Incompatible Element Ratios Research Articles

Geochemistry of Volcanic Rocks in Ponelo Island, North Gorontalo, Indonesia

Ponelo Island is located in the northern part of Sulawesi, which is still an enigma regarding the genesis of the volcanic rocks found on this island. Therefore, the objective of this study is to understand the petrogenesis and tectonic implication of these volcanic rocks. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) to obtain trace and rare earth elements is the method of this study. The volcanic rocks found on Ponelo Island consist of basalt and basaltic andesite rocks with a calc-alkaline affinity. The transition data suggested a highly fractionated cause of low transition element (Ni=17-38 ppm; Cr=13-47) compared to primary magma concentration, anomalies negative of Ba, Sr, and Ti of spider diagrams, and negative anomaly of Eu (Eu/Eu*=0.88-0.99). Relationship between low concentration between Ce/Y (0.74-0.76) and La/Yb vs Sm/Yb ratio indicated ~5% spinel-lherzolite mantle source partial melting. On the other hand, incompatible element ratios, such as Ba/Nb (39.03-45.28), Ba/Th (75.52-82.67), Rb/Nb (3.93-6.22), K/Nb (1772.22-2703.45), Ba/La=13.67-14.57, Th/La (0.17-0.18), La/Nb (2.91-3.16), depleted Nb/U (6-6.74), and also lack of xenolith or enclaves indicate cryptic crustal contamination. The slab-derived fluid indicated by ratios of Rb/Y (0/019-0/05), Nb/Y (0.10-0.11), Th/Yb (0.52-0.61), and Ba/La ratio (13.29-14.57). Ponelo volcanic rocks shows typical calc-alkaline island arc tectonic setting particularly with enrichment in ion lithophile element (LILE) and light rare earth elements (LREE) along with depletion in high field strenght elements (HFSE) and heavy rare earth elemets (HREE), as shown by spider diagrams.

Slab break-off of the Kalamaili oceanic slab revealed by the latest Carboniferous mafic–ultramafic rocks in eastern North Tianshan (NW China)

Identifying the tectonic transition from oceanic subduction to collision is crucial for tracking the final stage evolution of ancient orogenic belts. In this study, we present new geochronological and geochemical data for the Mozbaysay mafic–ultramafic complex in the Balikun area, eastern North Tianshan of the southern Central Asian Orogenic Belt. This complex had intruded into the late Carboniferous volcano-sedimentary rocks and is comprised mainly of hornblende gabbro and lherzolite. Zircon U-Pb ages of the hornblende-gabbros reveal that this complex was emplaced at ca. 305 Ma. Geochemical analyses suggest these mafic–ultramafic rocks are characterized by slight enrichment of light rare earth elements (LREEs) and relatively depleted heavy rare earth elements (HREEs), resembling enriched mid-ocean ridge basalt (E-MORB). They also exhibit restricted (87Sr/86Sr)i ratios (0.702396–0.704295) and εNd(t) values (+7.0 to +9.1), indicative of a depleted mantle source with minimal crustal contamination. Incompatible element ratios (i.e., Nb/Ta, Zr/Hf, Rb/Nb, and Ba/Nb) suggest the involvement of subducted slab-derived aqueous fluids in their mantle source. These collectively indicate that the parental magmas of the Mozbaysay mafic–ultramafic rocks may have been generated by a mixed mantle source consisting of the E-MORB-like asthenospheric mantle, subcontinental lithospheric mantle (SCLM), and hydrous fluids from subducted slab. Furthermore, a slab break-off model is proposed to explain the generation of these latest Carboniferous mafic–ultramafic rocks. Integrating these findings with regional geological data, we propose that the tectonic transition from subduction (slab roll-back) to collision (slab break-off) along the Kalamaili suture zone occurred at ca. 305–300 Ma.

Geochemical processes in the roots of the Azores magmatic systems

The Azores archipelago, situated east of the Mid-Atlantic Ridge, comprises volcanic islands arranged along sub-parallel spreading systems and rests on a thick oceanic crust. Magma is supplied directly from the roots of the volcanic systems. Located at or nearby the boundary between the crust and the mantle, they consist of mafic cumulates and mafic mush layers. This work focuses on tephra samples and a submarine lava younger than 40.000 years, collected from both central volcanoes and fissure zones. Our report details a new dataset of major, trace, and volatile elements analysed in glassy melt inclusions trapped in olivine (Fo75.8–85.6) which are extracted from cumulative bodies at the vicinity of the crust-mantle boundary. Their compositions cover a range from subalkaline to mildly alkaline basalt, and trachybasalt, which match those of Azores lavas. They registered a chemical evolution through fractional crystallisation of olivine alone, as well as olivine and clinopyroxene, as both the FeOt/MgO (1.4–3.1) and CaO/Al2O3 (0.4–1.0) ratios of the melt decrease. Incompatible element ratios of Zr (40–352 ppm), Ba (135–612 ppm), and Rb (5–77 ppm), as compared to Nb (5–82 ppm), exhibit variability within a limited but significant range of values. The ranges in the Nb/Zr, Ba/Nb and Rb/Nb ratios recorded by melt inclusions possibly reveal distinct geochemical sources (at least two), and mixing between partial melts as they move upward. The halogen signature is characteristic of the shallow mantle. The majority of melt inclusions show Cl/K ratio (0.06) similar to E-MORB, although some of them are comparable to N-MORB (Cl/K = 0.03). Their F/Nd ratio may achieve a rather high value (27.8).

