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APPLICATION OF THE ICP-MS TECHNIQUE TO TRACE ELEMENT ANALYSIS OF PERIDOTITES AND THEIR MINERALS 总被引：20，自引：0，他引：20
Inductively coupled plasma mass spectrometry (ICP-MS) has been applied to the determination of Rb, Sr, Ba, Zr, Hf, Nb, Ta, Th, U, Pb, Sc and 14 REE in peridotites and their constituent minerals: garnet, clinopyroxene, orthopyroxene and olivine. Determination limits for most elements are 1–10 ppb (in the solid sample), with accuracy and precision comparable to those of thermal ionisation and spark-source mass spectrometry. New data on international standards PCC-1, DTS-1, UB-N and JP-1 are reported. 相似文献
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IONOV D. A.; ASHCHEPKOV I. V.; STOSCH H.-G.; WITT-EICKSCHEN G.; SECK H. A. 《Journal of Petrology》1993,34(6):1141-1175
A suite of large and fresh peridotite xenoliths from a picritetuff deposit in the Cenozoic Vitim volcanic field, {small tilde}200km east of Lake Baikal, shows a continuous gradation from protogranularspinel through garnet–spinel to very abundant garnet peridotites.This includes composite nodules in which all these lithologiescoexist on the scale of a few centimeters. Garnet and many spinellherzolites are remarkably fertile in terms of their ‘basaltic’major element contents (CaO 30–37%, MgO 37–40%,Ca/Al=11, Cr/Al<013), whereas some garnet–spineland spinel peridotites are moderately depleted (Cr/Al 014–045).T estimates are 850–880C for the fertile spinel lherzolitesapparently brought up from shallow depths of 40–50 km.This contrasts with 980–1030C for depleted spinel peridotitesand 1000–1150C for the garnet-bearing peridotites forwhich equilibration pressures between 16 and 23 kbar are inferred.The data suggest that garnet and spinel peridotites coexistin the sub-Vitim mantle at a pressure of {small tilde}18 kbarover an interval of {small tilde}2 kbar, with the appearanceof garnet, and with the garnetto-spinel ratio in this transitionalzone primarily being controlled by bulk rock contents of Ca,Al, Cr, and Cr/Al ratios, in addition to P–T conditions. The Vitim peridotites show little evidence for metasomatic enrichment:they commonly show depletion of LREE compared with intermediateREE; this includes also rare amphibole-bearing veins. The fertilespinel and garnet lherzolites have very similar bulk rock majoroxide contents and REE distribution patterns; these featuresindicate a lack of significant chemical vertical mantle stratificationin that region. Garnet peridotites from Vitim show large differencesin modal and chemical composition from garnet peridotite xenolithsfrom Yakutian and South African kimberlites, suggesting distinctlithospheric mantle structure and composition in Archean cratonsand post-Archean mobile belts.
