Mid-ocean Ridge Basalt Research Articles

Off-rift Axis Channelized Melt and Lithospheric Metasomatism along Mid-ocean Ridges—A Case Study from Iceland on the Limits of Melt Channelling

AbstractMagmatism in Iceland is classically explained by the interaction of the mid-Atlantic ridge with the Iceland plume. The growth of Iceland through time is the result of volcanic activity at the rift axis. However, Holocene volcanism (0–11.5 ka) is not restricted to the rift zone (RZ) but also occurs off-axis, specifically in the western Snæfellsnes Volcanic Belt (SVB) and in the Southern Flank Zone (SFZ). The rift and off-axis postglacial volcanic zones are separated by a gap of ~60–80 km. While the volcanic activity of the SFZ seems correlated with the actual location of the Iceland plume, the plume relation to the SVB is uncertain. To address the origin and relationships between rift and off-rift magmas, we present new data from two transects perpendicular to the Reykjanes rift. The lavas in the SVB are characterized by transitional to alkaline compositions, with elevated incompatible trace element content. In contrast, the RZ volcanic rocks have tholeiitic compositions with trace element signatures slightly more enriched than Mid-Ocean Ridge Basalt (MORB). Rift and off-rift Iceland lavas are all characterized by positive Ba and Nb anomalies, particularly in alkaline rocks. Tholeiitic and alkaline lavas show distinct differentiation sequences, with the main difference being the delayed crystallization of plagioclase in the fractionating assemblage of alkaline magmas. We apply these sequences to calculate primary magma compositions, which are then used to constrain melting conditions. Geochemical modelling indicates that Iceland rift and off-rift magmas can be produced from a peridotitic mantle source if lithospheric processes are involved. We demonstrate that recycled crust in the form of pyroxenite is not required to generate Snæfellsnes alkaline lavas. The low solidus temperature and high productivity of pyroxenite favour early and more extensive melting producing primary magmas that are not sufficiently enriched in incompatible trace elements to explain the compositional variation of Snæfellsnes magmas. An alternative mechanism to involve pyroxenite in the source of Snæfellsnes lavas relates to the hybridization of recycled oceanic crust with peridotite, but such reacted pyroxenite requires specific compositions to reproduce the Ba and Nb anomalies. As an alternative, we suggest that Snæfellsnes alkaline lavas are the result of channelized low-degree melts produced on the periphery of the melting column at distances exceeding 65 km from the ridge axis. These melts accumulate and percolate into the lithosphere producing amphibole ± phlogopite-bearing hydrous cumulates. Incongruent melting of these cumulates via renewed magmatic activity and melt-peridotite reaction can reproduce the alkaline compositions observed in the SVB, including the Ba and Nb anomalies. Numerical simulations of melt extraction below mid-ocean ridges suggest that low-degree melts produced as far as ~65 km from the central ridge axis rise vertically to the base of the lithosphere and are then focussed towards the ridge axis in decompaction channels. We propose that these melts interact with hydrous cumulates previously formed during the development of decompaction channels at the lithosphere–asthenosphere boundary and acquire specific Ba and Nb anomalies. The mixing of these distal enriched melts with more depleted melts extracted from the central part of the melting regime explains the composition of RZ lavas. Alkaline lavas observed in the SFZ show numerous analogies with the Snæfellsnes magmas, suggesting that similar lithospheric processes control their chemistry. The high thermal regime in Iceland and thick lithosphere explains the difference between Icelandic tholeiite and typical MORB. Our results highlight the importance of mantle dynamics below mid-ocean ridges and lithospheric interaction to produce off-axis magmatism with enriched alkaline signatures.

Constraints of barium isotopes on recycling of ancient oceanic crust in the mantle of the South China Sea

As surface reservoirs (e.g. sediments and altered oceanic crust) have extremely high barium (Ba) concentrations and distinctive Ba isotope compositions compared to the mantle, Ba isotopes have the potential to trace crustal recycling in the mantle. The sediment components and fluids derived from altered oceanic crust in arc lavas and continental basalts have been identified using Ba isotopes, while the geochemical behavior of Ba in subducted oceanic crust remains enigmatic. A series of volcanic breccia that temporally evolved from carbonated silicate clasts to alkali basalts in the South China Sea (SCS) drilled by the Integrated Ocean Discovery Program (IODP) Site U1431 was suggested to originate from the mantle source contaminated by recycled oceanic crust. To understand the potential of using Ba stable isotopes to trace recycled oceanic crust in the mantle, we report high-precision Ba isotope compositions of these samples, together with alkali basalts from adjacent Hainan Island to constrain the Ba isotope characteristics of the Hainan mantle plume. The carbonated silicate clasts and alkali basalts from the SCS display extremely high Ba concentrations and are thus insensitive to magma evolution during ascent through the thin lithospheric mantle and proper to trace crustal materials in their mantle source using Ba isotopes. These samples have variable Ba isotope compositions ranging from δ138/134Ba = −0.10 to +0.05‰, implying local mantle isotopic heterogeneity underlying the SCS. The Ba isotope characteristics of some samples, which are much lighter than those of the mantle-derived carbonatites and global mid-ocean ridge basalts (MORBs), in conjunction with trace element ratios and radiogenic isotope compositions, reflect significant incorporation of recycled carbonated oceanic crust after subduction-related dehydration. The rising of the Hainan mantle plume offers the most reasonable interpretation of the entrainment of the recycled oceanic crust in the carbonated silicate magmatism beneath the SCS. Alkali basalts from Hainan Island display Ba isotope characteristics slightly heavier than the SCS carbonated silicate clasts and alkali basalts. It implies that the Hainan mantle plume underlying Hainan Island contains less recycled-oceanic-crust components than that underneath the SCS. This study highlights Ba isotopes as effective tracers of recycled oceanic crust in the mantle.

