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1.
Clinopyroxene and orthopyroxene megacrysts with lamellar intergrowths of pyroxenes and garnet rarely survive in pyroxenite layers from the exposed spinel-lherzolite massifs because of the emplacement history into the crust. Such features are remarkably preserved in some thick bands (up to 1 m) from the Freychinède ultramafic body (Ariège, French Pyrenees). These bands display a symmetrical zoning from the edges to the centre due to the concurrent decrease of orthopyroxene/clinopyroxene and spinel/garnet modal ratios. Textural and chemical data suggest that the present pyroxenite parageneses resulted from subsolidus recrystallization of magmatic assemblages composed of Al-rich orthopyroxene and clinopyroxene with minor spinel. These primary assemblages were changed by subsolidus recrystallization connected with an isobaric cooling at upper-mantle depth (45–50 km) from solidus temperature (1250°C) down to steady equilibrium temperature (950° C). The primary Al-rich ortho-and clinopyroxenes behaved differently on cooling. In a first stage, orthopyroxene exsolved concomitant Al-rich clinopyroxene and garnet, whereas clinopyroxene exsolved only Al-rich orthopyroxene. The garnet exsolution in clinopyroxene host is delayed to lower temperatures. This multistage process could account for the contrasting shapes of diffusion gradients adjacent to exsolved garnet, which tend to be flat in host-orthopyroxene and steep in host-clinopyroxene. An independent thermal modelling, together with available Al-diffusion data in clinopyroxene, allows us to define a fast magmatic cooling followed by a two-stage subsolidus cooling (35° C/year-6 from 1250° C to 1050° C and 9° C/year-6 to 900° C). This matches the contrasted exsolution sequences observed in the pyroxene megacrysts. 相似文献
2.
The lherzolites have recrystallized to plagioclase lherzolites consisting of olvine, pyroxenes, chromian spinel, plagioclase and pargasite at a depth of 20 to 25 km in the uppermost part of the mantle. It is believed that the garnet lherzolites and spinel lherzolites were originally derived from depths of 50–75 km and 30–50 km respectively. The clinopyroxenes contained about 10 mol. % of jadeite and Tschermak's molecules, respectively and the orthopyroxenes also included about 5–10% of Tschermak's component. Transported upward, the garnet was transformed through pyroxene-spinel symplectite to olivine, plagioclase and spinel aggregates, and most of the jadeite amd some Tschermak's components in the pyroxenes formed secondary pyroxenes and pargasite, and finally plagioclase under isochemical conditions. 相似文献
3.
Kevin Grant Jannick Ingrin Jean Pierre Lorand Paul Dumas 《Contributions to Mineralogy and Petrology》2007,154(1):15-34
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
watercpx/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
watercpx/oliv(gnt) = 22.2 ± 24.1) is a factor of four greater than mineral pairs from spinel-bearing xenoliths (D
watercpx/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. 相似文献
4.
R. C. Wallace 《Contributions to Mineralogy and Petrology》1975,49(3):191-199
Xenoliths up to a metre in length occur in a carbonatitic diatreme member of a lamprophyric dike swarm at Moeraki River, south Westland, New Zealand. The xenoliths reported here consist of Iherzolite (chromite, orthopyroxene, clinopyroxene and olivine) and harzburgite (chromite, olivine and orthopyroxene). A clinopyroxene xenocryst is also reported. Analyses of these phases are presented. The chemistry, low CaO and high Al2O3 and Na2O content of the clinopyroxenes; low CaO and high forsterite content of the olivine, suggests that these phases were in equilibrium under high pressures within the spinel Iherzolite field. An orthopyroxene-chromite intergrowth is described and is interpreted as the product of the re-equilibration of garnet in passing from the garnet Iherzolite field to the spinel Iherzolite field. 相似文献
5.
