Volatile control of contrasting trace element distributions in peralkaline granitic and volcanic rocks

Anomalous enrichments of Zr (>500 ppm), Zn (> 100 ppm), Nb (>25 ppm), Y (>60 ppm), Th (>20 pm), U (> 5 ppm), LREE (>230 ppm) and HREE (>35ppm), and high Rb/Sr (>5) characterize peralkaline granites, in contrast to their peraluminous and calc-alkaline equivalents. Within the peralkaline suite, comenditic and pantelleritic volcanics exhibit two- to five-fold increases in the concentrations of these trace elements over comagmatic granites. These cannot be explained by crystal- liquid fractionation processes, and require the evolution of a sodium-enriched fluid. Corresponding trace element increases in the granites in areas of alkali metasomatism support this argument, and reflect the partial confinement of this volatile phase within the high-level magma chambers. REE studies in particular might eventually allow an evaluation of the role of Cl⁻ versus F⁻ and CO₃-complexing in the evolution of the volatile fluid.     

Occurrence of lamproitic dykes at the northern Margin of the Indravati Basin,Bastar Craton,Central India

Occurrence of two lamproitic dykes intruding the basement granite near Khadka village at the northern margin of the Indravati Basin, Bastar craton is reported. Combined field, microscopy, XRD, EPMA and whole-rock geochemical investigations reveal that these lamproites were subjected to high degree of hydrothermal alteration as well as possible metamorphism. However, relicts of their original textures are well-preserved thereby providing important clues as to the nature of the protolith. Quartz, carbonate, chlorite and phlogopite constitute the bulk mineralogy whereas spinel, apatite and iron oxides are the accessory phases. Chemical composition of the groundmass spinels are strikingly similar to those from the lamproites. The Khadka lamproite dykes display high abundances of compatible elements such as Ni (238–396 ppm), Cr (484–892 ppm), and V (160–200 ppm) as well as high-field strength elements such as Zr (719–2057 ppm) and Nb (92–126 ppm) that resemble those in lamproites. Khadka lamproites also have high whole-rock REE abundances (ΣREE up to 1260 ppm) and display fractionated chondrite-normalized REE patterns (La/Yb= 113–237) which together with their average compatible and incompatible trace elemental ratios (e.g. Nb/Zr, Nb/La, Ba/Rb) are strikingly similar to those of the Mesoproterozoic Krishna lamproites of the Eastern Dharwar craton. Available field evidences suggest the Khadka lamproites to be of at least Palaeoproterzoic age (1.88 Ga) which makes them some of the oldest such rocks as yet documented from the Indian shield.     

The major and trace element chemistry of kaersutite and its bearing on the petrogenesis of alkaline rocks

New major and trace element analyses are presented for 7 kaersutites from basic alkaline rocks. K/Rb ratios lie between 1209 and 4276. Rb is low, averaging 6 ppm. Sr ranges from 532 to 1060 ppm and Ba from 181 to 701 ppm. Zr averages 109 ppm, Nb 44 ppm and V 390 ppm. There is a moderate enrichment in light REE. Zn correlates with FeO + Fe₂O₃ but the concentrations of Ni, Co, Cr, Cu and Pb are variable. Large variations in trace element concentration in kaersutites reflect only small variations in the melt when the distribution coefficient for a given element strongly favours the amphibole.Kaersutite is significant in the petrogenesis of alkaline rocks as a possible accessory phase in the upper mantle source regions, and as an important phase in the fractionation of basic alkaline liquids over a wide range of pressures.     

The Role of Lithospheric Mantle Heterogeneity in the Generation of Plio-Pleistocene Alkali Basaltic Suites from NW Harrat Ash Shaam (Israel)

