Light Rare Earth Element Depletion Research Articles

Geochemistry and Petrology of Some Proterozoic Banded Iron-Formations of the Quadrilatero Ferrifero, Minas Gerais, Brazil

This study presents detailed geochemical and petrologic data for 16 samples collected from four different iron-formations in the Early Proterozoic Minas Supergroup of the Quadrilatero Ferrifero. These banded ironformation samples represent the best preserved, unweathered, and least altered iron-formation precursors to the major hematite-rich ore deposits of the region. Since the 17th century, the Quadrilatero Ferrifero has been recognized as a major gold- and iron-rich province, which has resulted in it being the best-studied Precambrian region in Brazil. The banded iron-formations, locally known as itabirites, and associated rocks, including dolomites, all are part of the Proterozoic Minas Supergroup with a minimum Pb/Pb age determined on the dolomites of 2.4 Ga. The samples were obtained from fresh outcrops in the field, from fresh exposures in open pit mines or from deep diamond drill cores in four localities: the Santuario da Serra da Piedade, and the open pit iron ore mines of Pico do Itabirito, Aguas Claras, and Mutuca. All these have hematite as the only or principal iron oxide; maghemite was found in only one sample. This is in contrast to the common occurrence of magnetite as a major iron oxide in many Archean and Early Proterozoic iron-formations. Although all four localities have been affected by low-grade metamorphism and complex tectonics, none of the assemblages studied show any metamorphic reaction between the oxides and quartz, or between the oxides and carbonates and/or quartz to form metamorphic iron-rich silicates. All of the samples studied (except one that contains maghemite) consist of quartz-hematite, quartz-hematite-carbonate, or hematite-carbonate. It is unknown whether all or only part of the hematite represents a primary phase. In view of the deep lateritic conditions in much of Brazil it is likely that not all of the hematite reflects a primary Fe 3+ -rich oxide or hydroxide phase, but that some of the hematite is a secondary oxidation product. The overall major oxide bulk chemistry of these four iron-formations is very similar to that of most Archean and Proterozoic iron-formations studied except with respect to their very high Fe2O3 (due to hematite) and correspondingly very low FeO contents. Many of the REE patterns of the iron-formation samples show a considerable depletion in light REE, a small negative Ce anomaly and a positive Eu anomaly. Such positive Eu anomalies are common in most Archean and Proterozoic iron-formations and are considered to reflect the result of deep-ocean hydrothermal input into an otherwise highly seawater-dominated system. Such a hydrothermal origin is corroborated when the sum of the REE is plotted against Co + Ni + Cu with the resulting data points clustering in the hydrothermal deposits for metalliferous ocean bottom sediments. The carbon isotope compositions of the carbonate-containing iron-formations range from δ 13 C of ‐1.055 to ‐5.083, which coincides with a range of values for other Proterozoic iron-formations of South Africa (Kuruman iron-formation sequence).

Pb, Sr and Nd Isotopic Systematics of the Carbonatites of Sung Valley, Meghalaya, Northeast India: Implications for Contemporary Plume-Related Mantle Source Characteristics

A Pb/Pb–Pb/Pb age of 134± 20 Ma with a model INTRODUCTION l1 of 8·19 ± 0·02 has been obtained for the carbonatites from Carbonatites are carbonate-rich igneous rocks of mantle Sung Valley, Meghalaya, placing their time of emplacement very origin. They are found mainly in continental settings and close to the time assigned for the break-up of Gondwana. Initial their emplacement ages range from Late Archaean to eSr (+5·3 to+7·8), eNd (+1·7 to+2·3), Pb/Pb (19·02), Quaternary (Woolley, 1989). Their high abundances of Pb/Pb (15·67) and Pb/Pb (39·0) indicate a mantle Sr (average 7000 ppm) and Nd (average 250 ppm) source region with a somewhat higher Rb/Sr ratio than Bulk Earth, preclude significant changes in Sr and Nd isotopic ratios minor light rare earth element (LREE) depletion, and a timecaused by crustal contamination and provide a unique integrated enhancement of U/Pb. The isotopic characteristics of the means of characterizing the evolution of the mantle from Sung Valley carbonatites, however, differ significantly from those of Archaean to Recent times (Bell et al., 1982; Bell & basalts from Rajmahal and the Ninetyeast Ridge, which have a Blenkinsop, 1987). In the Indian Peninsula, carbonatites relatively more depleted upper-mantle source component, indicating have been reported from various geological settings with involvement of different mantle sources. The mantle source region of ages ranging from Early Proterozoic to Tertiary (Krishnathe carbonatites is characterized by an EM2–HIMU signature, murthy, 1988; Natarajan et al., 1994). Petrological, minsimilar to that observed for Ambadongar, western India, but different eralogical and chemical data are available for most of from the carbonatites of East Africa, which have an EM1–HIMU the Indian carbonatite complexes (Krishnamurthy, 1988). signature. It is speculated that the Sung Valley carbonatites were derived from a parental magma generated by partial melting of the However, isotopic data remain scarce and are confined sub-continental lithosphere, which was previously metasomatized to Sr/Sr analyses from Sevattur (Deans & Powell, by fluids derived from an EM2–HIMU type mantle plume. The 1968; Kumar & Gopalan, 1991), an Rb–Sr study on data suggest that the pre-130 Ma sub-continental mantle exhibited carbonatites of Hogenekal (Natarajan et al., 1994), an preferentially an EM2 type signature. isotopic study of Sr, Nd and Pb from the Ambadongar carbonatite complex (Veena et al., 1993; Simonetti et al., 1995) and a recent Pb/Pb age determination on the Newania and Sevattur carbonatites of India (Schleicher et al., 1997). This study attempts to define the combined Sr, Nd and Pb isotopic systematics of the Sung Valley

