Composition Of Lithosphere Research Articles

Geology Matters for Antarctic Geothermal Heat

AbstractGeothermal heat plays a vital role in Antarctic ice sheet stability. The continental geothermal heat flow distribution depends on lithospheric composition and ongoing tectonism. Heat‐producing elements are unevenly enriched in the crust over deep time by various geological processes. The contribution of crustal heat production to geothermal heat flow is widely recognized; however, in Antarctica, crustal geology is largely hidden, and its complexity has frequently been excluded in thermal studies due to limited observations and oversimplified assumptions. Li and Aitken (2024), https://doi.org/10.1029/2023GL106201 take a significant step forward, focusing on Antarctic crustal radiogenic heat. Utilizing gravity inversion and rock composition data, they show that the crustal heterogeneity introduces considerable variability to heat flow. However, modeling crustal heat production proves challenging because it lacks distinct associations with geophysical observables and has a narrow spatial association. Robust quantification of geothermal heat production and heat flow must incorporate explicit aspects of geology.

Continental Residual Topography Extracted From Global Analysis of Crustal Structure

AbstractContinental topography is dominantly controlled by a combination of crustal thickness and density variations. Nevertheless, it is clear that some additional topographic component is supported by the buoyancy structure of the underlying lithospheric and convecting mantle. Isolating these secondary sources is not straightforward, but provides valuable information about mantle dynamics. Here, we estimate and correct for the component of topographic elevation that is crustally supported to obtain residual topographic anomalies for the major continents, excluding Antarctica. Crustal thickness variations are identified by assembling a global inventory of 26,725 continental crustal thickness estimates from local seismological data sets (e.g., wide‐angle/refraction surveys, calibrated reflection profiles, receiver functions). In order to convert crustal seismic velocity into density, we develop a parametrization that is based upon a database of 1,136 laboratory measurements of seismic velocity as a function of density and pressure. In this way, 4,120 new measurements of continental residual topography are obtained. Observed residual topography mostly varies between ±1 and 2 km on wavelengths of 1,000–5,000 km. Our results are generally consistent with the pattern of residual depth anomalies observed throughout the oceanic realm, with long‐wavelength free‐air gravity anomalies, and with the distribution of upper mantle seismic velocity anomalies. They are also corroborated by spot measurements of emergent marine strata and by the global distribution of intraplate magmatism that is younger than 10 Ma. We infer that a significant component of residual topography is generated and maintained by a combination of lithospheric thickness variation and sub‐plate mantle convection. Lithospheric composition could play an important secondary role, especially within cratonic regions.

Jura-Cretaceous synorogenic magmatism and relations to supercontinental rifting in the northwestern U.S. Cordillera

AbstractChanges in lithospheric composition and strength associated with rifting of the Rodinian supercontinent (ca. 780–485 Ma) served to localize Mesozoic tectonic activity in the McCall region of west-central Idaho, USA. Although discrimination diagrams support slab failure melting, ca. 150–121 Ma calcic magmatism in the Hazard Creek complex and chemically similar intrusive rocks of northeastern Oregon, USA (e.g., Cornucopia stock), is best explained by progressive loading of oceanic crust. Westward-directed thrusting across the Laurentian continental margin and adjacent island-arc terranes (Olds Ferry/Wallowa; Blue Mountains province) led to partial melting of metabasaltic source rocks at pressures and depths sufficient for garnet stability (>10 kbar, ~35 km). Early pluton emplacement during right-oblique arc–continent collision overlapped in time with subduction zone reorganization, which involved a change in dip direction (westward to eastward) under the accreting Blue Mountains block. Ensuing magmatic activity predated and was concurrent with ca. 116 Ma thrust reactivation of the Wallowa/Olds Ferry boundary, which temporally overlapped with contractional deformation along the terrane–continent accretionary boundary. Later synorogenic magmatism was focused along the accretionary boundary, where rising anatectic melts were caught between the eastward-impinging/northward-migrating (obliquely colliding) terrane block and continental margin backstop (Rodinian rift architecture). Middle Cretaceous dextral transpression viewed in this manner precludes the need for an outboard “hit-and-run” collider (Insular superterrane) and thus, a post-accretion shear zone in western Idaho.