Suzanne Y. Wass 《Lithos》1979,12(2):115-132
Geochemical and textural data on clinopyroxenes in individual alkali basaltic flows from provinces in eastern Australia and the Massif Central can be used to differentiate four different modes of origin (three at high pressure) for these clinopyroxenes. Many single flows from the two provinces contain clinopyroxenes of three, or even four, of these origins. Rare flows contain core clinopyroxene with overgrowths of clinopyroxenes of two distinct generation. Each of the overgrowth is compositionally analogous to clinopyroxenes occurring in xenoliths or as discrete crystals in the same host lava. Such rimming relationships provide evidence that the host magma has undergone high pressure crystallisation and confirm that some xenoliths and megacrysts are cognate. With decreasing pressure the major changes in clinopyroxene chemistry are an increase in the ratio Allv/Alvi, a linear increase of atomic proportions of Ti and Al with decreasing Si, and an increase in the Ti: (100 Mg/(Mg + Σ Fe)) ratio of the pyroxenes. Al2O2 wt, % is an unreliable potential geobarometric indicator; consideration of tetrahedral and octahedral site occupancies by Al (Allv/Alvl ratio) is necessary. High-pressure fractionation dominated by clinopyroxene is postulated for some basaltic-composition, resulting in SiO2 depletion, alkali enrichment and decrease in the Mg/(Mg + Fe2+) value of the host magma. 相似文献
6.
Ultrahigh-pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps 总被引：1，自引：0，他引：1
M. JANÁK N. FROITZHEIM M. VRABEC E. J. KROGH RAVNA J. C. M. De HOOG 《Journal of Metamorphic Geology》2006,24(1):19-31
New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation. 相似文献
7.
名义无水矿物(NAMs)中的结构水在地球内部演化中扮演了重要角色.应用红外光谱对产自5个构造背景下的13个二辉橄榄岩进行了详细的结构水测定,探讨不同构造背景下橄榄岩结构水的含量和赋存机制.研究显示,全岩和橄榄石结构水含量按构造背景从高到低排序,依次为超高压地体、地幔柱、稳定克拉通、俯冲带和活化克拉通,反映了橄榄岩的水含量与构造环境具有显著相关性.在相同构造背景下,石榴石二辉橄榄岩比尖晶石二辉橄榄岩含更多结构水,表明上地幔水分布可能具有分层性.超高压地体和稳定克拉通样品中橄榄石具有[Si]空位导致的吸收峰3 611~3 613 cm-1,而缺乏由[Fe3+]引起的吸收峰3 325 cm-1和3 355 cm-1,表明地幔的还原性随深度增强. 相似文献
8.
山东昌乐位于华北克拉通东部,郯庐断裂带中段,广泛出露挟裹幔源捕虏体的新生代碱性玄武岩.为深入了解华北克拉通演化,依据岩相学特征、矿物化学成分特征,把昌乐碱性玄武岩中单斜辉石主要分为三大类:第1类为幔源捕虏体中原生单斜辉石,其主、微量元素含量呈现趋势不一致的解耦现象,且LREE、HREE各异特征表明其经历了多期、不同程度的地幔富集交代和部分熔融作用（低于10%的尖晶石相部分熔融）,交代熔体包括地幔富碱（K）富Al硅酸盐熔/流体,可能有碳酸盐熔体的贡献;第2类为幔源捕虏体中筛状单斜辉石,是部分熔融和熔体交代作用的共同产物,从无筛孔部位到筛孔周围远离筛孔部位和紧挨着筛孔部位,呈现出受熔体作用逐渐加强趋势,离筛孔较远部位记录了玄武质熔浆作用之前的一次富碱（K）富Al熔体交代的特征,而紧挨着筛孔的部位受晚期玄武质熔浆作用的影响最强;第3类为幔源捕虏体中反应边单斜辉石及玄武岩中斑晶单斜辉石,两者的化学成分及形成条件相似,显示HFSE正异常,LILE负异常,受OIB特征寄主玄武岩浆强烈的影响.昌乐新生代碱性玄武岩地幔捕虏体中单斜辉石显示经历多期"熔体作用"和不同程度的部分熔融特征,是研究区岩石圈地幔不均一性的体现. 相似文献
9.
