Clinopyroxene-matrix partitioning of K,Rb, Cs,Sr and Ba

Extremely pure samples of clinopyroxene phenocrysts from two volcanic rocks have been analyzed for K, Rb, Cs, Sr and Ba. In conjunction with matrix concentrations, partition coefficients are obtained which are in the range 0.001–0.004 for K, Rb, Cs and Ba. These values are lower than those in the literature by factors of 6–100 but are in good agreement with values determined experimentally at pressures of 15–30 kb by Shimizu (Geochim. Cosmochim. Acta38, 1974). Values for partition coefficients measured on separates of impure or cloudy pyroxenes from these same rocks were higher and similar to those in the literature. We suggest this effect is related to ‘trapping,’ during crystal growth, of liquid which is enriched in the larger ions (such as Rb and Cs) due to lack of diffusion equilibrium in the liquid. Partition coefficient values for olivine and plagioclase from one of these same rocks were also determined.     

Zonation des éléments en traces au cours de la croissance des cristaux dans les bains silicatés: l'exemple de Rb,Cs, Sr et Ba dans le système Qz-Ab-Or-H2O

An indirect method was used to study Na, K, Rb, Cs, Sr and Ba partition coefficients between crystals and silicate melt. Equilibria between a hydrothermal solution and the melt at 800°C and 2 kb and between a hydrothermal solution and crystals at 750°C and 2 kb were separately achieved.For major element partitioning (Na and K), the results obtained here are in good agreement with those of Tuttle and Bowen (1958) which allow us to follow crystal evolution during a fractional crystallization process where the growth of zoned crystals takes place.For minor elements Rb, Cs, Sr, Ba, melt/aqueous solution partition coefficients depend on Na/K as well as the silica content of the melt. These effects are rather small for Rb and Cs, but are much more important for the alkaline earths. The feldspar/aqueous solution partition coefficients also depend on Na/K.The variations of the partition coefficients feldspar/melt are complex in the part of the Qz-Ab-Or diagram located below the cotectic line.During fractional crystallization following the Rayleigh law (assuming that there are no kinetic phenomena) Sr (D > 10) is almost totally removed from the melt in the early stages whereas Cs (D < 0.1) remains in the melt during the whole process. Rb and Ba have partition coefficients closer to unity. The variation of these coefficients, due to changes in bulk composition of liquid and crystals during fractional crystallization, can lead to complex zoning with possible concentration maxima at some stages. Similar phenomena can be expected in non-ideal natural solid solutions, even if no discontinuities can be detected in the physicochemical evolution of the parent magma.     

Geochemistry of K‐feldspar and Muscovite in Rare‐element Pegmatites and Granites from the Totoral Pegmatite Field,San Luis,Argentina

The geochemistry of K‐feldspar for K, P, Sr, Ba, Rb, Cs, Ga, and of muscovite for the same elements plus Nb and Ta, was used for proving the parental relationships of S‐type granites and LCT (Li, Cs, Ta) rare‐element pegmatites in the southernmost pegmatitic field of the Pampean pegmatite province in Argentina. The variation of K/Rb‐Cs, K/Cs‐Rb, K/Rb‐Rb/Sr, K/Rb‐Ba in K‐feldspar from the granites and pegmatites show that they form an association with the evolutional sequence: granites → barren‐ to transitional pegmatites → beryl type, beryl‐columbite‐phosphate pegmatites → complex type of spodumene subtype pegmatites → albite‐spodumene type → albite type pegmatites. This sequence reflects the regional distribution of the different magmatic units. The Ta‐Cs diagram for muscovite reveals that none of the studied pegmatites exceed the threshold established in previous studies for being considered with important tantalum oxide mineralization. The granites and pegmatites constitute a rare‐element pegmatitic field in which different magmatic units form a continuous fractionation trend, extended from the less evolved granitic facies to the most geochemically specialized pegmatites     

Case studies on the origin of basalt: III. Petrogenesis of the Mauna Ulu eruption,Kilauea, 1969–1971

