Kiglapait geochemistry VI: Oxygen isotopes

The Kiglapait layered intrusion is the first major intrusion found to have all whole rock and calculated liquid δ¹⁸O values close to a normal uncontaminated gabbroic value of 6.0. The intrusion experienced no detectable oxygen isotope exchange with its surrounding rocks and cooling of the magma was conductive. The δ¹⁸O values of average whole rocks vary smoothly from 6.0 at the base of the Lower Zone to 6.3 at the top of the Upper Zone. The calculated liquid δ¹⁸O values lie practically superimposed on the whole rock trend. The whole-rock data and the modelled δ¹⁸O of the magma and cumulates rigorously demonstrate that the effect of incoming cumulus phases such as magnetite and augite on the δ¹⁸O of the liquid and rocks during fractional crystallization is negligible. The cancelling effects of complementary modal variations among the mafic mineral phases and feldspar, keep the δ¹⁸O of the whole rocks constant to within ±0.1 %.. The minor change in δ¹⁸O that does occur with fractionation is consistent with the enrichment of residual liquids in feldspar component and the increasing fractionation factor δ Liquid-Fsp with falling temperature.The δ¹⁸O values of the country rocks bracket the estimated δ¹⁸O of the Kiglapait magma. Modelling with oxygen isotopes indicates that contamination of the intrusion, indicated by published radiogenic Sr and Nd isotopic data, was minor. The most probable contaminant had δ¹⁸O?7.7 and the contamination most likely occurred at >99% solidified. Subsolidus oxygen isotope exchange with an external source appears to have been very minor.     

Kiglapait geochemistry VII: Yttrium and the rare earth elements

Based on 51 wholerock analyses by XRF and summation over the layered group, the Kiglapait Intrusion contains 4.7_?1.6^+1.2 ppm Y, which resides principally in augite and apatite. Using liquid compositions calculated by summation, the partition coefficient D^AUG/L_Y is 0.95 ± 0.12 from 84 to 97 PCS (percent solidified) and 1.5 ± 0.4 above 97 PCS. For feldspar, the most likely value for D is 0.028 ± 0.02 (N = 6).REE analyses for 13 whole rocks were interpreted with the aid of yttrium models to yield trends for wholerocks and liquids vs PCS. Summations over the rocks of the layered group gave La = 2.5, Ce = 5.8, Nd = 3.9, Sm = 1.0, Eu = 0.8, Tb = 0.17, Yb = 0.37, and Lu = 0.06 ppm, with 2 s.d. errors near ± 30%. All these elements are highly incompatible until the arrival of augite, which affects chiefly the HREE, and apatite, which affects all (but more strongly, the LREE). The net result is that after apatite arrival at 94 PCS, the liquid compositions are nearly constant, hence D^WR/L_REE ≈ 1.0. These results are compatible with the mineralogy of the intrusion and the estimated partition coefficients for feldspar, olivine, augite, apatite, and Fe-Ti oxide minerals. For pre-apatite liquids, D^FSP/L_REE vary regularly with the normative di content of the liquid and change by an order of magnitude, hence the bulk liquid composition must be considered in any attempt to invert the compositions of feldspars to parent liquids.The Eu anomaly at first decreases in Kiglapait liquids due to plagioclase fractionation, but then increases due to removal of augite and apatite with negative Eu anomalies. The features dominantly responsible for Eu partitioning are liquid structure and, for monoclinic ternary feldspars, crystal structure. The former is best monitored by the augite or diopside content of the liquid and the latter, by the K content of the feldspar.The chondrite-normalized REE pattern for the intrusion has La_N = 7.4, Lu_N = 1.6, (Ce/Yb)_N = 3.6, and Eu/Eu* = 2.4, indicating its feldspar-rich nature. The chilled margin of the nearby Hettasch Intrusion has a similar but more evolved pattern, corresponding roughly to the Kiglapait liquid at 70 PCS. As with other data, those for the REE suggest source differences for the two intrusions rather than a relationship due to fractionation.     