Geochemistry of Organic-Rich Black Shales Overlying the Natural Nuclear Fission Reactors of Oklo, Republic of Gabon

The organic-rich black shales of the Franceville Series' FB Formation overlying the uranium ores and natural nuclear fission reactors of Ohio, Gabon, are not notably metalliferous. Chromium, gold, silver, and barium are slightly enriched in average Ohio black shale (AOK) relative to black shale standard SDO-1. Geochemical variations among the black shale samples of the sedimentary sequence include enrichment in potassium, barium, chromium, and silver in the four lowermost samples, the presence of a bleached zone depleted in organic carbon lowermost in the sequence, and elevated rare earth element (REE) content in samples closest to the Oklo reactor zones. Hydrothermal activity has influenced the geochemistry of the black shale but is evidently not linked to reactor-driven processes. Chondrite-normalized REE patterns of Oklo black shale samples show slight enrichment in light REE and slight depletion in heavy REE, especially in the sample closest to the reactor zone. However, comparison of REE content with various petrographic facies in and near the Oklo reactors shows no apparent enrichment in fission product (intermediate) REE. With few exceptions, reactor facies all contain more REE than AOK The chondrite-normalized REE pattern of AOK resembles that of greywacke-shale turbidites of Archean greenstone belts. The paucity of uranium and manganese in AOK is a curious anomaly in an area of world class uranium and manganese deposits.

Infiltration of refractory melts into the lowermost oceanic crust: evidence from dunite- and gabbro-hosted clinopyroxenes in the Bay of Islands Ophiolite

Up to 3 km of dunitic rocks occur below crustal gabbro in the Blow Me Down massif (Bay of Islands Ophiolite, Newfoundland). Analyses of dunite- and gabbro-hosted clinopyroxene grains (cpx) for rare earth elements (REE), Zr, and Ti reveal three types of chondrite-normalized patterns: N-group patterns are similar to cpx grains as they would form by fractionation from a range of mid ocean ridge basalts (MORB). They are typical for a few higher level dunitic samples as well as mafic cumulates. F-group patterns show light REE depletion, very strong middle REE fractionation and a positive Zr anomaly and occur in dunites only. R-group patterns are severely depleted in both light and heavy REEs relative to MORB-like cpx and two samples of the group display a positive Ti anomaly. They are also restricted to dunitic rocks. The patterns are explained in a two stage model in which an established dunite sequence, dominated by MORB-type cumulate signatures (N-group), was infiltrated by extremely refractory melts. During infiltration of the refractory melt chromatographic fractionation occurred, transforming N-group dunites into F-group and R-group dunites. The F-group patterns are composite patterns: heavy REE, Ti ± Zr reflect the original MORB-like cumulate dunite host, light REEs indicate equilibrium with the infiltrating, refractory melts. Steep slopes in the middle REEs reflect the position of the chromatographic front. For more intense percolation of refractory melts, R-group patterns with a positive Ti anomaly will form by the same process. The rest of the R-group patterns displaying no positive Ti anomaly may represent either the most intensely reacted host rocks or these dunites derive directly as cumulates from refractory melts. Only small volumes of refractory melt (a 5 m column) are required to imprint the observed trace element pattern on the thick original dunite sequence. One of several possible origins for the refractory melts is transformation of original MORB-type melts by way of chromatographic fractionation within the highly depleted, residual uppermost mantle. In the framework of an oceanic spreading centre, the migrating, refractory liquids are considered a late event following the main constructive stage dominated by aggregated melts. The study demonstrates that highly refractory melts can exist under oceanic spreading centres dominated by a MORB-like cumulate and volcanic sequence.

Geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites, Central Anatolian Crystalline Complex, Turkey

Plagiogranites associated with the Sarikaraman ophiolite of the Central Anatolian Crystalline Complex, Turkey, closely resemble other plagiogranites from supra-subduction zone-type ophiolites of Neotethys. The ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite (ca. 10% by volume) than is usually associated with such bodies. The Sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex. The plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite. Field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber. Both the deep-seated trondhjemites and the volcanic rhyolites constitute the Sarikaraman plagiogranite suite. Geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites, which are characterised by (MORB-normalized) low HFS element contents with small negative NbTa anomalies and variably enhanced LIL element abundances. Unlike other plagiogranites, however, the Sarikaraman suite is not characterized by consistently low K 2O contents; a feature that reflects the variable mobilization of the LIL elements under lower greenschist facies conditions. The REE are uniformly enriched relative to the basic components of the complex, but have similar normalized patterns exhibiting mild light REE depletion. In terms of their origin, the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization (70–85% of clinopyroxene-feldspar ± amphibole) or partial melting (5–15% batch melting) of a gabbroic ‘source material’, although only the first process can produce most of the range of the plagiogranite compositions. As a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole.

