Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ⁵⁷Fe are in a range of ?0.25 to 0.14‰ for olivine, ?0.17 to 0.17‰ for orthopyroxene, ?0.21 to 0.27‰ for clinopyroxene, and ?0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)_N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.     

Erratum to: Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ⁵⁷Fe are in a range of −0.25 to 0.14‰ for olivine, −0.17 to 0.17‰ for orthopyroxene, −0.21 to 0.27‰ for clinopyroxene, and −0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)_N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.     

吉林双辽地区橄榄岩包体中熔体-岩石作用特征及其对地幔交代作用的启示

吉林双辽地区古近纪玄武岩中一方辉橄榄岩包体记录了上地幔交代作用的信息。原生斜方辉石被交代成因的单斜辉石和橄榄石所围绕,或形成反应边结构,或斜方辉石残留在次生单斜辉石中。这些反应结构仅出现在尖晶石的周围。电子探针分析表明次生单斜辉石具有高Mg#、Cr#和CaO/Al2O3比值,次生橄榄石高Mg#、CaO和Cr2O3,被交代的尖晶石边部高Cr#、CaO。由于交代作用并未影响橄榄岩体系的Mg#,而且熔体-岩石反应结构指示Opx(斜方辉石) Sp(尖晶石) 熔体(Ⅰ)→Cpx(单斜辉石) Ol(橄榄石) 熔体(Ⅱ),结合实验以及文献资料,认为双辽地区的岩石圈地幔受到了硅酸盐熔体的交代。这种交代导致橄榄岩中斜方辉石逐渐减少,单斜辉石和橄榄石逐渐增加,从而使方辉橄榄岩渐变成易剥橄榄岩。这种交代现象可能发生在软流圈-岩石圈接触带上的熔-岩反应区,暗示了在古近纪双辽地区岩石圈减薄和软流圈上涌导致的软流圈-岩石圈的相互作用仍在继续。     

安徽女山橄榄岩包体的氧同位素比值和微量元素组成：地幔交代作用

1本文的研究对象是9个来自安徽女山新生代玄武岩的橄榄岩包体,利用激光氟化技术系统分析了橄榄石、单斜辉石和斜方辉石的氧同位素比值;利用LA-ICPMS技术分析了单斜辉石的微量元素组成,结果显示,女山橄榄岩存在辉石与橄榄石之间的氧同位素不平衡分馏现象,其中△Cpx-O1^18O（=δ^18OCpx-δ^18OO1））出现低至-0．5‰的负值;合角闪石包体中的单斜辉石明显富集大离子亲石元素（LILE,如Sr、Th、LREE）和亏损高场强元素（HFSE,如Nb、Zr、Ti）,氧同位素的数据表明,女山上地幔曾经经历过俯冲洋壳流体参与的交代作用,根据单斜辉石和橄榄石之间氧同位素分馏值△Cpx-O1^18O与包体形成深度之间的负相关性,提出了一个“上升＋缓冲”的流体交代模型,虽然微量元素特征也表明女山上地慢受到过地壳流体的影响,但是氧同位素的不平衡分馏和LILE的富集／HFSE的亏损并不是同步的,对这种现象的可能解释是：引起氧同位素不平衡分馏的交代作用和造成女山单斜辉石微量元素特征的交代作用不是同一期的,即女山上地幔曾经发生过至少2次交代事件.     

Oxygen isotope evidence for subduction and rift-related mantle metasomatism beneath the Colorado Plateau–Rio Grande rift transition

