Incompatible Trace Elements Research Articles

Composition and age constraints on a Hadean enriched mantle source revealed by the Paleoarchean São Tomé layered intrusion, NE Brazil

Evidence for Hadean depleted mantle reservoir(s) is well-established by mantle-derived rocks from multiple Archean complexes showing excesses in 142Nd compared to the modern mantle. Yet, the existence of an early enriched mantle source, which should have concomitantly formed during these Hadean silicate differentiation event(s), has only been recently confirmed by well-resolved 142Nd deficits measured in Paleoarchean mafic amphibolites from the São José do Campestre massif of the Borborema Province, NE Brazil. To investigate the nature and extent of this early-formed enriched reservoir, a series of samples from the Seridó belt, NE Brazil, have been investigated for their geochemical and coupled 147,146Sm-143,142Nd isotopic compositions, focusing on the Paleoarchean São Tomé layered intrusion. The São Tomé intrusion is composed of layers of metamorphosed mafic and ultramafic rocks with whole-rock trace element compositions and high olivine crystal Ni contents consistent with an incompatible trace element enriched pyroxenitic mantle source. Most analyzed samples from the Seridó belt yielded negative µ142Nd, as low as ∼ -20 (representing the lowest 142Nd/144Nd ratio ever measured in terrestrial rocks), with an average µ142Nd for the São Tomé intrusion of -9.6 ± 1.4. The São Tomé samples interpreted as the least isotopically disturbed yielded an 147Sm-143Nd isochron age of 3551 ± 368 Ma, consistent with previous zircon U-Pb ages, with an initial ε143Nd = -1.7 ± 1.2. Coupling the long-lived 147Sm-143Nd and the short-lived 146Sm-142Nd systems for the São Tomé rocks suggests a Hadean enriched mantle source formed at ∼4.44 Ga with a 147Sm/144Nd of ∼0.18, which may be complementary to the early depleted mantle recorded by Eoarchean rocks from the North Atlantic Craton, possibly formed during magma ocean crystallization. These results also imply that the record of this Hadean enriched mantle source is not restricted to the São José do Campestre massif but extends to a larger portion of the Borborema Province.

New insights on the petrogenesis of the Koktokay No.3 pegmatitic dyke: Petrological and zirconological evidence from the Aral granitic complex (Xinjiang, China)

The Koktokay No.3 pegmatite dyke (KPD), containing numerous Li–Be–Nb–Ta–Cs mineral resources, is among the world’s most famous rare-metal deposits, and attracted much attention. Up to now, over thirty geochronological data have been reported ranging from 332 Ma to 120 Ma for the KPD, which causes uncertainty about the origin and petrogenesis of the pegmatite. In this contribution, whole-rock geochemistry and zircon geochronology have been conducted on the Aral biotite monzogranite (BMG), the Koktokay muscovite alkali-feldspar granite (MAG), and the KPD. Petrological and whole-rock geochemistry results reveal that the BMG is normal granite with medium SiO2 (mean of 67.23 %), enriched in compatible trace elements such as Ba, Sr and Zr, and slight negative Eu anomalies, while the MAG belongs to highly-fractionated granite with high SiO2 (mean of 73.92 %) and extensive negative Eu anomalies, and enriched in incompatible trace elements such as Rb, Ta, and U. Zircon morphology and LA–ICPMS analysis reveal that magmatic zircons from the BMG, hydrothermal zircons from the MAG and KPD yield lower intercept ages at 217.3 ± 2.4 Ma, 197.8 ± 4.7 Ma, and 195.4 ± 2.0 Ma, respectively. Combining with tectonic information and geochronological data of granitic activity and its mineralization in the Altay, this study explains the petrogenetic mechanism of the KPD after Wang et al. (2021)‘s metallogenic model: (1) In late stage of Middle Triassic, the collision between the Siberian plate and the Kazakhstan–Junggar plate caused a large number of thrust nappe faults and crust thickening in the Altay orogenic belt; At ∼217 Ma, the compression reached its peak, the crustal anatexis produced syn-collisional BMG (i.e., the Aral BMG); After the compressive peak, huge amount of granitic magma in deep-seated magma chamber underwent over 20 Myr of fractional crystallization, and the residual magma enriched in ore-forming materials (rare metal elements, volatile components, and aqueous fluids) occurred at the top of the magma chamber; (2) When the regional stress converted from compression to extension, the highly-fractionated residual magma ascended rapidly from the long-lived magma chamber along extensional faults at ∼195 Ma; The huge amount of melt-bearing fluids were exsolved from the residual magma in the course of its emplacement due to sharply decreasing pressure, and intruded into a large cavity generated by extensional fault; Along with slowly decreasing temperature, the melt-enriched fluids crystallized outside-in as (quasi-) concentric pegmatitic zones (i.e., KPD); (3) The residual magma which lost huge amount of fluid filled the lower space of the extensional system, and crystallized as post-collisional MAG (i.e., the Koktokay MAG). Based on the genetic relationship among tectonics, petrogenesis, and metallogeny, the proposed model shows material and energetic conversion processes from syn-collisional granites to post-collisional granites with granitic–pegmatitic deposits

Deconvolution of high and low-temperature alteration processes along the contact zones of basaltic-dike intrusions in basaltic host rocks of different permeabilities – implications for geothermal exploration

