A Nd isotopic study of the Kerguelen Islands: Inferences on enriched oceanic mantle sources

The origin of the highly differentiated igneous rocks of the Kerguelen Islands and the nature of their source regions have been investigated by a Nd isotopic study. The Nd isotopic compositions of syenites and granites are identical to those of gabbros and basalts and indicate a common source. The isotopic data preclude the involvement ofold continental crustal material in the genesis of these granitic and alkalic rocks. The data from the Kerguelen samples greatly extend the Nd-Sr isotopic correlation observed for uncontaminated basalts from the oceanic mantle. The large Nd isotopic variations in the Kerguelen samples could be explained by mixing of deep mantle material brought up by a plume and the upper oceanic mantle or by heterogeneities in the lower mantle. An important finding of this study is that there are enriched mantle sources under the oceanic regions. These enriched sources may be ancient in age and are compatible with the 2-b.y. age inferred from the Pb isotope data of these samples. Earth models in future must incorporate this feature of the oceanic mantle in a consideration of mantle-crust evolutionary relationships.     

Osmium-strontium-neodymium-lead isotopic covariations in mid-ocean ridge basalt glasses and the heterogeneity of the upper mantle

Osmium, strontium, neodymium, and lead isotopic data have been obtained for 30 hand picked samples of basaltic glass from the Pacific, Atlantic and Indian mid-oceanic ridges. Large variations in Os isotopic ratios exist in the glasses, from abyssal peridotite-like values to radiogenic compositions similar to oceanic island basalts (¹⁸⁷Os/¹⁸⁶Os and ¹⁸⁷Os/¹⁸⁸Os ratios range from 1.06 to 1.36 and from 0.128 to 0.163, respectively). Os isotopic and elemental data suggest the existence of mixing correlations. This relationship might be ascribed to secondary contamination processes; however, such a hypothesis cannot account for the negative correlation observed between Os and Nd isotopes and the existence of complementary covariations between Os and SrPb isotopes. In this case, OsSrNdPb isotopic variations are unrelated to late post-eruption or shallow level contamination. These relationships provide strong evidence that the Os isotopic composition of the samples are derived from the mantle and thus implies a global chemical heterogeneity of the oceanic upper mantle. The results are consistent with the presence of recycled oceanic crust in the mantle sources of mid-ocean ridge basalts, and indicate that the unique composition of the upper mantle below the Indian ocean results from its contamination by a mantle component characterized by radiogenic Os and particularly unradiogenic Nd and Pb isotopic compositions.     

Upwelling flow in Earth's mantle and sea-floor spreading

Upwelling flows in the Earth's mantle are accompanied by mass, momentum and energy transports from deep to upper layers. Those flows beneath the mid-ocean ridges give rise to sea-floor spreading. Mantle plumes, on the other hand, cause hot spots to be formed on the Earth's surface. Using the basic equations of fluid dynamics, temperature and velocity distributions in two-dimensional upwelling and cylindrical plumes can be obtained by an integral-relation method. Then the mass, momentum and energy transported to the lithosphere by these upwelling flows can readily be calculated. Based on those results we can more thoroughly discuss problems of plate dynamics, such as the driving mechanism of plate motion, the causes of formation of rift valleys over mid-ocean ridges, and the effect of mantle plumes on sea-floor spreading.     

New insights into the Re-Os systematics of sub-continental lithospheric mantle from in situ analysis of sulphides

The Os isotopic compositions of mantle rocks generally are considered to be established during melt-depletion events and to be robust to subsequent disturbances (e.g. metasomatism). Consequently, Os isotopes are used to date the main melting event that a mantle section has undergone, i.e. transformation of fertile asthenospheric material into a depleted, buoyant lithosphere. However, Os resides almost entirely in Fe-Ni-Cu sulphides, which can be very mobile under mantle conditions. In situ laser ablation multi-collector ICP-MS measurement of Re/Os isotopic ratios in sulphides from spinel peridotite xenoliths demonstrates that whole-rock Os-isotope signatures record the mixing of multiple sulphide populations. Sulphides residual after melting events have unradiogenic Os isotopic compositions reflecting ancient melt depletion; those introduced by later metasomatism events contain more radiogenic Os. Therefore, the whole-rock Os isotopic signature can be strongly altered by metasomatic processes, and studies of mantle-derived xenoliths show that such disturbance is quite common in the lithospheric mantle. Because melt-depletion ages estimated from individual sulphide inclusions are systematically older than those obtained from whole-rock analysis, caution is essential in the interpretation of the Os model ages derived from whole-rock analysis, and their use and abuse in geodynamic models. This work suggests that sulphide could become a key phase in unravelling the formation and evolution of the lithosphere.     