Spinel lherzolite and pyroxenite xenoliths from the Rio Puerco Volcanic Field, New Mexico, were analyzed for oxygen isotope ratios by laser fluorination. In lherzolites, olivine δ¹⁸O values are high (+5.5‰), whereas δ¹⁸O values for pyroxenes are low (cpx=+5.1‰; opx=+5.4‰) compared to average mantle values. Pyroxenite δ¹⁸O values (cpx=+5.0‰; opx=+5.3‰) are similar to those of the lherzolites and are also lower than typical mantle oxygen isotope compositions. Texturally and chemically primary calcite in pyroxenite xenoliths is far from isotopic equilibrium with other phases, with δ¹⁸O values of +21‰. The isotopic characteristics of the pyroxenite xenoliths are consistent with a petrogenetic origin from mixing of lherzolitic mantle with slab-derived silicate and carbonatite melts. The anomalously low δ¹⁸O in the pyroxenes reflects metasomatism by a silicate melt from subducted altered oceanic crust, and high δ¹⁸O calcite is interpreted to have crystallized from a high δ¹⁸O carbonatitic melt derived from subducted ophicarbonate. Similar isotopic signatures of metasomatism are seen throughout the Rio Puerco xenolith suite and at Kilbourne Hole in the southern Rio Grande rift. The discrete metasomatic components likely originated from the subducted Farallon slab but were not mobilized until heating associated with Rio Grande rifting occurred. Oxygen diffusion modeling requires that metasomatism leading to the isotopic disequilibrium between calcite and pyroxene in the pyroxenites occurred immediately prior to entrainment. Melt infiltration into spinel-facies mantle (xenoliths) prior to eruption was thus likely connected to garnet-facies melting that resulted in eruption of the host alkali basalt.     

Multiple metasomatism in Sulu ultrahigh-P garnet peridotite constrained by petrological and geochemical investigations

The pre‐pilot hole (PP1) of the Chinese Continental Scientific Drilling Project (CCSD) recovered drill core samples from a 118 m‐thick section of peridotites located at Zhimafang in the southern Sulu UHP terrane, China. The peridotites consist of phlogopite‐bearing garnet lherzolite, harzburgite, wehrlite and dunite. Some peridotite layers contain magnesite and Ti‐clinohumite, and are characterized by LREE and LILE enrichment and HFSE depletion. Phlogopite (Phl) occurs in the peridotite matrix and is LILE‐enriched with low Zr/Hf ratios (0.19–0.60). Phlogopite shows a mantle signature in H and O isotopes (δ¹⁸O: +5.4‰ to +5.9‰, and δD: ?76‰ to ?91‰). Ti‐clinohumite (Ti‐Chu) is Nb and Ta‐enriched and has higher Ti and HREE concentrations than phlogopite. Magnesite (Mgs) occurs as megacrysts, as a matrix phase, and as veins (±Phl ± Ti‐Chu), and contains low REE^total contents (<0.3 ppm) with a flat REE pattern. The δ¹⁸O values (+5.5‰ to +8.0‰) of magnesite are in the range of primary carbonatite, but the δ¹³C values (?2.4‰ to ?3.4‰) are slightly more positive than those of the mantle and of primary carbonatite. Petrochemical data indicate that the Zhimafang peridotite was subjected to three episodes of metasomatism, listed in succession from oldest to youngest: (1) crystallization of phlogopite in the mantle caused by infiltration of K‐rich hydrous fluid/melt; (2) formation of Mgs and Mgs ± Phl ± Ti‐Chu veins possibly caused by infiltration of mantle‐derived carbonatitic melt with a hydrous silicate component; and (3) replacement of magnesite, garnet and diopside by dolomite and secondary hydrous phases caused by a crust‐related, CO₂‐bearing, aqueous fluid. Stable isotopic compositions of phlogopite and magnesite indicate metasomatic agents for events (1) and (2) are from an enriched mantle. Multiple metasomatism imposed on mantle peridotite of variable composition led to significant compositional heterogeneity at all scales within the Zhimafang peridotite.     