Mantle Plume‐Lithosphere Interactions Beneath the Emeishan Large Igneous Province

AbstractThe formation of large igneous provinces (LIPs) has been widely believed to be linked to mantle plume activity. However, how the plume modifies the overlying lithosphere, particularly its compositional structure, remains uncertain. Here, we characterize the deep thermochemical structure beneath the Emeishan LIP (ELIP), which is a well‐known Permian plume‐related LIP in China, by taking a multi‐observable probabilistic inversion. Our results find a clear correlation between the lithospheric composition with the ELIP's concentric zones. We infer that the fertile feature of the lithospheric mantle in the ELIP's inner zone was caused by the plume‐derived fertile magmas which infiltrated into and chemically refertilized the ambient depleted lithosphere. This plume‐modified lithospheric compositional structure is likely to be preserved after the plume event, while the present lithospheric thermal structure has been mainly influenced by the subsequent thermal‐tectonic activity. Our results improve our understanding of the physicochemical interactions between the lithosphere and ancient plume.

Thermochemical Structure and Melting Distribution of the Upper Mantle Beneath Intraplate Volcanic Areas in Eastern South China Block

AbstractThe Eastern South China Block (SCB) has experienced complex Mesozoic‐Cenozoic tectonism and intraplate volcanism. However, due to a lack of exhaustive exploration of the upper mantle's thermochemical structure, it is difficult to determine the extent of the lithospheric modification and the mechanisms by which the volcanism generate. Here, we jointly invert Rayleigh wave dispersion, surface heat flow, geoid height, and elevation data to provide a comprehensive thermal and compositional structure of the upper mantle beneath eastern SCB and infer regions of partial melting. Our model reveals widespread lithospheric thinning in the eastern SCB and large variations of lithospheric composition with a more fertile eastern Lower Yangtze lithosphere than the lithosphere elsewhere, suggesting the lithosphere of the eastern Lower Yangtze is more severely modified than the rest of the SCB. Moreover, two high‐temperature anomalies are revealed: one beneath the eastern Lower Yangtze and the other beneath the Pearl River Delta region, associated with the Pacific plate subduction and Hainan plume, respectively. The high‐degree partial melting (∼6%) in the asthenosphere beneath the Lower Yangtze is responsible for the strong ongoing lithospheric modification and the young intraplate volcanism in the Nvshan and Subei areas. Small‐scale upper mantle convections triggered by the large mantle upwellings created a low value of ∼3% melts, possibly responsible for the intraplate volcanism in the coastal CB and less severe lithospheric modification. We demonstrate that the lithospheric thickness and its thermochemical state are the key factors that influence the composition and evolution of intraplate volcanism in the eastern SCB.

Ophiolite complex of Piedmont-Ligurian basin (Northern Apennines)

The Northern Apennines contain remnants of the Piedmont-Ligurian Basin (PLB) or ocean, which during the Late Mesozoic time (mainly in the Jurassic) separated the paleo-Europe (Iberian plate) from the southern paleocontinent Adria at the Africa promontory. The closure of the PLB and subsequent collision of Europe/Adria in Cretaceous-Cenozoic time led to the exhumation of the ophiolite complex of Northern Apennines in the Ligurian units. The paper gives information obtained by the authors during several field trips on the composition of the ophiolite complex of the Northern Apennines, representative of the composition and structure of the oceanic lithosphere. The latter, absent in the territory of Ukraine, is a key question to understanding the evolution of the oceanic crust, subduction processes, and formation of accretion wedges in the transitional zones to the continents. The Ligurian ophiolites of the PLB constitute an accessible and unique window to track the opening and evolution of the slow-spreading oceanic lithosphere. The Internal Ligurian ophiolites consist of km-scale gabbroic bodies intruded into depleted mantle peridotites and bear remarkable structural and compositional similarities to oceanic lithosphere from slow and ultra-slow spreading ridges. The External Ligurian ophiolites, associated with continental crust material and transition zone between the oceanic and continental crust, include mantle sequences retaining a subcontinental lithospheric origin. The gabbro-peridotite associations from the Internal Ligurian ophiolites were explored in the Bracco-Levanto ophiolite massif, which includes a km-scale gabbroic body, recalling the oceanic core complexes from modern spreading centres, intruded into the mantle peridotites. The peridotites and the gabbros from these ophiolites record a composite history involving deformation and alteration from high temperature to seafloor conditions. The top of the peridotites is covered by tectono-hydrothermal breccias (ophicalcites), radiolarites, and sedimentary breccias that testify to the exposure of peridotites at the seafloor. This succession is then covered by basalts pillow-lavas. Thus, the almost full section of the ophiolite complex is represented here.