Isotopic inferences on the chemical structure of the mantle

Variations in the isotopic composition of rocks derived from the upper mantle can be used to infer the chemical history and structure of the Earth's interior. The most prominent material in the upper mantle is the source of mid-ocean ridge basalts (MORB). The MORB source is characterized by a general depletion in incompatible elements caused by the extraction of the continental crust from the mantle. At least three other isotopically distinct components are recognized in the suboceanic mantle. All three could be generated by the recycling of near surface materials (oceanic crust, pelagic sediments, continental lithospheric mantle) into the mantle by subduction. Therefore, the isotope data do not require a compositionally layered mantle, but neither do they deny the existence of such layering. Correlations between the volumetric output of plume volcanism with the reversal frequency of the Earth's magnetic field, and between the geographic distribution of isotopic variability in oceanic volcanism with seismic tomography suggest input of deep mantle material to surface volcanism in the form of deep mantle plumes. Volcanism on the continents shows a much wider range in isotopic composition than does oceanic volcanism. The extreme isotopic compositions observed for some continental magmas and mantle xenoliths indicate long-term (up to 3.3 Gyr) preservation of compositionally distinct material in thick (>200 km) sections of continental lithospheric mantle.     

Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy

Trace element relationships of near-primary alkalic lavas from La Grille volcano, Grande Comore, in the Indian Ocean, as well as those of the Honolulu volcanic series, Oahu, Hawaii, show that their sources contain amphibole and/or phlogopite. Small amounts of each mineral (2% amphibole in the source of La Grille and 0.5% phlogopite plus some amphibole in the source of the Honolulu volcanics) and a range of absolute degrees of partial melting from 1 to 5% for both series are consistent with the observed trace element variation. Amphibole and phlogopite are not stable at the temperatures of convecting upper mantle or upwelling thermal plumes from the deep mantle; however, they are stable at pressure-temperature conditions of the oceanic lithospheric mantle. Therefore, the presence of amphibole and/or phlogopite in the magma source region of volcanics is strong evidence for lithospheric melting, and we conclude that the La Grille and the Honolulu series formed by melting of the oceanic lithospheric mantle.

The identification of amphibole ± phlogopite in the source region of both series implies that the metasomatism by fluids or volatile-rich melts occurred prior to melting. The presence of hydrous phases results in a lower solidus temperature of the lithospheric mantle, which can be reached by conductive heating by the thermal plumes. Isotope ratios of the La Grille and the Honolulu series display a restricted range in composition and represent compositional end-members for each island. Larger isotopic variations in shield lavas, represented by the contemporaneous Karthala volcano on Grande Comore and the older Koolau series on Oahu, reflect interaction of the upwelling thermal plumes with the lithospheric mantle rather than the heterogeneity of deep-seated mantle plume sources or entrainment of mantle material in the rising plume. Literature OsSr isotope ratio covariations constrain the process of plume-lithosphere interaction as occurring through mixing of plume melts and low-degree melts from the metasomatized oceanic lithospheric mantle.

The characterization of the lithospheric mantle signature allows the isotopic composition of the deep mantle plume components to be identified, and mixing relationships show that the Karthala and Koolau plume end-members have nearly uniform isotopic compositions. Based on independent arguments, isotopic variations on Heard and Easter islands have been shown to be a result of mixing between deep plume sources having distinct isotopic compositions with lithosphere or shallow asthenospheric mantle. To the extent that these case studies are representative of oceanic island volcanism, they indicate that interaction with oceanic lithospheric mantle plays an important role in the compositions of lavas erupted during the shield-building stage of plume magmatism, and that isotopic compositions of deep mantle plume sources are nearly uniform on the scale that they are sampled by melting.     

