Plume Source Research Articles

Combined petrography, noble gas, stable isotope and fluid inclusion chemistry of carbonatites from Uganda: Implications for the origin of the carbonatite melt in continental rift setting

An integrated petrographic, fluid inclusion, noble gas, and stable isotope study was performed for the Late Oligocene, spatially associated Sukulu and Tororo carbonatite complexes in SE Uganda in order to understand their genesis and provide additional constraints on the evolution of the East African Rift System (EARS). Both carbonatite complexes comprise mainly ring-shaped calcio‑carbonatites with accessory primary magmatic phases, including olivine, phlogopite, aegirine and pyrochlore. Absent or low Na deficiency on the A-site and high F contents (0.4–1.2 apfu) reveal the magmatic origin of pyrochlore. In contrast, the corroded rims of olivine, Sr-enrichments of the rims of apatite, as well as the occurrence of Sr- and Mg-rich lamellae in calcite, coupled with the presence of barite, REE-witherite, siderite, pyrite, bastnäsite and galena as secondary phases imply a hydrothermal overprint of the primary magmatic mineral assemblage.Fluid inclusions found in the cores of clustered apatite crystals are primary, two-phase H2O + NaCl aqueous inclusions in Sukulu-, and two-, or multi-phase H2O + NaCl ± NaHCO3 ± other solid inclusions in Tororo Complex. In the gas phase of some inclusions small amounts of CO2 or CH4 have been detected by micro-Raman analysis. The estimated trapping temperature of 330–425 °C and pressure of 465–1330 bars using the accepted denudation rates calculated for the EARS as independent barometers, reveal subsolidus crystallization conditions for the intercrystalline clustered apatite aggregates. Stable C-O isotope compositions of both carbonatite complexes suggest ultimately a mantle origin for the carbonate phase.Helium, neon and argon isotope compositions, liberated by crushing of bulk rock, apatite and calcite samples, show a systematic variation between the analyzed phases. Relatively low 3He/4He ratios of apatite and pyroxene separates (<5.3 Ra) are accounted for by radiogenic production of 4He in these minerals, whereas the air-corrected values in calcite reach as high as 9.6 Ra, characteristic for plume sources. Whole-rock Ne isotope ratios provide evidence for a depleted MORB mantle (DMM) trend. Combined (21Ne/22Ne)EX (air-corrected 21Ne/22Ne ratio extrapolated to Ne-B composition Holland and Ballentine, 2006; Trieloff et al., 2000) versus 4He/3He are indicative of mixing of a plume-like and a DMM-like component in the source region of the parental carbonatite magmas, which is in contrast to some recent interpretations of recent/sub-recent magmas in the EARS. Based on new noble gas data from this study we argue that in the early phase of the magmatic activity of the EARS, a low-viscosity carbonatite magma may have intruded into the shallow crust rapidly, without any significant contamination by the sub-continental lithospheric mantle.Combined fluid inclusion data and noble gas results collectively imply that the carbonatites worldwide are carbothermal (hydrothermal) rather than purely magmatic products and that they crystallized at sub-solidus temperatures from dominantly magma-derived fluids.