The geochemistry of the Dunedin Volcano: Strontium isotope chemistry

The isotopic composition of strontium has been determined for samples from the alkaline lavas of the Dunedin Volcano covering the range basalt, basanite, intermediate compositions, phonolite and quartz normative trachyte. The basaltic, intermediate and phonolitic rocks appear to be comagmatic and have similar low initial Sr⁸⁷/Sr⁸⁶ ratios around 0.7030, comparable with those of other alkaline provinces. The quartz normative trachytes have initial ratios significantly higher than those of the other rocks (0.7040) although their age is comparable. Contamination by sea water or crustal material could explain the higher initial ratios of the trachytes but it does not account for important features of their chemistry. It is suggested that the trachytes formed by partial melting involving an alkali feldspar-rich portion of older igneous rocks. Rb-Sr ages obtained are comparable with published K-Ar dates. The Rb-Sr age for the trachytes is 14.± 7 m.y. and the other alkali-enriched rocks give ages ranging within the limits of 14.4 to 12.0 m.y.     

Kiglapait Geochemistry II: Petrography

The mode of the Kiglapait intrusion carries 73 per cent feldspar,but average rocks near the base of the Upper Zone contain aslittle as 48 per cent feldspar. Olivine remained stable throughoutthe crystallization, but was locally suppressed by abundantcrystallization of augite and titano-magnetite. Red biotiteoccurs as rims on Fe–Ti oxide minerals and is probablyfluorine oxybiotite; its frequently similar occurrence in troctoliticrocks may, perversely, indicate dry magmas rather than dampones. Excluded modal components follow Rayleigh fractionation behaviour;their presence in trace amount permits estimation of residualporosity in the Lower Zone. This porosity diminishes directlywith accumulation rate from 0.14 to 0.03 over the first 80 percent of crystallization history. The saturation ratio of excludedmodal components is a well-behaved function of fraction solidified,and implies that the decrease in porosity continues above the80 per cent solidified level. The saturation ratio links withbulk composition, porosity, and F_L, allowing one of these parametersto be estimated from the others. The cumulus arrival of apatite is abrupt, but the earlier arrivalsof augite, oxide minerals, and sulfide each occur over an interval,followed by an interval of overproduction. This behaviour isattributed to feedback on concentration gradients generatedby a long history of plagioclase + olivine extraction, in theabsence of perfect stirring. Diffusion plays a role in the differentiationof large, slowly cooled magma systems because radial mixingby convection is inefficient. Inherited potential supersaturationis the inevitable result. This leads to modal irregularities,and to crystallization effectively on metastable extensionsof field boundaries. The track of the liquid in the Lower Zone is closely parallelto that in the system Fo–Di–An, but offset fromit by the combined effects of Ab and P (toward plagioclase)and Fa (away from plagioclase). The latter effect is important,with the result that the shedding of plagioclase by an ascendingmagma will be much less marked than predicted from iron-freeexperimental systems.     

Kiglapait Geochemistry III: Potassium and Rubidium

The regular geometry and completeness of the Kiglapait intrusion permit its bulk composition to be obtained by summation, and the composition of successive liquids to be obtained by subtraction. The summations for K and Rb give 1806 and 1.08 ppm, yielding Rfrsol|K/Rb= 1670 for the intrusion, taken as equal to the parent magma. R increases slightly from this initial value to 2000 at the end of crystallization where MgO approaches zero in the rocks. K and Rb are therefore closely coherent and their distribution coefficients can differ only by a small amount in the Kiglapait system.Apparent feldspar/liquid distribution coefficients (D^F/L) can be estimated from detailed plots of feldspar and liquid compositions against F_L. The Kiglapait data imply that these coefficients are linear 1:1 functions of plagioclase composition within experimental error, having values given by D_KF/L = 1.42? X_AnD_RbF/L = 1.13? X_An with minimum values of 0.75 and 0.49, respectively. The ratio $\text{R}{}^{\mathrm{F}}\text{R}{}^{\mathrm{L}}$ lies in the range of 1.53± 0.03 for the plagioclase composition range X_An= 0.34 to 0.67 showing that high-R rocks such as anorthosite crystallized from high-R liquids.The apparent feldspar distribution coefficients are much closer to 1.0 than common literature values. They can be reduced by assuming that the cumulate pile was continuously recharged by the circulating magma until an advanced stage of differentiation was reached, and assuming that alkalies were exchanged to the feldspars from the magma. When such an ‘aquifer recharge’ model is calibrated using olivine-liquid equilibria as a time marker for the liquid, the inferred minimum equilibrium values of the distribution coefficients are $\text{D}{}_{\mathrm{K}}\text{F}\text{L}$= 0.42, $\text{D}{}_{\mathrm{Rb}}\text{F}\text{L}$ = 0.25 at the base of the intrusion. Their variation is given by $\text{D}{}_{\mathrm{K}}\text{F}\text{L}\text{= 1.66?1.88X}{}_{\mathrm{An}}$, $\text{D}{}_{\mathrm{Rb}}\text{F}\text{L}\text{= 1.17?1.41X}{}_{\mathrm{An}}$, The equilibrium values are considered to be appropriate for deducing liquid compositions in plutonic bodies where alkali exchange can be shown or inferred to have been inhibited, such as in small intrusions. The apparent values are considered to be appropriate, even though they may be artificial, for large intrusions similar to the Kiglapait.The bulk K and Rb concentrations in the Kiglapait intrusion are consistent with a plagioclase-rich abyssal tholeiite magma. Clinopyroxene and olivine fractionation in the mantle may contribute to the production of such high-Rmagmas.     

