Average Upper Continental Crust Research Articles

The geochemical study of fluvio-lacustrine aquifers in the Kathmandu Basin (Nepal) and the implications for the mobilization of arsenic

The geochemical analyses of fluvial-lacustrine aquifer sediments of the Kathmandu Valley have been made as a step in assessing the environment for the mobilization of arsenic in groundwater. Elements measured by X-ray fluorescence (XRF) include 4 major oxides (Fe2O3, TiO2, CaO, P2O5) and 14 trace elements (As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc and TS). Elution tests of 15 selected core samples were also carried out to determine the potential leaching of arsenic from the sediments. The XRF results show that average bulk concentrations of the major oxides and trace elements are similar to modern unconsolidated sediments and average upper continental crust. However, the abundance of elements varies with grain size, with higher concentrations in finer-size grades. Variations in elemental abundances within the basin are strongly controlled by sediment facies. The elution tests show that greater amounts of arsenic are generally eluted from the fine sediments, although the rates are variable. The results overall suggest that As concentrations in the bulk sediments are not a controlling factor for elevated As in the Kathmandu Basin groundwater, and the roles of other factors such as redox conditions and organic matter contents are likely to be more significant.

Relative contributions of crust and mantle to the origin of the Bijli Rhyolite in a palaeoproterozoic bimodal volcanic sequence (Dongargarh Group), central India

New mineralogical, bulk chemical and oxygen isotope data on the Palaeoproterozoic Bijli Rhyolite, the basal unit of a bimodal volcanic sequence (Dongargarh Group) in central India, and one of the most voluminous silicic volcanic expressions in the Indian Shield, are presented. The Bijli Rhyolite can be recognized as a poorly sorted pyroclastic deposit, and comprises of phenocrystic K-feldspar + albite ± anorthoclase set in fine-grained micro-fragmental matrix of quartz-feldspar-sericite-chlorite-iron-oxide ± calcite. The rocks are largely metaluminous with high SiO2, Na2O + K2O, Fe/Mg, Ga/Al, Zr, Ta, Sn, Y, REE and low CaO, Ba, Sr contents; the composition points to an ‘A-type granite’ melt. The rocks show negative Cs-, Sr-, Eu- and Ti- anomalies with incompatible element concentrations 2–3 times more than the upper continental crust (UCC). LREE is high (La/Yb ∼ 20) and HREE 20–30 times chondritic. δ18Owhole-rock varies between 4.4 and 7.8‰ (mean 5.87±1.26‰).The Bijli melt is neither formed by fractionation of a basaltic magma, nor does it represent a fractionated crustal melt. It is shown that the mantle-derived high temperature basaltic komatiitic melts/high Mg basalts triggered crustal melting, and interacted predominantly with deep crust compositionally similar to the Average Archaean Granulite (AAG), and a shallower crustal component with low CaO and Al2O3 to give rise to the hybrid Bijli melts. Geochemical mass balance suggests that ∼ 30% partial melting of AAG under anhydrous condition, instead of the upper continental crust (UCC) including the Amgaon granitoid gneiss reported from the area, better matches the trace element concentrations in the rocks. The similar Ta/Th of the rhyolites (0.060) and average granulite (0.065) vs. UCC (0.13) also support a deep crustal protolith. Variable contributions of crust and mantle, and action of hydrothermal fluid are attributed for the spread in δ18Owhole-rock values. The fast eruption of high temperature (∼ 900°C) rhyolitic melts suggests a rapid drop in pressure of melting related to decompression in an extensional setting.