Plio-Pleistocene volcanism in the Golan and Galilee (northeasternIsrael) shows systematic variability with time and location:alkali basalts were erupted in the south during the Early Pliocene,whereas enriched basanitic lavas erupted in the north duringthe Late Pliocene (Galilee) and Pleistocene (Golan). The basaltsshow positive correlations in plots of ratios of highly to moderatelyincompatible elements versus the concentration of the highlyincompatible element (e.g. Nb/Zr vs Nb, La/Sm vs La) and indiagrams of REE/HFSE (rare earth elements/high field strengthelements) vs REE concentration (e.g. La/Nb vs La). Some of thesecorrelations are not linear but upward convex. ⁸⁷Sr/⁸⁶Sr ratiosvary between 0·7031 and 0·7034 and correlate negativelywith incompatible element concentrations and positively withRb/Sr ratios. We interpret these observations as an indicationthat the main control on magma composition is binary mixingof melts derived from two end-member mantle source components.Based on the high Sr/Ba ratios and negative Rb anomalies inprimitive mantle normalized trace element diagrams and the moderateslopes of MREE–HREE (middle REE–heavy REE) in chondrite-normalizeddiagrams, we suggest that the source for the alkali basalticend-member was a garnet-bearing amphibole peridotite that hadexperienced partial dehydration. The very high incompatibleelement concentrations, low K content, very low Rb contentsand steep MREE–HREE patterns in the basanites are attributedto derivation from amphibole- and garnet-bearing pyroxeniteveins. It is suggested that the veins were produced via partialmelting of amphibole peridotites, followed by complete solidificationand dehydration that effectively removed Rb and K. The requirementfor the presence of amphibole limits both sources to lithosphericdepths. The spatial geochemical variability of the basalts indicatesthat the lithosphere beneath the region is heterogeneous, composedof vein-rich and vein-poor domains. The relatively uniform ¹⁴³Nd/¹⁴⁴Nd(Nd = 4·0–5·2) suggests that the two mantlesources were formed by dehydration and partial melting of anoriginally isotopically uniform reservoir, probably as a resultof a Paleozoic thermal event. KEY WORDS: basanites; lithospheric heterogeneity; magma mixing; amphibole peridotite; pyroxenites     

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east–west along the southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the Birch–Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of mafic, intermediate and felsic composition. Tholeiitic basalts range compositionally from Mg# 59–26 evolving continuously to greater REE contents (La=2–19 ppm; Th/La_pm˜1), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Yb_n=1–8), high Th/Nb_pm ratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium andesites. The two suites are interpreted as a paired arc–back arc sequence. A third group of Nb-enriched basalts (NEB; Nb=9–18 ppm) extend to extremely high TiO₂, Ta, P₂O₅, Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas Zr/Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but flat trace element patterns and is interpreted as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean high-Al, high-La/Yb_n tonalites; they possess Yb ≤ 0.4 ppm, Y ≤ 6 ppm and Sc ≤ 8 ppm, with La/Yb_n of 19–30 and Zr/Sm of 50–59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere, often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched source that subsequently melts during induced mantle convection. The Archean adakite–NEB association erupted during development of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the Birch–Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs. Received: 2 June 1999 / Accepted: 29 December 1999     

Records of Crustal Metasomatism in the Garnet Peridotites of the Ulten Zone (Upper Austroalpine, Eastern Alps)

Peridotites in the Ulten Zone (Upper Austroalpine, Eastern Alps),occur as small bodies within lower-crustal rocks (gneisses andmigmatites) subducted at eclogite-facies conditions during theVariscan orogeny. They record a complex metamorphic and deformationevolution as indicated by the transition from coarse-grainedspinel-bearing peridotites to fine-grained garnet + amphibole-bearingperidotites, and are interpreted as portions of mantle wedgethat were incorporated in a downgoing slab of cold continentalcrust. The transition from spinel- to garnet-bearing assemblagewas accompanied by significant input of metasomatic agents,as shown by the crystallization of abundant amphibole. Herewe present trace-element mineral chemistry data for selectedUlten peridotites, with the aim of unravelling the nature ofthe metasomatic processes. Amphiboles display significant lightrare earth element (LREE) enrichment [Ce_N/Yb_N = 3·90–11·50;LREE up to (20–50) x C1], high Sr (150–250 ppm),K (1910–7280 ppm) and Ba (280–800 ppm) contents,and low concentrations of high field strength elements (HFSE)(Zr = 14–25 ppm, Y = 6·7–16 ppm, Ti = 1150–2500ppm, Nb = 2–7 ppm). On the basis of (1) the evidence formodal orthopyroxene decrease as a result of the garnet-formingreaction rather than abundant orthopyroxene crystallization,(2) the high modal amounts of amphibole (up to 23%) in the mostmetasomatized peridotites and (3) the strong large ion lithophileelement (LILE)/HFSE fractionation in amphiboles, we infer thatthe metasomatic agent was an H₂O–CO₂ fluid with a lowCO₂/H₂O ratio. Petrological investigations and geochronologicaldata indicate that the host metapelites experienced in situpartial melting and migmatization concomitantly with the garnet+ amphibole-facies recrystallization in the enclosed peridotites.We infer that the metasomatizing hydrous fluids could representthe residual fluids left after the crystallization of leucosomes,starting from water-undersaturated melts produced during migmatizationof the host gneisses. KEY WORDS: garnet peridotite; crustal metasomatism; amphibole; hydrous fluids     