* Present address: School of Earth Sciences, Macquarie University, N.S.W. 2109, Australia 相似文献
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Metasomatism-induced Melting in Mantle Xenoliths from Mongolia 总被引：29，自引：10，他引：19
Mantle xenoliths from two locations in Mongolia contain patchesof glass-phenocryst aggregates (‘melt pockets’)up to 1 cm in diameter, including one ‘composite’xenolith, which shows a complete transition from unaltered spinelIherzolite to a zone containing melt pockets surrounded by acpx and spinel-free peridotite matrix. We have analyzed majorelements by wet chemistry, X-ray fluorescence (XRF), and electronmicroprobe, trace elements by ion microprobe and inductivelycoupled plasma mass spectrometry (ICP-MS) techniques, and Srand Nd isotopes by mass spectrometry, to elucidate the originof these melt pockets. Petrographic and chemical evidence shows that the melt pocketswere formed neither by infiltration of the host basalt nor bydehydration melting of hydrous phases, such as amphibole. Instead,melting was induced by the interaction of a metasomatic fluidwith clinopyroxene and spinel. The reaction produced melts ofvariable composition, with SiO2 ranging from 52 to 68% and MgOfrom 4.5 to 0.5%. The melts contain euhedral grains of olivine,clinopyroxene, and spinel, and a large number of (now empty)vugs. The melt shows no sign of having invaded the Iherzolitematrix surrounding the pockets. There is some evidence for fractionalcrystallization, but some of the major element chemical trends,such as the negative correlation between Na2O and SiO2, cannotbe accounted for by such a mechanism. The glasses and clinopyroxenephenocrysts are very rich in light rare earth elements (LREE)and Sr, and completely dominate the bulk contents of these andsome other incompatible elements in the rocks with melt pockets.The invading fluid introduced high concentrations of LREE, Th,U, Pb, and Sr, but was relatively depleted in Ba, Rb, Nb, Ta,Zr, Hf, and Ti, and had unusually high Zr/Hf and Nb/Ta ratios. Ion microprobe analyses of fresh glass directly adjacent toclinopyroxene microphenocrysts yield a series of cpx-melt partitioncoefficients for REE and several other trace elements. DYb (cpx-melt)varies between 0–3 and 1.6 and is positively correlatedwith the A12O3+SiO2 and Na2O contents of the glass, and negativelycorrelated with MgO, FeO, and CaO contents. These correlationsare consistent with qualitative predictions from considerationsof silicate melt structure. The clinopyroxenes in the unaltered zones of the composite xenolithshow evidence of an earlier phase of metasomatism which enrichedCe, La, and Sr, but did not affect the other REE. Clinopyroxenesfrom these zones have high Nd values of + 14 and +19, indicatinga history of low Nd/Sm ratios. At the same time, 87Sr/86Sr ratiosare high (>0.704), indicating infiltration of relativelyradiogenic Sr during the early stage of metasomatism. Ion microprobetraverses show no zoning of La/Nd ratios. Therefore, there wasenough time to equilibrate the metasomatic effects in the graininteriors, and we estimate the time required for this equilibrationto be of the order of 105 years. In sharp contrast, the second, or main, metasomatic event thatcaused the formation of the melt pockets must have been extremelyshort-lived and probably lasted only hours or days before thexenolith was captured by the magma and erupted at the surface.This short duration is required by the preservation of freshglass and by the lack of equilibration of the melt pockets withtheir surrounding matrix. The isotopic compositions of Sr and Nd are identical betweenmelt pockets and host basalts in both localities. Therefore,we conclude that the metasomatic fluids were probably derivedfrom the same source rocks as the host basalts. We speculatethat the xenoliths originally resided in an upper-mantle regionwhich was intruded by a partially molten diapir. Volatiles wereexpelled from the unmelted margin of the diapir and invadedthe adjacent upper-mantle peridotites. The fluid infiltrationtriggered formation of the melt pockets, whereupon the materialwas picked up by rapidly ascending magma and erupted at thesurface. The fluids appear to have been poor in water, as nohydrous minerals are present among phenocryst or quench phasesin melt pockets. The major component of the fluid may have beenCO2 or liquid carbonate. 相似文献
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Mechanisms and Sources of Mantle Metasomatism: Major and Trace Element Compositions of Peridotite Xenoliths from Spitsbergen in the Context of Numerical Modelling 总被引：11，自引：6，他引：11
IONOV DMITRI A.; BODINIER JEAN-LOUIS; MUKASA SAMUEL B.; ZANETTI ALBERTO 《Journal of Petrology》2002,43(12):2219-2259