The trapped fluid phase in upper mantle xenoliths from Victoria,Australia: implications for mantle metasomatism

Mantle-derived xenoliths of spinel lherzolite, spinel pyroxenite, garnet pyroxenite and wehrlite from Bullenmerri and Gnotuk maars, southwestern Victoria, Australia contain up to 3 vol.% of fluids trapped at high pressures. The fluid-filled cavities range in size from fluid inclusions (1–100 m) up to vugs 11/2 cm across, lined with euhedral high-pressure phases. The larger cavities form an integral part of the mosaic microstructure. Microthermometry and Raman laser microprobe analysis show that the fluids are dominantly CO₂. Small isolated inclusions may have densities 1.19 g/cm³, but most inclusions show microstructural evidence of partial decrepitation during eruption, and these have lower fluid densities. Mass-spectrometric analysis of gases released by crushing or heating shows the presence of He, N₂, Ar, H₂S, CO_s and SO₂ in small quantities; these may explain the small freezing-point depressions observed in some inclusions. Petrographic, SEM and microprobe studies show that the trapped fluids have reacted with the cavity walls (in clinopyroxene grains) to produce secondary amphiboles and carbonates. The trapped CO₂ thus represents only a small residual proportion of an original volatile phase, which has undergone at least two stages of modification — first by equilibration with spinel lherzolite to form amphibole (±mica±apatite), then by limited reaction with the walls of the fluid inclusions. The inferred original fluid was a CO₂-H₂O mixture, with significant contents of (at least) Cl and sulfur species. Generation of this fluid phase in the garnet-peridotite stability field, followed by its migration to the spinel peridotite stability field, would provide an efficient mechanism for metasomatic enrichment of the upper mantle in LIL elements. This migration could involve either a volatile flux or transport in small volumes of silicate melt that crystallize in the spinel peridotite field. These observations suggest that some portions of the subcontinental upper mantle contain large reservoirs of free fluid CO₂, which may be liberated during episodes of rifting or magmatism, to induce granulite-facies metamorphism of the lower crust.     

U-Pb age of zircon from mantle peridotite nodules in Cenozoic alkali basalts of the Vitim Plateau,Transbaikal region

The U-Pb (SHRIMP-II) age of zircons from garnet-spinel peridotite nodules in Cenozoic alkali basalts of the Vitim Plateau, Transbaikal region were determined. Most of the zircons are euhedral and subhedral prismatic crystals with an elongation of 1.5–2.0. Fragments of crystals and nearly equant crystals with rounded edges are present as well. Rounded or irregular cores are observed in some grains. None of the zircons yielded an age that would correspond to the time of basalt eruption (21–2.35 Ma or younger). The youngest dates range from 135.2 ± 2.7 Ma to 141 ± 3 Ma (Early Cretaceous). Both concordant values and the lower intersection of discordia with concordia (138.8 ± 5.7 Ma) are within this age interval. The upper intersection corresponds to 1891 ± 26 Ma. A considerable part of the concordant values are grouped within the intervals (164.6 ± 1.6)–(183.4 ± 2.0) and (264.0 ± 7.3)–(295.7 ± 0.76) Ma (Early-Middle Jurassic and Early Permian, respectively). The older concordant values fall in the interval 1462 ± 19 to 1506 ± 4 Ma (Mesoproterozoic). Proterozoic age was obtained for cores of composite zircon grains. Zircons pertaining to all age intervals are enriched in REE relative to chondrite (except La). The chondrite-normalized REE patterns are positively sloped with an increase in contents from LREE to HREE. The LREE and HREE contents and the depth of the Eu minimum tend to increase with age. In composite zircons of Proterozoic age, cores are somewhat enriched in REE. It has been suggested that crystallization of zircon as a separate phase in peridotites extremely depleted in Zr was related to a low degree of partial melting. The melt that formed in the intergranular space and that was repeatedly enriched in Zr was not extracted from the solid framework of rock and crystallized in situ under the changed thermodynamic conditions in the upper mantle. The occurrence of zircons of several age intervals in peridotites testifies to the multistage evolution of the upper mantle and recurrent partial melting under various physicochemical conditions.     

