Thermomagnetic characteristics in late orogenic granites and gneisses of the southern Appalachian Piedmont

Thermomagnetic curves have been obtained for samples from 25 granites and gneisses in Georgia, South Carolina, and North Carolina. This data set can be divided into two distinct curve types: Type I includes rocks which exhibit no distinct Curie points and a linear decrease in magnetization with increasing temperature; Type II curves exhibit a distinct Curie point over a range of temperatures expected for magnetite (500–580°C). Synthetic samples constructed from magnetite and hematite powder exhibit Type I behavior when the hematite to magnetite ratio is high (e.g. 15 : 1). Examination of polished sections shows relatively coarse-grained magnetite only in Type II rocks. We interpret our data to indicate that Type I thermomagnetic curves are dominated by relatively large hematite/magnetite ratios while Type II granites are characterized by relatively coarse-grained magnetite. Type I granite samples have low magnetic susceptibility values (less than 4 × 10^?4 cgs) while most Type II granite samples have higher values. The Type I granites are invariably¹⁸O-enriched whereas Type II granites typically exhibit low¹⁸O/¹⁶O ratios. These relationships are consistent with previously reported correlations of susceptibility (generally indicative of magnetite content) and oxygen isotopic trends in the southern Appalachian Piedmont.     

Oxygen isotopic compositions of the late orogenic granites in the Southern Piedmont of the Appalachian Mountains,U.S.A., and their relationship to subcrustal structures and lithologies

Data were acquired from 143 whole rock samples from 20 late orogenic, post-metamorphic, Hercynian-age granitic plutons from the Piedmont of the southern Appalachians, principally in Georgia and South Carolina. These plutons exhibit a regional gradient in oxygen isotopic compositions in which the granites confined to the Inner Piedmont to the northwest are¹⁸O-enriched (11.4 to 7.9) whereas those toward the southeast within the Charlotte-Slate and portions of the Kiokee belts have distinctly lower¹⁸O/¹⁶O compositions (8.2 to 5.5); one body that lies along the southeastern edge of the Piedmont in South Carolina, however, appears to be anomalously¹⁸O-enriched (8.9). Most plutons display¹⁸O/¹⁶O variations of <1‰ although two vary by as much as 3–4‰. The regional oxygen isotopic pattern among plutons appears to correlate directly with: (1) regional Bouguer gravity patterns, in which the¹⁸O-enriched plutons occupy areas characterized by negative anomalies, whereas low-¹⁸O bodies are invariably restricted to regions of positive anomalies; (2)⁸⁷Sr/⁸⁶Sr data, where granites with δ¹⁸O values <8‰ have low initial strontium ratios of ～0.703 to 0.705, while¹⁸O-enriched plutons (>9‰) have ratios >0.710; (3) contrasting chemical and accessory mineral compositions, in which many¹⁸O-depleted granites have a number of I-type characteristics, whereas several of the most¹⁸O-enriched plutons exhibit a number of S-type features. It can be inferred from these data that the¹⁸O-enriched granites were formed from continental crustal protoliths that underlie much of the Inner Piedmont and portions of the Kiokee belt, whereas the low-¹⁸O plutons were derived from more mafic sources beneath the Charlotte-Slate and portions of the Kiokee belt. The overall correspondence between the regional¹⁸O/¹⁶O patterns exhibited by the granites and gravity data implies that these grantes may be essentially rooted to their protoliths, in turn suggesting that the large-scale translational movement recently proposed for the Southern Piedmont may have occurred prior to intrusion of these granites ～320 m.y. ago.     