Continental Deep Subduction Versus Subduction Cessation: The Fate of Collisional Orogens

AbstractThe contrasting fates of collisional orogens, that is, continental deep subduction or subduction cessation, are widely recognized by petrological, paleomagnetic and geophysical observations. However, the mechanisms of such different collisional modes, especially the dynamics of continental deep subduction, are controversial. In this study, we integrate the phase transition‐induced density evolutions into a thermo‐mechanical numerical model. Combing the systematic petrological‐thermo‐mechanical models with force balance analysis, we find that the high metamorphic transformation degree, mildly depleted mantle composition of the subcontinental lithosphere, and a long preceding oceanic slab, increase the driving force for continental deep subduction. Additionally, the rheologically weak continental crust and asthenospheric mantle decrease the resistance force and promote deep subduction. Otherwise, the continental subduction cessation mode is favored. The calculations of slab negative buoyancy indicate that the phase transition‐induced metamorphic densification of the subducted continental crust and the mildly to moderately depleted lithospheric mantle can provide a great slab pull force to sustain the continued continental deep subduction; however, the positive buoyancy of highly depleted Archean lithospheric mantle impedes deep subduction and causes subduction cessation. Based on systematic numerical models, we also evaluate the crustal mass balance or deficit in continental collisional system, which indicates that ∼12%–47% of pre‐collisional felsic crust could be subducted deeply with the sinking slab in the regime of continental deep subduction. In contrast, the recycled felsic crust is negligible in the regime of subduction cessation. Thus, the different modes of continental collision play a crucial role in the global crustal recycling and related mantle heterogeneities.

Mantle Flow Deflected by Arc–Continent Collision and Continental Subduction in Eastern Indonesia

Abstract We investigate seismic anisotropy across southeastern Indonesia where the Indo-Australian plate subducts beneath and collides with the Sunda-Banda arc. Geochemical, geodetic, and tomographic studies reveal an along-strike transition from oceanic subduction to continental subduction and collision near central Flores that is due to a change of lithospheric composition in the subducting plate. To investigate the anisotropic fabric and dynamics of the upper mantle surrounding this young (∼5 Ma) arc–continent collision, we perform shear wave splitting analysis on local and teleseismic S waves recorded by an array of broadband seismometers that crosses the subduction–collision boundary. Seismic anisotropy inferred from our local S dataset shows that anisotropic sources above the slab extend to depths exceeding 100 km. Analysis of teleseismic SKS and SKKS waves reveal a shift in subslab fast axes from trench-parallel to trench-perpendicular near the ocean–continent boundary in the lower plate, which we relate to regional subslab mantle flow being deflected around the subducted continental lithosphere. Along-strike variations in anisotropic fast axes from teleseismic phases overlap with distinct structural and tectonic boundaries that divide distinct regions of the collision, implying the effects of the collision transcend any one dataset and highlighting the complexity of collisional boundaries. These results shed light on the interaction between tectonic structure and mantle dynamics in an emergent collision, and help constrain the nature of upper mantle deformation in the early stages of collision.