Postorogenic alkali feldspar granite and associated pegmatites in West Avalonia: the petrology of the Neoproterozoic Georgeville Pluton, Antigonish Highlands, Nova Scotia

The 579.8 ± 2.2 Ma (40Ar-39Ar, muscovite) Georgeville Pluton in mainland Nova Scotia is an epizonal body consisting of alkali feldspar granite and related pegmatite. The pluton intrudes the ca. 619-608 Ma arc-related rocks of the Georgeville Group, which comprises part of West Avalonia, the largest terrane in the Canadian Appalachians. The granite is characterized by above-average SiO2, Th, Nb, Y, and Zr; very low CaO, TiO2, MgO, FeO, and MnO; and most notably by positively sloped rare earth element (REE) profiles generated by extreme light REE depletion. Tectonic discrimination diagrams suggest a within-plate environment, with many, but not all, geochemical and mineralogical features resembling A-type granites. Numerous local and regional geological constraints indicate that the pluton was intruded in a trancurrent setting following the cessation of Neoproterozoic arc-related magmatism along the West Avalonian portion of the Gondwanan continental margin. Geochemical data are consistent with derivation by partial melting of depleted crust or upper mantle followed by extreme fractionation, including REE-rich accessory phases.

Mid-Ocean Ridge Melting: Constraints from Lithospheric Xenoliths at Oahu, Hawaii

by melting a lherzolitic source. Instead, they reflect mixing between One plagioclase–spinel lherzolite and four spinel lherzolite xenoliths two components: (1) the pooled melt derived from lherzolitic source from Oahu, Hawaii, contain clinopyroxene grains that show homoin a passive melting regime with an Fmax of 30% (~80% of the geneous rare earth element (REE) abundances and smooth REE mixture) and (2) melt derived from garnet pyroxenite by 40% patterns with systematic depletion of light REE (LREE). These fractional melting (~20% of the mixture). This model produces five xenoliths are mid-ocean ridge basalt (MORB) magma-depleted 7·1 km of crust. residues with compositions that were not modified by later metasomatism. Trace element systematics of these xenoliths were used to investigate the melt production rate (dF/dP) within a 90-my-old residual mantle column (RMC). Such rates were calculated as

Mid-Ocean Ridge Melting: Constraints from Lithospheric Xenoliths at Oahu, Hawaii

One plagioclase–spinel lherzolite and four spinel lherzolite xenoliths from Oahu, Hawaii, contain clinopyroxene grains that show homogeneous rare earth element (REE) abundances and smooth REE patterns with systematic depletion of light REE (LREE). These five xenoliths are mid-ocean ridge basalt (MORB) magma-depleted residues with compositions that were not modified by later metasomatism. Trace element systematics of these xenoliths were used to investigate the melt production rate (dF/dP) within a 90-my-old residual mantle column (RMC). Such rates were calculated as ratios of the difference in extents of depletion (dF) to the difference in equilibrium pressures (dP) between two xenolith samples. The extents of melting were modeled from REE, Sr and Zr abundances in clinopyroxene; and equilibrium pressures were inferred from the two-pyroxene geobarometer of Mercier et al. (1984, Contributions to Mineralogy and Petrology, 85, 391–403). Equilibrium pressures range from 21 to 7 kbar and extents of melting vary from 2 to 8%. Together, these data constrain the maximum extent of melting in the garnet lherzolite stability field to be <2% and a dF/dP of 0.43%/kbar within the stability field of spinel lherzolite. Uncertainties in the estimates of equilibrium pressure and extent of depletion lead to a slightly broader range of dF/dP values (0.26-0.78%/kbar). These values are significantly lower than that of ∼1.2%/kbar suggested by most previous studies. With the best estimated mean dF/dP of 0.43%/kbar, only 3.9 km of crust could have been generated by melting lherzolite in the pressure range of 26–7 kbar. The thickness and composition of the crust that overlies the 90 Ma Oahu RMC require a higher extent of melting. Based on the REE abundances, most samples from the 90 Ma crust and East Pacific Rise can be explained as pooled melts derived from the lherzolitic source in a passive melting regime with a maximum extent of melting (Fmax) of 30%. This model produces a dF/dP of 4.4%/kbar in the pressure range of 7–2 kbar and generates an additional 1.8 km of crust. In detail, the high [Sm/Yb]DM ratios in most East Pacific Rise samples cannot be explained by melting a lherzolitic source. Instead, they reflect mixing between two components: (1) the pooled melt derived from lherzolitic source in a passive melting regime with an Fmax of 30% (∼80% of the mixture) and (2) melt derived from garnet pyroxenite by 40% fractional melting (∼20% of the mixture). This model produces 7.1 km of crust.