Absolute18O content of standard mean ocean water

The absolute values of the¹⁸O/¹⁶O ratio (R_s) and the relative¹⁸O content (X_s) in SMOW have been determined by comparing SMOW mass spectrometrically with well-defined synthetic mixtures of pure D₂¹⁸O and H₂¹⁶O. The results are:R_S = (2005.20 ± 0.45) × 10^?6, X_S = (2000.45 ± 0.45) × 10^?6     

Using magnetic susceptibility to assess soil degradation in the Eastern Rif,Morocco

The soil in the Rif, Morocco, is at serious risk because increasing anthropogenic pressures are gradually transforming large natural areas into farmland. The distribution of magnetic minerals within the soil profile can be used to assess soil development and degradation. The soils in the study area are severely eroded because of a combination of highly erodible soils, intense rainstorms and scarce vegetation cover. To sample of representative soil profiles, lithology, slope gradient and land use were considered. The ranges of magnetic susceptibility in the soil profiles distinguished between two primary soil groups. Magnetic susceptibility varied in the soil profile and along the soil toposequence, and the variations were related to the differences in the original magnetic composition and the influence of main erosion factors. Lithology is the main factor contributing to the variation in magnetic susceptibility. The magnetic susceptibility values in soils on Tertiary marls (χ = 13·5 × 10^?8 m³ kg^?1) differed significantly from those on Quaternary terraces (χ = 122·1 × 10^?8 m³ kg^?1). Slope affected the distribution of magnetic susceptibility because of the continuous loss of topsoil in some parts of the slope and the deposition of eroded soil in others. Elimination of the natural vegetation cover and a shift to cultivated land for cereals has had a negative impact on soil development and, on similar slopes and substrates, magnetic susceptibility decreased significantly in cultivated soils. The soils on steep slopes that had natural vegetation cover retained the magnetic minerals better than did those on gentler slopes that were under cultivation. Grazing, clearing and, especially, tilling has weakened the soil and made it much more vulnerable to erosion. An analysis of the main factors causing erosion will help to promote rational use of the land and to establish conservation strategies in such fragile agroecosystems. Copyright © 2009 John Wiley & Sons, Ltd.     

Sr and O isotopic characteristics of porphyries in the Qinling molybdenum deposit belt and their implication to genetic mechanism and type

A great deal of Mesozoic hypobatholithic granites and hypabyssal porphyries develop in the Qinling Mountains. The former has long been regarded as transformation type (or S-type), and the latter associated with Mo-mineralization regarded as syntexis type (or I-type) granitoids. Statistics show that Sr₁ and σ¹⁸O of hypabyssal porphyries respectively range from 0.705 to 0.714, and from 7.2‰ to 12.1‰, agreeing with those of hypobatholithes (Sr₁=0.705–0.710, σ¹⁸O=6.1‰–10.4‰), which indicates that they share similar material sources and petrogenic mechanism. Based on analysis of lithological, mineralogical and geochemical characteristics of these granitoids and on study of their petrogenic tectonic background and regional geophysical data, we argue that both the shallow-seated porphyries and deep-seated batholithes were the products of Mesozoic collision between South China and North China paleocontinents. Subsequently, all these grantitoids should be attributed to collision type.     

Channel stability and sediment source assessment in streams draining a Piedmont watershed in Georgia,USA

Streams can be classified as stable or unstable, depending on the stage of channel evolution. Many streams of the southern Piedmont in United States have high sediment loads and are listed as impaired under the total maximum daily load (TMDL) program and may be unstable. It is not clear as to what the target (reference) load or remediation measures should be for unstable streams. The objective of this study was to determine the relative channel stability for a typical southern Piedmont stream using rapid geomorphic assessments (RGAs) and sediment yield analysis. The results were supported through a sediment fingerprinting analysis. RGAs were performed along 52 reaches on the North Fork Broad River (NFBR) main stem and two tributaries. Annual sediment yields were calculated and compared with yields in the southern Piedmont for stable streams that are resilient to degradation or aggradation and unstable streams that are susceptible to such disturbances. Majority of the NFBR main stem was found to be unstable with signs of geomorphic instability in the form of degradation and aggradation. The estimated average annual sediment yield was 0·78 T ha^?1 year^?1. By comparison, the median annual yield is 0·20 T ha^?1 year^?1 for stable streams and 0·48 T ha^?1 year^?1 for unstable streams in the Piedmont ecoregion with comparable drainage basin size. We conclude that the NFBR is in an unstable stage of channel evolution. Sediment fingerprinting proved that majority of the stream‐suspended sediment emanated from eroding stream channels. The methods outlined in this study have implications for the reference condition and remediation efforts related to stream turbidity and stream channel restoration. Copyright © 2010 John Wiley & Sons, Ltd.     