Magmatic intrusions serve as crucial heat sources for geothermal systems, facilitating mass transfer, mineral transformations, and elemental exchange, which result in the formation of contact aureoles. While these processes have been extensively studied in large intrusive complexes in ancient geological formations, understanding of their occurrence in active geothermal systems remains limited. The extreme conditions present in active volcanic systems often obscure the geochemical processes occurring within host rock/intrusion zones. Uncertainties persist regarding whether relatively small dike intrusions (∼0.5 m thick) possess sufficient heat content to induce textural or geochemical changes in the surrounding wall rock, and what implications this may hold for future exploratory drilling projects. The analyses in this study were conducted on two distinct outcrops, each featuring 50-cm thick basaltic intrusions within both high- and low-permeability basaltic host rocks. The low permeability host rock hosts high-temperature mineral phases (>800 °C), such as sanidine + hedenbergite + albite-rich plagioclase in the contact zone, which we interpret to have formed during partial melting. Immobile, incompatible trace elements (such as Zr, Nb and La) retain the signatures of partial melting in both outcrops. We demonstrate that low degree partial melting (F ≈ 3–10 %) results in a compositional shift in the host rock from basalt to dacite and/or trachyandesite. Thermal modelling suggests that these small dikes, along with their partial melts in the contact zone, form over a very short period of time (< 1 day), but can elevate the ambient temperature. This type of events play an important role in the development of active geothermal systems. In theory, these small dikes do not pose a significant risk during geothermal drilling, unless they are too small to be detected during geophysical exploration.

Trace element abundances in mineral phases and the groundmass of ocean island basalts

Incompatible trace elements are important tracers in igneous petrogenesis for determining processes acting: (i) prior to partial melting on magma sources; (ii) during partial melting, fractional crystallization or contamination of magmas, and (iii) after emplacement of lavas or intrusive rocks close to Earth's surface. Of particular interest are the high field strength elements (HFSE: Ti, Zr, Nb, Hf, Ta) which are incompatible elements with high valence states that are also relatively immobile in aqueous fluids. Abundances of Ti, Ta, and Nb (TITAN) in ocean island basalts (OIB) have been suggested to reflect recycled oceanic crust in mantle source regions, due to retention of these elements in subducted eclogitic rutile. Some of these elements have also been used to determine ‘canonical’ trace element ratios, such as Nb/U and Ta/U, that are considered invariant during petrogenetic processes. In this contribution, we present an extensive set of in situ trace element data for olivine, clinopyroxene, oxide minerals and groundmass for a suite of well-studied ocean island basalt (OIB) lavas from the Canary Islands, Azores, Samoa, Tubuai and La Réunion. The data were obtained using laser-ablation inductively coupled plasma mass spectrometry, and we also report a new open-source reduction routine for abundance determination using this method. Mineral-groundmass partitioning behavior is determined for olivine, clinopyroxene and oxides, showing that these are consistent with published partition coefficients for clinopyroxene and olivine, but are more variable for oxide phases. This variability is due to the wide variety of oxide phases possible in OIB (e.g., Cr-spinel, ilmenite, titanomagnetite) and their formation within the crystallization sequence. The new data shows that Nb and Ta budgets are dominated by fine-grained groundmass, as are most other incompatible trace elements, while the Ti budget is divided between clinopyroxene, oxide and groundmass. Modally reconstructed TITAN anomalies closely reproduce bulk rock compositions in samples with >40 % groundmass. Olivine and clinopyroxene fractional crystallization have negligible effects on TITAN anomalies, while oxide fractionation has more marked effects, with Ti anomalies being most affected. Variability in the HFSE in OIB and their association with other elements (e.g., Nb/U and Ta/U) can reflect source characteristics, although careful screening for petrogenetic effects should be done prior to ascribing significance to TITAN anomalies or canonical trace element ratios.

Heavy halogen compositions of peridotite massifs in the Ivrea-Verbano Zone and implications for strong modification of mantle rocks

The lithospheric mantle affected by metasomatism is an important reservoir for halogens and often preserves significant heterogeneity in halogen abundances. However, the halogen compositions of the non-metasomatized mantle rocks with depleted geochemical features have not been well studied, impeding a comprehensive understanding of the halogen distribution in the mantle. The Balmuccia (BM) and Baldissero (BD) peridotite massifs in the Ivrea-Verbano Zone, Italian Alps, represent fragments of the sub-continental lithospheric mantle (SCLM) that are fresh and devoid of subduction-related mantle metasomatism. Here we examine the heavy halogen contents of well-characterized spinel-facies mantle peridotites (n = 20) and pyroxenites (n = 16) from the BM and BD massifs. The results indicate significant variation in whole rock halogen contents of peridotites and pyroxenites, with chlorine (Cl) ranging from <30 to 244 μg/g, bromine (Br) from <0.10 to 0.94 μg/g, and iodine (I) from <0.015 to 0.12 μg/g. Some peridotites and pyroxenites show rather low halogen contents (below the detection limit), consistent with their depleted features. However, a large number of samples display halogen contents far higher than the depleted MORB mantle (i.e., Cl: 3–10 μg/g) and peridotite xenoliths from the strongly metasomatized mantle, although ∼30 % of the halogens were leached out by ultrapure water or dilute nitric acid. The data suggest variable halogen enrichment in many mantle rocks. The halogen contents of different types of mantle rocks do not vary with major elements and incompatible trace elements (e.g., Nb), indicating that halogen compositions are not controlled by magmatic processes such as melting and melt-peridotite reaction. Instead, secondary fluid inclusions are abundant and mainly distributed as trails within olivine and pyroxene in both peridotites and pyroxenites, which were likely trapped during the exhumation of these mantle rocks. Their elevated halogen/Nb ratios compared with MORB mantle and strong correlations with Ba/Nb support the effect of fluids. Micro-Raman analyses identify hydrous serpentines within fluid inclusions. Moreover, mass balance calculation indicates that halogen enrichment is mainly caused by secondary fluids (> 90 %). Together, our findings highlight that the fluid percolation in the mantle rocks during exhumation can intensely modify the halogens, to a similar degree as mantle metasomatism, suggesting the sensitivity of halogens to secondary modification.

