Velocity structure of the continental upper mantle: evidence from southern Africa

The velocity model for southern Africa of Qiu et al. [Qiu, X., Priestley, K., McKenzie, D., 1996. Average lithospheric structure of southern Africa. Geophys. J. Int. 127, 563–587] is revised so as to satisfy both the regional seismic waveform data and the fundamental mode Rayleigh wave phase velocity data for the region. The revised S-wave model is similar to the original model of Qiu et al. except that the high velocity, upper mantle lid extends to 160 km depth in the revised model rather than to 120 km in the original model. Sensitivity tests of the regional seismic data show that the minimum velocity in the S-wave low velocity zone can be as high as 4.45 km s⁻¹ compared to 4.32 km s⁻¹ in the Qiu et al. model. The vertical S-wave travel time for the revised south African model is compared with the vertical S-wave travel times for the global tomographic models S12WM13 and S16B30, and they are found to be similar.     

Structure of the crust and uppermost mantle beneath southern Finland revealed by analysis of local events registered by the SVEKALAPKO seismic array

In 1998–1999, a large-scale seismic array was deployed in Finland as a part of the EUROPROBE/SVEKALAPKO subproject, involving 14 European universities and research institutes. The objective of the project was to map the deep lithosphere structure and thickness beneath the Fennoscandian Shield by means of teleseismic events. In addition, about 580 local seismic events were registered during the data acquisition period. Among them, only eight local earthquakes were recorded, the rest being quarry blasts from mining sites in Russia, Finland, Estonia and Sweden. In this study, we present the analysis of the seismic wave field from the strongest local events registered by the majority of the stations of the SVEcofennian–KArelian–LAPland–KOla Transect (SVEKALAPKO) array with the aim of mapping the structure of the upper mantle beneath the array. For this purpose, we selected the events corresponding to a single source type and compared these recordings with those from wide-angle reflection and refraction experiments in the area to identify the regional phases. The record sections of selected events demonstrate strong reflections (PmP) from the Moho boundary. The refracted Pn phases can be seen as first arrivals at distances of about 200–400 km from the source. At offsets of about 400–800 km, phases reflected from inhomogeneities in the uppermost mantle (P1) and double reflections from the Moho boundary (PmPPmP) were recorded.Results from 2D forward ray trace modeling of reflected and refracted P-waves along four profile swathes from SVEKALAPKO stations demonstrate that the mantle reflections originate from two different groups of boundaries beneath the array: one group of phases arrive from subhorizontal and gently dipping reflectors below the Moho boundary at a depth of 70–90 km, while the other group are phases originating from a depth of 100 to 130 km. Based on the irregular character of the first group of reflections, their different spatial orientation and correlation with the Moho offsets, we interpret the boundaries of this group as relicts of ancient subduction and collision processes. The second group of reflections can be explained by a transition from mechanically strong to mechanically weak lithosphere.     

Depleted lithosphere from the mantle wedge beneath Tres Lagos, southern Patagonia, Argentina

Anhydrous spinel lherzolite and harzburgite xenoliths from Tres Lagos, situated inboard of the Volcanic Arc Gap (VAG) in southernmost Patagonia, are samples of a depleted lithospheric mantle and can be divided into two major groups: metasomatized and non-metasomatized. Metasomatized samples, which are the minority, are partly mylonitized and their metasomatism is related to this tectonic process. A group of non-metasomatized samples have enriched whole rock LREE-abundances that are not consistent with the depleted LREE-abundances in their clinopyroxenes. Intergranular host basalt infiltration could be responsible for the whole rock LREE enrichments. Their Sr- and Nd-isotopic ratios have also been affected by host basalt infiltration, whereas their high Sr-isotopic ratios point to subsequent contamination by ground-water and/or Ca-rich surface solutions. Similar contamination is thought to cause the decoupling of Sr- and Nd-isotopes (high Sr- and Nd-isotopic ratios) observed in the non-metasomatized samples with depleted whole rock LREE. A two-stage partial melting process could be responsible for the origin of the Tres Lagos xenoliths. Model calculations have shown that in the first stage, 2% of batch melting took place in the garnet peridotite field and subsequently the residue experienced 2–8% batch melting in the spinel peridotite field. The Tres Lagos peridotites have not been affected by subduction-related metasomatic processes and they could represent an old lithospheric mantle.     

