Portion Of Mantle Research Articles

Use of a polyglycolide lactide cement plug restrictor in total hip arthroplasty.

The use of a polyglycolide lactide cement plug restrictor in cemented femoral fixation during total hip arthroplasty was evaluated. Femoral cement pressurization was evaluated in vitro in a cadaveric model and the host response to polymer degradation was evaluated in vivo in a canine total hip arthroplasty model. Sixteen embalmed anatomic specimen femurs were prepared for cement femoral fixation. The intramedullary canal was plugged with either an ultrahigh molecular weight polyethylene cement plug restrictor or a polyglycolide lactide cement plug restrictor. Peak pressures in the proximal, mid, and distal portions of the cement mantle were recorded during cement insertion, cement pressurization, and implant insertion. There was no difference between the two plug groups in peak pressures throughout the cement mantle during cement insertion, pressurization, or implant insertion. Total hip arthroplasty using a cementless acetabular component and a cemented femoral stem was performed in 24 dogs. The femoral intramedullary canal was plugged with a polyethylene or a biodegradable cement plug restrictor. The dogs were sacrificed at 7 weeks, 10 months, or 15 months. Radiographically, no osteolytic lesions were seen around either plug type. Histomorphometrically, the polyglycolide lactide plugs appeared intact at 7 weeks and partially degraded by 10 and 15 months. In both plug groups, a mild fibrohistiocytic reaction with infiltration of fibrocytes, histocytes, and endothelial cells was seen. No osteolysis was observed. The results of the current study show that femoral cement pressurization can be attained in vitro using a biodegradable cement plug restrictor and that for as long as 15 months in the in vivo canine model there were no adverse reactions associated with use of these plugs compared with conventional ultrahigh molecular weight polyethylene plugs.

Anisotropy in the oceanic lithosphere from the study of local intraplate earthquakes on the west flank of the southern East Pacific Rise: Shear wave splitting and waveform modeling

Shear wave splitting is observed on ocean bottom seismometer records from local, intraplate microearthquakes on the west flank of the East Pacific Rise at 18°S. Split times reach a maximum of about 0.2 s. For most of the incoming waves with long mantle paths (∼20 km), the polarization direction of the fast arrival is subparallel to the spreading direction, which we attribute to anisotropy caused by the strain‐induced preferential orientation of olivine. In contrast, for some records with short paths or shallower sources or propagation along the spreading direction, the fast direction is nearly parallel to the ridge axis. These polarizations are probably caused by seismic anisotropy from aligned cracks in the uppermost crust. In addition, for some events, the apparent splitting is frequency dependent. To explore the pattern of shear wave splitting that would be expected for nonvertical paths in an oceanic lithosphere with two distinct anisotropic layers, we generate synthetic seismograms for a variety of source depths and mechanisms. We employ a multidomain, pseudospectral method to simulate the elastic wave fields from point sources in an inhomogeneous, anisotropic medium. Splitting parameters measured from synthetic S waves demonstrate that the apparent fast direction is not always parallel to the symmetry axes and that, in some cases, fast directions at higher frequencies will be more characteristic of the shallower crustal anisotropy while fast directions at lower frequencies are dominated by the mantle portion of the path. Most of the observed characteristics of splitting can be reproduced if there is approximately 8% S wave anisotropy in the mantle and an average of about 6% S wave anisotropy in the upper crustal, seismic layer 2.

Single-crystal elastic properties of chondrodite: implications for water in the upper mantle

Chondrodite, a member of the humite group of minerals, forms by hydration of olivine and is stable over a range of temperatures and pressures that includes a portion of the uppermost mantle. We have measured the single crystal elastic properties of a natural chondrodite specimen at ambient conditions using Brillouin spectroscopy. The isotropic aggregate bulk (K) and shear (µ) moduli calculated from the single-crystal elastic moduli, Cij, are: KS=118.4(16) GPa and µ=75.6(7) GPa. A comparison of the structures and elasticity of olivine and chondrodite indicate that the replacement of O with (OH,F) in M2+O6 octahedra has a small effect on the elasticity of humite-group minerals. The slightly diminished elastic moduli of humite-group minerals (as compared to olivine) are likely caused by a smaller ratio of strong structural elements (SiO4 tetrahedra) to weaker octahedra, and perhaps a more flexible geometry of edge-sharing MO4(O,OH,F)2 octahedra. In contrast to the humite-olivine group minerals, the incorporation of water into garnets and spineloids leads to a more substantial decrease in the elastic properties of these minerals. This contrasting behavior is due to formation of O4H4 tetrahedra and vacant hydroxyl-bearing octahedra in the garnets and spineloids, respectively. Therefore, the mechanism of incorporation of H/OH into mineral phases, not only degree of hydration, should be taken into account when estimating the effect of water on the elastic properties of minerals. The bulk elastic wave velocities of chondrodite and olivine are very similar. If humite-like incorporation of OH is predominant in the upper mantle, then the reaction of OH with olivine will have a minor or possibly no detectable effect on seismic velocities. Thus, it may be difficult to distinguish chondrodite-bearing rocks from “anhydrous” mantle on the basis of seismically determined velocities for the Earth.

