The evolution of continental roots in numerical thermo-chemical mantle convection models including differentiation by partial melting

Incorporating upper mantle differentiation through decompression melting in a numerical mantle convection model, we demonstrate that a compositionally distinct root consisting of depleted peridotite can grow and remain stable during a long period of secular cooling. Our modeling results show that in a hot convecting mantle partial melting will produce a compositional layering in a relatively short time of about 50 Ma. Due to secular cooling mantle differentiation finally stops before 1 Ga. The resulting continental root remains stable on a billion year time scale due to the combined effects of its intrinsically lower density and temperature-dependent rheology. Two different parameterizations of the melting phase-diagram are used in the models. The results indicate that during the Archaean melting occurred on a significant scale in the deep regions of the upper mantle, at pressures in excess of 15 GPa. The compositional depths of continental roots extend to 400 km depending on the potential temperature and the type of phase-diagram parameterization used in the model. The results reveal a strong correlation between lateral variations of temperature and the thickness of the continental root. This shows that cold regions in cratons are stabilized by a thick depleted root.     

Electrical conductivity of hydrous basaltic melts: implications for partial melting in the upper mantle

The Earth’s uppermost asthenosphere is generally associated with low seismic wave velocity and high electrical conductivity. The electrical conductivity anomalies observed from magnetotelluric studies have been attributed to the hydration of mantle minerals, traces of carbonatite melt, or silicate melts. We report the electrical conductivity of both H₂O-bearing (0–6 wt% H₂O) and CO₂-bearing (0.5 wt% CO₂) basaltic melts at 2 GPa and 1,473–1,923 K measured using impedance spectroscopy in a piston-cylinder apparatus. CO₂ hardly affects conductivity at such a concentration level. The effect of water on the conductivity of basaltic melt is markedly larger than inferred from previous measurements on silicate melts of different composition. The conductivity of basaltic melts with more than 6 wt% of water approaches the values for carbonatites. Our data are reproduced within a factor of 1.1 by the equation log σ = 2.172 − (860.82 − 204.46 w ^0.5)/(T − 1146.8), where σ is the electrical conductivity in S/m, T is the temperature in K, and w is the H₂O content in wt%. We show that in a mantle with 125 ppm water and for a bulk water partition coefficient of 0.006 between minerals and melt, 2 vol% of melt will account for the observed electrical conductivity in the seismic low-velocity zone. However, for plausible higher water contents, stronger water partitioning into the melt or melt segregation in tube-like structures, even less than 1 vol% of hydrous melt, may be sufficient to produce the observed conductivity. We also show that ~1 vol% of hydrous melts are likely to be stable in the low-velocity zone, if the uncertainties in mantle water contents, in water partition coefficients, and in the effect of water on the melting point of peridotite are properly considered.     

Conditions for the origin of oxidized carbonate-silicate melts: Implications for mantle metasomatism and diamond formation

An experimental study on the origin of ferric and ferrous carbonate-silicate melts, which can be considered as the potential metasomatic oxidizing agents and diamond forming media, was performed in the (Ca,Mg)CO₃-SiO₂-Al₂O₃-(Mg,Fe)(Cr,Fe,Ti)O₃ system, at 6.3 GPa and 1350–1650 °C. At 1350–1450 °C and ?O₂ of FMQ + 2 log units, carbonate–silicate melt, coexisting with Fe^3 +-bearing ilmenite, pyrope-almandine and rutile, contained up to 13 wt.% of Fe₂O₃. An increase in the degree of partial melting was accompanied by decarbonation and melt enrichment with CO₂, up to 21 wt.%. At 1550–1650 °C excess CO₂ segregated as a separate fluid phase. The restricted solubility of CO₂ in the melt indicated that investigated system did not achieve the second critical point at 6.3 GPa. At 1350–1450 °C and ?O₂ close to CCO buffer, Fe^2 +-bearing carbonate–silicate melt was formed in association with pyrope-almandine and Fe^3 +-bearing rutile. It was experimentally shown that CO₂-rich ferrous carbonate-silicate melt can be an effective waterless medium for the diamond crystallization. It provides relatively high diamond growth rates (3–5 μm/h) at P,T-conditions, corresponding to the formation of most natural diamonds.     

