Degrees Of Melt Depletion Research Articles

Effective global mixing of the highly siderophile elements into Earth’s mantle inferred from oceanic abyssal peridotites

Late accretion occurred through addition of massive impactors to Earth, leading to potential heterogeneity in the distribution of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) within the mantle. Abyssal peridotites sample the present-day convecting mantle, which make them useful for examining the distribution of the HSE within the mantle. Here we report new HSE abundance data and 187Os/188Os ratios, in conjunction with mineral chemistry and bulk rock major- and trace-element compositions for abyssal peridotites from the fast-spreading Pacific Antarctic Ridge (PAR) and East Pacific Rise (Hess Deep), and for slow to intermediate spreading ridges from the Southwest Indian Ridge, Central Indian Ridge and Mid-Atlantic Ridge. These analyses expand the global abyssal peridotite Os isotope and HSE database, enabling evaluation of potential variations with spreading rate, from ultraslow (<20 mm/yr, full spreading rate) to fast (135–150 mm/yr). Accounting for likely effects from seawater modification and serpentinization, the Pacific data reveals heterogeneous and sometimes significant melt depletion for PAR (3–23% melt depletion; 187Os/188Os from 0.1189 to 0.1336, average = 0.1267 ± 0.0065; 2SD) and Hess Deep abyssal peridotites (15–20% melt depletion; 0.1247 ± 0.0027). Abyssal peridotites from fast to intermediate spreading ridges reveal no systematic differences in the distribution and behavior of the HSE or Os isotopes, or in degrees of melt depletion, compared with slow to ultraslow spreading ridges. These observations arise despite significant differences in melt generation processes at mid-ocean ridges, suggesting that the effects of ancient melt depletion are more profound on HSE compositions in abyssal peridotites than modern melting beneath ridges. Using global abyssal peridotites with Al2O3 content > 2 wt.%, the average composition of the primitive mantle is 0.3 ppb Re, 4.9 ppb Pd, 7.1 ppb Pt, 7.2 ppb Ru, 3.8 ppb Ir and Os, showing no Pd/Ir, but a positive Ru/Ir anomaly, relative to chondrites. There is ∼50% variation of the HSE abundances in the oceanic mantle, with much of this variation being observed at small length scales (<1 km) and due entirely to both modern and more ancient partial melting effects. Consequently, any significant HSE heterogeneities formed during late accretion or early Earth differentiation processes are no longer recognizable in the mantle sampled within ocean basins, implying generally efficient mixing of Earth’s mantle for these elements. By contrast, relatively ancient heterogeneity in Os and other radiogenic isotopes has been effectively preserved in the convecting mantle over the last ∼2 Ga, through recycling processes and through preservation and isolation of melt-depleted refractory residues.

The non-chondritic Ni isotope composition of Earth’s mantle

Nickel is a major element in the Earth. Due to its siderophile nature, 93% of Ni is hosted in the core and the Ni isotope composition of the bulk silicate Earth might inform on the conditions of terrestrial core formation. Whether Earth’s mantle is fractionated relative to the chondritic reservoir, and by inference to the core, is a matter of debate that largely arises from the uncertain Ni isotope composition of the mantle. We address this issue through high-precision Ni isotope measurements of fertile- to melt-depleted peridotites and compare these data to chondritic meteorites. Terrestrial peridotites that are free from metasomatic overprint display a limited range in δ60/58Ni (deviation of 60Ni/58Ni relative to NIST SRM 986) and no systematic variation with degree of melt depletion. The latter is consistent with olivine and orthopyroxene buffering the Ni budget and isotope composition of the refractory peridotites. As such, the average Ni isotope composition of these peridotites (δ60/58Ni = 0.115 ± 0.011‰) provides a robust estimate of the δ60/58Ni of the bulk silicate Earth. Peridotites with evidence for melt metasomatism range to heavier Ni isotope compositions where the introduction of clinopyroxene appears to drive an increase in δ60/58Ni. This requires a process where melts do not reach isotopic equilibrium with buffering olivine and orthopyroxene, but its exact nature remains obscure. Chondritic meteorites have variability in δ60/58Ni due to heterogeneity at the sampling scale. In particular, CI1 chondrites are displaced to isotopically lighter values due to sorption of Ni onto ferrihydrite during parent body alteration. Chondrites less extensively altered than the CI1 chondrites show no systematic differences in δ60/58Ni between classes and yield average δ60/58Ni = 0.212 ± 0.013‰, which is isotopically heavier than our estimate of the bulk silicate Earth. The notable isotopic difference between the bulk silicate Earth and chondrites likely results from the segregation of the terrestrial core. Our observations potentially provide a novel constraint on the conditions of terrestrial core formation but requires further experimental calibration.