Trace Element Contents of Mantle-Derived Magmas Through Time

Abstract A large compilation of quality-curated major and trace element data has been assembled to investigate how trace element patterns of mafic and ultramafic magmas have varied with time through particular settings from the Archean to the Phanerozoic, the primary objective being to recognise at what times particular patterns of variation emerge, and how similar these are to baseline data sets representing tectonic settings in the modern Earth. The most informative element combinations involve Nb, Th and the REE, where REE are represented by ‘lambda’ parameters describing slope and shape of patterns. Combinations of the ratios of Th, Nb, La and lambda values from Archean and early Proterozoic basalts and komatiites reveal a distinctive pattern that is common in most well-sampled terranes, defining a roughly linear trend in multi-dimensional space from compositions intermediate between modern n-MORB and primitive mantle at one end, towards compositions approximating middle-to-upper continental crust at the other. We ascribe this ‘Variable Th/Nb’ trend in most instances to varying degrees of crustal contamination of magmas with similar compositions to modern oceanic plateau basalts. Komatiites had slightly more depleted sources than basalts, consistent with the hypothesis of derivation from plume tails and heads, respectively. The most significant difference between Precambrian and Phanerozoic plume-derived basalts is that the distinctive OIB-like enriched source component appears to be largely missing from the Archean and Proterozoic geologic record, although isolated examples of OIB-like trace element characteristics are evident in datasets from even the oldest preserved greenstones. Phanerozoic intra-cratonic LIPs, such as the 260 Ma Emeishan LIP in China, have fundamentally different geochemical characteristics to Archean and Paleoproterozoic assemblages; the oldest Proterozoic LIP we have identified that has this type of ‘modern’ signal is the Midcontinent Rift at 1100 Ma. The data are consistent with plume tail sources having changed from being dominantly depleted in the Archean Earth to dominantly enriched in the Phanerozoic Earth, while plume head sources have hardly changed at all. Trace element patterns considered to be diagnostic of subduction are locally present but rare in Archean terranes and become more prevalent through the Proterozoic, although this conclusion is tempered by the large degree of overlap in compositional space between continental arc magmas and continental flood basalts. This overlap reflects the difficulty of distinguishing the effects of supra-subduction metasomatizm and flux melting from those of crustal contamination. Additional factors must also be borne in mind, particularly that trace element partitioning systematics may have been different in all environments in a hotter planet, and large-scale asthenospheric overturns might have been predominant over modern-style plumes in the Archean Earth. Some basaltic suites in particular Archean terranes, notably the western parts of both the Yilgarn and Pilbara cratons in Western Australia and parts of the Superior Craton, have restricted, but locally predominant, suites of basalts with characteristics akin to modern oceanic arcs, suggesting that some process similar to modern subduction was preserved in these particular belts. Ferropicrite magmas with distinctive characteristics typical of modern OIBs and some continental LIPs (notably Emeishan) are rare but locally predominant in some Archean and early Proterozoic terranes, implying that plume sources were beginning to be fertilised by enriched, probably subducted, components as far back as the Mesoarchean. We see no evidence for discontinuous secular changes in mantle-derived magmatism with time that could be ascribed to major mantle reorganisation events. The Archean–Proterozoic transition appears to be entirely gradational from this standpoint. The transition from Archean-style to Phanerozoic-style plume magmatism took place somewhere between 1900 Ma (age of the Circum-Superior komatiitic basalt suites) and 1100 Ma (the age of the Midcontinent Rift LIP).

Gone with the Wind: Microbial Communities Associated with Dust from Emissive Farmlands.

Dust is a major vehicle for the dispersal of microorganisms across the globe. While much attention has been focused on microbial dispersal in dust plumes from major natural dust sources, very little is known about the fractionation processes that select for the "dust microbiome." The recent identification of highly emissive, agricultural land dust sources in South Africa has provided the opportunity to study the displacement of microbial communities through dust generation and transport. In this study, we aimed to document the microbial communities that are carried in the dust from one of South Africa's most emissive locations, and to investigate the selective factors that control the partitioning of microbial communities from soil to dust. For this purpose, dust samples were generated at different emission sources using a Portable In-Situ Wind Erosion Lab (PI-SWERL), and the taxonomic composition of the resulting microbiomes was compared with the source soils. Dust emission processes resulted in the clear fractionation of the soil bacterial community, where dust samples were significantly enriched in spore-forming taxa. Conversely, little fractionation was observed in the soil fungal communities, such that the dust fungal fingerprint could be used to identify the source soil. Dust microbiomes were also found to vary according to the emission source, suggesting that land use significantly affected the structure and fractionation of microbial communities transported in dust plumes. In addition, several potential biological allergens of fungal origin were detected in the dust microbiomes, highlighting the potential detrimental effects of dust plumes emitted in South Africa. This study represents the first description of the fractionation of microbial taxa occurring at the source of dust plumes and provides a direct link between land use and its impact on the dust microbiome.

