The trace element geochemistry of Corsican ophiolites

Ultramafic rocks, gabbros, plagiogranites, dolerites and basaltic dikes and pillows lavas of the ophiolite slices of eastern Corsica have been metamorphosed in both oceanic and orogenic environments. The trace element content of the metabasaltic rocks which exhibit a tholeiitic differentiation trend towards a ferrobasaltic composition, is similar to that of oceanic basalts. The cumulate sequence is interpreted to have been formed by fractional crystallization from an olivine-tholeiite magma. The troctolites representing cumulates derived from melts representing liquid fractions in the range F=1-0.85, gabbros from melts with F=0.85-0.45, and ferrogabbros and plagiogranites from melts with F<0.45. An oceanic environment with moderate spreading rates and magmatic processes similar to present-day normal oceanic ridge segments is considered the more probable original setting of the Corsican ophiolites.Financial support by C.N.R. (Italy) and A.T.P. Géodynamique de la Méditerranée Occidentale C.N.R.S. (France)     

Petrology and trace element geochemistry of the Papuan Ultramafic Belt

New petrologic and geochemical data are presented for a suite of rocks from the Papuan Ultramafic Belt (PUB), Papua New Guinea. Tectonite harzburgites at the base of the ophiolite have extremely refractory, uniform mineralogy, and are exceptionally depleted in lithophile elements. These features are consistent with the proposed origin of these rocks as depleted upper mantle, residual after extraction of a basaltic melt. The tectonite peridotites are overlain by a thick sequence of layered ultramafic and mafic cumulates containing olivine, orthopyroxene, clinopyroxene and plagioclase as the major cumulus phases. Early cumulates are characterized by magnesian olivine Mg₉₀, orthopyroxene Mg₉₀ and calcic plagioclase An₈₆, and exhibit cryptic variation towards more iron-rich and sodic compositions. Abundances of incompatible elements in the cumulates are extremely low which, together with the nature of the cumulus phases, points to a magnesian olivine-poor tholeiite or magnesian quartz tholeiite parent magma(s) strongly depleted in incompatible elements. Highly fractionated iron-rich products of this parent magma type are represented by the LREE-depleted lavas in the overlying basalt sequence which, although resembling the most depleted mid-ocean ridge basalts (MORB) in terms of their low abundances of incompatible elements, have higher abundances of transition metals and lower abundances of Ti, HREE and other high valence cations compared to common MORB of similar Mg/(Mg+Fe) ratio.Eocene tonalites intruding the PUB are genetically unrelated to the ophiolites, and appear to be related to the Ti-poor high-Mg andesites of Cape Vogel and similar andesites and dacites at the northern end of the PUB. These rocks are considered to represent the early stages of island-arc magmatism associated with a northeastward-dipping subduction zone in the early Eocene immediately prior to emplacement of the PUB.     

Isotope and trace element geochemistry of Colorado Plateau volcanics

Basalts from the San Francisco Peaks and North Rim of Grand Canyon, nephelinites from the Hopi Buttes and Navajo minettes (Colorado Plateau) have been analyzed for trace element contents and Sr, Nd, Pb isotope compositions. The ages increase eastward from the Quaternary (basalt) to 5 Ma (nephelinite) and 30 Ma (minette) as does the depth of melt generation inferred from xenolith mineralogy and major element geochemistry.

The three rock types present an enrichment of incompatible elements (although minettes present negative concentration spikes for Nb, Zr, Ti, Ba, Sr) relative to other magma types. The chondrite-normalized Ce/Yb ratio changes from 8–22 (basalt) to 25–30 (nephelinite) and 33–60 (minette) and reflects small degrees of partial melting of a mantle source with a garnet/clinopyroxene ratio increasing with depth. The negative Eu anomaly present in minette, the low Sr/Nd and high Pb/Ce suggest the presence of a recycled continental crust component in their mantle source.

The ⁸⁷Sr/⁸⁶Sr ratio varies from 0.7032-0.7045 (basalt and nephelinite) to 0.7052-0.7071 (minette), while ε_Nd is remarkably more constant at +0.8 to +3.7 (nephelinite) and −2.6 to +2.2 (basalt and minette). Good linear correlations are observed in both ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagrams with basalt being the least and nephelinite the most radiogenic and indicate a 2.3 ±0.1 Ga age and a Th/U of 3.4.