Calcium isotope constraints on OIB and MORB petrogenesis: The importance of melt mixing

Lithological and isotopic heterogeneity in the mantle archive the mantle's melting history and fingerprint crustal recycling processes coupling the deep Earth with the terrestrial biosphere. Oceanic basalts, consisting of ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs), are invaluable tools for studying the chemical evolution of the mantle because they sample mantle heterogeneity at a variety of depths and length scales. In this study, we probe mantle heterogeneity by exploring the systematics of stable Ca isotopes in OIBs and MORBs. We find that OIBs from Iceland have δ44/40Ca values (∼0.85‰) that are indistinguishable from MORBs, while OIBs from Mangaia, the Azores, the Canary Islands and Hawaii have an average δ44/40Ca value ∼0.08‰ lower than the MORB-Iceland group. Moreover, MORB and Iceland δ44/40Ca values are relatively invariable, while OIBs range from 0.70‰ to 0.85‰ and correlate negatively with La/Lu and Nb/Y. The Ca isotope signature of the MORB-Iceland group is consistent with equilibrium fractionation during high-degree melting of average spinel/garnet peridotite with bulk silicate Earth (BSE) δ44/40Ca. Calcium-isotope variability in OIBs requires two-component mixing of melts with distinct δ44/40Ca values, one of which is a high-degree melt consistent with derivation from average peridotite with BSE δ44/40Ca. The melt-mixing endmember with isotopically light Ca (≤0.70‰) must be a deep, low-degree melt. This low-degree melt cannot be derived from a peridotite source with BSE δ44/40Ca, but instead requires either a source with lower-than-BSE δ44/40Ca or a different source mineralogy. The isotopically light, low-degree melt is consistent with derivation from a pyroxene- and garnet-rich source, but the effects of mineralogical heterogeneity in the source regions of OIBs cannot be easily distinguished from the possible influence of Ca isotope heterogeneity in the mantle. Regardless, mixing of melts from a shallow peridotite source and a deep source appears to be a ubiquitous part of OIB petrogenesis.

Major, trace and platinum-group element geochemistry of harzburgites and chromitites from Fuchuan, China, and its geological significance

Abstract The Fuchuan ophiolite, one of the oldest ophiolites in southern China, is located in the northeasternmost segment of the Jiangnan Orogen. It mainly consists of harzburgites, with minor dunites, pyroxenites and gabbro veins. To investigate the origin of the Fuchuan ophiolite and its related chromite ore deposit, bulk-rock major and minor element and platinum-group element (PGE) studies have been carried out. Major and trace element analyses show dual features of mid-ocean ridge basalt (MORB) and supra-subducted zone (SSZ) peridotites and high degrees of partial melting. The chondrite-normalized rare earth element (REE) patterns of the Fuchuan harzburgites are “U” shaped, reflecting the influence of light REE-enriched subducted melts or fluids. The Fuchuan chromitites originate from MORB-like melts and are enriched in IPGE and depleted in PPGE, which are basically consistent with high-Al chromitites worldwide, and the low PGE concentrations of chromitites reflect a lack of sulfur saturation of the parental melts during an early stage of their crystallization. Combining with the tectonic setting of the Jiangnan Orogen, we suggest that the Fuchuan peridotites and chromitites are believed to be formed in a back-arc spreading ridge between the Yangtze and Cathaysia blocks during 860–820 Ma.

The Early Cretaceous tectonic evolution of the Neo-Tethys: constraints from zircon U–Pb geochronology and geochemistry of the Liuqiong adakite, Gongga, Tibet

AbstractThe subduction model of the Neo-Tethys during the Early Cretaceous has always been a controversial topic, and the scarcity of Early Cretaceous magmatic rocks in the southern part of the Gangdese batholith is the main cause of this debate. To address this issue, this article presents new zircon U–Pb chronology, zircon Hf isotope, whole-rock geochemistry and Sr–Nd isotope data for the Early Cretaceous quartz diorite dykes with adakite affinity in Liuqiong, Gongga. Zircon U–Pb dating of three samples yielded ages of c. 141–137 Ma, indicating that the Liuqiong quartz diorite was emplaced in the Early Cretaceous. The whole-rock geochemical analysis shows that the Liuqiong quartz diorite is enriched in large-ion lithophile elements (LILEs) and light rare-earth elements (LREEs) and is depleted in high-field-strength elements (HFSEs), which are related to slab subduction. Additionally, the Liuqiong quartz diorite has high SiO2, Al2O3 and Sr contents, high Sr/Y ratios and low heavy rare-earth element (HREE) and Y contents, which are compatible with typical adakite signatures. The initial 87Sr/86Sr values of the Liuqiong adakite range from 0.705617 to 0.705853, and the whole-rock ϵNd(t) values vary between +5.78 and +6.24. The zircon ϵHf(t) values vary from +11.5 to +16.4. Our results show that the Liuqiong adakite magma was derived from partial melting of the Neo-Tethyan oceanic plate (mid-ocean ridge basalt (MORB) + sediment + fluid), with some degree of subsequent peridotite interaction within the overlying mantle wedge. Combining regional data, we favour the interpretation that the Neo-Tethyan oceanic crust was subducted at a low angle beneath the Gangdese during the Early Cretaceous.

