Effect of water on tholeiitic basalt phase equilibria: an experimental study under oxidizing conditions

To investigate the effect of water on phase relations and compositions in a basaltic system, we performed crystallization experiments at pressures of 100, 200 and 500 MPa in a temperature range of 940 to 1,220°C using four different water contents. Depending on the water activity, the oxygen fugacity varied between 1 and 4 log units above the quartz-magnetite-fayalite buffer. Addition of water to the dry system shifts the solidus > 250°C to lower temperatures and increases the amount of melt drastically. For instance, at 1,100°C and 200 MPa, the melt fraction increases from 12.5 wt% at a water content of 1.6 wt% to 96.3% at a water content of 5 wt% in the melt. The compositions of the experimental phases also show a strong effect of water. Plagioclase is shifted to higher anorthite contents by the addition of water. Olivine and clinopyroxene show generally higher MgO/FeO ratios with added water, which could also be related to the increasing oxygen fugacity with water. Moreover, water affects the partitioning of certain elements between minerals and melts, e.g., the Ca partitioning between olivine and melt. Plagioclase shows a characteristic change in the order of crystallization with water that may help to explain the formation of wehrlites intruding the lower oceanic crust (e.g., in Oman, Macquarie Island). At 100 MPa, plagioclase crystallizes before clinopyroxene at all water contents. At pressures > 100 MPa, plagioclase crystallizes before clinopyroxene at low water contents (e.g. < 3 wt%), but after clinopyroxene at H₂O in the melt > 3 wt%. This change in crystallization order indicates that a paragenesis typical for wehrlites (olivine–clinopyroxene–without plagioclase) is stabilized at low pressures typical of the oceanic crust only at high water contents. This opens the possibility that typical wehrlites in the oceanic crust can be formed by the fractionation and accumulation of olivine and clinopyroxene at 1,060°C and > 100 MPa in a primitive tholeiitic basaltic system containing more than 3 wt% water. The comparison of the experimental results with evolution trends calculated by the thermodynamic models “MELTS” and “Comagmat” shows that neither model predicts the experimental phase relations with sufficient accuracy.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Rheology of the upper mantle: Inferences from peridotite xenoliths

Stress estimates as a function of depth are obtained for peridotite xenoliths from the upper mantle of three types of tectonic environments by applying revised recrystallizedgrain-size paleopiezometry and pyroxene thermobarometry. The general increase in grain size with depth and hence decrease in deviatoric stress, observed previously, is confirmed but reversals in these trends are now established and remain enigmatic. Stresses and temperatures obtained are combined with a representative creep-flow law to calculate strainrate and viscosity profiles that appear to be physically reasonable. Profiles for the highthermal-gradient rift/ridge environments show a complexity that is interpreted as.a rheological discontinuity resulting from the emplacement of asthenospheric diapirs during late stages of continental rifting. Profiles for broad continental extension zones (C.E.Z.), believed to be most representative of oceanic upper mantle, fluctuate between 50 and 80 km, with a general small increase in strain rate and decrease in viscosity with depth; deepest samples apparently come from the base of the lithosphere. Profiles for the infracratonic mantle of southern Africa show nearly a uniform increase in strain rate to values greater than 10⁻¹⁴/sec, and a decrease in viscosity to lower than 10²¹ poise, at a depth of 230 km. These profiles may transect the mechanically defined lithosphere—asthenosphere transition at about 200 km and, if so, there is no evidence for a mechanical discontinuity at the boundary. This observation, coupled with evidence that the sense of shear is homogeneous for all mantle profiles constructed, clearly favors a model whereby lithospheric plates are dragged by thermal convection of the asthenosphere below. Sea-floor spreading rates and relative plate-velocity estimates are consistent with this interpretation but do not independently permit a definitive choice between the two favored models advanced to explain the driving force for plate motions.     

Melt migration in the upper mantle along the Romanche Fracture Zone (Equatorial Atlantic)