On mantle drag force for the formation of a next supercontinent as estimated from a numerical simulation model of global mantle convection

Three‐dimensional spherical mantle convection was simulated to predict future continental motion and investigate the driving force of continental motion. Results show that both the time required (≥300 Ma from the present) and the process for the next supercontinent formation are sensitive to the choice of critical rheological parameters for mantle dynamics, such as a viscosity contrast between the upper and lower mantles and a yield strength of the lithosphere. From all the numerical models studied herein, mantle drag force by horizontal mantle flow beneath the continents may mostly act as a resistance force for the continental motion in the process of forming a new supercontinent. The maximum absolute magnitude of the tensional and compressional stress acting at the base of the moving continents is in the order of 100 MPa, which is comparable to a typical value of the slab pull force.     

On the iron isotope heterogeneity of lithospheric mantle xenoliths: implications for mantle metasomatism, the origin of basalts and the iron isotope composition of the Earth

With the aim to better understand the cause of the iron isotope heterogeneity of mantle-derived bulk peridotites, we compared the petrological, geochemical and iron isotope composition of four xenolith suites from different geodynamic settings; sub-arc mantle (Patagonia); subcontinental lithospheric mantle (Cameroon), oceanic mantle (Kerguelen) and cratonic mantle (South Africa). Although correlations were not easy to obtain and remain scattered because these rocks record successive geological events, those found between δ⁵⁷Fe, Mg#, some major and trace element contents of rocks and minerals highlight the processes responsible for the Fe isotope heterogeneity. While partial melting processes only account for moderate Fe isotope variations in the mantle (<0.2 ‰, with bulk rock values yielding a range of δ⁵⁷Fe ± 0.1 ‰ relative to IRMM-14), the main cause of Fe isotope heterogeneity is metasomatism (>0.9 ‰). The kinetic nature of rapid metasomatic exchanges between low viscosity melts/fluids and their wall-rocks peridotite in the mantle is the likely explanation for this large range. There are a variety of responses of Fe isotope signatures depending on the nature of the metasomatic processes, allowing for a more detailed study of metasomatism in the mantle with Fe isotopes. The current database on the iron isotope composition of peridotite xenoliths and mafic eruptive rocks highlights that most basalts have their main source deeper than the lithospheric mantle. Finally, it is concluded that due to a complex geological history, Fe isotope compositions of mantle xenoliths are too scattered to define a mean isotopic composition with enough accuracy to assess whether the bulk silicate Earth has a mean δ⁵⁷Fe that is chondritic, or if it is ~0.1 ‰ above chondrites as initially proposed.     

Fluid-bearing alkaline carbonate melts as the medium for the formation of diamonds in the Earth''s mantle: an experimental study

We have experimentally studied the formation of diamonds in alkaline carbonate–carbon and carbonate–fluid–carbon systems at 5.7–7.0 GPa and 1150–1700 °C, using a split-sphere multi-anvil apparatus (BARS). The starting carbonate and fluid-generating materials were placed into Pt and Au ampoules. The main specific feature of the studied systems is a long period of induction, which precedes the nucleation and growth of diamonds. The period of induction considerably increases with decreasing P and T, but decreases when adding a C–O–H fluid to the system. In the range of P and T corresponding to the formation of diamonds in nature, this period lasts for tens of hours. The reactivity of the studied systems with respect to the diamond nucleation and growth decreases in this sequence: Na₂CO₃–H₂C₂O₄·2H₂O–C>K₂CO₃–H₂C₂O₄·2H₂O–C>>Na₂CO₃–C>K₂CO₃–C. The diamond morphology is independent of P and T, and is mainly governed by the composition of the crystallization medium. The stable growth form is a cubo-octahedron in the Na₂CO₃ melt, and an octahedron in the K₂CO₃ melt. Regardless of the composition of the carbonate melt, only octahedral diamond crystals formed in the presence of the C–O–H fluid. The growth rates of diamond varied in the range from 1.7 μm/h at 1420 °C to 0.1–0.01 μm/h at 1150 °C, and were used to estimate, for the first time, the possible duration of the crystallization of natural diamonds. From the analysis of the experimental results and the petrological evidence for the formation of diamonds in nature, we suggest that fluid-bearing alkaline carbonate melts are, most likely, the medium for the nucleation and growth of diamonds in the Earth's upper mantle.     