Hotspot–ridge interaction along the Southeast Indian Ridge near Amsterdam and St. Paul islands: helium isotope evidence

We report new helium isotope analyses for basaltic glasses recovered along the Southeast Indian Ridge (SEIR) between longitudes 77°E and 88°E. In this region, the SEIR shoals to a depth less than 1800 m as it crosses the Amsterdam–St. Paul (ASP) plateau. Atop the plateau, where the average sample spacing is ∼10 km, 3He/ 4He ratios range between 9.3 and 13.4 R A ( R A = atmospheric ratio) with significant variability over short distances. Away from the plateau, ridge segments show a more restricted 3He/ 4He range, between 7 and 9 R A. Elevated 3He/ 4He ratios on the plateau are evidence for a deep mantle plume component that has been injected into the sub-ridge mantle beneath the region. Ridge segments southeast of the plateau show lower 3He/ 4He (7.6–8.3 R A) than segments to the northwest (8.3–9.0 R A), suggesting either a systematically varying `background contamination' of the MORB asthenosphere in the region, or localized input of a `low 3He/ 4He' component along the southeastern ridge segments, possibly derived from the distant Kerguelen hotspot. A double peak structure exists in the regional 3He/ 4He spatial distribution, because there is also a strong plume-derived He isotope signal which is offset from the shallowest, thickest section of the ridge. High 3He/ 4He ratios (up to 14.1 R A) are present all along the ridge segment immediately to the northwest of the ASP plateau (segment H), where anomalies in K/Ti ratio are also observed. The high 3He/ 4He ratios persist to the northern end of segment H, beyond which they abruptly drop to MORB background levels, which apparently persist all the way to the Rodrigues Triple Junction. This pattern suggests that the high- 3He/ 4He ASP plume supplies a shallow north-northeastward mantle flow toward adjacent portions of the SEIR. There is a systematic difference in 3He/ 4He–K/Ti behavior between axial lavas from the ASP plateau and those from segment H, suggesting a higher He/Ti elemental abundance ratio in the plume source material beneath segment H compared to that beneath the ridge axis on the ASP plateau. These observations are consistent with a model in which plume material supplied to segment H is derived from the outer portions of the ASP plume, where removal of He (relative to Ti) has been less efficient than within the core of the plume located beneath the Amsterdam and St. Paul islands. These outer portions of the plume have lower temperatures and have undergone less partial melting, leading to a higher relative He/Ti ratio compared to the inner portions of the plume that feed portions of the sub-ridge mantle beneath the ASP plateau.

GLASSES IN MANTLE XENOLITHS: A TOOL FOR UNDERSTANDING MANTLE PROCESSES

A new perspective in the evolution of mantle material was developed along with the concept of metasomatism, that is peridotitic mantle could not only be depleted through melting process, but it could also get enriched by migration and reaction with fluids coming from deeper mantle portions. Anydrous mantle parageneses alone are not able to generate the wide magma spectrum which characterise, for example, alkaline intraplate magmatism and the presence of hydrous minerals, such as amphibole and phlogopite, has to be invoked. However, not every magmatic event occurring in the astenospheric or even lithospheric mantle reach the surface and, hence, may be detected. Most very-low degree partial melts, such as lamproites, lamprophyres or carbonatites, rarely, if ever, reach the surface, but, nevertheless, they may deeply modify the overlying mantle portions. In this context mantle glasses represent a powerful tool for investigating these basic petrogenetic problems. Their study may contribute to highlight a) which are the relationships between glasses, metasomatic processes and the formation of amphibole, phlogopite and other metasomatic phases, and b) which might be the relationships between glasses, geochemical components and basaltic magma production. In the following petrographical and geochemical (major and trace elements) features of three well-studied glass populations from oceanic and continental lithospheric mantle are reported. They were found within mantle xenoliths, in intra-plate magmatic settings, from Grande Comore island (Indian Ocean), Sal island (Cape Verde, Atlantic Ocean), and Veneto Volcanic Province (Southern Alps, Italy). Afterward, their characteristics will be compared with those of mantle glasses reported world wide. All studied xenoliths bear spinel as the only aluminiferous phase, then investigation is confined in the spinel stability field. The three groups of mantle xenoliths present quite distinctive geochemical features (both for major and trace element contents), such as Na2O/K2O ratio, and SiO2, CaO, TiO2, Zr, Sr, Rb and Ba contents, which allow to put some constrain on the nature of the original metasomatic agents. The final goals will be to identify i) what do glasses represent and which is the process controlling their formation, ii) what are the metasomatic fluids which might be responsible for their genesis and iii) what are geochemical markers which may be suitable for discriminating between Na- or Krich alkali-silicate vs carbonatite metasomatism.