Sedimentology and rare earth element geochemistry of phosphatic sediments from Sebastián Vizcaíno Bay, Baja California, Mexico

Surficial sediments rich in apatite pellets were recovered from an 80‐m‐deep area that runs parallel to the eastern coast of Vizcaino Bay, Baja California. This area is subject to intense seasonal upwelling, the sediments presenting elevated phosphate (P2O5), organic carbon (Corg), and carbonate concentrations. The pellets are moderately well sorted, concentrated mainly in the 3ø size class within a poorly sorted sediment matrix. Abundant nodular apatite and phosphatized bone fragments were found in only one of the pellet‐bearing samples. This sample has a P2O5concentration of over 20%, the ≥ — 2.5ø to 0.5ø grain size classes alone being formed of up to 32% P2O5. The high proportion of fragmented nodules and the presence of volcanic rock pebbles indicate that at least part of this deposit is allochthonous. The apatite pellets have a flat to concave‐downward heavy rare earth (HREE) shale normalized pattern and, except for La, are depleted in light rare earth elements (LREE). The close resemblance between the HREE patterns and the relatively high La concentration in the pellets of Vizcaino Bay, and the onland phosphorites of the southern Baja California Peninsula, suggest similar depositional histories for these deposits. No significant Ce anomalies were observed, but a negative Eu deviation is common in all of the pellet‐bearing sediments. The depletion of LREE in the pellets of Vizcaino Bay may indicate the preferential removal of these elements by weathering processes.

Rare earth elements, protoliths, and alteration at the Hemlo gold deposit, Ontario, Canada, and comparison with argillic and sericitic alteration in the Highland Valley porphyry district, British Columbia, Canada

Rare earth elements (REE), Sc, and Th are used to identify two volume-important protoliths for alteration and mineralization at the Hemlo gold deposit, Ontario: quartz-feldspar porphyry and greenstone (metasedimentary and metavolcanic rocks) of predominant intermediate-mafic composition. The REE geochemistry was essentially passive to the pervasive and characteristic microcline alteration at the deposit. However, using Al 2 O 3 as the immobile component, there is evidence for mobility of REE, particularly of light REE, TiO 2 , and Zr in other styles of alteration. Dilution of REE abundances by secondary minerals (pyrite, barite, quartz, and carbonates) averaged X 2.0, and loss of light REE to leaching averaged 56 percent in ores of the porphyry protolith, and X 1.6 and 68 percent, respectively, in ores of the greenstone protolith. Whereas most of the secondary minerals diluting REE abundances crystallized broadly contemporaneously with microcline, leaching of light REE most likely occurred later during the late calc-silicate alteration event. Depletion of light REE, as measured by a decrease in (La/Yb) N , increases with an increase in Au content for gold ores derived from both porphyry and greenstone protoliths. However, the REE geochemistry remains dominated by the protolith composition and, thus, is not a suitable primary tool for exploration for Hemlo-type gold deposits. Argillic alteration in the Highland Valley porphyry district, British Columbia, is uniformly enriched in REE (< or = X 2) compared with incipiently to intensely altered whole rocks and sericitic alteration. The argillic and sericitic alteration separates define a characteristic linear trend in the Al 2 O 3 -(CaO + Na 2 O)-K 2 O ternary diagram that is distinct from the composition field for aluminous rocks of the hanging wall of the Hemlo deposit. The geochemistry and regional occurrence of these latter rocks clearly indicate that they are metapelites.

Re-Os systematics of early proterozoic ferropicrites, Pechenga Complex, northwestern Russia: Evidence for ancient 187Os-enriched plumes

The Re-Os isotopic systematics of various ferropicritic flows and sills of the Pechenga Complex, Russia, have been examined. During crystallization about 1.98 Ga ago, many of these bodies became highly differentiated. In addition, some of the larger igneous units are associated with major NiCu ore deposits. The melts that produced these rocks have been termed ferropicritic because of their high FeO and MgO contents. They are also enriched in light rare earth elements (LREEs), TiO 2, Zr, and many other incompatible trace elements. Previous studies have concluded that the ferropicrites were most likely derived from an Fe-rich mantle plume that had a complex history of long-term LREE depletion (initial ϵ Nd = + 1.4), but that also experienced a LREE enrichment event within 200 Ma of the generation of the rocks. Whole rock samples believed to be most representative of primary melt compositions indicate that initial melt concentrations of rhenium and osmium were approximately 1.1 ppb and 0.5 ppb, respectively. The high primary melt concentrations presumably made the osmium contained in the melts relatively immune to the effects of crustal contamination. Nonetheless, all ore-bearing intrusions examined show osmium isotopic evidence for crustal contamination. For example, the initial γ Os for some primary magmatic sulfides from the Pilgujärvi intrusion average +46. Other ore-bearing intrusions, such as the Kammikivi sill, appear to have been similarly contaminated by crustal osmium during the injection of magma, with initial yo, values as high as +251. The seemingly high levels of crustal osmium may be attributed to the rapidly diminishing concentrations of osmium in the melts as the larger bodies differentiated, combined with localized in situ assimilation of the metasedimentary rocks that comprise the country rocks. The Re-Os systematics of some whole rock samples of both mineralized and sulfide-poor intrusions were affected by post-magmatic events, especially the greenschist grade metamorphism that impacted the rocks between about 1.7 and 1.8 Ga ago. The metamorphic effects are reflected in the recrystallization of many of the primary sulfides. As a consequence of this open-system behavior in many whole rock samples, the primary igneous Re-Os systematics of these rocks are best examined via analysis of magmatic phases such as chromite, olivine, clinopyroxene, and primary sulfides. Chromite and ilmenite+sulfide separates from two sulfide-poor lava flows, the Lammas and Keskitunturi, have characteristically low 187Re/ 188Os ( < l), and because of the limited age correction, precisely define the initial γ Os of these systems to be +6.0±0.7. Because of the identical initial compositions of the two, spatially distinct lava flows, and the fact that these flows were extruded onto only slightly older volcanic rocks, we conclude that the +6.0 value reflects the composition of the mantle source and not minor crustal contamination. Although 187Os-enriched, plume-derived systems are common during the Phanerozoic, this is the earliest known evidence for the existence of long-term, Re-enriched mantle reservoirs. The most commonly invoked model to explain 187Os enrichments in Phanerozoic systems, oceanic crustal recycling, in this instance requires that very large proportions of oceanic crust were recycled into the mantle source and that the event was likely very ancient. Other options, such as core-mantle interaction and a stratified mantle, are also discussed.

