Thermal convection in a heterogeneous mantle

Both seismology and geochemistry show that the Earth's mantle is chemically heterogeneous on a wide range of scales. Moreover, its rheology depends strongly on temperature, pressure and chemistry. To interpret the geological data, we need a physical understanding of the forms that convection might take in such a mantle. We have therefore carried out laboratory experiments to characterize the interaction of thermal convection with stratification in viscosity and in density. Depending on the buoyancy ratio B (ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly), two regimes were found: at high B, convection remains stratified and fixed, long-lived thermochemical plumes are generated at the interface, while at low B, hot domes oscillate vertically through the whole tank, while thin tubular plumes can rise from their upper surfaces. Convection acts to destroy the stratification through mechanical entrainment and instabilities. Therefore, both regimes are transient and a given experiment can start in the stratified regime, evolve towards the doming regime, and end in well-mixed classical one-layer convection. Applied to mantle convection, thermochemical convection can therefore explain a number of observations on Earth, such as hot spots, superswells or the survival of several geochemical reservoirs in the mantle. Scaling laws derived from laboratory experiments allow predictions of a number of characteristics of those features, such as their geometry, size, thermal structure, and temporal and chemical evolution. In particular, it is shown that (1) density heterogeneities are an efficient way to anchor plumes, and therefore to create relatively fixed hot spots, (2) pulses of activity with characteristic time-scale of 50–500 Myr can be produced by thermochemical convection in the mantle, (3) because of mixing, no ‘primitive’ reservoir can have survived untouched up to now, and (4) the mantle is evolving through time and its regime has probably changed through geological times. This evolution may reconcile the survival of geochemically distinct reservoirs with the small amplitude of present-day density heterogeneities inferred from seismology and mineral physics.     

云南地区上地幔各向异性研究

对云南23个数字地震台11次地震的SKS记录，采用理论切向分量与实测切向分量拟合的方法，确定了快S波的偏振方向和快、慢波之间的时间延迟．结果表明，除鹤庆台外，在各台都观测到了S波分裂现象；云南地区的快方向总体特征是北北东向，时间延迟变化范围为0.5～2.0ｓ．在地质构造复杂地区断层对分析的影响很大．分析表明，作为青藏高原与华南块体之间的过渡带，云南地区的S波快方向反映了印度板块向欧亚板块俯冲是该地区地球动力学的基本背景，而由于青藏高原隆起造成的康滇菱形块体的南东-南南东向运动是造成复杂构造、应力环境的重要因素．快方向与上地幔运动的方向存在差异，说明在云南地区低速层或者软流层的运动与地壳块体的运动之间存在着复杂的耦合作用，构造驱动力如同向北东方向张开的手掌．从时间延迟出发，推断各向异性层的厚度为60～225ｋｍ．其变化范围与低速层埋深的变化范围（104～260ｋｍ）相当，认为各向异性层顶面可能在地壳底部，也可能在低速层，且在不同地点是不相同的，这与云南及周边地区莫霍面变化剧烈有因果关系．进一步推断出上地幔的各向异性主要存在于岩石圈而不是整个上地幔．      

Noble gas systematics of the Réunion mantle plume source and the origin of primordial noble gases in Earth’s mantle

