Rb In Rocks Research Articles

High Degree of Differentiation and Enrichment of Li, Rb and Cs in Potassic-Ultrapotassic Volcanic Rocks: An Example from the Lhasa Block, Tibet

Post-collisional potassic-ultrapotassic volcanic rocks are widely developed in the Lhasa block, Qinghai-Tibet region. In this paper, it was observed for the first time that dilute alkali elements–Li, Rb, and Cs–exhibit supernormal enrichment in the research data of numerous potassic-ultrapotassic volcanic rocks in the Lhasa block. At the same time, there are few studies on the genesis of enrichment. Currently, 568 sets of published volcanic rock data and 8 sets of measured data in the Lhasa block are sorted in detail, and the genesis of dilute alkali element enrichment is explained by means of geochemical research methods. It is believed that the high degree of magmatic fractionation of potassic-ultrapotassic volcanic rocks in the Lhasa block is the main reason for the abnormal enrichment of dilute alkali elements such as Li, Rb, and Cs. The abnormal enrichment area is mainly located in the central and western parts of the Lhasa block with an age range of 25–13 Ma. The discrimination range of Zr/Hf and Nb/Ta with a high degree of differentiation of potassic-ultrapotassic volcanic rocks in the Lhasa block is divided by analogy with the research results of highly fractionated granites, which provides a reference for the study of the supernormal enrichment of dilute alkali elements in potassic-ultrapotassic volcanic rocks in this area.

Ore-forming elements diffusion and distribution in the altered host rock surrounding the Koktokay No. 3 pegmatite in the Chinese Altay

Petrography and geochemistry of the altered and unaltered host rocks surrounding the Koktokay No. 3 pegmatite revealed that the unaltered amphibolite is mainly composed of hornblende, plagioclase, and ilmenite. Beyond these primary components, the altered host rocks contain a few newly formed minerals, including biotite, tourmaline, chlorine, and muscovite. The alteration zone surrounding the Koktokay No. 3 pegmatite is limited to 2.0 m, characterized by biotitization, tourmalization, and chloritization. In the altered host rocks, the contents of SiO2, MgO, MnO, Na2O, and TiO2 did not vary greatly. However, Al2O3 showed a weak decreasing trend with the increasing distance from the pegmatite contact zone, while Fe2O3 and CaO showed an increasing trend. The contents of Li, Rb, and Cs in the altered host rocks were much higher than those in the unaltered host rocks, decreasing with distance from the contact. The chondrite-normalized rare earth element (REE) pattern of the altered and unaltered host rock was right-inclined from La to Lu, but enriched in light REEs over heavy REEs after hydrothermal alteration. An isocon plot shows that some oxides migrated in with an order of P2O5 > K2O > TiO2 > Al2O3 > SiO2 > MnO ≥ MgO, while others migrated out with an order of Na2O > CaO > Fe2O3. For REEs, the migration ratios are positive values with Cs > Rb > Li > Nb > Ta > Be, signifying that all REEs migrated from the exsolved magmatic fluid into the altered host rocks. It was concluded that diffusion was the only mechanism for migration of ore-forming elements in the alteration zone. The effective diffusion coefficients (D eff ) of LiF, RbF, and CsF were estimated under a fluid temperature of 500–550 °C. Using a function of concentration (C (x,t)) and distance (x), the order of migration distance was determined to be LiF > CsF > RbF, with diffusion times of (3.39 ± 0.35) × 106, (3.19 ± 0.28) × 105 and (6.33 ± 0.05) × 105 years, respectively.

Geochemical signals for determining the slip-weakening mechanism of an ancient megasplay fault in the Shimanto accretionary complex

[1] To investigate the slip behavior of a megasplay fault branching from a subduction boundary megathrust, we investigated the geochemistry of an out-of-sequence thrust that formed at 2.5–5.5 km depth. Rocks from the slip zone show major-element and fluid-immobile trace-element compositions that are consistent with disequilibrium flash melting. Distinct depletions of Li, Rb, and Cs in the slip-zone rocks indicate fluid–rock interactions at high temperatures (>350°C). These findings suggest a slip process in which high-temperature pore fluids were generated by frictional slip, but the thermally-enhanced pressure might not have reached a sufficient level to cause thermal pressurization, and the temperature continued to increase to attain melting of mica minerals. Comparison with slip zone that formed at a shallower depth (1–2 km), where only thermal pressurization occurred, suggests a transition from melt lubrication at depth to thermal pressurization at shallower depths along a megasplay faults.