Systematic behaviour of 3He/4He in Earth’s continental mantle

Helium isotopes are unrivalled tracers of the origins of melts in the Earth’s convecting mantle but their role in determining melt contributions from the shallower and rigid lithospheric mantle is more ambiguous. We have acquired new 3He/4He data for olivine and pyroxene separates from 47 well-characterised mantle xenoliths from global on- and off-craton settings. When combined with existing data they demonstrate a new systematic relationship between fluid-hosted 3He/4He and major and trace element composition of host minerals and whole rock. We show that a significant proportion (>70 %) of mantle peridotites from continental off-craton settings with depleted major element compositions (e.g., olivine Mg# ≥ 89.5) have 3He/4He in the range of modern-day mid-ocean ridge basalt (MORB) source mantle (7–9 Ra) and we propose that they represent underplated melt residues, which initially formed in the convecting upper mantle. Furthermore, we observe that off-craton mantle xenoliths with signatures often attributed to enrichment by melts or fluids from ‘ancient’ subducted oceanic lithosphere have lower 3He/4He (<7 Ra). Modest correlations between 3He/4He and whole rock incompatible trace element signatures commonly used as proxies for metasomatism by small-fraction carbonatite and silicate melts or C-O-H fluids characterise lithospheric mantle with 3He/4He ranging from 5 to 8 Ra.Using a numerical model that integrates temperature-dependent melt extraction from the upper mantle with in-situ radiogenic ingrowth of 4He in the continental mantle we show that the initial 3He/4He of continental lithosphere mantle has decreased over time. This is consistent with previous observations demonstrating that ancient (2.5–3.5 Ga) cratonic mantle has a depleted mineral chemistry (e.g., olivine Mg# = 91–94) and low 3He/4He (0.5–6.7 Ra), while continental off-craton mantle (<2.5 Ga) is more fertile (olivine Mg# = 88–92) and has less radiogenic 3He/4He (4–8.8 Ra). This relationship defines a ‘global lithospheric mantle array’ for intraplate peridotites on plots of 3He/4He vs olivine Mg#. Peridotites influenced by past and present subduction fluids, including those that contain amphibole, plot off this array. Our findings have broad implications for the 3He/4He signatures observed in continental magmas. Many of Earth’s deepest melts, i.e. proto-kimberlites, are characterised by relatively low 3He/4He. We attribute this to assimilation and incorporation of low 3He/4He cratonic mantle material during ascent of carbonate-rich melts through thick lithosphere, which overprints the original signatures. Moreover, our findings suggests that the lithospheric mantle acts as a long-term reservoir for other fluid-hosted volatiles (e.g., CO2, CH4, H2O), and in some cases able to sequester these over billion-year timescales until physio-chemical perturbation (e.g., during major rifting or heating events).

Recycled Oceanic Crust in the Source of the Intraplate Changbaishan‐Tianchi Volcano, China/North Korea

AbstractContinental intraplate volcano is an ideal probe to unravel the composition and structure of the deep Earth. The intraplate Changbaishan‐Tianchi volcano was one of the most hazardous eruptions on the Earth's planet. The long‐term activity of this volcano from the Pleistocene to 946 CE has erupted materials with a broad compositional range from basalt to rhyolite, which are expected to be associated with the continuous northeastward subduction of the Pacific plate, but the magma source remains controversial. In this paper, we present a comprehensive data set of in situ zircon Hf and O isotope data, combined with whole‐rock element and Sr‐Nd‐Pb isotope compositions, for selected eruptions of the Changbaishan‐Tianchi volcano, aiming to provide new insights into their magma source and the associated geodynamics. Radiogenic isotopic ratios and incompatible trace element compositions indicate that the erupted volcanic rocks at different stages, although with a varied differentiation degree, were derived from a common magma source characterized by a mixture of DM and EM1 end‐members. Zircon Hf and O isotopes are both relatively homogeneous for different lithologies and eruption stages, with the εHf(t) values varying between −5 and +5, and δ18O values between 3.58‰ and 5.97‰. Modeling of source mixing indicates that high‐temperature altered oceanic crust materials are an important component in the source of Changbaishan‐Tianchi volcano, likely derived from an ancient stagnant slab that has been reactivated by the subduction of the Pacific plate. This study demonstrates that the recycling of deeply subducted oceanic crust is potentially an important source and trigger for continental intraplate volcanism.

Discovery of mafic dikes in northeastern Tibet plateau belonging to the Emeishan large igneous province: Implications for paleocontinental reconstruction and mineral exploration