PbSrNd isotopic data of Indian Ocean ridges: new evidence of large-scale mapping of mantle heterogeneities

A PbSrNd isotope study of South West and East Indian Ridges confirms that the Indian Ocean belongs to a specific regional isotopic domain, as previously suggested by the results from islands of this ocean. The isotopic domain defined by the Indian MORB is indeed different from that of the North Atlantic and East Pacific Oceans. This demonstrates that the convective circulation of the upper mantle does not allow a rapid homogenization from one region to the other.The isotopic data of the Indian ridges can be interpreted by a contamination model, in which the depleted upper mantle (identical to that under the North Atlantic) is contaminated by two different types of contaminant, one corresponding to the source of the “central Indian Ocean” islands (Amsterdam, St. Paul, Marion, Prince Edward, Réunion, Rodriguez, Mauritius), and the other to a source similar to that of Walvis or Ninety East aseismic ridges. These two contaminants would have contributed to the ridge volcanism in different proportions over time.     

Q β structure of the crust and upper mantle in the eastern Sino-Korean paraplatform

Based on the long period surface wave data recorded by the China Digital Seismograph Network (CDSN), theQ _R of fundamental mode Rayleigh wave with periods from 10 s to 146 s is determined for the eastern Sino-Korean paraplatform in this paper. TheQ _β models of the crust and upper mantle are respectively obtained for the 4 paths, with the aid of stochastic inverse method. It shows that in the eastern Sino-Korean paraplatform, the average crustalQ _β is about 200, and that there exists a weak attenuation layer in the middle crust (about 10–20 km deep) which is possibly related to earthquake-prone layer. A strong attenuation layer (lowQ) of 70 km thick extensively exists in the uppermost mantle, with the buried depth about 80 km. The averageQ _R of fundamental mode Rayleigh wave is between the value of stable tectonic region and that of active tectonic region, and much close to the latter.     

The strontium isotopic composition of seawater,and seawater-oceanic crust interaction

The⁸⁷Sr/⁸⁶Sr ratio of seawater strontium (0.7091) is less than the⁸⁷Sr/⁸⁶Sr ratio of dissolved strontium delivered to the oceans by continental run-off (～0.716). Isotope exchange with strontium isotopically lighter oceanic crust during hydrothermal convection within spreading oceanic ridges can explain this observation. In quantitative terms, the current⁸⁷Sr/⁸⁶Sr ratio of seawater (0.7091) may be maintained by balancing the continental run-off flux of strontium (0.59 × 10¹² g/yr) against a hydrothermal recirculation flux of 3.6 × 10¹² g/yr, during which the⁸⁷Sr/⁸⁶Sr ratio of seawater drops by 0.0011. A concomitant mean increase in the⁸⁷Sr/⁸⁶Sr ratio of the upper 4.5 km of oceanic crust of 0.0010 (0.7029–0.7039) should be produced. This required⁸⁷Sr enrichment has been observed in hydrothermally metamorphosed ophiolitic rocks from the Troodos Massif, Cyprus.The post-Upper Cretaceous increase in the strontium isotopic composition of seawater(～0.7075–0.7091) covaries smoothly with inferred increase in land area. This suggests that during this period the main factor which has caused variability in the⁸⁷Sr/⁸⁶Sr ratio of seawater strontium could have been variation in the magnitude of the continental run-off flux caused by variation in land area. Variations in land area may themselves have been partly a consequence of variations in global mean sea-floor spreading rate.     