Geochemistry and isotopic constraints on the origin of the mesoproterozoic Rio Branco ‘anorogenic’ plutonic suite, SW of Amazonian craton, Brazil: high heat flow and crustal extension behind the Santa Helena arc?

The Rio Branco plutonic suite (RBS) occurs in the southwestern Amazonian craton, crops out in an area of 1500 km 2, and is emplaced into the ca. 1.79 Ga Alto Jauru terrane (Rio Negro/Juruena geochronological province). The RBS comprises basic (gabbro, diabase, and basalt) and felsic (porphyritic and rapakivi granite) rocks. Hybrid rocks (monzosyenite) with rapakivi-like textures indicate commingling and mixing among the basic and felsic magmas. Silica contents range 45–47% in the basic rocks (metaluminous) and 69–71% in the felsic rocks (slightly peraluminous–metaluminous). Lithogeochemical investigation also indicates higher contents of K 2O, Rb, Zr, and Ba in felsic rocks, comparable with results reported elsewhere for rapakivi granites. Trace element discrimination diagrams indicate that the RBS felsic and basic rocks have within-plate signatures. In addition, the felsic rocks have strongly fractionated REE patterns that show marked negative Eu anomalies, probably due to plagioclase fractionation. The basic rocks are similarly LREE enriched but display flatter patterns, characteristic of weakly fractionated gabbros. Single-grain IDTIMS U–Pb analyses yield an upper intercept age of 1427±10 (MSWD=1.7) for magmatic zircon from a granophyre of the RBS. This age contrasts significantly with an upper intercept age of 1471±8 Ma (with a concordant 207Pb/ 206Pb age of 1471±18 Ma) obtained for zircon from a sample of the basic group. The latter rocks show positive ɛ Nd(1420) ranging from +1.2 to +1.9 ( T DM=1.86−1.82 Ga), which indicates mantle-derivation, whereas the felsic ones yield ɛ Nd(1420) values from +0.2 to −1.0 ( T DM=1.80−1.73 Ga), indicating some older crust in their source. The RBS is interpreted to have formed at 1.47–1.42 Ga from a mixture of mantle source and crustal-derived magma. We propose high heat flow and an extensional environment for the origin of the RBS as a response to the inboard Santa Helena arc (ca. 1.45–1.42 Ga) that developed at the southwestern margin of the Amazonian craton at approximately the same time.

Precise determination of Rb, Sr, Ba, and Pb in geological materials by isotope dilution and ICP-quadrupole mass spectrometry following selective separation of the analytes

A new method was developed for the high precision measurement of Rb, Sr, Ba, and Pb concentrations in silicate rocks. This method is based on the combined use of isotope dilution and ICP quadrupole mass spectrometry, with matrix element removal. The concomitant separation of the analytes is achieved by extraction chromatography (Ba, Sr, Pb) and inorganic cation exchange chromatography (Rb), on two small tandem columns filled with 0.1 ml of “Sr resin” and 1 ml of zirconium (or titanium) phosphate, respectively. Compared to previous separation schemes based on cation- and anion-exchange, this method allows the rapid, concomitant isolation of four trace elements of great interest in geochemistry, with a greatly reduced volume of reagents. The capabilities of the method in terms of precision (equal to or better than 1% RSD) and accuracy are demonstrated by duplicate analyses of four international reference materials (BIR-1, BHVO-1, AGV-1 and RGM-1, from the US Geological Survey) representative of various silicate rock bulk compositions and spanning a large range (0.2–1000 µg g−1) of analyte concentrations.