High pressure elastic anisotropy of MgSiO 3 perovskite and geophysical implications

Using plane wave pseudopotential method within the local density approximation (LDA), we calculate single-crystal elastic constants ( c ij ) of orthorhombic MgSiO 3 perovskite, generally accepted to be the major component of the lower mantle, as a function of pressure up to 150 GPa. Our results are in excellent agreement with experimental data at zero pressure and compare favorably with other pseudopotential predictions over the pressure regime studied. Here we use our elastic constants to calculate anisotropy of seismic wave velocities as a function of pressure (depth). MgSiO 3 perovskite is shown to be highly anisotropic in all portions of the lower mantle and the nature of anisotropy changes significantly with depth. The absence of significant seismic anisotropy in most of the lower mantle suggests that MgSiO 3 perovskite assumes nearly random orientation in most of this region. Anisotropy at the topmost lower mantle suggested by some studies can be attributed to the preferred orientation of perovskite. However, anisotropy in the D″ layer is difficult to be attributed to preferred orientation of perovskite. Some other mechanisms including the presence of the aligned melt pockets and/or lattice preferred orientation of magnesiowustite are needed.

The unique anisotropy of the Pacific upper mantle

The development and interpretation of tomographic models of the Earth's mantle have usually proceeded under the assumption that fast and slow seismic velocity anomalies represent a spatially heterogeneous temperature field associated with mantle convection. Implicit in this approach is an assumption that either the effect of anisotropy on seismic velocities is small in comparison with isotropic thermal or compositional effects, or that the tomographic results represent the average isotropic heterogeneity, even if individual seismic observations are affected by anisotropic structure. For example, velocity anomalies in the upper portions of the oceanic mantle are commonly interpreted in terms of the progressive cooling1,2 (and localized reheating3) of amechanical and thermal boundary layer consisting of rigid oceanic lithosphere and an underlying, less viscous, asthenosphere. Here, however, we present results from a global three-dimensional tomographic model of shear-wave velocity which shows that the uppermost mantle beneath the central Pacific Ocean is considerably more complicated than this simple model. Over a broad area, with its centre near Hawaii, the seismic data reveal a regional anomaly in elastic anisotropy which produces variations of seismic velocities that are at least as large as those due to thermal effects. Because seismic anisotropy is an indicator of strain in Earth materials, our tomographic results canbe used to put constraints on both buoyancy forces (thermal effects) and flow patterns in the upper mantle.

THE Re-OsISOTOPE SYSTEM IN COSMOCHEMISTRY AND HIGH-TEMPERATURE GEOCHEMISTRY

▪ Abstract The Re-Os isotope sytem, based on the long-lived β−transition of187Re to187Os, has matured to wide use in cosmochemistry and high-temperature geochemistry. The siderophilic/chalcophilic behavior of Re and Os is different from that of the elements that comprise most other long-lived radiogenic isotope systems. Magmatic iron meteorites (IIIAB, IIAB, IVA, and IVB) have Re-Os isochrons that indicate asteroidal core crystallization within the first 10–40 million years of Solar System evolution. Rocks from Earth's convecting mantle show generally chondritic Re/Os evolution throughout Earth history that is explained by the addition of highly siderophile elements to the mantle after core formation via late accretion. Oceanic basalts have Os-isotope systematics that improve the detailed geological interpretation of extant mantle components. Some portions of ancient subcontinental lithospheric mantle are severely depleted in Re and have correspondingly subchondritic187Os/188Os, indicating long-term isolation from the convecting mantle during the Archean-Proterozoic. Magmatic ore deposits have differences in initial Os isotopic composition traceable to the crustal vs mantle sources of the platinum-group elements and base metals.

The helium paradoxes.