Preservation of an Archaean whole rock Re-Os isochron for the Venetia lithospheric mantle: Evidence for rapid crustal recycling and lithosphere stabilisation at 3.3 Ga

Re-Os and platinum group element analyses are reported for peridotite xenoliths from the 533Ma Venetia kimberlite cluster situated in the Limpopo Mobile Belt, the Neoarchaean collision zone between the Kaapvaal and Zimbabwe Cratons. The Venetian xenoliths provide a rare opportunity to examine the state of the cratonic lithosphere prior to major regional metasomatic disturbance of Re-Os systematics throughout the Phanerozoic. The 32 studied xenoliths record Si-enrichment that is characteristic of the Kaapvaal lithospheric mantle and can be subdivided into five groups based on Re-Os analyses. The most pristine group I samples (n=13) display an approximately isochronous relationship and fall on a 3.28±0.17Ga (95 % conf. int.) reference line that is based on their mean TMA age. This age overlaps with the formation age of the Limpopo crust at 3.35–3.28Ga. The group I samples derive from ∼50 to ∼170km depth, suggesting coeval melt depletion of the majority of the Venetia lithospheric mantle column. Group II and III samples have elevated Re/Os due to Re addition during kimberlite magmatism. Group II has otherwise undergone a similar evolution as the group I samples with overlapping 187Os/188Os at eruption age: 187Os/188OsEA, while group III samples have low Os concentrations, unradiogenic 187Os/188OsEA and were effectively Re-free prior to kimberlite magmatism. The other sample groups (IV and V) have disturbed Re-Os systematics and provide no reliable age information. A strong positive correlation is recorded between Os and Re concentrations for group I samples, which is extended to groups II and III after correction for kimberlite addition. This positive correlation precludes a single stage melt depletion history and indicates coupled remobilisation of Re and Os. The combination of Re-Os mobility, preservation of the isochronous relationship, correlation of 187Os/188Os with degree of melt depletion and lack of radiogenic Os addition puts tight constraints on the formation and subsequent evolution of Venetia lithosphere. First, melt depletion and remobilisation of Re and Os must have occurred within error of the 3.28Ga mean TMA age. Second, the refractory peridotites contain significant Re despite recording >40 % melt extraction. Third, assuming that Si-enrichment and Re-Os mobility in the Venetia lithospheric mantle were linked, this process must have occurred within ∼100Myr of initial melt depletion in order to preserve the isochronous relationship. Based on the regional geological evolution, we propose a rapid recycling model with initial melt depletion at ∼3.35Ga to form a tholeiitic mafic crust that is recycled at ∼3.28Ga, resulting in the intrusion of a TTG suite and Si-enrichment of the lithospheric mantle. The non-zero primary Re contents of the Venetia xenoliths imply that TRD model ages significantly underestimate the true depletion age even for highly depleted peridotites. The overlap of the ∼2.6Ga TRD ages with the time of the Kaapvaal-Limpopo collision is purely fortuitous and has no geological significance. Hence, this study underlines the importance of scrutiny if age information is to be derived from whole rock Re-Os analyses.

Iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts

Mineralogical variations in the Earth's mantle and the relative proportions of peridotitic versus enriched and potentially crustally-derived pyroxenitic domains within the mantle have important implications for mantle dynamics, magma generation, and the recycling of surface material back into the mantle. Here we present iron (Fe) stable isotope data (δ57Fe, deviation in 57Fe/54Fe from the IRMM-014 standard in parts per thousand) for peridotite and garnet–pyroxenite xenoliths from Oahu, Hawaii and explore Fe isotopes as tracer of both peridotitic and pyroxenitic components in the source regions of oceanic basalts. The pyroxenites have δ57Fe values that are heavy (0.10 to 0.27‰) relative to values for mid-ocean ridge and ocean island basalts (MORB; OIB; δFe57∼0.16‰) and the primitive mantle (PM; δFe57∼0.04‰). Pyroxenite δ57Fe values are positively correlated with bulk pyroxenite titanium and heavy rare earth element (REE) abundances, which can be interpreted in terms of stable isotope fractionation during magmatic differentiation and pyroxene cumulate formation. In contrast, the peridotites have light δ57Fe values (−0.34 to 0.14‰) that correlate negatively with degree of melt depletion and radiogenic hafnium isotopes, with the most depleted samples possessing the most radiogenic Hf isotope compositions and lightest δ57Fe values. While these correlations are broadly consistent with a scenario of Fe isotope fractionation during partial melting, where isotopically heavy Fe is extracted into the melt phase, leaving behind low-δ57Fe peridotite residues, the extent of isotopic variation is far greater than predicted by partial melting models. One possibility is derivation of the samples from a heterogeneous source containing both light-δ57Fe (relative to PM) and heavy-δ57Fe components. While pyroxenite is a viable explanation for the heavy-δ57Fe component, the origin of the depleted light-δ57Fe component is more difficult to explain, as melting models predict that even large (>30%) degrees of melt extraction do not generate strongly fractionated residues. Multiple phases of melt extraction or other processes, such as metasomatism, melt percolation or the assimilation of xenocrystic olivine with light δ57Fe values may need to be invoked to explain these light δ57Fe values; a caveat to this is that these processes must either preserve, or generate correlations between δ57Fe and Hf isotopes. Published variations in δ57Fe in mantle melting products, such as MORB and OIB, are also greater than predicted by melting models assuming derivation from δ57Fe-homogeneous mantle. For example, OIB from the Society and Cook-Austral islands, which have radiogenic Pb and Sr isotope compositions indicative of recycled components such as subduction modified, low-Pb oceanic crust and terrigenous sediments have heavy mean δ57Fe values (∼0.21‰) significantly distinct to those of other OIB and MORB, which could explained by the presence of heavy-δ57Fe pyroxenite cumulate or pyroxenitic melt components, whereas large degree partial melts, such as komatiites and boninites, display light Fe-isotopic compositions which may reflect sampling of refractory, light-δ57Fe mantle components. Iron stable isotopes may therefore provide a powerful new means of fingerprinting mineralogical variations within the Earth's mantle and identifying the mineralogy of depleted and enriched components within the source regions of volcanic rocks.

Composition of the mantle lithosphere beneath south-central Laurentia: Evidence from peridotite xenoliths, Knippa, Texas

Mantle xenoliths in ∼83 Ma basanites from south-central Texas provide a rare opportunity to examine the lithospheric mantle beneath southern Laurentia. These peridotites represent lithosphere at the boundary between Mesoproterozoic continental lithosphere and transitional Gulf of Mexico passive margin. Here we report petrographic, mineral, and major element data for 29 spinel peridotite xenoliths from Knippa and use these to characterize the lithospheric mantle beneath south central Texas. The xenoliths comprise spinel-bearing lherzolites and harzburgites with coarse, equigranular textures. Some peridotites contain veins of lizardite. There are no pyroxenites or eclogites. The peridotites contain olivine (Fo89-92), orthopyroxene (En89-92), clinopyroxene (Wo40-45En45-49Fs3-5), and spinel. Spinel Cr# (Cr/(Cr+Al)) distinguishes lherzolites (Cr# = 0.14–0.21) and harzburgites (Cr# = 0.25–0.36). Mineral and major element compositions indicate that the lherzolites are residues after <10% melt extraction from primitive upper mantle and the harzburgites formed by <15% melt extraction. Calculated oxygen fugacities indicate equilibration of the harzburgites at –1 to +0.61 and lherzolites at 0 to –2.6 log units with respect to fayalite-magnetite-quartz (FMQ) buffer, similar to lightly metasomatized spinel peridotites elsewhere. The degree of melt depletion and oxidation of the Knippa peridotites are consistent with present data sets for slightly metasomatized lithospheric mantle and/or backarc samples rather than forearc settings. Equilibration temperatures range from 824 to 1058 °C (mean= 916 °C), calculated at reference pressure of 2.0 GPa. Calculated mean seismic velocities Vs = 4.44 km/sec and Vp =7.87 km/sec show no systematic difference between lherzolites and harzburgites, and agree with present geophysical measurements of upper mantle velocity beneath Texas. The seismic velocities calculated for these samples will provide important constraints for interpretation of EarthScope and other geophysical data sets.