Supra-subduction zone ophiolite generated by the initial subduction of an Early Paleozoic island arc system abutting the northern North China Craton: Evidence from meta-igneous rocks

To constrain the evolution of an Early Paleozoic island arc belt abutting the North China Craton (NCC), this study presents new geochronological, geochemical, and Sr–Nd–Hf–O–Pb isotopic data for igneous rocks from the central part of Jilin Province within the southeastern part of the Central Asian Orogenic Belt. The igneous complex in this area hosts both mid-ocean ridge basalt (MORB)- and arc-type rocks that yield zircon 206Pb/238U ages of ca. 492 and 488–459 Ma, respectively. The MORB-type units in this area include metacumulate gabbros, metadiabases, and metabasalts that have normal MORB (N-MORB)-like compositions with (La/Yb)N values of 0.29–0.99, Th/Nb values of 0.07–0.52, whole rock εNd(t) values from +4.32 to +7.29, and initial 208Pb/204Pb values of 38.12–38.20. These compositions are indicative of derivation from the depleted asthenospheric mantle. The arc-type igneous rocks consist of amphibolite, epidiorite, and metagabbro with major- and trace-element and isotopic compositions that are similar to island arc tholeiite to calc-alkaline rocks, with (La/Yb)N values of 1.11–4.26, Th/Nb values of 0.11–2.25, zircon εHf(t) values from +6.03 to +15.30, zircon δ18O values from +4.32 to +5.1, whole-rock εNd(t) values from +2.09 to +7.25, and initial 208Pb/204Pb values of 38.09–38.11. These compositions are indicative of derivation from partial melting of the hydrous depleted mantle wedge material within an intra-oceanic arc setting. The MORB-type rocks formed within a forearc region of the SSZ ophiolite fragment, whereas the arc-type rocks record the evolution of this region from immature to mature island arc tectono-magmatic settings. Combining these new data with the results of previous research suggests that initial subduction of the Paleo-Asian oceanic plate abutting the northern NCC began during the latest Cambrian.

Noble gas isotope systematics in the Canary Islands and implications for refractory mantle components

Noble gas isotope systematics of ocean island basalts (OIB) provide evidence for relatively undegassed and primitive mantle sources. These OIB sources partly derive from the deep mantle by virtue of their distinctiveness from mid-ocean ridge basalts (MORB), which dominantly sample upper mantle. New helium, neon and argon isotope data are presented for Canary Islands lavas, carbonatites and cumulate and mantle xenoliths confirming 3He/4He ratios that are the same or lower than MORB, but that are heterogeneous (∼3–9.5RA) within and between islands in the archipelago. Neon and Ar isotope systematics for lavas are mostly within the range of air compositions. Harzburgite xenoliths from Lanzarote, which are interpreted to represent ancient refractory mantle residues, have distinct He-Ne-Ar isotope systematics from lavas or cumulate xenoliths. The harzburgites are characterized by forsteritic olivine (>Fo91) with uniformly low-3He/4He (6.6 ± 0.2RA), high 40Ar/36Ar (630–4900), and have Ne isotope compositions that range between air values or that are similar or more nucleogenic than depleted MORB mantle (DMM). Similar refractory mantle peridotites have been discovered as xenoliths at other OIB localities and are likely to be distinct from continental lithospheric mantle. Refractory mantle (RM), which by virtue of its high melting temperature is difficult to partially melt, may be a significant component in the convecting mantle and has potential to impart a cryptic noble gas signature to partial melts in intraplate, divergent and convergent margin settings. In this sense, RM may represent a ‘sixth mantle component’ after DMM, high-µ (high 238U/204Pb; HIMU), enriched mantle endmembers (EMI, EMII) and the ‘focus zone’ (FOZO). An RM-type component may be presented in the older eastern Canary Islands and possibly in recent rejuvenated volcanism from Teide (Tenerife). Canary Island intraplate volcanism samples multiple mantle components, confirming that the most gas-rich mantle sources involved in magmatism dominate OIB noble gas compositions.

Carbon stable isotope constraints on CO2 degassing models of ridge, hotspot and arc magmas

Carbon dioxide emissions from volcanoes are important parameters to constrain in order to fully understand the Earth system, especially the effect of volcanic forcing on climate. The characterization of carbon concentration in magmas has been used to constrain volcanic fluxes. Because of low CO2 solubility in silicate melts, however, CO2 is significantly degassed from magmas due to decompression during transfer to the surface. The measurement of the carbon stable isotope ratio (13C/12C expressed as δ13C-values) in natural submarine glasses has been a helpful geochemical tool to study magma degassing. Carbon stable isotope fractionation at magmatic temperature between CO2 in vesicles and carbonate ions dissolved in the melt is still large enough to cause variations in δ13C-values. This variability can be used to deduce the mode of degassing (open vs. closed system, equilibrium vs. kinetic) operating in a given magmatic system.In this study, I present a review of the existing carbon isotope data for magmas of three different settings (ridge, hotspot and arc). This review allows to (1) investigate the diversity of degassing modes operating within a given setting and (2) compare the prevailing degassing mode between these three settings.Except for rare undersaturated samples and for volatile-rich, vesicular popping rocks, Mid-Ocean Ridge Basalts (MORBs) are predominantly extensively degassed and supersaturated in CO2 reflecting incomplete degassing during their last degassing step. Such a behavior is also reflected in their vesicle-dissolved carbon isotopic fractionations that are generally smaller than equilibrium values stemming from kinetic/diffusive effects. By contrast, the CO2 + H2O gas phase in hotspot and arc magmas is predominantly in chemical equilibrium with the melt because of volatile-rich initial conditions (and thus larger vesicularity) enhancing vapor-melt chemical and isotopic equilibrium. This larger initial volatile content is responsible for the extensive open-system (Rayleigh distillation) degassing generally observed in hotspot and arc magmas.This review highlights the fact that one single degassing model cannot explain the geochemical evolution of all magmas. Also, no single model can be assigned to one specific type of magma (i.e., a significant diversity of degassing mode exists within a given type of magma, notably in MORBs). It is important to take into account these observations when degassing corrections are applied on the basis of noble gas (He/Ne or He/Ar) ratios. It appears helpful to characterize the carbon concentration and stable isotope ratios in vesicles and dissolved in the glass in order to (1) identify the mode of degassing operating in a given magmatic system and (2) apply the most appropriate degassing correction to these magmas. It is only after this critical correcting step has been achieved that an assessment of initial magmatic CO2 contents can be reasonably undertaken.