The ratio 3He/4He (R) plays a central role in models of mantle evolution that propose an undegassed lower mantle, rich in the primordial isotope 3He. A large primordial volatile-rich reservoir, a feature of recent models, is inconsistent with high-temperature accretion and with estimates of crustal and bulk Earth chemistry. High R can alternatively reflect high integrated 3He/(U+Th) ratios or low 4He abundances, as expected in refractory portions of the upper mantle. I show that high R materials are gas-poor and are deficient in radiogenic 4He compared with midocean ridge basalts. The seemingly primitive (i.e., high R) signatures in "hotspot" magmas may be secondary, derived from CO2-rich gases, or residual peridotite, a result of differential partitioning of U and He into magmas. A shallow and low 3He source explains the spatial variability and the temporal trends of R in ocean islands and is consistent with a volatile-poor planet. A shallow origin for the "primitive" He signature in ocean island basalts, such as at Loihi, reconciles the paradoxical juxtaposition of crustal, seawater, and atmospheric signatures with inferred "primitive" characteristics. High 238U/204Pb components in ocean island basalts are generally attributed to recycled altered oceanic crust. The low 238U/3He component may be in the associated depleted refractory mantle. High 3He/4He ratios are due to low 4He, not excess 3He, and do not imply or require a deep or primordial or undegassed reservoir. 40Ar in the atmosphere also argues against such models.

Re-Os Systematics in Orogenic Peridotite Massifs and Constraints on the Petrogenesis of Pyroxenites

Orogenic lherzolites are considered to represent large portions of the upper mantle that have been tectonically emplaced in the crust. They are composed of lherzolites, harzburgites, dunites and small proportions of pyroxenites of variable thickness and composition. The study of such massifs has provided much information on the spatial distribution of geochemical variations in the upper mantle. Because of the extreme compatibility of osmium and its resistance to secondary effects like metasomatism and retrograde alteration, The Re-Os isotopic system provides an additional perspective to help understanding the origin of orogenic lherzolites massifs. Here we investigate, Re-Os concentrations and isotopic compositions of peridotites and pyroxenites from three ultramafic massif: Beni-Bouzra (North Morocco), Ronda (South Spain) and Lherz (French Pyrenees). The Os isotopic ratios (1870S/1860S) for the peridotites range from 0.951 to 1.102 in good agreement with data published previously and with upper mantle value, while the concentration range from 0.531 to 8.740 ppb and is in general higher than those encountered in mantle xenoliths. It is widely admitted that orogenic massif come from the asthenospheric mantle that accreted into the lithosphere sub-continental (SCLM) via limited diapiric upwelling. Subsequently the correlation between Os isotopic ratios and Mg# or A1203 may result from ancient melt extraction (Fig. 3). Our study of mafic layers has focused on two familly of pyroxenites: spinel websterites and garnet pyroxenites. The garnet pyroxenites are more differentiated than spinel websterites and have high concentration in A1203, CaO end V and low concentration in MgO, Ni and Co. The osmium isotopic ratios of garnet pyroxenites range from 1.752 to 12.352 (Fig. 2). These values are higher than those encountered in the websterites (from 1.058 to 1.904) and are the result of a large Re/Os fractionation (160 to 900). Among the mafic samples, Os isotopic ratios are found to decrease strongly with increasing Os concentration. The negative correlation obtained between Re/Os and Mg# confirms the incompatible nature of Re. The pyroxenites data of Beni-Bouzra show a positive correlation (R 2 = 0.97, Fig. 1) between 1870s/1860s and lS7Re/lS6Os ratios which yield an apparent 'age' of 700 _+ 55 Ma. This age is in agreement with the model age obtained with 2~176 systematics for the same massif (590 +_ 150 Ma; Hamelin and Allbgre, 1988) and may reflect the time when was incorporated in the non-convecting SCLM. This event is younger than the stabilisation of, the Ronda ultramafic massif deduced for Os isotopes(~ 1.2 Ga, Reisberg and Lorand, 1995). The high Os isotopic ratios in mafic layers suggesting that this material cannot represent recent partial melting, or cumulates derived from recent melt of the surounding ultramafic rocks. They provide evidence for a process that occurred at least hundreds of millions of years ago. Comparison of the Os and Pb data for the peridotites and pyroxenites with the range observed in oceanic basalts shows that the radiogenic ratios in basalts could be produced by a mixture ofperidotitepyroxenite in the source which lends support to the hypothesis that the lithologies found in orogenic massifs are representative of the upper mantle, the 'marble cake' model (All~gre and Turcotte, 1986). However, the geochemical characteristics of pyroxenites, the zonation observed in some mafic layers and the positive correlation observed between the model age versus thickness (Fig. 4) suggests that this material represent high-pressure crystallization from melts of subducted oceanic crust (Pearson et al., 1993). Although in the basis of FeO-MgO-AI203 systematics, small proportion of Beni-Bouzra pyroxenites may represent subducted, metamorphosed oceanic crust directly recycled into the mantle (Kornprobst et al., 1991).