Three lithospheric source components are indicated: a) an OIB-type depleted mantle source, b) an end-member with unradiogenic Sr, Nd and Pb for basalt and nephelinite and c) a recycled crustal component for minette.     

Major and trace element geochemistry in Zeekoevlei, South Africa: A lacustrine record of present and past processes

This study reports a multi-parameter geochemical investigation in water and sediments of a shallow hyper-eutrophic urban freshwater coastal lake, Zeekoevlei, in South Africa. Zeekoevlei receives a greater fraction of dissolved major and trace elements from natural sources (e.g., chemical weathering and sea salt). Fertilizers, agricultural wastes, raw sewage effluents and road runoff in contrast, constitute the predominant anthropogenic sources, which supply As, Cd, Cu, Pb and Zn in this lake. The overall low dissolved metal load results from negligible industrial pollution, high pH and elevated metal uptake by phytoplankton. However, the surface sediments are highly polluted with Pb, Cd and Zn. Wind-induced sediment resuspension results in increased particulate and dissolved element concentrations in bottom waters. Low C/N ratio (10) indicates primarily an algal source for the sedimentary organic matter. Variation in sedimentary organic C content with depth indicates a change in primary productivity in response to historical events (e.g., seepage from wastewater treatment plant, dredging and urbanization). Primary productivity controls the enrichment of most of the metals in sediments, and elevated productivity with higher accumulation of planktonic debris (and siltation) results in increased element concentration in surface and deeper sediments. Aluminium, Fe and/or Mn oxy-hydroxides, clay minerals and calcareous sediments also play an important role in adsorbing metals in Zeekoevlei sediments.     

Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago

New lead, strontium and helium isotopic data, together with trace element concentrations, have been determined for basalts from the Cape Verde archipelago (Central Atlantic). Isotopic and chemical variations are observed at the scale of the archipelago and lead to the definition of two distinct groupings, in keeping with earlier studies. The Northern Islands (Santo Antão, São Vicente, São Nicolau and Sal) present Pb isotopic compositions below the Northern Hemisphere Reference Line (NHRL) (cf. Hart, 1984), unradiogenic Sr and relatively primitive ⁴He/³He ratios. In contrast, the Southern Islands (Fogo and Santiago) display Pb isotopes above the NHRL, moderately radiogenic Sr and MORB-like helium signatures. We propose that the dichotomy between the Northern and Southern Islands results from the presence of three isotopically distinct components in the source of the Cape Verde basalts: (1) recycled ∼1.6-Ga oceanic crust (high ²⁰⁶Pb/²⁰⁴Pb, low ⁸⁷Sr/⁸⁶Sr and high ⁴He/³He); (2) lower mantle material (high ³He); and (3) subcontinental lithosphere (low ²⁰⁶Pb/²⁰⁴Pb, high ⁸⁷Sr/⁸⁶Sr and moderately radiogenic ⁴He/³He ratios). The signature of the Northern Islands reflects mixing between recycled oceanic crust and lower mantle, to which small proportions of entrained depleted material from the local upper mantle are added. Basalts from the Southern Islands, however, require the addition of an enriched component thought to be subcontinental lithospheric material instead of depleted mantle. The subcontinental lithosphere may stem from delamination and subsequent incorporation into the Cape Verde plume, or may be remnant from delamination just before the opening of the Central Atlantic. Basalts from São Nicolau reflect the interaction with an additional component, which is identified as oceanic crustal material.     

Selenium and other trace element in phosphorites: A comparison between those of the Bayovar-Sechura and other provenances

Data of trace element composition of phosphorites are scarce and incomplete. Phosphorites of different origins can vary substantially in trace element contents. In this paper 20 trace element concentrations of 35 sample phosphorites are reported. The geographical provenance is: Bayovar-Sechura (Peru), Khouribga, Youssoufia and Boucraa (Morocco), Gafsa (Tunisia), Florida (USA), Idaho and Phosphoria Formation (USA), North Carolina (USA), Algeria, Israel, Senegal, Syria and Togo. Aqua regia extracts were used to estimate the “pseudototal” values, following standard procedures (ISO 11466, 2002) and measured by ICP-AES and ICP-MS.     

