MORB Composition Research Articles

Zircon dissolution and growth during metamorphism

Kohn et al. 2015 (this volume) present models of zircon dissolution and growth in hydrous MORB and metapelite bulk compositions under metamorphic conditions. They evaluate the propensity metamorphic minerals in equilibrium with zircon, to assess what minerals (besides zircon) affect whole rock Zr mass balance. Results indicate that mineral reactions over a range of metamorphic grades could play an important role in the dissolution and growth of metamorphic zircon, with Zr preferentially incorporated into melt, rutile, garnet, and hornblende with increasing temperatures and decreasing pressures.

Se–Te fractionation by sulfide–silicate melt partitioning: Implications for the composition of mantle-derived magmas and their melting residues

Partitioning of Se and Te has been measured between coexisting sulfide liquid, monosulfide solid solution (MSS) and silicate melt at 0.9–1.5 GPa, 1200–1300 °C, fO2 controlled near the fayalite–magnetite–quartz buffer (FMQ-1.2 to -1.6) and 3–22 wt% FeO in the silicate melt. Both elements are highly compatible in the sulfide phase relative to silicate liquid (Dsulfide phase/silicate liquid > 600), with the identity of the sulfide dictating the sense of Se–Te fractionation. Whereas the measured DTe/DSe is ∼5–9 for sulfide liquid/silicate liquid partitioning, MSS/silicate melt partitioning fractionates Te from Se in the opposite sense, with DTe/DSe of ∼0.5–0.8. At fixed fO2, DSulLiq/SilLiq values for both Se and Te decrease ∼8-fold over the range in silicate melt FeO content investigated. The relative values of DSulLiq/SilLiq for Cu to Se increase with increasing FeO in the silicate melt, such that DCu exceeds DSe only for melts with >11 wt% FeO. Hence the standard belief that DCu>DSe as indicative of sulfide removal should be carefully assessed in the context of the FeO content of the magmas involved.Assuming a chondritic mantle Se/Te, predicted MSS and sulfide liquid compositions are generally in accord with natural mantle sulfides, in terms of their designation as MSS or sulfide liquid, based on independent criteria. However, additional variability is likely due to Te redistribution in accessory platinum group minerals (PGM), or that some sulfides are metasomatic. Calculations show that the Se/Te ratio of silicate melt derived from a sulfide liquid-saturated mantle is significantly higher, and more variable, than for silicate melt in equilibrium with residual MSS; modest sulfide liquid removal at low pressure, however, likely obscures the Se/Te fractionation imposed by the source sulfide phase. Models indicate that the composition of MORB is consistent with melts produced from sulfide-bearing sources with chondritic Se/Te, and source sulfur contents higher than estimates for depleted mantle. The calculated composition of sulfide-saturated melting residues show relatively little deviation in Se/Te if MSS is residual, but a sharp drop in this ratio for sulfide liquid control. Although the data are scattered, a portion of the peridotite array near the primitive mantle composition is consistent with model trends, and suggests control by residual MSS.

The fall and rise of metamorphic zircon

Zircon geochronology and geochemistry are increasingly important for understanding metamorphic processes, particularly at extreme conditions, but drivers of zircon dissolution and regrowth are poorly understood. Here, we model Zr mass balance to identify P-T regions where zircon should dissolve or grow. Zirconium contents of major metamorphic minerals were assessed from published data and new measurements, and models were constructed of mineralogical development and zircon abundance for hydrous MORB and metapelitic compositions along representative P-T paths. Excluding zircon, the minerals rutile, garnet, and hornblende strongly influence Zr mass balance in metabasites, accounting for as much as 40% of the whole-rock Zr budget. Clinopyroxene and garnet contain more Zr than plagioclase, so breakdown of plagioclase at the amphibolite to eclogite facies transition, should cause zircon to dissolve slightly, rather than grow. Growth of UHP zircon is predicted over a restricted region, and most zircon grows subsequently at much lower pressure. In metapelites, zircon is predicted to undergo only minor changes to modal abundance in solid state assemblages. Partial melting, however, drives massive zircon dissolution, whereas melt crystallization regrows zircon. From a mass-balance perspective, zircon growth cannot be attributed a priori to the prograde amphibolite-eclogite transition, to UHP metamorphism, or to partial melting. Instead, zircon should grow mainly during late-stage exhumation and cooling, particularly during oxide transitions from rutile to ilmenite and melt crystallization. As predicted, most zircons from HP/UHP eclogites of the Western Gneiss Region and Papua New Guinea substantially postdate eclogite formation and maximum pressures.