Highly Refractory Peridotites on Macquarie Island and the Case for Anciently Depleted Domains in the Earth’s Mantle

Macquarie Island (Southern Ocean) is a fragment of Miocene ocean crust and upper mantle formed at a slow-spreading ridge system, uplifted and currently exposed above sea-level. The crustal rocks on the island have unusually enriched compositions and the strong signature of an enriched source requires low overall degrees of melt depletion in the underlying mantle. Peridotites on the island, however, are highly refractory harzburgites that can be modeled as residues of >20–25% of near-fractional melting from which all the free clinopyroxene was melted out. The peridotites have some of the highest spinel Cr-numbers (0·40–0·49) and lowest orthopyroxene-core Al2O3 concentrations (2·7–3·0 wt %) reported so far for oceanic peridotites. The peridotites were subsequently modified by melt–rock reactions underneath the Miocene ridge system. The refractory character of the peridotites is inconsistent with the slow-spreading ridge setting as well as with the enriched character of the overlying crust, and must indicate a previous depletion event; the peridotites are not the source residue of the overlying ocean crust on Macquarie Island. Osmium isotopic compositions of peridotite samples are very unradiogenic (187Os/188Os = 0·1194–0·1229) compared with normal abyssal peridotites and indicate a long-lived rhenium depletion. Proterozoic rhenium-depletion ages indicate that these rocks have preserved a memory of an old mantle melting event. We argue that the Macquarie Island harzburgites are samples from an anciently depleted refractory mantle reservoir that may be globally important, but that is generally overlooked because of its sterility; that is, its inability to produce basalts. This reservoir may preserve key information about the history of the Earth’s mantle as a whole.

Hf–Nd isotopic decoupling in continental mantle lithosphere beneath Northeast China: Effects of pervasive mantle metasomatism

Nd–Hf isotopic decoupling has frequently been observed in the continental and oceanic mantle, but its origin remains controversial. Here we present combined elemental and Sr–Nd–Hf isotopic study on peridotite xenoliths entrained in Cenozoic basalts from Shuangliao and Jiaohe in Northeast China, which provides insight into this issue. The data reveal a heterogeneous lithospheric mantle beneath Northeastern China, consisting of fertile (type I) to strongly refractory (type II) peridotites. Type I peridotites are largely shielded from late metasomatism, thus preserving information of depletion events. Nd model age suggests a Proterozoic lithospheric mantle beneath NE China. Type II peridotites are mostly refractory harzburgites and show ubiquitous enrichment of incompatible elements. They are further divided into two sub-groups. Clinopyroxenes from type IIa samples have high and wide Lu/Hf (0.34–1.3) and very radiogenic Hf isotopic ratios ( ε Hf = 44.4–63.8). Hf concentration is generally low (0.12–0.43 ppm) and plots along or slightly above the modeled partial melting depletion trend. In contrast, Nd content in type IIa clinopyroxenes is significantly higher than the modeled concentrations in residues at a given degree of melt depletion. The difference in enrichment of Hf and Nd translates to decoupling of Lu/Hf–Sm/Nd ratios and of Nd–Hf isotopes ( ε Nd = −1.3 to 8.4). Clinopyroxenes from most of type IIb peridotites have relatively low Lu/Hf ratios (0.04–0.24) and coupled Nd–Hf isotopes. Both Hf and Nd plot significantly above the depletion trend; their concentrations are governed by the equilibrium partitioning between percolating melt and peridotites. The distinct geochemical characteristics of type IIa and type IIb clinopyroxenes may have resulted from chromatographic percolation of small volumes of silicate melts, in which percolation fronts of incompatible elements are dependent on their relative incompatibilities.

Density structure and buoyancy of the oceanic lithosphere revisited

The density structure of the lithospheric and sublithospheric oceanic mantle is assessed with an integrating methodology that incorporates mineral physics, geochemical, petrological, and geophysical data. Compressibility, partial melting, and compositional layering are considered in addition to the standard thermal modelling. The results indicate that due to differences in the degree of melt depletion and crust segregation, the depth‐averaged density of old oceanic plates with thermal thicknesses of ∼105 ± 5 km is always lower than the density of the underlying sublithospheric mantle. Moreover, representative depth‐averaged density contrasts between the plate and the adiabatic mantle, , do not exceed values of ∼40 kg m−3, in contrast to what is assumed ( &gt; 70 kg m−3) in many geodynamic models. Thus, the role of in triggering/assisting processes such as subduction initiation may be less critical than previously thought.

Ancient melt extraction from the oceanic upper mantle revealed by Re–Os isotopes in abyssal peridotites from the Mid-Atlantic ridge