Mixing, fluid infiltration, leaching, and deformation (MILD) processes on the slab-mantle wedge interface at high T and P conditions: Records from the Dalrymple Amphibolite, Philippines

The whole rock compositions of the blocks and the surrounding matrix of the Dalrymple Amphibolite are investigated in this study to determine the protolith of the blocks and the effect of mechanical mixing and fluid infiltration in the matrix of this fossil slab-mantle wedge interface. The major and trace element contents of the metamafic blocks indicate their mid-oceanic ridge basalt origin similar to the mafic lavas of the crustal section of central Palawan Ophiolite. Similarities in their rare earth and trace element patterns indicate the genetic relationship between the mafic lavas of the Palawan Ophiolite and the metamafic blocks of the Dalrymple Amphibolite. This confirms that the metamafic blocks represent the basalt to gabbro section of the oceanic lithosphere of the subducting slab.The matrix surrounding the blocks exhibit highly variable phase assemblages. In order to determine its petrogenesis, we distinguished groups of components/elements which behave similarly (Group 1 TiO2, Al2O3, Zr, Th and the light rare earth elements; Group 2 Cr, Ni and MgO) based on statistical (correlation coefficient) analyses. These groups indicate mixing of metasedimentary (Group 1) and metamafic (Group 2) end-members to form the matrix. The mixing proportions of the end-members were estimated by employing regression analysis wherein the measured concentration of fluid immobile elements (Cr, Ni, Zr, TiO2 and Al2O3) in the matrix samples were fitted against a modelled concentration by changing the end-member and their relative proportions. The end-members and mixing ratio with the highest regression value (r2) was selected to obtain the modelled composition of the matrix. The modelled and the measured matrix compositions were then used as the original (unmetasomatized) rock and the altered rock respectively in the isocon analysis, assuming that TiO2, Al2O3, Cr, Nd, Zr, and Hf are immobile. This assumption is supported by the prevalence of kyanite, ilmenite and zircon in the matrix mineral assemblage.This procedural workflow helped distinguish end-member components, estimate their mixing ratios, and determine the effects of infiltrating fluids. In particular, the whole rock composition of the matrix was controlled by mixing of a subordinate amount of metamafic blocks in a metasedimentary-dominated shear zone. This is supported by the Cr-Nb content of rutile grains included in the matrix samples which indicate mixed metamafic and metapelitic signatures. The metamafic-metasedimentary dominated matrix in the Dalrymple Amphibolite contrasts with other high-pressure/temperature (P/T) type metamorphic terranes which are dominated by low T minerals (serpentine, Mg-chlorite, and talc) derived from an ultramafic end-member, and could be reflective of conditions in warmer subduction zones. Mass balance calculations further revealed that an early fluid infiltration event likely occurred following the mixing process. This preferentially leached out elements which are either fluid-mobile (e.g. CaO and SiO2) or are not incorporated into the growing minerals in the matrix. The strong control of mineral assemblage of the matrix in its chemistry is exhibited by a number of samples which showed variable degrees of losses and gains in elements traditionally interpreted to be fluid immobile (e.g. heavy rare earth elements and Y). A later hydration event linked to retrograde metamorphic stage imprinted gains of K2O, Rb, and Ba in the matrix samples with the growth of replacement minerals (e.g. muscovite on kyanite). This later fluid infiltration event possibly masked the original loss of these fluid-mobile elements in the matrix samples during the earlier fluid-rock interaction.

A plume broke up Columbia supercontinent: Evidence from the Mesoproterozoic metamafic rocks in the Tarim Craton, NW China

In the Mesoproterozoic, there was an epic transition on Earth, the final breakup of the Columbia supercontinent. Understanding the dynamic mechanism responsible for the final breakup of Columbia supercontinent is crucial for establishing Earth’s evolution that includes geology, environment and life. However, there is little research on what drove the breakup of the Columbia supercontinent. Here we focus on this issue by integrating new zircon U-Pb ages, Lu-Hf isotopes, and whole-rock geochemical data for metamafic rocks from the Oulongbuluke Block in the southeast of Tarim Craton. These data show that the protoliths of these metamafic rocks were emplaced at ca. 1.37–1.35 Ga and were divided into high-Fe and low-Fe groups. The high-Fe and low-Fe groups show geochemical character similar to normal mid-ocean ridge basalt (N-MORB) and enriched mid-ocean ridge basalt (E-MORB), respectively. The high-Fe group was formed by high degree (7%-9%) partial melting of spinel-phase lherzolite mantle source, whereas the low-Fe group was derived from low degree (5%-6%) partial melting of spinel-phase lherzolite mantle source. We propose that the high-Fe and low-Fe groups may have been derived from magmas from different parts of a mantle plume. The formation of the 1.37–1.35 Ga metamafic rocks in the Oulongbuluke Block may be related to the separation of the Tarim Craton into North and South Tarim cratons, which resulted in the opening of the initial Middle Tarim Ocean Basin. By comparison with the plume-induced Yanliao Rift of North China Craton and McArthur Basin of North Australia Craton, we present that these three cratons may potentially dominated by a superplume, forming circular rift zone and radiating dykes. This superplume triggered the continuous extension to most parts of Columbia and led to the final breakup of the Columbia supercontinent.

Geochemical and Geochronological Constraints of Permian-Triassic Magmatism on Oceanic Subduction and Continental Collision during the Eastern Paleo-Tethyan Evolution