The REE-Ti mineral chevkinite in comenditic magmas from Gran Canaria,Spain: a SYXRF-probe study

The REE-Ti silicate chevkinite has been recognised previously in Miocene ignimbrites from Gran Canaria, and in correlative offshore syn-ignimbrite turbidites. We have estimated the partition coefficients of REE, Y, Zr and Nb for chevkinite and co-existing peralkaline rhyolitic (comendite) glass using synchrotron-XRF-probe analyses (SYXRF) in order to evaluate the role of this mineral in the REE budget of felsic peralkaline magmas. The Zr/Nb ratio of the chevkinite is 1.55–1.7, strongly contrasting with Zr/Nb of 6.5 in the associated glass. Zr shows a three-fold enrichment in chevkinite relative to the residual melt, whereas Nb is enriched by a factor >10. The enrichment of Ce and La in chevkinite is even more significant, namely 19 wt(%) Ce and 12 wt(%) La, compared to 236 ppm Ce and 119 ppm La in the glass. Chevkinite/glass ratios are 988±30 for La, 806±30 for Ce, 626±30 for Pr, 615±40 for Nd, 392±50 for Sm, 225±30 for Eu, 142±25 for Gd, 72±20 for Dy. For trace elements, we derived Kd_TE of 74±25 for Y, >8 for Hf, >50 for Th, 15±5 for Nb and 3.55±0.4 for Zr. Mineral/glass ratios for co-existing titanite are 28±10 for La, 86±20 for Ce, 98±30 for Pr, 134±35 for Nd, 240±50 for Sm, 50±20 for Eu, 96±25 for Gd, 82±25 for Dy, 99±30 for Y, 45±10 for Nb and 3±0.5 for Zr. Based on these data, the removal of only 0.05 wt% of chevkinite from a magma with initially 300 ppm Ce would deplete the melt by 93 ppm to yield 207 ppm Ce in the residual liquid. Chevkinite thus appears, when present, to be the controlling mineral within the LREE budget of evolved peralkaline magmas.Editorial responsibility: I. Parsons

---

Trace-element composition and zoning in clinopyroxene- and amphibole-group minerals: Implications for element partitioning and evolution of carbonatites

The present work is a first comprehensive study of the trace-element composition and zoning in clinopyroxene- and amphibole-group minerals from carbonatites, incorporating samples from 14 localities worldwide (Afrikanda, Aley, Alnö, Blue River, Eden Lake, Huayangchuan, Murun, Oka, Ozernaya Varaka, Ozernyi, Paint Lake, Pinghe, Prairie Lake, Turiy Mys). The new electron-microprobe data presented here significantly extend the known compositional range of clinopyroxenes and amphiboles from carbonatites. These data confirm that calcic and sodic clinopyroxenes from carbonatites are not separated by a compositional gap, instead forming an arcuate trend from nearly pure diopside through intermediate aegirine–augite compositions confined to a limited range of CaFeSi₂O₆ contents (15–45 mol%) to aegirine with < 25 mol% of CaMgSi₂O₆ and a negligible proportion of CaFeSi₂O₆. A large set of LA-ICPMS data shows that the clinopyroxenes of different composition are characterized by relatively low levels of Cr, Co and Ni (≤ 40 ppm) and manifold variations in the concentration of trivalent lithophile and some incompatible elements (1–150 ppm Sc, 26–6870 ppm V, 5–550 ppm Sr, 90–2360 ppm Zr, and nil to 150 ppm REE), recorded in some cases within a single crystal. The relative contribution of clinopyroxenes to the whole-rock Rb, Nb, Ta, Th and U budget is negligible. The major-element compositional range of amphiboles spans from alkali- and Al-poor members (tremolite) to Na–Al-rich Mg- or, less commonly, Fe-dominant members (magnesiohastingsite, hastingsite and pargasite), to calcic–sodic, sodic and potassic–sodic compositions intermediate between magnesio-ferrikatophorite, richterite, magnesioriebeckite, ferri-nyböite and (potassic-)magnesio-arfvedsonite. In comparison with the clinopyroxenes, the amphiboles contain similar levels of tetravalent high-field-strength elements (Ti, Zr and Hf) and compatible transition elements (Cr, Co and Ni), but are capable of incorporating much higher concentrations of Sc and incompatible elements (up to 500 ppm Sc, 43 ppm Rb, 1470 ppm Sr, 1230 ppm Ba, 80 ppm Pb, 1070 ppm REE, 140 ppm Y, and 180 ppm Nb). In some carbonatites, amphiboles contribute as much as 25% of the Zr + Hf, 15% of the Sr and 35% of the Rb + Ba whole-rock budget. Both clinopyroxenes and amphiboles may also host a significant share (~ 10%) of the bulk heavy-REE content. Our trace-element data show that the partitioning of REE between clinopyroxene (and, in some samples, amphibole) and the melt is clearly bimodal and requires a revision of the existing models assuming single-site REE partitioning. Clinopyroxenes and amphiboles from carbonatites exhibit a diversity of zoning patterns that cannot be explained exclusively on the basis of crystal chemistry and relative compatibility of different trace-element in these minerals. Paragenetic analysis indicates that in most cases, the observed zoning patterns develop in response to removal of selected trace elements by phases co-precipitating with clinopyroxene and amphibole (especially magnetite, fluorapatite, phlogopite and pyrochlore). With the exception of magnesiohastingsite–richterite sample from Afrikanda, the invariability of trace-element ratios in the majority of zoned clinopyroxene and amphibole crystals implies that fluids are not involved in the development of zoning in these minerals. The implications of the new trace-element data for mineral exploration targeting REE, Nb and other types of carbonatite-hosted rare-metal mineralization are discussed.     