During the Mauna Ulu flank eruption on Kilauea, Hawaii, the concentrations in the lavas of the minor elements K, P, Na and Ti, and the incompatible trace elements (analyzed by isotope dilution) K, Rb, Cs, Ba, Sr, and the REE (except Yb) decreased monotonically and linearly with the time (or date) of the eruption. At the same time, the concentrations of the major elements and of Yb, and the ratios of K/Rb, K/Cs, Ba/Rb, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd remained constant. Most of the scatter in the raw concentration data is removed by a simple correction for olivine (plus chromite) fractionation previously established by Wright et al. (1975). These results are explained by simple equilibrium partial melting of a uniform source. The degree of melting increased by about 20% of the initial value during the course of the eruption. The trace element data are inverted by the method originated by Minster and Allègre (1978) and simplified by Hofmann and Feigenson (1983). The source has the following element (or isotope) ratios: K/Rb=501±7, Ba/Rb=14.0±0.5, Rb/Cs=95±7, Rb/Sr=0.0193 (+0.0045, –0.0090), (Ce/Ba)_CN= 1.1±0.1, (Sr/Ba)_CN=1.19 (+0.30, –0.19), ⁸⁷Sr/⁸⁶Sr=0.703521±0.000016, and ¹⁴³Nd/¹⁴⁴Nd=0.512966±0.000008. The REE pattern of the source has a nearly flat or slightly negative slope (=relative LREE enrichment) between Ce and Dy and a strongly positive slope between Dy and Yb. However, this relative HREE enrichment is poorly constrained by the analytical data, is highly model dependent and may not be a true source feature. The Yb concentration in the source is particularly poorly constrained because it is essentially constant in the melts. On the other hand, this special feature demonstrates that Yb must be buffered by a mineral phase with a high partition coefficient for Yb, namely garnet. The calculated clinopyroxene/garnet ratio in the source is roughly equal to one. In contrast, the source of Kohala volcano had previously been found to contain little or no garnet.     

The nature and origin of geochemical variation in Mid-Atlantic Ridge basalts from the Central North Atlantic

Seventy-two basalts from 58 dredge stations located along the Mid-Atlantic Ridge from 29°N to 59°N have been analyzed for ${}^{87}\text{Sr}{}^{86}\text{Sr}$ and for K, Rb, Cc, Sr and Ba. The Sr-isotope profile along the ridge has three distinct maxima, one coinciding with the Azores platform (0.70345), one at 45°N (0.70340) and the third at 35°N, in the vicinity of the Oceanographer Fracture Zone. Basalts from ridge segments between 29°N and 33°N, and 49°N and 59°N have ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios typical of ‘normal’ mid-ocean ridge basalts (0.70230–0.70280). Profiles of K, Rb, Cs, Sr, Bz, Rb/Sr and Ba/Sr are similar to the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ profile, but Rb/K, Cs/K and Ba/K show broad maxima between 35°N and 45°N.These variations result from chemical and isotopic heterogeneity in the mantle, and are interpreted as caused by a mantle plume beneath the Azores which mixes with the LIL-element-depleted asthenosphere. Additional plumes may exist beneath 45°N and 35°N.Compared to the LIL-element-depleted asthenosphere, the Azores mantle plume is 10 to 30 times enriched in LIL elements with very small (? 0.1) bulk crystal/melt partition coefficients (Rb, Cs, Ba, La). Mildly incompatible elements (0.1 < D < 1) (Sr, Sm, Yb) are only 0.8–3 times enriched. These, observations suggest that LIL element differences between these two mantle reservoirs resulted from processes involving solid-liquid equilibria and not vapor-solid or vapor-liquid equilibria. Isotope systematics indicate that neither mantle reservoir remained a closed system since the formation of the Earth, but it is not possible to determine the time at which heterogeneity first developed.     

Distribution of groups I and II elements between liquidus phases of fluorine-saturated Si-Al-Na-K-Li-H-O system