Clinopyroxene and orthopyroxene megacrysts containing garnet lamellae up to 1.2 mm thick as an exsolved phase are found rarely in kimberlites from Frank Smith and Bellsbank. Chemically the clinopyroxenes are characteristically subcalcic, being within the range of 100 Ca/Ca + Mg + Fe = 27 to 36, and the orthopyroxenes are characterized by high Al2O3 and Cr2O3. Immediately after crystallization during very slow cooling, clinopyroxene and orthopyroxene exsolve wide-spaced orthopyroxene and clinopyroxene phases parallel to (100) of the host phases, respectively, then both host and exsolved phases exsolve garnet lamellae. Topotactic relations between pyroxenes and garnet are determined by X-ray for the first time. Partitioning of major and minor elements among the coexisting clinopyroxene, orthopyroxene and garnet in pyroxene megacrysts is the same as that of the granular-type garnet peridotite xenoliths in Lesotho and South African kimberlies. Mineralogy and chemistry indicate that subcalcic clinopyroxene and orthopyroxene megacrysts contain respectively about 10 and 3 mole % of the garnet molecule in solid solution. 相似文献
10.
摘 要 利用穆斯堡尔谱仪对辽宁宽甸地幔岩包体中的矿物进行了三价铁的测定测试结果
与国内外已发表的数据接近。Fe
3＋ ／∑Fe 比值分别为：斜方辉石0∙066～0∙196单斜辉石
0∙216～0∙344尖晶石0∙283～0∙299石榴石0∙057～0∙223。依据电价平衡原理计算的 Fe
3＋
含量受 Si 4＋ 、Al 3＋ 和 Cr 3＋ 的电子探针分析误差影响很大。根据地幔矿物的电子探针分析结果
计算平衡温度、压力和氧逸度时二辉石 En、Ca 转移反应温度计和石榴石 斜方辉石 Al 压力
计的计算结果不超过温压计的一般误差范围而石榴石 单斜辉石 Fe
2＋
Mg 交换反应温度计
在忽略 Fe
3＋ 时计算的温度值可偏高100℃以上;采用橄榄石／斜方辉石／尖晶石组合氧压力计
计算的氧逸度值误差一般约在±0∙7个对数单位。 相似文献
11.
SUN Weidong PENG Zicheng CHEN Daogong ZHI Xiachen WANG Zhaorong GE NingjieDepartment of Earth & Space Sciences University of Science & Technology of Chin and Advanced Centre for Earth Sciences & Astronomy The Third World Academyof Sciences Hefei Anhui Province Zhu Xiling 《《地质学报》英文版》1998,72(2):180-190
This paper presents a study of the major and trace element compositions of fresh mantle-derived spinel lherzolite and harzburgite inclusions from Cenozoic alkaline basalt in Mount Lianshan and Mount Panshi, Liuhe County, Jiangsu Province. An estimation is made of the contents of the major elements and some of the trace elements in the primitive mantle source region of the area, from which the authors have obtained MgO/Al2O3= 7.86. The contents of MgO and Al2O3 are also obtained as 37.58% and 4.78% respectively based on the correlation of MgO-Al2O3. Then, the contents of various elements in the primitive mantle are calculated using their regression equations with MgO, and the compositor) of the primitive mantle, a basic issue in geochemistry study, is discussed on that basis. 相似文献
12.
Trace element compositions of minerals in garnet and spinel peridotite xenoliths from the Vitim volcanic field, Transbaikalia, eastern Siberia 总被引：12，自引：0，他引：12
Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volcanic province of Transbaikalia consist of garnet lherzolite, garnet–spinel lherzolite and spinel lherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20–23 kbar close to the garnet to spinel peridotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridotites, probably during formation of the lithosphere. The peridotites show petrographic and mineral chemical evidence for infiltration by an alkaline silicate melt shortly before their transport to the Earth's surface. The melt infiltration event is indicated petrographically by clinopyroxenes which mimic melt morphologies, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Earth's surface. Single-mineral thermometry gives reasonable temperature estimates of 1050±50°C, whereas two-mineral methods involving clinopyroxene are falsified by secondary components in clinopyroxene introduced during the melt infiltration event. Excimer Laser–ICP-MS analysis has been performed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock type. Orthopyroxene and garnet show only partial equilibration of trace elements with the infiltrating melt, whereas clinopyroxene and amphibole are close to equilibration with the melt and with each other. The incompatible element composition of the infiltrating melt calculated from the clinopyroxene and amphibole analyses via experimental mineral/melt partition coefficients is similar to the host alkali picrite, and probably represents a low melt fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series of events expected during the influence of an expanding rift region in the upper mantle, namely the progressive erosion of the lithosphere and the episodic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field. 相似文献
13.