Radial anisotropy in the crust and upper mantle beneath the Qinghai-Tibet Plateau and surrounding regions

Relative SV and SH wave speeds are generally attributed to radial seismic anisotropy which can be used as the indicator of crust/mantle deformation styles. Surface wave data were initially collected from events of magnitude Ms  5.0 and shallow or moderate focal depth occurring between 1980 and 2002: 713 of them generated Rayleigh waves and 660 Love waves, which were recorded by 13 broadband digital stations in Eurasia and India. Up to 1525 source-station Rayleigh waveforms and 1464 Love wave trains were earlier analysed by multiple filtering to obtain Love- and Rayleigh wave group velocity curves in the broad period range 10–105 s. We have performed tomographic inversion to obtain period-dependent group velocity and further shear wave velocity at 2° × 2°-sized grid-cells of a mesh covering the model region, after averaging azimuthal effects. Horizontally and vertically varying shear-wave velocities are observed, but the models of isotropic seismic velocity in the crust and upper mantle cannot fit simultaneously the inverted group-velocity dispersion curves due to the discrepancy in the transmission velocities of Love and Rayleigh waves, whose likely origin is the existence of radial anisotropy in the continental crust and topmost mantle. The strength of radial anisotropy computed from the Love–Rayleigh discrepancy and its spatial extent beneath the Qinghai-Tibet Plateau are shown as maps of percentage anisotropy at various depths down to 170 km and cross-sections along five profiles of reference. Areas in which radial anisotropy is in excess of 6% are found in the crust and upper mantle underlying most of the plateau, and even up to 10% in some places. The strength and spatial configuration of radial anisotropy seem to indicate the existence of a regime of horizontal compressive forces in the frame of the convergent Himalayan–Tibetan orogen, the laterally variation of the lithospheric rheology and the differential movement as regards the compressive driving forces.     

Ferropericlase—a lower mantle phase in the upper mantle

Experiments on compositions along the join MgO–NaA³⁺Si₂O₆ (A=Al, Cr, Fe³⁺) show that sodium can be incorporated into ferropericlase at upper mantle pressures in amounts commonly found in natural diamond inclusions. These results, combined with the observed mineral parageneses of several diamond inclusion suites, establish firmly that ferropericlase exists in the upper mantle in regions with low silica activity. Such regions may be carbonated dunite or stalled and degassed carbonatitic melts. Ferropericlase as an inclusion in diamond on its own is not indicative of a lower mantle origin or of a deep mantle plume. Coexisting phases have to be taken into consideration to decide on the depth of origin. The composition of olivine will indicate an origin from the upper mantle or border of the transition zone to the lower mantle and whether it coexisted with ferropericlase in the upper mantle or as ringwoodite. The narrow and flat three phase loop at the border transition zone—lower mantle together with hybrid peridotite plus eclogite/sediments provides an explanation for the varying and Fe-rich nature of the diamond inclusion suite from Sao Luiz, Brazil.     

Crustal and upper mantle S-wave velocity structure beneath the Bransfield Strait (West Antarctica) from regional surface wave tomography

Regional surface wave tomography in the sub-Antarctic Scotia Sea is helpful in revealing the nature of the crust and the S-wave seismic velocity profile beneath the Bransfield Strait. The joint use of our regional network, global seismographic network stations and local temporary arrays provide better lateral resolution than that obtained in our previous studies concerning the Scotia Sea region.Tomographic analysis of data obtained using 10 broad band seismic stations and more than 300 regional events, shows that the Bransfield Basin is characterised by a strong group velocity reduction of 8% with respect to the surrounding areas, in the period range from 15 s to 50 s.The crustal and upper mantle models of the eastern, central and western Bransfield Basin are obtained by joint inversion of Rayleigh and Love local dispersion curves from 15 s to 50 s. In addition our data set is expanded to a broader period interval (1–80 s), in central Bransfield Strait in order to better constrain the upper mantle and shallow crust.The main results can be summarized as follows: (a) the crust thins distinctly from W toward E; the variation is consistent with the type of volcanism, earthquake distribution and bathymetric observations, (b) low upper mantle velocities (soft lid) extend down to depths exceeding 70 km as a consequence of elevated temperatures, (c) the crust beneath the central Bransfield Basin displays continental characteristics with a gradually increasing S-wave velocity distribution versus depth analogous to the East African Rift structure of Kenya, (d) negative velocity gradients are present in the lower crust beneath the eastern Bransfield Basin; these could be interpreted as magmatic bodies originating from decompression melting of the mantle.     