Experimental data allow modeling the behavior of the named elements during formation of fluorine- saturated leucocratic rocks of silicic and alkaline compositions. The distribution of alkaline and alkaliearth elements is discussed at equilibrium between the silica-alumina melt with fluoride phases (crystalline and liquid) and with feldspar. Cryolite crystals form during saturation of silica-alumina melt of normal alkalinity with fluorine. Continuous solid solution of sodium-potassium cryolite is stable at 800°C. The equilibrium between melt and crystals continues up to the maximum molar fraction of 0.1 lithium end member in cryolite, at which two fluoride phases (crystalline and liquid) coexist with the silica-alumina melt of fixed composition. Separation of salt melts during late differentiation stages of granite and alkaline rocks is a regular process continuing the natural evolution of ore-magmatic systems. At equilibrium of two liquid phases, the silica phase is relatively enriched in potassium, and the fluoride phase is substantially enriched in sodium. This detected effect is the only currently possible mechanism for the occurrence of the potassium differentiation trends of granite melts. All effects related to crystallization cause enrichment in sodium. In other cases (with Ca, Sr, Mg, Rb, and Cs), separation of the second liquid phase acts in the same direction and enhances the action of crystallization. Comparison between partition coefficients allows derivation of the following affinity rows of alkaline elements for fluoride melt: Li > Na > K > Rb≈Cs and Mg > Ca > Sr > Ba. Hence, the known rule for joining strong bases with strong acids and weak bases with weak acids is fulfilled.     

Determination of Thirteen Rare-Earth Elements by High-Performance Liquid Chromatography in Thirty and of K, Rb, Cs, Sr and Ba by Isotope Dilution Mass Spectrometry in Eighteen International Geochemical Reference Samples

This study reports the results of thirteen rare-earth elements in thirty geochemical reference samples. Three alkali (K, Rb, Cs) and two alkaline earth (Sr, Ba) elements were also determined in eighteen reference samples. The analytical procedure involved is based on spiked samples and later measurement of the rare-earth elements by high-performance liquid chromatography and the alkalies and alkaline earths by thermo-quartz mass spectrometric isotope dilution (THQ-MSID). Whenever possible, the results are compared with literature values.     

熔体-溶液体系中元素分配系数:新资料及其研究方向

基于近几年来国内外学者测得的熔体-溶液体系中F、Cl、P、CO_2、B、Au、Pb、Zn、Cu、Fe、Mg、Li、Rb、Cs、K、Na、Ca、Sr、Ba、Co、Ni、Be、Mo、W、Si、Al、REE、Sc、Ti、U、Th、Zr、Hf、Nb、Pt、Ir、Pd等元素的分配系数,从元素性质、溶液成分(阴离子和阳离子)、熔体成分和物理化学条件等方面进一步分析总结了分配系数变化规律,并提出了预测未知元素分配系数的元素性质准则、溶液成分准则、熔体成分准则和物理化学条件准则,最后指出了这一研究领域今后的研究方向。     

The geochemistry and evolution of early precambrian mantle

Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and ⁸⁷Sr/⁸⁶Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times. Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution:

1. starting with primordial Sr, a short period of relatively rapid ⁸⁷Sr/⁸⁶Sr growth, followed by Rb depletion;
2. a period between ≧ 3.5 b.y. and ～ 1.7 b.y. when closed-system Sr isotope evolution occurred (with Rb/Sr ～ 0.023);
3. development of large-scale Rb/Sr heterogeneities in the mantle at ～ 1.7 b.y., leading to a present-day mantle with ⁸⁷Sr/⁸⁶Sr ranging from 0.7023 to 0.7065 and Rb/Sr ranging from ～ 0 to 0.065.

     

Two-liquid partition coefficients: Experimental data and geochemical implications

Partition coefficients for Cs, Ba, Sr, Ca, Mg, La, Sm, Lu, Mn, Ti, Cr, Ta, Zr, and P between immiscible basic and acidic liquids in the system K₂O-Al₂O₃-FeO-SiO₂ were experimentally determined at 1,180 °C and 1 atm. Phosphorus is most strongly enriched in the basic melt (by a factor of 10), followed by rare earth elements, Ta, Ca, Cr, Ti, Mn, Zr, Mg, Sr, and Ba (enriched by a factor of 1.5). Of the elements studied, only Cs is enriched in the acidic melt. The two-liquid partition coefficients of Zr, Ta, Sm, and Mn are constant for concentrations ranging from <0.1% to as high as 1 wt.-%, suggesting that Henry's law is applicable in silicate melts (at least for these elements) to concentrations well above typical trace element levels in rocks. The strong relative preference of many elements for the basic melt implies that the structural characteristics of basic melts more readily permit stable coordination of cations by oxygen. Partitioning of elements between crystal and liquid in a magma must therefore be influenced by the composition (and consequent structure) of the liquid.Application of the two-liquid partition coefficients to possible occurrences of liquid immiscibility in magmas reveals that typical basalt-rhyolite associations are probably not generated by two-liquid phase separation. However, liquid immiscibility cannot be discounted as a possible origin for lamprophyric rocks containing felsic segregations.     