Metamorphic garnet pyroxenite from the 540-600 m main borehole of the Chinese Continental Scientific Drilling (CCSD) project

Compared to other mafic and ultramafic rocks from the CCSD main borehole as well as from the outcrops, rocks from the 540-600 m section is extraordinary in terms of its geophysical as well as geochemical properties. It consists of > 70% hematite-ilmenite garnet pyroxenite (HI-GPX) and < 30% intercalated rutile garnet pyroxenite (R-GPX). Whole-rock geochemical data show that HI-GPXs have: (1) relatively high V with an average of 606 ppm, but lower Nb and Ta; (2) highest TFeO, Fe₂O₃/FeO ratio, and highly variable but strong positive Eu anomalies with Eu/Eu? up to 2.9; (3) anomalously high V/Sc ratios ranging from 8.39 to 43.23, average 15.03; and (4) high amounts of hematite-ilmenite solid solutions with a very fine intergrowth structure down to nanometer scale. V/Sc ratios in the CCSD garnet pyroxenites are correlated negatively with MgO, but positively with Fe₂O₃/FeO ratios. Both suites of pyroxenites have similar rare earth elements and high field strength elements geochemistry. These features demonstrate that these pyroxenites were formed from metamorphism of high-Fe and/or -Ti gabbroic cumulates. This can account not only for low high field strength elements (HFSE) and rare earth elements (REE) but also low Nb/Ta and Zr/Hf ratios in these rocks. Seemingly negative correlation between Nb/Ta and Zr/Hf in the CCSD metabasites, not significantly affected by UHP metamorphism, is also consistent with the silicate differentiation trend in a basaltic magma chamber.     

Depositional conditions,characteristics and source of rare earth elements in carbonate strata of the Albian Asu River Group,Middle Benue Trough,north central Nigeria

Major, trace and rare earth elements (REE) concentrations in limestone beds of the Asu River Group within the Middle Benue Trough were measured to understand the depositional conditions, characteristics and source of REE. The limestone has high content of CaO (Average of 46.55%), followed by SiO₂ (Average of 7.90 %), Fe₂O₃(t), MgO and Al₂O₃. The limestones are depleted in most of the trace elements (Co, V, Rb, Ba, Zr, Y, Nb, Hf and Th) when compared with the Post-Archean Australian Shale (PAAS). The observed large variations in ΣREE contents among various limestones of the present study (12.22 to 142.53ppm) are mainly due to the amount of terrigenous matter present in them. The characteristics of non-seawater-like REE patterns, elevated REE concentrations, high La_N/Yb_N ratios and low Y/Ho ratios, suggest that the observed variations in ΣREE contents are mainly controlled by the amount of detrital sediments in the limestones of the Asu River Group in the middle Benue trough. The observed variations in Ce contents and Ce anomalies in the studied samples resulted from detrital input. The limestones show positive Mn* values (0.30 to 0.78) and low contents of U (～0.60–3.20 ppm) suggesting that they were deposited under oxygen-rich environment.     