Chemical fluxes in the Tonga Subduction Zone: Evidence from the Southern Lau Basin

Chemical and isotopic data for volcanic glasses from the Valu Fa Ridge (VFR) in the southern Lau Basin closely resemble the composition of volcanics from the nearby Tonga arc. The most primitive basalts from the VFR were used to reconstruct the primary magma composition which is believed to reflect the composition of the bulk crust. The magma production rate at the Valu Fa Ridge was estimated (0.7−1.5×106 kg/a/m), and chemical fluxes of various elements have been calculated based on these values and the primary magma composition. The chemical and isotopic composition of VFR lavas is totally consistent with the release of fluid‐soluble elements from the subducting altered oceanic crust and sediments. Mass balance calculations indicate the important role of the altered basaltic basement as a source of H2O, K, Rb, Cs, Sr, U, and Pb in supra‐subduction zone rocks.

Direct ICP-MS determination of trace and ultratrace elements in geological materials after decomposition in a microwave oven. Part II. Quantitation of Ba, Cs, Ga, Hf, In, Mo, Nb, Pb, Rb, Sn, Sr, Ta and Tl

A new method has been developed for the rapid determination of traces of Ba, Cs, Ga, Hf, In, Mo, Nb, Pb, Rb, Sn, Sr, Ta and Tl in silicate rocks and lake, stream and river sediments. The method involved dissolution of samples in a microwave oven by heating in a pressure decomposition Teflon vessel with a mixture of HF + HNO 3 + HCl + H 3BO 3 + EDTA followed by direct multielement determination using inductively coupled plasma-mass spectrometry ( ICP MS ). The method is faster than conventional dissolution of samples by open vessel acid digestion and fusion and determination by instrumental methods. The accuracy and precision of the developed method were tested by replicate analyses of a number of international geochemical reference samples of established trace element contents. Satisfactory correlation with the “recommended” or “consensus” values was found and recoveries were in most cases 95–100%. New values for Ga, In, Nb and Tl in several international geochemical reference materials are first reported in this paper.

Determination of traces of Cs, Li and Rb in rocks by laser enhanced ionization spectrometry without preconcentration

Methods for lithium, cesium and rubidium determinations in rock samples using a laser spectrometer for laser enhanced ionization (LEI) with flame atomization were developed. The analyte atoms were excited with one- and two-step schemes from both the ground state and excited states. The schemes yielded detection limits down to 10 −1 pg ml in aqueous solutions. The effect of sample macrocomponents on the analytical signal was shown to be mainly the appearance of additive multiphoton ionization background due to CaOH molecules in the flame. The CaOH ionization-excitation spectrum was produced to permit the selection of the atom excitation schemes which are optimum with respect to the background. The detection limits in rocks are 0.002, 0.001 and 0.5 ppm for cesium, lithium and rubidium, respectively. The absence of chemical interferences permits the calibration to be conducted with aqueous standard solutions. Cesium, Li and Rb determinations were made in a standard magnesian basalt, low-alkali tholeiite basalt, alaskite granite, trap and some other samples. The accuracy of the analysis was tested by the standard addition method and by comparision with the certified values obtained by atomic absorption spectrometry and other techniques, where available.

Computer-aided INAA Measurements and Geochemical Classification of Various Basaltic Rocks from Costa Rica

The basaltic rocks of Costa Rica show a wide range of chemical compositions. By means of INAA measurements a correlation matrix has been derived and there is a positive correlation of the REE and Th with U, Hf, Ta and Rb in basaltic rocks and a negative correlation of the REE with Co, Sc, Cs and Fe.

Analysis of silicate rocks for Rb, Sr, Y, Zr and Nb by Energy‐Dispersive X‐Ray Fluorescence

AbstractA procedure is described for accurate and precise routine analysis for the trace elements Rb, Sr, Y, Zr and Nb in silicate rocks by Energy‐Dispersive X‐Ray Fluorescence analysis with Ag secondary target excitation. The trace elements are analyzed on pressed powder pellets. Primary standardization is relative to international reference rocks and matrix absorption corrections are based on the Compton scatter method. The computer program for spectrum analysis incorporates advanced non‐linear least squares peak fitting techniques. Overall precision for a single measurement can be as good as 2% relative. Analysis of established U.S.G.S. reference samples AGV‐1, GSP‐1 and G‐2 shows that accuracies of better than 5% relative are obtained. Supplementary data for Zn, Ga, Pb and Th show poorer precision.