Mineral water contents, together with the elements and isotopes of minerals and whole‐rock, were determined for garnet pyroxenites enclosed by ultrahigh‐pressure (UHP) metamorphic gneiss at Hujialin in the Sulu orogen. The results suggest that the garnet pyroxenites were generated in the Triassic by metasomatic reaction of the mantle wedge peridotite with hydrous felsic melts derived from partial melting of the deeply subducted continental crust. Measured water contents vary from 523 to 1213 ppm for clinopyroxene, and 55 to 1476 ppm for garnet. These mineral water contents are not only correlated with mineral major and trace element abundances but also relatively homogenous within single mineral grains. Such features preclude significant disturbance of the mineral water contents during pyroxenite exhumation from the mantle depth to the surface and thus indicate preservation of the primary water contents for the UHP metasomatites. The garnet pyroxenites are estimated to have bulk water contents of 424–660 ppm, which are higher than those for the MORB source, similar to or higher than those for the OIB sources and close to the lower limit for the arc magma source. The relationships between contents of mineral water and some elements suggest that the high water contents of garnet pyroxenites are primarily determined by the abundance of water‐rich clinopyroxene. Calculated whole‐rock H2O/Ce ratios are 63–145, higher than those for Hawaiian garnet pyroxenites and SWIR abyssal pyroxenites. These observations suggest that metasomatic pyroxene‐rich lithologies have the capacity to contribute high H2O concentrations and variable H2O/Ce ratios to the mantle. This lends support to the interpretation that the source of some intraplate basalts may be a heterogeneous mixture of peridotite and pyroxenite. On the other hand, the high water contents of garnet pyroxenites suggest that the ultramafic metasomatites are an important water reservoir in the mantle wedge. 相似文献
14.
徐淮地区镁铁质岩石捕虏体单斜辉石中石榴石和石英的出溶 总被引：7，自引：0，他引：7
徐淮地区早侏罗世侵入杂岩中榴辉岩,石榴辉石岩和单斜辉石岩捕虏体单斜辉石中可以观察丰富的出溶石英针和石榴石,黝帘石及角闪石的出溶叶片,榴辉岩中出溶石英针的绿辉石核部比其边部相对富含FeO和MgO,贫SiO2,Al2O3和CaO。在石榴辉石岩和单斜辉石岩捕虏体中具有出溶石榴石的单斜辉石。从靠近出溶石榴石的一侧向其核部,Al2O3,Na2O和TiO2含量降低,MgO,SiO2和CaO含量增加,单斜辉石中定向石英针的出溶表明曾经存在有超高压条件下（≥25×10^8Pa)稳定的过硅质绿辉石。单斜辉石中出溶石榴石表明温压条件的降低可能是引起出溶的一个主要原因,捕虏体中的矿物组合和岩相学特征表明它们曾经遭受了榴辉岩相和角闪岩相退化变质作用,这与因压力和温度降低引起矿物出溶的结果相吻合。 相似文献
15.
Mg-Al Sapphirine- and Ca-Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya 总被引：2，自引：0，他引：2
Basanites of the Chyulu Hills (Kenya Rift) contain mafic Mg–Aland Ca–Al granulite xenoliths. Their protoliths are interpretedas troctolitic cumulates; however, the original mineral assemblageswere almost completely transformed by subsolidus reactions.Mg–Al granulites contain the minerals spinel, sapphirine,sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxeneand garnet, whereas Ca–Al granulites are characterizedby hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase,quartz, clinopyroxene, corundum, and garnet. In the Mg–Algranulites, the first generation of orthopyroxene and some spinelmay be of igneous origin. In the Ca–Al granulites, hibonite(and possibly some spinel) are the earliest, possibly igneous,minerals in the crystallization sequence. Most pyroxene, spineland corundum in Mg–Al and Ca–Al granulites formedby subsolidus reactions. The qualitative P–T path derivedfrom metamorphic reactions corresponds to subsolidus cooling,probably accompanied, or followed by, compression. Final equilibrationwas achieved at T 600–740°C and P <8 kbar, inthe stability field of sillimanite. The early coexistence ofcorundum and pyroxenes (± spinel), as well as the associationof sillimanite and sapphirine with clinopyroxene and the presenceof hibonite, makes both types of granulite rare. The Ca–Alhibonite-bearing granulites are unique. Both types enlarge thespectrum of known Ca–Al–Mg-rich granulites worldwide. KEY WORDS: granulite xenoliths; corundum; sapphirine; hibonite; Kenya Rift 相似文献
16.
