Sedimentary Phosphate Fractions Related to Calcite Precipitation in an Eutrophic Hardwater Lake (Lake Alserio, Northern Italy)

P-fractional composition has been studied in a 64-cm long sediment core collected in an eutrophic hardwater lake (Lake Alserio, northern Italy) in an attempt to identify the main mechanisms (chemical or biologic) controlling CaCO₃ precipitation in the water column. The results of the sedimentary phosphate fractionation showed that the most important P fraction was an organic fraction extracted with hot NaOH: org-P_alkali (26% of P_sed). A digestion of the supernatant of the P-fraction bound to CaCO₃ allowed the detection of a large pool of org-P (19% of P_sed). Although the nature of this fraction is unclear, we suggest that it may be involved in the biologically mediated precipitation of CaCO₃. The high CaCO₃ concentration in the entire vertical sediment profile, as well as the presence of inorg- and org-P bound to Ca in both laminated and non-laminated zones, suggest that the CaCO₃ precipitation is not a recent process but is a process that has been occurring in the lake for some time. Accordingly, the change from a homogeneous sediment to a laminated sequence might be caused by the eutrophication process of Lake Alserio occurring since the 1960s, which has allowed the preservation of the varved sediment as a consequence of the drastic reduction in bioturbation.     

Emplacement mechanisms of late-orogenic granites: structural and geochemical evidence from southern Finland

The country rock in southern Finland formed mainly during the Svecofennian orogeny ca. 1.9 Ga ago. The middle and lower crust was partially melted 1.83 Ga ago due to crustal thickening and subsequent extension. During this event, S-type migmatites and granites were formed along a 100×500 km zone. This Late Svecofennian Granite–Migmatite zone (LSGM zone) is a large crustal segment characterised by roughly E–W trending sub-horizontal migmatites and granites. Combined ductile E–W shear movements and NNW–SSE compressional movements defined a transpressional tectonic regime during the emplacement. Partial melts that moved through the crust pooled as granite sheets or froze as migmatites. Major transpressive shear zones border the LSGM zone, which forms a tectonic and metamorphic zone that crosscuts the earlier Svecofennian granitoids. Based on field observations and geochemical data from two sets of outcrops, we show that the great volumes of late-orogenic granites and migmatites in southern Finland were transported and emplaced as small chemically variable batches, possibly extracted from different protoliths. These melt batches were transported along repeatedly activated channels and collected at some horizontal level in the crust. In the Nagu area, the melt batches were trapped under a roof-layer of amphibolite and the whole complex was synchronously folded into open folds with steep axial surfaces and E–W trending fold axes. The sheets of microcline granite are, in places, strongly sheared; the microcline phenocrysts are imbricated and subsequent deformation of the microcline phenocrysts indicates syn-tectonic movements of the layers as well as a syn-tectonic mechanism for the late-magmatic fractionation. Depending on the degree of crystallisation of the individual melt batches during shearing at different intensities, the granites have slightly different appearances. Some sheared zones show a cumulate-like trace element geochemistry, indicating that melt fractions were expelled from the system, producing layers of deformation enhanced fractionated granites and cumulate layers. Our interpretation is that the Nagu area shows shear-assisted fractionation mechanisms in granitic melts, and that similar processes are responsible for the fractionation trends seen in the sub-horizontal sheeted granites in Hämeenlinna at higher levels in the crust.     

西秦岭江里沟复式岩体LA-ICP-MS锆石U-Pb年龄、地球化学和Hf同位素特征及其地质意义

古特提斯洋在三叠纪时期沿现今勉略缝合带的闭合,标志着华北板块(NCB)和扬子板块(YZB)的最终拼贴完成。秦岭造山带西段发育印支期花岗岩浆活动,为该地区的成矿作用提供了有利的条件。对这些岩浆岩的形成时代、地化属性、演化、来源等的研究将有助于了解该时期的构造背景及动力学环境。本文以西秦岭北亚带的江里沟复式岩体作为研究对象,通过LA-ICP-MS锆石U-Pb测年、锆石Hf同位素测试和主微量元素分析,对该岩体进行了系统的研究。结果显示:江里沟复式岩体的三种主要岩性为斑状黑云母二长花岗岩、花岗斑岩和细粒花岗岩,加权平均年龄分别为:229.1±1.8Ma(MSWD=0.82)、222.5±1.4Ma(MSWD=0.41)和217.1±1.8Ma(MSWD=1.4),指示该岩体形成于晚三叠世。其地球化学性质具有高硅(SiO_2=72.67%~77.84%),富碱(K_2O+Na_2O=7.67%~8.75%),弱过铝质(A/CNK=1.01~1.06),高度分异(岩体由早至晚DI及Rb/Sr均升高,DI=88.9~95.36,Rb/Sr=1.76~12.19),富集Rb、Th、U、Ta、Zr、Hf和HREE元素特点,亏损Ba、Sr、P、Ti等元素;三个侵入体均发育明显负Eu异常(δEu=0.54~0.62、0.26~0.47、0.20~0.60);P_2O_5含量与SiO_2呈明显的负相关关系,而Y的含量反之,综合判定认为,岩体属高分异I型花岗岩。岩石形成温度从早至晚明显降低(平均由786℃至755℃)。εHf(t)值少数0外,绝大多数介于0.1~4.8之间,相应tDM2=0.95~1.25Ga。综合推断认为,江里沟岩体的原始岩浆是由部分熔融的岩石圈地幔与中新元古代的地壳混合,形成的混合岩浆,后经强烈的结晶分异作用形成江里沟复式岩体。     