Some Crystallo-chemical Relations of Nephelines and Feldspars on Stjerny, North Norway

Chemical, X-ray, and optical data are given for coexisting nephelinesand potassium feldspars separated from a nepheline syenite,and of nepheline and albite from pegmatites. Li, Rb, Cs (detectedonly in one nepheline), Ba, and Sr were determined, in additionto the major elements. (Pb and Tl were not detected by the spectrographicmethod used.) Some peculiarities of the mineral structures which are consideredto be important for the partition of elements between the coexistingphases are discussed. The importance of vacancies in nephelines,and their probable restriction to the large cavities in thestructure, is emphasized. It is pointed out that since low concentrations of Rb, Cs, Pb,and Tl are found in both nepheline and feldspar, they cannotbe caused by an unfavourable nepheline structure. They are eithera property of the parent magma or are the result of the physicalconditions of crystallization (i.e. escaping gas phase). Thelow concentrations of Sr, and especially of Ba, in nephelinesare in contrast to the high contents of these elements in thecoexisting feldspars. This is explained by considering the nephelinestructure. The unit cell contains eight structural sites forthe large cations (Na, K, Ca), and there are eight valenciesto satisfy. Six of the sites are smaller than the other two,and preferentially incorporate Na (and Ca) ions. The two largersites will tend to remain as vacant sites. Those which haveto be filled in order to satisfy the charge balance, will preferentiallyincorporate the large univalent ions (K and H₃O). The higher-chargedions (Ba² and Sr²) will pull the anions closer together andreduce the size of the site. They are, therefore, rejected fromthese positions. When nephelines coexist with feldspar, theywill incorporate the maximum amount of Si possible at the temperatureof formation. The substitution of Al for Si simultaneously withthe entry of a divalent cation (as in feldpspar) is, therefore,not possible except in non-feldspathic, extremely desilicifiedrocks. Thus, the entry of a divalent cation will not effectivelyclose the vacant sites. The somewhat higher contents of Sr relativeto Ba in the nephelines is because Sr is more easily incorporatedin the small sites (with Na and Ca).     

Trace-element distribution in lavas from Anjouan and Grande Comore,western Indian Ocean

The elements Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr and Ba have been determined by X-ray fluorescence for 65 basaltic and differentiated lavas from Anjouan, while Sc, V, Cr, Co, Ni, Cu, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba and Hf have been determined by spark-source mass spectrometry for selected lavas from Anjouan and Grande Comore, the most recently formed of the Comores Archipelago. Basaltic lavas studied range through nephelinite, basanite, alkali basalt and hypersthene-normative basalt, while differentiated lavas belong mainly to the trends: alkali basalt - trachyte and basanite - phonolite. The results indicate that during magmatic fractionation behaviour of large-ion elements such as Rb, Sr, Y, Zr, Ba and Nb is controlled by size/charge criteria, resulting in their exclusion from crystallising phases until the late trachytic and phonolitic stages. These elements are clearly fractionated by amphibole, plagioclase and alkali feldspar. Variation of transition elements due to crystal-liquid differentiation is largely in accord with the predictions of crystal field theory. The behaviour of Zn is not readily accounted for. Fractionation of K/Rb, K/Cs and probably Zr/Nb and discrepancies in abundance levels of large-ion elements between the main basaltic types are best accounted for in terms of high-pressure processes and probably also reflect inherent features of source-region geochemistry, coupled with the effects of variable partial melting.     

Trace element partitioning between apatite and silicate melts

We present new experimental apatite/melt trace element partition coefficients for a large number of trace elements (Cs, Rb, Ba, La, Ce, Pr, Sm, Gd, Lu, Y, Sr, Zr, Hf, Nb, Ta, U, Pb, and Th). The experiments were conducted at pressures of 1.0 GPa and temperatures of 1250 °C. The rare earth elements (La, Ce, Pr, Sm, Gd, and Lu), Y, and Sr are compatible in apatite, whereas the larger lithophile elements (Cs, Rb, and Ba) are strongly incompatible. Other trace elements such as U, Th, and Pb have partition coefficients close to unity. In all experiments we found D_Hf > D_Zr, D_Ta ≈ D_Nb, and D_Ba > D_Rb > D_Cs. The experiments reveal a strong influence of melt composition on REE partition coefficients. With increasing polymerisation of the melt, apatite/melt partition coefficients for the rare earth elements increase for about an order of magnitude. We also present some results in fluorine-rich and water-rich systems, respectively, but no significant influence of either H₂O or F on the partitioning was found. Furthermore, we also present experimentally determined partition coefficients in close-to natural compositions which should be directly applicable to magmatic processes.     