Contrasting18O enrichment and origins of High Himalayan and Transhimalayan intrusives

Twelve analysed leucogranites of the High Himalaya in Bhutan (Chung La, Mönlakarchung) and Garhwal (Badrinath) are among the most¹⁸O-enriched granites known (11.5–12.4‰ δ¹⁸O_SMOW with two exceptions) and separate minerals show good isotopic concordance. The data strongly support an origin of the granites by anatexis of continental basement such as the Indian crystalline basement sheet or slab, undercut by the Main Central Thrust, of which five samples were analysed. In contrast, the pre-collision Transhimalayan (Gangdese) batholiths to the north of the Indus-Tsangpo suture, as exemplified by the Ladakh intrusives, show an initially oceanic trend of δ¹⁸O vs. SiO₂ that becomes gradually somewhat enriched with respect to Hachijo-Jima. While not completely outside the range of enrichment that seems possible by fractional crystallisation, this could tie in with the⁸⁷Sr inhomogeneities reported by Honegger et al. [9], which may be due to assimilation of variably radiogenic Eurasian continental basement. For both the leucogranites and Ladakh intrusives the¹⁸O levels and the concordance between minerals rule out significant cumulative water/rock ratios in syn- or post-magmatic interaction with subsurface waters.     

87Rb–87Sr history of the Norton County enstatite achondrite

⁸⁷Rb⁸⁷Sr analysis of the Norton County achondrite has been achieved with special attention to the rubidium analysis. Enstatite crystals and polycrystalline material give an “age” of 4.48 ± 0.04 × 10⁹ years and an initial ratio ⁸⁷Sr/⁸⁶Sr_I= 0.7005 ± 0.0004 (λ = 1.39 × 10^?11yr^?1, maximum errors). The feldspar component of the meteorite contains about 70% of the strontium and 30% of the rubidium of the whole sample, and does not lie on the isochron. Its model age relative to the strontium initial ratio of Allende is 4.6 × 10⁹ years. The data are consistent with a complex history dealing with an incomplete isotopic reequilibration of the meteorite, 120 m.y. after its formation at 4.6 × 10⁹ years, with an initial ratio similar to that of Allende.     

Petrogenesis of S-type Ladakh granite and mafic microgranular enclaves in the southern margin of Ladakh batholith: An evidence of crust–mantle interaction during the collision between Indian and Eurasian plates

The previous studies revealed the I-type Ladakh magmatism in the Andean-type southern margin of the Ladakh batholith (LB) was related to the subduction of the Neotethyan Ocean and India-Eurasia collision. However, LB's S-type granitic magmatism and associated mafic microgranular enclaves (MMEs) are poorly constrained. Here, we present the new data for S-type Ladakh granite (LG) and associated monzodiorite MMEs in the Andean-type orogeny in the southern margin of the Eurasian plate. The low SiO₂ (47.4–53.9 wt%), high K₂O (1.56–3.21 wt%), Mg^# (52–65), continental-arc tracer patterns, and slightly depleted to evolved Sr-Nd isotopic composition ((⁸⁷Sr/⁸⁶Sr)i = 0.7047–0.7166; ℇ_Nd (t = 50 Ma) = (+1.40 to −8.92)) for MME suggest that they were derived from the phlogopite-bearing deep lithospheric mantle-source at a depth of 5.4–10.5 km depth with 810–870°C, 1.4–2.8 kbar, and enriched by sediment-melts addition into the mantle-wedge from subducting Neotethyan Oceanic slab. The mantle-derived ascending hot mafic magma mixing with felsic magma of the ancient northern Indian margin-derived, generates monzodiorite MME by assimilation and magma mixing processes. Plagioclase, amphibole, and biotite chemistry support the magma mixing processes. LG are characterized by high SiO₂ (63.4–75.0 wt%), K₂O (3.93–5.67 wt%), CaO/Na₂O ratio of >0.3, differentiation index (90.27–97.46), normative corundum (1.0–2.8), A/CNK values (1.00–1.18), hypersthene (0.7–5.7), and low Al₂O₃, MgO, TiO₂, Fe₂O₃. They also exhibit peraluminous, variable tracer elemental abundances, variable (⁸⁷Sr/⁸⁶Sr)i ratios (0.6967–0.7191), and high whole rock ℇ_Nd (t = 50 Ma) values of −4.15 to −11.92) and ancient two-stage Nd model age of 1160 and 1858 Ma. These features suggest that S-type Ladakh granites were derived from the melting of ancient metagreywacke-dominated metasedimentary rocks of the northern Indian margin by a large amount of mafic magma underplating after subducted Neotethyan slab-rollback. The formation of LG and MMEs related to the Andean-type orogeny in the southern margin of the Eurasian plate.     