Xenoliths from southern Africa: a perspective on the lower crust

Summary Granulite-facies xenoliths from southern African kimberlites yield temperatures from 780–900°C and pressures from 10–15 kb. They have Nd model ages of 1.4–3.7 Ga and retain Rb-Sr mica and Sm-Nd garnet ages from 0.14–1 Ga, significantly older than kimberlite emplacement ages. It is argued that low abundances of U, K and Rb in lower crustal rocks can be either primary features resulting from plagioclase-dominated cumulate processes, or secondary effects caused by the granulite-facies breakdown of mica in wet intermediate-acid rocks. Combined trace element and isotope arguments suggest that contamination with lower crustal material does not significantly affect the geochemistry of flood basalts from the Karoo, the Parana and the Deccan.

Energy-Dispersive X-Ray Fluorescence Analysis of Trace Elements in Carbonate Rocks

A method for the energy-dispersive x-ray fluorescence analysis of the trace elements Zn, Rb, Sr, Zr, and Ba in carbonate rocks (limestone and dolomite) is described. This method utilizes silicate rock standards, loose powder samples, and a matrix correction based on scattered radiation. Values for several international carbonate rock standards are calculated and compare favorably with neutron activation analysis results.

Geochemistry and petrogenesis of ophiolites from Northern Pindos (Greece)

The ophiolitic complex of Northern Pindos (Greece) contains ocean-floor basalts and low-Ti mafic rocks. The former rocks are similar to recent mid-ocean ridge basalts with a light REE depletion and a La/Yb ratio < 2. The low-Ti rocks resemble boninites in their high Mg and very low Ti and Zr contents and in their REE patterns which have convex-downwards shape with a slight light REE enrichment. However, their Zr/Ti, Ti/V and Zr/Y ratios are lower than in boninites. Both rock-types could be generated by dynamic partial melting of a rising upper mantle diapir. Slight enrichment in light REE, Sr, Rb and Ba in low-Ti rocks could be the result of either metasomatic or alteration processes. Although a subduction zone origin of the sequence is possible, the geochemical data do not necessarily imply such a setting.

The rare alkalies in hydrothermal alteration at Wairakei and Broadlands, geothermal fields, N.Z.

Drillcores and waters from Wairakei and Broadlands geothermal areas New Zealand have been analyzed for Li, Rb, Cs, Na, K, Mg, Ca, Al, Ti, Mn, and Be. The drillcores were altered to various degrees at temperatures below 300°C in slightly alkaline chloride water, probably derived from rock-water interaction in untapped horizons at higher temperature. It changes its composition as it leaches Ca and Na from the rock and adds K, Rb, Cs and Li. Evaluation of these changes in relation to the dimensions of the altered zone under observation suggests that a high mass ratio of water to rock ( e.g., 100) and a period of up to 1 million years are responsible for the present stage of alteration. Increase of K and Rb in the altered rocks is a result of the formation of abundant adularia in addition to illite. The K Rb ratio of the rock decreases during alteration but remains higher than that of the fluid. Only clay materials and zeolites that preferentially absorb Rb give slightly lower K Rb ratios than the fluid. The mineral phases responsible for the uptake of lithium during alteration are chlorite (300 ppm Li) and quartz (up to 430 ppm Li). Li uptake in quartz is considered to be the mechanism by which Al-rich quartz crystallises from alumino-silicates. Li Al atomic ratios of 0.3–0.57 and Al concentrations up to 3000 ppm have been observed. Relatively small concentrations of Cs are found in potassic minerals ( e.g. 10 ppm Cs in adularia, 44 ppm Cs in illite). However, 240 ppm Cs are found in wairakite from Wairakei equilibrated at 235°C. Lower equilibration temperatures may lead to higher cesium concentrations. This effect, in conjunction with a more concentrated hydrothermal fluid, could explain a content of 4500 ppm Cs in wairakite extracted from a drillcore taken in the El-Tatio geothermal field in Chile.