Anisotropic seismic properties of the upper mantle beneath the Torre Alfina area (Northern Apennines, Central Italy)

Central Italy is an active tectonic area that has been recently studied by several regional mantle, Pn and SKS, studies which revealed the presence of a strong regional anisotropy. In this paper, we present the first petrophysical results on the only mantle xenoliths from Central Italy, which place new constraints on the upper mantle structures of this region. The Torre Alfina mantle xenoliths are very small in size, from few millimetres to about 1.5 cm. They are mainly dunites and harzburgites, with subordinate lherzolites and wehrlites. Since olivine and spinel are always present, they should have crystallised in the spinel-bearing lherzolite field. Their mineralogical composition is ol+spl±opx±cpx. Both olivines and pyroxenes are present as porphyroclasts and as neoblasts. The xenoliths show different degrees of recrystallization. Geothermobarometry on these xenoliths give a temperature range of 1040±40 °C and a pressure estimate of about 1.5 GPa, corresponding to 50 to 60 km depth. Previous seismic studies have estimated the Moho to be at 20 to 25 km in this region, hence the xenoliths come from a hot mantle, probably asthenospheric, below a lithosphere of about 25 to 40 km in thickness below the Moho. We measure the crystallographic preferred orientation (CPO) of olivines and pyroxenes using a SEM and the Electron Back Scattered Diffraction (EBSD) technique. The CPO shows all three axes of olivine are tightly clustered: [100] axis is typically more tightly clustered than [010] and [001] is the most widely distributed axis. The fabric strength expressed by the integral J index, varies from 4.5 to 25.9, and decreases with the degree of recrystallization. We use CPO data to calculate anisotropic seismic properties of the xenoliths. They are very homogenous and probably statistically representative of the mantle below the Torre Alfina area. Vp ranges from 8.4 to 9.1 km/s, Vs₁ from 4.8 to 5.0 km/s. The seismic anisotropy is more variable; AVp ranges from 9.8% to 19.3% and AVs from 7.3% to 13.4%. The majority of the xenoliths display an orthorhombic seismic symmetry, but xenoliths with a transverse isotropic behaviour have also been observed.

We consider four geodynamic models for the source region of the xenoliths (extension, shear, upwelling, slab tilted), defined by different orientations of the structural reference frame, and we calculated for each model the variation of the seismic properties with temperature, pressure and volume fraction of orthopyroxene. After comparing this variation of calculated seismic parameters with seismic observations from the region, we form the hypothesis that the xenoliths come from either an extensional tectonic zone (lineation X and foliation plane XY horizontal) or transcurrent shear zone (lineation X horizontal and foliation plane XY vertical) and that the mantle beneath Torre Alfina is composed by 70% olivine and 30% orthopyroxene forming an anisotropic layer of about 160 or 110 km in thickness, respectively.     

吉林南部辉南-靖宇地区岩石圈地幔氧化-还原状态及研究意义

吉林省南部辉南-靖宇地区第四纪碱性玄武岩中的地幔包体主要为尖晶石相二辉橄榄岩和方辉橄榄岩。二辉橄榄岩和方辉橄榄岩的平衡温度分别为770~1000℃和850~1025℃,对应的氧逸度 (f_O2)值分别为FMQ -0.70至+0.34 (均值为FMQ -0.06) 和FMQ -0.46至+0.05 (均值为FMQ -0.15),它们与深海橄榄岩(abyssal peridotites)以及软流圈地幔的f_O2相似。橄榄岩的f_O2值,连同其全岩化学成分(如Mg^#、Al₂O₃、CaO、Ni、Co和Cr)和矿物化学成分(如橄榄石的Fo、尖晶石的Cr^#和Mg^#,以及辉石的Mg^#)特征,表明辉南-靖宇地区龙岗火山群下面的岩石圈地幔很可能是在晚中生代以来,伴随着华北克拉通和扬子板块的碰撞以及来自东侧太平洋板块和北侧蒙古-额霍次克(Mongolo-Okhotsk)板块分别向西和向南的俯冲叠加,原来的古老岩石圈失衡、塌陷(拆沉？),取而代之的深部软流圈底辟、上涌,又经历了低度部分熔融的产物。     

A classification scheme for mantle-derived garnets in kimberlite: a tool for investigating the mantle and exploring for diamonds