Geochemistry of Spodosols formed in holocene till, Norra Storfjället Massif, northern Sweden

Silt and clay size fractions of soils, from a transect of six Spodosols formed in the Norra Storfjallet Massif, were analyzed by neutron activation to determine the degree to which pedogenic processes have influenced the distribution of macro, micro and trace elements. The distributions of Mg, Ca and Fe, together with Co, Cr and other trace elements in the profiles, suggest the presence of different parent materials, with A and E horizons arising from an influx of aeolian sediment. Translocation processes, both physical and chemical, occurred in the soil concentrating Fe and Br in the spodic (Bs) horizons of the profiles. The rare earth elements (REEs) are predominantly associated with the heavy mineral fraction of the soil material. The distributions of chondrite normalized REEs patterns of the profiles indicate that light rare earth element (LREE) concentrations increase with horizon depth. The depletion of LREEs in the upper soil horizons confirms the presence of material that is chemically different from that in the lower horizons, thus indicating a distinct chemical difference from the local glacial deposits.

Petrography and geochemistry of the Paleoproterozoic Hotazel Iron-Formation, Kalahari manganese field, South Africa; implications for Precambrian manganese metallogenesis

New data are presented concerning the petrography and geochemistry of the Paleoproterozoic Hotazel iron-formation of the Kalahari manganese field, South Africa. Mineralogically and texturally, the Hotazel iron-formation is very similar to Paleoproterozoic banded iron-formations of the Superior type, comprising laminae of chert, iron oxides (magnetite, hematite), iron silicates (greenalite, minnesotaitc, stilpnomelane, riebeckite, Fe mica), carbonates (calcite, ankerite, minor siderite), and pyrite. The major chemical constituents of the Hotazel rocks are SiO 2 , Fe oxides, CaO, and MgO, whereas MnO, Al 2 O 3 , Na 2 O, and K 2 O contents are negligible (below 1 wt %). Elements of detrital derivation (Ti, Zr, Rb) and transition metals such as Zn, Cu, Ni, Co, and V are present in low concentrations, on the order of a few tens of parts per million.Stratigraphically higher, the Hotazel banded iron-formation shows a conspicuous increase in CaO contents that is coupled with a subtle decrease in bulk-rock Fe number ratios (Fe (super 2+) /(Fe (super 2+) + Fe (super 3+) )), implying a gradual transition to a more oxidizing environment. Shale-normalized rare earth element (REE) patterns are similar to those of older banded iron-formations of the Transvaal Supergroup, exhibiting light REE depletion and negative Ce anomalies comparable to modern shallow-level seawater. Occasional weak positive Eu anomalies in iron silicate-rich rocks indicate minor admixtures of a hydrothermal component below the chemocline. The similarities between the Hotazel iron-formation and most Superior-type banded iron-formations, the lack of transition metal enrichments, and the absence of strongly positive Eu anomalies in the Hotazel rocks challenge volcanogenic exhalative models proposed previously for the genesis of the interbedded Kalahari Mn deposits.