Geochemistry, petrogenesis and tectonic setting of late Neoproterozoic Dokhan-type volcanic rocks in the Fatira area, eastern Egypt

 The Neoproterozoic Dokhan volcanics of the Fatira area in eastern Egypt comprise two main rock suites: (a) an intermediate volcanic suite, consisting of basaltic andesite, andesite, dacite, and their associated pyroclastic rocks; and (b) a felsic volcanic suite composed of rhyolite and rhyolitic tuffs. The two suites display well-defined major and trace element trends and a continuum in composition with wide ranges in SiO₂ (54–76%), CaO (8.19–0.14%), MgO (6.96–0.04%), Sr (983–7 ppm), Zr (328–95 ppm), Cr (297–1 ppm), and Ni (72–1 ppm). They are enriched in LILEs (Rb, Ba, K, Th, Ce) relative to high field strength elements (Nb, Zr, P, Ti) and show strong affinity to calc-alkaline subduction-related rocks. However, their undeformed character, their emplacement temporally and spatially with post-orogenic A-type granite, and their high Zr/Y values suggest that their emplacement follow the cessation of subduction in eastern Egypt in an extensional-related within-plate setting. Major and trace element variations in the intermediate volcanics are consistent with their formation via partial melting of an enriched subcontinental lithospheric mantle source followed by a limited low-pressure fractional crystallization of olivine and pyroxene before emplacement. The LILE enrichment relative to HFSE is attributed to the inheritance of a subduction component from mantle material which constituted the mantle wedge during previous subduction events in eastern Egypt. The evolution of the whole volcanic spectrum was governed mainly by crystal/melt fractionation of amphibole, plagioclase, titanomagnetite, and apatite in the intermediate varieties and plagioclase, amphibole, biotite, Fe–Ti oxides, apatite, and zircon in the felsic varieties. At each stage of evolution, crystal fractionation was accompanied by variable degrees of crustal contamination. Received: 28 June 1998 / Accepted: 25 August 1999     

Hydrothermal alteration and evolution of Zr-Th-U-REE mineralization in the microgranite of Wadi Ras Abda,North Eastern Desert,Egypt

Ras Abda plutonic suite, North Eastern Desert of Egypt, consists predominantly of Neoproterozoic calc-alkaline older granites. Minor exposures of pink microgranite are occurring along Wadi Ras Abda within the older granites. Previous studies on this area demonstrated that the microgranite is altered in some parts and contains anomalous concentrations of rare metal elements (Zr, Th, and U). These altered and mineralized zones are re-assessed using field observations, chemical analysis, and by the application of various transmitted light and electron microscopic techniques. The rare metals exist as mineral segregation grew freely into open cavities of the microgranite and concordant with the NNE strike-slip fault movement. The mineralized zones contain an assemblage of secondary magnetite, zircon, uranothorite, columbite-(Mn), fergusonite-(Y), and allanite-(Ce). The extreme abundance of zircon in the mineralized zone, along with other evidence, indicates a hydrothermal origin of this zircon together with associated rare metals. The geochemical investigation and mass balance calculations revealed extreme enrichment of Zr, Th, U, Y, Nb, Ta, and REE. Post-magmatic hydrothermal alterations resulted in such pronounced chemical and mineralogical heterogeneity. The hydrothermal fluids are thought to be oxidizing, alkaline and of medium temperature (>?250 °C). The average contents of the elements Zr (1606 ppm), Th (1639 ppm), U (306 ppm), Nb (955 ppm), and REE (1710 ppm) in the mineralized microgranite reach sub-economic levels and could be a potential source of these elements.     

黄河和长江沉积角闪石亲石元素地球化学特征对比与物源辨识

为辨识黄河和长江入海沉积物中角闪石的物源差异,对采自黄河口段、长江口段以及废黄河口和苏北沿岸,共26个样点、38组粒度粗细不同的碎屑角闪石进行了矿物元素地球化学测试,获得了这些角闪石群体的50种常量和微量元素含量值.结果表明:不同粒级测量的同源角闪石元素含量除少数大离子活泼元素相对偏差较大之外,大部分元素含量差异性较小...     