A new empirical method has been devised for classification of mantle-derived garnets in kimberlite. Simple chemical screens have been developed to distinguish between garnets from different parageneses, based on Mg, Fe, Ca, Cr, Ti and Na values of published analyses of garnets from >2000 ultramafic xenoliths in kimberlite. Although crustal garnets are typically uncommon as xenocrysts in kimberlite, the first step in the classification is to screen these from the mantle population, using data from >600 garnet-bearing crustal rocks. Such a screen may also prove useful in evaluating the source (crust vs. mantle) of garnet in kimberlite exploration samples. Subsequent steps divide mantle garnets into eclogite, peridotite and Cr-poor megacryst groupings, and sub-groups of the peridotite (lherzolite, harzburgite, wehrlite) and eclogite (Groups I and II and A, B, C and grospydite) populations. Important features of this classification include the fact that it is based on distinctions between groups of fundamental geological significance (e.g., peridotite vs. eclogite) and it is based on a large, well-documented and well-understood xenolith database. As it utilizes oxide values and molar ratios of major and minor elements, the rationale for the screens is readily understood and it is simple to use.     

The electrical conductivity of upper-mantle rocks: water content in the upper mantle

The electrical conductivity of upper-mantle rocks—dunite, pyroxenite, and lherzolite—was measured at ∼2–3 GPa and ∼1,273–1,573 K using impedance spectra within a frequency range of 0.1–10⁶ Hz. The oxygen fugacity was controlled by a Mo–MoO₂ solid buffer. The results indicate that the electrical conductivity of lherzolite and pyroxenite are approximately half and one order of magnitude higher than that of dunite, respectively. A preliminary model involving water and iron content effects on the electrical conductivity was derived and is summarized by the relation:

Imaging the continental upper mantle using electromagnetic methods

The internal structure of the continental lithosphere holds the key to its creation and development, and this internal structure can be determined using appropriate seismic and electromagnetic methods. These two are complementary in that the seismic parameters usually represent bulk properties of the rock, whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix, such as the presence of a conducting mineral phase, e.g. carbon in graphite form, or of a fluid phase, e.g. partial melt or volatiles. In particular, conductivity is especially sensitive to the top of the asthenosphere, generally considered to be a region of interconnected partial melt. Knowledge of the geometry of the lithosphere/asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create, modify, and destroy the lithosphere. Accordingly, collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own. This paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural-source magnetotelluric technique, and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep-probing electromagnetic surveying. The resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust. Examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere–asthenosphere boundary. Also, the observations from laboratory measurements on candidate mineralogies representative of the mantle, such as olivine, are presented.     

An investigation of upper mantle heterogeneity beneath the Archaean and Proterozoic crust of western Canada from Lithoprobe controlled-source seismic experiments

Observations of upper mantle reflectivity at numerous locations around the world have been linked to the presence of a heterogeneous distribution of rock types within a broad layer of the upper mantle. This phenomenon is observed in wide-angle reflection data from Lithoprobe's Alberta Basement Transect [the SAREX and Deep Probe experiments of 1995] and Trans-Hudson Orogen Transect [the THoRE experiment of 1993]. SAREX and Deep Probe image the Archaean lithosphere of the Hearne and Wyoming Provinces, whereas THoRE images the Archaean and Proterozoic lithosphere of the Trans-Hudson Orogen and neighbouring areas.Finite-difference synthetic seismograms are used to constrain the position and physical properties of the reflective layer. SAREX/Deep Probe modelling uses a 2-D visco-elastic finite-difference routine; THoRE modelling uses a pseudospectral algorithm. In both cases, the upper mantle is parameterized in terms of two media. One medium is the background matrix; the other is statistically distributed within the first as a series of elliptical bodies. Such a scheme is suitable for modelling: (1) variations in lithology (e.g., a peridotite matrix with eclogite lenses) or (2) variations in rheology (e.g., lenses of increased strain within a less strained background).The synthetic seismograms show that the properties of heterogeneities in the upper mantle do not change significantly between the two Lithoprobe transects. Beneath the Trans-Hudson Orogen in Saskatchewan, the layer is best modelled to lie at depths between 80 and 150 km. Based on observations from perpendicular profiles, anisotropy of the heterogeneities is inferred. Beneath the Precambrian domains of Alberta, 400 km to the west, upper mantle heterogeneities are modelled to occur between depths of 90 and 140 km. In both cases the heterogeneous bodies within the model have cross-sectional lengths of tens of kilometers, vertical thicknesses less than 1 km, and velocity contrasts from the background of − 0.3 to − 0.4 km/s. Based on consistency with complementary data and other results, the heterogeneous layer is inferred to be part of the continental lithosphere and may have formed through lateral flow or deformation within the upper mantle.     