Textural and petrological data of mantle peridotites sampled in the central and western parts of the Romanche Fracture Zone (Equatorial Atlantic) during the oceanographic expedition PRIMAR-96 (Russian R/V Gelendzhik) are presented. The studied rocks are mantle peridotites carrying patches, pockets and veins/dikes of magmatic origin, interpreted to be the product of various extents of magma impregnation on mantle partial melting residues. Estimated partial melting degrees based on clinopyroxene Ti/Zr ratios are in the ranges 5–13% and 18–20%. In highly impregnated samples, refertilization of residual peridotite minerals precludes a correct evaluation of the degree of melting. Magmatic products occur as pl±cpx±opx±ol±sp aggregates with various textural features. Interstitial pl-rich patches and gabbroic pockets are interpreted to derive from magma migration through the upper mantle by diffusive porous flow in the ductile part of the lithosphere and melt–rock reactions. Metasomatism of the host peridotites is testified by Ti and Cr increase in spinel and Ti, Sr, Zr, Y and LREE increase in clinopyroxene. Veins and dikes reflect channeled magma migration focused by brittle failures at shallower lithospheric levels. Minor or no chemical changes occurred in peridotites impregnated along fractures. The compositions of magmatic minerals in impregnated peridotites are consistent with derivation from variably fractionated melts of probably MORB type. Barometric estimates suggest that the Romanche peridotites were impregnated at minimum depths of ca. 9–12 km. Thermometric estimates for the peridotite hosts are in the range 750–1050 °C. The spread in temperature values is partly ascribed to localized heating by migrating melts of relatively cold peridotites. Our data and the occurrence of both fertile and depleted peridotites in a neighbouring area along the western Romanche FZ are in accord with the hypothesis of small-scale (<100 km) mantle heterogeneity along this fracture zone.     

Continuation of the San Andreas fault system into the upper mantle: Evidence from spinel peridotite xenoliths in the Coyote Lake basalt, central California

The Coyote Lake basalt, located near the intersection of the Hayward and Calaveras faults in central California, contains spinel peridotite xenoliths from the mantle beneath the San Andreas fault system. Six upper mantle xenoliths were studied in detail by a combination of petrologic techniques. Temperature estimates, obtained from three two-pyroxene geothermometers and the Al-in-orthopyroxene geothermometer, indicate that the xenoliths equilibrated at 970–1100 °C. A thermal model was used to estimate the corresponding depth of equilibration for these xenoliths, resulting in depths between 38 and 43 km. The lattice preferred orientation of olivine measured in five of the xenolith samples show strong point distributions of olivine crystallographic axes suggesting that fabrics formed under high-temperature conditions. Calculated seismic anisotropy values indicate an average shear wave anisotropy of 6%, higher than the anisotropy calculated from xenoliths from other tectonic environments. Using this value, the anisotropic layer responsible for fault-parallel shear wave splitting in central California is less than 100 km thick. The strong fabric preserved in the xenoliths suggests that a mantle shear zone exists below the Calaveras fault to a depth of at least 40 km, and combining xenolith petrofabrics with shear wave splitting studies helps distinguish between different models for deformation at depth beneath the San Andrea fault system.     

Formation of plagioclase-bearing peridotite and plagioclase-bearing wehrlite and gabbro suite through reactive crystallization: an experimental study

Plagioclase-bearing peridotites are commonly associated with gabbroic rocks sampled around the Moho Transition Zone. Based on mineral chemistry, texture, and spatial relations, the formation of plagioclase-bearing peridotites has been attributed to impregnation of basalt into residual peridotites. We conducted reactive dissolution and crystallization experiments to test this hypothesis by reacting a primitive mid-ocean ridge basalt with a melt-impregnated lherzolite at 1,300 °C and 1 GPa and then cooling to 1,050 °C as pressure decreased to 0.7 GPa. Crystallization during cooling produced lithologic sequences of gabbro–wehrlite or gabbro–wehrlite–peridotite, depending on reaction time. Wehrlitic and peridotitic sections contain significant amounts of plagioclase interstitial to olivine and clinopyroxene and plagioclase compositions are spatially homogeneous. Clinopyroxene in the wehrlite–peridotite section is reprecipitated from the melt and exhibits poikilitic texture with small rounded olivine chadacrysts. Mineral composition in olivine and clinopyroxene varies spatially, both at the scale of the sample and within individual grains. Olivine grains that crystallized close to the melt–peridotite interface are enriched in iron due to their proximity to the basaltic melt reservoir. Consistent with many field studies, we observed gradual spatial variation in olivine and clinopyroxene composition across a lithologically sharp boundary between the gabbro and wehrlite–peridotite. Plagioclase compositions show no obvious dependence on distance from the melt–rock interface and were precipitated from late-stage trapped melts. Compositional trends of olivine, pyroxene, and plagioclase are consistent with previous experimental results and natural observations of the Moho Transition Zone. Different lithological sequences form based primarily on the melt–rock ratio, composition of the melt and host peridotite, and thermochemical conditions, but are expected to grade from gabbro to wehrlite or troctolite to peridotite. Plagioclase-bearing peridotite represents the low melt–rock ratio end member where pyroxene is only partially replaced by olivine and melt, whereas dunite is expected to form where melts overwhelm and consume all other phases. This study confirms that under nominally anhydrous conditions, the gabbro–wehrlite–plagioclase-peridotite sequence can be formed by reaction between basalt and lherzolite and subsequent crystallization at intermediate to low pressures. Melt–rock reaction is a fundamental process in the formation of new crust at the shallowest part of the melting column where pyroxene-undersaturated melts percolate through depleted peridotite.     

Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa

Laboratory experiments on natural, hydrous basalts at 1–4 GPa constrain the composition of “unadulterated” partial melts of eclogitized oceanic crust within downgoing lithospheric slabs in subduction zones. We complement the “slab melting” experiments with another set of experiments in which these same “adakite” melts are allowed to infiltrate and react with an overlying layer of peridotite, simulating melt:rock reaction at the slab–mantle wedge interface. In subduction zones, the effects of reaction between slab-derived, adakite melts and peridotitic mantle conceivably range from hybridization of the melt, to modal or cryptic metasomatism of the sub-arc mantle, depending upon the “effective” melt:rock ratio. In experiments at 3.8 GPa, assimilation of either fertile or depleted peridotite by slab melts at a melt:rock ratio 2:1 produces Mg-rich, high-silica liquids in reactions which form pyrope-rich garnet and low-Mg# orthopyroxene, and fully consume olivine. Analysis of both the pristine and hybridized slab melts for a range of trace elements indicates that, although abundances of most trace elements in the melt increase during assimilation (because melt is consumed), trace element ratios remain relatively constant. In their compositional range, the experimental liquids closely resemble adakite lavas in island-arc and continental margin settings, and adakite veins and melt inclusions in metasomatized peridotite xenoliths from the sub-arc mantle. At slightly lower melt:rock ratios (1:1), slab melts are fully consumed, along with peridotitic olivine, in modal metasomatic reactions that form sodic amphibole and high-Mg# orthopyroxene.     

Partitioning of iron and magnesium between crystals and partial melts in peridotite upper mantle

The iron-magnesium distribution coefficient, $$K'_D = (X_{\Sigma {\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{\Sigma {\text{FeO}}} )^{{\text{liquid}}} ,$$ has frequently been used as a means of testing whether experimental and natural silicate liquids could have been in equilibrium with olivine of mantle composition. It is shown here that this K′ _D decreases with increasing oxygen fugacity (xxx) for a hydrous partial melt in equilibrium with a natural spinel peridotite assemblage under pressure and temperature conditions corresponding to those of the upper mantle (from 0.52 at the xxx of the iron-wüstite buffer to 0.04 at the xxx of the magnetite-hematite buffer). K′ _D also increases with increasing pressure, with decreasing temperature, and probably with increasing Mg/(Mg+∑ Fe) of the parental peridotite, suggesting that $$K_D = (X_{{\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{{\text{FeO}}} )^{{\text{liquid}}}$$ also increases with increasing pressure and decreasing temperature. Thus, unless these four variables (P, T, xxx, silicate composition) are known for a natural magma, K′ _D and probably K _D are variables, and the Mg/(Mg+∑ Fe) of such a magma cannot be correlated to that of the parent. The K _D determined at 1 atm pressure by Roeder and Emslie has frequently been used to test whether the Mg/(Mg+∑ Fe) ratios of experimentally formed liquids at high pressure in equilibrium with olivine of known Fo content represent the equilibrium Mg/(Mg+Fe²⁺) of this liquid, assuming that ∑Fe=Fe²⁺ and that K′ _D does not vary with P, T, and composition of the system. Published data demonstrate that the oxygen fugacities of the experimental designs employed by different laboratories vary between those of the magnetite-hematite and magnetite-wüstite buffers (6 orders of magnitude), resulting in K′ _D between 0.04 and 0.31 at 1050° C and 15 kbar, for example. Thus, published arguments as to whether the quenched liquids represent equilibrium compositions based on iron-magnesium partitioning are inadequate. The effects of P, T, xxx, and the composition of the starting material must also be considered.     