Geochemistry of Cenozoic microtektites and clinopyroxene-bearing spherules

We have determined the major and trace element compositions of 176 individual microtektites/spherules from the Australasian, Ivory Coast, and North American microtektite and clinopyroxene-bearing (cpx) spherule layers. Trace element contents for up to 30 trace elements were determined by instrumental neutron activation analysis (INAA), and major element compositions were determined using energy dispersive X-ray (EDX) analysis in combination with a scanning electron microscope (SEM). In addition, petrographic data were obtained for the cpx spherules using the SEM and EDX. This is the first trace element study of individual Australasian microtektites, and the data revealed the presence of a previously unrecognized group of Australasian microtektites with high contents of Ni (up to 471 ppm). In previous studies the high-Mg (HMg) Australasian microtektites were thought to be related to the HMg Australasian tektites, but our trace element data suggest that the high-Ni (HNi) Australasian microtektites, rather than the high-Mg microtektites, are related to the high-Mg Australasian tektites. We find that Cenozoic microtektites/spherules from a given layer can be distinguished from microtektites/spherules from other layers as a group, but it is not always possible to determine which layer an individual microtektite/spherule came from based only on trace element compositions. The cpx spherules and most of the microtektites have Cr, Co, and Ni contents that are higher than the average contents of these elements in the upper continental crust, suggesting the presence of a meteoritic component. The highest Cr, Co, and Ni contents are found in the cpx spherules (and low-Si cpx-related microtektites). Unetched to slightly etched cpx spherules have Ni/Cr and Ni/Co ratios that generally lie along mixing curves between the average upper continental crust and chondrites. The best fit appears to be with an LL chondrite. The moderately to heavily etched cpx spherules have values that lie off the mixing curves in a direction that suggests Ni loss, probably as a result of solution of a Ni-rich phase (olivine?). The Ni-rich Australasian microtektites also have Ni values that lie close to mixing curves between the average upper continental crust and chondrites. However, both the cpx spherules and HNi Australasian microtektites appear to have Ir (and to a lesser extent Au) contents that are much too low to have Ni/Ir ratios similar to chondritic values. We have no explanation for the low-Ir and -Au contents except to speculate that they may be the result of a complex fractionation process. The Ivory Coast and North American microtektites do not have high enough siderophile element contents to reach any firm conclusions regarding the presence of, or nature of, a meteoritic component in them. Trace element compositions are consistent with derivation of the Cenozoic microtektite/spherule layers from upper continental crust. The normal Australasian microtektites appear to have been derived from a graywacke or lithic arenite with a range in clay and quartz content. The source rock for the high-Mg Australasian microtektites is not known, but the HMg microtektites do not appear to be normal Australasian microtektites that were simply contaminated by meteorites or ultramafic rocks. The average Ivory Coast microtektite composition can be matched with a mixture of target rocks at the Bosumtwi crater. The average composition of the North American microtektites suggests an arkosic source rock, but with graywacke and quartz-rich end members. However, we could not match the composition of the North American microtektites with lithologies in impact breccias recovered from the Chesapeake Bay impact structure that is believed to be the source crater. Likewise, we could not match the composition of the cpx spherules with mixtures of basement rocks and overlying sedimentary deposits (for which compositional data are available) at the Popigai impact crater that may be the source crater for the cpx spherules. This may be because the cpx spherules were derived, in large part, from clastic surface rocks (sandstones and shales) for which no compositional data are available.

Sources of osmium to the modern oceans: new evidence from the 190Pt-186Os system

High precision Os isotope analysis of young marine manganese nodules indicate that whereas the composition of modern seawater is radiogenic with respect to 187Os/188Os, it has 186Os/188Os that is within uncertainty of the chondritic value. Marine Mn nodule compositions thus indicate that the average continental source of Os to modern seawater had long-term high Re/Os compared to Pt/Os. Analyses of loess and freshwater Mn nodules support existing evidence that average upper continental crust (UCC) has resolvably suprachondritic 186Os/188Os, as well as radiogenic 187Os/188Os. Modeling the composition of seawater as a two-component mixture of oceanic/cosmic Os with chondritic Os compositions and continentally-derived Os demonstrates that, insofar as estimates for the composition of average UCC are accurate, congruently weathered average UCC cannot be the sole continental source of Os to seawater. Our analysis of four Cambrian black shales confirm that organic-rich sediments can have 187Os/188Os ratios that are much higher than average UCC, but 186Os/188Os compositions that are generally between those of chondrites and average-UCC. Preferential weathering of black shales can result in dissolved Os discharged to the ocean basins that has a much lower 186Os/188Os than does average upper crust. Modeling the available data demonstrates that augmentation of estimated average UCC compositions with less than 0.1% additional black shale and 1.4% additional ultramafic rock can produce a continental end-member Os isotopic composition that satisfies the requirements imposed by the marine Mn nodule data. The interplay of these two sources provides a mechanism by which the 187Os/188Os of seawater can change as sources and weathering conditions change, yet seawater 186Os/188Os varies only minimally.