Interaction of model peridotite with H2O-KCl fluid: Experiment at 1.9 GPa and its implications for upper mantle metasomatism

Mineralogical and geochemical data suggest that chloride components play an important role in the transformation and partial melting of upper mantle peridotites. The effect of KCl on the transformation of hydrous peridotite rich in Al₂O₃, CaO, and Na₂O was examined in experiments aimed at studying interaction between model NCMAS peridotite with H₂O-KCl fluid under a pressure of 1.9 GPa, temperatures of 900–1200°C, and various initial H₂O/KCl ratios. The experimental results indicate that KCl depresses the solidus temperature of the hydrous peridotite: this temperature is <900°C at 1.9 GPa, which is more than 100°C lower than the solidus temperature (1000–1025°C) of hydrous peridotite in equilibrium with KCl-free fluid. The reason for the decrease in the melting temperature is that the interaction of KCl with silicates prevails over the effect of chloride on the water activity in the fluid. Experimental data highlight the key role of Al₂O₃ as a component controlling the whole interaction process between peridotite and H₂O-KCl fluid. Garnet, spinel, and pargasite-edenite amphibole in association with aluminous orthopyroxene are unstable in the presence of H₂O-KCl fluid at a chloride concentration in the fluid as low as approximately 2 wt % and are replaced by Cl-bearing phlogopite (0.4–1.1 wt % Cl). Interaction with H₂O-KCl fluid does not, however, affect clinopyroxene and forsterite, which are the Al poorest phases of the system. Chlorine stabilizes phlogopite at relatively high temperatures in equilibrium with melt at temperatures much higher than the solidus (>1200°C). The compositional evolution of melt generated during the melting of model peridotite in the presence of H₂O-KCl fluid is controlled, on the one hand, by the solubility of the H₂O-KCl fluid in the melt and, on the other hand, by phlogopite stability above the solidus. At temperatures below 1050°C, at which phlogopite does not actively participate in melting reactions, fluid dissolution results in SiO₂-undersaturated (35–40 wt %) and MgO-enriched (up to 45 wt %) melts containing up to 4–5 wt % K₂O and 2–3 wt % Cl. At higher temperatures, active phlogopite dissolution and, perhaps, also the separation of immiscible aqueous chloride liquid give rise to melts containing >10 wt % K₂O and 0.3–0.5 wt % Cl. Our experimental results corroborate literature data on the transformation of upper mantle peridotites into phlogopite-bearing associations and the formation of ultrapotassic and highly magnesian melts.     

兴蒙造山带橄榄岩捕掳体中石榴石次变边对地幔组成转变的启示

地幔橄榄岩捕虏体中石榴石次变边的形成过程对理解地幔的构造演化和转变具有非常重要的意义。兴蒙造山带锡林浩特地区新生代玄武岩携带的石榴石橄榄岩捕虏体中的石榴石普遍发育冠冕状次变边结构。本文通过对石榴石及其次变边进行详细的岩相学和电子探针分析,探讨石榴石次变边的成因及其揭示的岩石圈地幔经历的深部过程。根据次变边矿物组成的不同,将其分为原始的次变边（R1和R2）和交代的次变边（MR1和MR2）。原始的次变边中,新鲜的石榴石由内向外依次被放射状且矿物颗粒较细的R1和粒状且矿物颗粒较粗的R2包围,且R1通常比R2宽。R1主要组成矿物为Opx+Sp+Melt1/Pl±Cpx,R2主要组成矿物为Opx+Sp+Cpx。与R2及橄榄岩捕虏体相比,R1的斜方辉石和单斜辉石具有较高的Al₂O₃含量和较低Mg^#值及SiO₂含量。与橄榄岩捕虏体相比,R1和R2中的尖晶石均具有较低的Cr^#值和较高的Mg^#值。R1的斜长石为钙长石,熔体成分与斜长石相比具有偏高的MgO和FeO含量。计算的R1的全岩成分与新鲜的石榴石一致,是石榴石等化学分解的产物。R2的全岩成分比新鲜的石榴石具有偏高的MgO和偏低的SiO₂及Al₂O₃含量,是石榴石和橄榄石反应的产物。交代的次变边是由原始的次变边受到部分或完全的交代作用形成的。完全交代的次变边仍然保留原始次变边的双圈层结构,而未完全交代的次变边则仅在原始次变边的局部出现。交代的次变边中,矿物颗粒较细的核部（MR1）和矿物颗粒较粗的边部（MR2）主要矿物组成一致,皆为Ol+Cpx+Sp+Melt2。与原始的次变边相比,MR1和MR2中的橄榄石和单斜辉石均具有较高的Mg^#值,单斜辉石同时具有较高Ca/Al比值（>8）,尖晶石具有较高的Cr^#值和较低的Mg^#值,熔体较富SiO₂、Na₂O和K₂O含量。这些现象说明交代的次变边可能是碳酸盐熔/流体交代原始的次变边消耗斜方辉石生成橄榄石和单斜辉石形成的,这与岩相学观察到的单斜辉石中包裹斜方辉石残余体一致。此外,同一样品中R1的平衡温度略高于R2的平衡温度,且二者均高于橄榄岩的平衡温度。因此,锡林浩特地区石榴石橄榄岩至少经历了两阶段的退变质作用：第一阶段为橄榄岩自石榴石相抬升至尖晶石相,且受到地幔上涌的加热作用,导致石榴石和橄榄石进行缓慢的反应形成R2;第二阶段是在连续减压且加热的背景下,第一阶段残余的石榴石发生快速等化学分解反应,形成R1。退变质作用之后,石榴石原始的次变边又经历了碳酸盐熔/流体的交代作用形成MR1和MR2,最终被寄主玄武岩携带至地表。所以,石榴石次变边的形成记录了新生代时期兴蒙造山带经历的广泛的地幔上涌和多次的地幔隆升,以及地幔交代作用,为研究深部地幔过程提供了重要证据。华北克拉通晚中生代时期经历了强烈的岩石圈伸展运动并伴随着软流圈的上涌,这些过程同样会造成岩石圈地幔的减压和加热,从而导致石榴石相橄榄岩向尖晶石相转变,这可能也是华北克拉通岩石圈地幔转变的机制之一。