Earliest Paleoproterozoic arc magmatism in the western Yangtze Block, South China and its geological implications

The Archean – Early Paleoproterozoic tectono-magmatic history of the Yangtze Block is inadequately constrained owing to the scarcity of coeval magmatic outcrops. Herein, we report on the Earliest Paleoproterozoic (2498 ± 17 Ma) meta-diabase dike in the Bajiaojing area of the western Yangtze Block, which may represent the oldest known mafic magmatism in the block. Whole-rock geochemical and mineral chemical proxies (e.g., U/Nb, Sc/Yb, and La/Yb) demonstrated that the Bajiaojing meta-diabase was likely emplaced in a continental arc setting. Positive zircon εHf(t) values (> 8) and incompatible elemental ratios (e.g., Gd/Yb, Sm/Yb, and Lu/Hf) further indicated that the mafic dike was derived from a depleted asthenospheric mantle source, which was modified by fluid and sediment from the subducting slab. The subduction-related modification also accounted for the compositional heterogeneity in the mantle source, which was observed in the wide range of Zr/Nb (1.05–4.38), Ce/Pb (6.11–81.59), and Nb/U (7.96–78.72) values. By combining the Precambrian evolutionary history of the Yangtze Block, we determined that an essential magmatic episode occurred in the Earliest Paleoproterozoic that was likely linked to crustal growth in the block. Therefore, the 2.5 Ga magmatism represented by the Bajiaojing meta-diabase was a potential provenance for the synchronous detrital zircons from Precambrian basement rocks.

The relationship between iron redox states and H2O contents in back-arc basin basaltic glasses from the North Fiji Basin

Island-arc basalts (IAB) from convergent plate margins tend to have both higher iron oxidation states (higher Fe3+/Fe2+) and higher H2O contents than the mid-ocean ridge basalts (MORB) produced by seafloor spreading, which raises the question of whether these two characteristics are causally related. Back-arc basin basalts (BABB) may help with this question, in that they are products of seafloor spreading, like MORB, but sourced from supra-subduction mantle, like IAB. Here we examine the relationship between Fe3+/Fe2+ (determined by XANES spectroscopy) and H2O contents in BABB glasses from the North Fiji Basin (NFB). The glasses cover a range of compositions from 6.1 to 8.5 wt% MgO, and from 0.16 to 1.6 wt% H2O, and have also been analysed for trace elements and Sr-Nd-Pb-isotopes. Measured Fe3+/Fe2+ range from MORB-like to slightly higher values and are positively correlated with H2O and heavy-halogen contents (Cl, Br and I). The correlation is due to lower Fe2+ rather than higher Fe3+ compared to the MORB array. This suite of BABBs is not significantly enriched in trace elements other than H2O and the heavy halogens, with MORB-like Ba/Th and other incompatible-element ratios, suggesting that the H2O and halogens may have come from dehydration of subducted lithospheric serpentinites under subsolidus conditions. Na2O is unusually variable for a suite of samples from such a limited area, and is not correlated with H2O, precluding a relationship between H2O and degree of melting. The link between H2O and low Fe2+ may be explained by H2O-rich fluids entering the transcrustal magma plumbing system, where they increase the proportion of olivine and augite to plagioclase crystallizing during the crustal evolution of the magmas. In this situation, relatively small additions of H2O can have an enhanced effect on major-element chemistry if the crustal evolution proceeds by cycles of replenish-mix-tap-crystallize (RMTX) rather than by simple fractional crystallization without repeated replenishment and tapping. The absence of any increase in Fe3+ with H2O, together with the lack of any correlation between H2O and Fe3+/Fe2+ in MORB glasses generally, suggest that various mechanisms, including more extensive olivine fractionation, were responsible for elevating Fe3+/Fe2+ in H2O-rich basalts from convergent margin settings. Proxy methods based on concentrations of elements with redox-variable partition coefficients (V, Eu) are not sensitive enough to record the subtle variations in Fe3+/Fe2+ observable by XANES spectroscopy.