The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton: Links between mantle metasomatism,thermal state and diamond potential

Although the diamond potential of cratons is linked mainly to thick and depleted Archean lithospheric keels, there are examples of craton-edge locations and circum-cratonic Proterozoic terranes underlain by diamondiferous mantle. Here, we use the results of comprehensive major and trace-element studies of detrital garnets from diamond-rich Late Triassic (Carnian) sedimentary rocks in the northeastern Siberia to constrain the thermal and chemical state of the pre-Triassic mantle and its ability to sustain the diamond storage. The studied detrital mantle-derived garnets are dominated by low- to medium-Cr lherzolitic (～45%) and low-Cr megacrystic (～39%) chemistries, with a significant proportion of eclogitic garnets (～11%), and only subordinate contribution from harzburgitic garnets (～5%) with variable Cr₂O₃ contents (1.2–8.4 wt.%). Low-Cr megacrysts display uniform, “normal” rare-earth element (REE) patterns with no Eu/Eu* anomalies, systematic Zr and Ti enrichment (mainly within 2.5–5), which are evidence of their crystallization from deep metasomatic melts. Lherzolitic (G9) garnets exhibit normal or humped to MREE-depleted sinusoidal REE patterns and elevated Nd/Y (up to 0.33–0.41) and Zr/Y ratios (up to 7.62). Rare low- to high-Cr harzburgitic (G10) garnets have primarily “depleted”, sinusoidal REE-patterns, low Ti, Y and HREE, but vary significantly in Zr-Hf, Ti and MREE-HREE contents, Nd/Y (within 0.1–2.4) and Zr/Y (1.53–19.9) ratios. The observed trends of chemical enrichment from the most depleted, harzburgitic garnets towards lherzolitic (including high-Ti high-Cr G11-type) garnets and megacrysts result from either voluminous high-temperature metasomatism by plume-derived silicate melts or recurrent mobilization of less voluminous kimberlitic or related carbonated mantle melts, rather than the initially primitive, fertile nature of the Proterozoic SCLM. Calculated Ni-in-garnet temperatures (primarily within ～1150–1250 °C) indicate their derivation from at least ～220 km thick Cr-undersaturated lithosphere at the relevant Devonian to Triassic thermal flow of ～45 mW/m² or cooler. We suggest the existence of rare harzburgitic domains in the primarily lherzolitic diamond-facies SCLM beneath the northeastern Siberian craton at least by Triassic, whereas the abundance of eclogitic garnets, predominance of E-type inclusions in placer diamonds and specific morphologies argue for diamondiferous eclogites occurring within a ～50–65 kbar diamond window of the Olenek province by the same time.     

东北和华北地区上地幔成分和热状态对比及对中国东部岩石圈减薄的启示

地幔成分与其上覆地壳年龄存在相关关系,年龄越老,地幔越亏损玄武质组分.本文对产于东北和华北地区的尖晶石相橄榄岩包体的成分进行了统计分析,结果显示东北地区橄榄岩包体比华北地区包体更亏损玄武质组分.这说明东北岩石圈地幔比华北上地幔更难熔,但其上覆地壳年龄却远小于华北地区地壳的年龄.这种地壳年龄和地幔组分之间的解耦暗示东北和华北地区的岩石圈地幔形成之后发生了大规模的改造.华北地区的壳幔解耦与中生代岩石圈减薄和增生有关,而东北地区的壳幔解耦则是该区地壳的多期改造和中生代岩石圈减薄和增生等过程综合作用的结果.两地区地幔成分的差异显然与部分熔融程度的不同有关,但影响部分熔融程度的因素很多,目前尚不能确定.包体的平衡温度统计和地温线对比显示东北岩石圈的地温梯度低于华北的地温梯度,可能是东北地区岩石圈减薄的时间要早于华北地区,或者华北岩石圈减薄程度可能大于东北地区的结果,因此中国东部岩石圈减薄存在时空不均一性.     