Formation mechanism of the North–South Gravity Lineament in eastern China

Gravity lineaments are well recognized along plate boundaries and in the interiors of oceanic plates. However, the formation mechanism of intracontinental gravity lineaments remains poorly understood. The North–South Gravity Lineament (NSGL), which extends >4000 km from Russia to South China, is characterized by an abrupt change in the observed Bouguer gravity anomaly, ranging from −100 mGal in the west to −40 mGal in the east over a distance of <100 km. Here we employ gravity modeling along a deep seismic sounding profile that transects the NSGL to delineate the relative contributions of the thickness and composition of the crustal and upper mantle and the stagnant slab in the mantle transition zone to the observed gravity anomaly, and to ultimately constrain the formation mechanism of the NSGL. Our results indicate that crustal thickness is the dominant factor in the formation of the NSGL, with lithospheric composition also playing a role. Combining our results with those of previous studies, lateral variations in the lithospheric mantle composition and crustal/lithospheric thickness across the NSGL can be attributed to lithospheric thinning and mantle replacement in the eastern part of the North China Craton during the late Mesozoic. This was triggered by the subduction of the Paleo-Pacific plate and subsequently led to the persistence of a stagnant slab and big mantle wedge. This study highlights the fundamental need for integrated geophysical–petrological analyses to better understand the surface response to deep tectonic processes.

Petrogenetic evolution of the Zhuopan potassic alkaline complex, western Yunnan, SW China: Implications for heterogeneous metasomatism of lithospheric mantle beneath Simao and western Yangtze block

Potassic alkaline rocks are instrumental in unveiling the composition and geodynamic evolution of mantle lithosphere. The Zhuopan alkaline complex, located in the western Simao block, SW China, exhibits features of high potassium, enrichment in LILEs, and depletion in HFSEs. These same elemental features and SrNd isotopic compositions are also evident in the coeval clinopyroxenitic and syenitic rocks, indicating a comagmatic origin. In this paper, we present new data and discuss the petrogenesis of these rocks. We show that magmas parental to the Zhuopan alkaline complex are derived from highly enriched clinopyroxene-rich lithospheric mantle. These alkaline magmas emplaced in shallow crustal level and experienced crustal assimilation during ascent and through magma chamber processes. Pyroxenite is of cumulitic origin dominated by clinopyroxene and apatite. It formed at an early stage and received more effective crustal contamination. Pyroxene syenite, syenite and nepheline syenite are compositional mixture of variably evolved melt with incompletely segregated liquidus phases (e.g., clinopyroxene, biotite, alkali-feldspar, apatite, nepheline). An important observation is that the Dy/Yb ratio, LILE and LREE abundances of the Zhuopan complex are significantly higher than those of the contemporaneous mafic volcanic rocks in the western Yangtze block, reflecting different sources and source histories. The lithospheric mantle beneath Simao and western Yangtze blocks must have been heterogeneously metasomatized by oceanic subduction-related fluids and melts. Particularly, the mantle source for the Zhuopan alkaline rocks exhibits feature of carbonatite metasomatism. This is further supported by our modeling where melts derived from marine carbonate play a crucial role.

Recycling of Paleo-Asian Ocean carbonates and its influence on the lithospheric composition of the North China Craton

Recycling of Paleo-Asian Ocean carbonates and its influence on the lithospheric composition of the North China Craton

Serpentinization, Carbonation, and Metasomatism of Ultramafic Sequences in the Northern Apennine Ophiolite (NW Italy)