Plume Source Localization on Enceladus by Sequential Monte Carlo Method

Water vapor plumes emanating from the geyser vents at Enceladus’s south pole area invite the possibility of direct access to the subsurface liquid reservoir to acquire pristine biological material if it exists. Any descending lander adapted for plume localization is required to not only explore the icy plume environment during its descent, but it must also infer the location of the landing target—the plume source—autonomously. Compared with existing scenarios of terrestrial plume source localization methods, the source likelihood map (SLIM) method for an Enceladus mission offers a more extensive search area, a higher maneuver velocity, and a shorter search time. This paper investigates a particle-based odor source localization (pOSL) approach that offers the prospect of targeting one of the plume sources by autonomously measuring the concentration field. Reasons for the negative likelihood and overfitting issues associated with Bayesian SLIM are analyzed to build a novel probabilistic model. By implementing this model, the proposed pOSL algorithm evaluates the observation likelihood via posterior maximization method and estimates the source location via the sequential Monte Carlo method. The pOSL algorithm resolves difficulties associated with other methods while reducing the time complexity from to . The numerical simulations illustrate that the proposed approach is feasible and permits accurate targeting of Enceladus’s geyser vents.

Petrogenesis of Pleistocene Basalts from the Western Snake River Plain, Idaho

Abstract We present new geochemical, Sr, Nd, and Pb isotope, and 40Ar/39Ar data from Pleistocene basalts of the Western Snake River Plain (WSRP), Idaho, USA to explore their petrogenesis and to investigate the nature of the lithosphere at the western boundary of the North American craton. The basalts are divided into three groups based on their geochemical and isotopic characteristics. Prior to ∼1 Ma, volcanoes in the WSRP erupted iron-rich tholeiites (FeB1), but subsequent volcanism was dominated by concurrent eruptions of mildly alkaline, alumina-rich lavas (AlB) and iron-rich tholeiites (FeB2) with isotopic signatures similar to the AlB lavas. New 40Ar/39Ar dates of AlB and FeB2 basalts range from 0·920 ± 0·049 to 0·287 ± 0·014 Ma. MELTS models of FeB1 differentiation trends indicate that the range of compositions in this suite can be produced by 10–15 % crystallization of olivine and plagioclase at low pressure using the least evolved FeB1 composition as a parental magma; isotopic ratios can be produced via combined assimilation of a Miocene rhyolite and fractional crystallization. Additional modeling suggests that parental magmas at AlB centers were produced by 3–12 % equilibrium melting of a garnet–spinel-enriched mantle source, slightly different from that proposed for the youngest mildly alkaline lavas of the eastern and central Snake River Plain. Our new geochemical, isotopic, and geochronological data for the FeB2 basalts suggests that they are related to AlB-type magmas via a combination of fractional crystallization and assimilation of evolved mafic crust. MELTS models suggest that crystallization of an AlB parental melt at a depth of 6–8 km (2·5 kbar) could produce residual liquids having many of the major oxide characteristics of FeB2 ferrobasalts. Sr–Nd–Pb isotopic signatures of these three suites indicate a dominant contribution from an enriched plume source. FeB1 lavas are probably products of mixing between melts of an enriched plume mantle source (represented by Imnaha and Steens Basalts of the Columbia River Basalt Group) and isotopically heterogeneous sub-continental lithospheric mantle (SCLM) that has been isolated from the convecting mantle since the Archean. Isotopic ratios of FeB2 and AlB lavas capture mixing between enriched plume mantle and a more isotopically homogeneous ancient SCLM domain characteristic of the eastern and central Snake River Plain, with a coupled decrease in lithospheric contribution and degree of partial melting through time to the present. Mixtures of enriched asthenospheric reservoirs with lithospheric mantle have been proposed for neighboring volcanic fields to the east along the strike of the Yellowstone–SRP hotspot track, and to the west owing to differences in the mantle underlying the boundary of the North American craton and accreted terranes. Our petrogenetic model for the Pleistocene WSRP basalts suggests that there is also a lateral, across-strike gradient in the geometry and interaction of enriched plume mantle and ancient lithosphere. We reiterate suggestions that the WSRP is a lithosphere-scale conduit connecting initial plume-head impingement in east–central Oregon with the subsequent Yellowstone–SRP hotspot plume-tail track.