Due to the lack of paleomagnetic data and paleoclimatic records, the paleocontinental belonging of the Yidun terrane in northeastern Tibet plateau is debated. In this paper, we use the distribution of the Emeishan large igneous province (ELIP) to put the issue to rest. The ELIP is the product of mantle plume activity, which took place at 260 ± 3 Ma in the western margin of the Yangtze craton. The Yangtze craton is the northern part of the South China paleocontinental block. Our new discovery of four mafic dikes in the Yidun terrane with ages and chemistry like the ELIP strongly supports a new interpretation that this terrane was an integral part of the Yangtze craton during the ELIP magmatic event. Baddeleyites from these mafic dikes yield the U–Pb ages of 257.1 ± 4.8, 259.8 ± 8.5, 260.2 ± 4.5 and 261.5 ± 6.8 Ma for each of these dikes, which are similar to the zircon U-Pb ages of the mafic–ultramafic intrusions of the ELIP within the Yangtze craton. The Yidun mafic dikes and some high-Ti basalts of the ELIP within the Yangtze craton have similar Sr-Nd isotope compositions and mantle-normalized incompatible trace element distribution patterns, indicating similar mantle source compositions. Mixing calculation using Sr-Nd isotope compositions shows that the parental magmas for the Yidun mafic dikes experienced only 2–3 % crustal contamination. Partial melting modelling shows that the primary magmas for the Yidun mafic dikes were produced by 5–6 % mantle partial melting at the depths > 85 km below surface, consistent with mantle plume-related magmatism in a continental setting. The Yidun terrane is a new exploration frontier for magmatic Fe-Ti-V oxide deposits that are present elsewhere in the ELIP.

A record of magmatic differentiation in plutonic xenoliths from Santorini (Greece)

Plutonic xenoliths from volcanic arcs provide unique insights into transcrustal magmatic systems in subduction zone settings. At Santorini volcano in the Central Aegean Volcanic Arc (Greece), plutonic xenoliths occur throughout a sequence of lavas and pyroclastic rocks erupted within the last ~360 ka. They are mineralogically variable, ranging from troctolites to olivine gabbros, gabbros, gabbronorites, and diorites. Thermobarometric calculations based on mineral and melt inclusion compositions indicate equilibration over a range of temperatures (1100 to 750 °C) at shallow to mid-crustal depths (P &lt;400 MPa), but there is no evidence for crystallisation at lower crustal depths. Oxygen isotope data of mineral separates and calculated δ18O melt values are in line with extensive closed-system fractional crystallisation at magmatic temperatures, without a requirement for extensive assimilation of the subvolcanic continental basement. The xenolith minerals compositionally overlap with phenocrysts from the volcanic rocks, but they also contain evidence for the presence of highly evolved melt compositions in the form of melt inclusions with extremely silica-rich compositions (up to 82 wt.% SiO2) and high enrichments of incompatible trace elements coupled with increasing negative Eu anomalies in clinopyroxenes. Since these characteristics correlate systematically with differentiation indices and rock type, they are interpreted to reflect melt evolution via fractional crystallisation as the dominant differentiation process with no significant role of reactive porous flow. These observations highlight that trapped melt fractions can influence mineral compositional variations in the plutonic xenoliths, and in turn the mineral compositions demonstrate a melt compositional variability not preserved in the volcanic rock record.

Melt-Rock Reaction in the Lower Oceanic Crust and the Influence on the Evolution of Mid-Ocean Ridge Basalts at the Central Indian Ridge (7°50′–8°30′S)

Abstract Various crustal processes shape both the lower oceanic crust and mid-ocean ridge basalts (MORBs). To better understand how these crustal processes influence MORB compositions, we conducted comprehensive petrographic and geochemical investigations on gabbroic rocks and erupted lavas dredged from a segment of the Central Indian Ridge (CIR) spanning from 7°50′S to 8°30′S. The petrographic and geochemical analyses of the gabbroic rocks revealed evidence of melt-rock reaction through reactive porous flow in olivine gabbro and gabbro. This process resulted in distinctive features in clinopyroxene, including disequilibrium textures with a troctolite/anorthosite matrix, complex variations in Mg#-Cr-Ti [Mg# = molar Mg/(Mg + Fe2+)] relationships, and considerable enrichment and fractionation of incompatible trace elements. A significant finding of our study is the close resemblance of trace element ratios in MORB and olivine-hosted melt inclusions to those of melts in equilibrium with clinopyroxene from olivine gabbro and gabbro with Sr anomaly (Sr/Sr* = SrN/sqrt[PrN*NdN]; N refers to chondrite-normalized values) greater than ~0.7. This observation strongly indicates that the composition of MORB is influenced by the melt-rock reaction taking place in the lower oceanic crust. Furthermore, our findings suggest that evolved melts in equilibrium with clinopyroxene having Sr/Sr* values lower than ~0.7 are less likely to erupt onto the seafloor and are instead trapped within the lower oceanic crust. Oxide gabbronorite is characterized by coarse-granular, pegmatitic textures and exhibits mineralogically and chemically more evolved characteristics compared to olivine gabbro and gabbro. It is inferred that the oxide gabbronorite formed through the in situ freezing of highly evolved melts within a melt-rich layer. Finally, we present a comprehensive model for melt evolution in the lower oceanic crust at the 7°50′S–8°30′S CIR by integrating all petrological and geochemical data obtained from gabbroic rocks, MORB, and olivine-hosted melt inclusions. This holistic model contributes to a better understanding of the intricate processes governing MORB composition in the context of the lower oceanic crust dynamics at slow-spreading ridges.

