Towards a Better Understanding of the Fibrolite Problem: the Effect of Reaction Overstepping and Surface Energy Anisotropy

Trachytes from the Euganean Hills District (Italy) contain metapeliticxenoliths that have been pyrometamorphosed during incorporationin the melt. In xenoliths containing sillimanite crystallizedduring a previous regional HT/LP metamorphism, fibrolite systematicallynucleates at the grain boundaries of sillimanite prisms andwithin plagioclase crystals. Ternary feldspar thermometry showsthat plagioclase in contact with sillimanite plots along the750°C solvus that reflects near-equilibrium conditions ofregional metamorphism. Plagioclase containing fibrolite plotscloser towards the 950°C solvus, reflecting the tendencyof plagioclase to re-equilibrate at high temperature duringpyrometamorphism by a fibrolite-forming reaction:     

湘南火山岩深源包体组合及岩石圈岩石学模型

湘南早白垩世（１３２～１３５）Ｍａ基性火山岩中含有橄榄岩类、辉长岩类、角闪麻粒岩相酸性岩包体，这三类包体的岩石学特征及地球物理参数（ｖｐ，ρ），分别与岩石圈地球物理模型中的上地幔，下地壳及中地壳相吻合。包体的岩相学、主要元素、ＲＥＥ及稀有元素地球化学特征及同位素年龄的研究结果表明，现在的岩石圈中的下地壳部分是元古代（１１４１Ｍａ±６７Ｍａ）由底板垫托垂直增生形成的；中地壳与世界范围内的中地壳和中国华北及扬子克拉通相比偏酸性，而下地壳则偏基性，以富Ａｌ２Ｏ３为其特征。这一富Ａｌ２Ｏ３的下地壳包体除了在美国亚利桑那以外，在世界范围内还未曾见过报道。     

内蒙古阿巴嘎旗新生代玄武岩中超镁铁岩包体的特征

内蒙古东部阿巴嘎旗及其以北地区第四纪更新世钠质碱性火山岩中发现的超镁铁岩包体,主要为尖晶石二辉橄榄岩、方辉橄榄岩、纯橄榄岩、橄榄二辉岩、尖晶石单斜辉石岩和单斜辉石巨晶以及辉长岩、花岗闪长岩、石英长石片麻岩等壳源包体。超镁铁岩包体和巨晶的岩相学特征显示它们不是寄主玄武岩岩浆分离结晶堆积体;其主要元素、稀土元素特征与阿尔卑斯型橄榄岩相似,其组成接近于原始地幔岩值;同位素地球化学特征表明它们与寄主岩-玄武岩浆不是同源的;尖晶石二辉橄榄岩包体形成的平衡温度和压力范围分别为1056.9℃～1103.1℃,压力为1.50～1.84GPa,形成相当深度为49.5～60.72km。辉石岩包体形成温度为887.1℃～958.6℃,压力为1.25～1.4GPa,形成相当深度为41.25～46.2km。寄主玄武岩形成温度1390℃～1450℃,压力1.75～2.20GPa,形成相当深度为60～70km。可以认为,研究区在新生代发生了软流圈地幔上涌,导致岩石圈减薄以及地壳的拉张,大量岩浆物质沿构造通道溢出地表。其中超镁铁岩包体与寄主玄武岩浆之间没有熔体与残余物这种成因联系,它们是寄主玄武岩浆在上升过程中捕获的岩石圈地幔碎块。     

Fe-rich Dunite Xenoliths from South African Kimberlites: Cumulates from Karoo Flood Basalts