The Origins of Yakutian Eclogite Xenoliths

Owing to the association with diamonds, eclogite xenoliths have received disproportionate attention given their low abundance in kimberlites. Several hypotheses have been advanced for the origin of eclogite xenoliths, from the subduction and high-pressure melting of oceanic crust, to cumulates and liquids derived from the upper mantle. We have amassed a comprehensive data set, including major- and trace-element mineral chemistry, carbon isotopes in diamonds, and Rb–Sr, Sm–Nd, Re–Os, and oxygen isotopes in ultrapure mineral and whole-rock splits from eclogites of the Udachnaya kimberlite pipe, Yakutia, Russia. Furthermore, eclogites from two other Yakutian kimberlite pipes, Mir and Obnazhennaya, have been studied in detail and offer contrasting images of eclogite protoliths. Relative to eclogites from southern Africa and other Yakutian localities, Udachnaya eclogites are notable in the absence of chemical zoning in mineral grains, as well as the degree of light rare earth element (LREE) depletion and unradiogenic Sr; lack of significant oxygen, sulfur, and carbon isotopic variation relative to the mantle; and intermineral radiogenic isotopic equilibration. Several of these eclogites could be derived from ancient, recycled, oceanic crust, but many others exhibit no evidence for an oceanic crustal protolith. The apparent lack of stable-isotope variation in the Udachnaya eclogites could be due to the antiquity of the samples and consequent lack of deep oceanic and biogenically diverse environments at that time. Those eclogites that are interpreted to be non-recycled have compositions characteristic of Group A eclogites from other localities that also have been interpreted as being directly from the mantle. At least two separate and diverse isotopic reservoirs are suggested by Nd isotopic whole-rock reconstructions. Most samples were derived from typical depleted mantle. However, two groups of three samples each indicate both enriched mantle and possible ultra-depleted mantle present beneath Yakutia during the late Archean and early Proterozoic. The vast majority of eclogites studied from the Obnazhennaya pipe also exhibit characteristics of Group A eclogites and are probably derived directly from the mantle. However, the eclogites from the Mir kimberlite are more typical of other eclogites world-wide and show convincing evidence of a recycled, oceanic crustal affinity. We concur with the late Ted Ringwood that eclogites can be formed in a variety of ways, both within the mantle and from oceanic crustal residues.

Stable and radiogenic isotope study of eclogite xenoliths from the Orapa kimberlite, Botswana

Eclogite xenoliths from Orapa can be accurately classified as Group I or Group II on the basis of Na 2O in garnet and K 2O in clinopyroxene. Group I xenoliths are commonly diamondiferous while Group II xenoliths are diamond-free. Both xenolith varieties may contain graphite. Isotopic character is to some degree correlated with major- and trace-element chemistry. Group II samples with Ca-poor garnet have clinopyroxenes with radiogenic 87 Sr 86 Sr (0.705–0.709) and the least radiogenic 143 Nd 144 Nd (0.5122–0.5125). Group I eclogites with higher Ca and Fe in garnets have less radiogenic 87 Sr 86 Sr (0.702–0.7066) and bulk-Earth or higher 143 Nd 144 Nd ratios. Group I eclogites have more radiogenic 206 Pb 204 Pb (18.6–19) than Group II xenoliths (16.5–18.6). In contrast, Group II xenoliths have more variable and, in some cases, more radiogenic 206 Pb 204 Pb (36.6–39.3) than Group I xenoliths (38.3–38.4). The Sr, Sm, Nd and Pb concentrations of minerals in Orapa Group I eclogite xenoliths are much lower than in Group II samples. All the Group II xenoliths are inferred to be enriched in light rare-earth elements while Group I xenoliths are probably characterised in many cases by light rare-earth element depletion. Constituent garnet and clinopyroxene in both Group I and II eclogite xenoliths are essentially in isotopic equilibrium at the time of pipe emplacement. Mineral as well as calculated whole-rock 143 Nd 144 Nd compositions of most of the Group I eclogites are too close to bulk-Earth and depleted-mantle estimates in order to obtain useful model age information. Depleted-mantle model ages derived from the much lower 143 Nd 144 Nd compositions of the Group II eclogite xenoliths range from 661 to 1248 My, with an average clinopyroxene model age of 908 My and an average whole-rock model age of 1016 My. On the basis of an observed covariation of O and Sr isotopic compositions the entire Orapa Group I eclogite xenolith suite can be modelled as mixtures of oceanic basalt with or without a few percent of ocean floor sediment. The Group II xenoliths might have crystallised from a melt which derives from a protolith with time-averaged LREE depletion. Their radiogenic Sr isotope character could be due to interaction of the melt with metasomatised lithosphere, or might be a superimposed metasomatic signature.

The geochemistry and petrogenesis of the late-Cretaceous picrites and basalts of Curaçao, Netherlands Antilles: a remnant of an oceanic plateau

The island of Curacao in the southern Caribbean Sea is composed mainly of a thick sequence (>5 km) of pillow lavas, grading upwards from picrites at the base of the exposed section, to basalts nearer the top. Modelling suggests that picrites are related to the basalts by fractional crystallisation. Initial radiogenic isotope ratios of the picrites have a restricted compositional range: ɛNd=+6.1 to +6.6, 87Sr/86Sr=0.70296–0.70319; whereas the basalts display a wider range of compositions: ɛNd=+6.6 to +7.6, 87Sr/86Sr=0.70321–0.70671. This variation in isotope ratios between basalts and picrites may be due to the assimilation of altered oceanic crust (or possibly partial melts of such crust) by a picritic magma along with fractional crystallisation. The relatively narrow range of Nd and Pb isotopic compositions in the Curacao lavas suggests either that the source region was homogeneous, or that melts from a heterogeneous mantle source were well mixed before eruption. Chondritic to slightly light rare earth element enriched patterns, combined with long-term light rare earth element depletion (positive ɛNd), suggest that the lavas were formed by polybaric melting of spinel lherzolite, with small a contribution from garnet lherzolite melts. High-MgO lavas, the absence of a subduction related chemistry, and the chemical similarity to other oceanic plateaux, suggest a mantle plume origin for the Curacao lava succession. The Curacao volcanic sequence is part of an oceanic plateau formed at about 88–90 Ma, fragments of which are dispersed around the Caribbean as well as being obducted onto the western margin of Colombia and Ecuador. The occurrence of high-Mg lavas throughout this Cretaceous Caribbean–Colombian igneous province requires anomalously hot mantle (>200° C hotter than ambient upper mantle) over a large part of a putative plume head, which is inconsistent with some mantle plume models.