Mineral and whole-rock chemical data for peridotite xenolithsin basaltic lavas on Spitsbergen are examined to reassess mechanismsof melt–fluid interaction with peridotites and their relativerole versus melt composition in mantle metasomatism. The enrichmentpatterns in the xenoliths on primitive mantle-normalized diagramsrange from Th–La–Ce ‘inflections’ inweakly metasomatized samples (normally without amphibole) toa continuous increase in abundances from Ho to Ce typical foramphibole-bearing xenoliths. Numerical modelling of interactionbetween depleted peridotites and enriched melts indicates thatthese patterns do not result from simple mixing of the two end-membersbut can be explained by chromatographic fractionation duringreactive porous melt flow, which produces a variety of enrichmentpatterns in a single event. Many metasomatized xenoliths havenegative high field strength element and Pb anomalies and Srspikes relative to rare earth elements of similar compatibility,and highly fractionated Nb/Ta and Zr/Hf. Although amphiboleprecipitation can produce Nb–Ta anomalies, some of thesefeatures cannot be attributed to percolation-related fractionationalone and have to be a signature of the initial melt (possiblycarbonate rich). In general, chemical and mineralogical fingerprintsof a metasomatic medium are strongest near its source (e.g.a vein) whereas element patterns farther in the metasomatic‘column’ are increasingly controlled by fractionationmechanisms. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; trace elements; theoretical modelling 相似文献
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Chemical Variations in Peridotite Xenoliths from Vitim, Siberia: Inferences for REE and Hf Behaviour in the Garnet-Facies Upper Mantle 总被引：6，自引：2，他引：6
Peridotite xenoliths in a Miocene picrite tuff from the Vitimvolcanic province east of Lake Baikal, Siberia, are samplesof the off-craton lithospheric mantle that span a depth rangefrom the spinel to garnet facies in a mainly fertile domain.Their major and trace element compositions show some scatter(unrelated to sampling or analytical problems), which is notconsistent with different degrees of partial melting or metasomatism.Some spinel peridotites and, to a lesser degree, garnet-bearingperidotites are depleted in heavy rare earth elements (HREE)relative to middle REE (MREE), whereas some garnet peridotitesare enriched in HREE relative to MREE, with Lu abundances muchhigher than in primitive mantle estimates. Clinopyroxenes fromseveral spinel peridotites have HREE-depleted patterns, whichare normally seen only in clinopyroxenes coexisting with garnet.Garnets in peridotites with similar modal and major elementcompositions have a broad range of Lu and Yb abundances. Overall,HREE are decoupled from MREE and Hf and are poorly correlatedwith partial melting indices. It appears that elements withhigh affinity to garnet were partially redistributed in theVitim peridotite series following partial melting, with feweffects for other elements. The Lu–Hf decoupling may disturbHf-isotope depletion ages and their correlations with meltingindices. KEY WORDS: garnet peridotite; lithospheric mantle; Lu–Hf isotope system; Siberia; trace elements 相似文献
6.
Several spinel peridotite xenoliths from Spitsbergen have Sr–Ndisotopic compositions that plot to the right of the ‘mantlearray’ defined by oceanic basalts and the DM end-member(depleted mantle, with low 87Sr/86Sr and high 143Nd/144Nd).These xenoliths also show strong fractionation of elements withsimilar compatibility (e.g. high La/Ce), which cannot be producedby simple mixing of light rare earth element-depleted peridotiteswith ocean island basalt-type or other enriched mantle melts.Numerical simulations of porous melt flow in spinel peridotitesapplied to Sr–Nd isotope compositions indicate that thesefeatures of the Spitsbergen peridotites can be explained bychemical fractionation during metasomatism in the mantle. ‘Chromatographic’effects of melt percolation create a transient zone where thehost depleted peridotites have experienced enrichment in Sr(with a radiogenic isotope composition) but not in Nd, thusproducing Sr–Nd decoupling mainly controlled by partitioncoefficients and abundances of Sr and Nd in the melt and theperidotite. Therefore, Sr–Nd isotope decoupling, earlierreported for some other mantle peridotites worldwide, may bea signature of metasomatic processes rather than a source-relatedcharacteristic, contrary to models that invoke mixing with hypotheticalSr-rich fluids derived from subducted oceanic lithosphere. Pbisotope compositions of the Spitsbergen xenoliths do not appearto be consistently affected by the metasomatism. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; radiogenic isotopes; theoretical modelling 相似文献
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