Contrasting Cu isotopes in mid-ocean ridge basalts and lower oceanic crust: Insights into the oceanic crustal magma plumbing systems

Magma plumbing systems exert strong controls on the formation and evolution of the oceanic crust during crust accretion. However, plumbing system dynamics based largely on mid-ocean ridge basalts (MORBs) are extremely difficult to constrain because the MORBs are aggregated melts and original information may have been blurred. Copper isotopic compositions of lower crustal gabbroic cumulates effectively archive early-stage histories because sulfides constitute the dominant Cu budget of gabbroic cumulates and are largely isolated from subsequent magma filtering processes. Here, we present Cu isotopes of a suite of gabbroic cumulates from the ultraslow-spreading Southwest Indian Ridge (Hole U1473A), and MORBs from the South Mid-Atlantic Ridge and East Pacific Rise. The δ65Cu of the MORBs from this study (+0.04 ‰ to +0.19 ‰) match those of previous MORB analyses, indicating uniform δ65Cu at various spreading rates. In contrast, the δ65Cu of gabbroic rocks vary significantly (−1.14 ‰ to 0.87 ‰), exceeding by far the range known for peridotites. The Cu budget of these gabbroic rocks is hosted in sulfides and the mantle-like S isotopic compositions of sulfides indicate their origin of igneous processes. These results thus suggest that magma accumulation and sulfide segregation would lead to notable Cu isotopic fractionation in the lower oceanic crust. We propose that repeated recharging of primitive melts and efficient mixing of melts within the plumbing system can buffer the removal of early 63Cu-rich sulfides from sulfide-saturated MORBs. The discrepancy in δ65Cu between lower oceanic crust and MORBs is a natural consequence of continuous replenishment that occurs concurrently with melt-rock interaction, mixing, crystallization and extraction, irrespective of the oceanic settings. The consistent δ65Cu in MORBs (0.09 ± 0.08 ‰) and komatiites (0.06 ± 0.06 ‰) further indicates that their Cu isotopes reflect the mean mantle source composition, providing a robust δ65Cu for bulk silicate Earth (BSE) of 0.08 ± 0.08 ‰ (2sd). Accordingly, we propose that in open sub-ridge plumbing systems, other incompatible element isotopes and ratios of elements with similar incompatibility in MORBs as Cu isotopes suggest, could also represent mean mantle source compositions.

Major, Volatile, Ore, and Trace Elements in Magmatic Melts in the Earth’s Dominant Geodynamic Environments. I. Mean Concentrations

Data from our original database, which includes more than 2 600 000 analyses for 75 elements of mineral-hosted melt inclusions and quench glasses in volcanic rocks, are generalized to calculate the mean concentrations of major, volatile, ore, and trace elements in magmatic melts from the following dominant geodynamic environments: (I) spreading zones of oceanic plates (mid-oceanic ridges), (II) environments affected by mantle plumes in oceanic plates (oceanic islands and lava plateaus), (III, IV) environments related to subduction processes (III is zones of arc magmatism on the oceanic crust, and IV is zones of magmatism in active continental margins in which magma-generating processes involve the continental crust), (V) environments of continental rifts and areas with continental hotspots, and (VI) environments of backarc spreading. A histogram of SiO2 distribution in natural magmatic melts shows a bimodal distribution: one of the maxima falls onto SiO2 concentrations of 50–52 wt % and the other onto 72–76 wt %. The most widely spread melts contain 62–66 wt % SiO2. Mean temperatures and pressures are calculated for each of the environments. The normalized multielemental patterns presented for environments I through VI show the ratios of the mean concentrations of elements in magmatic melts of mafic, intermediate, and felsic composition to the concentrations in the primitive mantle. Mean ratios of incompatible, trace, and volatile components (H2O/Ce, K2O/Cl, Nb/U, Ba/Rb, Ce/Pb, etc.) are evaluated for the melts of each of the environments. The variations in these ratios are calculated, and it is demonstrated that the ratios of incompatible elements are mostly statistically significantly different in the different environments. The differences are particularly significant between the ratios of the most differently incompatible elements (e.g., Nb/Yb) and some ratios involving volatile components (e.g., K2O/H2O).

Basalts from MAR at 13°15′–13°40′N: What mixed?

We present new data on chemical composition of basalt glasses collected within the geochemical anomaly of the Mid-Atlantic ridge at 13°25′–13°40′N, where fresh quenched lavas occur dominantly in asymmetric off-axial structures. The collected samples indicate wide compositional ranges from strongly depleted D-MORB with 0.04 wt% K2O, K2O/TiO2 = 0.08 and (La/Sm)N = 0.69 to the most enriched E-MORB with 0.92 wt% K2O, K2O/TiO2 = 0.42 and (La/Sm)N = 2.45. Clear correlations are traced between incompatible element ratios (K2O/TiO2, La/Sm, Nb/Zr), concentrations of MgO, K2O, H2O, and 87Sr/86Sr ratio, suggesting variable contribution to the compositional diversity of the glasses from two distinct source materials, one of which is depleted mantle. Testing the hypothesis of pyroxenite-derived melts as an enriched end-member failed to reproduce the observed REE patterns, (Sr/Zr)N, and (Ba/Nb)N ratios, and the K2O–K2O/TiO2 relations. Two alternative hypotheses – melts derived from melting remnants of continental crust (1) or oceanic core complex (OCC) gabbro (2) – are suggested based on the complementary character of the REE patterns of the basalt glasses and partial melts compositions calculated at 1000 °C/50 MPa from either bulk continental crust as estimated by Taylor and McLennan (1995) (1) or OCC gabbro sampled in the study area (2). Complementarity of the two sources with the D-MORB holds also for the other geochemical parameters: (Sr/Zr)N vs (Ba/Nb)N, (La/Sm)N vs (Rb/Nb)N, and for the K2O–K2O/TiO2 relations. The first hypothesis calls upon partial melting of continental crustal material preserved in the vicinity of ridge-transform fault system, while the second suggests melt-rock interaction within the off-axial structures of the pre-existed gabbro assemblages, with concurrent melting of the wall-rock material and dissolution of the resulting melt in the intruded magma. Both scenarios, operating either separately or jointly, might contribute to the MORB geochemical variability documented in the present study.