Tracking the lithium isotopic evolution of the mantle using carbonatites

Carbonatites are mantle-derived, intraplate magmas that provide a means of documenting isotopic variations of the Earth's mantle through time. To investigate the secular Li isotopic evolution of the mantle and to test whether Li isotopes document systematic recycling of material processed at or near the Earth's surface into the mantle, we analyzed the Li isotopic compositions of carbonatites and spatially associated mafic silicate rocks. The Li isotopic compositions of Archean (2.7 Ga) to Recent carbonatites (δ⁷Li = 4.1 ± 1.3 (n = 23, 1σ)) overlap the range typical for modern mantle-derived rocks, and do not change with time, despite ongoing crustal recycling. Thus, the average Li isotopic composition of recycled crustal components has not deviated greatly from the mantle value (~ + 4) and/or Li diffusion is sufficiently fast to attenuate significant heterogeneities over timescales of 10⁸ years. Modeling of Li diffusion at mantle temperatures suggests that limited δ⁷Li variation in the mantle through time reflects the more effective homogenization of Li in the mantle compared to radiogenic isotope systems. The real (but limited) variations in δ⁷Li that exist in modern mantle-derived magmas as well as carbonatites studied here may reflect isotopic fractionation associated with shallow-level processes, such as crustal assimilation and diffusive isotopic fractionation in magmatic systems, with some of the scatter possibly related to low-temperature alteration.     

The longevity of subcontinental lithospheric mantle beneath Jiangsu-Anhui Region

The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in two laboratories of USTC, China and CRPG, France, respectively. The¹⁸⁷Os/¹⁸⁸Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC) and from 0.117 to 0.131 (17 samples, CRPG). The Os isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good correlation with the major element compositions. With the¹⁸⁷Os/¹⁸⁸Os-Al₂O₃ alumichron, the samples yield a model age of 2.5 ± 0.1 Ga (data of USTC) and 1.9 ± 0.1 Ga (data of CRPG), late Archean to early Pro-terozoic. The two samples with the lowest¹⁸⁷Os/¹⁸⁸Os ratio (0.119 and 0.117) have the T_RD (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle, but residual fractions of Proterozoic mantle.     

Mantle heterogeneity beneath the southern Mid-Atlantic Ridge: trace element evidence for contamination of ambient asthenospheric mantle

We report new trace element data for an extensive suite of quench basalt glasses dredged from the southern Mid-Atlantic Ridge (MAR) between 40°S and 52.5°S. Ratios between highly incompatible trace elements are strongly correlated and indicate a systematic distribution of incompatible element enriched mid-ocean ridge basalt (MORB) (E-type: Zr/Nb=5.9-19, Y/Nb=0.9-8.4, (La/Sm)_n=1.0-2.9) and incompatible element depleted MORB (N-type: Zr/Nb=30-69, Y/Nb=11-29, (La/Sm)_n=0.48-0.79) along this section of the southern MAR. A notable feature of N-type MORB from the region is the higher than usual Ba/Nb (4-9), La/Nb (1.2-2.4) and primitive mantle normalised K/Nb ratios (>1). Ba/Nb ratios in E-type MORB samples from 47.5 to 49°S are especially elevated (>10). The occurrence and geographic distribution of E-type MORB along this section of the southern MAR can be correlated with the ridge-centred Shona and off-axis Discovery mantle plumes. In conjunction with published isotope data for a subset of the same sample suite [Douglass et al., J. Geophys. Res. 104 (1999) 2941], a model is developed whereby prior to the breakup of Gondwana and the opening of the South Atlantic Ocean, the underlying asthenospheric mantle was locally contaminated by fluids/melts rising from the major Mesozoic subduction zone along the south-southwest boundary of Gondwana, leaving a subduction zone geochemical imprint (elevated (K/Nb)_n and ⁸⁷Sr/⁸⁶Sr ratios, decreased ¹⁴³Nd/¹⁴⁴Nd ratios). Subsequent impingement of three major mantle plume heads (Tristan/Gough, Discovery, Shona) resulted in heating and thermal erosion of the lowermost subcontinental lithosphere and dispersal into the convecting asthenospheric mantle. With the opening of the ocean basin, continued plume upwelling led to plume-ridge interactions and mixing between geochemically enriched mantle derived from the Shona and Discovery mantle plumes, material derived from delamination of the subcontinental lithosphere, and mildly subduction zone contaminated depleted asthenospheric mantle.     