Tracking particle-associated processes in nearshore environments by use of 234Th/238U disequilibrium

Measurement of excess ²³⁴Th (t_1/2 = 24.1 days) in surface sediment from 12 stations throughout Long Island Sound, U.S.A., demonstrates: (1) a mean (summer) sediment inventory of 3.6 dpm/cm² consistent with complete, nearly instantaneous removal of ²³⁴Th from the overlying water and capture within the estuary, and (2) preferential association of excess ²³⁴Th with small particles and inventory build-ups in muddy bottom areas. There may also be a tendency for higher inventories in areas of high physical or biogenic reworking of surface sediments. A range of particle reworking rates (0–5 cm) from <0.01 × 10^?6 to 1.6 × 10^?6 cm²/s is found in the Sound with most values ～0.2?0.5 × 10^?6 cm²/s. The inventory and reworking patterns demonstrate the high mobility, both horizontal and vertical, of particles in the estuary on ²³⁴Th decay time scales and are unequivocal evidence for control of reactive element distribution in the water column by the muddy regions of the basin.     

18O/16O ratios in cherts associated with the saline lake deposits of East Africa

The cherts formed from sodium silicate precursors in East African saline, alkaline lakes have δ¹⁸O values ranging from 31.1 to 44.1. The δ¹⁸O values correlate in general with lake salinities as inferred from geologic evidence, indicating that most chert was formed from its precursor in contact with lake water trapped at the time of deposition. A few of the analyzed cherts probably formed in contact with dilute meteoric water. From the widely varying δ¹⁸O values we conclude that precursors were transformed to chert in fluids of widely varying salinity and a_Na⁺/a_H⁺ ratio.     

87Rb/87Sr study of diogenites

The five diogenites, Johnstown, Roda, Ellemeet, Shalka and Tatahouine, give scattered data in the⁸⁷Rb/⁸⁶Sr,⁸⁷Sr/⁸⁶Sr diagram. This can result from a disturbance which occurred later than 4.45 Ga ago. However, it is shown that if samples of sufficient size were analyzed, there meteorites could plot on the eucrite isochron and are thereby in agreement with a genetic relation between eucrites, howardites and diogenites. The age of eucrite differentiation from diogenites has been computed using data from the two families yielding an age of 4.47±0.1Ga(2σ) (λ=1.42×10^?11a^?1), the initial⁸⁷Sr/⁸⁶Sr ratio being BABI.     

Petrogenesis and tectonic implications of Late-Triassic high εNd(t)-εHf(t) granites in the Ailaoshan tectonic zone(SW China)

High εNd(t)-εHf(t) granites are robust evidence for crustal growth. In this paper we report results of petrologic, geochronological and geochemical investigations on the Huashiban granites from the Ailaoshan tectonic zone in western Yunnan(SW China). Zircon grains separated from the two samples(10HH-119 A and 10HH-120A) yield the weighted mean 206Pb/238 U ages of 229.9 ± 2.0 Ma and 229.3 ± 2.3 Ma, respectively, interpreted as the crystallization ages of the granites. Based on our results, in combination with the existing U-Pb geochronological data for the Ailaoshan metamorphic rocks, we propose that the Ailaoshan Group might be a rock complex composed of the Mesoproterozoic, Neoproterozoic, Hercynian, Indosinian and Himalayan components, rather than a part of the crystalline basement of the Yangtze block. The zircon grains show highly depleted Lu-Hf isotope compositions, with positive εHf(t) values ranging from 8.4 to 13.1. The Huashiban granites have high SiO2(72.66 wt%–73.70 wt%), low Mg#(0.28–0.34) with A/CNK=1.01–1.05, and can be classified as peralumious high-K calc-alkaline I-type granites. A synthesis of these data indicates that the Ailaoshan tectonic zone had evolved into a post-collisional setting by the Late-Triassic(229 Ma). Genesis of the Huashiban high εNd(t)-εHf(t) granites involved into two processes:(1) underplating of the sub-arc mantle into the lower crust, and(2) remelting of the juvenile crustal materials in response to the upwelling of the asthenospheric mantle in the post-collisional setting.     