Rare earth and trace elements in igneous and high-temperature metamorphic minerals of oceanic gabbros (MARK area, Mid-Atlantic Ridge)

SIMS analyses have been carried out on clinopyroxenes, plagioclases and amphiboles of six gabbroic samples from Holes 921–924 of the Ocean Drilling Program Leg 153 sited in the MARK area of the Mid-Atlantic Ridge at the ridge–transform intersection, to investigate the rare earth, trace and volatile element distribution in the lower ocean crust during igneous crystallization and higher grade metamorphic conditions. The metagabbros underwent granulite to subgreenschist facies conditions through three main tectono-metamorphic phases: (1) ductile regime (750 < T < 1000 °C and P ≈ 0.3 GPa); (2) transitional regime (600 < T < 700 °C and P ≈ 0.2 GPa); (3) brittle regime (350 ≤ T < 600 °C and P < 0.2 GPa). Igneous clinopyroxenes show C1-chondrite normalized patterns depleted in LREE, and nearly flat for HREE. The rare earth and trace element distributions in igneous clinopyroxenes and plagioclases indicate that these minerals act as REE reservoirs, and comprise the main contribution to the overall rock content. The abundances in igneous minerals reflect the degree of fractionation of the parent liquids. In metamorphic clinopyroxenes recrystallized in anhydrous assemblages, the REE and trace elements patterns mimic those of the primary ones. Conversely, clinopyroxene re-equilibrated in amphibole-bearing assemblages shows a significant increase in REE, Ti, Zr, Y and V, a negative Eu anomaly, and slight decreases in Sr and Ba. An overall increase of REE and some trace elements is evident in hydrous assemblages, with preferential partitioning in the amphibole. It shows high Ti (18196–22844 ppm), LREE depleted patterns and La_N/Sm_N=0.10–0.33, La_N/Yb_N=0.10–0.30. Amphiboles from granoblastic assemblages show homogeneous patterns with no or a positive anomaly for Ti_N and negative anomalies for Sr_N and Zr_N. Volatiles in amphibole are low, with Cl/F < 1; H₂O% is significantly lower than the stoichiometric ratio (1.33–1.53%). The composition of the clinopyroxene and amphibole recrystallized in low-strain domains records evidence of incomplete re-equilibration, and element diffusion and partitioning is in part controlled by the textural site. The possible origins of the fluids involved in the metamorphic recrystallization are discussed: (1) remobilization from igneous amphibole; (2) exsolution from evolved melts; (3) introduction of seawater-derived fluids modified in rock-dominated systems; (4) injection of highly evolved hydrous melts during the metamorphic process. Received: 6 September 1999 / Accepted: 6 March 2000     

Metasomatism in the subcontinental mantle beneath the Eastern Carpathians (Romania): new evidence from trace element geochemistry

A wide range of trace elements have been analysed in mantle xenoliths (whole rocks, clinopyroxene and amphibole separates) from alkaline lavas in the Eastern Carpathians (Romania), in order to understand the process of metasomatism in the subcontinental mantle of the Carpatho-Pannonian region. The xenoliths include spinel lherzolites, harzburgites and websterites, clinopyroxenites, amphibole veins and amphibole clinopyroxenites. Textures vary from porphyroclastic to granoblastic, or equigranular. Grain size increases with increasing equilibrium temperature of mineralogical assemblages and results from grain boundary migration. In peridotites, interstitial clinopyroxenes (cpx) and amphiboles resulted from impregnation and metasomatism of harzburgites or cpx-poor lherzolites by small quantities of a melt I with a melilitite composition. Clinopyroxenites, amphibole veins and amphibole clinopyroxenites are also formed by metasomatism as a result of percolation through fracture systems of large quantities of a melt II with a melanephelinite composition. These metasomatic events are marked by whole-rock enrichments, relative to the primitive mantle (PM), in Rb, Th and U associated in some granoblastic lherzolites and in clinopyroxene and amphibole veins with enrichments in LREE, Ta and Nb. Correlations between major element whole-rock contents in peridotites demonstrate that the formation of interstitial amphibole and clinopyroxene induced only a slight but variable increase of the Ca/Al ratio without apparent modifications of the initial mantle composition. Metasomatism is also traced by enrichments in the most incompatible elements and the LREE. The Ta, Nb, MREE and HREE contents remained unchanged and confirm the depleted state of the initial but heterogeneous mantle. Major and trace element signature of clinopyroxene suggests that amphibole clinopyroxenites and some granoblastic lherzolites have been metasomatized successively by melts I and II. Both melts I and II were Ca-rich and Si-poor, somewhat alkaline (Na > K). Melt I differed from melt II in having higher Mg and Cr contents offset by lower Ti, Al, Fe and K contents. Both were highly enriched in all incompatible trace elements relative to primitive mantle, showing positive anomalies in Rb, Ba, Th, Sr and Zr. They contrasted by their Ta, Nb and LREE contents, lower in melt I than in melt II. Melts I and II originate during a two-stage melting event from the same source at high pressure and under increasing temperature. The source assemblage could be that of a metasomatized carbonated mantle but was more likely that of an eclogite of crustal affinity. Genetic relationships between calc-alkaline and alkaline lavas from Eastern Carpathians and these melts are thought to be only indirect, the former originating from partial melting of mantle sources respectively metasomatized by the melts I and II. Received: 17 March 1997 / Accepted: 14 July 1997     