AbstractFluid‐rock interaction in ultramafic rocks considerably affects the chemical and isotopic composition of the oceanic lithosphere. We present a geochemical and petrological study of serpentinites and ophicalcites of the Northern Apennine ophiolite, Italy. This ophiolite sequence represents fragments of Jurassic oceanic lithosphere that have been denuded by low angle detachment faults, exposing peridotites on the ocean floor and triggering hydrothermal alteration. Seawater circulation is documented by (Jurassic) seawater‐like 87Sr/86Sr values and δ13C values of 1.1–3.0‰ in carbonate veins of the ophicalcites. Bulk rock ophicalcites have low 87Sr/86Sr values of 0.70489–0.70599, elevated SiO2 contents, and talc druses filling calcite veins that record Si‐metasomatism. In contrast, underlying serpentinites have 87Sr/86Sr values above Jurassic seawater values. Bulk rock δD and δ18O values of ophicalcites and serpentinites suggest interaction with an evolved seawater‐derived and/or magmatic fluid. These chemical signatures result from a complex history of serpentinization, carbonation, and metasomatism. Multiphase water‐rock interaction includes infiltration of basement‐derived fluids during initial mantle upwelling within an opening ocean basin, followed by localized high‐temperature fluid infiltration, extensive seawater circulation resulting in carbonation, and oxidation near the seawater‐exposed surface, and finally, fluid‐rock interaction with overlying mafic lithologies leading to Si‐metasomatism.The studied sequence represents an excellent example of the evolution from serpentinite to ophicalcite during continuous uplift and exposure of ultramafic rocks on the seafloor and documents the complex hydrothermal evolution of ultramafic rocks associated with this process. The extensive chemical transformation of mantle peridotites likely has an impact on geochemical cycles and subduction zone processes.

The nature of the southern West African craton lithosphere inferred from its electrical resistivity

The West-African craton is defined by a combination of Archean and Palaeoproterozoic rocks that stabilised at ~2 Ga towards the end of the Paleoproterozoic Eburnean Orogeny, and therefore may reflect the transition from Archean to modern tectonic processes. Exploring its present lithospheric architecture aids further understanding of not only the craton’s stability through its history but also its formation. We investigate the lithospheric structure of the craton through analysing and modelling magnetotelluric (MT) data from a 500-km-long east-west profile in northern Ghana and southern Burkina Faso crossing part of the Baoulé-Mossi Domain and reaching the Volta Basin in the south-eastern part of the craton. Although the MT stations are along a 2D profile, due to the complexity of the structures characterising the area, 3D resistivity modelling of the data is performed to obtain insights on the thermal signature and composition of the subcontinental lithosphere beneath the area. The thermal structure and water content estimates from different resistivity models highlight a strong dependence on the starting model in the 3D inversions, but still enable us to put constraints on the deep structure of the craton. The present‐day thermal lithosphere‐asthenosphere boundary (LAB) depth is estimated to be at least 250 km beneath the Baoulé-Mossi domain. The area likely transitions from a cold and thick lithosphere with relatively low water content into thinner, more fertile lithosphere below the Volta Basin. Although the inferred amount of water could be explained by Paleoproterozoic subduction processes involved in the formation of the Baoulé-Mossi domain, later enrichment of the lithosphere cannot be excluded

Thermal architecture of cratonic India and implications for decratonization of the Western Dharwar Craton: Evidence from mantle xenoliths in the Deccan Traps

The mantle beneath the Western Dharwar Craton of the Indian shield comprises a suite of refractory and fertile peridotites and mafic granulites. Detailed petrographic studies coupled with new mineral analysis and geothermobarometric estimations permit to decipher the thermal architecture and get an insight into the evolution of this ancient craton. The refractory rocks are coarse grained harzburgites/dunites, whereas the more fertile ones are at times, porphyroclastic lherzolites. Both show a similar range of equilibration temperatures and pressures indicating intermixing between the two at various levels. The peridotites contain undeformed interstitial REE-enriched clinopyroxene, phlogopite, apatite and carbonates recording post-kinematic modal and cryptic metasomatic events in the Precambrian cratonic lithosphere. Xenoliths of mafic granulite contain layers of clinopyroxenite which also vein the granulite. The P-T range of the granulites overlaps that of the ultramafic rocks.This study in combination with previous investigations reveals a distinct change in the thermal architecture of the craton from a warm/hot geotherm in the Proterozoic to a highly perturbed, still hotter geotherm of the Palaeocene. The Cenozoic thermotectonic rifting episodes heated, refertilized and thinned the bulk of the cratonic lithosphere beneath the Western Dharwar Craton, which has witnessed the most re-activation among cratons of the Indian shield. The waning of the Deccan Traps volcanism in Palaeocene time saw the reworking of ancient cratonic lithosphere and its replacement by non-cratonic, juvenile mantle and magmatic accretions, indicated by compound xenoliths. Differing petrological and geochemical characteristics of refractory xenoliths and fertile lherzolites serve to constrain the relative timing and composition of non-cratonic lithosphere. By the end of the Palaeocene the Western Dharwar Craton was characterised by a thermal high, an attenuated continental lithosphere (60–80 km), and a thin crust (<10- ~ 21 km), reflecting the decratonization of at least the western part of the Western Dharwar Craton.