Trace element partition coefficients (D's) for up to 13 REE, Nb, Ta, Zr, Hf, Sr and Y have been determined by SIMS analysis of seven garnets, four clinopyroxenes, one orthopyroxene and one phlogopite crystallized from an undoped basanite and a lightly doped (200 ppm Nb, Ta and Hf) quartz tholeiite. Experiments were conducted at 2–7.5 GPa, achieving near-liquidus crystallization at relatively low temperatures of 1080–1200°C under strongly hydrous conditions (5–27 wt.% added water). Garnet and pyroxene DREE show a parabolic pattern when plotted against ionic radius, and conform closely to the lattice strain model of Blundy and Wood (Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454). Comparison, at constant pressure, between hydrous and anhydrous values of the strain-free partition coefficient (D0) for the large cation sites of garnet and clinopyroxene reveals the relative importance of temperature and melt water content on partitioning. In the case of garnet, the effect of lower temperature, which serves to increase D0, and higher water content, which serves to decrease D0, counteract each other to the extent that water has little effect on garnet–melt D0 values. In contrast, the effect of water on clinopyroxene–melt D0 overwhelms the effect of temperature, such that D0 is significantly lower under hydrous conditions. For both minerals, however, the lower temperature of the hydrous experiments tends to tighten the partitioning parabolas, increasing fractionation of light from heavy REE compared to anhydrous experiments.
Three sets of near-liquidus clinopyroxene–garnet two-mineral D values increase the range of published experimental determinations, but show significant differences from natural two-mineral D's determined for subsolidus mineral pairs. Similar behaviour is observed for the first experimental data for orthopyroxene–clinopyroxene two-mineral D's when compared with natural data. These differences are in large part of a consequence of the subsolidus equilibration temperatures and compositions of natural mineral pairs. Great care should therefore be taken when using natural mineral–mineral partition coefficients to interpret magmatic processes.
The new data for strongly hydrous compositions suggest that fractionation of Zr–Hf–Sm by garnet decreases with increasing depth. Thus, melts leaving a garnet-dominated residuum at depths of about 200 km or greater may preserve source Zr/Hf and Hf/Sm. This contrasts with melting at shallower depths where both garnet and clinopyroxene will cause Zr–Hf–Sm fractionation. Also, at shallower depths, clinopyroxene-dominated fractionation may produce a positive Sr spike in melts from spinel lherzolite, but for garnet lherzolite melting, no Sr spike will result. Conversely, clinopyroxene megacrysts with negative Sr spikes may crystallize from magmas without anomalous Sr contents when plotted on mantle compatibility diagrams. Because the characteristics of strongly hydrous silicate melt and solute-rich aqueous fluid converge at high pressure, the hydrous data presented here are particularly pertinent to modelling processes in subduction zones, where aqueous fluids may have an important metasomatic role. 相似文献
17.
Major and trace elements and water contents were analyzed in 16 peridotite xenoliths embedded by the Cenozoic basalts in Pingnan (southeastern Guangxi Province), to constrain the chemical composition and evolution of the lithospheric mantle located in the central part of the South China Block (SCB). The peridotites are mainly moderately refractory harzburgites and lherzolites (Mg#-Ol?=?90.3–91.7) and minor fertile lherzolites (Mg#-Ol?=?88.9–89.9). Clinopyroxenes in the peridotites show LREE-depleted pattern, and commonly exhibit negative anomalies in Nb and Ti, suggesting the peridotites probably represent residues after 1–10% of partial melting without significant mantle metasomatism. Water contents range from 146 to 237 ppm wt. H2O in clinopyroxene, and from 65 to 112 ppm wt. H2O, in orthopyroxene but are below detection limit (2 ppm wt. H2O) in olivine. Calculated bulk water contents, based on the mineral modes and partition coefficient, range from 14 to 83 ppm wt. H2O (average 59 ppm wt. H2O). There is a correlation between melting indices (such as Mg#-Ol, Ybn in clinopyroxene) and water contents in clinopyroxene and orthopyroxene, but no correlation is observed between the whole-rock water contents and the redox state (Fe3+/∑Fe ratios in spinel), suggesting that water contents in the peridotites are mainly controlled by the degree of partial melting rather than by oxygen fugacity. The lithospheric mantle beneath the interior of the SCB may not be compositionally stratified; fertile and moderately refractory mantle coexist at the similar depths. Geochemical data and water contents of the studied peridotites are similar to the proposed MORB source and indicate that the ancient refractory lithospheric mantle was irregularly eroded or reacted by the upwelling asthenosphere, and eventually replaced by juvenile fertile accreted mantle through the cooling of the asthenosphere. 相似文献
18.