U-Pb zircon and geochemical evidence for bimodal mid-Paleozoic magmatism and syngenetic base-metal mineralization in the Yukon-Tanana terrane, Alaska

New SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic Yukon-Tanana terrane in east-central Alaska help define the tectonic setting of mid-Paleo-zoic magmatism and syngenetic hydrothermal Zn-Pb-Ag mineralization along the ancient Pacific margin of North America. We compare data from similar greenschist-facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks, in the Wood River area of the Alaska Range and the Salcha River area of the Yukon-Tanana Upland, and from amphibolite-facies augen gneiss and mafic gneiss (amphibolite) in the Goodpaster River area of the upland. Allowing for analytical uncertainties, igneous crystallization age ranges of 376–353 Ma, 378–346 Ma, and 374–358 Ma are indicated by 13 new SHRIMP U-Pb dates for the Wood River, Salcha River, and Goodpaster River areas, respectively. Bimodal magmatism is indicated by Late Devonian crystallization ages for both augen gneiss (371 ± 3 and 362 ± 4 Ma) and associated orthoamphibolite (369 ± 3 Ma) in the upland and by stratigraphic interleaving of mafic and felsic rocks in the Alaska Range. Metabasites in all three study areas have elevated HFSE (high field strength element) and REE (rare earth element) contents indicative of generation in a within-plate (extensional) tectonic setting. Within-plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the Bonnifield district in the Wood River area and for metarhyolite tuff interlayered with the carbonaceous Nasina assemblage, which hosts sedimentary exhalative sulfide occurrences in the Salcha River area. Most of the other felsic metaigneous samples from the Alaska Range and the Yukon-Tanana Upland have geochemical signatures that are similar to those of both average upper continental crust and continental-margin arc rocks generated in thick continental crust. Given the absence in our study areas of intermediate-composition magmatic products generally found in most arcs, and the presence of bimodal magmatism, the alkalic within-plate chemistry of the mafic rocks and some of the felsic rocks, and the widespread occurrence of interlayered carbonaceous sedimentary rocks indicative of deposition within a restricted marine basin or submerged continental margin, we consider it most likely that this prolonged Late Devonian to Early Mississippian magmatic episode resulted from attenuation of the ancient continental margin of western North America, rather than development of an arc, as proposed by many others.

Sediment geochemistry of the Yamuna River System in the Himalaya: Implications to weathering and transport

Bed sediments of the Yamuna River and its tributaries in the Himalaya (Yamuna River System, YRS) have been analyzed for major elements and trace metals (Sr, Ba, Ni, Cu, Co, Zn, Pb and Cr). These results have been used to characterize chemical weathering and transport in the Himalaya, to assess relative mobility of elements during weathering and to understand heavy metal association. Concentrations of major and trace elements of YRS sediments vary between 20 and 50%. In general, elemental variability reduces when data are analyzed individually for the major rivers, suggesting that tributaries draining diverse lithology contribute significant variations. Comparison of sediment chemistry with composition of source rocks and average Upper Continental Crust (UCC) suggests significant loss of Na, K, Ca and Mg from source rocks during weathering, the degree of loss being more for Ca and Na. Chemical index of alteration (CIA) for YRS sediments averages at 59, indicating that weathering in the basin is of moderate intensity. This inference is also supported by major ion chemistry of YRS waters and is attributed to steep gradient and enhanced physical erosion in the basin. Available results seem to indicate that Na and Sr are effectively more mobile than Ba, which is thought to be a combined effect of higher solubility of Na and Sr, and the affinity of Ba to be adsorbed onto solid phase. Heavy metals show significant positive correlation with Al and weak correlation with Fe, Mn and P. These observations suggest that metal concentrations are controlled mainly by clay mineral abundances, and that Fe-Mn oxides and organic matter may be playing less significant role. Heavy metal concentrations of YRS sediments are lower than those of suspended particulates of the Yamuna river, presumably due to higher clay mineral abundances in the latter. Strong association of metals with Al, and lower metal concentrations in bed sediments compared to suspended matter underscores the importance of sediment transport and mineral sorting in influencing the YRS sediment chemistry. Enrichment factor and geo-accumulation index calculated for heavy metals in YRS sediments suggest that they are mainly of natural origin and that anthropogenic activities exert little influence on their abundances.