Early Ordovician magmatism in the Sierra de Ancaján, Sierras Pampeanas (Argentina): implications for the early evolution of the proto-Andean margin of Gondwana

The Ancajan pluton is a small-scale (ca. 5.34 km2) elongated igneous body of granodiorite to monzogranite composition that crops out in the Sierra the Ancajan (Eastern Sierras Pampeanas), intruding marbles and metasedimentary rocks of the Ediacaran Ancajan series. New SHRIMP and LA-MC-ICP-MS U–Pb zircon analyses from one granodiorite sample yielded a likely Ordovician crystallization age of ca. 473 Ma. Inherited Cambrian, Neoproterozoic, Mesoproterozoic and Paleoproterozoic ages have also been recorded. The Ancajan granitoids are calc-alkalic, magnesian and slightly peraluminous with medium to high K2O (2.44–3.74 wt%) and high Na2O (4.05–4.51 wt%) contents. These geochemical characteristics are comparable to those of the Ordovician Na-rich (TTG-like) magmatism of the Foreland Famatinian Domain (FFD) in the Sierras de Cordoba. Therefore, the Ancajan pluton could represent the northernmost outcrop of such magmatism. Isotopes data (Sr/Sri = 0.7052–0.7055; eNdi = − 0.7 to − 0.4; TDM = 1.24–1.27 Ga) and inherited zircon ages suggest that the parental magma probably resulted from partial melting of a combined source, mainly formed by older subcontinental mantle with mafic–ultramafic oceanic lithosphere composition, along with recycling/assimilation of continental crust involving Pampean granitoids and/or Ediacaran to Cambrian sedimentary protoliths. This interpretation is coincident with that postulated for the Ordovician Na-rich granitoids from the Sierras de Cordoba. The evidence shown here further implies that metasedimentary rocks of the Sierra de Ancajan were part of the continental upper plate during the Famatinian subduction, and corroborate the previous imbrication of the Ancajan and Puncoviscana series during the early Cambrian Pampean Orogeny.

Neotethyan Subduction Ignited the Iran Arc and Backarc Differently

AbstractMost arcs show systematic temporal and spatial variations in magmatism with clear shifts in igneous rock compositions between those of the magmatic front (MF) and those in the backarc (BA). It is unclear if similar magmatic polarity is seen for extensional continental arcs. Herein, we use geochemical and isotopic characteristics coupled with zircon U‐Pb geochronology to identify the different magmatic style of the Iran convergent margin, an extensional system that evolved over 100 Myr. Our new and compiled U‐Pb ages indicate that major magmatic episodes for the NE Iran BA occurred at 110–80, 75–50, 50–35, 35–20, and 15–10 Ma. In contrast to NE Iran BA magmatic episodes, compiled data from MF display two main magmatic episodes at 95–75 and 55–5 Ma, indicating more continuous magmatism for the MF than for the BA. We show that Paleogene Iran serves as a useful example of a continental arc under extension. Our data also suggest that there is not a clear relationship between the subduction velocity of Neotethyan Ocean beneath Iran and magmatic activity in Iran. Our results imply that the isotopic compositions of Iran BA igneous rocks do not directly correspond to the changes in tectonic processes or geodynamics, but other parameters such as the composition of lithosphere and melt source(s) should be considered. In addition, changes in subduction zone dynamics and contractional versus extensional tectonic regimes influenced the composition of MF and BA magmatic rocks. These controls diminished the geochemical and isotopic variations between the magmatic front and backarc.