Experimental determination of partition coefficients for Rb,Sr and Ba between alkali feldspar and silicate liquid

Partitioning of Rb, Sr and Ba between alkali feldspar and a synthetic granitic melt has been determined at 8 kb and 720 to 780°C for a single quaternary granite composition. The results suggest that Henry's law is obeyed by Rb up to ~0.8 wt.% Rb₂O in both the liquid and in the alkali feldspar. The measured D values for Rb range from 0.77 to 1.1. For Ba, Henry's Law is obeyed up to ~0.6 wt.% BaO in the liquid and ~5 wt.% BaO in the alkali feldspar. D values for Ba range from 6.4 to 14. For Sr there is only a crude relationship between concentration in the liquid and concentration in the alkali feldspar at concentrations greater than ~0.6 wt.% SrO in the liquid and ~0.4 wt.% SrO in the alkali feldspar. D values for Sr range from 1.2 to 5.0. Partitioning of Sr is apparently sensitive to the concentration of Ba in the system and this partly explains the failure of Sr to obey Henry's Law.Linear least-squares fits to the partitioning data as a function of temperature suggest inverse correlation between temperature and D values. Rb shows only a slight temperature effect whereas Ba and Sr appear to be rather strongly affected by temperature, but the temperature range examined here is small compared to the scatter in the data making these trends relatively uncertain. Other factors that appear to affect partitioning, especially of Sr, are growth rate, development of sector zoning and Or content of the alkali feldspar. These factors severely limit the use of partitioning of these elements in alkali feldspar as geothermometers.The technique for measuring growth rates utilized here combined with measurement of trace element depletion in diffusion boundary layers adjacent to the alkali feldspar crystals makes it possible to estimate diffusivities for Ba and Sr. These estimates suggest a difference of 2 orders of magnitude between diffusivities for Ba and Sr in a vapor-saturated melt and those measured by HOFMANN and MAGARITZ (1976) for a dry obsidian glass.     

Trace and minor element geochemistry of the rhyolitic volcanic rocks,Central North Island,New Zealand

Analytical data are presented for the following elements: Cs, Rb, Ba, K, Sr, Ca, Na, Fe, Mg, Cu, Co, Ni, Li, Sc, V, Cr, Ga, Al, Si, La, Y, and Zr. Eight samples were analysed by the spark source method for rare earths, Tl, Pb, Hf, Sn, Nb, Mo, Bi, and In. In addition to data on rhyolitic volcanics, a small number of intermediate volcanics and eugeosynclinal sediments were analysed for comparative purposes. The following features are shown by the trace element data:

1. The rhyolitic rocks have consistently lower concentrations of most trace and minor elements when compared with recent estimates of average concentrations in granites. None of the criteria for strong fractionation (e.g. low K/Rb, Ba/Rb and K/Cs ratios) are present.
2. The data do not indicate any systematic differences between the rhyolitic lavas and ignimbrites, although the very young rhyolitic pumices are consistently more “basic” in their element concentrations compared to the other rhyolitic analyses.
3. The residual glasses (and devitrified matrices) are depleted, relative to the total rock compositions, in Fe, Mg, Ca, Sr, V, Sc, and Al, and enriched in Cs, Rb, K, Ba, and Si. Zr is depleted in the residual glasses separated from rhyolites, but not in the andesitic residual matrices.
4. The rare earth fractionation patterns of the rhyolitic and andesitic extrusives are very similar, being intermediate between chondritic and sedimentary patterns i.e., there is no evidence of strong fractionation. The rhyolitic patterns also indicate a slight Eu depletion.
5. Comparable trace and minor element behaviour (with the possible exception of Zr) seems to exist through the rhyolite-andesite compositional range. This is supported by the whole rock-residual liquid trends for the various elements studied, which broadly coincide with the observed whole rock trends, both through the rhyolitic-andesitic compositonal range, and within the rhyolitic compositional range.