Kiglapait Geochemistry IV: The Major Elements

The summed bulk composition of the Kiglapait intrusion is SiO₂ 47.46 (31). TiO₂ 0.81 (20), Al₂O₃ 19.46 (48), Fe₂O₃ 1.91 (46), FeO 9.36 (36), MnO 0.15 (01). MgO 8.04 (21), CaO 9.27 (46). Na₂O 3.13 (05), K₂O 0.22 (01), P₂O₅, 0.09 (01), and BaO 0.02 (01). total 99.98 wt% (estimated standard deviations of the last two decimals in parentheses). The CIPW norm isap 0.34, mt 2.78, il 1.52, or 1.11, ab 26.27, an 38.64, (fsp 66.47), di 5.38, hy 2.45, ol 21.58. Molar X_An is 0.58; molar x_fo is 0.65.The variation curves for oxides in calculated liquids are explained in detail by the petrographic variations shown in earlier papers of this series. The Kiglapait FeO + MnO:MgO:Alkalies (FMA) trend reaches higher iron concentrations than the Skaergaard liquid and constitutes a new limiting case of the Fenner trend of fractional crystallization, implying low oxygen fugacity and silica activity. The end-stage Kiglapait ferrosyenite lies on the FeO + MnO-Alkalies sideline, being essentially devoid of MgO.Except in$\text{K and K}\text{Rb.}$the Kiglapait composition is the same within experimental error as the chilled margin of the nearby Hettasch intrusion (BERG, 1980.), but slight systematic differences show that the Hettasch composition is slightly more fractionated or represents a smaller fraction of melting at the source. Both are low-augite troctolite compositions, and they require troctolitic parent liquids. Such liquids are related to common augite-rich basalts by addition of spinel, and they imply exhaustion of clinopyroxene in the source. Except for their low augite and high Al₂O₃, the Kiglapait and Hettasch compositions resemble modern MORB and suggest a depleted source.     

Strontium isotopic geochemistry of Tianqiao Pb-Zn deposit,Southwest China

Tianqiao carbonate-hosted Pb-Zn deposit, controlled by NW-trending F37 thrust fault and NW-trending Tianqiao anticline, is located in the eastern part of Sichuan-Yunnan-Guizhou （SYG） Pb-Zn metallogenic province, southwestern Yangtze Block, southwest China. Ore bodies in this deposit are hosted in the Devonian-Carboniferous carbonate rocks, and ore minerals include sphalerite, galena and pyrite, while the gangue minerals are dominated by calcite and dolomite. Using high-precision solid thermal ionization mass spectrometry （TIMS）, this paper reports the strontium isotopic compositions （0.7119 to 0.7167） of sulfide samples from the Tianqiao deposit in order to trace the origin of hydrothermal fluids. Compared with the country rocks, the calculated 87Sr/86Sr200 Ma values of sulfide range from 0.7118 to 0.7130, higher than those of the age-corrected Devonian to Permian sedimentary rocks （0.7073 to 0.7101） and the Middle Permian Emeishan flood basalts （0.7078 to 0.7039）, but lower than those of the age-corrected Proterozoic basement rocks （such as the Kunyang and Huili Groups, 87Sr/86Sr200 Ma=0.7243 to 0.7288）. This implies a mixed strontium source between the older basement rocks and the younger cover sequences. Together with geologic and previous isotopic evidences, we considered that the fluids＇ mixing is a possible mechanism for sulfide precipitation in the Tianqiao deposit.     