Strontium and neodymium isotopic evidence for the heterogeneous nature and development of the mantle beneath Afar (Ethiopia)

Neodymium isotope and REE analyses of recent volcanic rocks and spinel lherzolite nodules from the Afar area are reported. The¹⁴³Nd/¹⁴⁴Nd ratios of the volcanic rocks range from 0.51286 to 0.51304, similar to the range recorded from Iceland. However, the⁸⁷Sr/⁸⁶Sr ratios display a distinctly greater range (0.70328–0.70410) than those reported from the primitive rocks of Iceland. Whole rock samples and mineral separates from the spinel lherzolite nodules exhibit uniform¹⁴³Nd/¹⁴⁴Nd ratios (ca. 0.5129) but varied⁸⁷Sr/⁸⁶Sr ratios in the range 0.70427–0.70528.The SrNd isotope variations suggest that the volcanic rocks may have been produced by mixing between two reservoirs with distinct isotopic compositions. Two possible magma reservoirs in this area are the source which produced the “MORB-type” volcanics in the Red Sea and Gulf of Aden and the anomalous source represented by the nodule suite. The isotopic composition of the volcanics is compatible with mixing between these two reservoirs.It is shown that the anomalous source with a high⁸⁷Sr/⁸⁶Sr ratio cannot have been produced by simple processes of partial melting and mixing within normal mantle. Instead the high⁸⁷Sr/⁸⁶Sr is equated with a fluid phase. A primitive cognate fluid, subducted seawater or altered oceanic lithosphere may have been responsible for the generation of the source with a high⁸⁷Sr/⁸⁶Sr ratio.     

Thermochemical convection in the mantle with oceanic crust recirculation

Basalts of mid-ocean ridges are depleted in incompatible elements that have passed into the continental crust. Basalts of hot spots (oceanic islands and igneous provinces) have a chemical composition close to the primary uniform mantle and are even somewhat enriched in incompatible elements. At present, for explaining the reason for this difference, there are different qualitative schemes of differentiation and mixing of substance in the mantle. In the present work, the results of numerical modeling of the two-component thermochemical convection in the mantle are given. They quantitatively demonstrate with which parameters in the mantle the layers of different chemical composition can remain unchanged. Models with different density contrasts and with variable viscosity are examined. The times of the partial mixing of layers depending on the values of these parameters are calculated. For retaining the stratified mantle for two Ga, the density contrast must be more than 2%. If the layer D″ contains a substance of the primary composition, then, its upper boundary can be the place of origin of the plumes that feed the hot spots of the Earth. The enrichment in the incompatible elements and the variety of the chemical composition of hot spots can be explained by the mixing of the substance of the slowly eroded D″ layer and the oceanic crust accumulated in it.     

Viscosity distribution in the mantle convection models

Viscosity is a fundamental property of the mantle which determines the global geodynamical processes. According to the microscopic theory of defects and laboratory experiments, viscosity exponentially depends on temperature and pressure, with activation energy and activation volume being the parameters. The existing laboratory measurements are conducted with much higher strain rates than in the mantle and have significant uncertainty. The data on postglacial rebound only allow the depth distributions of viscosity to be reconstructed. Therefore, spatial distributions (along the depth and lateral) are as of now determined from the models of mantle convection which are calculated by the numerical solution of the convection equations, together with the viscosity dependences on pressure and temperature (PT-dependences). The PT-dependences of viscosity which are presently used in the numerical modeling of convection give a large scatter in the estimates for the lower mantle, which reaches several orders of magnitude. In this paper, it is shown that it is possible to achieve agreement between the calculated depth distributions of viscosity throughout the entire mantle and the postglacial rebound data. For this purpose, the values of the volume and energy of activation for the upper mantle can be taken from the laboratory experiments, and for the lower mantle, the activation volume should be reduced twice at the 660-km phase transition boundary. Next, the reduction in viscosity by an order of magnitude revealed at the depths below 2000 km by the postglacial rebound data can be accounted for by the presence of heavy hot material at the mantle bottom in the LLSVP zones. The models of viscosity spatial distribution throughout the entire mantle with the lithospheric plates are presented.     