Peralkaline silicic melts of island arcs, active continental margins, and intraplate continental settings: Evidence from the investigation of melt inclusions in minerals and quenched glasses of rocks

Based on the analysis of data on the composition of melt inclusions in minerals and quenched glasses of igneous rocks, we considered the problems of the formation of peralkaline silicic magmas (i.e., whose agpaitic index, the molar ratio AI = (Na₂O + K₂O)/Al₂O₃, is higher than one). The mean compositions of peralkaline silicic melts are reported for island arcs and active continental margins and compared with the compositions of melts from other settings, primarily, intraplate continental areas. Peralkaline silicic rocks are rather common in the latter. Such rocks are rare in island arcs and active continental margins, but agpaitic melts were observed in inclusions in phenocrysts of plagioclase, quartz, pyroxene, and other minerals. Plagioclase fractionation from an alkali-rich melt with AI < 1 is considered as a possible mechanism for the formation of peralkaline silicic melts (Bowen’s plagioclase effect). However, the analysis of available experimental data on plagioclase-melt equilibria showed that natural peralkaline melts are almost never in equilibrium with plagioclase. For the same reason, the melting of the majority of crustal rocks, which usually contain plagioclase, does not produce peralkaline melts. The existence of peralkaline silicic melt inclusions in plagioclase phenocrysts suggests that plagioclase can crystallize from peralkaline melts, and the plagioclase effect may play a certain role. Another mechanism for the formation of peralkaline silicic magmas is the melting of alkali-rich basic and intermediate rocks, including the spilitized varieties of subalkali basalts.     

Melting of hydrous pyroxenites with alkali amphiboles in the continental mantle: 2. Trace element compositions of melts and minerals

The trace element compositions of melts and minerals from high-pressure experiments on hydrous pyroxenites containing K-richterite are presented. The experiments used mixtures of a third each of the natural minerals clinopyroxene, phlogopite and K-richterite, some with the addition of 5% of an accessory phase ilmenite, rutile or apatite. Although the major element compositions of melts resemble natural lamproites, the trace element contents of most trace elements from the three-mineral mixture are much lower than in lamproites. Apatite is required in the source to provide high abundances of the rare earth elements, and either rutile and/or ilmenite is required to provide the high field strength elements Ti, Nb, Ta, Zr and Hf. Phlogopite controls the high levels of Rb, Cs and Ba.Since abundances of trace elements in the various starting mixtures vary strongly because of the use of natural minerals, we calculated mineral/melt partition coefficients (D_Min/melt) using mineral modes and melting reactions and present trace element patterns for different degrees of partial melting of hydrous pyroxenites. Rb, Cs and Ba are compatible in phlogopite and the partition coefficient ratio phlogopite/K-richterite is high for Ba (1 3 6) and Rb (12). All melts have low contents of most of the first row transition elements, particularly Ni and Cu ((0.1–0.01) × primitive mantle). Nickel has high D_Min/melt for all the major minerals (12 for K-richterite, 9.2 for phlogopite and 5.6 for Cpx) and so behaves at least as compatibly as in melting of peridotites. Fluorine/chlorine ratios in melts are high and D_Min/melt for fluorine decreases in the order apatite (2.2) > phlogopite (1.5) > K-richterite (0.87). The requirement for apatite and at least one Ti-oxide in the source of natural lamproites holds for mica pyroxenites that lack K-richterite. The results are used to model isotopic ageing in hydrous pyroxenite source rocks: phlogopite controls Sr isotopes, so that lamproites with relatively low ⁸⁷Sr/⁸⁶Sr must come from phlogopite-poor source rocks, probably dominated by Cpx and K-richterite. At high pressures (>4 GPa), peritectic Cpx holds back Na, explaining the high K₂O/Na₂O of lamproites.     