Experimental geochemistry of Pu and Sm and the thermodynamics of trace element partitioning

The partitioning of Pu and Sm between diopside/liquid and whitlockite/liquid has been investigated experimentally to evaluate the geochemical coherence of Pu and the light REEs. ${}^{\mathrm{Pu}}\text{D}{}^{\mathrm{Sm}}\text{D}$ is 2̃ for both diopsidic pyroxene and whitlockite. This small amount of fractionation would be decreased further if Pu were compared to Ce or Nd. Our experimental results thus validate the suggestion that Pu behaves as a LREE during igneous processes in reducing environments.Our data and the data of Ray et al. (1983) indicate that temperature rather than melt composition is the most important control on elemental partitioning. This is true even though we demonstrate that additions of only 1–2 wt.% of P₂O₅ to the diopside-anorthite-albite system change ^PuD_cpx by a factor of two. Our data suggest that P₂O₅ in aluminosilicate melts serves as a complexing agent for the actinides and lanthanides.     

Comparative geochemistry of shell phosphorites and dictyonema shales of the Baltic

Major and trace element composition of the Ordovician Obolus phosphorites and associated Dictyonema shales were determined by ICP-MS and chemical and microchemical elemental analyses. Relative to the phosphorites, the Dictyonema shales are substantially enriched in a variety of trace elements, except for As, Be, Co, Y, REE, Sr, and Pb. The Obolus phosphorites show enrichment of As, Bi, Hg, Mo, La, Y, Pb, and Sr and depletion of Ag, Ba, Be, Cd, Cr, Cu, Hf, Ni, Sc, Sn, U, V, Zn, and Zr relative to the world average phosphorite composition. The average trace element composition of the Dictyonema shales is close to the mean shale composition, except for higher contents of Mo, Hg, Pb, Se, Ta, Te, Th, V, and U and lower contents of Ba, Bi, Cd, Co, Re, Sr, and Zn. The results suggest that the change from phosphate sedimentation in aerated environments to anoxic carbonaceous sedimentation was accompanied by changes in the composition and concentration of trace elements in the sediment. Both facies show similar trends of trace element distribution indicative of the stability of the composition of seawater and terrigenous sediment input.     

Major and trace element geochemistry in upper Ganga river in the Himalayas, India

Chemical weathering and resulting water compositions in the upper Ganga river in the Himalayas were studied. For the first time, temporal and spatial sampling for a 1 year period (monthly intervals) was carried out and analyzed for dissolved major elements, trace elements, Rare Earth Elements (REE), and strontium isotopic compositions. Amounts of physical and chemical loads show large seasonal variations and the overall physical load dominates over chemical load by a factor of more than three. The dominant physical weathering is also reflected in high quartz and illite/mica contents in suspended sediments. Large seasonal variations also occur in major elemental concentrations. The water type is categorized as HCO₃^––SO₄^2––Ca²⁺ dominant, which constitute >60% of the total water composition. On an average, only about 5–12% of HCO₃^– is derived from silicate lithology, indicating the predominance of carbonate lithology in water chemistry in the head waters of the Ganga river. More than 80% Na⁺ and K⁺ are derived from silicate lithology. The silicate lithology is responsible for the release of low Sr with extremely radiogenic Sr (⁸⁷Sr/⁸⁶ Sr>0.75) in Bhagirathi at Devprayag. However, there is evidence for other end-member lithologies for Sr other than carbonate and silicate lithology. Trace elements concentrations do not indicate any pollution, although presence of arsenic could be a cause for concern. High uranium mobilization from silicate rocks is also observed. The REE is much less compared to other major world rivers such as the Amazon, perhaps because in the present study, only samples filtered through <0.2 m were analysed. Negative Eu anomalies in suspended sediments is due to the excess carbonate rock weathering in the source area.     