Enrichment of heavy calcium isotopes in saprolite due to secondary mineral formation

Calcium stable isotopes were analyzed in well-characterized saprolite samples developed on a Mesozoic metadiabase dike near Cayce, South Carolina, to determine the degree and sense of isotopic fractionation during chemical weathering. The δ44/40Ca values (relative to NIST SRM 915a) of the saprolites, measured using double-spike TIMS, vary significantly (0.91‰ to 1.26‰), and are generally higher than the δ44/40Ca value of the unweathered metadiabase (0.98‰), which overlaps, within analytical uncertainties, with the δ44/40Ca estimate of the bulk silicate Earth. The δ44/40Ca values of the saprolites negatively correlate with bulk density, and Ti-normalized calcium concentrations, and positively correlate with chemical index of alteration (CIA) values. These trends reflect loss of light Ca isotopes to the hydrosphere during metadiabase weathering. Using insoluble incompatible elements ratios (e.g., Th/Nb) we estimate that the influence of aeolian dust is minimal (3% - 11%) and therefore unable to explain the observed variation in δ44/40Ca values of the saprolites. Selective weathering of rock-forming minerals like plagioclase and clinopyroxene cannot explain the high δ44/40Ca values (> 1 ‰) of the saprolites based on mass balance constraints. Samples with higher kaolinite/smectite ratio have higher δ44/40Ca values and lower concentrations of Ca, which suggests the loss of Ca from smectite-rich clays during progressive weathering. In this case, the lighter isotopes are preferentially lost, leaving the residual clays enriched in heavier isotopes of Ca. The δ44/40Ca values of saprolites correlate positively with δ26Mg and negatively with δ7Li values of the same samples, which further suggests that Ca stable isotopic variability of the Cayce metadiabase saprolites is controlled by formation of clay minerals and progressive loss of Ca during saprolitization. The enrichment of heavy Ca isotopes in saprolites with progressive loss of Ca can be modeled using Rayleigh distillation, with apparent fractionation factors between saprolites and fluid (α) of 1.00005 to 1.00015.

Slow changes in lava chemistry at Kama‘ehuakanaloa linked to sluggish mantle upwelling on the margin of the Hawaiian plume

Abstract Temporal variations in lava chemistry at active submarine volcanoes are difficult to decipher due to the challenges of dating their eruptions. Here, we use high-precision measurements of 226Ra-230Th disequilibria in basalts from Kama‘ehuakanaloa (formerly Lō‘ihi) to estimate model ages for recent eruptions of this submarine Hawaiian pre-shield volcano. The ages range from ca. 0 to 2300 yr (excluding two much older samples) with at least five eruptions in the past ∼150 yr. Two snapshots of the magmatic evolution of Kama‘ehuakanaloa (or “Kama‘ehu”) are revealed. First, a long-term transition from alkalic to tholeiitic volcanism was nearly complete by ca. 2 ka. Second, a systematic short-term fluctuation in ratios of incompatible elements (e.g., Th/Yb) for summit lavas occurred on a time scale of ∼1200 yr. This is much longer than the ∼200-yr-long historical cycle in lava chemistry at the neighboring subaerial volcano, Kīlauea. The slower pace of the variation in lava chemistry at Kama‘ehu is most likely controlled by sluggish mantle upwelling on the margin of the Hawaiian plume.

Zircon U–Pb Dating and Geochemical Characteristics of Chahannuo Gabbros in the Northern Margin of Qaidam Basin, Northern Tibetan Plateau

Late Paleozoic–early Mesozoic intrusive rocks are distributed widely along the northern margin of the Qaidam Basin in the northern Tibetan Plateau. To constrain the tectonic evolution, we carried out petrological, chronological, and geochemical studies of the Chahannuo gabbros. LA-ICP-MS Zircon U–Pb dating yields an age of 255.0 ± 0.9 Ma for the gabbros, which confirms the existence of Indosinian tectono-magmatic activity on the northern margin of the Qaidam Basin. The Chahannuo gabbros have low whole-rock SiO2, Fe2O3 contents, and high Al2O3 contents, which suggests a calc-alkaline affinity. In addition, the gabbros have high MgO, Cr, and Ni contents and Mg#, similar to those predicted of the regional basaltic melts, and indicating that they were affected mainly by fluid from the subducted slab. The Chahannuo gabbros are characterized by arc-like trace element patterns, with enrichment in LREE and LILE, and depletion in HREE and HFSE. No obvious negative Eu anomalies also indicate that no significant magmatic differentiation has occurred. The low Nb/La ratio and Ti content in gabbros samples suggests that the Chahannuo gabbros were partially contaminated by the crust during their formation. The Chahannuo gabbros have high incompatible element ratios (Rb/Sr, Th/Nd, and Th/La), which are closer to the category of enriched mantle. Combing our data with previous data from contemporaneous magmatism in the region, we suggest that the Chahannuo gabbros formed in a continental arc environment related to the northward subduction of the Paleo-Tethyan oceanic plate.