Modelling of the phase relations in high‐pressure and ultrahigh‐pressure eclogites

AbstractPhase relations of basic rocks under high pressure (HP) and ultrahigh pressure (UHP) metamorphic conditions are modelled on the basis of a MORB composition. The calculated pseudosections predict that basic rocks will contain glaucophane, garnet, omphacite, lawsonite, phengite, quartz with or without talc under HP‐lawsonite eclogite subfacies conditions (1.8–2.8 GPa, 500–600°C). In these assemblages, the pyrope content (Xpy) in garnet mainly increases with temperature rising, the grossular content (Xgr) chiefly decreases with pressure rising, and the silica content (Si‐) in phengite increases linearly with increasing pressure; their contents are subtly affected by variations in bulk‐rock composition. Thus, the isopleths of garnet and phengite compositions in P–T pseudosections potentially present a robust geothermobarometric method for natural glaucophane‐bearing HP eclogites. Under low‐T UHP conditions (>2.8 GPa, 550–650°C), a common assemblage for basic rocks is predicted to be garnet + omphacite + lawsonite + phengite + talc + coesite + phengite. In this assemblage, the Xpy steadily increases as temperature rises and the Si‐phengite increases with pressure rising, whereas the Xgr is very sensitive as pressure changes. The peak P–T conditions for low‐T UHP eclogites can be determined using the isopleths of maximum Xpy and Si‐phengite in P–T pseudosections. Under medium‐T UHP conditions (>2.8 GPa and >650°C), basic rocks are predicted commonly to contain garnet + omphacite + lawsonite + phengite + coesite. In this assemblage, the Xpy mostly depends on bulk‐rock compositions, whereas the Xgr and Si‐phengite regularly increase, respectively, as temperature and as pressure rises, and thus, can provide good thermobarometric constraints for medium‐T UHP eclogites. The decompression of these HP and UHP assemblages are modelled to be dominated by lawsonite dehydration reactions, which will result in disappearance of lawsonite and formation of glaucophane, epidote and/or kyanite with releasing a large amount of bound fluid.

Constraints from Os-isotope variations on the origin of Lena Trough abyssal peridotites and implications for the composition and evolution of the depleted upper mantle

The Lena Trough is a highly oblique, sparsely magmatic, ultra-slow spreading center located at the smallest distance between North America and Eurasia in the Arctic basin. Competing models suggest that it is either floored by oceanic mantle abyssal peridotites (APs) exposed by lithospheric necking, or by subcontinental mantle exposed in a still juvenile rift. To distinguish between these hypotheses, we have examined mineral major and trace element and whole rock Os-isotope variations in Lena Trough peridotites. Lena Trough peridotites are predominantly LREE-depleted, similar to other AP suites, and have 187Os/188Os ranging from ∼0.118 to 0.130 (Ave.=0.1244). This distribution is nearly identical to that of abyssal peridotites globally. Both the REE patterns and the Os-isotope distribution of the Lena Trough peridotites differ starkly from subcontinental mantle xenoliths sampled at Svalbard adjacent to Lena Trough. This suggests that Lena Trough is a site of oceanic spreading, although mid-ocean ridge volcanism as such has not yet begun.Highly refractory APs from several settings have Os- and Hf-isotope compositions indicating ancient (>1 Ga) melt depletion. Some researchers have proposed that at least some APs do not directly sample the convecting upper mantle source of MORB, but instead sample highly melt-depleted residues either entrained in the convecting mantle or present as a buoyant “slag” floating atop the less-depleted MORB-source mantle. However, ocean island peridotite xenoliths and APs reveal an essentially identical, non-Gaussian distribution of Os-isotopes and also span a similar range in Hf-isotopes. The similar mean and distribution of Os-isotopes between APs and ocean island xenoliths indicate that these two sample types derive from the same heterogeneous mantle reservoir. This similarity is inconsistent with the AP “slag hypothesis” due to the significantly greater depth of origin of ocean island xenoliths with respect to APs.Global correlation between peridotite fertility indices (e.g., whole rock Al2O3, spinel Cr#) and 187Os/188Os in both APs and ocean island xenoliths suggests ancient melting events have played a role in their petrogenesis, with an average age of melt depletion of ∼1.5 Ga. However, APs display a much steeper correlation than ocean island xenoliths, but have a similar y-intercept. The contrasting trends can be explained by recent on-ridge melt extraction superimposed on an ancient melt-depletion trend. The average Os-isotope composition of both APs and ocean island xenoliths is distinctly sub-chondritic, and requires significantly greater average melt depletion of the upper mantle (>10%) than suggested by previous inversions of MORB compositions.