The Paleozoic-Aged University Foidolite-Gabbro Pluton of the Northeastern Part of the Kuznetsk Alatau Ridge, Siberia: Geochemical Characterization, Geochronology, Petrography and Geophysical Indication of Potential High-Grade Nepheline Ore

Geological, geochemical and ground magnetic techniques are used to characterize the University alkaline-gabbroid pluton and crosscutting N-S trending alkaline dikes, located northeast of the Kuznetsk Alatau ridge, Siberia. Trace element concentrations and isotopic compositions of the igneous units were determined by XRF, ICP-MS and isotope analysis. The Sm-Nd age of subalkaline (melanogabbro, leucogabbro 494–491 Ma) intrusive phases and crosscutting alkaline dikes (plagioclase ijolite, analcime syenite 392–389 Ma) suggests two stages of activity, likely representing separate events. The subalkaline and alkaline rocks are characterized by low silicic acidity (SiO2 = 41–49 wt %), wide variations in alkalinity (Na2O + K2O = 3–19 wt %; Na2O/K2O = 1.2–7.2 wt %), high alumina content (Al2O3 = 15–28 wt %) and low titanium content (TiO2 = 0.07–1.59 wt %). The new trace element data for subalkaline rocks (∑REE 69–280 ppm; La/Yb 3.7–10.2) of the University pluton and also the crosscutting younger (390 Ma) alkaline dikes (∑REE 10–1567 ppm; La/Yb 0.7–17.8 ppm) both reflect an intermediate position between oceanic island basalts (OIBs) and island arc basalts (IABs). The presence of a negative Nb–Ta anomaly and the relative enrichment in Rb, Ba, Sr, and U indicate a probable interaction of mantle plume material with the lithospheric mantle beneath previously formed accretion complexes of subduction zones. The isotopic signatures of strontium (εSr(T) +3.13–+28.31) and neodymium (εNd(T) +3.2–+8.7) demonstrate the evolution of parental magmas from a plume source from moderately depleted PREMA mantle, whose derivatives underwent selective crustal contamination.

Towards Fast Plume Source Estimation with a Mobile Robot.

The estimation of the parameters of an odour source is of high relevance for multiple applications, but it can be a slow and error prone process. This work proposes a fast particle filter-based method for source term estimation with a mobile robot. Two strategies are implemented in order to reduce the computational cost of the filter and increase its accuracy: firstly, the sampling process is adapted by the mobile robot in order to optimise the quality of the data provided to the estimation process; secondly, the filter is initialised only after collecting preliminary data that allow limiting the solution space and use a shorter number of particles than it would be normally necessary. The method assumes a Gaussian plume model for odour dispersion. This models average odour concentrations, but the particle filter was proved adequate to fit instantaneous concentration measurements to that model, while the environment was being sampled. The method was validated in an obstacle free controlled wind tunnel and the validation results show its ability to quickly converge to accurate estimates of the plume’s parameters after a reduced number of plume crossings.

The volatile budget of Hawaiian magmatism: Constraints from melt inclusions from Haleakala volcano, Hawaii