The Jinshajiang–Ailaoshan–Song Ma orogenic belt (JASB), as a vital segment of the eastern Paleo-Tethyan tectonic zone, is one of the most important zones in which to study the Paleo-Tethyan tectonic evolution. We have undertaken an integrated geochronological, petrological, and geochemical study of mafic rocks from the JASB to reveal the subduction and closure processes of the eastern Paleo-Tethyan Ocean during the Permian to Triassic. In conjunction with previous magmatic and metamorphic records in the JASB, three important tectonic stages are identified: (1) Early Permian to Early Triassic (ca. 288–248 Ma). Most of the Early Permian to Early Triassic mafic rocks have normal mid-ocean ridge basalt (N-MORB)- or enriched MORB (E-MORB)-like rare earth elements (REE) and trace element-normalized patterns with positive εNd(t) and εHf(t) values and negative Nb and Ta anomalies. Their La/Nb ratios and εNd(t) values show that approximately 3%–15% of slab-derived fluid accounts for the generation of these rocks. These characteristics suggest that the mafic rocks formed in an arc/back-arc basin setting at this stage. Additionally, the Early Permian mafic rocks are mainly exposed in the Jomda–Weixi–Yaxuanqiao–Truong Son magmatic rock belt (JYTB) on the western side of the JASB, indicating that the westward subduction of the Jinshajiang–Ailaoshan–Song Ma Paleo-Tethys Ocean (JASO) began in the Early Permian. Middle Permian mafic rocks are exposed in the Ailaoshan-Day Nui Con Voi metamorphic complex belt and the JYTB on both sides of the JASB. We propose that the bipolar subduction of the JASO occurred in the Middle Permian and ended in the Early Triassic. (2) Middle Triassic (ca. 248–237 Ma). The mafic rocks at this stage have LREE- and LILE-enriched patterns, negative Nb and Ta anomalies and negative εNd(t) values. Their variable εHf(t), εNd(t) values and La/Nb ratios show that these mafic rocks were highly affected by crustal material (ca. 16%). Considering the Middle Triassic high-pressure (HP) metamorphism and massive Al-enriched felsic magmatism in the JASB, these rocks may have formed in a collisional setting between the South China Block (SCB) and the North Qiangtang–Simao–Indochina Block (QSIB) during the Middle Triassic. (3) Late Triassic (ca. 235–202 Ma). The mafic rocks at this stage have negative εNd(t) and εHf(t) values and show terrestrial array characteristics. The εNd(t) values and La/Nb ratios show that approximately 30% of crustal components account for the generation of these rocks. Combined with the contemporaneous bimodal magma and metamorphism during the Late Triassic, we suggest that these rocks may have formed in a postcollisional extensional setting associated with magma diapir.

Barium stable isotopic composition of chondrites and its implication for the Earth

Barium is a refractory and fluid mobile element that is highly incompatible in basaltic systems. The elemental and isotopic compositions of Ba are used to trace fluid processes or sediment recycling during subduction. However, the Ba isotopic composition of primitive meteorites (chondrites), which represents the possible building blocks of terrestrial planets, is presently poorly known. Here, we report high precision Ba isotopic compositions of major types of chondrites using a double-spike method. Chondritic meteorites without clear terrestrial weathering show relatively homogeneous Ba isotopic distributions and have an average δ137/134Ba (the permil deviation of the 137Ba/134Ba ratio from the SRM3104a standard) value of 0.09 ± 0.09‰ (2SD, N = 14). Given the refractory and lithophile behavior of Ba and the limited variations of Ba isotope composition between various chondrite groups, this value can represent the Ba isotopic composition of the bulk Earth and the Bulk Silicate Earth (BSE). It is actually consistent with the present estimate of BSE (0.05 ± 0.06‰, Li et al., 2020; Nan et al., 2022) based on the measurements of mid-ocean ridge basalts and carbonatites. Small Ba isotope variations are observed between carbonaceous chondrite groups where CO and CV show slightly heavier (by 0.09‰) Ba isotopic compositions than CI and CM. Significant light stable Ba isotopes enrichment has previously been documented in calcium, aluminum-rich inclusions (CAIs) in the Allende CV chondrite, and the Ba isotopic differences between CO and CV might result from the various proportions of CAIs based on a mass balance calculation. However, the parent body of CI and CM might have experienced aqueous alteration and secondary carbonates are most likely responsible for their light Ba isotopic signatures.

Geochemical evidence for incorporation of subducting sediment-derived melts into the mantle source of Paleozoic high-Mg andesites from northwestern Tianshan in western China

Abstract Although high-Mg andesites (HMA) have attracted increasing attention due to their unique geochemical composition and important geological significance, there is no consensus on their petrogenesis. The present study indicates that the subducting terrigenous sediment-derived hydrous melts were incorporated into the mantle source of Paleozoic HMA in northwestern Tianshan, western China. These HMA are composed of basaltic andesite and andesite. They generally exhibit arc-type trace element distribution patterns and weakly enriched Sr-Nd-Hf isotope compositions. Some of them show remarkably higher Ba/La, Ba/Th, Rb/Nb, and U/Th ratios than normal mid-ocean ridge basalts (MORB). Furthermore, most of them are characterized by higher Th/Nb, Th/Yb, and Th/Nd ratios but lower Nb/U ratios relative to normal MORB, similar to those of terrigenous sediments. Whole-rock Nd isotopes covary with Rb/Nb and Th/Yb ratios, indicating contributions from both oceanic crust-derived aqueous solutions and terrigenous sediment-derived hydrous melts. Together with their high zircon δ18O values, it appears that the mantle source of the target HMA contains terrigenous sediment-derived hydrous melts in addition to subducting oceanic crust-derived aqueous solutions. In the studied HMA, the andesite generally exhibits higher contents of large-ion lithophile elements and light rare earth elements and more enrichments in Sr-Nd-Hf isotopes than the basaltic andesite. These differences indicate that the mantle source of andesite would contain more subducting sedimentderived hydrous melts than that of basaltic andesite. As a consequence, relatively Si-rich and Si-poor pyroxenite sources were respectively generated as the mantle sources of the andesite and basaltic andesite. This qualitative interpretation is verified by quantitative modeling of the geochemical transfer from subducting oceanic crust into the mantle wedge. Model calculations indicate that the addition of ~3% oceanic crust-derived aqueous solutions and 4%–12% terrigenous sediment-derived hydrous melts into the mantle wedge peridotite can account for the geochemical compositions of the target HMA. Therefore, the HMA in northwestern Tianshan provide the geochemical evidence for the crust-mantle interaction during the oceanic subduction in the Paleozoic. As such, the subducting terrigenous sediment-derived hydrous melts play a dominant role in the composition of the lithochemically fertile, geochemically enriched mantle sources and thus in the origin of HMA above oceanic subduction zones.