Accretion of the Lower Oceanic Crust at Fast-Spreading Ridges: Insights from Hess Deep (East Pacific Rise, IODP Expedition 345)

Abstract At mid-ocean ridges, melts that formed during adiabatic melting of a heterogeneous mantle migrate upwards and ultimately crystallize the oceanic crust. The lower crustal gabbros represent the first crystallization products of these melts and the processes involved in the accretion of the lowermost crust drive the chemical evolution of the magmas forming two thirds of Earth’s surface. At fast-spreading ridges, elevated melt supply leads to the formation of a ⁓6-km-thick layered oceanic crust. Here, we provide a detailed petrochemical characterization of the lower portion of the fast-spread oceanic crust drilled during IODP Expedition 345 at the East Pacific Rise (IODP Holes U1415), together with the processes involved in crustal accretion. The recovered gabbroic rocks are primitive in composition and range from troctolites to olivine gabbros, olivine gabbronorites and gabbros. Although textural evidence of dissolution-precipitation processes is widespread within this gabbroic section, only the most interstitial phases record chemical compositions driven by melt-mush interaction processes during closure of the magmatic system. Comparing mineral compositions from this lower crustal section with its slow-spreading counterparts, we propose that the impact of reactive processes on the chemical evolution of the parental melts is dampened in the lower gabbros from magmatically productive spreading centres. Oceanic accretion thereby seems driven by fractional crystallization in the lower gabbroic layers, followed by upward reactive percolation of melts towards shallower sections. Using the composition of clinopyroxene from these primitive, nearly unmodified gabbros, we estimate the parental melt trace element compositions of Hess Deep, showing that the primary melts of the East Pacific Rise are more depleted in incompatible trace elements compared to those formed at slower spreading rates, as a result of higher melting degrees of the underlying mantle.

The oxygen fugacity of intermediate shergottite NWA 11043: implications for Martian mantle evolution

Shergottite meteorites, classified as depleted, intermediate, or enriched based on incompatible trace elements and specific radiogenic isotope compositions (Sr, Nd, and Hf isotope ratios), point to multiple Martian mantle source regions. The oxygen fugacity (fO2) of these mantle regions, determined from early crystallizing minerals using the olivine-pyroxene-spinel oxybarometer, appears to correlate with incompatible trace element enrichment and isotope compositions. However, values derived from the vanadium-in-olivine oxybarometer challenge this correlation, hinting at potential biases in oxybarometry or complexities in the redox conditions of the Martian mantle. By analyzing the intermediate shergottite Northwest Africa (NWA) 11043 with various oxybarometers, this study deduced its origin from a reduced mantle source, with an average fO2 value of −0.77 ± 0.35 relative to the iron-wüstite (IW) buffer. Notably, these values coincide with those of depleted shergottites, which represent the depleted Martian mantle region. This redox similarity between intermediate and depleted shergottites contrasts with earlier notions that postulated intermediate shergottites as a mix of depleted and enriched mantle derivatives. Moreover, intermediate shergottites such as NWA 11043, Elephant Moraine (EETA) 79001A, and Allan Hills (ALH) 77005 display 176Hf/177Hf values akin to those of depleted shergottites, suggesting that intermediate mantle components can be separated from the depleted mantle source at approximately 2.2 Ga based on model age calculations. Therefore, there presents a consistent redox state between mantle magma sources of both intermediate and depleted shergottites since the Hesperian period, while enriched shergottites lean toward more oxidized conditions past source formation.This study prompts a reassessment of conventional theories, emphasizing the nuanced redox evolution of the Martian mantle across distinct mantle source regions and underscoring the complexity of the redox evolution of the Martian mantle. The emergence of chemically diverse mantle reservoirs might predominantly arise from early magma ocean differentiation processes, albeit with inherent oxidation nuances. The differences in fO2 observed between intermediate and depleted shergottites underscore the need for more in-depth studies to decipher Martian mantle differentiation and evolution.

Geologic fingerprints of the Rodinia breakup in the western North China Craton: Constraints from the Early Neoproterozoic (ca. 825–810 Ma) mafic volcanic and intrusive rocks in the Langshan tectonic belt

As an important component of the East Asian continental block, the North China Craton (NCC) is crucial for reconstructing Rodinia in the Neoproterozoic. However, whether the NCC is a component of the Rodinia supercontinent and records its breakup remains enigmatic. Here, we identified Langshan gabbro and mafic dikes with ages of ca. 829–826 Ma and mafic volcanic rocks with ages of 810 ± 3 Ma in the northeastern Alxa Block of the western NCC. The Langshan gabbro shows flat rare earth element (REE) patterns [(La/Yb)N = 1.3–1.5] and slight enrichment in most incompatible trace elements, similar to enriched mid-ocean ridge basalt. The mafic volcanic rocks show the geochemical features of oceanic island basalt, including high total alkaline content (7.86–8.47 wt%), enriched in light REEs (La/YbN = 8.0–9.1), and insignificant Eu anomalies. Together with contemporaneous felsic volcanic rocks identified in previous studies, the mafic volcanic rocks (ca. 810 Ma) define a bimodal association. The mafic intrusive and associated dike swarms from ca. 825–810 Ma along with the coeval Jinchuan ultramafic–mafic intrusions, followed by rifting sedimentary successions, were formed in an extensional rift setting, supporting the geologic fingerprints of the Rodinia breakup in the western NCC. The well-matched mafic magmatic barcodes, similar provenance, and sedimentary successions preserved in the Adelaide superbasin of southeastern Australia and the Langshan rift basin of the western NCC support a possible link between the NCC and Australia during the Early Neoproterozoic.