High-magnesium igneous associations record final-stage geodynamic process of the southeastern segment of the Central Asian Orogenic Belt

Much controversy exists regarding the evolution and termination of accretionary orogens where typical continent-continent collision features are lacking. The Central Asian Orogenic Belt provides a key opportunity to explore the genetic relationships between the formation of high-Mg igneous associations and the evolution of accretionary orogens. In this study, two-stage Triassic magmatic events were identified in the southeastern segment of the Central Asian Orogenic Belt (including ca. 247 Ma andesites and ca. 222 Ma diorites), through precise secondary ion mass spectrometry (SIMS) and laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) zircon dating. The Olenekian andesites show typical sanukitoid affinities with high TiO2, MgO, Ni, and Cr contents, and enrichment in large ion lithophile elements (LILEs; K, Ba, and Sr). They yield depleted Hf isotopic compositions and high δ18O values, which along with Hf-O isotope modeling results indicate that they originated from the partial melting of mantle wedge peridotites with input of 20% terrigenous sediments. The Norian diorites have moderate SiO2, relatively high MgO, Cr, and Ni contents, Mg# values of 63−65, high Sr, low Y and Yb contents, relevant high La/Yb ratios, and negligible Eu anomalies, which makes them comparable to high-Mg andesites with adakitic affinity. Their depleted isotopic compositions are similar to coeval A-type granites that resulted from lithospheric delamination after the final collision. In addition, with high K2O/Na2O ratios and incompatible element ratios similar to those of the continental crust (e.g., Nb/U, Ce/Pb, and Nd/Sm), they conform to an origin in which partial melting of delaminated mafic lower crust subsequently interacted with mantle materials. These two-stage abnormal Triassic high-Mg rocks archive a hot slab window triggered by the break-off of the Paleo-Asian oceanic slab and lower crustal delamination related to collapse of the southeastern segment of the Central Asian Orogenic Belt. These findings provide beneficial information for the reconstruction of the East Asian blocks in Pangea.

Enrichment of Incompatible Elements in Alkaline Syenites in Large Igneous Provinces Due to Magma Replenishment and Reactive Porous Flow in a Mush Reservoir

AbstractUnderstanding the petrogenesis of alkaline syenites is important for constraining the mechanisms of rare earth element (REE) and rare-metal mineralization. Here we report a detailed petrological and geochemical study of early Permian syenitic rocks from the southwestern Tarim large igneous province (TLIP) in the Wajilitag area (China). We use these data to investigate the complex magmatic processes responsible for the enrichment of incompatible elements (e.g. REEs and rare metals) in these rocks. The Wajilitag syenitic rocks comprise early hornblende syenite (281 Ma) and later nepheline syenite (278 Ma), both of which are spatially and temporally associated with mafic intrusions (i.e. gabbro and diabase). These syenitic rocks show continuous variations in major elements with the mafic rocks. They also have similar Sr–Nd–Hf isotopic compositions with the nearby mafic rocks, denoting that the Wajilitag syenitic rocks should be derived from the melts represented by these mafic rocks. However, these syenitic rocks, particularly the nepheline syenites, are typically characterized by extreme enrichment of incompatible elements, which is hard to be accounted by simple fractional crystallization of mafic magmas. Some clinopyroxene phenocrysts in the Wajilitag nepheline syenites show oscillatory zoning with strikingly increasing and then gradually decreasing MgO contents, which recorded replenishment of mafic magma at the interval. Low-MgO clinopyroxene mantles and/or rims have much higher incompatible element contents (e.g. Nb, Ce, and Zr) and ratios (e.g. Ce/Y) than their cores, which could reflect melt injection controlled by reactive porous flow in a mush reservoir of a crustal magma chamber. Such processes may also cause the high Ce and/or Nb contents of the mantles and/or rims of zoned titanite and apatite phenocrysts. The injection of reactive porous flow melts is the key process that produces the extreme enrichment of incompatible elements in the alkaline syenites from the western TLIP, as well as other plume-related alkaline syenites that host world-class REE and rare-metal deposits.