华北克拉通东部显生宙地幔演化

华北克拉通东部显生宙以来的地幔可以划分为3种类型:克拉通型地幔,大陆活动带型地幔和大陆裂谷型地幔。1 700 Ma—古生代末,地幔属于克拉通型:ε(Nd,t)值高于-5,为弱富集型;层圈相互作用以幔源的熔体和/或流体与古老的岩石圈地幔的作用为主,但规模较小,范围局部。100 Ma以前的中生代地幔属于“大陆活动带型”:ε(Nd,t)值低,在-5以下,为富集型;地幔中含有地壳的组分,层圈相互作用以下地壳与弱化的岩石圈地幔之间的作用为主;发生的时间为190~100 Ma,高峰期在130 Ma左右;发生的部位邻近莫霍面,导源的岩浆多为钙碱性系列,部位浅,活动范围广泛。100 Ma至新生代,地幔属于“大陆裂谷型”:为亏损型的软流圈地幔,ε(Nd,t)值高,几乎均为正值。层圈相互作用转变为软流圈岩石圈地幔之间的作用,转变的时间具有约40 Ma的过渡时期,前锋开始于100~109 Ma,导源的岩浆大致沿NWW和NEE向的大型断裂带分布。进一步证实了软流圈地幔上隆的不均匀性和主动性。     

Mesozoic thermal evolution of the southern African mantle lithosphere

The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150–200 °C isobaric temperature range at 5–6 GPa confirms original interpretations of a conductive geotherm, which is perturbed at depth, and therefore does not record steady state lithospheric mantle structure.

Xenoliths from both Archean and Proterozoic terranes record conductive limb temperatures characteristic of a “cratonic” geotherm (40 mW m⁻²), indicating cooling of Proterozoic mantle following the last major tectonothermal event in the region at 1 Ga and the probability of thick off-craton lithosphere capable of hosting diamond. This inference is supported by U–Pb thermochronology of lower crustal xenoliths [Schmitz and Bowring, 2003. Contrib. Mineral. Petrol. 144, 592–618].

The entire region then suffered a protracted regional heating event in the Mesozoic, affecting both mantle and lower crust. In the mantle, the event is recorded at 150 Ma to the southeast of the craton, propagating to the west by 108–74 Ma, the craton interior by 85–90 Ma and the far southwest and northwest by 65–70 Ma. The heating penetrated to shallower levels in the off-craton areas than on the craton, and is more apparent on the southern margin of the craton than in its western interior. The focus and spatial progression mimic inferred patterns of plume activity and supercontinent breakup 30–100 Ma earlier and are probably connected.

Contrasting thermal profiles from Archean and Proterozoic mantle result from penetration to shallower levels of the Proterozoic lithosphere by heat transporting magmas. Extent of penetration is related not to original lithospheric thickness, but to its more fertile character and the presence of structurally weak zones of old tectonism. The present day distribution of surface heat flow in southern Africa is related to this dynamic event and is not a direct reflection of the pre-existing lithospheric architecture.     

A top to bottom lithospheric study of Africa and Arabia

We study the lithospheric structure of Africa, Arabia and adjacent oceanic regions with fundamental-mode surface waves over a broad period range. Including group velocities with periods shorter than 35 s allows us to examine shallower features than previous studies of the whole continent. In the process, we have developed a crustal thickness map of Africa. Main features include crustal thickness increases under the West African, Congo, and Kalahari cratons. We find crustal thinning under Mesozoic and Cenozoic rifts, including the Benue Trough, Red Sea, and East, Central, and West African rift systems, along with less abrupt crustal thickness changes at passive continental margins. We also find crustal thickness differences in North Africa between the West African Craton and East Saharan Shield. Crustal shear wave velocities are generally faster in oceanic regions and cratons, and slower in more recent crust and in active and remnant orogenic regions. Deeper structure, related to the thickness of cratons and modern rifting, is generally consistent with previous work. Under cratons we find thick lithosphere and fast upper mantle velocities, while under rifts we find thinned lithosphere and slower upper mantle velocities. However, we also find the lack of a thick cratonic keel beneath the central portion of the Congo Craton. There are no consistent effects in areas classified as hotspots, indicating that there seem to be numerous origins for these features. Finally, it appears that the African Superswell, which is responsible for high elevation and uplift over large portions of Africa, has had a significantly different impact (as indicated by features such as temperature, time of influence, etc.) in the north and the south. This is consistent with episodic activity at shallow depths, which is well-expressed in northeastern Africa and Arabia today.     