Melt Enrichment of Shallow Depleted Mantle: a Detailed Petrological, Trace Element and Isotopic Study of Mantle-Derived Xenoliths and Megacrysts from the Cameroon Line

Major element, trace element and Sr-Nd-Pb isotopic compositions ofultramafic xenoliths and megacrysts from the continental Cameroon line provide evidence for metasomatism of the uppermost lithospheric mantle by enriched melts during the Mesozoic The megacrysts probably crystallized within the lower continental crust from melts similar to the host magmas. All the xenoliths originated as depleted residues after the extraction of basaltic melts, but some indicate evidence of interaction with enriched partial melts before entrainment. The U—Pb isotopic data on garnet are consistent with cooling through > 900°C at >300 Ma. The Sm—Nd isotope systematics in constituent phases appear to have been in equilibrium on a xenolith scale at the time of entrainment, indicating derivation from mantle that remained at temperatures > 600°C until eruption. Spinel Iherzolites that show simple light rare earth element (LREE) depletions are characterized by isotopic compositions that are comparable with, but slightly more depleted than Atlantic NMORB, suggesting that the unmetasomatized sub-continental lithosphere of the Cameroon line may be isotopically similar to that of sub-oceanic lithosphere. The Nd-depleted mantle model ages of these xenoliths are consistent with late Proterozoic depletion, similar in age to much of the overlying continental crust. In contrast, samples that have LREE-enriched clinopyroxenes (La/Yb —4-7—9-4) contain trace amounts of amphibole, are enriched in U and have more radiogenic Pb and Sr. These xenoliths yield U-Pb and Sm—Nd model ages consistent with Mesozoic enrichment, in agreement with the age of enrichment of the source regions of the basalts, as deduced from Pb isotopic compositions. Clinopyroxenes record three orders of magnitude enrichment in U and LREE accompanied by progressive K depletion associated with the growth of trace amphibole, with K/ U ratios that range from 12 000 to 1. The ratios of the trace elements thought to have similar bulk D in mantle melting, Ce/ Pb, Ba/Rb and Nd/Sr ratios, display regional variations related to the time integrated history of enrichments indicated by Nd isotopic compositions. Mass balance calculations suggest that the melts responsible for the most recent enrichment of the lithosphere had higher La/Yb and U/Pb than Cameroon line host magmas, and were probably the product of small degrees of partial melting associated with the earliest stages of the breakup of Pangea.

Pb-Sr-Nd isotopic compositions and trace element geochemistry of megacrysts and melilitites from the Tertiary Urach volcanic field: source composition of small volume melts under SW Germany

The Urach volcanic field is unique within the Tertiary–Quaternary European volcanic province (EVP) due to more than 350 tuffaceous diatremes and only sixteen localities with extremely undersaturated olivine melilitite. We report representative Pb-Sr-Nd isotopic compositions and incompatible trace element data for twenty-two pristine augite, Cr-diopside, hornblende, and phlogopite megacryst samples from the diatremes, and seven melilitite whole rocks. The Pb isotopic compositions for melilitites and comagmatic megacrysts have very radiogenic 206Pb/204Pb ratios of 19.4 to 19.9 and plot on the northern hemisphere mantle reference line (NHRL). The data indicate absence of an old crustal component as reflected in the high 207Pb/204Pb ratios of many basalts from the EVP. This inference is supported by 206Pb/204Pb ratios of ∼17.6 to 18.3 and ɛNd of ∼−7.8 to +1.6 for five phlogopite xenocryst samples reflecting a distinct and variably rejuvenated lower Hercynian basement. The 87Sr/86Sr ratios of 0.7033 to 0.7035 in the comagmatic megacrysts are low relative to their moderately radiogenic Nd isotopic compositions (ɛNd +2.2 to +5.1) and consistent with a long-term source evolution with a low Rb/Sr ratio and depletion in light rare-earth elements (LREE). The melilitite whole-rock data show a similar range in Nd isotopic ratios as determined for the megacrysts but their Sr isotopic compositions are often much more radiogenic due to surface alteration. The REE patterns and incompatible trace element ratios of the melilitites (e.g. Nb/Th, Nb/U, Sr/Nd, P/Nd, Ba/Th, Zr/Hf) are similar to those in ocean island basalts (OIB); negative anomalies for normalized K and Rb concentrations support a concept of melt evolution in the lithospheric mantle. Highly variable Ce/Pb ratios of 29 to 66 are positively correlated with La/Lu, La/K2O, and Ba/Nd and interpreted to reflect melting in the presence of residual amphibole and phlogopite. The data suggest an origin of the melilitites from a chemical boundary layer very recently enriched by melts from old OIB sources. We suggest that the OIB-like mantle domains represent low-temperature melting heterogeneities in an upwelling asthenosphere under western Europe.