Fe-rich dunite xenoliths within the Kimberley kimberlites compriseolivine neoblasts with minor elongated, parallel-oriented ilmenite,and rarely olivine porphyroclasts and spinel. Compared withtypical mantle peridotites, olivines in the Fe-rich duniteshave lower forsterite (Fo_87–89) and NiO contents (1300–2800ppm), which precludes a restitic origin for the dunites. Chrome-richspinels are remnants of a metasomatic reaction that producedilmenite and phlogopite. Trace element compositions differ betweenporphyroclastic and neoblastic olivine, the latter having higherTi, V, Cr and Ni and lower Zn, Zr and Nb contents, documentingtheir different origins. The dunites have high ¹⁸⁷Os/ ¹⁸⁸Osratios (0·11–0·15) that result in youngmodel ages for most samples, whereas three samples show isotopicmixtures between Phanerozoic neoblasts and ancient porphyroclasticmaterial. Most Fe-rich dunite xenoliths are interpreted to berecrystallized cumulates related to fractional crystallizationof Jurassic Karoo flood basalt magmatism, whereas the porphyroclastsare interpreted to be remnants from a much earlier (probablyArchaean Ventersdorp) magmatic episode. The calculated parentalmagma for the most primitive olivine neoblasts in the Fe-richdunites is similar to low-Ti Karoo basalts. Modelling the crystalfractionation of the inferred parental magma with pMELTS yieldselement fractionation trends that mirror the element variationof primitive low-Ti Karoo basalts. KEY WORDS: dunite xenoliths; fractional crystallization; Karoo; large igneous province; pMELTS; Re–Os; trace elements     

东北和华北地区上地幔成分和热状态对比及对中国东部岩石圈减薄的启示

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

Os, Nd, and Sr isotopic and chemical compositions of ultramafic xenoliths from Kurose, SW Japan: Implications for contribution of slab-derived material to wedge mantle

We examined seven ultramafic xenoliths from 1~3 Ma alkali olivine basalt reefs near the Eurasian continent and one sample of the host alkali basalt to identify the mantle wedge material and to constrain the origin and evolution of mantle beneath SW Japan. Six xenoliths are from Kurose and one xenolith is from Takashima, northern part of the Kyushu islands, SW Japan. The Sr and Nd isotopic ratios vary from 0.70416 to 0.70773 and from 0.51228 to 0.51283, respectively. The Kurose and Takashima xenoliths have higher Sr isotopic ratios and lower Nd isotopic ratios than those of the peridotite xenoliths from the other arc settings such as Simcoe and NE Japan.

The Kurose xenoliths have less radiogenic Os isotopic ratios (¹⁸⁷Os/¹⁸⁸Os = 0.123–0.129) than the primitive upper mantle (PUM) estimate and limited variation compared to the other arc xenoliths. Their Os isotope compositions are rather similar to the ultramafic xenoliths from NE and east China. In addition, the samples of the Kurose and Takashima xenoliths plot along a mixing line between ultramafic xenoliths from SE and NE China and a slab component in Sr–Nd–Os isotopic space. Our results suggest that fragments of continental lithospheric mantle from the China craton may exist beneath Kurose and Takashima after the Sea of Japan expansion when the Japanese islands were rifted away from the Eurasian continent during Miocene. Later magmatism due to subduction of the Philippine Sea Plate beneath the SW Japan arc around 15 Ma ago may have introduced fluids or melts derived from slab component, interpreted to be oceanic sediments rather than altered oceanic crust, that possibly modified the original composition of the lithospheric mantle sampled by the peridotite xenoliths from Kurose and Takashima.     

Water partitioning between mantle minerals from peridotite xenoliths

The speciation and amount of water dissolved in nominally anhydrous silicates comprising eight different mantle xenoliths has been quantified using synchrotron micro-FTIR spectroscopy. Samples studied are from six geographic localities and represent a cross-section of the major upper mantle lithologies from a variety of tectonic settings. Clinopyroxene contains between 342 and 413 ppm H₂O. Orthopyroxene, olivine and garnet contain 169–201, 3–54 and 0 to <3 ppm H₂O, respectively. Pyroxenes water contents and the distribution of water between ortho- and clinopyroxene is identical regardless of sample mineralogy (D _water^cpx/opx = 2.1 ± 0.1). The total water contents of each xenolith are remarkably similar (113 ± 14 ppm H₂O). High-resolution spectroscopic traverses show that the concentration and speciation of hydrous defects dissolved in each phase are spatially homogeneous within individual crystals and identical in different crystals interspersed throughout the xenolith. These results suggest that the amount of water dissolved in the silicate phases is in partial equilibrium with the transporting melt. Other features indicate that xenoliths have also preserved OH signatures of equilibrium with the mantle source region: Hydroxyl stretching modes in clinopyroxene show that garnet lherzolites re-equilibrated under more reducing conditions than spinel lherzolites. The distribution of water between pyroxenes and olivine differs according to xenolith mineralogy. The distribution of water between clinopyroxene and olivine from garnet peridotites (D _water^cpx/oliv(gnt) = 22.2 ± 24.1) is a factor of four greater than mineral pairs from spinel-bearing xenoliths (D _water^cpx/oliv(sp) = 88.1 ± 47.8). Such an increase in olivine water contents at the spinel to garnet transition is likely a global phenomenon and this discontinuity could lead to a reduction of the upper mantle viscosity by 0.2–0.7 log units and a reduction of its electrical resistivity by a factor of 0.5–0.8 log units.     