Pre-eruptive volatile contents recorded by melt inclusions from ocean island settings such as Hawaii constrain the extent of deep Earth outgassing on geologic timescales by mantle plume activity. However, melt inclusions trapped from a partially degassed magma will not reflect the original volatile content of the primary melt, and relatively silicic (>45 wt% SiO2) shield-stage tholeiites are more likely to have been affected by degassing during fractionation at shallow depths. In contrast, magmas associated with post-shield volcanoes erupt volatile-rich melts that ascend quickly from the source region and crystallize in deep reservoirs. As a result, melt inclusions from post-shield volcanoes may be more likely to preserve undegassed melt compositions that can be used to determine the primary melt and source volatile contents. In this study, we analyzed melt inclusions from Haleakala Volcano (East Maui, Hawaii) to estimate the volatile budget of Hawaiian post-shield magmas. Melt inclusions from Haleakala contain up to 1.3 wt% CO2, 1.2 wt% H2O, and about 2000 ppm S. We calculate that melts from Haleakala were derived from a parental melt with ~0.7 wt% H2O and ~0.7 wt% CO2 and experienced ~15–40% polybaric, fluid-saturated crystallization starting at 5–7 kbar. Post-shield melt inclusions from Haleakala have H2O/Ce and CO2/Ba compositions that are intermediate between shield-stage melt inclusions from Kilauea and the alkaline North Arch Volcanic Field, which is consistent with a scenario in which primary melts from Haleakala are influenced by the volatile-rich auto-metasomatized periphery of the composite plume source.

Fine‐Scale Structure of Earth's Deep Mantle Resolved Through Statistical Analysis of Hawaiian Basalt Geochemistry

AbstractHawaiian volcanoes belong to two geographically and geochemically distinct trends, the Loa and Kea trends. The cause of this dichotomy is still strongly debated. One of the prevailing hypotheses is that the two trends originate in the deep mantle where the Hawaiian mantle plume straddles two geophysically and geochemically distinct domains at the core‐mantle boundary (CMB). New high‐precision multi‐isotopic (Pb, Hf, Nd, and Sr) compositions of lavas from three key volcanoes, Lō‘ihi, Kohala, and Haleakalā, show transitional signatures between Loa and Kea compositions that call into question the degree of physical independence between the two trends. Statistical analysis of multi‐collector inductively coupled plasma mass spectrometer or triple‐spike Pb (n &gt; 800) and Sr, Nd, and Hf data (n &gt; 400) for shield tholeiites from the entire Hawaiian Islands (&lt;5.5 Ma) identifies six unique geochemical groups, and for the first time, documents large‐scale heterogeneities in the Kea trend. The spatial orientation of the six geochemical groups shows that the bilateral zonation of the plume source at the CMB is gradational, and that the Hawaiian mantle plume periodically entrains large‐scale ephemeral geochemical heterogeneities on million‐year, regional timescales. These geochemical heterogeneities are stretched vertically during transit of the plume to the surface and are observed in the lavas from the Hawaiian Islands. These results provide evidence that the large low shear velocity province in the deep Pacific is thermochemical and highly heterogeneous.

Sr-Nd-Hf-O isotope constraints on crustal contamination and mantle source variation of three Fe-Ti-V oxide ore deposits in the Emeishan large igneous province