海洋钙同位素分馏机制及其古海洋学应用

钙在海洋中是离子浓度仅次于钠和镁的金属元素,在海洋生物过程及矿物形成中都占据重要的地位。随着同位素分析仪器精度的提高,钙同位素等非常规稳定同位素的研究逐渐成为地球化学的热门,钙同位素在海洋环境中的分馏机制及其在古海洋学中的应用研究不断完善。系统介绍了钙同位素高精度分析的方法和原理,海洋环境中无机成因和有机成因2种钙同位素分馏的机制和应用;通过大量的文献调研综述了钙同位素地球化学研究在古海洋参数恢复上的应用。通过有孔虫Globigerinoides sacculifer的钙同位素具有对温度敏感、温感公式简单、后生成岩作用对其影响小和分析材料完整易得等优点,并同其他传统地质温度计联用,为古海洋研究提供精确的海洋表面温度(Sea Surface Temperature,SST);利用钙同位素值温度相关性较小的钙质壳体来恢复古海洋钙通量;利用无机成因碳酸盐岩盖帽的钙同位素恢复海水CO2-3浓度和研究甲烷渗漏对古海洋环境的影响。     

Alkali-calcic and alkaline post-orogenic (PO) granite magmatism: petrologic constraints and geodynamic settings

The end of an orogenic Wilson cycle corresponds to amalgamation of terranes into a Pangaea and is marked by widespread magmatism dominated by granitoids. The post-collision event starts with magmatic processes still influenced by subducted crustal materials. The dominantly calc-alkaline suites show a shift from normal to high-K to very high-K associations. Source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle, affected by dehydration melting and generating more and more K- and LILE-rich magmas. In the vicinity of intra-crustal magma chambers, anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves. The post-collision event ends with emplacement of bimodal post-orogenic (PO) suites along transcurrent fault zones. Two suites are defined, (i) the alkali-calcic monzonite–monzogranite–syenogranite–alkali feldspar granite association characterised by [biotite+plagioclase] fractionation and moderate [LILE+HFSE] enrichments and (ii) the alkaline monzonite–syenite–alkali feldspar granite association characterised by [amphibole+alkali feldspar] fractionation and displaying two evolutionary trends, one peralkaline with sodic mafic mineralogy and higher enrichments in HFSE than in LILE, and the other aluminous biotite-bearing marked by HFSE depletion relative to LILE due to accessory mineral precipitation. Alkali-calcic and alkaline suites differ essentially in the amounts of water present within intra-crustal magma chambers, promoting crystallisation of various mineral assemblages. The ultimate enriched and not depleted mantle source is identical for the two PO suites. The more primitive LILE and HFSE-rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere. Present rock compositions are a mixture of major mantle contribution and various crustal components carried by F-rich aqueous fluids circulating within convective cells created around magma chambers. In favourable areas, PO suites pre-date a new orogenic Wilson cycle.     

大别造山带祝家铺辉长岩的铂族元素特征

采用镍锍火试金法结合ICP-MS分析了祝家铺14个辉长岩样品中的Ir，Ru，Rh，Pt和Pd的含量。结果显示其PGE的含量较低，原始地幔标准化后的PGE分布模式呈正斜率型，PPGE相对原始地幔略微亏损，而IPGE强烈亏损，Pd／Ir值(22—138)远高于相应的地幔比值，表明铂族元素发生了分异。对祝家铺辉长岩的铂族元素研究表明，在其源区发生过硫化物的分异作用，地壳的混入可能促进了硫化物的饱和。祝家铺辉长岩中铂族元素的分异是因为在地幔部分熔融和岩浆演化过程中，PPGE主要受硫化物控制，而Ir则存在于非硫化物相如尖晶石、可能还有合金之中。     

Origin of a Mesozoic granite with A-type characteristics from the North China craton: highly fractionated from I-type magmas?