Oxidized mantle sources of HIMU- and EM-type Ocean Island Basalts

Oxygen fugacity (fO2) is a fundamental variable in igneous petrology with utility as a potential tracer of recycled high Fe+3/ΣFe surficial materials in the sources of mantle-derived lavas. It has been postulated that ocean island basalts (OIB) have elevated fO2 relative to mid-ocean ridge basalts (MORB) owing to higher average Fe+3/ΣFe in their source regions. To examine this issue, trace-element systematics of olivine grains are reported from OIB lavas with HIMU (high-μ; Mangaia, Canary Islands), enriched mantle (EM; Samoa; São Miguel in the Azores Islands) and depleted MORB mantle (DMM; Pico in the Azores Islands) Sr-Nd-Pb-Os isotope signatures, in order to constrain the fO2 of each magmatic system. Despite sampling distinct mantle reservoirs based on radiogenic isotope systematics, these OIB suites show similar fO2, ranging from +0.8 to +2.6 ΔFMQ, with an average of +1.5 ± 0.8 ΔFMQ, largely higher than MORB using the same oxybarometry method at +0.6 ± 0.2 ΔFMQ. The studied OIB show no correlation between fO2 and bulk rock isotopic ratios or calculated parental magma compositions. The lack of correlations with isotopic signatures likely results from radiogenic isotope signatures being hosted in volumetrically minor trace element enriched mantle lithologies, while Fe+3/ΣFe ratios that act as the primary control on fO2 are controlled by the volumetrically dominant mantle component. Higher fO2 in many OIB relative to MORB implies that they sample a uniformly high Fe+3/ΣFe plume source mantle that may be the result of either a common oxidized oceanic crust-rich reservoir parental to many modern plume lavas, or preservation of un-degassed and oxidized mantle domains formed shortly after core formation.

Iron isotope compositions of subduction-derived rocks: Insights from eclogites and metasediments of the Münchberg Massif (Germany)

The Fe isotope systematics of subducted lithologies are crucial for the understanding of redox-dependent mass transfer in subducting slabs, with consequences for the compositions of arc magmas and of the deep mantle. We investigated eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif to unravel whether their Fe isotope compositions are dominated by the igneous/sedimentary protolith signature, by low-temperature seawater alteration, or by later fluid-rock interactions during the subduction-exhumation cycle. Although the eclogites are thought to be derived from a continental rather than oceanic setting (possibly a rift-drift transition stage), they have mid-ocean ridge basalt (MORB)-like major and trace element compositions. They are often moderately oxidized compared to MORB (Fe3+/ΣFe = 0.06 to 0.30). Their δ56Fe values (+0.00 to +0.17‰; mean + 0.08 ± 0.01‰) mostly resemble those of MORB (+0.07 to +0.17‰). The metagabbros, which are derived from a more enriched mantle source than the eclogites, yielded heavier δ56Fe values (+0.09 to +0.22‰) similar to ocean island basalts, whereas those of the paragneisses (+0.03 to +0.10‰) are typical for pelitic sediments.It appears that the Fe isotope compositions of the igneous protoliths are largely preserved and little if any Fe was mobilized during the diverse fluid-rock interaction stages. The parental magma of the eclogites was probably somewhat isotopically lighter than similarly differentiated MORB magmas, perhaps due to the presence of metasomatized, isotopically light peridotites in the subcontinental lithospheric mantle (SCLM) source. Although it is possible that δ56Fe values were slightly modified during seawater alteration and/or metamorphic fluid-rock interactions in some of the eclogites, the impact of fluid-rock interactions on the major element compositions of the eclogites appears to be small. Furthermore, the scarcity of metamorphic veins in the Münchberg Massif argues against significant Fe mobilization in the slab. We suggest that continental eclogites tend to retain their magmatic δ56Fe values throughout the subduction-exhumation cycle, whereas δ56Fe values of oceanic eclogites may often be dominated by seafloor alteration with potential local modifications in the slab close to fluid channels. The remarkable robustness of the Fe isotope compositions of continental eclogites suggests that they may be used to reconstruct protolith mantle source properties despite the complex post-magmatic history.

Nd-Hf isotopic systematics of the arc mantle and their implication for continental crust growth

The 143Nd/144Nd and 176Hf/177Hf values of depleted mid-ocean ridge basalt (MORB) mantle have long been used to calculate the Nd and Hf model ages and constrain the growth rate of the continental crust. However, because continental crust forms principally at subduction zones, the appropriate 143Nd/144Nd and 176Hf/177Hf for use in model age calculations are those of the mantle wedge that underlies the juvenile magmatic arcs. In this study, we compile the Nd and Hf isotopic compositions of the modern island arc volcanic rocks. The average 143Nd/144Nd value obtained for the arc mantle is 0.512959 (εNd = +6.3), with upper and lower 95% confidence uncertainties of 0.513115 (εNd = +9.3) and 0.512613 (εNd = −0.5), respectively. The average obtained for 176Hf/177Hf is 0.283144 (εHf = +13.2) with upper and lower limits of 0.283272 (εHf = +17.7) and 0.282927 (εHf = +5.5), respectively, at the 95% confidence level. The Nd and Hf isotopic compositions of the modern island arc volcanic rocks can be divided into three populations, the high, transitional and low ε arc mantle populations, that reflect a combination of mantle heterogeneity and additions of different slab-derived components to the mantle wedge. The high ε arc mantle (HAM) population (εNd > +5.7 and εHf > +10) and the transitional ε arc mantle (TAM) population (εNd < +5.7 and εHf > +10) were metasomatized by the transfer of unradiogenic Nd and Hf from pelagic sediments, and Fe-Mn crusts and nodules, to the mantle wedge, the HAM by fluid dominate process and the TAM by a combination of sediment melts and fluids. In contrast, melts derived from subducted sediments, particularly zircon-rich turbidites, play a more important role in the low ε arc mantle population (LAM, εNd < +5.7 and εHf < +10). The average of both Nd and Hf isotopic data for the arc mantle, together with the 95% confidence limits, are used to define the arc mantle growth curves and their uncertainties. We suggest that these arc mantle growth curves should be used to calculate Nd and Hf model ages and constrain the growth rate of the continental crust.