Melt migration in the mantle beneath spreading zones and formation of replacive dunites: a review

The state of the art of the problem of mantle melt transport through the upper mantle beneath spreading zones is reviewed. The geochemical data and physical properties of the mantle constraining the melt migration processes are considered. The review concentrates on the ways of mantle magma transport and mechanisms of the localization of diffuse intergranular porous melt percolation into the channel flow. The hypothesis of the reaction of migrating mantle magmas with wall peridotites and the formation of replacive dunites as a result of this process are discussed. We examine the publications on the field, theoretical, and geochemical studies of the reactive melt migration and its role in the dunite formation in the mantle sections of the ophiolites. Alternative models of the dunite origin in the mantle are also tested. The results of experimental studies of the melt/peridotite interaction are presented.     

Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence

This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and dunites crop out at Mt. Arpone and Mt. Musinè. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that <1–5% fractional melting of spinel-facies DMM produced the injected melts. Plagioclase peridotites are hybrid rocks resulting from pre-existing spinel peridotites and variable enrichment of plagioclase and micro-gabbroic material by percolating melts. The impregnating melts attained silica-saturation, as testified by widespread orthopyroxene replacement of olivine, during open system migration in the lithosphere. At Mt. Musinè, coarse granular spinel harzburgite and dunite bodies replace the plagioclase peridotites. Most of these replacive, refractory peridotites have interstitial magmatic clinopyroxene with trace element compositions in equilibrium with MORB, while some Cpx have REE-depleted patterns suggesting transient geochemical features of the migrating MORB-type melts, acquired by interaction with the ambient plagioclase peridotite. These replacive spinel harzburgite and dunite bodies are interpreted as channels exploited for focused and reactive migration of silica-undersaturated melts with aggregate MORB compositions. Such melts were unrelated to the silica-saturated melts that refertilized the pre-existing plagioclase peridotites. Finally, MORB melt migration occurred along open fractures, now recorded as gabbroic dikes.

Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.     

CO2-CO fluid inclusions in a composite peridotite xenolith: implications for upper mantle oxygen fugacity

Fluid inclusions occur in a composite xenolith from the Lunar Crater Volcanic Field, Nevada, U.S.A. The xenolith is an amphibole-bearing wehrlite that is cut by an andesine-amphibole vein. The compositions of individual fluid inclusions in both portions of the xenolith have been determined using microthermometry and micro Laser-Raman spectroscopy. Fluids in the host wehrlite are nearly pure CO₂ (>99 mol%) whereas those in the vein contain from 8.5 to 12.0 mol % CO in CO₂. Chemical modelling shows that the composition of the vein fluids at T _room is representative of the composition at the high P, T conditions of trapping. Graphite has not been observed by optical microscopy in any of the fluid inclusions. Graphite is probably absent (although stable at T<800° C) most probably because of the kinetically unfavorable CO decomposition reaction and rapid quenching. By combining the measured fluid compositions with fluid P-V-T data and the chemical equilibrium CO₂CO +1/2 O₂, we have calculated the oxygen fugacity of the fluid inclusions at 1200° C: log 8.6 (vein) and –6 (host). If the of the fluid in the vein represents that in equilibrium with the magma that crystallized to produce the vein, then the of the basalt magma is near QFM at 1200° C and 10.3 kbar. This is similar to values reported for extrusive basaltic lavas. If the much lower intrinsic oxygen fugacity-values for divines and spinels from alkali basalt nodules are representative of upper mantle conditions, then oxidation of basaltic magmas must occur in the upper mantle prior to ascent to the surface. Implications for the origin of CO₂-rich fluids and carbon isotope geochemistry are also discussed.     

Melt migration and melt-rock reaction in the Alpine-Apennine peridotites:Insights on mantle dynamics in extending lithosphere

The compositional variability of the lithospheric mantle at extensional settings is largely caused by the reactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments.The Alpine-Apennine(A-A)ophiolites are predominantly constituted by mantle peridotites and are widely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition and slow-to ultraslow-spreading settings.Structural and geochemical studies on the A-A mantle peridotites have revealed that they preserve significant compositional and isotopic heterogeneity at variable scale,reflecting a long-lived multi-stage melt migration,intrusion and melt-rock interaction history,occurred at different lithospheric depths during progressive uplift.The A-A mantle peridotites thus constitute a unique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreading environments.In this work,we review field,microstructural and chemical-isotopic evidence on the major stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discuss their consequences in creating chemical-isotopic heterogeneities at variable scales and enhancing weakening and deformation of the extending mantle.Focus will be on three most important stages:(i)old(pre-Jurassic)pyroxenite emplacement,and the significant isotopic modification induced in the host mantle by pyroxenite-derived melts,(ii)melt-peridotite interactions during Jurassic mantle exhumation,i.e.the open-system reactive porous flow at spinel facies depths causing bulk depletion(origin of reactive harzburgites and dunites),and the shallower melt impregnation which originated plagioclase-rich peridotites and an overall mantle refertilization.We infer that migrating melts largely originated as shallow,variably depleted,melt fractions,and acquired Si-rich composition by reactive dissolution of mantle pyroxenes during upward migration.Such melt-rock reaction processes share significant similarities with those documented in modern oceanic peridotites from slow-to ultraslow-spreading environments and track the progressive exhumation of large mantle sectors at shallow depths in oceanic settings where a thicker thermal boundary layer exists,as a consequence of slow-spreading rate.     