We report geochronological, geochemical and isotopic data for the Mesozoic Shangshuiquan granite from the northern margin of the North China craton. The granite is highly fractionated, with SiO₂ > 74%. Occurrence of annitic biotite, high contents of alkalis (K₂O + Na₂O), Rb, Y, Nb and heavy rare earth elements, high FeO_t/MgO, low contents of CaO, Al₂O₃, Ba, and Sr, and large negative Eu anomalies, makes it indistinguishable from typical A-type granites. A mantle-derived origin for the rocks of the granite is not favored because their high initial ⁸⁷Sr/⁸⁶Sr (≥0.706) and low ε_Nd (t) (<−15) are completely different from either those of the lithospheric or asthenospheric mantle. In fact, their Sr–Nd isotopes fall within the range of Sr–Nd isotopic compositions of the Archean granulite terrains and are comparable to those of Mesozoic crustal-derived I-type granitoids in the region. Therefore, the Shangshuiquan granite is considered to be dominantly derived from partial melting of the ancient lower crust. Its parental magmas prove to be similar to I-type magmas and to have undergone extensive fractionation during its ascent. This is supported by the fact that some of the nearby Hannuoba feldspar-rich granulite xenoliths can be indeed regarded as the early cumulates in terms of their mineralogy, chemistry, Sr–Nd isotopes and zircon U–Pb ages and Hf isotopes. It is furthermore argued that some of highly fractionated granites worldwide, especially those with A-type characteristics and lacking close relationship with unfractionated rocks, may in fact be fractionated I-type granites. This suggestion can explain their close temporal and spatial associations as well as similar Sr–Nd isotopes with I-type granites. Our study also sheds new light on the petrogenesis of deep crustal xenoliths.     

土壤气中He、Ar组分在金家地区油气化探中应用研究

对济阳凹陷南缘与鲁西隆起接合部开展了土壤气Ｈｅ、Ａｒ组分及Ａｒ同位素测定，结果表明Ｈｅ的丰度从５．１—６μＬ／Ｌ（Ｓ．Ｔ．Ｐ），δＨｅ值从－４％－１６％，平均２．５９％；４０Ａｒ／３６Ａｒ比值从２５５—３２５，δ（４０Ａｒ／３６Ａｒ）值从＋１１％－１１％，Ｈｅ异常主要分布在研究区北部，而Ａｒ等值图分布比较复杂。Ｈｅ、Ａｒ、烃三组分之间的组合关系表明有３个远景区，最有意义的是中深层远景区。此外，根据地质分析及Ａｒ同位素特征表明中深层异常远景区不仅是与已知的第三系油田有关，而且还与深部Ｃ—Ｐ煤系地层有关，故属煤成气异常。     

13C/12C- und 18O16O-Signaturen von Calcit-Abscheidungen in Drainagesystemen

¹³C/¹²C- and ¹⁸O/16O-signatures of Calcite Precipitations in Drainage Systems Measurements of drainage waters show two distinct processes of calcite precipitation: 1. reprecipitation of calcium carbonate previously dissolved in groundwaters and 2. absorption of atmospheric CO₂ by alkaline solutions. Both processes may be distinguished by the stable isotopes of oxygen and carbon. Calcite precipitated from carbonate groundwater yields δ¹³C ≈ ?13%₀ (PDB) and δ¹⁸O ≈ 24%₀ (SMOW), whereas calcite produced by CO₂-absorption shows δ¹³C ≈ ?25%₀ (PDB) and δ¹⁸O ≈ 10%₀ (SMOW).     

The fidelity of melt inclusions as records of melt composition

A series of experiments created melt inclusions in plagioclase and pyroxene crystals grown from a basaltic melt at 1,150°C, 1.0 GPa to investigate diffusive fractionation during melt inclusion formation; additionally, P diffusion in a basaltic melt was measured at 1.0 GPa. Melt inclusions and melts within a few 100 microns of plagioclase–melt interfaces were analyzed for comparison with melt compositions far from the crystals. Melt inclusions and melt compositions in the boundary layer close to the crystal–melt interface were similar, but both differ significantly in incompatible element concentrations from melt found greater than approximately 200 microns away from the crystals. The compositional profiles of S, Cl, P, Fe, and Al in the boundary layers were successfully reproduced by a two-step model of rapid crystal growth followed by diffusive relaxation toward equilibrium after termination of crystal growth. Applying this model to investigate possible incompatible element enrichment in natural melt inclusions demonstrated that at growth rates high enough to create the conditions for melt inclusion formation, ∼10⁻⁹–10⁻⁸ m s⁻¹, the concentration of water in the boundary layer near the crystal was similar to that of the bulk melt because of its high diffusion coefficient, but sulfur, with a diffusivity similar to major elements and CO₂, was somewhat enriched in the boundary layer melt, and phosphorus, with its low diffusion coefficient similar to other high-field strength elements and rare earth elements, was significantly enriched. Thus, the concentrations of sulfur and phosphorus in melt inclusions may over-estimate their values in the bulk melt, and other elements with similar diffusion coefficients may also be enriched in melt inclusions relative to the bulk melt. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.