Magmatic response to arc-arc amalgamation: Insights from latest Paleozoic igneous rocks from the Gangou section of the Eastern Tianshan

Multiple arc systems were developed in response to Neoproterozoic to Mesozoic consumption of the Paleo-Asian Ocean and their magmatic evolution is crucial for understanding the arc-arc amalgamation in the Central Asian Orogenic Belt. Here, we report whole-rock geochemical data as well as simultaneous in situ zircon U-Pb and Lu-Hf isotope results for the late Paleozoic magmatic rocks associated with the shear zones in the Gangou section of the Eastern Tianshan to constrain their petrogenesis and arc-arc amalgamation processes. We obtained ∼307 Ma and ∼270 Ma zircon U-Pb ages for the Late Carboniferous diorites and Middle Permian andesites, respectively. These rocks have high Mg# (∼65 and ∼47, respectively) values, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), with depleted zircon εHf(t) values (+12.1 to +18.1), suggesting an origin from partial melting of the depleted mantle with various degree of differentiation. Furthermore, our results also provided ∼276 Ma zircon U-Pb ages for the Middle Permian granitic porphyries and ∼256 Ma zircon U-Pb ages for the Late Permian diabase rocks. The granitic porphyries are geochemically similar to A2-type granites, i.e., high K2O + Na2O, FeOT/MgO, Ga/Al and low CaO, Sr and Ba. They have high zircon εHf(t) (+9.0 to +13.7) values, indicating a possible derivation from juvenile lower crust. The diabase rocks show depletion of light rare earth elements (LREEs), resembling normal mid-ocean ridge basalt. These rocks have depleted zircon Hf isotopic compositions (εHf(t) = +8.9 to +15.8), demonstrating a possible derivation from partial melting of the depleted mantle. The available geochemical data from the Eastern Tianshan show that the Permian mafic rocks along the shear zones possess higher Nb/La and lower Ba/La ratios than their counterparts in the Carboniferous. These contrasting features imply that the Carboniferous mafic rocks originated from a metasomatic mantle wedge, while the Permian mafic rocks were likely derived from the depleted mantle with addition of enriched asthenosphere components. The high zircon saturation temperatures (mostly > 800 °C) of the Permian A-type granitoids and diverse magma sources of coeval mafic rocks also imply possible asthenosphere upwelling. Therefore, we propose that the Late Carboniferous magmatic rocks were likely resulted from subduction of the North Tianshan oceanic plate, while the spatial and temporal evolution of the Permian magmatic rocks were probably attributed to large-scale dextral transcurrent tectonics associated with arc-arc amalgamation.

Geochemistry of basaltic blueschists from the Deyader Metamorphic Complex (Makran Accretionary Prism, SE Iran): New constraints for magma generation in the Makran sector of the Neo-Tethys

• The Deyader Complex in the Makran includes a blueschist HT-LP metaophiolitic unit. • Blueschist volcanic protoliths show N-MORB, E-MORB, and P-MORB affinities. • Volcanic protoliths were erupted in mid-ocean ridge featuring plume-ridge interaction. • Blueschists show geochemical features similar to those of the Makran ophiolites. The North Makran Domain in the Makran Accretionary Prism consists of an imbricate stack of Mesozoic Neo-Tethyan ophiolitic and metaophiolitic units. The Deyader Complex is an important metamorphic unit of this Domain and includes tectonic slices of HP-LT blueschists derived from upper oceanic crust protoliths. The volcanic protoliths consist of basalts with Mg#=75.1–62.7, SiO 2 = 41.59–48.02 wt%, and TiO 2 = 0.79–1.53 wt%. Based on incompatible and REE contents and ratios, three geochemical types can be distinguished. Group 1 basalts show trace element compositions, and La N /Yb N and La N /Sm N ratios <1 resembling those of normal-type (N-) mid-ocean ridge basalts (MORB). Compared to N-MORBs, Group 2 basalts show slight enrichment in Th, Ta, Nb, as well as La N /Yb N and La N /Sm N ratios = ∼1 similar to those observed in enriched-type (E-) MORB . Group 3 basalts show a transitional-alkaline nature (Nb/Y = 0.7–1.0), marked Th, Ta, Nb enrichment and high La N /Yb N (4–7) and La N /Sm N (2–3.5) ratios resembling those of plume-type (P-) MORB. Trace element and REE petrogenetic models show that N-MORB protoliths were generated from a depleted MORB mantle source (DMM), whereas E-MORB and P-MORB protoliths were generated from partial melting of a DMM source that was metasomatized to variable extents by OIB-type chemical components. The Deyader blueschists protoliths show close geochemical and petrogenetic similarities with all the North Makran ophiolites, suggesting that they were formed in a large Late Jurassic – Cretaceous oceanic basin that was strongly affected by mantle plume activity and different extents of plume-ridge interaction.