Melting of hydrous pyroxenites with alkali amphiboles in the continental mantle: 1. Melting relations and major element compositions of melts

Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite + K-richterite, some including 5% of accessory phases, have been conducted at 15 and 50 kbar. The assemblages represent probable source components that contribute to melts in cratonic regions, but whose melt compositions are poorly known. A main series of starting compositions based on MARID xenoliths consisted of a third each of clinopyroxene (CPX), phlogopite (PHL) and K-richterite (KR) with or without 5% ilmenite, rutile or apatite. Additional experiments were run without KR and with higher proportions of accessory phases. Melt traps were used at near-solidus temperatures to facilitate accurate analysis of well-quenched melts, for which reversal experiments demonstrate equilibrium.Results show that KR melts rapidly and completely within 50 °C of the solidus, so that melts reflect the composition of the amphibole and its melting reaction. Melts have high SiO₂ and especially K₂O but low CaO and Al₂O₃ relative to basaltic melts produced from peridotites at similar pressures. They have no counterparts amongst natural rocks, but most closely resemble leucite lamproites at 15 kbar. KR and PHL melt incongruently to form olivine (OL) and CPX at 15 kbar, promoting SiO₂ contents of the melt, whereas orthopyroxene OPX is increasingly stable at lower lithosphere pressures, leading to an increase in MgO and decrease in SiO₂ in melts, which resemble olivine lamproites. Melts of mica pyroxenites without KR are richer in CaO and Al₂O₃ and do not resemble lamproites. These experiments show that low CaO and Al₂O₃ in igneous rocks is not necessarily a sign of a depleted peridotite source. Accessory phases produce melts exceptionally rich in P₂O₅ or TiO₂ depending on the phases present and are unlike any melts seen at the Earth’s surface, but may be important agents of metasomatism seen in xenoliths. The addition of the 5% accessory phases ilmenite, rutile or apatite result in melting temperatures a few ten of degrees lower; at least two of these appear essential to explain the compositions of many alkaline igneous rocks on cratons.Melting temperatures for CPX + PHL + KR mixtures are close to cratonic geotherms at depths > 130 km: minor perturbations of the stable geotherm at >150 km will rapidly lead to 20% melting. Melts of hydrous pyroxenites with a variety of accessory phases will be common initial melts at depth, but will change if reaction with wall-rocks occurs, leading to volcanism that contains chemical components of peridotite even though the temperature in the source region remains well below the melting point of peridotite. At higher temperatures, extensive melting of peridotite will dilute the initial alkaline melts: this is recognizable as alkaline components in basalts and, in extreme cases, alkali picrites. Hydrous pyroxenites are, therefore, components of most mantle-derived igneous rocks: basaltic rocks should not be oversimplified as being purely melts of peridotite or of mixtures of peridotite and dry pyroxenite without hydrous phases.     

Possible mantle phase transitions by the formation of SiO<Subscript>2</Subscript> peroxides: Implications for mantle convection

On the basis of quantum–chemical calculations of the linear to isomeric bent transition of the SiO₂ molecule, it is suggested that the bent to linear transition of SiO₂ forms can occur in melted mantle minerals of the lower mantle. This may be important for the formation of the peculiarities of mantle convection and origination of plumes.     

华北东部大陆地幔橄榄岩组成、年龄与岩石圈减薄

对比分析了华北东部地块陆下岩石圈地幔橄榄石Mg#值和单斜辉石的REE配分形式。报道了汉诺坝和鹤壁橄榄岩中不同产状硫化物的激光MC-ICPMS原位Re-Os年龄和信阳橄榄岩中锆石的U-Pb年龄和信阳橄榄岩锆石的U-Pb年龄。在这些资料基础上,进一步讨论了华北东部岩石圈中、新生代时的减薄机制。原位分析在揭示岩石圈深部过程的细节上,有比全岩分析更大的优越性,并揭示出了在华北深部有中元古代(14亿年)和新元古代(7～8亿年)热活动的记录。岩石圈拆沉作用不能很好地解释古老难熔地幔、过渡型地幔和新生饱满地幔并存的事实;同时,单纯的熔体-橄榄岩相互作用也难以解释中、新生代岩石圈的减薄过程和新生地幔单斜辉石中出现强烈LREE亏损现象,即历史复杂的克拉通岩石圈向历史明显简单的“大洋型”地幔的转换。因此,华北东部岩石圈减薄包括地幔伸展、熔-岩作用、侵蚀置换等复杂过程。这些过程可能包括:(1)早中生代时,扬子地块向北俯冲碰撞所引起华北岩石圈的熔/流体交代富集作用、地幔伸展和受扰动软流圈物质上涌并侵蚀被改造了的岩石圈;(2)晚中生代—古近纪,因太平洋俯冲的热扰动致使软流圈物质进一步的强烈侵蚀作用引起岩石圈的巨大减薄;(3)晚第三纪以来的软流圈热沉降作用所带来的小幅度岩石圈增厚过程。岩石圈先大幅减薄、后小幅增厚实现了最终的地幔置换和岩石圈整体的减薄过程。喷发时代为100Ma的阜新玄武岩所捕获的橄榄岩主体是饱满的,说明华北东部部分地区在此之前曾有过地幔置换作用。     