Neoproterozoic dyke swarms from southern Sinai (Egypt): geochemistry and petrogenetic aspects

Neoproterozoic ( ca 0.6 Ga) basic to acidic dyke swarms are widespread in southern Sinai (Egypt). Two mineralogically and geochemically distinct subalkaline basic-intermediate dyke suites are spatially closely associated in the northern portions of the Arabian-Nubian Shield. Augite bearing dykes (CpxD) correspond to basalts and basaltic trachyandesites, while the amphibole bearing dykes (AmpD) are basaltic andesites and andesites. The acidic dykes have rhyolitic compositions. CpxD define trends typical of tholeiitic suites and are characterised by high TiO 2 and incompatible elements (e.g. light REE and HFSE), and are similar to high TiO 2 continental basalts. In contrast, the AmpD define a calcalkaline suite. Modelling shows that differentiation of both CpxD and AmpD suites is compatible with fractional crystallisation occurring at relatively low and high fO 2 (QFM versus NNO buffer) and H 2O (0–0.5 versus ≥2 wt%) conditions, respectively. Major and trace element relationships ruled out that CpxD and AmpD were derived from a similar parental magma, but require distinct primary melts and different source mantle. For the AmpD, crustal contamination cannot be ruled out, but it is difficult to evaluate the role of crustal components derived from a subducting slab, from that which might be due to crystalline basement. The source mantle of the Amp magmas was H 2O enriched by fluids released from a subducting slab, and this allowed the genesis of melts with high SiO 2. The source of the Cpx magmas would instead correspond to lithospheric mantle portions poorly affected by fluid release but characterised by a relatively high content of TiO 2 and incompatible elements. A fractional crystallisation genesis for the acidic dykes starting from Cpx or Amp magmas is not supported by trace element Rayleigh's calculations and the SiO 2 gap (60–71 wt%). Therefore, the possibility that they formed by the melting of lower basic-intermediate continental crust or underplated basalts cannot be ruled out. On the whole, the close association of tholeiitic (CpxD) and talc-alkaline (AmpD) magmas in space and time suggests that the studied Sinai dykes can be related to late extensional stages of the Pan-African cycle.

Proteins of morula-like cells in hemolymph of the giant clam, Tridacna derasa.

The morula-like cell, a hemocyte packed with many large (about 3 microns in diameter) electron-dense granules, is found only in the hemolymph of giant clams belonging to the Tridacnidae. To clarify the function of the morula-like cell, we investigated its proteins, especially those found in the large granules. Proteins with molecular weights of 64 kDa, 17 kDa and 7.4 kDa were found to be specific to this type of hemocyte. N-terminal amino acid sequence analysis revealed that the 17-kDa and 7.4-kDa proteins were novel proteins rich in aromatic amino acids. Rabbit polyclonal antibody against a synthetic peptide of the 7.4-kDa protein reacted not only with that protein but also with a larger molecular weight (about 16-kDa) protein in the morula-like cell. Examination of the N-terminal amino acid sequences showed that the 16-kDa protein is distinct from the 17-kDa protein, and Western blot analysis suggested that it is a precursor of the 7.4-kDa protein. The zooxanthellate portion of clam mantle and kidney contained proteins immunoreactive to the antibody, but the azooxanthellate portion of the mantle did not contain any immunoreactive protein. These results suggest that the morula-like cells interact with the zooxanthellae.

Anatexis of lunar cumulate mantle in time and space: Clues from trace-element, strontium, and neodymium isotopic chemistry of parental Apollo 12 basalts