Strontium isotope geochemistry of megacrysts and host basalts from southeastern Australia

Strontium isotopic data for megacrysts and lavas from six eruptive centers within the Newer Basalts province of southeastern Australia show that megacrysts of clinopyroxene are in isotopic equilibrium with associated basalts, but that megacrysts of kaersutite, ferrokaersutite, orthopyroxene and anorthoclase may exhibit slight disequilibrium with their host basalts. Furthermore, the anorthoclase megacrysts may be either more or less radiogenic than their hosts. The ⁸⁷Sr/⁸⁶Sr ratios for 14 basalts from throughout the province vary from 0.7035 to 0.7045 and it is proposed that anorthoclase, amphibole and orthopyroxene megacrysts which crystallized in isotopic equilibrium with one magma may have been caught up in a pulse of a later magma of a different isotopic composition. The variations in the ⁸⁷Sr/⁸⁶Sr ratios for the basalts are attributed to variations in the isotopic composition of their source regions. Such isotopic heterogeneity is supported by published data for ultramafic xenoliths which occur in the Newer Basalts lavas.     

湘中锡矿山锑矿床的Sr同位素地球化学

对湘中锡矿山锑矿床围岩灰岩、硅化灰岩、煌斑岩和脉石矿物进行了系统的Sr同位素研究。结果表明，矿区围岩发生了隐性蚀变，灰岩中Sr亏损，而^87Sr/^86Sr高于同时代的海相碳酸盐，这种隐性蚀变很可能是水／岩反应所致。矿体附近的硅化灰岩中Sr更加亏损，而^87Sr/^86Sr明显增加。成矿期方解石的^87Sr/^86Sr较高，成矿体系中变化的W／R比造成了方解石中^87Sr/^86Sr值的明显波动。成矿流体为一富放射成因^87Sr的溶液。成矿流体来自或流经基底地层，流体中的Sr由基底碎屑岩提供，矿质Sb也可能主要来自富Sb的元古宇基底。水／岩反应的理论模拟显示，锡矿山成矿流体中的Sr约为3．0μg/g，^87Sr/^86Sr为０．７１７；蚀变－成矿体系为一开放体系，矿石的沉淀机制主要为水／岩反应，成矿体系中W／R 比较高。     

Strontium in coral aragonite: 3. Sr coordination and geochemistry in relation to skeletal architecture

Use of the coral Sr palaeothermometer assumes that the Sr in coral skeletons is substituted randomly for Ca in the aragonite structure. The presence of Sr in additional phases e.g., strontianite, or the non random distribution of Sr across metal sites in aragonite, would complicate the Sr/Ca-sea surface temperature relationship. We have used Sr K-edge microEXAFS (extended X-ray absorption fine structure) to determine the structural state of Sr across selected microvolumes of four coral skeletons (Porites lobata, Acropora palmata, Pavona clavus, and Montastrea annularis). We used a 5 × 3 μm beam to analyse specific areas of the coral skeletal architecture, i.e., centres of calcification, fasciculi, and dissepiments. All EXAFS analyses refine, within error, to an ideally substituted Sr in aragonite, and we found no evidence of strontianite or partly ordered structural states. Anisotropy in the first shell responses results from the fact that the analysed microvolumes are not necessarily averaged for the responses of all crystal orientations in the aragonite. Although secondary ion mass spectrometry confirmed that Sr/Ca composition can vary substantially between skeletal components, we find no evidence for any contrast in Sr structural state. Sr heterogeneity may result from kinetic effects, reflecting complex disequilibrium processes during crystal precipitation, or biological effects, resulting from variations in the composition of the calcifying fluid which are biologically mediated.     

Kiglapait Mineralogy I: Apatite, Biotite, and Volatiles

Electron microprobe analyses show that Upper Zone apatites inthe Kiglapait intrusion are fluorine rich and contain minorchlorine and hydroxyl (calculated). Apatite from the Outer BorderZone has a higher Cl content. The refractive indices of UZ apatites have the following ranges: = 1.6345–1.6379, = 1.6326–1.6352, and B = 0.0020–0.0028.The birefringence is low for apatites with these refractiveindices. Some Outer and Upper Border Zone apatites have higherindices of refraction and normal birefringence. Fractional crystallization of the basaltic Kiglapait magma producedcumulus apatite beginning at the 94 per cent solidified levelwhen P₂O₅ reached saturation in the liquid. The amounts of P₂O₅and modal apatite decreased gradually from the 94 per cent tothe 99.99 per cent solidified level as the liquid was depletedin P₂O₅. F and Cl appear to be equally partitioned between theliquid and apatite because no fractionation trends are notedbetween the two halogens. There is a slight decrease in thecalculated ratio OH/F in apatite which suggests possible depletionof OH in the liquid with fractionation. Kiglapait apatites appear to be stoichiometric, based on microprobechemistry, refractive index, and unit cell dimensions. However,infrared absorption analyses show no detectable water, whichleaves approximately 11 per cent of the monovalent anion siteunaccounted for. Anion deficiencies in apatites from low-H₂Oenvironments may be explained either by substitution of O forF, or domains of tetracalcium phosphate. Non-cumulus biotite occurs in minor quantities in the intrusion.Electron microprobe analyses of Upper Zone biotites show thatthey contain an average (by weight) pf 0.4 per cent F, 0.07per cent Cl, and 4.0 per cent H₂O (calculated). The volatile chemistry of the Kiglapait intrusion is calculatedfrom apatite and biotite chemistry. The intrusion contains anestimated 900 ppm P₂O₅, 166 ppm F, and 12 ppm Cl. There is amaximum of 68 ppm H₂O using calculated H₂O from microprobe data,or a minimum of 8 ppm using H₂O from infrared analysis. It isproposed that the anhydrous basaltic Kiglapait magma was a secondpartial melt of amphibole-bearing mantle rock.     