The role of bromide and iodide ions in the salinization mapping of the aquifer of Glafkos River basin (northwest Achaia,Greece)

Seawater intrusion causes many problems for groundwater quality, whereas natural remediation is time consuming. However, in cases where groundwater replenishment is feasible, groundwater quality remediation is possible and rapid. The alluvial aquifer in the lowland of the Glafkos River basin, which extends south of Patras city, was for over 30 years the major water source supplying the broader area. Groundwater quality has been degraded due to seawater intrusion, caused by overpumping and generally by inappropriate groundwater management. During the last decade, groundwater quality has been remedied due to diminished groundwater abstractions. The remediation rate was further higher because of rapid discharge of the brackish groundwater, through wells with freely flowing water in the coastal area, where, however, groundwater quality remains low. This paper deals with the hydrogeochemical processes that take place in the area. It is ascertained that ion exchange and mineral dilution processes are dominant. The ion relations between chloride, bromide and iodide, as well as the distribution maps of their concentrations, were used to determine the spatial distribution of the seawater intrusion front. In the lower part of the area in a distance from 1000 and 1500 m from the coast, the rBr^?/rCl^? ratio showed low values (<2·5 × 10^?3) similar to those of seawater. The rI^?/rCl^? ratio also presented low values (<7 × 10^?5), with the lowest one (2·7 × 10^?5) detected along the coastline. In the upper part of the area, a gradual change of those ratios was observed upstream, until they receive values similar to those of the surface waters of Glafkos River. Copyright © 2007 John Wiley & Sons, Ltd.     

Bashikaogong-Shimierbulake granitic complex, north Altun, NW China:Geochemistry and zircon SHRIMP ages

The Bashikaogong-Shimierbulake granitoid complex is about 30 km long and 2―6 km wide, with an area of 140 km2, located at the north margin of the Bashikaogong Basin in the north Altun terrain. It intruded into schist, metapelite and metatuff of Precambrian ages. This granitoid complex consists of darkish quartz diorite, grey granite, pink granite and pegmatite. Geochemically, the quartz diorite has I-type granite affinity and belongs to Calc-alkaline sereies, and the other gran- ites have S-type affinity and to high-K calc-alkaline series. Zircon SHRIMP U-Pb dating shows that the quartz diorite has a bigger age than those of other granites, which is 481.6±5.6 Ma for quartz diorite, 437.0±3.0 Ma―433.1±3.4 Ma for grey granite and 443±11 Ma―434.6±1.6 Ma for pink granite, re- spectively. Combined with regional geology, we think that the quartz diorite formed in tectonic envi- ronment related to oceanic crust subduction and the granites in post-collision.     

36Cl and53Mn in Antarctic meteorites and10Be-36Cl dating of Antarctic ice

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) has been measured in twenty Antarctic meteorites by neutron activation analysis.³⁶Cl (t_1/2 = 3.0 × 10⁵years) has been measured in fourteen of these objects by tandem accelerator mass spectrometry. Cosmic ray exposure ages and terrestrial ages of the meteorites are calculated from these results and from rare gases.¹⁴C (t_1/2 = 5740years) and²⁶Al (t_1/2 = 7.2 × 10⁵years) data. The terrestrial ages range from 3 × 10⁴ to 5 × 10⁵ years. Many of the L3 Allan Hills chrondrites seem to be a single fall based on these results. In addition,¹⁰Be (t_1/2 = 1.6 × 10⁶years) and³⁶Cl have been measured in six Antarctic ice samples. The first measurements of¹⁰Be/³⁶Cl ratios in the ice core samples demonstrate a new dating method for ice.     