Deep mantle roots and continental emergence: implications for whole-Earth elemental cycling,long-term climate,and the Cambrian explosion

Elevations on Earth are dominantly controlled by crustal buoyancy, primarily through variations in crustal thickness: continents ride higher than ocean basins because they are underlain by thicker crust. Mountain building, where crust is magmatically or tectonically thickened, is thus key to making continents. However, most of the continents have long passed their mountain building origins, having since subsided back to near sea level. The elevations of the old, stable continents are lower than that expected for their crustal thicknesses, requiring a subcrustal component of negative buoyancy that develops after mountain building. While initial subsidence is driven by crustal erosion, thermal relaxation through growth of a cold thermal boundary layer provides the negative buoyancy that causes continents to subside further. The maximum thickness of this thermal boundary layer is controlled by the thickness of a chemically and rheologically distinct continental mantle root, formed during large-scale mantle melting billions of years ago. The final resting elevation of a stabilized continent is controlled by the thickness of this thermal boundary layer and the temperature of the Earth’s mantle, such that continents ride higher in a cooler mantle and lower in a hot mantle. Constrained by the thermal history of the Earth, continents are predicted to have been mostly below sea level for most of Earth’s history, with areas of land being confined to narrow strips of active mountain building. Large-scale emergence of stable continents occurred late in Earth’s history (Neoproterozoic) over a 100–300 million year transition, irreversibly altering the surface of the Earth in terms of weathering, climate, biogeochemical cycling and the evolution of life. Climate during the transition would be expected to be unstable, swinging back and forth between icehouse and greenhouse states as higher order fluctuations in mantle dynamics would cause the Earth to fluctuate rapidly between water and terrestrial worlds.     

On the implications of the coupled evolution of the deep planetary interior and the presence of surface ocean water in hydrous mantle convection

We investigate the influence of the deep mantle water cycle incorporating dehydration reactions with subduction fluxes and degassing events on the thermal evolution of the Earth as a consequence of core–mantle thermal coupling. Since, in our numerical modeling, the mantle can have ocean masses ∼12 times larger than the present-day surface ocean, it seems that more than 13 ocean masses of water are at the maximum required within the planetary system overall to partition one ocean mass at the surface of the present-day Earth. This is caused by effects of water-dependent viscosity, which works at cooling down the mantle temperature significantly so that the water can be absorbed into the mantle transition zone and the uppermost lower mantle. This is a result similar to that without the effects of the thermal evolution of the Earth's core (Nakagawa et al., 2018). For the core's evolution, it seems to be expected for a partially molten state in the deep mantle over 2 billion years. Hence, the metal–silicate partitioning of hydrogen might have occurred at least 2 billion years ago. This suggests that the hydrogen generated from the phase transformation of hydrous-silicate-hosted water may have contributed to the partitioning of hydrogen into the metallic core, but it is still quite uncertain because the partitioning mechanism of hydrogen in metal–silicate partitioning is still controversial. In spite of many uncertainties for water circulation in the deep mantle, through this modeling investigation, it is possible to integrate the co-evolution of the deep planetary interior within that of the surface environment.     

A rare composite xenolith from Salt Lake Crater,Oahu: high-pressure fractionation and implications for kimberlitic melts in the Hawaiian mantle