Whole-rock geochemical variations and evolution of the arc-derived Murihiku Terrane, New Zealand

Arc-flank volcaniclastic sedimentation in the Murihiku Terrane of New Zealand lasted about 120 million years from Late Permian to Early Cretaceous time. Despite the effects of pervasive zeolite-facies alteration, whole-rock geochemical parameters for sandstones, siltstones and tuffs record changes in source-rock composition, both in time and along the length of the depositional basin. Sandstones are considered to give a more reliable indication of the state of evolution of the source volcanic arc than do the siltstones. The siltstones commonly contain detrital white mica flakes that are generally lacking in the sandstones, and are possibly of distal continental origin. Some also contain very fine felsic ash particles. Average abundances and normalized multi-element diagrams are used to estimate proportions of three model end-member source constituents, average upper-continental crust (UCC), high-K rhyolite (RHY) and basaltic andesite (AND). Sandstone provenance for the Southland Syncline sector changed from a predominantly basaltic-andesite source in Late Permian to early Middle Triassic time, for example, UCC:RHY:AND = 0:17:83 in the Early to early Middle Triassic, to highly felsic in the Middle to Late Triassic, reaching UCC:RHY:AND = 2:74:24 in the Late Triassic Oretian Stage. A UCC component became increasing significant from latest Triassic upward and the proportion of mafic to felsic volcanism increased again, with UCC:RHY:AND = 15:30:35 in the Middle Jurassic Temaikan Stage. Mix modelling suggests that along-arc source proportions varied, with greater mafic and upper continental crust contributions in the northern Kawhia segment than in the Southland segment. These patterns may be explained by deposition at an oceanic Aleutian-type arc margin, with transition to a continental oceanic arc character induced either by arc evolution and dissection, forearc sliver translation, or underplating of rafted microcontinental fragments.

Geology and geochemistry of Jurassic pelagic sediments, Scisti silicei Formation, southern Apennines, Italy

Abstract A better understanding of genesis and palaeoenvironmental setting of the Scisti silicei Formation (Lagonegro units, southern Italy) was achieved by means of geochemical analysis integrated with new stratigraphic information. Data show that major and trace element geochemistry of ancient clay-rich beds and banded cherts add new insights into the Mesozoic evolution of the Lagonegro basin. Sedimentary contributions to Jurassic shales sampled during this study were mainly derived from two major sources: (i) a dominant terrigenous fine-grained component, having affinity with average upper continental crust that had not undergone intense weathering and (ii) biogenic siliceous material. The latter component occurs in clay-rich layers from the “basal member” of the Scisti silicei Formation. Composition varies up section and accounts for changes in the detrital supply due to bathymetric oscillations. The compositional variations from the basal to the overlying member are consistent with a distal source passing in time to a more “proximal” source, as indicated by sharp changes in the concentrations of detrital elements (Ti, Zr and Nb). It is likely that increased detrital input occurred through turbidity current deposition. Finally, the chemical features of the clay-rich layers from the upper cherty portion of the studied succession imply a progressive deepening of the basin. The lack of any mafic and hydrothermal contributions in the Jurassic shales as well as the continental nature of detrital input suggests that the Lagonegro basin was located between two carbonate platforms, in accordance with the classical restoration of the African–Apulian palaeomargin. Thus, the basin acted as a preferential sink connected to the African cratonic areas through a southern entry point.