Distribution of Ferric Iron in some Upper-Mantle Assemblages 总被引：16，自引：5，他引：11
The distribution of ferric iron among the phases of upper-mantlerocks, as a function of pressure (P), temperature (T) and bulkcomposition, has been studied using 57Fe Mssbauer spectroscopyto determine the Fe3+/Fe ratios of mineral separates from 35peridotite and pyroxenite samples. The whole-rock Fe3+ complementof a peridotite is typically shared approximately evenly amongthe major anhydrous phases (spinel and/or garnet, orthopyroxeneand clinopyroxene), with the important exception of olivine,which contains negligible Fe3+. Whole-rock Fe3+ contents areindependent of the T and P of equilibration of the rock, butshow a well-defined simple inverse correlation with the degreeof depletion in a basaltic component. Fe3+ in spinel and inboth pyroxenes from the spinel Iherzolite facies shows a positivecorrelation with temperature, presumably owing to the decreasein the modal abundance of spinel. In garnet peridotites, theFe3+ in garnet increases markedly with increasing T and P, whereasthat in clinopyroxene remains approximately constant. The complexnature of the partitioning of Fe3+ between mantle phases resultsin complicated patterns of the activities of the Fe3+ -bearingcomponents, and thus in calculated equilibrium fO2, which showlittle correlation with whole-rock Fe3+ or degree of depletion.Whether Fe3+ is taken into account or ignored in calculatingmineral formulae for geothermobarometry can have major effectson the resulting calculated T and P. For Fe-Mg exchange geothermometers,large errors must occur when applied to samples more oxidizedor reduced than the experimental calibrations, whose fO2 conditionsare largely unknown. Two-pyroxene thermometry is more immuneto this problem, and probably provides the most reliable P—Testimates. Accordingly, the convergence of P—T valuesderived for a given garnet peridotite assemblage may not necessarilybe indicative of mineral equilibrium. The prospects for thecalculation of accurate Fe3+ contents from electron microprobeanalyses by assuming stoichiometry are good for spinel, uncertainfor garnet, and distinctly poor for pyroxenes. KEY WORDS: mantle; oxidation; partitioning; peridotite; thermobarometry
*Corresponding author. Present address: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700, Victoria, B.C., V8W 2Y2, Canada 相似文献
19.
《International Geology Review》2012,54(15):1940-1974
Experimental synthesis of spinel peridotite phase assemblages for a range of compositions that mimic natural samples is used to derive a set of empirical geothermometers and geothermobarometers represented by multiple linear regression best-fit surfaces that link the variables of temperature, pressure, and composition. The calibrated geothermometers use reactions that govern the solubility of Al and Cr in both pyroxenes and the Mg–Fe exchange between silicates and spinel. Geothermobarometers map the Mg–Fe exchange between coexisting olivine and clinopyroxene and pyroxenes and Ca–Mg exchange between coexisting pyroxenes. Application of the geothermometers and geothermobarometers to suites of naturally occurring samples indicates that while reactions governing the Cr and Al solubility and solvus of orthopyroxene give useful estimates of ‘original’ mantle temperatures and pressures, respectively, comparable reactions for clinopyroxene yield estimates that are variably dependent on the transport phase of the sample suites. Temperature and pressure estimates from reactions governing Mg and Fe exchange between silicates and spinel and coexisting silicates are all sensitive to the later transport stage of the samples. 相似文献
20.
Ca-rich Garnet-Clinopyroxene Rocks at Hujialin in the Su-Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates? 总被引：3，自引：0，他引：3
Layers of Ca-rich garnet–clinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the Su–Lu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards Fe–Ti gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crust–mantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped P–T pathsfor the garnet–clinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnet–clinopyroxene rocks; Su–Lu terrane; UHP metamorphism 相似文献