Clinopyroxene REE Geochemistry of the Red Hills Peridotite, New Zealand: Interpretation of Magmatic Processes in the Upper Mantle and in the Moho Transition Zone

The Red Hills peridotite in the Dun Mountain ophiolite of SouthIsland, New Zealand, is assumed to have been produced in a paleo-mid-oceanridge tectonic setting. The peridotite is composed mostly ofharzburgite and dunite, which represent residual mantle andthe Moho transition zone (MTZ), respectively. Dunite channelswithin harzburgite blocks of various scales represent the MTZcomponent. Plagioclase- and clinopyroxene-bearing dunites occursporadically within common dunites. These dunites representproducts of melt–wall-rock interaction. Chondrite-normalizedrare earth element (REE) patterns of MTZ clinopyroxenes showa wide compositional range. Clinopyroxenes in plagioclase dunitesare extremely depleted in light REE (LREE) ([Lu/La]_N >100),and are comparable with clinopyroxenes in abyssal peridotitesfrom normal mid-ocean ridges. Interstitial clinopyroxenes inthe common dunite have flatter patterns ([Lu/La]_N 2) comparablewith those for dunite in the Oman ophiolite. Clinopyroxenesin the lower part of the residual mantle harzburgites are evenmore strongly depleted in LREE ([Lu/La]_N = 100–1000) thanare mid-ocean ridge peridotites, and rival the most depletedabyssal clinopyroxenes reported from the Bouvet hotspot. Incontrast, those in the uppermost residual mantle harzburgiteand harzburgite blocks in the MTZ are less LREE depleted ([Lu/La]_N= 10–100), and are similar to those in plagioclase dunite.Clinopyroxenes in the clinopyroxene dunite in the MTZ are similarto those reported from mid-ocean ridge basalt (MORB) cumulates,and clinopyroxenes in the gabbroic rocks have compositions similarto those reported from MORB. Strong LREE and middle REE (MREE)depletion in clinopyroxenes in the harzburgite suggests thatthe harzburgites are residues of two-stage fractional melting,which operated initially in the garnet field, and subsequentlycontinued in the spinel lherzolite field. The early stage meltingproduced the depleted harzburgite. The later stage melting wasresponsible for the gabbroic rocks and dunite. Strongly LREE–MREE-depletedclinopyroxene in the lower harzburgite and HREE-enriched clinopyroxenein the upper harzburgite and plagioclase dunite were formedby later reactive melt migration occurring in the harzburgite. KEY WORDS: clinopyroxene REE geochemistry; Dun Mountain ophiolite; Moho transition zone; orogenic peridotite; Red Hills     

西沙群岛基底火山碎屑岩中单斜辉石的矿物化学特征及其地质意义

本文利用电子探针对南海西沙群岛琛航岛珊瑚礁底部火山碎屑岩中的单斜辉石矿物的化学特征进行了研究。结果表明,该单斜辉石属于富钙透辉石,部分有正环带结构,从核部到边部Ca、Fe、Ti的含量逐渐增加,是岩浆正常结晶顺序的反映,说明该区域的岩浆演化是向着富Ca、Fe、Ti方向发展的。主量元素数据显示,单斜辉石具有低Si高Al的特征(SiO₂=41.40%～48.44%,Al₂O₃=5.54%～10.20%),且AlⅣ含量较高,说明母岩浆为不饱和碱性岩浆系列;此外,单斜辉石Ca含量偏高,Ca/(Ca+Mg+Fe)值在46.1%～51.4%之间,推测是母岩浆的高Ca含量导致了大量高钙辉石的产出。结合西沙海域的地震和构造资料,推测琛航岛珊瑚礁的基底是玄武质火山碎屑岩组成的平顶状海山,系岩浆穿过断裂发育的岩石圈层在西沙群岛的海底喷发,随后火山碎屑物质经过堆积、固结作用而形成;该火山碎屑岩的原岩为板内碱性玄武岩。     

Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N:Evidence from High MgO MORB and Melt Inclusions

Mid-ocean ridge basalts(MORBs)from East Pacific Rise(EPR)13°N are analysed for major and trace elements,both of which show a continuous evolving trend.Positive MgO-Al_2O3 and negative MgO-Sc relationships manifest the cotectic crystallization of plagioclase and olivine,which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts.However,the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO.Thus,MORB samples are believe...     