Tectono-magmatic evolution of the Philippine Sea Plate: A review

• Geological and geophysical characteristics of the PSP. • Tectonic evolution of the PSP as a key to the west Pacific trench-arc-basin system. • Subduction initiation, origin of basins and intraplate magmatism. • Relationship between subduction input and arc/back-arc output. The West Pacific Margin incorporates 75% of global marginal basins/back-arc basins. These basins and their adjacent trenches and island arcs constitute a huge trench-arc-basin system created by the interactions among three tectonic plates (the Pacific, Eurasian, and Indian plates) since about 50 Ma. Located among these three plates, the Philippine Sea Plate (PSP) is surrounded by a series of subduction zones, e.g., the Philippine Trench to the west, the Ryukyu Trench and the Nankai Trough to the north, the Izu-Bonin-Mariana (IBM) Trench to the east, and the Yap and Palau trenches to the south. Plate reconstruction shows that the PSP was originally located near the equator, and it has gradually moved northward since the Early Cenozoic. In addition, due to multiple subducted slab rollback events and the related multiple rifting of the proto-IBM Arc, several important tectonic units have been formed, including back-arc basins and relic/active arcs. The back-arc basins include the West Philippine Basin (WPB, spreading at 65-35 Ma), the Shikoku Basin (SKB, spreading at 27-15 Ma), the Parece Vela Basin (PVB, spreading at 29-15 Ma), and the Mariana Trough (MT, spreading from 5 Ma to present), and the main arcs include the Relic Kyushu-Palau Arc (ridge) (KPR, termination at 29 Ma), the Relic West Mariana Arc (ridge) (WMR, termination at 6Ma), the Active IBM Arc (since 11 Ma), and the Yap Arc (since 24 Ma). The geochemistry of these main tectonic units is as follows. The WPB has undergone back-arc spreading and intraplate magmatism, producing many rock types such as back-arc basin basalts (BABBs) similar to normal mid-oceanic ridge basalt (NMORB) and/or enriched mid-oceanic ridge basalt (EMORB) in the central basin, and oceanic island basalts (OIBs) in the Benham Rise and the Urdaneta. In addition, several Late Mesozoic arc terranes (e.g., the Amami Plateau, the Daito Ridge, and the Huatun Basin) developed in the WPB. The SKB mainly contains back-arc basin basalts (BABBs). After termination of the spreading in the SKB, two groups of seamounts with ages of 15-7 Ma formed. One group has undergone subduction. The other group (the Kinan Seamount Chain) is characterized by an OIB and mixed mantle source (Indian Ocean MORB and enriched mantle type one (EM1) geochemical signatures. The PVB basement rock is composed of back-arc basin basalts (BABBs). However, the southern PSP basin is very complex. The Mariana Trough mainly contains BABBs, with island arc basalts (IABs) occurring in the spreading center near the island arc. The IBM Arc mainly contains 52-51 Ma fore-arc basalts, 49-45 Ma boninites, and <45 Ma mature island-arc magmatic rocks. The two relic arcs (the KPR and the WMR) mainly contain IABs. The basement rocks in these back-arc/arc tectonic units are isotopically similar to Indian Ocean MORB. Thus, the West Pacific Ocean, e.g., the PSP, is an ideal place for studying several important geological processes, including interactions between large-scale tectonic plates, subduction initiation, back-arc spreading, intra-oceanic subduction, intraplate magmatism in a back-arc basin, and the geochemical cycle of a subduction zone.

RETRACTED: Fe3+ partitioning between clinopyroxene and silicate melt at 1–2.5 GPa: Implications for Fe3+ content of MORB and OIB source mantle

Pyroxene is the chief reservoir of Fe 3+ in upper mantle peridotite, but experiments exploring pyroxene/melt Fe 3+ partitioning ( D Fe3+ pyx/liq ) have been restricted to 100 kPa and pyroxene with low alumina. Here we present Fe 3+ partitioning experiments between clinopyroxenes (cpx) and mafic melt at elevated pressures (1–2.5 GPa). Experiments were conducted with f O2 buffered and modulated by Ru + RuO 2 and Fe-Pt alloy capsules, respectively, between ΔQFM −2.7 and +5.1. Fe 3+ /Fe T of both cpx and melt were determined by Fe K -edge X-ray absorption near edge structure spectroscopy. The experimentally synthesized cpx compositions (Al 2 O 3 = 2.36–6.01 wt.%, CaO = 19.33–22.21 wt.%) approximate those expected in basalt source regions. We find that Fe 3+ is moderately incompatible in cpx and D Fe3+ cpx/liq correlates with cpx Al 2 O 3 content, increasing from 0.05 ± 0.09 to 0.81 ± 0.04. D Fe3+ cpx/liq also increases with increasing f O2 . Comparison between experimentally synthesized cpx with those from natural peridotites indicates influences of both temperature and composition on Fe 3+ /Fe T for cpx in spinel and garnet peridotites. The combined effects of decreased pyroxene Al 2 O 3 concentration and pyroxene mode with progressive partial melting of peridotite diminishes the bulk partition coefficients of Fe 3+ , leading to greater Fe 2 O 3 contents in high degree partial melts, and this accounts for an inverse relationship between Na 2 O and Fe 2 O 3 observed in mid-ocean ridge basalts (MORB). Comparison to numerical experiments with pMELTS and the model of Jennings and Holland (2015) show that these models overpredict D Fe3+ cpx/liq for partial melting of the mantle, and so they do not accurately determine the relationship between the f O2 and Fe 2 O 3 of peridotite in basalt source regions. To estimate the Fe 3+ /Fe T ratio of the mantle source of MORB, we modeled liquid Fe 2 O 3 during isentropic batch melting of peridotite at three potential temperatures (1320 °C, 1400 °C, and 1440 °C) for peridotitic sources with Fe 3+ /Fe T ratios between 0.02–0.06. A source with an Fe 3+ /Fe T ratio of 0.038 ± 0.007 matches most of the span of natural MORB. This ratio is similar to that typical of continental lithospheric mantle sampled by xenoliths, but lower than that surmised by several recent experimental and thermodynamic studies. Considering this source Fe 3+ /Fe T but extending the partial melting calculations to higher pressures (2.5–4 GPa) reveals that bulk D Fe3+ perid/liq significantly decreases for garnet peridotite relative to spinel peridotite because the cpx become significantly less aluminous with increasing pressure. This results in high pressure partial melts with greater liquid Fe 3+ /Fe T ratios. Therefore, elevated Fe 3+ /Fe T ratios observed from some oceanic island basalts (OIB), such as those from Hawaii and Iceland, reflect in part the differences in conditions of melting and may not require mantle source regions more oxidized than those that produce MORB.