Genetic implications of minor-element and Sr-isotope geochemistry of alkaline rock complexes in the Wet Mountains area, Fremont and Custer counties, Colorado

Concentrations of Rb, Sr, and REE (rare earth elements), and Sr-isotopic ratios in rocks of the Cambrian alkaline complexes in the Wet Mountains area, Colorado, show that rocks formed as end-products of a variety of magmas generated from different source materials. The complexes generally contain a bimodal suite of cumulus mafic-ultramafic rocks and younger leucocratic rocks that include nepheline syenite and hornblende-biotite syenite in the McClure Mountain Complex, nepheline syenite pegmatite in the Gem Park Complex, and quartz syenite in the complex at Democrat Creek. The nepheline syenite and hornblende-biotite syenite at McClure Mountain (535±5m.y.) are older than the syenitic rocks at Democrat Creek (511±8m.y.). REE concentrations indicate that the nepheline syenite at McClure Mountain cannot be derived from the hornblende-biotite syenite, which it intrudes, or from the associated mafic-ultramafic rocks. REE also indicate that mafic-ultramafic rocks at McClure Mountain have a source distinct from that of the mafic-ultramafic rocks at Democrat Creek.

Terrestrial geochemistry of Cd, Bi, Tl, Pb, Zn and Rb

About 2000 common magmatic, metamorphic and sedimentary rocks and rockforming minerals contained in 465 individual samples have been analyzed for 6 trace metals and potassium with high precision, mainly by combined distillation and AAS methods. Estimates of average abundances in the continental crust are: 98 ppb Cd. 82 ppb Bi. 490 ppb Tl, 14.8 ppm Pb, 77 ppm Zn and 98 ppm Rb (K/Rb: 223). These averages are close to the mean concentrations of the 6 elements in sedimentary and in low to medium grade metamorphic rocks. In relation to the upper mantle the earth's crust has very effectively accumulated Rb, Pb, Tl (and Bi). Cd and Zn are equally distributed between the upper and lower crust. Bi, Tl, Rb, Pb and K are accumulated in the upper relative to the lower continental crust by factors between 3.5 and 1.4. This is mainly due to higher concentrations in granites and lower abundances in granulites relative to gneisses and schists. The five metals form large ions with bulk coefficients less than one for the partition between metamorphic rocks and anatectic granitic melts. The major hosts of Rb, Tl, Pb and Bi in rocks are minerals with 8- to 12-coordinated sites such as mica, K-feldspar, plagioclase etc. (except for some preference of Bi for sphene and apatite). As examples of significant correlations those of Pb with Tl, K, Bi and Rb in mafic rocks and of Bi with K, Rb, Tl and Pb in sedimentary rocks can be reported. In granites and gneisses hydroxyl containing Fe 2+-Mg-silicates are major host minerals for Zn and Cd. Except in some carbonate rocks Cd has no preference for Ca minerals.

Rb and Sr in rocks from Osoreyama volcano.

Rb and Sr in rocks from Osoreyama volcano.

Rb,Sr and Sr isotopic composition in vocanic rocks from Younger Ontake, Kiso-Ontake volcano.

Rb,Sr and Sr isotopic composition in vocanic rocks from Younger Ontake, Kiso-Ontake volcano.

Trace elements and anorthosite genesis

Terrestrial massif anorthosites have gained new interest for the understanding of the deep zones of the crust and for the reconstitution of its history in Proterozoic time. The purpose of this paper is to show how trace elements can enlighten two controversial questions in the problem of anorthosites, namely the nature of the parental magma and the process which gives rise to related acidic rocks. Data obtained on rocks and minerals coming from the Rogaland anorthositic province, South Norway, are presented together with those available in the literature. The Sr and Ba in plagioclase and their relationship with Ca and K, K and Rb in rocks and plagioclases, rare earth elements (REE) in cumulate minerals and in various liquids, 87Sr/ 86Sr initial ratios on rocks and minerals as well as a few data on transition elements and on 18O/ 16O ratios are discussed in the different sections. Quantitative modelling of the behaviour of trace elements is realized mainly by graphical methods in the Bjerkrem-Sogndal layered lopolith and in the Hidra body, both andesine-type massifs. The major conclusions are as follows: (1) The parental magma of the andesine-type massifs has a jotunitic (hypersthene-monzodioritic) composition characterized by variable K/Rb ratios (from 480 to 1700), by the absence of an Eu anomaly, by variable REE contents (from 50 to 220 for La chondrite-normalized content) and by La/Yb ratios almost constant (from 6 to 12), as well as by high Ti and Fe contents, by transition elements indicating calc-alkaline affinities and by 87Sr/ 86Sr initial ratios similar to those of rocks derived from the upper mantle or the deep crust. (2) A jotunitic composition appears not to be compatible with the parental magma of the labradorite-type massif anorthosite. The relationship between andesine anorthosite and labradorite anorthosite cannot be described simply in terms of fractional crystallization. (3) Fractional crystallization can explain the succession of rocks from andesine anorthosite to leuconorite, to norite and finally to acidic rocks. In some cases however, namely the Bjerkrem-Sogndal lopolith in Rogaland, contamination by supracrustal material must be invoked and seems to have superimposed its effects on those of fractional crystallization. (4) Deformation, granulation and recrystallization do not appear to fractionate both the major and trace elements of the plagioclase, except Ti which is lowered.