18O/16O ratios in lunar fines and rocks

¹⁸O/¹⁶O ratios have been measured for Luna 20 and Apollo 15 fines and Apollo 15 rocks.Isotopic composition and fractionation between minerals are compared with previous results.Partial fluorination experiments on Luna 20 soil and Apollo 15021 extreme fines show large¹⁸O enrichments in grain surfaces. These results are discussed.     

Iron ions Fe<Superscript>2+</Superscript> and Fe<Superscript>3+</Superscript> in a magnetic model of the sedimentary medium

The concentration of rock-forming elements, the static magnetic susceptibility κ, spectra of electron paramagnetic resonance, and their relative intensities I are studied in samples from a borehole drilled in Cenozoic sedimentary deposits of southern Western Siberia. All measured values experience appreciable irregular variations with depth. A linear dependence exists between κ and I within the range of their medium and large values; κ and I have maximum values in the same sample, and κ_max = 1920 × 10^?6SI, κ_min = 210 × 10^?6 SI, and κ_av = 630 × 10^?6 SI. The magnetic properties of the samples are controlled by Fe²⁺ ions present in clastic material and by microphases (clusters) with Fe³⁺ ions of the goethite and lepidocrocite type present in the cement. The theoretically possible magnetic susceptibility of the Fe²⁺ ion system (provided that all iron exists in this form) is quite comparable with κ_min but, even with very high concentrations of Fe²⁺, does not reach half of κ_av: (154 < κ(Fe²⁺) < 254) × 10^?6 SI. Anomalously high values of κ are due to a large number of clusters with Fe³⁺ ions if structural units FeOOH do not dissociate and the interaction of the clusters with hydroxides of aluminum and precipitation medium impedes the process of their coagulation. Otherwise, the cluster sizes gradually increase, an antiferromagnetic structure develops in clusters, and the magnetic susceptibility decreases.     

Cosmic-ray-produced36Cl and53Mn in Allan Hills-77 meteorites

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) has been determined by neutron activation in nine Allan Hills-77 meteorites. Additionally,³⁶Cl (_1/2 = 3.0 × 10⁵years) has been measured in seven of these objects using tandem accelerator mass spectrometry. These results, along with¹⁴C (t_1/2 = 5740years) and²⁶Al (7.2 × 10⁵ years) concentrations determined elsewhere, yield terrestrial ages ranging from 0.1 × 10⁵ to 7 × 10⁵ years. Weathering was not found to result in⁵³Mn loss.     

Extra-terrestrial 53Mn in Antarctic ice

The reasons why⁵³Mn (a cosmogenic radionuclide with a half-life of 3.7 × 10⁶ y) appears as one of the best indicators of the presence of interplanetary dust are summarized. This paper reports the detection of⁵³Mn in pre-1952 snow samples collected on the Eastern Antarctic Plateau in the vicinity of Plateau Station. The measurements were carried out by neutron activation and X-ray spectrometry on three samples weighing a few hundred kg and covering each the time interval 1935–1950. The specific activity of⁵³Mn was found to be (0.82 ± 0.17) disint.min^?1/10³ tons of snow, corresponding to a deposition rate at Plateau Station of (2.2 ± 0.5) × 10^?5 disint. min^?1 m^?2 y^?1. The mean global deposition rate would be three times higher if⁵³Mn were assumed to behave in the same way as stratospheric⁹⁰Sr. By comparing this figure with existing data on the meteorite flux reaching the earth and with the galactic and solar production rates of⁵³Mn, it is concluded that the bulk of the⁵³Mn found at Plateau Station is associated with interplanetary dust in which it had been produced by the action of solar protons on iron. The deposition rate of extra-terrestrial dust-borne iron must be between 1.3 × 10^?5 and 1.3 × 10^?4 g m^?2 y^?1 at Plateau Station. These results support jointly with other studies the concept of an interplanetary zodiacal cloud of dust with a chemical composition and density not essentially different from chondritic meteorites, with a relatively ‘flat’ grain size distribution and a mass influx to the earth of the order of 10⁵ tons/y.