Hf and O isotopic evidence for metamorphic crystallization of zircon during contact metamorphism of Fenniaolin metabasalts, Tananao complex, Taiwan

Abstract The eHf and δ18O compositions of zircons separated from an amphibolite in the Tananao complex of northeast Taiwan have been analyzed in order to test the hypothesis that these zircons are metamorphic in origin. A previous study found the U–Pb age of 18 zircons to be 86 ± 1 Ma. The 176Hf/177Hf ratio of 13 of these zircons is rather constant with an average value of 0.283263 ± 0.000037 (2σ SD) and a weighted mean of initial eHf(t) value of + 19.2. The δ18O compositions of these same 13 grains and of 7 other grains vary from + 1.2 to + 2.0‰ with a weighted mean of + 1.5 ± 0.43‰ (2σ SD). The very positive eHf(t) values show that the protolith of this amphibolite had a primitive composition, consistent with N-MORB. The low δ18O values are not consistent with typical mantle values of ~ 5.3; rather we interpreted them to reflect metamorphic growth of zircon at greenschist facies conditions in a basalt already hydrothermally altered by seawater. In keeping with other studies, we conclude [1] that the host amphibolite was basalt of MORB composition extensively altered by seawater shortly after extrusion, [2] that the crystallization of the metamorphic zircons was caused by contact metamorphism of the host amphibolite by the 87 Ma intrusion of the adjacent granodiorite, and [3] that this metamorphism was not caused by the Neogene collision of the Philippine Sea plate with the south China margin.

Comparison of major, volatile, and trace element contents in the melts of mid-ocean ridges on the basis of data on inclusions in minerals and quenched glasses of rocks

Using our database on major, trace, and volatile element contents in melt inclusions in minerals and quenched glasses of volcanic rocks reported in the literature, we compared the mean contents of 71 chemical elements in melts from the mid-ocean ridges (MORB) of the Atlantic, Pacific, and Indian oceans and determined the mean MORB composition for all the oceans of the Earth (global MORB composition). Mean ratios of incompatible trace and volatile components (H2O/Ce, K2O/Cl, Nb/U, Ba/Rb, Ce/Pb, Nb/U, etc.) were calculated for magmatic melts from all the oceans. Variations of these parameters were estimated, and significant differences between the melts of the Atlantic and Pacific oceans were established.

The hydrogen isotopic composition and water content of southern Pacific MORB: A reassessment of the D/H ratio of the depleted mantle reservoir

In this paper, we re-investigate the isotopic composition of hydrogen in MORB and the possible effects of contamination on δD and water content. A suite of 40 N-MORB from the Pacific–Antarctic ridge, far from any hotspot, was analyzed for chlorine content by electron microprobe and for water content and δD with silica tubes. Cl concentrations (from 29 to 2400 ppm) indicate widespread contamination, more intense with faster spreading rates, while water contents (from 840 to 7800 ppm) are mainly controlled by igneous processes. δD values range from −76 to −48‰, with an average value of −61‰. The lack of correlation between Cl content and either H2O/Ce or δD indicate that contamination has a negligible effect on δD for our samples, which is therefore characteristic of the mantle below the Pacific–Antarctic ridge. We suggest that the 20‰ lower δD value reported for the North Pacific and North Atlantic is highly unlikely from geodynamical arguments. We propose that the convecting mantle is characterized by a δD of −60±5‰, as supported by the most recent data from North Atlantic N-MORB.