The data are finally discussed in the light of the possible origin of the rhyolitic magmas. It is believed that the analytical data presented are qualitatively consistent with the recently proposed idea that the magmas are derived by partial fusion of the associated Triassic-Jurassic eugeosynclinal greywacke-argillite sedimentary sequence.     

Trace element constraints on pegmatite genesis: Tin Mountain pegmatite,Black Hills,South Dakota

The Tin Mountain pegmatite is a small, zoned granitic body that is extremely enriched in Rb and Li, but has moderate concentrations of Sr and Ba. These trace elements are modelled using granitic distribution coefficients in order to test the potentials of partial melting of metasedimentary rocks and fractionation of a less-evolved granitic melt to have produced the parental liquid to the Tin Mountain pegmatite. Batch melting of any reasonable metasedimentary source rock would likely have yielded melts that were either insufficiently enriched in Rb and Li to be the parental liquid, or that had Sr and Ba concentrations that were much higher than those estimated for the parental liquid. The modelling of simple fractional crystallization and equilibrium crystallization of a granitic melt within the compositional range of the spatially associated Harney Peak Granite gives calculated melt compositions with either lower Sr and Ba concentrations or inadequate Rb and Li enrichments, to be the parent liquid of the pegmatite. At least two variants from simple crystal-liquid fractionation models can, however, successfully account for the derivation of the parent liquid: 1) generation of a Rb-, Li-, Ba- and Sr-rich granitic melt (outside of the compositional range of the sampled portions of the Harney Peak Granite complex) by low degrees of partial melting of metasedimentary rocks found in the Black Hills, followed by moderate extents of fractional or equilibrium crystallization, 2) derivation from Harney Peak granite via a complex, multi-stage crystal-liquid fractionation process, such as progressive equilibrium crystallization.     

Partition of Sr,Ba, Ca,Y, Eu2+, Eu3+, and other REE between plagioclase feldspar and magmatic liquid: an experimental study

Plagioclase feldspar/magmatic liquid partition coefficients for Sr, Ba, Ca, Y, Eu²⁺, Eu³⁺ and other REE have been determined experimentally at 1 atm total pressure in the temperature range 1150–1400°C. Natural and synthetic melts representative of basaltic and andesitic bulk compositions were used, crystallizing plagioclase feldspar in the composition range An₃₅–An₈₅. Partition coefficients for Sr are greater than unity at all geologically reasonable temperatures, and for Ba are less than unity above approximately 1060°C. Both are strongly dependent upon temperature. Partition coefficients for the trivalent REE are relatively insensitive to temperature. At fixed temperature they decrease monotonically from La to Lu. The partition of Eu is a strong function of oxygen fugacity. Under extreme reducing conditions D_Eu approaches the value of D_Sr.     

Rare earth and other trace element mobility during mylonitization: a comparison of the Brevard and Hope Valley shear zones in the Appalachian Mountains, USA

In progressing from a granitoid mylonite to an ultramylonite in the Brevard shear zone in North Carolina, Ca and LOI (H₂O) increase, Si, Mg, K, Na, Ba, Sr, Ta, Cs and Th decrease, while changes in Al, Ti, Fe, P, Sc, Rb, REE, Hf, Cr and U are relatively small. A volume loss of 44% is calculated for the Brevard ultramylonite relative to an Al–Ti–Fe isocon. The increase in Ca and LOI is related to a large increase in retrograde epidote and muscovite in the ultramylonite, the decreases in K, Na, Si, Ba and Sr reflect the destruction of feldspars, and the decrease in Mg is related to the destruction of biotite during mylonitization. In an amphibolite facies fault zone separating grey and pink granitic gneisses in the Hope Valley shear zone in New England, compositional similarity suggests the ultramylonite is composed chiefly of the pink gneisses. Utilizing an Al–Ti–Fe isocon for the pink gneisses, Sc, Cr, Hf, Ta, U, Th and M-HREE are relatively unchanged, Si, LOI, K, Mg, Rb, Cs and Ba are enriched, and Ca, Na, P, Sr and LREE are lost during deformation. In contrast to the Brevard mylonite, the Hope Valley mylonite appears to have increased in volume by about 70%, chiefly in response to an introduction of quartz. Chondrite-normalized REE patterns of granitoids from both shear zones are LREE-enriched and have prominent negative Eu anomalies. Although REE increase in abundance in the Brevard ultramylonites (reflecting the volume loss), the shape of the REE pattern remains unchanged. In contrast, REE and especially LREE decrease in abundance with increasing deformation of the Hope Valley gneisses. Mass balance calculations indicate that ≥95% of the REE in the Brevard rocks reside in titanite. In contrast, in the Hope Valley rocks only 15–40% of the REE can be accounted for collectively by titanite, apatite and zircon. Possible sites for the remaining REE are allanite, fluorite or grain boundaries. Loss of LREE from the pink gneisses during deformation may have resulted from decreases in allanite and perhaps apatite or by leaching ofy REE from grain boundaries by fluids moving through the shear zone. Among the element ratios most resistant to change during mylonitization in the Brevard shear zone are La/Yb, Eu/Eu*, Sm/Nd, La/Sc, Th/Sc, Th/Yb, Cr/Th, Th/U and Hf/Ta, whereas the most stable ratios in the Hope Valley shear zone are K/Rb, Rb/Cs, Th/U, Eu/Eu*, Th/Sc, Th/Yb, Sm/Nd, Th/Ta, Hf/Ta and Hf/Yb. However, until more trace element data are available from other shear zones, these ratios should not be used alone to identify protoliths of deformed rocks.     