Petrogenesis of Butajira-Kibet Quaternary basaltic rocks; Central Main Ethiopian Rift

Major and trace element data are presented for Quaternary basalts from Butajira-Kibet area, close to the western escarpment of the Central Main Ethiopian Rift to investigate the petrogenetic processes involved and the nature of mantle source compositions. The data indicate that the basalts are mildly alkaline characterized by moderate to high contents of MgO (6.85-9.00 wt. %) and TiO2 (2.40-2.92 wt. %). In the primitive mantle-normalized multi-element diagram, they exhibit troughs at Th, U, Rb and K and peaks at Ba, Nb, Ta, La, Pb and Sr with positive Eu/Eu*(1.01-1.17). Incompatible element ratios e.g., Ba/Nb (8.92-16.25), Rb/Nb (0.42-0.70), K/Nb (233.00-340.00) and Nb/U (47.97-61.05) suggest that the basalts are unaffected by crustal contamination. The acquired data do not evidence fractionation of plagioclase and clinopyroxene. On the other hand, relationships between major elements and their ratios (e.g., Na2O and Ca2O/Al2O3) and trace element ratios (e.g., Zr/Y and (La/Sm)N) indicate partial melting of a clinopyroxene - rich mantle at medium pressure. Chondrite - normalized REE diagram is characterized by relatively flat HREE (TbN/YbN=1.73-2.13) and significantly enriched HREE (higher than 10-times chondritic values) patterns. LREE/MREE versus MREE/HREE modelling of non-modal batch melting indicates that the basalts are formed by 1.5-3% partial melting of spinel peridotite mantle source with minor contribution from garnet-bearing peridotite. Primitive mantle-normalized multi-element diagram and trace element ratios, such as Nb/Zr, Nb/U, Ba/La, Rb/Zr and Ba/Nb, suggest that the studied basalts are not affected by carbonatite and/or hydrous phase metasomatism. Rather, they are closely similar to the OIB source (mainly EM-I) and the SCLM. They are most likely the result of significant interaction of melts from a mantle plume and the SCLM.

Architectural and Compositional Diversity of Early Earth Ocean Floor Evidenced by the Paleoarchean Nondweni Greenstone Belt, South Africa

Abstract The nature of the early Archean ocean floor remains a topic of important debate. There are relatively few well-preserved occurrences worldwide where such terrains may be studied in detail because of structural dismemberment, metamorphic overprinting and pervasive early stage hydrothermal alteration to recent weathering. The 3.41-Ga dominantly mafic formations of the Nondweni Greenstone Belt (NGB) covering 270 km2 in the south-eastern Kaapvaal Craton comprise submarine volcanics that exhibit a wide range of textural features, including pillows, chill zones and brecciated flow tops, and various spinifex textures, including the rare platy pyroxene type, cumulate layers, and tuffs. Channelized subaqueous lava lakes that underwent fractionation are capped by thick spinifex-textured units and pillows. Early stage seafloor alteration is regionally variable, ranging from intense to minimal, with preservation of original mineralogy in many areas. Mafic volcanic rocks of the NGB contrast with those of the Barberton Greenstone Belt both in the style of volcanism and in the associated compositional range of komatiitic basalt to basalt with a complete absence of high-Mg komatiites. Olivine-phyric rocks, or derivatives thereof, are largely absent and pyroxene is the main controlling phase with orthopyroxene in the most primitive komatiitic basalts and clinopyroxene in the evolved lava lake sequences. The abundance of orthopyroxene typifies the long-standing silica-enriched character of the Kaapvaal Craton. Three exceptionally well-preserved and well-exposed sequences were studied utilizing hand-drilled samples and deep coring providing unprecedented stratigraphic and textural detail and field controls for more than 400 samples. A unifying feature of the mafic volcanics of the NGB is the range of compositions and ratios of incompatible elements most clearly illustrated by a series of high- and low-Ti compositional lineages reflecting differing sources or degrees of mantle partial melting. Sharp boundaries between high- and low-Ti flow successions indicate sudden changes in the melting regimes or the interaction of flow sequences from different volcanic centres. Th/Nb ranges from 0.1 to 0.2 and reveals crustal contamination of primitive lavas. The primary magma that gave rise to the most primitive komatiitic basalts with 19.5% MgO was derived from partial melting of a mantle plume source in the garnet stability field. Trace element modeling shows that the sequences studied in detail have been modified by fractionation and crustal contamination with the most likely contaminant being the Ancient Gneiss Complex (3.43–3.66 Ga), which is extensively exposed in Eswatini and probably underlies the Paleoarchean terrains in the southern Kaapvaal Craton. The geotectonic setting was likely that of a submerged felsic crustal platform as enclaves within an oceanic plateau.

Effects of Mantle Hybridization by Interaction with Slab Derived Melts in the Genesis of Alkaline Lavas across the Back-Arc Region of South Shetland Subduction System