The recognition of hydrothermal processes in the trace element geochemistry of uranium- and thorium-enriched tuffaceous rocks, Northwest Iran

Radiometrically anomalous tuffaceous rocks occur as conformable lenses in a volcano-sedimentary sequence of Neogene age near the village of Sarkhanlu, northwest Iran. The anomalous rocks have been extensively altered by hydrothermal fluids, and kaolinization and silicification are widespread. 250 samples of the anomalous and background tuffs and other sediments have been analyzed for Fe, Mn and 18 trace elements. Enhanced levels of Ag, As, Mo, Pb, Sn, Sr, Th, U, W and Zr indicate an alkaline volcanic origin of the tuffaceous material, and the elements of this association dominate the statistical analysis of the geochemical data. Maximum U and Th values are among the highest ever recorded in extrusive volcanic rocks. A zone of kaolinization contains increased levels of U, Th, Pb and Sr relative to the unaltered country rock, although nine other elements are depleted in the zone. This association of elements is recognized in the radiometrically anomalous lithologies, particularly in the kaolinized tuffs, and indicates the availability of U in the hydrothermal fluids causing the alteration. Although U mineralization has not yet been found at Sarkhanlu, comparisons with the similar geological environment in central Italy and elsewhere suggest possible exploration criteria for the future. Distinct sedimentary U associations have been defined in tuffs with only background radioactivity and in dark, calcareous shales.     

Isotope and Trace Element Geochemistry of Fuchuan Ophiolite Suite,Anhui,China

The Fuchuan ophiolite suite in Shexian County, Anhui Province, was formed in the Middle-Late Proterozoic. It is characterized by varying Nd [∈_Nd (T) =0.7−3.8], Sr [∈_Sr(T) = 30.7−53.9] and O(δ¹⁸O=3.2−11.0%.) and low ratios of Nd/La (<0.8), Ti/Y (<350) and Ti/V (<30). These characteristics, in combination with geological features, indicate that the ophiolite suite was formed in the axial part of the back-arc basin of the Jiangnan ancient island arc at the southeastern margin of the Yangtze Plate. The varying ∈_Nd (T) was caused by the contamination of the underlying inmature sialic crust during the formation of the ophiolite and variations in ∈_Sr(T) and δ¹⁸ O may have resulted from hydrothermal alteration by seawater during or shortly after its formation.     

Application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis

Carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium. This wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid-rock interaction. Careful consideration of the uncertainties associated with trace element partitioning, sample heterogeneity and fluid-rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis, goundwater evolution, ancient seawater chemistry and isotope stratigraphy. A principal uncertainty involved in the application of Sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of Sr between mineral and solution. This variation may be a function of precipitation rate, mineral stoichiometry, crystal growth mechanism, fluid composition and temperature. Calcite and dolomite in ancient limestones commonly have significantly lower Sr concentrations (20–70 p.p.m.) than would be expected from published trace element distribution coefficient values and Sr/Ca ratios of most modern sedimentary pore waters. This discrepancy probably reflects the uncertainties associated with determining distribution coefficient values. As techniques improve for the analytical measurement and theoretical modelling of Sr concentration and isotopic variations, the petrological analysis of carbonate samples becomes increasingly important. The presence of even small percentages of non-carbonate phases with high Rb concentrations and high ⁸⁷ Sr⁸⁶ Sr values, such as clay minerals, can have significant effects on the measured ⁸⁷ Sr/⁸⁶ Sr values of carbonate rocks, due to the decay of ⁸⁷Rb to ⁸⁷ Sr. For example, a Permian marine limestone with 50 p.p.m. Sr and 1 p.p.m. Rb will have a present-day ⁸⁷ Sr/⁸⁶ Sr value that is >2 × 10^?4 higher than its original value. This difference is an order of magnitude greater than the analytical uncertainty, and illustrates the importance of assessing the need for and accuracy of such corrections. A quantitative evaluation of the effects of water-rock interaction on Sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers. Geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water-rock interaction, including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals. Quantitative modelling may also be used to construct diagnostic fluid-rock interaction trends that are independent of distribution coefficient values, and to distinguish between mixing of mineral end-members and fluid-rock interaction.     