Major and trace element compositions of georgiaites: Clues to the source of North American tektites

Abstract— Electron microprobe and laser ablation, inductively‐coupled plasma mass spectrometer analyses of 24 georgiaites show that these tektites are all Si‐rich (79–83 wt% SiO2) glasses with variable major and trace element abundances (e.g., FeO varies from 2.1 to 3.7 wt%). Glass compositions are similar to but not identical with average upper continental crust. For example, georgiaites are light rare earth element enriched with small negative Eu anomalies (Eu/Eu*=0.73‐0.86) and La‐Th‐Sc systematics are intermediate between that of Archean and post‐Archean continental crust. When the georgiaite data are placed in the context of data for all North American tektites, triangular arrays appear on some oxide‐oxide plots (e.g., FeO‐MgO). Large variations in refractory element abundances and ratios compared to the variation in SiO2 favors mixing over volatilization as a cause of the compositional variation. If all the tektites formed as a result of a single impact, then triangular arrays in oxide‐oxide variation diagrams require at least three source components. These components include a Si‐rich material, probably a quartz‐rich sand that was predominant in the formation of georgiaites. Two relatively silica‐poor and Fe‐rich components have compositional characteristics similar to shales and greywackes. The La‐Th‐Sc systematics of the georgiaites and most other North American tektites are distinctive and could potentially be used to link the tektites to Eocene sediments at the Chesapeake Bay impact structure.

Wind-blown origin of the Pliocene red clay formation in the central Loess Plateau, China

In order to determine the depositional processes of the Pliocene red clay formation deposited directly beneath the Plio–Pleistocene loess in the Chinese Loess Plateau, four red clay sections spanning over 400 km are studied. Grain size analysis of closely spaced samples in the sections shows that the particles of the red clay are very fine with the sand fraction (>63 μm) being negligible, and that all the curves indicating changes in different grain size parameters have a similar pattern. The grain size records in the upper part of the four sections are almost identical with the median grain size centered at 4–8 μm. The REE patterns of 28 samples from the Jiaxian red clay section are all characterized by LREE enrichments, relatively flat HREE and slight negative Eu anomaly, being similar to those of eolian loess and the average upper continental crust. These lines of evidence point to a wind-blown origin of the red clay deposits. Accumulation of the loess–soil sequences in the Chinese Loess Plateau during the past 2.6 Ma can be therefore regarded as the continuation of the Pliocene atmospheric dust deposition. Observations of spatial grain size changes in the Loess Plateau suggest that the eolian red clay might be transported mainly by the westerlies, differing significantly from the overlying loess that was transported essentially by the East-Asia winter monsoonal winds.

Loess geochemistry and its implications for particle origin and composition of the upper continental crust

Chemical and Nd Sr isotopic compositions of loess samples from Argentina, Europe and Spitsbergen were analyzed to examine the nature of source terrains, the origin of silt-size particles and the suitability of using loess as starting material for estimating the average chemical composition of the upper continental crust. From the relations between Na 2O/Al 2O 3 and K/ 2O/Al 2O 3 ratios and CIA values (chemical index of alteration), the loess protoliths must have undergone previous sedimentary differentiation and subjected to moderate chemical weathering. REE patterns are remarkably uniform with (La/Yb) N ≈ 10 , which is characteristic of the upper continental crust (UCC). Negative Eu anomalies, expressed in Eu/Eu * ratios, vary from 0.65 for European loess to 0.8 for Pampean loess from Argentina. All loess deposits have nearly constant La/Th or Th/U ratios, which are very similar to those of the average UCC or post-Archean shales. These ratios are not fractionated in size-fractions relative to the whole-rock values. Nd and Sr isotopic compositions clearly distinguish Argentinean loess ( 87Sr 86Sr = 0.706–0.709, ε Nd(0) = −6to−1.5 ) from all other loess deposits ( 87Sr 86Sr = 0.712–0.730, ε Nd(0) = −13to−8 ). The REE and isotopic results clearly indicate a significant contribution of young Andean volcanics to the Pampean loess deposits, whereas multi-recycled and well-mixed ancient sediments are principal sources for the other deposits. The present results reinforce the earlier conclusion reached by S.R. Taylor, S.M. McLennan and M.T. McCulloch [Geochemistry of loess, continental crustal composition and crustal model ages, Geochim. Cosmochim. Acta 47 (1983) 1897–1905] that the average chemical composition of UCC can be obtained from eolian deposits as well as from fine-grained clastic sediments.