Crystallization and alteration of quartz monzonite, Iron Springs mining district, Utah; relation to associated iron deposits

The Iron Springs mining district of southwestern Utah contains the largest iron deposits in the western United States. These are related to emplacement, at minimum depths of 1 to 2.3 km, of three Miocene laccoliths of porphyritic quartz monzonite. When emplaced, the magma was nearly half crystallized, on the verge of brittle behavior, and contained phenocrysts of plagioclase, amphibole, biotite, and clinopyroxene. Granophyric dikes in quartz monzonite formed from residual liquid, but miarolitic cavities in these dikes are lined with alpha quartz, indicating that deposition continued from a volatile-rich phase below the solidus. Pervasive high-temperature alteration led to: replacement of augite by diopside; oxidation of magnetite and ilmenite; replacement of amphibole (completely) and biotite (partially) by aggregates of diopside, alkali feldspars, smectite-chlorite, magnetite, ilmenite, and calcite; albitization of groundmass alkali feldspar; exchange of Mg for Fe and of F for OH in biotite; and exchange of F and OH for Cl in apatite. This alteration produced whole-rock enrichment in Fe (super 3+) , Na, and H 2 O, and losses in Fe (super 2+) , Ca, and K. Total Fe was effectively unchanged. A chilled, relatively impermeable margin, surrounding the pervasively altered interior of each laccolith, was mechanically bonded to wall rock more strongly than to the magmatic interior, and deformed with the wall rock during subsequent intrusion and structural readjustment. In this peripheral shell, clinopyroxene, amphibole, and biotite were less altered. Where the roof of each laccolith had a radius of curvature of less than 2 km, a heterogeneous jointed zone formed below the peripheral shell. In this zone, pervasively altered quartz monzonite was fractured by renewed intrusion or by structural readjustment of roof rocks. Along the resulting joints, quartz monzonite was altered to bleached zones (selvages) during a second stage of alteration. Within these selvages, complete decomposition of biotite that had survived pervasive alteration was accompanied by further growth of diopside, albitization of feldspars, and dissolution of some apatite. Unlike early pervasive alteration, selvage alteration was fracture controlled and caused strong whole-rock depletion in light REE, total Fe, Mg, P, Rb, Y, Ba, and H 2 O (super -) , but caused enrichment in Na. Magnetite-apatite veins commonly formed along the joint. Adjacent to zones of selvage joints, strata-bound orebodies of magnetite, hematite, and apatite formed in limestone around the three laccoliths. Iron-bearing fluid was derived by subsolidus alteration from quartz monzonite; there is no evidence supporting immiscible separation of iron-oxide-rich liquid from silicate magma. Mass balance calculations indicate that the Fe, P, S, Al, and Mg in the orebodies could have been entirely supplied by selvage alteration of 40 km 3 of quartz monzonite, in agreement with the exposed and confidently inferred volume of altered intrusive rock. A fourth pluton, lacking magnetite veins, orebodies, and selvages, was emplaced closer to the surface and against sandstone and share rather than limestone. Iron liberated from hydrous mafic phenocrysts in this body was oxidized in place, forming disseminated hematite.

Rare earth element geochemistry of the metamorphosed volcanogenic massive sulfide deposits of the Manitouwadge mining camp, Superior Province, Canada; a potential exploration tool?

Trace and rare earth element (BEE) geochemistry indicates that rocks in the Manitouwadge mining camp were derived from three major lithological units: mafic volcanic rocks, acid volcanic tuffs, and dacites. The Geco, Willroy, Willecho, and Big Nama Creek Cu-Zn-Ag deposits are contained within the 2720 Ma acid volcanic tuffs. Our study suggests that the thickness of the felsic volcanic unit (acid tuffs and volcaniclastics) may be significantly underestimated in the area. Medium- to high-grade metamorphism (upper amphibolite, lower granulite) gave rise to specific alteration assemblages which crystallized during several episodes of deformation and plutonism. Because major element geochemistry of the rocks was modified during ore- forming processes and later by several episodes of metamorphism and tectonism, the present alteration minerals merely reflect the cumulative effect(s) of these episodes and are often misleading in the identification of the protoliths. Thus, due to extensive alteration, the protoliths of various lithological units can be positively identified only from trace and rare earth element geochemistry. Extensive (up to 97%) light REE depletion was observed in the felsic volcanic rocks around the Geco, Willroy, and Willecho deposits. The positive correlation between BEE mobility and mineralization suggests that REE geochemistry can be successfully used in the exploration for volcanogenic massive sulfide deposits around the Manitouwadge mining camp. The absence of a well-defined discordant alteration pipe at Geco and localized enrichment in Fe, Al, + or - K at the Willroy, Willecho, Big Nama Creek, and Geco deposits implies that they may be highly metamorphosed equivalents of Mattabi-type deposits where the original iron-rich carbonates have recrystallized to calc-silicate or amphibole-rich assemblages during regional metamorphism. Localized enrichment in Fe, Al, and K in the felsic tuffs could be used to identify possible alteration pipes or semicomformable alteration zones associated with mineralization.