A composite xenolith of olivine-bearing garnet clinopyroxenite wall rock intruded by two spinels + garnet veins is described. Vein minerals exhibit textural evidence of a reaction relationship with the mineral phases in the wall rock. Wall rock clinopyroxene contains exsolved blebby garnet and very fine lamellar exsolution of orthopyroxene, indicating that this xenolith had undergone considerable subsolidus cooling. Garnet-clinopyroxene thermometry suggests that the xenolith last equilibrated in the mantle at a temperature of about 1,060 (ᆭ °C). The spinels in the veins are of two kinds: pleonaste (that occurs with vein garnet) and a high-Mg, high-Al titanomagnetite (MAT spinels). Intriguingly, the MAT spinels are chemically very similar to the spinels found as groundmass in kimberlites, are moderately subhedral to euhedral, have a weakly developed cumulate texture, and, at places, show a reaction relation with the pleonaste + garnet (cumulate?) assemblage in the vein. Based on petrographic, chemical, and phase equilibrium considerations, we propose the following evolutionary history of this composite xenolith. (1) In the first stage the olivine-bearing garnet clinopyroxenite formed as crystal extracts (cumulates) as a result of high pressure fractionation of an alkaline melt in the deepest levels of Hawaiian lithosphere/uppermost asthenosphere (100-110 km). (2) In the second stage, igneous veining (the melt composition of this vein is not precisely known but could be kimberlitic) occurs in the already existing wall rock resulting in the precipitation of pleonaste + garnet. A reaction relation between the igneous veins and the wall rock also characterizes this stage. (3) The last igneous episode in this xenolith is recorded by MAT spinels in the wall rock and their precipitation close to the previous pleonaste + garnet veins. The last igneous stage could well be due again to high pressure fractionation of a kimberlitic melt (the residual melt after precipitation of pleonaste + garnet). The time relationship between exsolution and the later igneous veining stages is not known. The MAT spinels are not a result of sub-solidus solvus processes as partial reaction (melt present) between the pleonaste + garnet (from the second igneous stage) and MAT spinel exists, pointing to the igneous nature of the MAT spinel. Based on striking similarity between the MAT spinels in our xenolith and those found as groundmass in kimberlites, we propose that the veining stages could well have been kimberlitic. Thus, even though kimberlitic melts are not seen on the Koolau shield, this particular xenolith clearly shows the existence of such melts at great depths beneath Hawaii. We also propose that the initial wall rock, which represents crystal extracts (even though it does not exhibit definitive cumulate texture) as a result of high-pressure fractionation of an alkaline melt and subsequent veining episodes, are of pre-Koolau age. This implies that the Koolau shield volcano may have had a pre-shield alkalic stage.     

Mechanisms of orthopyroxene dissolution in silica-undersaturated melts at 1 atmosphere and implications for the origin of silica-rich glass in mantle xenoliths

Experiments dissolving orthopyroxene (En₉₃) in a variety of Si-undersaturated alkaline melts at 1 atmosphere and variable f _O2 demonstrate that orthopyroxene dissolves to form olivine, Si-rich melt and clinopyroxene. These phases form a texturally and chemically distinct boundary layer around the partly dissolved orthopyroxene crystals. The occurrence of clinopyroxene in the boundary layer is due to inward diffusion of Ca from the solvent melt to the boundary layer causing clinopyroxene saturation. Compositional profiles through the solvent and the boundary layer for a number of experiments demonstrate rapid diffusion of cations across the boundary layer – solvent interface. SiO₂ diffuses outward from the boundary layer whereas CaO and Al₂O₃ diffuse toward the Si-enriched boundary layer melt. The rate of Al diffusion is slower under reducing conditions compared to the rates in experiments performed in air. Concentrations of FeO and MgO in the boundary layer and solvent are approximately equal indicating rapid diffusion and attainment of equilibrium despite ongoing crystallisation of clinopyroxene within the boundary layer. The behaviour of Na₂O and K₂O is strongly affected by f _O2. Under reducing conditions Na₂O and K₂O concentrations are approximately equal in the boundary layer and solvent indicating normal diffusion down the concentration gradient and attainment of equilibrium. Under oxidising conditions, K₂O and to a lesser extent Na₂O, have compositional profiles indicative of uphill diffusion likely due to their preference for more polymerised Si- and Al-rich melts. Under reduced conditions Al-enrichment in the boundary layer melt is not as extreme and uphill diffusion did not occur. The composition of the solvent melt after the experiments indicates that it was contaminated by the boundary layer by convective mixing due to the onset of hydrodynamic instabilities brought on by density and viscosity contrasts between the two melts. Despite using a wide variety of solvent melt compositions we find that the boundary layer melts converge toward a common composition at high SiO₂ contents. The composition of glass generated by orthopyroxene dissolution at 1 atmosphere is similar in many respects to Si-rich glass found in many orthopyroxene-rich mantle xenoliths that have been attributed to high pressure in situ processes including mantle metasomatism. The results of this study suggest that at least some Si-rich melts are likely to have formed by dissolution of xenolith orthopyroxene at low pressure possibly by their Si-undersaturated host magmas. Received: 30 August 1996 / Accepted: 15 April 1998     