藏北羌塘戈木错渐新世碱性钾质火山岩中绿核辉石的成因:岩浆补给—混合过程

藏北戈木错渐新世碱性钾质火山岩中单斜辉石斑晶普遍出现正环带、反环带和韵律环带结构,多具有"绿核辉石"的结构特征。通过矿物电子探针测得"绿核辉石"的核部有高Mg#和低Mg#两种不同成分,两者都不同程度发育有筛孔状熔蚀结构,环带结构主要发育在辉石的幔部到边部,与绿色核部存在明显的成分间断,幔部环带结构的成分变化范围相对较窄。"绿核辉石"的成分和结构特征反映了同源碱性钾质岩浆在壳内岩浆房中发生了岩浆补给-混合作用,复杂的环带结构记录了混合岩浆的结晶过程。     

Ca-rich Garnet-Clinopyroxene Rocks at Hujialin in the Su-Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates?

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     

Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral-Fluid Equilibria and Petrogenetic Grids

The role of clinopyroxene in producing grandite garnet is evaluatedusing data from an ultrahigh-temperature metamorphosed calc-silicategranulite occurrence in the Eastern Ghats Belt, India. ‘Peak’pressure–temperature conditions of metamorphism were previouslyconstrained from associated high Mg–Al granulites as c.0·9 GPa, >950°C, and the rocks were near-isobaricallycooled to c. 750°C. Grandite garnet of variable compositionwas produced by a number of reactions involving phases suchas clinopyroxene, scapolite, plagioclase, wollastonite and calcite,in closely spaced domains. Compositional heterogeneity is preservedeven on a microscale. This precludes pervasive fluid fluxingduring either the peak or the retrograde stage of metamorphism,and is further corroborated by computation of fluid–rockratios. With the help of detailed textural and mineral compositionalstudies leading to formulation of balanced reactions, and usingan internally consistent thermodynamic dataset and relevantactivity–composition relationships, new petrogenetic gridsare developed involving clinopyroxene in the system CaO–Al₂O₃–FeO–SiO₂–CO₂–O₂in T–aCO₂–fO₂ space to demonstrate the importanceof these factors in the formation of grandite garnet. Two singularcompositions in garnet-producing reactions in this system arededuced, which explain apparently anomalous textural relations.The possible role of an esseneite component in clinopyroxenein the production of grandite garnet is evaluated. It is concludedthat temperature and fO₂ are the most crucial variables controllinggarnet composition in calc-silicate granulites. fO₂, however,behaves as a dependent variable of CO₂ in the fluid phase. Externalfluid fluxing of any composition is not necessary to producechemical heterogeneity of garnet solid solution. KEY WORDS: grandite garnet; role of clinopyroxene; internal buffering; oxidation–decarbonation equilibria     

Revised activity–composition models for clinopyroxene and amphibole

Recent activity–composition models for clinopyroxene and amphibole are revised to provide better consistency with observed phase relations in natural rocks. For clinopyroxene, the calibration in NCFMAS is retained, but the incorporation of acmite is revised to improve the partitioning of ferric iron between coexisting clinopyroxenes. For amphibole, the NCFMASH calibration is retained, but the addition of ferric iron is changed to provide consistency with the clinopyroxenes. The thermodynamics of orthoamphibole (gedrite) is also adjusted to resolve an unrelated inconsistency. The effects of these improvements are illustrated through comparison of calculated pseudosections produced with the existing and new models with natural data from lawsonite eclogites.     

Oxidation Experiment of Natural Megacrystal Clinopyroxene: Implications for Assignment of Mössbauer Spectra