In an effort to elucidate the processes of lunar mantle melting, and the magma evolution of mare basalts within Oceanus Procellarum on the western lunar near-side, we have analyzed seven fine-grained to vitrophyric Apollo 12 basalts for trace-elements; five of these also have been analyzed for Nd and Sr isotopic compositions. These samples represent all three main groups identified among the Apollo 12 mare basalts and have been proposed as parental melts to their respective groups, i.e., olivine-, pigeonite-, and ilmenite-basalts. The sources for these low-Ti mare basalts are postulated to have formed from crystallization of a global magma ocean. LiBe systematics, combined with REE data, indicate that the specific sources for the Apollo 12 low-Ti mare basalts were generated after 82–94% crystallization of this lunar magma ocean. In fact, it seems that all mare basalts analyzed from the Apollo collections were generated from cumulates precipitated in the last 20% of the magma ocean. Chemical compositions of fine-grained pigeonite and olivine basalts are consistent with 7–9% nonmodal (in proportions not defined by experimental petrology and phase equilibria) melting of a source consisting of 48% olivine, 30% calcic clinopyroxene, and 22% pigeonite (as per Neal et al., 1994b). SmNd and RbSr abundance data also suggest that the pigeonite- and olivine-basalt source contained from 0.3 to 0.5% trapped residual liquid from the magma ocean. The compositions of the two fine-grained ilmenite basalts are consistent with 5–7% partial melting of a source with subequal proportions of olivine (45.5%) and pigeonite (42.5%) and lesser amounts of clinopyroxene (11.5%) and entrained plagioclase (0.5%). Furthermore, the ilmenite source was nearly devoid of trapped liquid (<0.15%). A few of these samples do indicate minor post-extrusive fractionation, but most of the samples are considered to be unfractionated, primitive magmas that are parental to the other mare basalts. Isotopic systematics of the Apollo 12, fine-grained, parental basalts are consistent with their derivation from two distinct mantle source regions. Both of these sources were LREE depleted for extended periods of time: up to 600 million years for the ilmenite-basalt source and up to 900 million years for the pigeonite- and olivine-basalt source. Due in part to the relatively small proportion of low Sm Nd , trapped, residual magma-ocean liquid in the source (<0.15%), the Nd isotopic compositions of the ilmenite basalts are among the most radiogenic ever analyzed from the Moon (ϵ Nd = +10.5 to +11.2, at 3.2 Ga). The mantle source for the olivine and pigeonite basalts contained a higher proportion of trapped, residual, magma-ocean liquid (0.3 to 0.5%), thus yielding less radiogenic Nd isotopic signatures (ϵ Nd = +4.3 to +4.7, at 3.2 Ga). By integrating this information on parental, low-Ti, Apollo 12 basalts with mare basalt and picritic glass data from other landing sites, as well as telescopic and remote-sensing data, we propose a model for melting of the lunar interior. The upper 400–500 km of the lunar mantle is a consequence of incipient melting of the Moon and formation of a global magma ocean. This magma ocean became progressively enriched in incompatible elements as it precipitated the cumulate upper mantle. This incompatible-earliest, extensive mare magmas (high-Ti mare basalts) were generated at shallow depths in the mantle from cumulate source-regions that had trapped relatively large proportions of this incompatible-element enriched, residual, magma ocean liquid. These source regions also contained the late-crystallizing phase ilmenite and, thus, generated high-Ti magmas. The trapped liquid component contained elevated abundances of heat-producing elements (K, U, and Th), increasing the fertility of associated source-regions. With time, melting moved progressively deeper in the mantle to source-regions with less of the residual, heat-producing, magma ocean liquid. Due to density contrasts, the ilmenite-bearing upper portions of the lunar mantle sank into the cumulate pile, possibly carrying more fertile material with it and allowing melting of more Mg-enriched source regions (to form the high-Ti picritic glass beads). Thus, the major controlling factors in the melting of the lunar interior could be the proportion of trapped, magma-ocean liquid in the cumulate source and the sinking of more fertile, ilmenite-bearing material into the lower mantle.

Sound velocities of clinohumite, and implications for water in Earth's upper mantle

The velocities of longitudinal and shear acoustic waves were measured for clinohumite, a possible host for water in the upper mantle. Brillouin scattering measurements on a natural crystal under ambient conditions indicate that the hydration of olivine to clinohumite lowers most of the single‐crystal elastic moduli, as well as the bulk and shear moduli of a polycrystalline aggregate. However, the velocities of mantle rocks in which all olivine is hydrated to clinohumite are only 1–1.5% lower than those of the equivalent dry rocks under ambient conditions. These results suggest that in shallow, cold portions of the uppermost mantle (for example the seismic lid), water may be difficult to identify seismologically if all of it is bound up in humite‐group minerals. Negative velocity anomalies in the shallow mantle may require a different explanation than the presence of hydrous minerals.

Dniepr-Donets Rift: deep structure and evolution from DSS profiling

Forward modelling has been applied to refraction and wide-angle reflection traveltime data along two profiles crossing the intracratonic Dniepr-Donets Rift (DDR), a Devonian sedimentary basin located in the southwestern part of the Eastern European Platform between the Ukrainian Shield and the Voronezh Massif. The 2-D P-wave velocity models show that the crystalline crust beneath the rift basin is thinner and characterised by higher velocity values (of almost 1.0 km/s) than beneath the Shield. Differences along the strike of the DDR are also indicated. The two profiles cross the Dniepr and Donets segments of the DDR and reveal, respectively, sedimentary infill thickness of 7 and 20 km and crystalline crustal thicknesses of 31 and 17 km. In both cases, the crystalline crust comprises two distinct velocity layers. Beneath the Dniepr basin, the Moho lies at 38 km and the upper mantle velocity is 8.4 km/s. In the Donets region, the crustal layers are underlain by velocity boundaries at depths of 37 and 42 km with velocities of 7.6 and 8.0 km/s, respectively. The velocity models are interpreted to indicate that a key process in the formation and evolution of the DDR was the exchange of material between the crust and uppermost mantle. It is hypothesised that the extended and faulted pre-rift shield crust was intruded by mantle melts during rifting and that crust/mantle material exchange may also have occurred during the subsequent cooling and subsidence stage of development. Rifting was less intense in the Dniepr segment of the DDR compared to the Donets. To explain other differences in uppermost mantle structure between the two segments of the DDR, it is postulated that density reduction of some portion of the Donets subcrustal mantle occurred in response to compressional deformation and inversion during the Permian Uralian orogeny.