Effect of melt composition on basalt and peridotite interaction: laboratory dissolution experiments with applications to mineral compositional variations in mantle xenoliths from the North China Craton

Interaction between basaltic melts and peridotites has played an important role in modifying the lithospheric and asthenospheric mantle during magma genesis in a number of tectonic settings. Compositions of basaltic melts vary considerably and may play an important role in controlling the kinetics of melt–peridotite interaction. To better understand the effect of melt composition on melt–peridotite interaction, we conducted spinel lherzolite dissolution experiments at 2 GPa and 1,425 °C using the dissolution couple method. The reacting melts include a basaltic andesite, a ferro-basalt, and an alkali basalt. Dissolution of lherzolite in the basaltic andesite and the ferro-basalt produced harzburgite–lherzolite sequences with a thin orthopyroxenite layer at the melt–harzburgite interface, whereas dissolution of lherzolite in the alkali basalt produced a dunite–harzburgite–lherzolite sequence. Systematic variations in mineral compositions across the lithological units are observed. These mineral compositional variations are attributed to grain-scale processes that involve dissolution, precipitation, and reprecipitation and depend strongly on reacting melt composition. Comparison of mineral compositional variations across the dissolution couples with those observed in mantle xenoliths from the North China Craton (NCC) helps to assess the spatial and temporal variations in the extent of siliceous melt and peridotite interaction in modifying the lithospheric mantle beneath the NCC. We found that such melt–rock interaction mainly took place in Early Cretaceous, and is responsible for the enrichment of pyroxene in the lithospheric mantle. Spatially, siliceous melt–peridotite interaction took place in the ancient orogens with thickened lower crust.     

Fluid inclusions in upper mantle xenoliths from Northern Patagonia,Argentina: Evidence for an upper mantle diapir

Summary Three generations of fluid inclusions can be recognized in upper mantle xenoliths from alkali basalts of the Somoncura Massif, Northern Patagonia, Argentina. The first (early, primary) one consists of dense CO₂ inclusions which were trapped in the mantle-crust boundary zone (22–36 km minimum trapping depth). Their co-genetic relationship with silicate melt inclusions enables us to constrain their minimum trapping temperature at 1200°C, indicating a high temperature event in a cooler environment. The late (pseudosecondary and secondary) generations of fluid inclusions were classified in accordance with their homogenization temperature to liquid CO₂ (L1) and vapor CO₂ (L2) phase. The minimum trapping depth for the first of the late inclusions (L1) is about 16 km. In spite of the uncertainties related to this value, L1 inclusions indicate that the upper mantle rocks, of which samples were delivered by the basalts, had some residence time in the middle crust where they experienced a metasomatic event. The fact that this event did not destroy the earlier inclusions, places severe constraints on its duration. The second late inclusions (L2) are low-pressure CO₂ inclusions with a minimum trapping depth of only 2 km, presumably a shallow magma chamber of the host basalts. The succession of fluid inclusions strongly points toward a fairly fast uprising upper mantle underneath Northern Patagonia. The petrology and mineral chemistry of the peridotitic xenoliths support this view. Extensive partial melting and loss of these melts is indicated by the preponderance of harzburgites in the upper mantle underneath Northern Patagonia, a fairly unusual feature for a continental upper mantle. That depletion event as well as several metasomatic events — including those which left traces of fluid inclusions — are possibly related to a high-speed diapiric uprise of the upper mantle in this area. The path can be traced from the garnet peridotite stability field into the middle crust, a journey which must have been unusually fast. Differences in rock, mineral, and fluid inclusion properties between geographic locations suggest a diffuse and differential type of diapirism. Future studies will hopefully help to map the full extent and the highs and lows of this diapir and elucidate questions related to its origin and future.