Rheology of the lower crust beneath the northern part of North China: Inferences from lower crustal xenoliths from Hannuoba basalts, Hebei Province, China

Lower crustal xenoliths brought up rapidly by basaltic magma onto the earth surface may provide di-rect information on the lower crust. The main purpose of this research is to gain an insight into the rheology of the lower crust through the detailed study of lower crustal xenoliths collected from the Hannuoba basalt, North China. The lower crustal xenoliths in this area consist mainly of two pyroxene granulite, garnet granulite, and light-colored granulite, with a few exception of felsic granulite. The equilibration temperature and pressure of these xenoliths are estimated by using geothermometers and geobarometers suitable for lower crustal xenoliths. The obtained results show that the equilibration temperature of these xenoliths is within the range of 785―900℃, and the equilibrium pressure is within the range of 0.8―1.2 GPa, corresponding to a depth range of 28―42 km. These results have been used to modify the previously constructed lower crust-upper mantle geotherm for the studied area. The dif-ferential stress during the deformation process of the lower crustal xenoliths is estimated by using recrystallized grain-size paleo-piezometer to be in the range of 14―20 MPa. Comparing the available steady state flow laws for lower crustal rocks, it is confirmed that the flow law proposed by Wilks et al. in 1990 is applicable to the lower crustal xenoliths studied in this paper. The strain rate of the lower crust estimated by using this flow law is within the range of 10-13―10-11 s-1, higher than the strain rate of the upper mantle estimated previously for the studied area (10-17―10-13 s-1); the equivalent viscosity is estimated to be within the range of 1017―1019Pa·s, lower than that of the upper mantle (1019―1021 Pa·s). The constructed rheological profiles of the lower crust indicate that the differential stress shows no significant linear relation with depth, while the strain rate increases with depth and equivalent vis-cosity decrease with depth. The results support the viewpoint of weak lower continental crust.     

Geodynamic implications of flattened tabular equigranular textured peridotites from the Bakony-Balaton Highland Volcanic Field (Western Hungary)

Peridotite xenoliths showing unusual tabular equigranular textures (addressed as flattened tabular equigranular) were found in Neogene alkali basalts from the Bakony-Balaton Highland Volcanic Field (Western Hungary), Carpathian-Pannonian Region. The olivines have a characteristic crystallographic preferred orientation (CPO) with [0 1 0]-axes perpendicular to the foliation and the [1 0 0]- and [0 0 1]-axes forming a continuous girdle in the foliation plane. Contrarily, the CPO pattern of orthopyroxene is much more scattered, although a single maximum can be observed in [0 0 1] axes subparallel to the plane of foliation. In case of olivine, the activation of (0 1 0)[1 0 0] and also probably (0 1 0)[0 0 1] is suggested. The deformation micro-mechanisms of orthopyroxenes are suggested to be a combination of intracrystalline glide on the (1 0 0)[0 0 1] system and some kind of other mechanism resulting in quite scattered patterns.We suggest that the unusual orientation patterns of olivines and orthopyroxenes are the result of the complex tectonic evolution of the region. The flattened tabular equigranular xenoliths represent a structural domain within the subcontinental lithospheric mantle beneath the volcanic field with particular seismic characteristics. The occurrence of flattened domains in the upper mantle may considerably influence the percolation and residence time of the mantle melts and fluids, which could promote or prevent melt/wall-rock interaction.