     

Review of melting experiments on carbonated eclogite and peridotite: insights into mantle metasomatism

Experimental studies on the partial melting of eclogite and peridotite provide important clues on mantle metasomatism. Here, we review results from some of the recent experiments and show that melting of carbonated eclogite and peridotite can produce carbonatitic to carbonated silicate melt, in which carbonates melt preferentially before Ti oxides and silicates. Low-degree melting results in carbonatitic melt coexisting with Ti oxides and silicates. This process also leads to the fractionation between some high-field strength elements (Nb, Ta, Zr, Hf, and HREE) and highly incompatible elements (U and Th) in the melt. When Ti oxides are nearly exhausted in eclogite, extremely high TiO₂ contents (e.g. 19 wt.%) are present in the melt with marked concentration of Nb and Ta. These results help to explain the features of carbonatitic metasomatism and the Nb–Ta spike in oceanic island basalts as identified in experimental studies. These studies also explain the reducing conditions that stabilize diamond in the deep mantle (>150 km) as well as the occurrence of diamond at different depths reported in various studies. Melting in such a reduced mantle can happen through redox reaction between diamond, pyroxene, and olivine, in which the initial liquid is a carbonated silicate melt. However, the theoretical oxygen fugacity (fO₂) in the asthenosphere is much lower than that predicted by the reaction and requires elevated fO₂, which can be caused by the addition of relatively oxidized materials from the lower mantle, deep asthenospheric material, and various recycled components. A combination of these processes generates locally oxidized domains in the deep mantle.     

Isochemical breakdown of garnet in orogenic garnet peridotite and its implication to reaction kinetics