Magma-carbonate interaction and recycled oceanic crust in the mantle source are thought to have played a critical role in the genesis of magmatic Fe-Ti-V oxide ore deposits in the Emeishan mantle plume-related large igneous province in southwestern China. To test these hypotheses, we have carried out a combined study of zircon Hf-O isotopes and whole-rock Sr-Nd isotopes on three representative magmatic Fe-Ti-V oxide ore deposits associated with different types of wall-rock in the province: the Hongge and Taihe deposits with granitoid wall-rocks in most places, and the Panzhihua deposit with marble wall-rocks. Diopside-garnet marble xenoliths are common in the host gabbros of the Taihe deposit. The zircon separates used in this study are all from the mineralized units of the host layered mafic–ultramafic intrusions. The selected zircon grains yield U-Pb isotopic ages of 258.2 ± 2.8 Ma for Hongge intrusion, 263.3 ± 2.2 Ma for Taihe intrusion, and 257.6 ± 2.1 Ma for Panzhihua intrusion. The Hf-O isotopes of the zircon separates are εHf(t) from 3.1 to 4.7 and δ18O from 5.77 to 6.45‰ for Hongge, εHf(t) from 8.3 to 9.8 and δ18O from 4.12 to 4.56‰ for Taihe, and εHf(t) from 4.6 to 6.3 and δ18O from 5.49 to 6.10‰ for Panzhihua. The zircon δ18O values of the Taihe deposit are significantly lower than the mantle zircon values (5.3 ± 0.3‰), whereas those of the Hongge and Panzhihua deposits are higher than the mantle values. The initial 87Sr/86Sr and εNd(t) of the whole rock samples from the deposits are similar, from 0.7047 to 0.7052 and from 2.3 to 3.1, respectively. The observed decoupling of zircon Hf-O isotopes (low δ18O and high εHf(t) values) in the Taihe deposit is most consistent with a high-temperature altered, low δ18O oceanic gabbroic component in the plume source for this deposit. All of the isotope data together show that the parental magmas for these deposits experienced different types of crustal contamination. Contamination with the Precambrian metamorphic country rocks is much more significant in the parental magma for the Hongge deposit than those for the Panzhihua and Taihe deposits. On the other hand, contamination with marble country rocks is more pronounced in the parental magmas for the Panzhihua and Taihe deposits than in the parental magma for the Hongge deposit. An important new finding from this study is that these deposits are not linked to the same component of the mantle plume nor a single type of crustal contamination.

Guidance, Navigation, and Control System for Landing near Plume Source on Enceladus

Saturn’s moon Enceladus is one of the most promising candidates in the solar system for hosting microbial life. Plumes emanating from its south pole are ejecting water directly from the ocean to space. The Enceladus Explorer (EnEx) lander mission concept aims to land near a plume source and deploy a melting probe to sample liquid pockets under the plumes for life. Landing there would be exceptionally challenging due to the rough canyonous topography, polar lighting conditions, a surface covered by superfine snow, and strict planetary protection regulations. In this work the Guidance, Navigation, and Control (GN&C) system and operations for the critical approach phase of such a landing are investigated. An advanced GN&C system is defined, comprising extended Kalman filter simultaneous localization and mapping (EKF-SLAM)–based navigation, fuzzy-reasoning-based hazard detection and avoidance, and convex guidance. To verify the landing feasibility and further refine the concept, the system is investigated using a closed-loop simulation tool we created. The performance of each function is analyzed separately with sensitivity and worst-case analyses. A Monte Carlo simulation encompassing all functions is performed to estimate mission success statistics.

Sources and transport of fluid and heat at the newly-developed Theistareykir Geothermal Field, Iceland

Successful management of geothermal energy requires detailed understanding of physical and chemical conditions within the field prior to exploitation. It is thus crucial to identify fluids involved and their residence times, as well as the heat source, so as to assess the potential of the resource in terms of energy production. To this end, a geochemical study of relatively undisturbed fluids from the newly-developed Theistareykir geothermal field, Northern Volcanic Zone, Iceland was carried out on production wells, fumaroles, and mud pots. Noble gas (He, Ne, Ar, Kr, and Xe) elemental and isotopic abundances and stable isotopes (δ18O and δ2H) were measured to determine the system fluid sources and dynamics as exploitation proceeds. Results of this study, together with previously published data, show that four fluid sources are present: modern and local meteoric water (48.9%); sub-modern meteoric water from regional highlands precipitation (10.6%); pre-Holocene glaciated meteoric water (40.4%) with strongly depleted δ2H values of −127‰, calculated 40K-40Ar* fluid residence times from 57 ± 20 ka to 92 ± 30 ka and a (U/Th)-4He fluid residence times from 96 ± 50 ka to 160 ± 80 ka; and, finally, 3He-rich magmatic fluids. Concomitant enrichment in 18O and radiogenic 4He suggests that some fluids reside a long time in the reservoir, exchanging O and He with reservoir rocks. Maximum estimated helium isotopic ratios, 3He/4He (R), of 11.45 Ra (Ra = atmospheric ratio) show that the magma beneath Theistareykir is a depleted mid-ocean ridge basalt (MORB) mantle (DMM), with less influence (8.7 to 12.7%) of the Icelandic mantle plume source. Calculated heat (Q)/3He ratios plotted vs. R/Ra and 4He/36Ar ratios suggest that convective heat transport dominates the eastern part of the field where the magmatic heat source is located, while in other parts of the field, heat conduction seems to be dominant. Boiling and phase separation exists in the field, as indicated by δ18O values which fall to the left of the Global Meteoric Water Line in a δ18O vs. δ2H plot, but Q/3He ratios indicate that boiling affects only 1–10% of the fluid reservoir. With this obtained knowledge, any subsequent changes in the field conditions during the exploitation phase of Theistareykir can be better understood, helping to sustainably manage the resource.