The Mössbauer spectra of natural megacrystal clinopyroxene are usually fitted by 4 sets of symmetric doublets, A‐A', B‐B', C‐C' and D‐D', respectively, in terms of increasing Qs value in literature. But the assignments of those doublets are quite different, except the D‐D' doublet assigned to Fe³⁺at the lattice site M***1 in previous papers. Particularly, the assignment and interpretation of the C‐C' doublet are diverse. The oxidation experiments of natural megacrystal clinopyroxene collected from the Hannuoba basalt, North China, were performed under controlled conditions of temperature at 1000°C and oxygen fugacity of FMQ buffer in 1, 2, ***3 and 5 days respectively. The oxidized samples were then measured by X‐ray diffraction spectrometry and Mössbauer spectrometry. The oxidation of clinopyroxene is characterized by Fe²⁺ → Fe³⁺at M1 under the subsolidus conditions, which is consistent with the increase of the area of the D‐D' doublet when the heating time increases. Accordingly, the area of the A‐A' and B‐B' doublets decreases with the increasing heating time. However, the area of C‐C' keeps almost constant. Therefore, the 4 sets of doublets can be assigned correctly as follows: A‐A' doublet to Fe⁺²at M1, B‐B' Fe²⁺at M1, C‐C' Fe²⁺at M2, and D‐D' Fe³⁺at M1. The M1 site splitting is due to the NNN (Next Nearest Neighbour) effect of the M2 site. The amount of Fe³⁺at the tetrohedral site of megacrystal clinopyroxene is negligible according to this study.     

Genesis of the Kamioka Skarn Deposits: An Important Role of Clinopyroxene Skarn and Graphite-bearing Limestone in Precipitating Sulfide Ore

Abstract: A genetical relationship between skarn formation and mineralization is investigated for the Kamioka skarn deposits which are the largest Zn-Pb producer in Japan. In the Mozumi deposit, one of main deposits in the Kamioka mining area as well as Tochibora and Maruyama, clinopyroxene skarn was generally subjected to later replacement by garnet or magnetite–calcite–quartz during the Zn-Pb mineralization. The replacement of hedenbergitic clinopyroxene by andraditic garnet resulted in the formation of diopsidic clinopyroxene relicts. With the progress of replacement, the S/So value (So: an estimated area occupied by an original clinopyroxene grain in a thin section, S: a total area of relict clinopyroxene fragments) which is an index of the degree of replacement decreases from 0. 7 to 0. 1, and the hedenbergite mole percent of relict clinopyroxene decreases drastically from about 65 to less than 40. A close association of andraditic garnet and sphalerite suggests that heden-bergitic clinopyroxene skarn played an important role to reduce the relatively oxic ore-forming fluid enriched in Zn²⁺ and SO4^2– and to precipitate sphalerite from the fluid. Ferrous iron in the hedenbergitic clinopyroxene skarn was oxidized to form andraditic garnet. Besides this garnet formation, the mineral assemblage of magnetite–calcite–quartz replaced the clinopyroxene skarn at the time of mineralization. In both cases, the reduction of relatively oxic ore-forming fluid by hedenbergitic clinopy-roxene skarn at the later stage brought about the precipitation of sulfide minerals. In contrast, these types of later replacement are not found in the Tochibora deposit. Instead, graphite-bearing crystalline limestone and relatively fresh clinopyroxene skarn are common. Mineralized clinopyroxene skarn has high graphite carbon contents relative to barren one, suggesting that the amount of graphite in the skarn was an important controlling factor for mineralization. It is very likely that the graphite played a role of reducing agent during the mineralization in the Tochibora deposit.     

Eruption episodes and magma recharge events in andesitic systems: Mt Taranaki,New Zealand

Eruption episodes, where a series of eruption events are generically related, can include the eruption of a wide spectrum of volcanic activity over decadal periods. This paper concentrates on the opening phases of an eruption episode which occurred approximately 1800 yrs BP from Mt Taranaki, New Zealand. These events spanned the eruption of differing bulk compositions and styles from two distinct vent locations; an andesitic sub-plinian eruption from the summit vent and a scoria cone-building eruption of basaltic magma from a satellite vent. Compositional profiles and zoning textures of plagioclase, amphibole and clinopyroxene phenocrysts from the opening andesitic event show evidence of magma mixing and subsequent crystallisation just prior to the initiation of the eruption episode. Titanomagnetite grain morphology and Ti variation suggest that the magma mixing event occurred within a few days to weeks before the eruption acting as a trigger for it. We present a magmatic model which is constrained by the petrological observations and eruptions of the episode. In this model magma differentiation at depth causes its rise and recharging of a mid-crustal magma storage area at 5–7 km. Although the recharging magma differed slightly in oxygen fugacity and temperature, it was compositionally and physically similar enough to the residing andesitic magma to allow efficient mixing. The petrological characteristics described here can be readily observed and enable identification of mixing events in other recent eruption episodes.