Metamorphic history of glaucophane‐paragonite‐zoisite eclogites from the Shanderman area, northern Iran

AbstractThe Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields <11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.

Garnet growth as a proxy for progressive subduction zone dehydration

The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological water-CO 2 cycle. While models exist to predict slab volatile release from hydrous phases, it is challenging to reconstruct and test these fluid fluxes in nature. Here we show that the growth of garnet may be used as a proxy for devolatilization at blueschist to lower eclogite facies conditions in subduction zones. Using thermodynamic analysis including the effects of garnet fractionation and fluid removal, we show the proportional relationship between garnet and water production in two end-member crustal lithologies (pelitic sediment and hydrated mid-oceanic-ridge basalt [MORB]) in three representative subduction geotherms. Dehydrating minerals such as lawsonite, chlorite, amphibole, and epidote contribute to garnet growth, especially between ∼1.4 and 3.0 GPa where geophysical models and observations predict dehydration. The average production ratio for altered MORB compositions is 0.52 (wt% water as fluid per vol% garnet) in cooler geotherms (Honshu [Japan] and Nicaragua) and 0.27 in hotter geotherms (Cascadia [North America]), whereas for pelite the production ratios are about half (0.24 and 0.13, respectively). Garnet growth correlates with production of 3.3–5.9 wt% water in hydrated MORB and 1.8–3.1 wt% water in pelite, representing 42%–100% of the water lost between 0.5 and 6.5 GPa from a fully saturated starting material. Garnet abundance, its pressure-temperature growth span, and its growth chronology may be used to recognize, reconstruct, and test models for progressive subduction zone dehydration.

Multiple ways of producing intermediate and silicic rocks within Thingmúli and other Icelandic volcanoes

Major and trace element compositions of rocks and coexisting phenocrysts of the Thingmuli volcano suggest a revision of the existing models for the formation of intermediate and silicic melts in Iceland. The new data define two compositional tholeiitic trends with a significant gap between them. A high-iron trend (HFe) contains 6–14 wt% total FeO in silicic rocks with c. 1 wt% MgO, as well as sodic plagioclase and hedenbergite phenocrysts. A low-iron trend (LFe) contains 3–5 wt% FeO at c. 1 wt% MgO, which is typical of Iceland but higher than MORB compositions. The most evolved phenocrysts of the LFe trend do not reach iron-rich end members. The HFe trend is interpreted as a result of fractional crystallization; numerical modelling using the MELTS algorithm suggests that crystallization took place under redox conditions constrained to one-log unit below the fayalite-magnetite-quartz oxygen buffer (FMQ-1). The LFe trend is explained by a combination of mixing between rhyolite and ferrobasalt, assimilation of hydrated crust and fractional crystallization under higher redox conditions (FMQ). The two trends and the gap are best defined in a plot of Mg# versus SiO2 that is useful to unravel petrogenetic processes. For example, intermediate and silicic rocks of the Holocene volcanic systems of spreading rifts (e.g. Krafla), propagating rifts (e.g. Hekla) and off-rifts (Oraefajokull) also fall into high- and low-iron fields and outline a gap similar to Thingmuli. The identification of two compositional trends in erupted intermediate and silicic volcanic products shows that processes in the deep roots of single volcanic systems are highly diverse and likely controlled by local variations in the thermal gradients and the extend of hydrothermal alteration. Generalizations about the relationship between the compositions of intermediate and silicic rocks and plate tectonic setting, therefore, should be avoided.