An isochemical kelyphite (orthopyroxene+spinel+plagioclase) that has nearly the same bulk chemical composition as the precursor garnet was found within a matrix of ordinary kelyphites (orthopyroxene+clinopyroxene+spinel±amphibole) in garnet peridotites from the Czech part of the Moldanubian Zone. It was shown that the kelyphitization of garnet took place in three stages: (1) the garnet-olivine reaction, accompanied by a long-range material transfer across the reaction zone, and (2) the isochemical breakdown of garnet, essentially in a chemically-closed system, and finally, (3) an open-system hydration reaction producing a thin hydrous zone (amphibole+spinel+plagioclase), which is located between the isochemical kelyphite and relict garnet. The presence of relict garnet suggests that this breakdown reaction of the second stage did not proceed to a completion probably being hindered by the formation of the hydrous zone at the reaction front. It was found by electron back-scattered diffraction method that orthopyroxene and spinel do not show any topotaxic relationship in the first type of kelyphite; whereas they show locally topotaxic relationship in the isochemical kelyphite. The transition from the first type to the second type of kelyphite is discussed on the basis of the detailed observations in the transition zone between the two kelyphites. More widespread occurrence of isochemical kelyphite is expected to occur in orogenic peridotites as well as from xenoliths brought by volcanics.     

Garnet peridotite from Colorado Plateau ultramafic diatremes: Hydrates,carbonates, and comparative geothermometry

Crystal fragments of pyrope from diatremes of ultramafic microbreccia in the Navajo Province of the Colorado Plateau contain inclusions of olivine, pyroxene, spinel, chlorite, amphibole, chlorapatite, and dolomite. The included suite supports earlier hypotheses that hydrous phases and carbonates were primary parts of some garnet peridotite assemblages in the Plateau lithosphere. Garnets with spinel and orthopyroxene inclusions likely all were sampled at pressures less than 36 kb and perhaps as low as 15–20 kb; no evidence was found for inclusions from greater depths. Temperature estimates are 800°–900° C for garnet-clinopyroxene equilibration, but only 500°–700° C for garnetolivine equilibration. Inherent differences between geothermometry methods account for only part of the discrepancy. Pronounced Fe-Mg zoning in garnet at olivine contacts and the lack of such zoning at clinopyroxene contacts are evidence that the difference in part relates to relative reequilibration rates with cooling. The garnet-olivine temperature estimates may be the best approximations to mantle temperatures before eruption. Our data are compatible both with the hypothesis that the garnet peridotite was emplaced in the mantle by large-scale, horizontal transport in the lithosphere and with the hypothesis that rocks were sampled from Precambrian lithosphere cooled to temperatures like those along a low heat flow geotherm. Discordances between the geothermometers here and in other lherzolite localities may be keys to evaluating tectonic histories of lherzolite masses.     

Abundance and distribution of PGE and Au in the island-arc mantle: implications for sub-arc metasomatism

Ultramafic xenoliths from a veined mantle wedge beneath the Kamchatka arc have non-chondritic, fractionated chondrite-normalized platinum-group element (PGE) patterns. Depleted (e.g., low bulk-rock Al₂O₃ and CaO contents) mantle harzburgites show clear enrichment in the Pd group relative to the Ir group PGEs and, in most samples, Pt relative to Rh and Pd. These PGE signatures most likely reflect multi-stage melting which selectively concentrates Pt in Pt–Fe alloys while strongly depleting the sub-arc mantle wedge in incompatible elements. Elevated gold concentrations and enrichment of strongly incompatible enrichment (e.g., Ba and Th) in some harzburgites suggest a late-stage metasomatism by slab-derived, saline hydrous fluids. Positive Pt, Pd, and Au anomalies coupled with Ir depletions in heavily metasomatized pyroxenite xenoliths probably reflect the relative mobility of the Pd and Ir groups (especially Os) during sub-arc metasomatism which is consistent with Os systematics in arc mantle nodules. Positive correlations between Pt, Pd, and Au and various incompatible elements (Hf, U, Ta, and Sr) also suggest that both slab-derived hydrous fluids and siliceous melts were involved in the sub-arc mantle metasomatism beneath the Kamchatka arc.     