Rubidium as a guide to ore in Chilean porphyry copper deposits

Hypogene mineralization in porphyry copper deposits is characteristically associated with zones of potassic alteration. The mineralization and hydrothermal alteration are thought to be produced by an aqueous phase which has separated from a crystallizing magma. Due to the similarity in geochemical characteristics of K and Rb, Rb can be incorporated in K minerals that crystallize from a magma. The Rb ion is only 0.14 Aa larger than the K ion, but this size difference results in a higher bonding energy between K and O than between Rb and O. Therefore, when K minerals crystallize from a magma, K is preferentially removed with respect to Rb. The Rb tends to concentrate in the residual melt. If an aqueous phase separates from a magma during late stages of crystallization, it should have relatively higher Rb concentrations and low K/Rb ratios. High Rb concentrations and low K/Rb ratios might therefore be expected in rocks that have undergone potassic alteration.Wide zones of potassic alteration are closely associated with the orebodies at the Chilean porphyry copper deposits in the El Teniente (Braden), Rio Blanco, and Los Bronces (Disputada) mines. Therefore, the distribution of Rb and its relationship to ore and alteration at the three deposits is determined. As K replaces Ca in plagioclase, and Sr is geochemically related to Ca, the distribution of Sr is also determined. Background values for Rb and Sr are difficult to determine, as all accessible parts of the intrusions near the deposits are hydrothermally altered. Background values for intrusive rocks were obtained from 17 samples of unaltered intrusive bodies not associated with porphyry copper deposits.The following conclusions, though based on the study at El Teniente, Rio Blanco, and Los Bronces, may also be applicable to other porphyry copper deposits.1. Rocks that have undergone potassic or sericitic alteration have a two- to threefold increase in Rb and low K/Rb ratios. As these types of alteration characteristically accompany mineralization in porphyry copper deposits, Rb distribution may be useful to locate favorable alteration zones for porphyry copper exploration. The highest anomalies are found in mafic host rocks, which have low Rb background values, though anomalies can also be detected in rocks as siliceous as quartz monzonite.2. A twofold increase in Rb in the propylitic zone at El Teniente produces an anomaly which extends more than 400 meters beyond the margin of the orebody. No corresponding increase in K was observed. The Rb anomaly represents an enlarged target enclosing the zones of alteration commonly associated with porphyry copper mineralization.3. Weathering does not affect the distribution of Rb in rocks from the leached capping at El Teniente. Due to this immobility of Rb, weathering of the potassic zone in porphyry copper deposits should not destroy the Rb anomaly. The distribution of Rb could be a useful tool to locate potassic alteration zones in areas of tropical weathering, for example, in the southwest Pacific and in Central America.4. Addition of Rb and removal of Sr during hypogene alteration can produce high Rb/Sr anomalies which extend out through all of the zones of hydrothermal alteration. However, care must be taken in interpreting the significance of Rb/Sr anomalies because they may simply indicate extensive removal of Sr during normal weathering processes.5. Contouring the Rb values may be useful for quantitatively mapping the extent of K metasomatism.6. Anhydrite is readily dissolved and removed by acid meteoric waters. In deposits where anhydrite is an abundant hypogene alteration product, Sr depletion can be used as a quantitative guide to the degree of supergene activity.