Isolated buoyant convection in a two-layered porous medium with an inclined permeability jump

Abstract

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.

The Deccan Volcanic Province (DVP), India: A Review

The Deccan Volcanic Province (DVP), India: A Review

Ca. 2.1 Ga Mahakoshal Supracrustal Belt: An allochthonous terrain in Central India Tectonic Zone

Back-arc magmatism is perhaps extremely rare in a Paleoproterozoic mobile tectonic regime. The present study provides a detailed account on the petrogenesis of basalts that are interleaved with marine sedimentary rocks in the Agori Formation, which overlies the Chitrangi Formation in the ~2.1 Ga Mahakoshal supracrustal belt within the Central India Tectonic Zone. The basalts are sub alkaline, tholeiitic in nature. They have moderate concentrations of TiO2 (~0.9 wt%), Nb (2.6–5.2 ppm), and Zr (59–105 ppm), Zr/Nb (18–24) and Zr/Y (3.1–3.6) ratios, mildly enriched chondrite normalized rare earth element patterns (LaN/YbN = 1.6–2.8), and negative Nb and Ti anomalies on a primitive mantle normalized trace element variation diagram. Post-magmatic low grade metamorphism, alteration and trace element mobility, and/or contamination by Archean continental crust, if any, cannot relate to these patterns. The positive initial εHf (2.1 Ga) + 2.24 to +5.15 and εNd(2.1 Ga) + 1.02 to +1.54 isotopic compositions in these basalts are variable, yet consistent with a juvenile mantle source. We ascribe the isotopic variability in the Agori basalts to reflect the general isotopic variability in the depleted mantle at 2.1 Ga. Consequently, the basalts were produced by decompression partial melting in the shallow upper mantle in a back-arc setting. Subduction zone magmatic processes produced the arc volcanics in the upper stratigraphic unit (Agori), whereas, a plume source generated the ferropicrites in the underlying Chitrangi Formation. The evolution of the Mahakoshal supracrustal belt predates the collision and interlocking of North and South India cratonic blocks, which resulted in the formation of Central India Tectonic Zone. Hence, it represents an allochthonous plume-arc accreted terrain in the CITZ.

Identification of chondritic krypton and xenon in Yellowstone gases and the timing of terrestrial volatile accretion

Identifying the origin of noble gases in Earth's mantle can provide crucial constraints on the source and timing of volatile (C, N, H2O, noble gases, etc.) delivery to Earth. It remains unclear whether the early Earth was able to directly capture and retain volatiles throughout accretion or whether it accreted anhydrously and subsequently acquired volatiles through later additions of chondritic material. Here, we report high-precision noble gas isotopic data from volcanic gases emanating from, in and around, the Yellowstone caldera (Wyoming, United States). We show that the He and Ne isotopic and elemental signatures of the Yellowstone gas requires an input from an undegassed mantle plume. Coupled with the distinct ratio of 129Xe to primordial Xe isotopes in Yellowstone compared with mid-ocean ridge basalt (MORB) samples, this confirms that the deep plume and shallow MORB mantles have remained distinct from one another for the majority of Earth's history. Krypton and xenon isotopes in the Yellowstone mantle plume are found to be chondritic in origin, similar to the MORB source mantle. This is in contrast with the origin of neon in the mantle, which exhibits an isotopic dichotomy between solar plume and chondritic MORB mantle sources. The co-occurrence of solar and chondritic noble gases in the deep mantle is thought to reflect the heterogeneous nature of Earth's volatile accretion during the lifetime of the protosolar nebula. It notably implies that the Earth was able to retain its chondritic volatiles since its earliest stages of accretion, and not only through late additions.