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Fluid-Einschlüsse in Erdmantel-Xenolithen von Nord-Patagonien: Evidenz für einen Diapir im oberen Erdmantel

Zusammenfassung Erdmantel - Xenolithe in Alkali-Basalten des Somoncure Massivs, Nord-Patagonien, Argentinien, führen drei Generationen von Fluid-Einschlüssen. Die erste (frühe, primäre) Generation besteht aus dichten CO₂-Einschlüssen, welche offenbar in der Mantel-Kruste Grenzzone (22–36 km Minimum-Tiefe) eingeschlossen wurden. CO₂-Einschlüsse sind kogenetisch mit Silikat-Schmelzeinschlüssen. Dies erlaubt die Abschätzung der Einschließ-Temperatur mit minimal 1200°C, was auf ein Hochtemperatur-Ereignis in einer deutlich kühleren Umgebung hinweist. Die späten (pseudosekundäre und sekundäre) CO₂- Fluid-Einschlüsse bilden zwei Generationen von denen die eine in die flüssige (L1), die andere in die Dampfphase (L2) homogenisieren. Die minimale Einschließ-Tiefe für die L1 Generation ist etwa 16 km. Dies bedeutet - auch bei Berücksichtigung der mit diesem Wert verbundenen Ungenauigkeit - daß diese Erdmantel-Gesteine einige Zeit in der mittleren Erdkruste verbrachten und ein metasomatisches Ereignis erlebten, bevor sie von den Basalten zur Erdoberfläche gebracht wurden. Die Tatsache, daß dieses Ereignis die frühen Einschlüsse nicht zerstörte, kann nur bedeuten, daß es von kurzer Dauer war. Die L2-Generation besteht aus Niedrigdruck CO₂-Einschlüssen mit einer Minimum-Einschließtiefe von nur 2 km. Dies könnte in einer seichten Magmakammer des Wirt Basaltes geschehen sein.Die Abfolge von Fluid-Einschlüssen deutet auf einen relativ schnell aufsteigenden oberen Erdmantel unterhalb von Patagonien hin. Die Petrologie und Mineralchemie der peridotitischen Xenolithe unterstützen das. Die Vorherrschaft von Harzburgiten im Erdmantel unterhalb von Nord-Patagonien deutet auf umfangreiche Bildung partieller Schmelzen und deren Abfuhr hin — eine für einen kontinentalen Mantel ungewöhnliche Situation. Sowohl die Verarmungsereignisse, als auch die metasomatischen Veränderungen (einschließlich jene, welche Spuren in Form von Fluid Einschlüssen hinterließen) machen das Vorhandensein eines schnell aufsteigenden Daipirs im oberen Erdmantel dieser Gegend wahrscheinlich. Der Aufstieg kann vom Stabilitätsbereich der Granat-Peridotite bis in die mittlere Kruste verfolgt werden und muß daher relativ schnell erfolgt sein. Unterschiede in Gesteins-, Mineral und Fluid-Eigenschaften zwischen verschiedenen Lokalitäten legen einen diffusen und differenziellen Diapirismus nahe. Zukünftige Studien sollten es ermöglichen, das Gesamtausmaß und die unterschiedlichen Aufstiegshöhen des Diapirs zu kartieren und Hinweise auf seine Entstehung und zukünftige Entwicklung zu erhalten.

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With 5 Figures     

班公湖-怒江缝合带东段丁青地幔橄榄岩成因: 来自钻孔ZK02地幔橄榄岩矿物学及地球化学特征约束

丁青蛇绿岩位于班公湖-怒江缝合带东段,是该缝合带出露面积最大的蛇绿岩。为查明岩体成因,在丁青东岩体中实施了一口165.19m的钻孔。除最顶部有约0.5m厚的第四系残坡积物外,其余均为地幔橄榄岩。结合显微镜鉴定将岩心划分出17个岩性单元层,岩性主要以方辉橄榄岩为主,夹少量纯橄岩和含铬铁矿纯橄岩。地幔橄榄岩中橄榄石的Fo变化于88.79~93.73,铬尖晶石的Cr^#变化于44.33~81.66,揭示丁青地幔橄榄岩可能经历过约20%~40%的中高度部分熔融作用;全岩地球化学分析表明其具有富镁（MgO=45.98%~49.45%）、贫铝（Al₂O₃=0.19%~1.37%）和贫钙（CaO=0.28%~0.70%）的特点,属于熔融程度较高的地幔残余物质。岩石具有明显不同于阿尔卑斯蛇绿岩的轻稀土元素富集特征,指示区内地幔橄榄岩先经历了较强程度的部分熔融,后经历了俯冲消减过程中的流体交代。利用地幔橄榄岩中的铬尖晶石成分计算母熔体Al₂O₃含量对应的FeO/MgO值,与不同构造环境原始岩浆成分相比较,发现丁青地幔橄榄岩母熔体大多处于玻安岩中。纯橄岩氧逸度估算FMQ=-3.05~-0.71,方辉橄榄岩氧逸度FMQ=-3.89~+1.47,显示丁青地幔橄榄岩有俯冲作用的参与。通过丁青钻孔岩心的研究,提出丁青东岩体可能形成于俯冲带之上的弧前环境这一观点。