The role of calcium phosphates and silicates in the storage and transport of phosphorus at the upper-to-lower mantle transition: An experimental study to 25 GPa in a model peridotitic bulk composition

Calcium (Ca) phosphates are major hosts for phosphorus, halogens and incompatible trace elements in the Earth’s crust and mantle and, by supplying almost all phosphorus to the biosphere, are indispensable for sustaining life on Earth. To better understand the role of Ca-phosphates in the deep silicate Earth portion of the global phosphorus cycle at depths of the transition zone, we performed experiments in the P-T range of 15–25 GPa and 1600–2000 °C using a moderately fertile peridotite doped with 3 % synthetic β-Ca3(PO4)2 and 1 % of a trace element mix containing a range of HFSE, LILE, REEs, Cl and Br. Tuite is stable to at least 25 GPa at which pressure its upper T-stability limit is between 1700 and 1750 °C. At 15 GPa, by comparison, the upper T-stability limit is located between 1750 and 1800 °C, indicating a negative slope of the tuite-out reaction(s). Thus, tuite is stable to at least 700-km depth with a maximum T-stability close to or slightly above the average current mantle adiabat (ACMA) at 20–25 GPa and significantly above the ACMA at 15 GPa. Beyond the P-T stability limit of majoritic garnet, tuite is the principal subsolidus phosphorus carrier in the uppermost lower mantle. Beyond the P-T stability limit of tuite, phosphorus is likely to either (i) enter a Ca3(PO4)2 polymorph with an even higher density, (ii) form an entirely new phosphate phase or (iii) be accommodated in lower mantle silicates in six-fold coordination. Within the P-T range of our experiments, the phases encountered show the following trend of phosphorus contents:Pmelt>Pmajoritic garnet > Polivine ≅ Pringwoodite > Pdavemaoite ≅ Pwadsleyite > Pclinoenstatite ≅ Pbridgmanite ≅ Pferropericlase. Even when coexisting with tuite, bridgmanite and ferropericlase show phosphorus contents < 100 µg/g, and davemaoite has phosphorus contents that do not exceed 390 µg/g, testifying to the very limited phosphorus storage capacity of the lower mantle. Model calculations based on the phosphorus contents of ocean island basalts (OIBs) compared with those of mid ocean ridge basalts (MORBs) indicate that the OIB source is significantly richer in phosphorus than the MORB source and, for a wide range of OIB phosphorus contents, is likely to contain modal apatite. The relative importance of Ca-phosphates and silicates as phosphorus carriers in the peridotitic upper and uppermost lower mantle is strongly dependent upon pressure and the bulk phosphorus content. With increasing P, phosphorus is progressively transferred from Ca-phosphates to silicates with an ensuing decrease in the modal amount of the Ca-phosphates. Dependent upon the bulk phosphorus content, this may cause the Ca-phosphates to disappear before the upper-to-lower mantle boundary is reached, leaving majoritic garnet as the major phosphorus host. Breakdown of garnet will subsequently cause tuite to re-appear in the uppermost lower mantle and to resume its role of the principal subsolidus phosphorus carrier. For sufficiently high bulk phosphorus contents, tuite will remain stable throughout the transition zone, thereby maintaining phosphorus saturation in the coexisting silicates, and will eventually be joined by the tuite newly generated by garnet breakdown.

Young KREEP-like mare volcanism from Oceanus Procellarum

The Moon’s mare volcanism predominantly occurs within the Procellarum KREEP Terrane (PKT), which is widely thought to be associated with KREEP components within the lunar interior. The Chang’e-5 (CE-5) mission sampled a young (2 Ga) mare basalt Em4/P58 unit of northern Oceanus Procellarum. The geochemistry of the CE-5 mare basalt enables assessment of mantle source compositions which are essential to understand the thermo-chemical mechanism for prolonged volcanism during secular cooling of the Moon. Geochemical compositions of the CE-5 bulk soil, breccias, and basalt clasts from various depths within the drill core consistently display high concentrations of incompatible trace elements (ITE: ∼ 0.3 × high-K KREEP; ∼ 5 μg/g Th) with KREEP-like inter-element ratios, for example for La/Sm, Nb/Ta, and Zr/Y. Exotic impact ejecta, extensive magma differentiation (<70 % fractional crystallization) and significant assimilation of KREEP materials during magma transit and eruption cannot account for the ITE contents and ratios or radiogenic isotope compositions (e.g., εNdinitial of + 8 to + 9 and εHfinitial of + 40 to + 46) of the CE-5 basalts; instead, partial melting of their mantle source played a dominant role. The Chang’e-5 basalt is a chemically evolved low-Ti mare basalt (Mg# of ∼ 34) with enriched KREEP-like ITE compositions but high long-term time-integrated Sm/Nd and Lu/Hf ratios, which represent a hitherto unsampled type of mare basalt. It formed by melting of an augite-rich mantle source (late-stage magma ocean cumulates containing > 30–60 % augite, and little or no ilmenite), with a small amount of late-stage interstitial melt that resembles KREEP (∼1–1.5 modal %, equivalent to 0.2–0.3 μg/g Th in the mantle source). The voluminous mare basalts making up the Em4/P58 unit (>1500 km3) provide compelling evidence for large-scale, ITE enriched young mare magmatism within Oceanus Procellarum. In combination with remote sensing data and with the unique Th-rich Apollo 12 basalt fragment 12032,366–18 (impact ejecta likely from Oceanus Procellarum), this implies that significant portions of the FeO- and Th-rich mare regions of the western PKT may also have formed in a similar way.