The pollution of marine sediments by trace elements in the coastal region of Togo caused by dumping of cadmium-rich phosphorite tailing into the sea

 The marine coastal sediments from Togo have been analysed for the trace elements Cd, Cr, Cu, Ni, Pb, Sr, V, Zn and Zr to ascertain the geo-ecological impact of dumping of phosphorite tailings into the sea. Trace element concentrations ranged from 2–44 ppm for Cd, 22–184 ppm for Cu, 19–281 ppm for Ni, 22–176 ppm for Pb, 179–643 ppm for Sr, 38–329 ppm for V, 60–632 ppm for Zn and 18–8928 ppm for Zr. Regional distribution of trace elements in the marine environment indicates that the concentrations of Cr, Cu, Ni, Pb, V, Sr and Zn increase seawards and along the coastal line outwards of the tailing outfall, whereas Cd and Zr showed reversed spatial patterns. Sorting and transport of phosphorite particles by coastal currents are the main factors controlling the distribution of particle-bound trace metals in the coastal environment. The Cd, Sr and Zn concentrations decrease with decreasing grain size in marine coastal sediments, whereas Cr, Cu, Ni and Zn concentrations increase with decreasing grain size. Percolation and shaking experiments were carried out in laboratory using raw phosphate material and artificial sea water. Enhanced mobilization of Cd from phosphorites by contact with the sea water was observed. Received: 11 May 1998 · Accepted: 20 October 1998     

Roles of Melting and Metasomatism in the Formation of the Lithospheric Mantle beneath the Leizhou Peninsula, South China

This study characterizes the nature of fluid interaction andmelting processes in the lithospheric mantle beneath the Yingfenglingand Tianyang volcanoes, Leizhou Peninsula, South China, usingin situ trace-element analyses of clinopyroxene, amphibole andgarnet from a suite of mantle-derived xenoliths. Clinopyroxenesfrom discrete spinel lherzolites exhibit large compositionalvariations ranging from extremely light rare earth element (LREE)-depletedto LREE-enriched. Trace-element modelling for depleted samplesindicates that the Leizhou lherzolites are the residues of amantle peridotite source after extraction of 1–11% meltgenerated by incremental melting in the spinel lherzolite fieldwith the degree of melting increasing upwards from about 60km to 30 km. This process is consistent with gradational meltingat different depths in an upwelling asthenospheric column thatsubsequently cooled to form the current lithospheric mantlein this region. The calculated melt production rate of thiscolumn could generate mafic crust 5–6 km thick, whichwould account for most of the present-day lower crust. The formationof the lithospheric column is inferred to be related to Mesozoiclithosphere thinning. Al-augite pyroxenites occur in compositexenoliths; these are geochemically similar to HIMU-type oceanisland basalt. These pyroxenites postdate the lithospheric columnformation and belong to two episodes of magmatism. Early magmatism(forming metapyroxenites) is inferred to have occurred duringthe opening of the South China Sea Basin (32–15 Ma), whereasthe most recent magmatic episode (producing pyroxenites withigneous microstructures) occurred shortly before the eruptionof the host magmas (6–0·3 Ma). Trace-element traversesfrom the contacts of the Al-augite pyroxenite with the spinelperidotite wall-rock in composite xenoliths record gradientsin the strength and nature of metasomatic effects away fromthe contact, showing that equilibrium was not attained. Significantenrichment in highly incompatible elements close to the contacts,with only slight enrichment in Sr, LREE and Nb away from thecontact, is inferred to reflect the different diffusion ratesof specific trace elements. The observed geochemical gradientsin metasomatic zones show that Sr, La, Ce and Nb have the highestdiffusion rates, other REE are intermediate, and Zr, Hf andTi have the lowest diffusion rates. Lower diffusion rates observedfor Nb, Zr, Hf and Ti compared with REE may cause high fieldstrength element (HFSE) negative anomalies in metasomatizedperidotites. Therefore, metasomatized lherzolites with HFSEnegative anomalies do not necessarily require a carbonatiticmetasomatizing agent. KEY WORDS: China; lithosphere; mantle xenoliths; clinopyroxene trace elements; mantle partial melting; mantle metasomatism; trace-element diffusion rates     