Possible tectonic patterns along the eastern margin of Gondwanaland from numerical studies of mantle convection

Two end-member scenarios have been proposed for the tectonic situation along the eastern margins of Gondwanaland before Zealandia was formed ca. 100 million years ago (Ma), namely: (1) A subduction zone located far from the eastern margin of Zealandia, wherein Zealandia may have separated from Gondwanaland by plume push of an active hotspot plume.; (2) A subduction zone located along the eastern margin of Gondwanaland, wherein Zealandia possibly separated from Gondwanaland via trench/subduction retreat. Assuming that the thermal structure of the deep mantle and source of hotspot plumes remained relatively stationary over the last hundred million years, major hotspot plumes with a large buoyancy flux did not exist under Zealandia; the eastern margins of Gondwanaland were far from two large low-shear-velocity provinces under the Africa–Atlantic and South Pacific regions. Herein, through numerical studies of three-dimensional global mantle convection, we examined the mantle convection and surface tectonic patterns at ~100 Ma. The present model considered the real configuration of Gondwanaland at the model surface to observe long-term variations of mantle convection and the resulting surface tectonic conditions. The results demonstrate that the extensive subduction zones developed preferentially along the eastern margin of Gondwanaland when the temperature anomaly of the lower mantle was primarily dominated by high-temperature regions under present-day Africa–Atlantic and South Pacific regions. The results of this study support one of the proposed hypotheses, where the breakup at the eastern margins of Gondwanaland at ~100 Ma occurred via trench/subduction retreat.

Merging of two plumes from area sources with applications to cooling towers

A mathematical model is developed to describe the merging of adjacent plumes arising from nonpoint sources. Essential to this model is a characterization of how plume boundary curvature modifies entrainment and defines the height of merger. The nonpoint nature of the plume sources considered in this study serves as an extension to past irrotational flow theory, which focuses primarily on point source plumes.

Mantle plume dynamics at the rear of a retreating slab

SUMMARY In this paper, we present 3-D numerical simulations in which a compositional mantle plume rises through a shallow mantle layer at the rear of a retreating slab. The slab–plume buoyancy flux ratio Bs/Bp is varied from 3 to 30 over nine slab–plume simulations. The plume causes an overall decrease of the slab retreat rate from 15 to 7 % in the corresponding range $B_s/B_p=[3,30]$. The retreat rate decrease occurs in two stages: the first decrease occurs remotely when plume and slab are hundreds of kilometres apart; the second decrease is linked to the slab–plume-head impact. Continuous tracking of key positions along the plume head and conduit together with velocity profiling further shows a very close interplay between the conduit and plume head dynamics. In particular, the combination of the slab– and plume(head)–induced flows at the rear of the slab increases the advection and tilt of the conduit and causes its flaring with height in the direction parallel to the trench. As a result, the conduit source slowly drifts away from the slab by hundreds of kilometres and flares by one and a half times its original radius over 30 Myr of plume head spreading. The conduit tilt and flaring result in an increasingly unbalanced azimuthal distribution of the incoming plume flux from the feeding conduit into the head. These changes in the feeding conditions coupled with the mantle flow at the level of the plume head lead to the asymmetric spreading of plume material beneath the plate in the preferential direction that is parallel to the trench. Upon its arrival against the slab, it can be a front of buoyant material set to subduct along the slab width that has widened up to two and a half times its initial dimension. In nature, it is expected to extend from hundreds to thousands of kilometres depending on the slab–plume buoyancy flux ratio. To our knowledge, this study is the first to highlight the inter-relationships between a plume head and its feeding conditions in the plume source region.