Geochemical heterogeneities of metagraywackes from the Sandur schist belt: implications for active plate margin processes

Metagraywackes found at four stratigraphic levels in different blocks of the Sandur schist belt represent unmixed sediments, exhibiting great diversity in petrography, mineralogy, major, trace and rare earth element (REE) geochemistry. Most of these rocks have a low (Q + F) C ratio and compared to AUCC (Average Upper Continental Crust) are depleted in incompatible elements like Rb, Ba, Th, U, Nb, Ta, K, Na, Ca, Ce and Sr and enriched in compatible elements such as Mg, Cr, Ni and Sc. They are enriched in Fe 2O 3 (upto 25%) and their Mg K ratio varies from <1 to 1000. La Sc , Th Sc , Th Yb and Co Th ratios, along with several diagnostic geochemical twins such as UTh, HfZr and NbTa, indicate that upper continental crust has contributed marginally to the composition of most of these graywackes. In each block, except one, two types of REE patterns are found. Type I patterns have moderate ∑REE, La N Yb N ≤ 3 and no significant Eu anomalies. On the other hand, type II patterns show LREE enrichment resulting in significant increase in ∑REE and La N Yb N between 5 and 15. Type II patterns are common in the Deogiri block. Type I patterns are similar to those found in modern deep sea forearc basins. Significant negative Eu anomalies are found in a few cases of type II graywackes where recycling of sediments such as micaceous quartzite is evident. La:Th:Sc and Co Th , Th Sc and La Sc ratios show that most of these graywackes were supplied from a source that was mainly basic volcanic rocks, banded iron formations and cherts. However, significant compositional variations are found from block to block and within the same block. These are attributed to the proportional differences in their local sources. The overall composition indicates that the graywackes of different blocks have been deposited in various environments of oceanic subduction complex and have not sampled much of the upper continental crust. The compositional, structural and metamorphic discontinuities observed in the belt are best explained by a small plate tectonic model, involving accretion of diverse blocks to a pericontinental passive margin succession. It is argued that even in the Archaean, volcanosedimentary processes at young undifferentiated arcs involving ocean (floor) ridge basalts (AORB) have generated turbidites which in many geochemical features resemble those found in modern forearc basins.

Weathering of granodioritic crust, long-term storage of elements in weathering profiles, and petrogenesis of siliciclastic sediments

The bulk composition and mineralogy of the Toorongo Granodiorite, Australia, are similar to average upper continental crust (AUCC). Weathering characteristics of the Toorongo profile consequently provide insight into large-scale chemical weathering of the upper crust. In situ weathered materials of the profile do not reflect parent granodiorite composition in quartz-plagioclase-K-feldspar (Q-P-K) or in quartz-feldspar-rock fragment (Q-F-L) compositional space. Intensive in situ weathering precludes sands, derived from mature weathering profiles through erosion, from reflecting their provenance. Where intensive chemical weathering has occurred, clay minerals and oxyhydroxides of the profile, and by inference muds derived therefrom, contain much more chemical information about provenance than do associated sands. Actinides, rare earth elements (REEs), many transition metals, and metalloids have accumulated in deep parts of the weathering profile at concentrations much greater than observed in the fresh granodiorite. Mass balance considerations require the bulk of these elements to have been derived from previously weathered, and now eroded, granodiorite. These elements were, and are, continually cycled from the intensely weathered uppermost soil zone to deeper, less weathered, zones of the profile where they accumulate. The profile therefore represents a large, continental elemental storage reservoir, the storage capacity of which has increased over time. Wherever erosion is sufficiently slow and chemical weathering sufficiently rapid, mature weathering profiles may become large, long-term storage reservoirs for actinides, REEs, and many other elements. The total REE contents of extremely weathered soil material are somewhat less than in the parent granodiorite, but they are enriched twofold to threefold in the zone of intermediate weathering relative to parent. Similar variations in total REEs are observed in some muds when normalized to their source (AUCC). These differences are attributed to a combination of chemical weathering and selective mass wasting of profiles. Homogenization of detritus in large sedimentary basins, however, produces muds with REE patterns and total REE contents similar to source (AUCC). Nd/Sm ratios are not influenced by chemical weathering, although both elements are mobilized by weathering and become enriched by over 200% relative to parent rock. Constancy of Nd/Sm in the profile indicates that Nd-Sm model ages derived from soils and sediments are not affected by chemical weathering. The least mobile trace elements of the profile are Sc, Cu, Nb, and Ta, but others are more mobile. Thorium, for example, is mobilized during weathering of the Toorongo Granodiorite and displays a twofold increase in the profile, as does the Th/Sc ratio. The ratio, however, varies by more than a hundredfold in major rock types so that Th/Sc (and other ratios) provides valuable information about provenance, although sensitivity is diminished somewhat by the effects of chemical weathering.