Trace elements and their variation along seam profiles of certain coal seams of Middle and Upper Barakar formations (Lower Permian) in East Bokaro coalfield,district Hazaribagh,Bihar, India

Trace-element data are presented for the first time for any coal seam in India, across a full working section, based on systematically collected channel samples of coal, together with their maceral composition. The trace-element variation curves along the seam profile are presented together with group maceral compositions of Kargali Bottom, Kargali Top, Kargali, Kathara, Uchitdih, Jarangdih Bottom, Jarangdih, and Jarangdih Top seams, East Bokaro coalfield. The Kathara and Uchitdih seams have also been sampled at two other localities and lateral variation in data in their trace-element and maceral compositions is also evaluated.The East Bokaro coals have: Ba and Sr > 1000 ppm; Mn < 450 ppm; Zr < 400 ppm; Ni and V < 250 ppm; Cr < 185 ppm; La < 165 ppm; Cu, Nb, and B < 125 ppm; Pb, Co and Y < 75 ppm; Ga, Sn, Mo, In and Yb < 15 ppm; Ag 2 ppm; and Ge 7 ppm. Petrographically, the coals are dominant in vitrinite (33–97%), rare in exinite (<15%), and semifusinite (0.8–49%) is the dominant inertinite maceral, with variable mineral and shaly matter (11–30%), graphic representation of trace elements versus vitrinite, inertinite, and coal ash indicates the affinity of (a) vitrinite with Cu, Ni, Co, V, Ga and B; (b) inertinite with Nb and B; and (c) coal ash (mineral matter) with Pb, Cu, Ni, La, Mn and Y; Ba, Cr, Sr, Zr, Cu and Ni are of organic as well as inorganic origins.The trend of the variation patterns and average compositions of the different seams are shown to be distinct and different. The variation along the same profile is inferred to be different for different seams of the coalfield.Trace-element data for certain coals of seams from different coalfields in the Gondwana basins of India are presented. There is a wide difference for each of these basins with respect to certain elements. This is suggestive of the proportions of Cu, Ni, V, Y, Ba, Sr, Cr, B, Zr and Ag, characterizing the different Gondwana Basins.     

金红石微量元素电子探针分析

金红石电子探针微量元素分析一般以人工合成的氧化物来作为监测标样,尚较缺乏对金红石标样进行系统地测试分析。本文运用CAMECA SXFive电子探针对金红石标样R10进行微量元素分析,根据金红石中主要微量元素在地质学中的应用,本次共分析了Al、Si、Ti、Fe、Cr、Zr、V、Nb、Ta等9个元素,Ti、Si元素作为本次分析的监测元素。本文通过调整加速电压和电流、背景和峰值测试时长以及干扰谱峰处理等来提高微量元素分析精度和准确度。分析结果显示,其中,Zr(780±29×10~(-6))(1SD,n=25)、Nb(2799±66×10~(-6))、V(1276±33×10~(-6))、Fe(4309±34×10~(-6))、Cr(718±31×10~(-6))的分析结果与二次离子质谱(SIMS)和激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)的推荐值在误差范围内一致。大部分元素数据波动范围在10%以内,V、Fe元素的数据波动范围仅在5%以内。V、Nb和Fe测试精度比前人电子探针分析结果有较大提高。金红石Zr测试误差传递给金红石Zr温度计给出的温度误差一般22℃。本文还对金红石Zr温度计应用、提高Ta元素分析精度和准确度、金红石Fe~(3+)分析等问题进行了探讨。     

Experimental cpx/melt partitioning of 24 trace elements

Cpx/melt partition coefficients have been determined by ion probe for 24 trace elements at natural levels in an alkali basalt experimentally equilibrated at 1,380°C and 3 GPa. One goal was to intercompare Ds for both high-field-strength elements and rare earth elements (REE) in a single experiment. Relative to the REE spidergram, Hf and Ti show virtually no anomaly, whereas Zr exhibits a major negative anomaly. Other incompatible elements (Ba, K, Nb) fall in the range of published values, as do elements such as Sr, Y, Sc, Cr and V. Pb shows a value intermediate between La and Ce. Values for Be, Li and Ga are reported for the first time, and show that Be is as incompatible as the light REEs whereas Li and Ga are somewhat more compatible than the heavy REE.