Exhumation of oceanic eclogites: thermodynamic constraints on pressure, temperature, bulk composition and density

AbstractThe exhumation mechanism of high‐pressure (HP) and ultrahigh‐pressure (UHP) eclogites formed by the subduction of oceanic crust (hereafter referred to as oceanic eclogites) is one of the primary uncertainties associated with the subduction factory. The phase relations and densities of eclogites with MORB compositions are modelled using thermodynamic calculations over a P–T range of 1–4 GPa and 400–800 °C, respectively, in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) system. Our modelling suggests that the mineral assemblages, mineral proportions and density of oceanic crust subducted along a cold P–T path are quite different from those of crust subducted along a warm P–T path, and that the density of oceanic eclogites is largely controlled by the stability of low‐density hydrous minerals, such as lawsonite, chlorite, glaucophane and talc. Along a cold subduction P–T path with a geotherm of ~6 °C km−1, lawsonite is always present at 1.1 to >4.0 GPa, and chlorite, glaucophane and talc can be stable at pressures of up to 2.3, 2.6 and 3.6 GPa respectively. Along such a P–T path, the density of subducted oceanic crust is always lower than that of the surrounding mantle at depths shallower than 110–120 km (< 3.3–3.6 GPa). However, along a warm subduction P–T path with a geotherm of ~10 °C km−1, the P–T path is outside the stability field of lawsonite, and the hydrous minerals of chlorite, epidote and amphibole break down completely into dry dense minerals at relatively lower pressures of 1.5, 1.85 and 1.9 GPa respectively. Along such a warm subduction P–T path, the subducted oceanic crust becomes denser than the surrounding mantle at depths >60 km (>1.8 GPa). Oceanic eclogites with high H2O content, oxygen fugacity, bulk‐rock XMg [ = MgO/(MgO + FeO)], XAl [ = Al2O3/(Al2O3 + MgO + FeO)] and low XCa [ = CaO/(CaO + MgO + FeO + Na2O)] are likely suitable for exhumation, which is consistent with the bulk‐rock compositions of the natural oceanic eclogites on the Earth's surface. On the basis of natural observations and our calculations, it is suggested that beyond depths around 110–120 km oceanic eclogites are not light enough and/or there are no blueschists to compensate the negative buoyancy of the oceanic crust, therefore explaining the lack of oceanic eclogites returned from ultradeep mantle (>120 km) to the Earth's surface. The exhumed light–cold–hydrous oceanic eclogites may have decoupled from the top part of the sinking slab at shallow depths in the forearc region and are exhumed inside the serpentinized subduction channel, whereas the dense–hot–dry eclogites may be retained in the sinking slab and recycled into deeper mantle.

Contrasting settings of serpentinite bodies in the northwestern Zagros Suture Zone, Kurdistan Region, Iraq

AbstractProtrusions and lenses of serpentinite–matrix mélanges occur at several places along the thrust faults of the Zagros Suture Zone. They separate the lower allochthonous thrust sheet, the ‘Lower Allochthon’ (i.e. Walash–Naopurdan nappe), of Paleocene–Eocene age from sediments of the Arabian platform and the upper thrust sheet of Mesozoic, ophiolite-bearing terranes termed the ‘Upper Allochthon’ (i.e. Gemo–Qandil nappe). The serpentinite–matrix mélanges occur mostly as stretched bodies (slices) on both sides of the Lower Allochthon (Hero, Halsho and Pushtashan (HHP) and Galalah, Qalander and Rayat (GQR)). Their overall chondrite-normalized rare earth element (REE) patterns form two main groups. Group One exhibits enrichment in the total REEs (> 1 × chondrite) whereas the Group Two pattern shows depletion (i.e. < 1 × chondrite). Bulk-rock MORB-normalized profiles of Group Two are almost flat in the MREE–HREE region with flattening profiles in the Gd–Lu range (> 3 times the MORB composition). In comparison with Group One, Group Two has extremely high REE content and displays variable depletions in the moderately incompatible high-field-strength elements (HFSEs) (Zr, Hf, Y) relative to their adjacent REEs. The REEs in the GQR serpentinite–matrix mélanges have a noticeably high LREE content, and a positive Eu anomaly, and their HREE content never reaches more than 1 × chondrite (i.e. < 0.01 to 1 × chondrite). The latter indicates that the hemipelagic sedimentary, melt-like components (i.e. high LREE, U/La, La/Sm and low Ba/Th) control the geochemical peculiarities of this type of serpentinite. The HHP serpentinite–matrix mélanges, however, are either equally divided between the two REE pattern groups (e.g. Hero, Halsho) or inclined towards Group One (e.g. Pushtashan). Contrary to GQR serpentinites, the variation in HHP serpentinite–matrix mélanges spans a compositional spectrum from U/La-rich to more Ba/Th-rich. Such ratio variations reflect the large variation in these two subducted sedimentary components (i.e. carbonate and hemipelagic sediment mix). The obvious differences in the trace element signatures of the GQR and HHP serpentinite–matrix mélanges might be related to plate tectonic parameters such as convergence rate, subduction age and thickness and type of subducted slab. It is more likely that the influx of subducted components to the mantle wedge relied heavily on the composition of the sedimentary inputs. These vary considerably with time from the relatively deepwater hemipelagic sediments (Qulqula Radiolarite Formation) to platform carbonate sediments (Balambo limestone). The trace element signatures of the GQR and HHP serpentinite–matrix mélanges might suggest multi-staging of the allochthonous sheet emplacement on the Arabian platform sediments.