AbstractLate Miocene to Late Pleistocene alkaline lavas in the northernmost part of the Antarctic Peninsula and its off-lying islands are the latest stage of magmatic activity that took place in response to lithospheric extension in the back-arc region of the South Shetland subduction system. The alkaline magmatism occurred much later than the main pulse of Cretaceous arc magmatism and generated basaltic extrusive rocks during several sub-aqueous/sub-glacial and sub-aerial eruption periods. The suite consists primarily of alkali olivine basalts with oceanic island basalt (OIB)-like trace element signatures, characterized by elevated highly to less incompatible element ratios compared to MORB. The samples have higher 87Sr/86Sr (0.70301–0.70365), and lower 143Nd/144Nd (0.51283–0.51294) and 176Hf/177Hf (0.28291–0.28298) than depleted MORB mantle. Their lead isotope ratios vary within a limited range with 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb ratios of 18.797–18.953, 15.577–15.634, and 38.414–38.701, respectively. Sr, Nd, Hf and Pb isotope systematics suggest involvement of diverse source materials in the genesis of the alkaline magmas. Evaluation of radiogenic isotope and trace element data indicates that the source of the alkaline melts had a complex petrogenetic history, reflecting the effects of mantle hybridization along the slab mantle interface through interaction of mantle wedge peridotites with volatile-bearing, siliceous melts produced by melting of subducted sediments and basaltic oceanic crust. Hf-Nd isotope and trace element projections further demonstrate that the metasomatizing melt was likely generated by eclogite partial melting at sub-arc to post-arc depths, in equilibrium with a garnet-bearing residue and involved breakdown of high field strength elements (HFSE) retaining phases. Consumption of metasomatic amphibole during partial melting of hybridized peridotite at the wet solidus appears to have had a significant effect on the final melt compositions with high HFSE, Na and H2O contents.

Petrology of continental, OIB-like, basaltic volcanism in Saudi Arabia: Constraints on Cenozoic anorogenic mafic magmatism in the Arabian Shield

Continental basaltic volcanism in the Arabian Shield of Saudi Arabia has distinctive mineralogical and geochemical features important for understanding the composition of its mantle source and melting conditions. The studied Cenozoic basaltic rocks (19.3–15 Ma) occur at Jabal Al Gharib and Jabal Al Ghuraybayn areas within the Al Qasr quadrangle and southwest of Hail City in Saudi Arabia. They show similar chondrite-normalized REE patterns, suggesting that they were derived from a common mantle source. Their OIB-like features, silica undersaturated nature, and incompatible trace element ratios all reveal an asthenospheric mantle source and argue against crustal contamination process, subduction contribution, and interaction with a lithospheric mantle. Moreover, the elevated compatible trace element concentrations of Ni, Cr, and Co and the low and relatively narrow range of FeOtotal/MgO ratios argue for their nearly primitive nature and indicate that the role of crystal fractionation processes was minor during the evolution of the studied rocks. The estimated temperatures indicate that the sequence of crystallization likely began with olivine at 1,378–1,475°C, then clinopyroxene at 1,137–1,214°C, and soon after followed by plagioclase at 1,096–1,108°C. Plagioclase geo-hygrometers reveal 0.63–1.41 wt% water contents, suggesting crystallization under hydrous conditions. The basalts have geochemical features such as high (K2O + Na2O)/TiO2, Zr/Hf, and Nb/Ta ratios and negative K anomalies which suggest a carbonated peridotite mantle source. Their bulk-rock compositions are consistent with <5% CO2 in their peridotite melts. Highly incompatible elemental ratios supported by REE modeling indicate that they were generated by low degrees (4–10%) of partial melting of a garnet-bearing lherzolite mantle source. The garnet signature in the source region suggests a deeper origin exceeding ∼85 km. The basanite compositions fall within the range of alkaline OIB and intracontinental basalts formed in the rifted region. Moreover, they show geochemical characteristics typical of Cenozoic rift-related Oman basanites and other Arabian intraplate volcanic rocks. The basaltic volcanism in Saudi Arabia seems to be the result of melting asthenospheric mantle source in response to the lithospheric extension that is spatially and temporally linked to Red Sea rifting and triggered anorogenic mafic magmatism due to passive mantle upwelling beneath the Arabian Shield.

Constraints on the Magma Source and Rift Evolution From Geochemistry of the Stratoid Flood Basalts (Afar, Ethiopia)

AbstractThe relationship between rifting and continental flood basalt eruptions is debated, and a control by mantle plume is commonly invoked for flood basalts production. In this work, we investigate the relationship between magmatism and rifting by studying the flood basalts erupted in Afar (4.5–0.6 Ma), known as the Stratoid and Gulf Series. We present new field observations and petrography, major and trace elements analyses and mineral chemistry of lavas collected during a regional campaign in Afar. The Series are characterized by E‐MORB magmatism and residual amphibole in the mantle source, consistent with the contribution of metasomatized sub‐continental lithospheric mantle during partial melting. Differences in garnet‐compatible elements indicate a shallower melting column for the oldest and youngest products (4.5–2.6 Ma Lower Stratoid Series; 1.1–0.6 Ma Gulf Series), and deeper for the products erupted at 2.6–1.1 Ma (Upper Stratoid Series). Incompatible element ratios (Th/Nb, Th/Zr) indicate a higher degree of partial melting for the Gulf with respect to the Upper Stratoid Series. Accordingly with independent geophysical and stratigraphic evidence, we explain our results with rift re‐localization: the Pliocene rift caused thinning of the lithosphere and the Lower Stratoid eruptions in Southern Afar, then the Pleistocene rift jumped to Central Afar under a less‐extended lithosphere, producing the Upper Stratoid and, subsequently, as stretching of the lithosphere localized, the Gulf Series formed. Our findings suggest that rift migration and localization can exert a fundamental control on the spatial variability and character of flood basalts without requiring variations on the activity of the mantle plume.