     

Global trench migration velocities and slab migration induced upper mantle volume fluxes: Constraints to find an Earth reference frame based on minimizing viscous dissipation

Since the advent of plate tectonics different global reference frames have been used to describe the motion of plates and trenches. The difference in plate motion and trench migration between different reference frames can be substantial (up to 4 cm/yr). This study presents an overview of trench migration velocities for all the mature and incipient subduction zones on Earth as calculated in eight different global reference frames. Calculations show that, irrespective of the reference frame: (1) trench retreat always dominates over trench advance, with 62–78% of the 244 trench segments retreating; (2) the mean and median trench velocity are always positive (retreating) and within the range 1.3–1.5 cm/yr and 0.9–1.3 cm/yr, respectively; (3) rapid trench retreat is only observed close to lateral slab edges (< 1500 km); and (4) trench retreat is always slow far from slab edges (> 2000 km). These calculations are predicted by geodynamic models with a varying slab width, in which plate motion, trench motion and mantle flow result from subduction of dense slabs, suggesting that trench motion is indeed primarily driven by slab buoyancy forces and that proximity to a lateral slab edge exerts a dominant control on the trench migration velocity. Despite these four general conclusions, significant differences in velocities between such reference frames remain. It is therefore important to determine which reference frame most likely describes the true absolute velocities to get an understanding of the forces driving plate tectonics and mantle convection. It is here proposed that, based on fluid dynamic considerations and predictions from geodynamic modelling, the best candidate is the one, which optimises the number of trench segments that retreat, minimizes the trench–perpendicular trench migration velocity (v_T) in the centre of wide (> 4000 km) subduction zones, maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge, minimizes the average of the absolute of the trench–perpendicular trench migration velocity (|v_T|) for all subduction zones on Earth, and minimizes the global upper mantle toroidal volume flux (_To) that results from trench migration and associated lateral slab migration (i.e. slab rollback or slab roll-forward). Calculations show that these conditions are best met in one particular Indo-Atlantic hotspot reference frame, where 75% of the subduction zones retreat, v_T in the centre of wide subduction zones ranges between − 3.5 and 1.8 cm/yr, 83% of the trench segments located within 2000 km of the closest lateral slab edge retreat, the average of |v_T| is 2.1 cm/yr, and _To = 456 km³/yr (lower limit) and 539 km³/yr (upper limit). Inclusion of all the incipient subduction zones on Earth results in slightly greater fluxes of 465 km³/yr (lower limit) and 569 km³/yr (upper limit). It is also found that this reference frame is close to minimizing the total sub-lithospheric upper mantle volume flux (_K) associated with motion of continental keels located below the major cratons. It is stressed, however, that _K is an order of magnitude smaller than _To, and thus of subordinate importance. In conclusion, the Indo-Atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities.     

Depleted and enriched matter in the upper mantle of Spitsbergen: Evidence from the study of mantle xenoliths

Doklady Earth Sciences -     

Amphibole—Bearing Peridotite Xenoliths from Nushan,Anhui Province：Evidence for Melt Percolation Process in the Upper mantle and Lithospheric uplift

The spinel peridotite xenoliths of Group I in Quaternary basanites from Nushan,Anhui province,can be classified as two suites:a hydrous suite characterized by the ubiquitous occurrence of (Ti-) pargasite and an anhydrous suite.The nineral chemistry reveals that the anhydrous suite and one associated phlogopite-bearing lherzolite are equilibrated under temperature conditions of 1000-1100℃,whereas amplhibole-bearing peridotites display distinct disequilibrum features,indicating partial reequilibration from 1050 to 850℃ and locally down to 750℃. The amplhbole-bearing peridotites were probably the uppermost part of the high temperature anhydrous suite which was modally modifed by fractionating H2O-rich metasomatic agent during regional upwelling.This relatively recent lithospheric uplift event followed an older uplift event recognized from pyroxene unmixing of domains in local equilibrium,as well as the dominant deformation texture in the anhydrous suite.The first thermal disturbance can be linked with the regional extension and widespread basaltic volcanism in Jiangsu-Anhui provinces since Early Tertiary and the formation of the nearby Subei(North Jiangsu) fault-depression basin during the Eocene,while the second event in association with the formation of amphiboles probably indicates the continuation but diminution of upwared mantle flux since Neogene in response to the change in tectonic regime for eastern china.     