Strontium isotope geochemistry of Icelandic geothermal systems and implications for sea water chemistry

Chemical and Sr isotopic analyses have been made of waters from 16 geothermal sites in Iceland with particular reference to the systems at Reykjanes and Svartsengi for which compositions of geothermal sea water and fresh and hydrothermally-altered rocks have been compared. The alkalies display mixing relationships indicating a hydrothermal input of Rb and K to local meteoric and sea waters as do results for Sr and Ca involving high-temperature fluids. ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the geothermal waters of meteoric origin parallel those of associated rocks but are higher. Ratios for geothermal sea waters are 0.7042 (Reykjanes) and 0.7040 (Svartsengi), lower than for normal sea water (0.7092) because of leaching of Sr from rocks followed by partial removal into alteration minerals, of which epidote and chlorite may be most important. Consequently, associated hydrothermally-altered rocks have been subject to significant Sr isotopic contamination by sea water Sr raising ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios from 0.7032 for fresh rock to 0.7038–0.7042 (Reykjanes) and to 0.7039–0.7041 (Svartsengi). Altered basalt is only ~50% equilibrated isotopically with geothermal sea water, at a water/rock ratio of ~2, but is internally equilibrated whereas palagonitized rocks (water/rock ratio of 3 to 4) are close to Sr isotopic equilibrium with associated sea water but show significant internal Sr disequilibrium. Hydrothermal input is unlikely to be important in the oceanic mass balance of Sr but is likely to be highly significant in controlling the strontium isotopic composition of sea water.     

Kiglapait Mineralogy III: Olivine Compositions and Rayleigh Fractionation Models

Olivines, present throughout the layered cumulates of the KiglapaitIntrusion, record extreme iron enrichment during fractionalcrystallization. Mn is less compatible than Fe; the exchangecoefficient K_D for Mn/Fe (OL-LIQ) has values near 0.95 but theydrop to 0.7 near the Lower Zone-Upper Zone boundary. Ca is depletedby more than a factor of 30 relative to experimental valuesat 5 kbar pressure. Attempts to find a sink for Ca in the LowerZone without invoking liquid as a reactant are unconvincing,and questions of metastability arise. Cumulus olivines varysystematically to lower Fo contents with stratigraphic height.Using equilibrium values for the exchange coefficient K_D (Fe/Mg,OL-LIQ), and the observed mode of olivine and residual porosity,a Rayleigh fractionation calculation reproduces the observedLower Zone trend without recourse to multiple injections ofmagma. An anomalously Fe-rich region above the Main Ore Bandin the Upper Zone is ascribed to ponding of Fe-rich magma atthe floor of the intrusion until later, buoyant residual magmaentrained the Fe-rich residua. Such Fe-rich ponds probably accountfor the abnormal Feenrichment shown by some other Fenner-trendlayered intrusions. Summation over the observed crystal compositionsyields values of X_Mg that do not represent the equilibrium liquid,but that reduce to it by a simple multiplier p = 0.78. Thisresult means that if K_D is known, the high-temperature, liquidusolivine compositions can be retrieved, or vice versa. Becausethe observed olivine compositions are supported by realisticforward models, there is no need to invoke evolved boundary-layerliquids to explain the in situ crystallization of Kiglapaitcumulates. Classical cumulate theory suffices. KEY WORDS: olivine; Rayleigh fractionation; Kiglapait Intrusion; cumulative theory     