Mineralogy and geochemistry of lavas from the submarine lower caldera walls of Santorini Volcano (Greece)

Santorini Volcano, located in the central sector of the South Aegean volcanic arc, is one of the most active and potentially dangerous volcanoes in Europe. It has hosted Plinian eruptions over the last 350 ka of which at least four eruptions were accompanied by caldera collapses. Even though Santorini Volcano is considered a major threat, the main focus of research, thus far, has been on the comparatively young and subaerial deposits, whereas older stages of volcanism have been poorly studied. Our study focusses on samples from the submarine inner caldera walls and gives new insights into the early evolutionary stages of Santorini Volcano, contributing to a better understanding of its eruptive history and thus potential risks. The submarine lava successions were sampled along the inner caldera wall by a remotely operated vehicle (ROV) during R/V POSEIDON cruise 511 in 2017.The recovered lavas comprise two magmatic series, a low-K basaltic series overlain by a medium- to high-K series, which includes basaltic andesites, andesites and occasional dacites to trachytes. Major and trace element compositions and mineral zonation patterns suggest that fractional crystallisation and periodic magma replenishment were the dominant processes controlling magma evolution of both magmatic series. In addition, repeated magma mixing events played an important role, as indicated by zonation patterns in plagioclase and clinopyroxene ante- and phenocrysts. The thickness of the submarine lava successions (≥100 m) and the occurrence of similarly zoned plagioclase throughout indicate long-lived magma plumbing systems characterised by repeated differentiation and magma mixing. Furthermore, the incompatible element and radiogenic isotope ratios indicate a heterogeneous mantle source for Santorini magmas, which reflects the relative contribution of subduction (sedimentary) input and mantle wedge influx. A lava sample from the northern submarine caldera wall probably represents a deep level of the original Peristeria volcano. 40Ar/39Ar dating of andesite lavas from the southern submarine caldera produced ages of ~255 ka, whereas one basalt lava produced an age of 309 ± 30 ka. Interestingly, the new ages of both groups fall within an age gap in the volcanic history of Santorini Volcano. Even though it was not possible to unequivocally correlate the sampled submarine lava series to known subaerial units, the major and trace element compositions, and Sr-Nd-Pb isotopic compositions of our intermediate lavas show a strong similarity to subaerial lavas of Peristeria volcano, the second oldest major stage of Santorini volcanism. It however seems more likely that we have sampled a thus far unknown stage of volcanism at Santorini Volcano.

Generation of Continental Alkalic Mafic Melts by Tholeiitic Melt–Mush Reactions: a New Perspective from Contrasting Mafic Cumulates and Dikes in Central Tibet

Abstract Alkalic basalts from continental regions have been widely used to fingerprint chemically and lithologically distinct components in the mantle. However, superimposed on mantle source effects are the effects of intra-crustal magma differentiation, which may be underestimated when exclusively considering fractional crystallization (FC) in melt-rich magma chambers. Here we present extensive mineralogical and geochemical data for mafic cumulates and dolerite dikes from central Tibet to demonstrate that tholeiitic magma differentiation occurred in crystal-rich mushes and melt-filled fractures. The mafic cumulates provide a snapshot of the percolation of water-poor (<0.9 wt% H2O), oxide-undersaturated, tholeiitic melts through slightly hotter mushes (50–60% crystals) with a framework of primitive olivine ± plagioclase crystals. The highly corroded olivines indicate that thermo-chemical disequilibrium between the later percolating melts and pre-existing crystal framework triggered melt–mush reactions in an open system. High-temperature (>1060°C) reactions were accompanied by a decreasing melt mass, causing the initial tholeiitic melts to evolve into alkalic mafic melts during slow cooling. This process involved the dissolution and reprecipitation of olivine, plagioclase, and clinopyroxene as well as the late crystallization of high-TiO2 (up to 3.4 wt%) clinopyroxene and kaersutite. In contrast, the same tholeiitic magmas (i.e. dolerite dikes) that intruded the cold country rocks along fractures produced silica-rich residual melts saturated in Fe–Ti oxides and sodic plagioclase by FC during rapid cooling. The residual melt was always in equilibrium with the instantaneous crystal during closed-system FC, so there was no reaction between the interstitial melt and adjacent crystal even after a high degree of crystallization. Alkalic mafic melts that can be formed by melt–mush reactions exhibit strong enrichment of incompatible element contents (e.g. Ti and alkali and rare earth elements) and ratios (e.g. Ce/Y), but little Si enrichment and Mg depletion, compared with the parental tholeiitic magma, which cannot be reproduced by simple FC. We argue that tholeiitic melt–mush reactions at mid- to lower-crustal pressures (≥0.5 GPa) are a new mechanism for the generation of interstitial alkalic mafic melts in continental mush reservoirs. Continental alkalic basalts could be produced by this mechanism if such interstitial melts were concentrated into melt lenses by compaction and then transported rapidly upward to the surface by focused flow.