Occurrence,chemistry, and origin of immiscible silicate glasses in a tholeiitic basalt: A TEM/AEM study

The occurrence and chemistry of immiscible silicate glasses in a tholeiite mesostasis from the Umtanum formation, Washington, were investigated with transmission electron microscopy and analytical electron microscopy (TEM/AEM). TEM observation reveals isolated, dark globules (2.1 micron or less in diameter) randomly distributed in a transparent matrix glass interstitial to plagioclase laths. The globules less than 0.3 micron and larger than 0.8 micron fall beyond the linear relationship defined by the 0.3–0.8 micron globules in a plot of the logarithm of number versus size. Large globules (0.7 micron or larger in diameter) range from homogeneous to heterogeneous in optical properties and chemistry. Homogeneous globules are completely glassy, whereas heterogeneous globules contain crystalline domains. AEM analyses show that the globules have high Si, Fe, Ca, and Ti with subordinate Mg, Al, P, S, Cl, K, and Mn, which gives high normative fa, px, il, and ap. The matrix glass consists dominantly of Si with low Al and minor Na and K, yielding a high normative qz, or, ab, and an.It is proposed that the silicate liquid immiscibility occurs by reaction of network-modifying cations (NMCs) with dominantly chain-like anionic units in the parental melt to form less polymerized, NMC-bearing units and highly polymerized, Si-rich units. The globules nucleated metastably under supercooled conditions, and medium-size globules become either larger or smaller at lower temperatures. Internal nucleation of NMC-rich phases occurred in some larger globules upon cooling.     

Composition of the Siberian cratonic mantle: evidence from Udachnaya peridotite xenoliths

Bulk compositions and mineral analyses for forty-one, large, garnet- and spinel-facies peridotite xenoliths from the Udachnaya kimberlite in the central Siberian platform have many similarities to those of well-studied peridotites from the Kaapvaal craton in southern Africa. Coarse Mg-rich lherzolites and harzburgites with equilibration temperatures below 1000 °C are abundant and are believed to form the principal rock type in the Siberian lithosphere. The low-temperature Udachnaya peridotites have an average mg number [Mg/(Mg+Fe)] of 92.6 with a wide dispersion in modal enstatite, ranging to over 40 wt%. High-temperature peridotites are relatively richer in Fe and Ti and are commonly deformed, with porphyroclastic or mosaic-porphyroclastic textures, some of the latter having fluidized enstatite. The Udachnaya peridotites have experienced late-stage metasomatism before, during and after eruption. Garnets and pyroxenes in many of the high-temperature rocks are zoned, probably by reaction with melt prior to eruption. Virtually all the peridotites contain secondary diopside, inhomogeneous on a micron scale, that mantles primary orthopyroxene. It is believed to have crystallized along with lesser amounts of intergranular calcite and monticellite during eruption. Bulk analyses for total Fe in many specimens are higher than whole-rock Fe calculated from the electron probe analyses and the modes. The magnitude of the difference between the two measurements of total Fe correlates with loss-on-ignition, suggesting that Fe has been introduced during serpentinization following eruption. These late metasomatic processes have thus affected some major as well minor and trace element compositions. The similarities in bulk composition of peridotites from Udachnaya and the Kaapvaal are evidence of a common origin. Low-temperature cratonic peridotites differ from oceanic peridotites in having higher mg numbers (>92) and in having relatively high but wide-ranging modal enstatite (Mg/Si = 1.06–1.49 weight fraction). The Udachnaya low-temperature peridotites have an inverse correlation between FeO (calculated from the probe analyses and modes) and SiO₂. This correlation is also present in the Kaapvaal data but is complicated by a greater range in fertility that produces a positive variation of Fe with Si. A negative trend for Fe/Si can be seen within a portion of the Kaapvaal data, that for low-Ca harzburgites, in which the variation in fertility is restricted. The negative trends for Fe/Si can be interpreted as a consequence of either segregation of olivine and orthopyroxene by metamorphic differentiation or partial sorting during cumulate formation. Received: 18 June 1996  / Accepted: 11 February 1997