The relationships between rock-derived lead and iron in natural waters

The relationships between Fe and Pb released to the hydrocycle via rock weathering is examined in terms of field observations and laboratory experiments. Rock, soil, and unpolluted water samples from granodiorite, basalt, and carbonate terrains were analyzed for their Pb, Fe, and other element (Ca, Mg, Na, K, Si, Al, Mn) concentrations. Samples of granodiorite, soil developed on the granodiorite, basalt, and dolomite rocks, as well as biotite and feldspar minerals, were also used for laboratory leaching experiments. Fe/Pb ratios found in granodiorite, basalt, and insoluble residue of carbonate bedrocks are preserved within an order of magnitude in surface and shallow groundwater reservoirs draining these rocks. With the exception of carbonate rocks, leachates of these rocks have Fe/Pb ratios similar to that of the bedrock. Laboratory experiments suggest that Pb is adsorbed and coprecipitates with Fe-rich particles even in the presence of higher concentrations of competing ions (e.g., Mg 2+). Complexation of Pb by ethylenediaminetetraacetate (EDTA), however, substantially decreases the amount of Pb bound to surface sites (ligand competition). Based on our results we suggest that in early phases of the weathering cycle, Pb both coprecipitates and is adsorbed on surfaces of Fe-rich particles. Because of complementary transport mechanisms and comparable rates of release from common source rock and soil minerals, the molar ratio of Fe to natural (rock-derived) Pb in the particulate load of rivers (in many cases it includes particles which pass through 0.45 μm pore size filters) is similar to that of the upper continental crustal rock, approximately 6500. This ratio can vary in watersheds which drain rocks with different Fe Pb ratios than the average upper continental crust, in rivers loaded with dissolved natural organic matter, or in rivers polluted with Pb or with anthropogenic chelators (secondary pollution effect).

Parental basalts in subduction zones: Implications for continental evolution

Basaltic liquid is parental to new crust in subduction zones. It occurs as inclusions of glass in crystals of olivine in andesites and dacites as well as basalts. Volcanoes which have never erupted a basaltic rock nevertheless have basaltic glasses included in extruded crystals. The basaltic liquids are rich in A12O3 and Cl and have higher Cl/K2O than associated andesitic liquids. Because decrease in Cl/K20 with increase in K2O and SiO2 implies vapor saturation, most andesitic liquids develop in vapor‐saturated environments. Estimated concentrations of H2O and CO2 in the liquids indicate that vapor saturation is probably restricted to the crust. Consequently, most andesite is a derivative magma generated from basaltic magma within the crust. New crust which is formed in subduction zones is basaltic in composition, not andesitic like the average upper continental crust. Consequently, continental crust is either basaltic in bulk or becoming less silicic. Alternatively, some other processes transport silica‐poor materials from the crust to the mantle, leaving a silicic crustal residue. The chlorine in the parental basaltic liquids probably is supplied by the complete destruction of amphibole and the extraction of about 10% partial melt in subducted oceanic lithosphere or other mantle. The basaltic composition of parental liquids in subduction zones is probably established in hot mantle overlying the Benioff zone, not the Benioff zone itself.