Serpentinization and infiltration metasomatism in the Trinity peridotite,Klamath province,northern California: implications for subduction zones

The Trinity peridotite was emplaced over metabasalts and metasedimentary rocks of the central metamorphic belt along the Devonian Trinity thrust zone. Three metamorphic events can be recognized in the Trinity peridotite: (1) antigorite (D= –63 to –65%.) formation related to regional underthrusting of the central metamorphic belt; (2) contact metamorphism associated with Mesozoic dioritic plutons; and (3) late-stage formation of lizardite ± brucite and chrysotile (D= –127 to –175%.) due to infiltration of meteoric waters. Abundant relict phases indicate incomplete reactions and strongly suggest that the availability of H₂O was a controlling factor during serpentinization.Antigorite (event 1) formed as a result of infiltration into the Trinity peridotite of mixed H₂O-CO₂ fluids derived from the underlying central metamorphic belt. Foliation defined by magnetite veins and shear zones within antigorite serpentinites are subparallel to the Trinity thrust. The assemblage Fo + Atg + Chl + Mag ± Tr ± Carb reflects partial hydration of peridotite at 425–570° C. Talc-rich serpentinite formed along the thrust as a result of the infiltration of silica-bearing fluids. Metasomatic mass-balance calculations based on silica solubilities and the extent of antigorite serpentinization suggest that 80–175 volumes of fluid have passed through a given volume of original peridotite at the Trinity thrust.The Trinity thrust probably represents a Devonian subduction zone. Thermodynamic calculations suggest that hydration reactions account for 30–35% of the total heat released by the cooling Trinity peridotite. By analogy, similar hydration reactions are to be expected in the overlying mantle wedge of a subduction zone which act to retard cooling of the hanging wall, just as dehydration reactions delay heating of the downgoing slab. Metasomatic zones formed in peridotite at the Trinity thrust may reflect similar metasomatic processes to those proposed to occur in the mantle wedge above a subducting slab.     

赣东北蛇绿岩地幔橄榄岩岩石成因及其地质意义

赣东北蛇绿岩作为华南少有的前寒武纪蛇绿岩长期都是华南大地构造研究中的热点。该蛇绿岩地幔橄榄岩以方辉橄榄岩为主,稀土总量为0.83×10-6~2.62×10-6,远低于原始地幔的含量,而Mg O含量高于原始地幔。稀土元素表现为LREE相对富集,微量元素表现为大离子亲石元素(LILE)富集,高场强元素Nb明显亏损的特征,这表明赣东北地幔橄榄岩具有亏损地幔源区特征,同时也有不同程度的俯冲带流体的交代特征。HREE模拟的部分熔融程度为10%~30%;同时,赣东北蛇绿岩地幔橄榄岩中同时包含Cr#60和Cr#60两种不同成因的铬尖晶石,说明其经历了MOR(mid-ocean ridge)和SSZ(supra-subduction zone)两种构造环境。结合前人资料,笔者推测赣东北蛇绿岩早先可能形成于洋中脊环境(约1060 Ma),随后在洋内俯冲作用下(约970 Ma),位于俯冲带上部的地幔橄榄岩受到了来自俯冲带的流体/熔体的交代,经历了SSZ环境的改造。     

Experimental determination of rare earth element partitioning between hydrous silicate melt,amphibole and garnet peridotite minerals at upper mantle pressures and temperatures

Partition coefficients of Ce, Sm and Tm involving garnet peridotite minerals, amphibole and hydrous silicate melt have been determined experimentally in the temperature and pressure ranges 950–1075°C and 10–25 kbar.Only several parts per million to several tens of parts per million of rare earth element (REE) can dissolve in the minerals before the crystal-liquid partition coefficients begin to vary as a function of REE content. The concentration ranges of constant partition coefficient increase with increasing temperature and are also positively correlated with the magnitude of the crystal-liquid partition coefficients. The upper concentration limits of constant partition coefficient and the value of the crystal-liquid partition coefficient for REE decrease in the order garnet > clinopyroxene > amphibole > orthopyroxene > olivine.Partition coefficients may vary by at least an order of magnitude as a function of bulk composition of the liquid phase (e.g. changing from basaltic to andesitic). The approximate ranges of the values of the partition coefficients as a function of bulk liquid composition are as follows:

$\text{Ce}\text{Sm}\text{Tm}\text{K}{}^{\mathrm{<mtext>ga-liq</mtext>}}\text{0.01–0.1}\text{0.3–3.4}\text{1–10}\text{K}{}^{\mathrm{<mtext>cpx-liq</mtext>}}\text{0.05–0.4}\text{0.09–0.7}\text{0.04–0.4}\text{K}{}^{\mathrm{<mtext>amph-liq</mtext>}}\text{0.04–0.4}\text{0.08–0.8}\text{0.07–0.7}\text{K}{}^{\mathrm{<mtext>opx-liq</mtext>}}\text{0.04–0.1}\text{0.05–0.1}\text{0.08–0.1}\text{K}{}^{\mathrm{<mtext>ol-liq</mtext>}}\text{0.01–0.02}\text{0.01–0.02}\text{0.01–0.02}$

where the values increase with increasing acidity of the melt.     

Fluid and silicate glass inclusions in ultramafic and mafic xenoliths from Hierro,Canary Islands: implications for mantle metasomatism

Fluid and solid inclusions have been studied in selected samples from a series of spinel-bearing Crdiopside-and Al-augite-series ultramafic (harzburgites, lherzolites, and olivine-clinopyroxene-rich rocks), and gabbroic xenoliths from Hierro, Canary Islands. In these samples several generations of fluid inclusions and ultramafic-and mafic-glass inclusions may be texturally related to different stages of crystal growth. The fluid inclusions consist of pure, or almost pure, CO₂. The solid inclusions in the ultramafic xenoliths comprise early inclusions of devitrified ultramafic glass, sulphide inclusions, as well as polyphase inclusions (spinel+clinopyroxene±glass±other silicates) believed to have formed from trapped basaltic melts. Vitreous basaltic glass±CO₂±sulphide±silicates are common as secondary inclusions in the ultramafic xenoliths, and as primary inclusions in the gabbroic xenoliths. Microthermometry gives minimum trapping temperatures of 1110° C for the early ultramafic-and mafic-glass inclusions, and a maximum of 1260–1280° C for late inclusions of host basaltic glass. In most samples the CO₂ inclusions show a wide range in homogenization temperatures (-40 to +31° C) as a result of decrepitation during ascent. The lowest homogenization temperatures of about-40° C, recorded in some of the smallest CO₂ inclusions, indicate a minimum depth of origin of 35 km (12 kbar) for both the Cr-diopside-and Al-augite-series xenoliths. The gabbroic xenoliths originate from a former magma chamber at a depth of 6–12 km.Contribution no. 100 of the Norwegian programme of the International Lithosphere Project     

The origin of reaction textures in mantle peridotite xenoliths from Sal Island, Cape Verde: the case for “metasomatism” by the host lava

Reaction zones around minerals in mantle xenoliths have been reported from many localities worldwide. Interpretations of the origins of these textures fall into two groups: mantle metasomatic reaction or reaction during transport of the xenoliths to the surface. A suite of harzburgitic mantle xenoliths from Sal, Cape Verde show clear evidence of reaction during transport. The reactions resulted in the formation of olivine–clinopyroxene and Si- and alkali-rich glass reaction zones around orthopyroxene and sieve-textured clinopyroxene and sieve textured spinel, both of which are associated with a Si- and alkali-rich glass similar to that in the orthopyroxene reaction zones. Reaction occurred at pressures less than the mantle equilibration pressure and at temperatures close to the liquidus temperature of the host magma. In addition, there is a clear spatial relation of reaction with the host lava: reaction is most intense near the lava/xenolith contact. The residence time of the xenoliths in the host magma, determined from Fe–Mg interdiffusion profiles in olivine, was approximately 4 years. Our results cannot be reconciled with a recent model for the evolution of the mantle below the Cape Verde Archipelago involving mantle metasomatism by kimberlitic melt. We contend that alkali-rich glasses in the Sal xenoliths are not remnants of a kimberlitic melt, but rather they are the result of reaction between the host lava or a similar magma and xenolith minerals, in particular orthopyroxene. The formation of a Si- and alkali-rich glass by host magma–orthopyroxene reaction appears to be a necessary precursor to formation of sieve textured spinel and clinopyroxene.