Calculated phase equilibria for MORB compositions: a reappraisal of the metamorphic evolution of lawsonite eclogite

Pseudosections calculated with thermocalc predict that lawsonite-bearing assemblages, including lawsonite eclogite, will be common for subducted oceanic crust that experiences cool, fluid-saturated conditions. For glaucophane–lawsonite eclogite facies conditions (500–600 °C and 18–28 kbar), MORB compositions are predicted in the NCKMnFMASHO system to contain glaucophane, garnet, omphacite, lawsonite, phengite and quartz, with chlorite at lower temperature and talc at higher temperature. In these assemblages, the pyrope content in garnet is mostly controlled by variations in temperature, and grossular content is strongly controlled by pressure. The silica content in phengite increases linearly with pressure. As the P–T conditions for these given isopleths are only subtly affected by common variations in bulk-rock compositions, the P–T pseudosections potentially present a robust geothermobarometric method for natural glaucophane-bearing eclogites. Thermobarometric results recovered both by isopleth and conventional approaches indicate that most natural glaucophane–lawsonite eclogites (Type-L) and glaucophane–epidote eclogites (Type-E) record similar peak P–T conditions within the lawsonite stability field. Decompression from conditions appropriate for lawsonite stability should result in epidote-bearing assemblages through dehydration reactions controlled by lawsonite + omphacite = glaucophane + epidote + H2O. Lawsonite and omphacite breakdown will be accompanied by the release of a large amount of bound fluid, such that eclogite assemblages are variably recrystallized to glaucophane-rich blueschist. Calculated pseudosections indicate that eclogite assemblages form most readily in Ca-rich rocks and blueschist assemblages most readily in Ca-poor rocks. This distinction in bulk-rock composition can account for the co-existence of low-T eclogite and blueschist in high-pressure terranes.

Grain growth kinetics of majorite and stishovite in MORB

Grain growth rates of majorite and stishovite, in their aggregates with MORB composition, were studied at 18GPa and temperatures ranging from 1873 to 2223K. The rates of grain growth were expressed by Gn−G0n=k0t exp(−H*/RT) where G is the grain size at time t (s), and G0 is the initial grain size. We determined the grain growth kinetic parameters for majorite, where n=9.1±1.3, H*=950±249kJ/mol, and logk0=−32.5±2.3m9.1/s for the graphite capsule and logk0=−31.3±3.9m9.1/s for the Re capsule. Kinetic parameters for stishovite were determined to be n=6.6±0.9, H*=599±226kJ/mol, and logk0=−30.6±0.9m6.6/s for the graphite capsule and logk0=−29.0±1.1m6.6/s for the Re capsule.

Thermal diffusivity of MORB-composition rocks to 15 GPa: implications for triggering of deep seismicity

We have measured the thermal diffusivity of eclogite and majorite with a model MORB composition at pressures of 3 and 15 GPa, respectively. Both phase assemblages show inverse dependences of their thermal diffusivities on temperature: D eclogite=9(10)×10−10+7(1)×10−4/T(K) m 2/s and D majorite=6.2(5)×10−7+3.0(5)×10−4/T(K) m 2/s. The values for majorite are in good agreement with previous measurements for other garnets and are considerably lower than thermal diffusivities of wadsleyite and ringwoodite, which are the main components of the mantle transition zone. We discuss the implications of the low thermal conductivity of subducted oceanic crust in the transition zone for the triggering of deep seismicity.