Revisiting the Petrogenesis of Pyroclastic Glass Bead Deposits at the Apollo 15 and 17 Sites

Abstract The Apollo 15 low-titanium and Apollo 17 high-titanium pyroclastic glass beads are among the most primitive magmatically derived samples obtained from the Moon. Two key samples, the low-Ti Apollo 15426 green glass clod and the high-Ti Apollo 74220 orange glass are morphologically distinct, where the Apollo 15 beads are larger (~107 μm along maximum axis) and more fractured, and the Apollo 17 are smaller (~42 μm) and less fractured. In this study, holohyaline beads as well as crystallized beads were examined from both samples using petrography, electron microprobe analysis, and laser-ablation inductively coupled plasma mass spectrometry. Crystallized beads show compositional variability in major, minor, and trace elements and enable examination of magmatic mineral fractionation processes during cooling of both deposits. The Apollo 15426 beads experienced variable olivine crystallization, whereas the Apollo 74220 beads experienced both olivine and ilmenite crystallization. Holohyaline beads from both deposits show more limited major, minor, and trace element variability than their crystallized counterparts. Trace element abundance data for individual holohyaline beads show that in Apollo 74220, they are tightly clustered at ~30× Carbonaceous Ivuna chondrite [CI] with negative Eu anomalies and subchondritic Nb/Ta, interpreted to reflect the presence of late-stage magma ocean cumulates overturned into an otherwise primitive mantle source. Incompatible trace element abundances for holohyaline beads in 15426 are supra-chondritic from ~8× CI, to &amp;gt;80× CI, with pronounced relative depletions in Sr and Eu for the most incompatible element enriched beads, which represent a distinct bead group within the deposit. Apollo 15426 beads have elevated Ni and Co abundances at the edges of the beads compared to their centers. These data are interpreted to reflect a more complex magmatic evolution of the 15426 deposit, beginning with (i) initial magma generation, storage, and assimilation within shallower low- and high-Ca pyroxene bearing magma ocean cumulates (15B,C); (ii) mobilization of the earlier magmas by more recently generated primitive magmas (15A); (iii) eruption and crystallization of some beads (15D,E); and (iv) later jumbling of the deposit, possible impact contamination and addition of exotic basaltic bead components (J Group). In contrast, the 74220 data show no discernable difference between Ni and Co abundances at the edges and centers supporting prior observations for limited melt fractionation and an absence of meteoritic components. Both deposits are likely to have been formed in the presence of a transient atmosphere. Using 74220 melt compositions from this study, post-entrapment crystallization abundances range from 266 to 1130 μg/g for H2O, 36 to 68 μg/g for F, 441 to 832 μg/g for S, and 0 to 2.31 μg/g for Cl, consistent with prior studies and suggesting up to ~0.1 wt % H2O in the melt, with considerably less in the source. The role that late-stage magma ocean cumulates rich in ilmenite and high-Ca pyroxene might play in modifying this volatile element estimate, however, casts remaining doubt on the volatile element abundance and evolution of the primitive Moon.

Compositional zoning of the Otowi Member of the Bandelier Tuff, Valles caldera, New Mexico, USA

Abstract The Otowi Member of the Bandelier Tuff erupted at ca. 1.60 Ma from the Valles caldera (New Mexico, USA). It consists of as much as 400 km3 (dense rock equivalent) of strongly differentiated high-silica rhyolite and shows systematic upward variations in crystallinity, mineral chemistry, and trace element concentrations through its thickness, but the major element composition is almost constant and is near the low-pressure granite minimum. Incompatible trace elements in whole pumice fragments and glasses show well-correlated linear covariations. Upward zoning to lower abundances of incompatible trace elements is accompanied by development of overgrowths on quartz and alkali feldspar, although earlier-formed interiors of quartz and feldspar have near-constant compositions throughout the tuff, modified by cation diffusion in the case of feldspar. Melt inclusions in remnant quartz cores show diverse Pb isotope ratios, pointing to a wide range of distinct protoliths that contributed rhyolitic melt to the Otowi magma. Mineral thermometers suggest a modest temperature gradient through the melt body, perhaps of 40 °C, at the time of eruption. Chemical, textural, and mineralogical variations and volume-composition relations through the tuff are consistent with an origin for zoning by melting of a high-crystallinity cumulate layer beneath cognate supernatant liquid to produce denser, remobilized liquid of accumulative composition (i.e., the “modified mush model”). Melting may have occurred in several episodes. The latest of these episodes, probably thousands of years prior to eruption, introduced new rhyolitic liquid into the system and was associated with a thermal excursion, recorded in core compositions of pyroxene, during which much of the earlier crystal mass was dissolved. This left inherited cores and interiors of accumulated quartz and feldspar mantled with new growth having less-evolved compositions (higher Ti, Sr, and Ba). Changing solubility of zircon during cumulate melting produced a reversal of Zr concentrations. There is no clear petrologic evidence of a recharge eruption trigger; nonetheless, compositional zoning resulted mainly from repeated recharge-induced remobilization of quartz-feldspar cumulate. The Otowi system was built, evolved, and modified by several events over the course of a few hundred thousand years.

Petrogenesis of a new type of intrusive shergottite: olivine-gabbro Northwest Africa 13227

Petrologic investigations of martian meteorite Northwest Africa (NWA) 13227 indicate it is an olivine-gabbroic shergottite, a relatively new shergottite group, which differs from previously described gabbroic shergottites due to relatively high quantities of olivine. NWA 13227 is comprised of phenocrystic, oscillatory-zoned pyroxene and olivine, set in a matrix of maskelynite, Fe-Cr-Ti oxides, phosphates, and sulfides. It displays gabbroic and poikilitic textures in 2D from back-scattered electron images, and in 3D from X-ray Computed Tomography (XCT) imaging, suggesting affinities to both poikilitic and gabbroic shergottites. Measured εHf and εNd values of bulk rock (-19.7 and −5.9, respectively) and its chondrite-normalized La/Yb ratio of 1.13 indicate the specimen is derived from a mantle reservoir relatively enriched in incompatible trace elements and is similar to that which produced most ‘enriched shergottites.’ Based on the Ti/Al ratio of pyroxene, phosphorous zoning in olivine, and minor components in phosphates and oxides, we infer that NWA 13227 began crystallizing under reducing conditions of QFM–2.6 and temperatures of ∼ 1100 °C, consistent with conditions in Mars’ lower crust/upper mantle. The sample finished crystallizing at or near the surface under redox conditions between QFM–0.5 to QFM–0.1 and temperatures of ∼ 850 °C. The volatile element compositions in apatite indicate that NWA 13227 experienced degassing during the last stages of crystallization. The timing of crystallization is estimated at 225 Ma ± 50 Ma using a Lu-Hf and Sm-Nd source versus age model.