Kiglapait Geochemistry I: Systematics, Sampling, and Density

The Kiglapait intrusion affords many opportunities for evaluatingplutonic fractionation processes. Estimates of boundary conditionsinclude emplacement of anhydrous high alumina basaltic magmaat 4 kbar and about 1250 °C, initial crystallization nearthe WM buffer, and fractionation to Mg-free ferrosyenite whichcrystallized at 960°C, somewhat above the WM buffer. Thelast ferrosyenites represent fractionation to 0.01 per centof the initial volume. Plagioclase varies from An₆₇ to An₁₀, olivine from Fo₆₉ to Fo₀,and augite from En₇₃ to En₀. The specific gravity of the intrusionis 2.93, varying between 2.87 and 3.18 on smoothed models. Crystals accumulated chiefly at the floor. As they did so, themagma depth decreased as the square root of the volume fractionof liquid. The volume fraction solidified was roughly proportionalto time. Cooling was slower than the t relation because of hotsurroundings on one side. The crystallization time was about10⁶ yr, and the average accumulation rate was about 1 cm/yr.Average crystal concentrations of 3–300 ppm are impliedfor the nucleation zone. The cooling rate corresponds to crystallizationof 2 x 10⁷ kg/yr per km² roof area, about 550 times slower thana lava lake. Calculated liquid densities range from 2.67 to 2.88 g/cm³ athigh temperatures. Feldspar almost surely did not sink in themagma, but nevertheless it accumulated mainly on the floor.Cooperative sinking with mafics as proposed by Coats, combinedwith oscillatory nucleation as proposed by Wager, may accountfor this paradox. Oscillatory nucleation leading to feldspar-supersaturatedliquids is supported by laboratory evidence on the feldspar-likestructure of liquids and the concave-úp plagioclase liquidusin systems involving olivine. Both lines of evidence imply highpolymerization of feldspar-rich liquids, particularly in slowprocesses. Such polymerization can help to explain the genesisof primary Eu anomalies and anorthositic magmas in additionto the floor accumulation of feldspar and rhythmic layering.     

Strontium and hydrogen isotope geochemistry of fresh and metabasalt dredged from the Mid-Atlantic Ridge

Fresh basalt and metabasalt dredged from the Mid-Atlantic Ridge were studied for Na, K, Rb, Sr, and H₂O(+) contents, and strontium and hydrogen isotope ratios. Na, K, Rb, and Sr contents of these samples are within the range of those of oceanic tholeiite. H₂O(+) content, strontium, and hydrogen isotope ratios vary widely. The variation in water content of metabasalt is apparently related to the chlorite content. The metamorphic temperature was about 550 °C based on the estimated δD value of chlorite. There is positive linear relationship between water content and strontium isotope ratio. Based on this relationship, the variation of strontium isotope ratio of the metabasalt was interpreted as follows: complete exchange occurred between strontium in the chlorite portion of the metabasalt and strontium in sea water (⁸⁷Sr/⁸⁶Sr ratio=0.7090), while the original strontium (⁸⁷Sr/⁸⁶Sr∼0.7023) was retained in the non-altered portion of the basalts.     

Strontium isotope geochemistry of groundwaters and streams affected by agriculture,Locust Grove,MD

The effects of agriculture on the isotope geochemistry of Sr were investigated in two small watersheds in the Atlantic coastal plain of Maryland. Stratified shallow oxic groundwaters in both watersheds contained a retrievable record of increasing recharge rates of chemicals including NO₃⁻, Cl, Mg, Ca and Sr that were correlated with increasing fertilizer use between about 1940 and 1990. The component of Sr associated with recent agricultural recharge was relatively radiogenic (⁸⁷Sr/⁸⁶Sr=0.715) and it was overwhelming with respect to Sr acquired naturally by water–rock interactions in the oxidized, non-calcareous portion of the saturated zone. Agricultural groundwaters that penetrated relatively unoxidized calcareous glauconitic sediments at depth acquired an additional component of Sr from dissolution of early Tertiary marine CaCO₃ (⁸⁷Sr/⁸⁶Sr=0.708) while undergoing O₂ reduction and denitrification. Ground-water discharge contained mixtures of waters of various ages and redox states. Two streams draining the area are considered to have higher ⁸⁷Sr/⁸⁶Sr ratios and NO₃⁻ concentrations than they would in the absence of agriculture; however, the streams have consistently different ⁸⁷Sr/⁸⁶Sr ratios and NO₃⁻ concentrations because the average depth to calcareous reducing (denitrifying) sediments in the local groundwater flow system was different in the two watersheds. The results of this study indicate that agriculture can alter significantly the isotope geochemistry of Sr in aquifers and streams and that the effects could vary depending on the types, sources and amounts of fertilizers added, the history of fertilizer use and groundwater residence times.     