中央インド洋（15-18°S)の中央海嶺軸産玄武岩の希ガス同位体組成

中央インド洋（15-18°S)の中央海嶺軸産玄武岩の希ガス同位体組成

Chemical variations and regional diversity observed in MORB

An assemblage of MORB analyses ( n = 792 samples), including a suite of new, high-precision LA-ICP-MS measurements ( n = 79), has been critically compiled in order to provide a window into the chemical composition of these mantle-derived materials and their respective source region(s), commonly referred to as the depleted MORB mantle (DMM). This comprehensive MORB data set, which includes both “normal-type” (N-MORB, defined by (La/Sm) N < 1.00) and “enriched-type” samples (E-MORB, (La/Sm) N ≥ 1.00), defines a global MORB composition that is more enriched in incompatible elements than previous models. A statistical evaluation of the true constancy of “canonical” trace element ratios using this data set reveals that during MORB genesis Ti/Eu, Y/Ho and Ce/Pb remain constant at the 95% confidence-level; thus, the ratios recorded in MORB (Ti/Eu = 7060 ± 1270, 2σ; Y/Ho = 28.4 ± 3.6, 2σ; Ce/Pb = 22.2 ± 9.7, 2σ) may reflect the composition of the DMM, presuming the degree of source heterogeneity, component mixing and conditions of melting/crystallization of the DMM are adequately recorded by global MORB. Conversely, Ba/Th, Nb/U, Zr/Hf, Nb/Ta, Sr/Nd, and Th/U are shown to fractionate as a function of MORB genesis, and thus these ratios do not faithfully record the composition of the DMM. Compared to samples from the Pacific and Indian Oceans, MORB derived from Atlantic ridge segments are characterized by statistically significant (≥ 95% confidence-level) enrichments in both highly incompatible elements (e.g., light REE, TITAN group elements, Sr, Ba, Pb, Th, and U) as well as less incompatible elements (e.g., heavy REE), indicating: i) a prominent recycled source component; ii) variable proportions of pyroxenite in the Atlantic source region; and/or, most likely iii) smaller degrees of melting and/or greater extents of fractional crystallization due to slower ridge spreading rates. Conversely, Pacific MORB has the most depleted regional signatures with regard to highly incompatible elements (e.g., Ba, Pb, Th, and U), likely due to faster ridge spreading rates. Indian Ocean MORB exhibit limited variation in incompatible element enrichments/depletions but are generally the most depleted in more compatible elements (e.g., Ti, Cr, Sc, and heavy REE), potentially due to distinct source characteristics or deep source melting in the garnet field. Atlantic, Pacific and Indian MORB can also be distinguished by trace element ratios, particularly Ce/Pb and Th/U, which is distinct at the > 99% confidence-level. Global MORB, and by inference the DMM, are characterized by enrichments in Y/Ho and depletions in Th/U relative to the chondritic ratios, and are complementary to the continental crust. However, the median of global MORB and the bulk continental crust both have sub-chondritic Ti/Eu and Nb/Ta ratios, suggesting an under-represented Ti- and Nb-rich reservoir in the Earth, potentially refractory, rutile-bearing eclogite at depth in the mantle.

Nitrogen recycling in subducted oceanic lithosphere: The record in high- and ultrahigh-pressure metabasaltic rocks