Subduction polarity reversal facilitated by plate coupling during arc-continent collision: Evidence from the Western Kunlun orogenic belt, northwest Tibetan Plateau

Abstract Subduction polarity reversal usually involves the break off or tearing of the downgoing plate (DP) along the continent-ocean transition zone, in order to initiate subduction of the overriding plate (OP) with opposite polarity. We propose that subduction polarity reversal can also be caused by DP-OP coupling and can account for the early Paleozoic geological relationships in the Western Kunlun orogenic belt in the northwestern Tibetan Plateau. Our synthesis of elemental and isotopic data reveals transient (~2 m.y.) changes in the sources of early Paleozoic arc magmatism in the southern Kunlun terrane. The early-stage (ca. 530–487 Ma) magmatic rocks display relatively high εNd(t) (+0.3 to +8.7), εHf(t) (−3.6 to +16.0), and intra-oceanic arc-like features. In contrast, the late-stage (485–430 Ma) magmatic rocks have predominantly negative εNd(t) (−4.5 to +0.3), εHf(t) (−8.8 to +0.9), and higher incompatible trace elements (e.g., Th), similar to the sub-continental lithospheric mantle beneath the Tarim craton. This abrupt temporal-spatial variation of arc magmatism, together with the detrital zircon evidence, indicate that subduction polarity reversal of the Proto-Tethys Ocean occurred in a period of ~10 m.y., consistent with the time interval reflected by ophiolite age. This rapid polarity reversal corresponds with the absence of ultrahigh-pressure (UHP) metamorphic and post-collisional magmatic rocks, features normally characteristic of slab break-off or tearing. Numerical modeling shows that this polarity reversal was caused by plate coupling during arc-continent collision. This coupling modified the normal succession of arc-continent collision events, preventing slab break-off or tearing-induced buoyant rock rebound and asthenosphere upwelling. Our model successfully explains early Paleozoic orogenesis in the Western Kunlun orogenic belt and may be applied elsewhere where post-collisional magmatic and UHP rocks are absent.

Cold deep subduction of Indian continental crust and release of ultrahigh‐pressure fluid during initial exhumation: Insights from coesite‐bearing eclogite‐vein systems in Kaghan Valley, Pakistan

AbstractThe ultrahigh‐pressure (UHP) eclogites from the Kaghan Valley in Pakistan, which formed by the deep subduction of the Indian plate beneath the Asian plate in the Eocene, contain complex metamorphic vein systems (including both isolated veins and vein networks), with mineral assemblages of epidote + quartz + kyanite + phengite ± omphacite ± garnet. The investigations on the Kaghan UHP eclogite‐vein systems provide important insights into the mechanism and timing of metamorphic dehydration, fluid flow, and fluid–rock interaction in the deeply subducted Indian continental slab as well as the chemical characteristics of slab‐derived, aqueous fluids. Abundant lawsonite pseudomorphs, characterized by prismatic aggregates of epidote, kyanite, and quartz porphyroblasts, are first recognized in the Kaghan eclogites. This observation, in combination with the occurrence of coesite pseudomorphs in epidote porphyroblasts as well as the coexistence of epidote and coesite in the eclogite zircon, indicates the previous existence of UHP lawsonite in these eclogites. Petrological studies and phase equilibrium modelling reveal clockwise P–T trajectories for the Kaghan eclogites that are featured by prograde vectors in lawsonite‐stability regions with peak conditions of 3.0–3.4 GPa/650–690°C, followed by isothermal decompression and lawsonite breakdown under UHP conditions during the initial exhumation stage. The results of metamorphic evolution, together with in situ epidote and bulk Sr isotopic analyses, indicate that the fluids responsible for vein systems are most likely derived from the breakdown of UHP lawsonite in the eclogites. SIMS U–Pb dating of metamorphic zircons from the eclogites, integrated with the Raman analysis of inclusions in zircons, indicates that the UHP dehydration of eclogites occurred at 46.4 ± 1.2 and 46.8 ± 0.9 Ma. Analyses of hydrothermal zircons from the veins yielded slightly younger ages of 44.7 ± 1.0 and 44.9 ± 1.4 Ma, which represent the timing of fluid flow and/or vein crystallization during exhumation of the UHP rocks. Mass‐balance calculation results, in combination with the vein compositions, show that the fluid flow and fluid‐eclogite interaction led to the transfer of Si, Al, Ca, K, and incompatible trace elements from the eclogites into the fluids, from which the vein systems crystallized. This study indicates cold deep subduction of Indian continental crust along low geothermal gradients (6–7°C/km). The UHP fluid liberation and channelized fluid flow occurred during the initial exhumation of the cold Indian slab and are expected to induce the transfer of H2O and incompatible trace elements from the Indian slab to the Asian lithosphere, which potentially contributes to the formation of post‐collisional magmas. Moreover, we suggest that metamorphic vein systems in UHP lawsonite eclogites offer important constraints on the occurrence and timing of fast slab exhumation in continental subduction‐collision zones.