Strontium isotope geochemistry of groundwater affected by human activities in Nandong underground river system, China

The Nandong Underground River System (NURS) is located in a typical karst area dominated by agriculture in SE Yunnan Province, China. Groundwater plays an important role in the social and economical development in the area. The effects of human activities (agriculture and sewage effluents) on the Sr isotope geochemistry were investigated in the NURS. Seventy-two representative groundwater samples, which were collected from different aquifers (calcite and dolomite), under varying land-use types, both in summer and winter, showed significant spatial differences and slight seasonal variations in Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios. Agricultural fertilizers and sewage effluents significantly modified the natural ⁸⁷Sr/⁸⁶Sr ratios signature of groundwater that was otherwise dominated by water-rock interaction. Three major sources of Sr could be distinguished by ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations in karst groundwater. Two sources of Sr are the Triassic calcite and dolomite aquifers, where waters have low Sr concentrations (0.1-0.2 mg/L) and low ⁸⁷Sr/⁸⁶Sr ratios (0.7075-0.7080 and 0.7080-0.7100, respectively); the third source is anthropogenic Sr from agricultural fertilizers and sewage effluents with waters affected having radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7080-0.8352 for agricultural fertilizers and 0.7080-0.7200 for sewage effluents, respectively), with higher Sr concentrations (0.24-0.51 mg/L). Due to the overlapping ⁸⁷Sr/⁸⁶Sr ratios, it is difficult to distinguish the sources of Sr in groundwater samples contaminated by agricultural fertilizers or sewage effluents based only on their ⁸⁷Sr/⁸⁶Sr ratios. However, ⁸⁷Sr/⁸⁶Sr ratios do provide key information for natural and anthropogenic sources in karst groundwater.     

Kiglapait Mineralogy II: Fe-Ti Oxide Minerals and the Activities of Oxygen and Silica

Cumulus titanomagnetite and subordinate ilmenite first appearin the Upper Zone of the Kiglapait intrusion. They arrive graduallyand then reach abnormal abundances before falling to a sustainedcotectic mode near seven volume per-cent. The most Ti-rich titanomagnetites(to Usp ₆₆) are preserved in ore bands (layers) which solidifiedby adcumulus growth leading to the complete expulsion of interstitialsilicate liquid. Analyses from three of these ore bands, appliedto the solution model of Lindsley (1977), form a single lineararray in f_o2 versus 1/T ?K, with log f_o2 = (–28,283 ?89)/T + 11.03 ? 0.25. This array implies log f_o2 = –9.65at 1094 ?C, the model temperature of the Main Ore Band, consistentwith primary mineral compositions Ilm₈₉, Usp₇₉ and a weightmode of 18 per cent ilmenite. Silicate rocks yield another linear array, i.e. log f_o2 = (–37,910? 102)/T + 22.57 ? 0.61. This array is ascribed to closure ofsubsolidus reactions from initial compositions near Usp_79–80Ilm₉₀. The center of gravity of the data falling on this arraysuggests a primary mode of about 50 per cent ilmenite for thesilicate rocks, implying somewhat more reducing conditions ofcrystallization than for the ore bands. The modal overproductionrepresented by the ore bands is attributed to super-saturationin oxygen, which is demonstrated by the Al-depleted compositionsof titanomagnetite in ore bands, by direct evidence for elevatedf_o2 at the top of the Main Ore Band, and by abnormally magnesiansilicate mineral compositions in and near the ore bands. The primary titanomagnetite composition for average rocks isestimated at Usp₈₀ for the base, and Usp₇₃ for the top of theUpper Zone, from rock and mineral chemistry and observed textures.The idealized magma path for the Upper Zone runs from (T andlog f_o2) 1154 ?C, –9.0 to 960 ?C, –12.2. The orebands lie above this path and are interpreted as lying on themetastable extension of the Lower Zone path, which originatesat 1250 ?C, –8.1, on the WM buffer at 4 kbar total pressure. Silica activity is estimated from mineral compositions nearthe ore bands as applied to the FMQ equilibrium, and mappedfor the Lower Zone by an adjustment downward from the En-Fo-Silequilibrium, with resultant values near 0.55 relative to quartz= 1.0. The logarithmic oxygen/silica activity ratio (OSAR) coincideswith that of the Skaergaard intrusion in the Lower Zones. TheSkaergaard OSAR is offset downward from the Kiglapait trendduring MZ time, and remains below it at the end of crystallization.The more highly silicated Skaergaard magma was initially moreoxidized than the Kiglapait magma, but this relation was reversedafter the loss of olivine in the Skaergaard intrusion, as couldhave been predicted from theory and the mineralogy of the twointrusions.     