Geochemistry of blackwall sequences in the Habachtal emerald deposit,Hohe Tauern,Austria Part 1: Presentation of geochemical data

Summary The Habachtal emerald deposit, Hohe Tauern, is composed of blackwall sequences of the type: serpentinite — talc schist — ±chlorite schist or actinolite schist — biotite schist —albite gneiss and/or micaschist. 2 serpentinites, 33 blackwall rocks, 9 micaschists, 10 albite gneisses, and 5 aplitic gneisses were analyzed for major elements, and for Li, Be, Cr, Ni, Zn, Zr, Sn, in 36 samples also for Sc, Cu, Rb, Sr, Cs, Ba, W. The blackwall formation is due to a metasomatic exchange involving a transfer of Mg from the serpentinite to the silicic country rock, and of Si, Ca, K, and Al from the country rock to the serpentinite. Some of the trace elements were also mobile: Compared to serpentinite, Li and Be were enriched in all the blackwall rocks, and Sn and Cs in the actinolite, chlorite, and biotite schists; Sr was concentrated in the dolomite-bearing talc schists, and Zn, Rb, and Ba predominantly in the biotite schists.

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Geochemie der Blackwall-Folgen in der Smaragd-Lagerstätte Habachtal, Hohe Tauern, Österreich. Teil 1: Darstellung der geochemischen Daten

Zusammenfassung Die Smaragd-Lagerstätte Habachtal, Hohe Tauern, besteht aus Blackwall-Folgen vom Typ: Serpentinit — Talkschiefer — ±Chloritschiefer oder Aktinolithschiefer — Biotitschiefer — Albitgneis und/oder Glimmerschiefer. Von 2 Serpentiniten, 33 Blackwall-Gesteinen, 9 Glimmerschiefern, 10 Albitgneisen und 5 Aplitgneisen wurden chemische Analysen der Hauptelemente und von Li, Be, Cr, Ni, Zn, Zr, Sn vorgelegt; 36 Proben wurden auch auf Sc, Cu, Rb, Sr, Cs, Ba und W analysiert. Die Blackwall-Bildung geht auf einen metasomatischen Austausch zurück, bei dem Mg aus dem Serpentinit ins Nebengestein, Si, Ca, K und Al aus dem Nebengestein in den Serpentinit transportiert wurden. Daneben waren auch einige Spurenelemente mobil: Im Vergleich zum Serpentinit wurden Li und Be in allen Blackwall-Gesteinen, Sn und Cs in den Aktinolith-, Chlorit- und Biotitschiefern angereichert; Sr wurde(n) in den dolomitführenden Talkschiefern, Zn, Rb und Ba hauptsächlich in den Biotitschiefern konzentriert.

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With 5 Figures     

Effect of Natural Organic Acids on Mobilization of Macro- and Microelements from Rocks

Doklady Earth Sciences - Experimental study of leaching of the main cations (Na, K, Mg, and Ca), alkaline and alkali-earth microelements (Li, Rb, Cs, Be, Sr, and Ba), heavy metals (Mn, Fe, Co, Ni,...