New noble gas data of ultramafic xenoliths from Réunion Island, Indian Ocean, further constrain the characteristics of primordial and radiogenic noble gases in Earth’s mantle plume reservoirs. The mantle source excess of nucleogenic ²¹Ne is significantly higher than for the Hawaiian and Icelandic plume reservoirs, similar to excess of radiogenic ⁴He. ⁴⁰Ar/³⁶Ar of the Réunion mantle source can be constrained to range between 8000 and 12 000, significant ¹²⁹Xe and fission Xe excess are present. Regarding the relative contribution of primordial and radiogenic rare gas nuclides, the Réunion mantle source is intermediate between Loihi- and MORB-type reservoirs. This confirms the compositional diversity of plume sources recognized in other radioisotope systematics. Another major result of this study is the identification of the same basic primordial component previously found for the Hawaiian and Icelandic mantle plumes and the MORB reservoir. It is a hybrid of solar-type He and Ne, and ‘atmosphere-like’ or ‘planetary’ Ar, Kr, Xe (Science 288 (2000) 1036). ²⁰Ne/²²Ne ratios extend to maximum values close to 12.5 (Ne-B), which is the typical signature of solar neon implanted as solar corpuscular radiation. This suggests that Earth’s solar-type noble gas inventory was acquired by small (less than km-sized) precursor planetesimals that were irradiated by an active early sun in the accretion disk after nebular gas dissipation, or, alternatively, that planetesimals incorporated constituents irradiated in transparent regions of the solar nebula. Previously, such an early irradiation scenario was suggested for carbonaceous chondrites which follow common volatile depletion trends in the sequence CI–CM–CV–Earth. In turn, CV chondrites closely match Earth’s mantle composition in ²⁰Ne/²²Ne, ³⁶Ar/²²Ne and ³⁶Ar/³⁸Ar. This indicates that mantle Ar could well be a planetary component inherited from precursor planetesimals. However, a corresponding conclusion for mantle Kr and Xe is less convincing yet, but this may be just due to the lack of appropriate ‘meteoritic’ building blocks matching terrestrial composition. Alternatively, heavy noble gases in Earth’s mantle could be due to admixing of severely fractionated air, but this effect must have affected all mantle sources to a very similar extent, e.g. by global subduction before the last homogenization of the mantle reservoirs.     

Saharan Dust Transport and High-Latitude Glacial Climatic Variability: The Alboran Sea Record

Millennial to submillennial marine oscillations that are linked with the North Atlantic's Heinrich events and Dansgaard–Oeschger cycles have been reported recently from the Alboran Sea, revealing a close ocean-atmosphere coupling in the Mediterranean region. We present a high-resolution record of lithogenic fraction variability along IMAGES Core MD 95-2043 from the Alboran Sea that we use to infer fluctuations of fluvial and eolian inputs to the core site during periods of rapid climate change, between 28,000 and 48,000 cal yr B.P. Comparison with geochemical and pollen records from the same core enables end-member compositions to be determined and to document fluctuations of fluvial and eolian inputs on millennial and faster timescales. Our data document increases in northward Saharan dust transports during periods of strengthened atmospheric circulation in high northern latitudes. From this we derive two atmospheric scenarios which are linked with the intensity of meridional atmospheric pressure gradients in the North Atlantic region.     

Experimental investigation of the origin of majoritic garnet inclusions in diamonds

 Multianvil experiments were carried out at 10–15 GPa and 1600–1700 °C to match the compositions of majoritic garnet inclusions from diamonds, and to determine the compositions of other phases potentially coexisting with these inclusions in the source. Most experiments produced coexisting majoritic garnet, diopsidic clinopyroxene, one or more (Mg,Fe)₂SiO₄ polymorphs, and quenched carbonatic melt. The experimental garnets had relatively high Ca and Fe contents similar to the observed Ca and Fe contents of the inclusions. The resulting Si contents confirmed that the depth of origin of the inclusion with the highest Si content did not exceed 410 km, thus none of the majoritic garnet inclusions found so far originated in the transition zone (410–660 km). The evidence from inclusions and experiments is consistent with the presence of an eclogite layer occurring globally between 200 and 410 km. Compositional variations observed among more than 100 majoritic garnet inclusions with their Si content, which is a measure of pressure and depth, are consistent with the origin of the eclogite layer by crystal fractionation in a magma ocean. The compositions of olivine coexisting with majoritic garnet in the experimental products had the average Fe/(Fe + Mg) ratios between 0.16 and 0.28. Inclusions with such high Fe contents have not been found; the Fe/(Fe + Mg) ratio of the olivine inclusions in diamonds usually varies between 0.05 and 0.09. Hence, the mantle between 200 and 410 km may not contain olivine. In the absence of olivine, the discontinuity at 410 km is most likely a chemical boundary between the 200-km-thick eclogite layer and a more mafic transition zone. Received: 15 March 2001 / Accepted: 14 September 2001