Strontium isotopic geochemistry of Pan-African/Brasiliano rocks,Chapada copper deposit,Goiás,Brazil

Whole rock and mineral Rb-Sr isochrons from the upper amphibolite grade metamorphic host rocks of the Chapada copper deposit in Goiás, Brazil, yield an age of 561±9 Ma, with a⁸⁷Sr/⁸⁶Sr initial (I) of 0.70414±0.00005, for the crystallization of pre-metamorphic volcanic and igneous rocks and 532 ±1 Ma for their metamorphism during the Pan-African/Brasiliano orogeny (700-450 Ma). A porphyritic diorite stock which intruded the host schists yields a mineral isochron age of 534±16 Ma (I=0.70415±0.00004). The low I values determined from these isochrons suggest that the host schists at Chapada were derived from upper mantle or lower crust material and are characteristic of magmatic products associated with island arcs. The Early Cambrian age of the host rocks at Chapada contradicts the previously assigned mid-Proterozoic age and suggests that Pan-African/Brasiliano magmatic activity may have been more extensive in central Brazil than previously thought.

---

Zusammenfassung Das Trägergestein der Chapada Kupferlagerstätte in Goiás ist in metamorpher Hoch-Amphibolitfazies ausgebildet. Gesamtgesteins- und individuelle Mineral Rb-Sr-Isochronen mit einem⁸⁷Sr/⁸⁶Sr-Initialwert (I) von 0,70414±0.00005 ergeben für die Kristallisation der Vulkanite und Magmatite, die die Metamorphose vordatieren, ein Alter von 561±9 Mio Jahre. Die Datierung der Metamorphose während der panafrikanisch/brasilianischen Orogenese (700 bis 450 Mio Jahre) ergibt 532±1 Mio Jahre. Das Mineral-Isochronenalter eines Dioritkörpers porphyrischen Gefüges, der in die Schiefer intrudierte, beträgt 534±16 Mio Jahre (I=0,70415 ±0,00004). Das Ergebnis niedriger Initial-Werte dieser Isochronen ergibt für die Schiefer bei Chapada ein Ausgangs-Material im Bereich des oberen Mantels oder der tieferen Kruste; diese Ausgangswerte sind charakteristisch für magmatische Tätigkeit in Verbindung mit Inselbögen. Das unterkambrische Alter des Trägergesteins bei Chapada steht im Widerspruch zu dem bisher angenommenen mittelproterozoischen Alter und ist ein Hinweis auf eine mögliche Phase intensiverer pan-afrikanisch/brasilianischer magmatischer Aktivität in Zentral-Brasilien als ursprünglich angenommen wurde.

---

Résumé Les roches qui contiennent le gisement de cuivre de Chapada (Goias, Brésil) sont des schistes cristallins appartenent au facies dupérieur des amphibolites. Dans des volcanites et des roches orthomagmatiques appartenant à cet ensemble, des isochrones Rb-Sr sur roches totales et sur minéraux donnent un âge de 561±9 Ma avec un rapport initial de 0,70414±0,00005 pour la cristallisation pré-métamorphique et un âge de 532±1 Ma pour le métamorphisme, lié à l'orogenèse pan-africaine / brésilienne. Un pluton de diorite porphyrique, qui intrude les schistes cristallins donne, par isochrone sur minéraux, un âge de 534±16 Ma (rapport initial de 0,70415±0,00004). Les valeurs basses des rapports initiaux de ces diverses roches suggèrent que les schistes de Chapada sont dérivés de matériaux mantelliques ou crustaux profonds et présentent les caractères des produits magmatiques associés aux arcs insulaires. Leur âge éo-cambrien, en contradiction avec l'âge mésoprotérozoïque admis jusqu'ici, permet de penser que l'activité magmatique pan-africaine/brésilienne dans le centre du Brésil a été plus intense que ce qu'on croyait jusqu'ici.

---

Chapada Gois . 0,70414±0,00005, 561± 9 , 532±1 , . . - / 700 450 /. , , 534±16 / 1 = 0,70415 = 0,00004 /. , Chapada , . . - ; , . , - - , , , .