Lead-strontium isotopic variations along the East Pacific Rise and the Mid-Atlantic Ridge: a comparative study

We have determined the Pb and Sr isotopic compositions in a number of fresh young oceanic basalts from the East Pacific Rise (between 20°N and 21°S latitudes), and from the Mid-Atlantic Ridge (between 65°N and 10°N).The results confirm the PbSr isotopic correlation for mid-ocean ridges basalts obtained by Allègre et al. [1], Cohen et al. [2], Dupréand Allègre [3], and the correlation between isotopic variation and the compatible trace elements ratios variation [1].A comparison between the Atlantic and Pacific results reveals that there is a wider range of values for the Atlantic than for the Pacific. After filtering the short wavelengths, a good correlation is obtained between long-wavelength bathymetric and isotopic variations for the Atlantic.The preferred model proposed to explain these differences involves the constant presence of hot spots under ridges. On slow-spreading ridges like the Atlantic, the hot spots signature is clearly visible in both bathymetry and isotopic ratios. On fast-spreading centers, the hot spot signature in both the bathymetry and isotopic signature may be diluted by the rapid supply of material coming from the asthenosphere.However, an alternative explanation for which no hot spot influence is found on the East Pacific Rise cannot be definitely ruled out.In two occurrences, south of the Hayes fracture zone (Atlantic), large isotopic heterogeneities are observed within a single dredge. This does not contradict the concept of regional isotopic regularities, but suggests that blob injection and source mixing may be observed at very different scales under the ridges.     

Azores mantle blob: Rare-earth evidence

Rare earths (RE) in basalts erupted within the rift of the Mid-Atlantic Ridge show a progressive change from light-RE enriched to depleted patterns from the Azores Platform (40°N) down to 33°30′N. South, the pattern remains light-RE depleted as along other “normal ridge” segments. A progressive increase in chemical variability of the basalts towards the Azores is also noted.The latitudinal RE profile and corresponding ΣFeO/ΣFeO + MgO variations, together, indicate that the origin of these basalts cannot be accounted for simply by considering variable extents of partial melting of a single mantle source and subsequent fractional crystallization during the ascent of the magmas. These two processes produce only second-order effects on the RE patterns. The data requires the presence of a distinct, light-RE richer, mantle source beneath the Azores Platform relative to that of south of 33°30′N and an intermediate zone where both mantle types mix. The relative contribution of the Azores mantle source to the mix appears to decrease fairly regularly southward along the ridge and becomes negligible at 33°30′N. Increasing chemical variability of the basalts towards the Azores is probably caused by correspondingly larger extent of fractional crystallization at shallow depth, and/or greater variability in the extent of partial melting, apparently subsequent to, and superimposed on the mixing of the two mantle sources.The combined morphological, geophysical and RE evidence along the profile are consistent with a model suggesting upwelling of a major blob (plume) under the Azores Plateau; and reveal the present extent of the blob's overflow and mixing with the asthenosphere depleted in large ionic lithophile trace elements. The influence of the Azores blob is geochemically detectable up to 1000 km southwestward beneath the ridge axis.     

Inversion of gravity anomalies over spreading oceanic ridges

Models of spreading ocean ridges are derived by Bayesian gravity inversion with geophysical and geodynamic a priori information. The aim is to investigate the influence of spreading rate, plate dynamics and tectonic framework on crust and upper mantle structure by comparing the Mid Atlantic Ridge (MAR), the Indian Ocean Ridge (IND) and the East Pacific Rise (PAC). They differ in mean spreading rate, dynamic settings, as attached slabs, and plume interaction. Topography or bathymetry, gravity, isostasy, seismology and geology, etc. are averaged along the ridges and guide the construction of initial 2D models, including features as mean plumes, i.e. averaged along the ridge. This is a gross simplification, and the results are considered preliminary.Three model types are tested: (a) the temperature anomaly; (b) asthenospheric rise into thickening lithosphere; (c) a crustal root as had been anticipated before seafloor spreading was discovered. Additional model components are a mean plume, a non-compensated ridge uplift, an under-compensated asthenospheric rise, e.g. of partially molten material, and seismic velocity models for P and S waves. Model type (c), tends to permute to model type (b) from thick crust to thin axial lithosphere. Model type (a) renders ‘realistic’ values of the thermal expansivity, but is insufficient to fit the gravity data; partial melt may disturb the simple temperature effect. A combination of (a) and (b) is most adequate. Exclusive seismic velocity models of S or P waves do not lead to acceptable densities nor to adequate gravity fitting. The different ridges exhibit significant differences in the best models: ATL and IND show an axial mass excess fostering enhanced ridge push, and ATL, in addition, suggests a mean plume input, while PAC shows an axial mass deficit reducing ridge push, most probably due to dominance of slab pull in the force balance.Goodness of the gravity fit alone is no justifiable criterion for goodness of model, indeed minor modifications to each model within the uncertainties of the assumptions can make the fit arbitrarily good. Goodness of model is quantified exclusively by a priori information.     