Xenolith evidence for polybaric melting and stratification of the upper mantle beneath South China

Mantle xenoliths from Hainan and Qilin, South China have been studied to constrain the nature of the upper mantle and mantle processes beneath a continental margin. The extremely low Ti (160–245 ppm) contents in clinopyroxenes from some spinel lherzolites, indicative of high degrees of partial melting are inconsistent with the relatively high clinopyroxene modes (7.4–13%) in these samples. This inconsistency could be due to polybaric melting that started in the garnet stability field, then, after the breakdown of garnet to pyroxene and spinel, continued in the spinel stability field. Polybaric melting, due to adiabatic decompression of upwelling mantle, would leave a residual mantle in which the degree of depletion decreases with depth. The predicted stratified lithospheric mantle is evidenced by the negative correlation between the forsterite content in olivine and the equilibration temperature, proportional to the depth in the lithosphere from which the xenolith was derived. The lower part of the lithospheric mantle beneath South China consists predominantly of fertile and moderately depleted peridotites, which are either devoid of LREE enrichment, or show the trace element signature of incipient metasomatism, and plot within the Phanerozoic mantle domain. In contrast, the upper part of the mantle contains harzburgite and cpx-poor lherzolite, which are strongly affected by metasomatism of melt/fluid of highly variable composition. The anomalously high orthopyroxene mode (up to 47%) makes some of these refractory samples compositionally similar to the Proterozoic/Archean mantle. Their low equilibrium temperature (800–900 °C) points to the presence of old lithospheric relicts in the uppermost mantle beneath South China. Such lithosphere architecture may have resulted from partial replacement of Archean–Proterozoic lithosphere by asthenosphere that rose adiabatically subsequent to lithospheric thinning during the Cenozoic.     

Layered mantle at the Karelian Craton margin: P–T of mantle xenocrysts and xenoliths from the Kaavi–Kuopio kimberlites, Finland

Peridotitic clinopyroxene (cpx) and pyrope garnet xenocrysts from four kimberlite pipes in the Kaavi–Kuopio area of Eastern Finland have been studied using major and trace element geochemistry to obtain information on the vertical compositional variability of the underlying mantle. The xenocryst data, when combined with the petrological constraints provided by peridotite xenoliths, yield a relatively complete section through the lithospheric mantle. Single-grain cpx thermobarometry fits with a 36-mW/m² geotherm calculated using heat flow constraints and xenolith modes and geophysical properties. Ni thermometry on pyrope xenocrysts gives 700–1350 °C and, based on the cpx xenocryst/xenolith geotherm, indicates a wide sampling interval, ca. 80–230 km. Plotting pyrope major and trace element compositions as a function of temperature shows there are three distinct layers in the local lithospheric mantle:

(1) A low-temperature (<850 °C) harzburgite layer distinguished by Ca-rich but Ti-, Y- and Zr-depleted pyropes. The xenoliths originating from this layer are all fine-grained garnet-spinel harzburgites with secondary cpx.

(2) A variably depleted lherzolitic, harzburgitic and wehrlitic horizon from 950 to 1150 °C or 130 to 180 km.

(3) A deep layer from 180 to 240 km composed largely of fertile material.

The peridotitic diamond window at Kaavi–Kuopio stretches from the top of the diamond stability field at 140 km to the base of the harzburgite-bearing mantle at about 180 km, implying a roughly 40-km-wide prospective zone.     

华北克拉通上地幔变形及其动力学意义

华北克拉通从稳定到破坏的演化过程对有关地球动力学的经典理论提出了挑战,研究其独特的演化历史是固体地球科学研究的一项重要内容。上地幔矿物晶体的各向异性记录了上地幔发生构造变形的信息,研究上地幔地震波各向异性能够揭示现今和构造历史时期所发生的构造运动。本文总结了近年来作者在华北克拉通地区所进行的高密度、覆盖广泛的地震波横波分裂观测研究结果。横波分裂的快轴方向与绝对板块运动方向的不一致,以及横波分裂参数快速的空间变化特征表明了华北克拉通的SKS横波分裂主要反映上地幔的变形。观测结果表明：鄂尔多斯块体保留了克拉通较弱的各向异性特征,其西端体现了元古代克拉通拼合的变形特征; 中新生代华北克拉通破坏事件以不同的机制主导了华北克拉通中部和东部的上地幔变形,在东部地区北西—南东向的拉张应力作用使得快轴方向平行于拉张方向,而在中部则因受到较厚岩石圈的阻挡使得地幔流动改变了方向,因此造成了北东和北北东向的岩石圈拉张。