西藏松多榴辉岩的矿物原位微量元素地球化学研究

对松多榴辉岩中单矿物进行的LA-ICP-MS原位微区微量元素分析研究结果表明,石榴石主要富集中、重稀土元素和Y,同时具有高丰度的Sc、V、Cr和Co等元素;绿辉石中的微量元素以中稀土元素、Sr、Sc、V、Cr、Co、Ni和Ti为主,含有一定量的Zr、Hf等。石榴石、绿辉石、角闪石和绿帘石中均显示轻稀土元素亏损的特点,表明在退变质过程中没有发生明显的富轻稀土元素的外来流体交代作用,因而其微量元素矿物地球化学的某些特点不同于苏鲁地区的榴辉岩。石榴石变斑晶中某些元素(如Ti、Zr)的分带性暗示了榴辉岩在紧随峰期变质之后的折返过程中发生了降压增温过程。榴辉岩主要变质矿物中微量元素的分配显然受到矿物主量元素的分配所控制,如MgO在石榴石和绿辉石之间的分配对Ni、Co、Ti分配的控制以及CaO的分配对Sr、Y、REE分配的控制等。退变质过程中矿物的形成或分解以及物理化学条件的改变都可以引起矿物间微量元素的重新分配。由绿辉石退变质而形成的角闪石,较之原先的绿辉石,其微量元素配分曲线总体特征会发生变化,但元素总体丰度相近,某些元素特点相似,又反映了绿辉石和角闪石之间的成生联系。金红石是Ti、Nb、Ta、Zr、Hf的主要赋存矿物,而与之共生的绿帘石所表现出来的高场强元素的亏损特征表明了金红石的存在所带来的影响。     

赣中古塘地区锡石LA-ICP-MS U-Pb年代学及微量元素地球化学特征

江西中部相山矿田及其周边铀多金属成矿作用复杂。近些年,在其南部古塘地区地表新发现了一处锡石矿化带,赋存于加里东期花岗岩中。通过对该矿点锡石开展原位LA-ICP-MS U-Pb定年和微量元素分析,获得其²⁰⁷Pb/²⁰⁶Pb-²³⁸U/²⁰⁶Pb谐和年龄为150.44±2.78 Ma、下交点年龄为150.64±2.78 Ma,两者相近,可以代表该锡石矿的成矿年龄,与矿化带周边的燕山期山心单元花岗岩成岩年龄（152.4±1.1 Ma）相近。微量元素中,高场强元素Ti、Nb、Ta富集明显。与Sn容易发生类质同象的Mn、Nb、Hf、Ta等4种元素彼此正相关性明显,Fe和Mo呈正相关关系,却与Nb、Mn、Hf、Ta均呈现负相关性。在W-Fe图解中,处于花岗岩岩浆热液型锡矿床的右侧,说明研究区成矿物质应该主要来自岩浆热液,而且成矿温度较高。古塘地区锡石矿与山心单元花岗岩虽然具有相近的成矿、成岩年龄,但由于后者具有极高的氧逸度,不利于Sn元素的富集,而与Mo成矿有关;与山心单元花岗岩中可见辉钼矿矿囊而未发现锡矿化的地质现象保持一致。

     

REE and Trace Element Geochemistry of Yinachang Fe-Cu-REE Deposit, Yunnan Province, China

1IntroductionThebehaviorsoftherare earthelements (REE)duringmagmatism ,metamorphismandmeta somationareofgreatgeologicalinterestbecauseoftheirapplicationsingeochronologyandgeochemi caltracing (e .g .reviewsbyFaure ,1 986;LipinandMckay ,1 989) .Becauseoftheirsimilargeo chemicalbehaviorsandlowsolubilitiesinwater,theREEshaveprovedthemselvestobeapowerfultoolforthecharacterizationofprovenanceswhenappliedtosedimentaryrocks (McDanieletal.,1 994) .Althoughtheirbehaviorsunderhydrothermalconditionsar…     

Isotope and trace element geochemistry of sediments from the Barbados Ridge-Demerara Plain region,Atlantic Ocean