Cholinergic neurons from different subdivisions of the basal forebrain lack connectional specificity for cerebral cortical target sites in vitro

Basal forebrain cholinergic neurons send their axons to cerebral cortex in a topographically organized projection. Experiments tested the hypothesis that this topographic organization results from target preferences of the cholinergic neurons. Slices containing either medial septum or substantia innominata were grown in co-culture with slices of lateral neocortex and hippocampus. Cholinergic neurons from septum and from substantia innominata projected axons into neocortex and hippocampus, without obvious differences in pattern or density. These data suggest that basal forebrain cholinergic neurons can innervate any portion of the cerebral mantle.

In vitro measurement of strain in the bone cement surrounding the femoral component of total hip replacements during simulated gait and stair-climbing.

The strains in the cement mantle surrounding the cemented femoral component of a total hip replacement were measured in vitro, using strain gauges embedded within the cement mantle adjacent to the femoral component in femurs from cadavers under physiologic loads simulating both single-limb stance and stair-climbing. Cement strains in the most proximal portion of the cement mantle were measured with and without full contact of the collar of the femoral stem on the cortex of the medial portion of the femoral neck during both loading conditions. To our knowledge, these are the first studies to contrast by direct measurement the strain profile in the cement mantle of a cemented femoral component under simulated stair-climbing with that occurring under simulated single-limb stance. They extend the findings from finite element analyses and from clinical specimens retrieved at autopsy in identifying those regions of the cement mantle most likely to fail. At two specific foci, the magnitude of the strain in the cement mantle approaches values that could lead to early fatigue failure of the cement. The two regions in which the strains were highest (greater than 1,000 microstrain) were the most proximal portions of the cement mantle and near the tip of the femoral component. Although these two regions are recognized areas of high strain and also common sites of cement debonding and cement mantle failure, the strain-gauge studies showed that the magnitude of cement strains in the proximal portion of the cement mantle were highest during stair-climbing; in contrast, high strains at the tip region occurred in both gait and stair-climbing. Contact between the collar and the medial portion of the femoral neck reduced the strain in the proximal portion of the cement mantle not only in single-limb stance but in stair-climbing as well. The level of strain recorded in these studies for a simulated person weighing 115 pounds (52 kg) could lead to cement fracture during extended in vivo service life of a cemented femoral component, from either single-limb stance or stair-climbing. This risk would be increased if a void or defect existed in the cement mantle at these sites. Moreover, the increase in strain in the cement mantle was linear with increases in body weight between 100 and 200 pounds (45 and 91 kg) of spinal load, indicating that strains in a heavy patient could readily exceed the fatigue limit of the cement, particularly if a stress riser such as a pore in the cement or a sharp corner of the prosthesis were present. These data reemphasize the need to continue efforts to develop methods to strengthen bone cement and to reduce those factors that increase the strain in the cement mantle of cemented femoral components of total hip arthroplasty, particularly proximally and near the tip.

Tibet uplift and erosion

The 5-km-high Tibetan plateau is an outstanding topographic feature on the Earth today. Its horziontal extent, elevation, and location cause significant effects on modern atmospheric circulation and climate, so the history of uplift of the surface of Tibet is linked to Cenozoic climate changes, at local, regional, and perhaps global scales. Geological and geophysical studies of the plateau are contributing data on the present and past deformation of the Tibetan lithosphere that has formed the plateau, primarily during the Cenozoic. The principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history. Different parts of Tibet probably had different uplift histories, but presently available data are not sufficient for distinguishing many contrasts. In one scenario, Cretaceous and Early Cenozoic north-south distributed shortening of Tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early Miocene. Thinning of the high-density mantle portion of the lithosphere during the Miocene may then have allowed the thick Tibetan crust to rise close to its present elevation (perhaps higher) before ∼8 Ma. Since then, slow east-west extension of Tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late Cenozoic.Erosion of Tibet, unlike narrow mountain belts, has been unable to match the uplift of a broad plateau. Orographic precipitation and efficient river networks concentrate erosion on the edges, while the interior is protected from significant erosion despite its lofty elevation. The southern edge of the plateau, the Himalaya, has suffered a minimum of 25 km of denudation since the Miocene, while central Tibet shows little or no sign of major erosion since that time. The Gangdese arc in southern Tibet was rapidly eroded during the mid-Miocene when >4 km of rock apparently were removed from the surface, as shown by mineral cooling ages. This pulse of erosion was probably caused by a combination of local thrust-system movement and changes in base level and precipitation due to relative elevation changes between the Gangdese and the Himalaya to the south. The modern long-wavelength flatness of Tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface.