Editorial: Volumes, Timescales, and Frequency of Magmatic Processes in the Earth's Lithosphere

Heat, mass, and fluid transfer processes related to the formation and growth of the continental crust along convergent and divergent plate boundaries, and the formation, modification, and recycling of the continental crust are key research themes in the solid Earth Science community. Establishing the link between magma generation, transport, emplacement, and eruption can therefore significantly improve our understanding of crust-forming processes associated with plate tectonics, and, particularly, help determining the architecture, and composition of the Earth's lithosphere.

Pacific Lithosphere Evolution Inferred from Aitutaki Mantle Xenoliths

Abstract Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), major and trace element abundances, and 187Re–187Os systematics are reported for xenoliths and lavas from Aitutaki (Cook Islands), to investigate the composition of Pacific lithosphere. The xenolith suite comprises spinel-bearing lherzolites, dunite, and harzburgite, along with olivine websterite and pyroxenite. The xenoliths are hosted within nephelinite and alkali basalt volcanic rocks (187Os/188Os ∼0·1363 ± 13; 2SD; ΣHSE = 3–4 ppb). The volcanic host rocks are low-degree (2–5%) partial melts from the garnet stability field and an enriched mantle (EM) source. Pyroxenites have similar HSE abundances and Os isotope compositions (Al2O3 = 5·7–8·3 wt %; ΣHSE = 2–4 ppb; 187Os/187Os = 0·1263–0·1469) to the lavas. The pyroxenite and olivine websterite xenoliths directly formed from—or experienced extensive melt–rock interaction with—melts similar in composition to the volcanic rocks that host the xenoliths. Conversely, the Aitutaki lherzolites, harzburgites and dunites are similar in composition to abyssal peridotites with respect to their 187Os/188Os ratios (0·1264 ± 82), total HSE abundances (ΣHSE = 8–28 ppb) and major element abundances, forsterite contents (Fo89·9±1·2), and estimated extents of melt depletion (&amp;lt;10 to &amp;gt;15%). These peridotites are interpreted to sample relatively shallow Pacific mantle lithosphere that experienced limited melt–rock reaction and melting during ridge processes at ∼90 Ma. A survey of maximum time of rhenium depletion ages of Pacific mantle lithosphere from the Cook (Aitutaki ∼1·5 Ga), Austral (Tubuai’i ∼1·8 Ga), Samoan (Savai’i ∼1·5 Ga) and Hawaiian (Oa’hu ∼2 Ga) island groups shows that Mesoproterozoic to Neoproterozoic depletion ages are preserved in the xenolith suites. The variable timing and extent of mantle depletion preserved by the peridotites is, in some instances, superimposed by extensive and recent melt depletion as well as melt refertilization. Collectively, Pacific Ocean island mantle xenolith suites have similar distributions and variations of 187Os/188Os and HSE abundances to global abyssal peridotites. These observations indicate that Pacific mantle lithosphere is typical of oceanic lithosphere in general, and that this lithosphere is composed of peridotites that have experienced both recent melt depletion at ridges and prior and sometimes extensive melt depletion across several Wilson cycles spanning periods in excess of two billion years.

Osmium isotope evidence for a heterogeneous 3He/4He mantle plume beneath the Juan Fernandez Islands

