrained asthenosphere predominant in the MORB-like tholeiites. Two lines of evidence such as association of the Fariman complex with pelagic to neritic sedimentary rocks and the tectonic position at the boundary of two continental blocks defined by ophiolites and accretionary complexes of different ages suggest formation in an oceanic domain. Thus, we interpret it as a fragment of an oceanic plateau, which escaped subduction and was accreted as exotic block in the Paleo-Tethyan suture zone.

Influence of electron dynamics on the enhancement of double-pulse femtosecond laser-induced breakdown spectroscopy of fused silica

Femtosecond laser pulse train induced breakdown of fused silica was studied by investigating its plasma emission and the ablated crater morphology. It was demonstrated that the electron dynamics in the ablated fused silica play a dominant role in the emission intensity of induced plasma and the volume of material removal, corresponding to the evolution of free-electron, self-trapped excitons, and the phase change of the fused silica left over by the first pulse. For a fluence of 11 J/cm2, the maximum plasma intensity of double-pulse irradiation at an interpulse delay of 120 ps was about 35 times stronger than that of a single-pulse, while the ablated crater was reduced by 27% in volume. The ionization of slow plume component generated by the first pulse was found to be the main reason for the extremely high intensity enhancement for an interpulse delay of over 10 ps. The results serve as a route to simultaneously increase the spatial resolution and plasma intensity in laser-induced breakdown spectroscopy of dielectrics.

He, Ne and Ar ‘snapshot’ of the subcontinental lithospheric mantle from CO2 well gases

The subcontinental lithospheric mantle (SCLM) constitutes a significant portion of the upper mantle sourcing magmatic volatiles to the continents above, yet its geochemical signature and evolution remain poorly constrained. Here we present new interpretation of noble gas datasets from two magmatic CO2 fields in the SW US, namely Bravo Dome and Sheep Mountain, which provide a unique insight into the volatile character of the SCLM sourcing the Cenozoic volcanism in the region. We identify that reduction of 3He/4Hemantle ratio within the Sheep Mountain CO2 field can be attributed to radiogenic production within the SCLM. Using a Reduced Chi-Squared minimisation on the variation of derived 4He/21Necrust ratios within samples from the Sheep Mountain field, combined with a radiogenically raised 21Ne/22Nemantle end member, we resolve 3He/4Hemantle ratios of 2.59 ± 0.15 to 3.00 ± 0.18 Ra. These values correspond with a 21Ne/22Nemantle value of 0.136. Using these 3He/4Hemantle end member values with 21Nemantle resolved from Ne three component analysis, we derive the elemental 3He/22Nemantle of 2.80 ± 0.16 and radiogenic 4He/21Ne*mantle range of 1.11 ± 0.11 to 1.30 ± 0.14. A second Reduced Chi-Squared minimisation performed on the variation of 21Ne/40Arcrust ratios has allowed us to also determine both the 4He/40Armantle range of 0.78 to 1.21 and 21Ne/40Armantle of 7.66 ± 1.62 to 7.70 ± 1.54 within the field. Combining these ratios with the known mantle production ranges for 4He/21Ne and 4He/40Ar allows resolution of the radiogenic He/Ne and He/Ar ratios corresponding to the radiogenically lowered 3He/4Hemantle ratios. Comparing these values with those resolved from the Bravo Dome field allows identification of a clear and coherent depletion of He to Ne and He to Ar in both datasets. This depletion can only be explained by partial degassing of small melt fractions of asthenospheric melts that have been emplaced into the SCLM. This is the first time that it has been possible to resolve and account for both the mantle He/Ne and He/Ar ratios within a SCLM source. The data additionally rule out the involvement of a plume component in the mantle source of the two gas fields and hence any plume influence on the Colorado Plateau Uplift event.