Magnesite-bearing inclusion assemblage in natural diamond

A significant mineral assemblage has been found as an inclusion in a natural diamond from the Finsch kimberlite pipe of South Africa: a euhedral rhombohedron-shaped magnesite (MgCO 3) crystal (d ∼ 30 μm) co-exists with several idiomorphic olivine [(Mg 1.86Fe 0.14)SiO 4] grains (d ∼ 80 μm). Many tiny anatase (TiO 2) particles (d ∼ 2–5 μm) and microcrystallites (d < 1 μm) of diamond and disordered graphite are attached to the surface of the magnesite grain. Structural and compositional characterization of the inclusion phases was achieved by micro-Raman spectroscopy and electron microprobe analysis. The occurrence of this syngenetic multiphase inclusion assemblage in a natural diamond provides unambiguous evidence for the existence in the Earth's mantle of magnesite, which has been proposed as a major carbon reservoir in most of the mantle. Both the formation and preservation aspects of the assemblage have been investigated. The mineralogy of the assemblage indicates that carbonated peridotite formed the surrounding petrologic environment. The inclusion assemblage suggests two reactions involving the decomposition of carbonates in mantle peridotite during decompression, which, in part, may explain the paucity of magnesite that has been found in other mantle rocks. The chemical inertness and low compressibility of the host diamond must have been critical to the preservation of this magnesite-bearing assemblage. The incorporation of a pure TiO 2 phase in a peridotitic diamond inclusion and its occurrence in the anatase structural form further emphasize the unusual conditions that allowed both the formation and preservation of this multiphase inclusion. The P-T-fO 2 conditions defined by the inclusion assemblage are represented by the intersection of the graphite-diamond transition curve and the enstatite-magnesite-olivine-diamond buffer. The oxygen fugacity range represented by the inclusion assemblage is below that of the quartz-fayalite-magnetite and CCOCO 2 buffers in the P-T range common to most diamonds. However, the co-existence of diamond, graphite, magnesite, olivine and anatase in this inclusion assemblage represents the highest oxygen fugacity at which olivine could be stable with both diamond and magnesite; that is, the highest oxidation state under which a mantle diamond can be stabilized in a peridotite environment. The diamond-carbonate-silicate co-existence wedge is relatively restricted in P-T-fO 2 space. Therefore, the P-T-fO 2 conditions implied by this and other diamond inclusion assemblages lead to two significant implications for mantle petrology: (1) the conditions for diamond formation are very limited if carbonates are major carbon sources for diamonds; (2) given the low- fO 2 conditions inferred for portions of the Earth's mantle, carbonates may rarely occur in peridotites, and much of the carbon in the mantle may be locked in reduced phases.

High-temperature creep in fine-grained polycrystalline CaTiO 3, an analogue material of (Mg, Fe)SiO 3 perovskite

High-temperature creep in fine-grained polycrystalline CaTiO 3 perovskite was studied to get some insight into the rheology of the lower mantle. Polycrystalline specimens with grain-size ranging from 5 to 63 μm were prepared by hot-pressing and subsequently deformed under room pressure or at 300 MPa confining pressure, temperatures of T = 1453–1573K, and stresses of σ = 6–100 MPa under the controlled oxygen fugacity of fO 2 = 0.24 × 10 5−10 −10 Pa. Under all the tested conditions, the strain rate increases nearly linearly with stress and is inversely proportional to the second power of grain-size, a result consistent with the volume diffusion creep model. The comparison of the creep data with the diffusion coefficients and point defect models suggests that the diffusion of titanium or calcium ions through vacancy mechanism may be the rate-controlling process and that impurities play an important role. Both creep and grain-growth kinetics are shown to be significantly enhanced in the temperature range of T = 1515–1525K, presumably related to a structural phase transformation. A deformation mechanism map is constructed based on our data on diffusion creep and on the data on dislocation creep in coarse-grained polycrystals. Conditions under which diffusion creep dominates in CaTiO 3 perovskite are wide which is presumably due to the role of twin boundaries to suppress dislocation creep but not diffusion creep. Assuming that the creep properties of (Mg, Fe)SiO 3 perovskite are similar to CaTiO 3 when compared at the same homologous temperature, the present results suggest that diffusion creep may dominate in a significant portion of the lower mantle and therefore significant rheological softening may occur associated with grain-size reduction after the transformation of spinel to perovskite + magnesiowustite. Also the structural phase transformation from the orthorhombic to higher symmetry structures (such as the tetragonal structure) in perovskite may reduce the creep strength.