Comprehensive major-, trace-element and rhenium–osmium (Re–Os) isotope data are presented for abyssal peridotites from Ocean Drilling Program (ODP) Leg 209, in the North Atlantic. The samples are from a single core (Site 1274A) located on the western wall of the axial rift valley of the Mid-Atlantic ridge, and their study allows elemental and isotope information to be precisely related to spatial variations in primary lithology, serpentinisation and seafloor weathering. The harzburgites and dunites at this site are highly serpentinised (with the degree of serpentinisation increasing with depth below the sea floor). Petrographic observations and variations of fluid mobile elements (such as Ba, Sr, U, and Re) are consistent with seawater interaction in the upper part of the core. Nevertheless, major and trace element indicators of the extent of melt depletion indicate extreme melt loss, and suggest that these are amongst the most depleted abyssal peridotites recovered thus far. Despite the evidence for extensive serpentinisation and sea floor weathering all of the samples possess 187Os/ 188Os isotope compositions that are less radiogenic than estimates for the primitive upper mantle (PUM), lower than any yet reported for abyssal peridotites, and consistent with melt loss over, at least, the past 1.5 Ga. Single sulphide Re–Os data show evidence for recent recrystallisation or diffusional modification either due to partial melting or seawater alteration. However, some grains are extremely unradiogenic ( 187Os/ 188Os = 0.114) providing unequivocal evidence for at least some degree of melt depletion at ca. 2 Ga. Taken with recently published data these results suggest that ancient melt depletion may be a widespread feature of the oceanic upper lithosphere, even though evidence for this depleted reservoir has not yet been observed in the Re–Os chemistry of mid-ocean ridge basalts.

STRUCTURAL PETROLOGY OF THE OTHRIS PERIDOTITE (GREECE): MELT IMPREGNATION IN MANTLE LITHOSPHERE

We present the results of a structural and petrological study of mantle rocks of the strongly dismembered Othris Ophiolite (Central Greece). The Othris Ophiolite is a remnant of the Mesozoic Tethyan ocean basin. It overlies a series of thrust sheets, which have the characteristics of an Upper Triassic-Jurassic passive margin sequence overlying Triassic volcanics associated with rifting. Earlier work on the mantle rocks of the Othris Ophiolite (e.g. Menzies, 1973) has revealed a wide range of compositions, from strongly depleted harzburgites to fertile plagioclase-lherzolites. Menzies (1973) interpreted these plagioclase-bearing rocks - which contain abundant melt-relics - as fertile mantle rocks undergoing incipient melting and incomplete meltextraction (‘MORB-source’ rocks). Our mapping of the mantle section of this ophiolite has confirmed its compositional heterogeneity. In addition, we find a large (micro)structural variability. The geometry of the mantle massif is dominated by a km-wide mantle shear zone, juxtaposing two blocks with distinct compositional and microstructural characteristics. The western block comprises coarse grained harzburgites with large (100 m scale) dunite bodies. These harzburgites contain interstitial orthopyroxene, and late stage amphibole. In contrast, the eastern block comprises harzburgitic tectonites, which are generally fine-grained. The top of this block consists of plagioclase- lherzolites and -harzburgites. The plagioclase-lherzolites contain abundant melt relics, such as plagioclase ± clinopyroxene in small lenses and gabbroic dykes. We favour an interpretation in which plagioclase and clinopyroxene in these rocks are the cumulate phases of a basaltic melt which impregnated previously depleted harzburgites, rather than that they represent the products of incipient melting of fertile peridotites. Some of these plagioclaseperidotites are heavily serpentinised, and intruded by (unaltered) doleritic dykes, suggesting that this (seafloor) serpentinisation pre-dated the cessation of magmatic activity, and that the plagioclase-peridotites were emplaced to shallow levels in the ocean. The composition of ophiolitic mantle sections are usually interpreted to reflect degree of melt-extraction. It is generally assumed that mainly harzburgitic mantle sections are formed by a high degree of melt depletion corresponding to fast spreading environments. Ophiolites with lherzolitic mantle sections are thought to correspond to low degrees of melt extraction in slow spreading or rift settings. In Othris, however, we argue that the compositional variability reflects degree of melt-impregnation and refertilisation rather than depletion. Microstructural analysis shows that the melt impregnation recorded in the Othris mantle rocks occurred mainly at sub-solidus temperatures. This indicates that the thermal lithosphere may have reached well into the mantle during magmatic activity, suggesting the presence of a thin crust and/or a thick lithosphere at the Othris paleo-ridge. Freezing of melt in the mantle part of the oceanic lithosphere will lead to incomplete separation of residual mantle and magmatic crust, and a thick Moho Transition Zone (MTZ). Such a situation can be expected at a ridge with a relatively cold structure, i.e. at a slow spreading ridge, in particular near a transform fault (Fig. 1). In contrast, at fast spreading ridges - such as the Oman paleo-ridge - super-solidus conditions exist up to the Moho level and possibly into the lower crust during magmatism. This leads to a narrow MTZ, as observed in the Oman Ophiolite (Boudier and Nicolas, 1995), a well-developed cumulate sequence and a thick crust.