This paper provides the first measurements of the nitrogen (N) concentrations and isotopic compositions of high- and ultrahigh-pressure mafic eclogites, aimed at characterizing the subduction input flux of N in deeply subducting altered oceanic crust (AOC). The samples that were studied are from the Raspas Complex (Ecuador), Lago di Cignana (Italy), the Zambezi Belt (Zambia) and Cabo Ortegal (Spain), together representing subduction to 50–90 km depths. The eclogites contain 2–20 ppm N with δ 15N air values ranging from −1 to +8‰. These values overlap those of altered oceanic crust, but are distinct from values for fresh MORB (for the latter, ∼1.1 ppm N and δ 15N air ∼ −4‰). Based on N data in combination with other trace element data, the eclogite suites can be subdivided into those that are indistinguishable from their likely protolith, AOC, with or without superimposed effects of devolatilization (Lago di Cignana, Cabo Ortegal), and those that have experienced metasomatic additions during subduction-zone metamorphism (Zambezi Belt, Raspas). For the former group, the lack of a detectable loss of N in the eclogites, compared to various altered MORB compositions, suggests the retention of N in deeply subducted oceanic crust. The metasomatic effects affecting the latter group can be best explained by mixing with a (meta)sedimentary component, resulting in correlated enrichments of N and other trace elements (in particular, Ba and Pb) thought to be mobilized during HP/UHP metamorphism. Serpentinized and high-pressure metamorphosed peridotites, associated with the eclogites at Raspas and Cabo Ortegal, contain 3–15 ppm N with δ 15N air values ranging from +3 to +6‰, significantly higher than the generally accepted values for the MORB mantle (δ 15N air ∼ −5‰). Based on their relatively high N contents and their homogeneous and positive δ 15N values, admixing of sedimentary N is also indicated for the serpentinized peridotites. One possible pathway for the addition of sediment-derived N into eclogites and peridotites involves mixing with fluids along the slab–mantle wedge interface. Alternatively, sedimentary N could be incorporated into peridotites during serpentinization at bending-related faults at the outer rise and, during later deserpentinization, released into fluids that then infiltrate overlying rocks. Deep retention of N in subducting oceanic crust should be considered in any attempt to balance subduction inputs with outputs in the form of arc volcanic gases. If materials such as these eclogites and serpentinized peridotites are eventually subducted to beyond sub-arc depths into the deeper mantle, containing some fraction of their forearc–subarc N inventory (documented here), they could deliver isotopically heavy N into the mantle to potentially be sampled by plume-related magmas.

Two contrasting magmatic types coexist after the cessation of back-arc spreading

Amongst island arcs, Izu–Bonin is remarkable as it has widespread, voluminous and long-lived volcanism behind the volcanic front. In the central part of the arc this volcanism is represented by a series of seamount chains which extend nearly 300 km into the back-arc from the volcanic front. These back-arc seamount chains were active between 17 and 3 Ma, which is the period between the cessation of spreading in the Shikoku Basin and the initiation of currently active rifting just behind the Quaternary volcanic front. In this paper we present new age, chemical and isotopic data from the hitherto unexplored seamounts which formed furthest from the active volcanic front. Some of the samples come from volcanoes at the western limit of the back-arc seamount chains. Others are collected from seamounts of various sizes which lie on the Shikoku Basin crust (East Shikoku Basin seamounts). The westernmost magmatism we have sampled is manifested as a series of volcanic edifices that trace the extinct spreading centre of the Shikoku Basin known as the Kinan Seamount Chain (KSC). Chemically, enrichment in fluid-mobile elements and depletion in HFSE relative to MORB indicates that the back-arc seamount chains and the East Shikoku Basin seamounts have a significant contribution of slab-derived material. In this context these volcanoes can be regarded as a manifestation of arc magmatism and distinct from the MORB-like lavas of the Shikoku back-arc basin. 40Ar/ 39Ar ages range from 15.7 to 9.6 Ma for the East Shikoku Basin seamounts, indicating this arc magmatism started immediately after the Shikoku Basin stopped spreading. Although the KSC volcanoes are found to be contemporaneous with the seamount chains and East Shikoku Basin seamounts, their chemical characteristics are very different. Unlike the calc-alkaline seamount chains, the KSC lavas range from medium-K to shoshonitic alkaline basalt. Their trace element characteristics indicate the absence of a subduction influence and their radiogenic isotope systematics reflect a mantle source combining a Philippine Sea MORB composition and an enriched mantle component (EM-1). One of the most remarkable features of the KSC is that their geochemistry has a distinct temporal variation. Element ratios such as Nb/Zr and concentrations of incompatible elements such as K 2O increase with decreasing age and reach a maximum at ca. 7 Ma when the KSC ceased activity. Based on the chemical and temporal information from all the data across the back-arc region, we have identified two contrasting yet contemporaneous magmatic provinces. These share a tectonic platform, but have separate magmatic roots; one stemming from subduction flux and the other from post-spreading asthenospheric melting.