Geophysical images of the creeping segment of the San Andreas fault: implications for the role of crustal fluids in the earthquake process

High-resolution magnetotelluric (MT) studies of the San Andreas fault (SAF) near Hollister, CA have imaged a zone of high fluid content flanking the San Andreas fault and extending to midcrustal depths. This zone, extending northeastward to the Calaveras fault, is imaged as several focused regions of high conductivity, believed to be the expression of tectonically bound fluid pockets separated by northeast dipping, impermeable fault seals. Furthermore, the spatial relationship between this zone and local seismicity suggests that where present, fluids inhibit seismicity within the upper crust (0–4 km). The correlation of coincident seismic and electromagnetic tomography models is used to sharply delineate geologic and tectonic boundaries. These studies show that the San Andreas fault plane is vertical below 2 km depth, bounding the southwest edge of the imaged fault-zone conductor (FZC). Thus, in the region of study, the San Andreas fault acts both as a conduit for along-strike fluid flow and a barrier for fluid flow across the fault. Combined with previous work, these results suggest that the geologic setting of the San Andreas fault gives rise to the observed distribution of fluids in and surrounding the fault, as well as the observed along-strike variation in seismicity.     

Apatite in the mantle: implications for metasomatic processes and high heat production in Phanerozoic mantle

The abundance of apatite in Phanerozoic mantle may be greatly underestimated. This study shows that apatite has a widespread occurrence in Phanerozoic lithospheric mantle and can be divided into two geochemically distinct types using halogen content, presence or absence of structural CO₂, Sr and trace element (especially U, Th, and light rare earth) ratios and abundances, and association with either metasomatised mantle wall-rock peridotites (Apatite A) or high-pressure magmatic crystallisation products (Apatite B). Apatite A is inferred to result from metasomatism by CO₂- and H₂O-rich fluids derived from a primitive mantle source region, while Apatite B compositions are consistent with crystallisation from magmas within the carbonate–silicate compositional spectrum.

The presence of significant apatite in the lithospheric mantle is important not only for the geochemical budget but also for assessing heat production and heat flow in the mantle. The measured U and Th contents of mantle apatite average 60 and 200 ppm, respectively and 0.5% apatite would dominate heat production. Metasomatised mantle may also contain amphibole and mica with K₂O and clinopyroxene with detectable U and Th. In lithospheric mantle with a thickness of 70 km, this abundance of apatite would result in mantle heat flow contribution of about 12 mW/m², a significant proportion of the total “normal” mantle heat flow of about 18 mW/m².     

Deep array electromagnetic sounding on the Baltic Shield: External excitation model and implications for upper mantle conductivity studies

The BEAR array of simultaneous electromagnetic (EM) observations probes the deep crustal and upper mantle conductivity structure of the Baltic Shield searching for the lithosphere–asthenosphere boundary beneath. The adequate interpretation of the results of this unique high latitude natural field EM sounding requires proper understanding of the actual external excitation conditions because conventionally used plane wave model assumptions may be substantially violated in the vicinity of inhomogeneous polar sources. The paper presents an overview of the morphology and statistics of source distortions in the BEAR EM field transfer functions (TF) and the ways of their suppression. The stability of the final TF estimates obtained with the exclusion of intensive non-stationary auroral effects is further justified. The external excitation model effective for the whole BEAR observation period is inferred from the array distribution of the inter-station geomagnetic transfer functions. The model is supported by the results of polar ionosphere–magnetosphere current system studies, based on the simultaneous ground and satellite geomagnetic observations, and sets bounds for the “plane wave” approach in the BEAR data interpretation to avoid unfounded inferences on the upper mantle electrical properties. The signatures of the lithosphere–asthenospere boundary under Fennoscandia derived from the BEAR data are summarized and its resolution within the traditional plane wave interpretational paradigm is analysed assuming the presented external source pattern and estimated TF uncertainties caused by the source inhomogeneity.