Twenty-four piston core sediment samples and 13 sediments and 3 basalts from DSDP Leg 78 Site 543 were analyzed for Sr, Nd and Pb isotopic compositions. The results show sediment with highly radiogenic Pb${}^{206}\text{Pb}{}^{204}\text{Pb}$ up to 19.8) and rather radiogenic Sr and unradiogenic Nd has been deposited in the region since the Cretaceous. The source of this sediment is probably the Archean Guiana Highland, which is drained by the Orinoco River. Pb and Sr isotopic compositions and sediment thickness decrease and${}^{143}\text{Nd}{}^{144}\text{Nd}$ increases northward due to a decrease in turbiditic component. This decrease is partly due to the damming action of basement ridges. Rare earth concentrations in the sediments are somewhat low, due to the abundance of detrital and biogenic components in the sediment and rapid sedimentation rates. Both positive and negative Ce anomalies occur in the surface sediments, but only positive Ce anomalies occur in the Site 543 sediments. It is unlikely that sediment subducted to the source region of Lesser Antilles arc magmas could be the cause of negative Ce anomalies in those magmas.Isotopic compositions of Site 543 basalts show some effect of contamination by seawater-basalt reaction products and sediments. Beyond this, however, they are typical of “normal” depleted MORB.     

The geochemistry of redox sensitive trace metals in sediments

We analyzed the redox sensitive elements V, Mo, U, Re and Cd in surface sediments from the Northwest African margin, the U.S. Northwest margin and the Arabian Sea to determine their response under a range of redox conditions. Where oxygen penetrates 1 cm or less into the sediments, Mo and V diffuse to the overlying water as Mn is reduced and remobilized. Authigenic enrichments of U, Re and Cd are evident under these redox conditions. With the onset of sulfate reduction, all of the metals accumulate authigenically with Re being by far the most enriched. General trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes: oxic, reducing where oxygen penetrates ≤1 cm, and anoxic conditions. Using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations, we calculate the area of sediments below 1000 m water depth in which oxygen penetration is ≤1 cm to be 4% of the ocean floor. We conclude that sediments where oxygen penetrates ≤1 cm release Mn, V and Mo to seawater at rates of 140%–260%, 60%–150% and 5%–10% of their respective riverine fluxes, using the authigenic metal concentrations and accumulation rates from this work and other literature. These sediments are sinks for Re, Cd and U, with burial fluxes of 70%–140%, 30%–80% and 20%–40%, respectively, of their dissolved riverine inputs. We modeled the sensitivity of the response of seawater Re, Cd and V concentrations to changes in the area of reducing sediments where oxygen penetrates ≤1 cm. Our analysis suggests a negligible change in seawater Re concentration, whereas seawater concentrations of Cd and V could have decreased and increased, respectively, by 5%–10% over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10%–20% if the area increased by a factor of 3. The concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of Cd/Ca and V/Ca ratios observed in foraminifera shells over the last 40 kyr. This implies that the area of reducing sediments in the ocean deeper than 1000 m (4%) has not been greater than twice the present value in the recent past.     

On the occurrence,trace element geochemistry,and crystallization history of zircon from in situ ocean lithosphere

We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a _TiO2 ≈ 0.7, a _SiO2 ≈ 1.0, pressure ≈ 2 kbar); intra-sample variation is typically ~60–150°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680–740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)_N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.     

Assessment of trace element geochemistry of Hampton Roads harbor and lower Chesapeake Bay area sediments

Acid extractable Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb. and Zn were determined in sediments from the Inner Virginia Shelf, and from shipping channels in the lower Chesapeake Bay and Hampton Roads, Virginia, harbor system. Data were evaluated by a variety of techniques Levels of Cd, Cu, Pb, and Zn exceeded average crustal abundances for most of the study sites. Cumulative frequency curves suggested that there were two major populations for all metals and perhaps a third and smaller, one for Cd, Cr, and Mn Plots of metal vs Fe indicated no anthropogenic inputs of metals for shelf and Chesapeake Bay channel sites, but suggested anthropogenic influences for all metals in several of the inshore sites. Enrichment factor calculations showed enrichment of Cd, Pb, and Zn with respect to average crustal abundances for all sites and of Cu for the industrial harbor system. A recommendation of this study for evaluation of environmental geochemical metals data is to utilize mean concentrations, cumulative frequency plots, and metal vs Fe and/or enrichment factor calculations when evaluating the pollution status of sediments.