Upper mantle structure in the ocean-continent transition zone: Kamchatka

We consider results from modeling the crustal and upper mantle velocity structure in Kamchatka by seismic tomography and compare these with gravity data and present-day tectonics. We found a well-pronounced (in the physical fields) vertical and lateral variation for the upper mantle and found that it is controlled by fault tectonics. Not only are individual lithosphere blocks moving along faults, but also parts of the Benioff zone. The East Kamchatka volcanic belt (EKVB) is confined to the asthenospheric layer (the asthenosphere lens) at a depth of 70–80 km; this lens is 10–20 km thick and seismic velocity in it is lower by 2–4%. The top of the asthenosphere lens has the shape of a dome uplift beneath the Klyuchevskoi group of volcanoes and its thickness is appreciably greater; overall, the upper mantle in this region is appreciably stratified. A low-velocity heterogeneity (asthenolith) at least 100 km thick has been identified beneath the Central Kamchatka depression; we have determined its extent in the upper mantle and how it is related to the EKVB heterogeneities. Gravity data suggest the development of a rift structure under the Sredinnyi Range volcanic belt. The Benioff zone was found to exhibit velocity inhomogeneity; the anomalous zones that have been identified within it are related to asthenosphere inhomogeneities in the continental and oceanic blocks of the mantle.     

Os isotope heterogeneity of the upper mantle: Evidence from the Mayarí–Baracoa ophiolite belt in eastern Cuba

In an attempt to quantify the extent of geochemical heterogeneity within a restricted and well dated portion of the upper mantle, 27 chromite separates from the 90 My old chromite deposits in the Mayarí–Baracoa ophiolite belt in eastern Cuba have been investigated for platinum group element (PGE) concentrations and Re–Os isotopic systematics. The samples are characterized by systematically subchondritic initial ¹⁸⁷Os/¹⁸⁸Os ratios and substantial heterogeneity. The initial ¹⁸⁷Os/¹⁸⁸Os ratios vary with chromite chemistry and with geographical distribution, reflecting differences in the Os isotopic evolution for the different upper mantle sections represented by the ophiolite. Accordingly, the Os isotope data might be divided into three groups. In the Moa–Baracoa district, where the chromite bodies are located in the mantle–crust transition zone, the calculated initial γOs values average − 0.97 ± 0.69 (n = 13). In the Sagua de Tanamo district, where chromite chemistry is highly variable and their location in relation the mantle sequence is less clear, the initial γOs values are intermediate, with an average of − 1.77 ± 0.80 (n = 7). In the Mayarí district, where the chromite bodies are located in the lower part of the mantle sequence, initial γOs values average − 2.66 ± 0.29 (n = 7). These subchondritic (i.e. negative) initial γOs values are most simply explained by Re depletion during ancient partial melting and/or melt percolation events.The Os isotope heterogeneity documented here indicates a high degree of geochemical complexity on small to intermediate length scales in the upper mantle. Our results, in combination with data on chromites from the literature, show that an “average present-day Os isotopic composition” for the hypothetical depleted MORB mantle (DMM) reservoir cannot be precisely established beyond the statement that it is “broadly chondritic”. Indeed, the upper mantle cannot be considered a sufficiently homogeneous geochemical “reservoir” to serve meaningfully as a baseline against which geochemical “anomalies” are evaluated. On the other hand, our findings are consistent with the “Statistical Upper Mantle Assemblage” or “SUMA”-concept, according to which a high level of geochemical heterogeneity is maintained in the upper mantle at all relevant length scales, as a result of the plate-tectonic cycle and intra-mantle processes such as melt-migration and metasomatism.