Mantle plume models have been widely applied to explain the formation of ocean island basalts (OIB), with high-3He/4He in their lavas being explained by sampling of a primitive deep mantle source. The Juan Fernandez Islands have 3He/4He (7.8–18 RA) similar to or higher than in mid-ocean ridge basalts (MORB; 8 ± 1 RA) and have been used to both support and refute the mantle plume hypothesis. Ambiguity regarding the origin of the Juan Fernandez Islands primarily originates from interpretation of mantle source signatures between the lava series from the two main islands, Robinson Crusoe and Alexander Selkirk. To examine this issue, we report new whole-rock and olivine separate 187Os/188Os ratios and major-, trace-, and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances. The HSE and trace element abundances in Juan Fernandez main shield lavas can be explained by up to 30% olivine removal, together with spinel crystallization at 1–5 kbar, whereas Robinson Crusoe rejuvenated lavas can be reproduced by higher-pressure fractional crystallization (up to 10 kbar). An assemblage of 30 modal % olivine and 1–5 modal % spinel, combined with the additional contribution of primary melt trapped in olivine inclusions reproduces the range of HSE compositions observed in Juan Fernandez Archipelago olivine grains. Ratios of 187Os/188Os for Juan Fernandez lavas are generally less radiogenic than global OIB and show no correlation with indices of fractionation, indicating that they reflect mantle source compositions. Younger basanite lavas from Robinson Crusoe represent rejuvenated volcanism dominantly from a depleted lithospheric mantle source (187Os/188Os < 0.13) mixed with a high-3He/4He component from the main shield building stage. These lavas are similar in origin and composition to other Pacific rejuvenated lavas (e.g., Samoa, Hawaii). Robinson Crusoe main shield lavas are from a high-3He/4He (>18 RA) and enriched mantle source (187Os/188Os = 0.1312) similar to the ‘C’ or ‘FOZO’ component, whereas Alexander Selkirk lavas are consistent with a dominant contribution from a depleted, low-3He/4He (<10 RA) mantle component. The mantle sources of the shield lavas yield subtle variations in Sr-Nd-Os-Pb isotope space and have no clear variations with relative or absolute abundances of the HSE or trace elements. These results are consistent with a heterogeneous mantle plume model, with initial eruption of lavas from a primitive high-3He/4He mantle source ∼4 million years ago (Ma) to form Robinson Crusoe Island, which also led to 3He enrichment of the oceanic lithosphere, followed by eruption of Alexander Selkirk lavas from a more depleted mantle source at ∼2 Ma. Juan Fernandez lavas show that mantle heterogeneity preserved in OIB can occur over short-timescales (<2 Ma) and can impact lithospheric compositions, leading to eruption of rejuvenated lavas with unusual isotopic characteristics.

The relationship between mantle potential temperature and oceanic lithosphere buoyancy

The Earth's mantle potential temperature (TP) is thought to have cooled by ∼250∘C since the Archean, causing a progressive change in both the structure and composition of oceanic lithosphere. These variables affect the negative buoyancy of subducting slabs, which is known to be an important force in driving plate motions. However, the relationship between TP and slab buoyancy remains unclear. Here, we model the formation and subduction of oceanic lithosphere as a function of TP, to investigate how TP influences the buoyancy of subducting slabs, and by extension how buoyancy forces may have changed through time. First, we simulate isentropic melting of peridotite at mid-ocean ridges over a range of TP (1300–1550∘C) to calculate oceanic lithosphere structure and composition. Second, we model the thermal evolution of oceanic plates undergoing subduction for a variety of scenarios (by varying lithospheric thickness, slab length and subduction velocity). Finally, we integrate the structural, compositional and thermal constraints to forward model subduction metamorphism of oceanic plates to determine down-going slab density structures. When compared with ambient mantle, these models allow us to calculate buoyancy forces acting on subducting slabs. Our results indicate that oceanic lithosphere derived from hotter mantle has a greater negative buoyancy, and therefore subduction potential, than lithosphere derived from cooler mantle for a wide range of subduction scenarios. With respect to the early Earth, this conclusion supports the viability of subduction, and models of subduction zone initiation that invoke the concept of oceanic lithosphere being primed to subduct. However, we also show that decreases to lithosphere thickness and slab length, and reduced crustal hydration, progressively reduce slab negative buoyancy. These results highlight the need for robust estimates of early Earth lithospheric properties when considering whether subduction was operative at this time. Nevertheless, our findings suggest that subduction processes on the early Earth may have been uniformitarian.