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1.
Summary The strongly peraluminous, P- and F-rich granitic system at Podlesí in the Krušné Hory Mountains, Czech Republic, resembles
the zonation of rare element pegmatites in its magmatic evolution (biotite → protolithionite → zinnwaldite granites). All
granite types contain disseminated Nb-Ta-Ti-W-Sn minerals that crystallized in the following succession: rutile + cassiterite
(in biotite granite), rutile + cassiterite → ferrocolumbite (in protolithionite granite) and ferrocolumbite → ixiolite →
ferberite (in zinnwaldite granite). Textural features of Nb-Ta-Ti-W minerals indicate a pre-dominantly magmatic origin with
only minor post-magmatic replacement phenomena. HFSE remained in the residual melt during the fractionation of the biotite
granite. An effective separation of Nb + Ta into the melt and Sn into fluid took place during subsequent fractionation of
the protolithionite granite, and the tin-bearing fluid escaped into the exocontact. To the contrast, W contents are similar
in both protolithionite and zinnwaldite granites.
Although the system was F-rich, only limited Mn-Fe and Ta-Nb fractionation appeared. Enrichment of Mn and Ta was suppressed
due to foregoing crystallization of Mn-rich apatite and relatively low Li content, respectively. The content of W in columbite
increases during fractionation and enrichment in P and F in the melt. Ixiolite (up to 1 apfu W) instead of columbite crystallized
from the most fluxes-enriched portions of the melt (unidirectional solidification textures, late breccia). 相似文献
2.
The most evolved rocks of the Pilansberg alkaline complex are aegirine lujavrites in which three varieties of eudialyte are recognized on the basis of textural relationships and composition. Manganoan eudialyte-I is a relict orthomagmatic phase occurring as poikilitic plates or as relict grains in pseudomorphed euhedral phenocrysts. Late eudialyte-II ranges in composition from manganoan eudialyte through kentbrooksite to taseqite-like varieties and is considered to be formed by cation exchange with eudialyte-I and alkaline fluids. Eudialyte-III is a hydrothermal phase replacing eudialyte-II, and has either taseqite-like (5–7.3 wt.% SrO, < 2.0 wt.% REE 2O 3) or kentbrooksite (< 1.5 wt.% SrO, 8.5 wt.% REE 2O 3) compositions. Three styles of replacement of eudialyte-I and -II are recognizable. Type 1 involves replacement by complex aggregates of zircon, fergusonite-(Ce), allanite-(Ce), britholite-(Ce), titanite, pyrochlore, albite and potassium feldspar, i.e. a “miaskitic” paragenesis. Type 2 alteration consists of complex aggregates dominated by deuteric Na–Zr-silicates (?catapleiite), stronalsite, strontium-apatite and lamprophyllite replacing eudialyte-I and -II and relicts of the “miaskitic paragenesis”, i.e. a highly sodic “agpaitic-to-hyperagpaitic” paragenesis. Type 3 replacement involves mantling of any residual eudialyte-II and zircon, and replacement of deuteric Na–Zr-silicates by eudialyte-III together with barytolamprophyllite as late hydrothermal phases. Further alteration and replacement resulted in the superposition of natrolite, britholite, pyrochlore, allanite and diverse Ba- and Mn-based minerals onto the types 2 and 3 assemblages, and ultimately to the deposition of allanite-(La), La-dominant REE carbonates and rarely a silica phase. All of the alteration styles are considered to have occurred in situ under subsolidus conditions (< 450 °C) by interaction of pre-existing eudialyte and other minerals with deuteric, sodium- and chlorine-bearing aqueous fluids. The evolution of the replacement products is from a miaskitic through an agpaitic to a hyperagpaitic paragenesis and ultimately back to a low agpaitic-to-miaskitic assemblage, reflecting changes in the a(Na +)/ a(Cl −) ratio and alkalinity of the deuteric/hydrothermal fluids. 相似文献
3.
通过野外和室内研究结果表明,石桥片麻岩是由沉积岩变质组成。在化学成分上,表现为高硅（SiO 2=75.22%～76.42%）、低铝（Al 2O 3=11.02%～12.29%）、富碱（Na 2O+K 2O=8.00%～8.68%）和贫钙（CaO=0.30%～0.82%）,富集Rb、Ba、Th、U等大离子亲石元素,贫Nb、Ta、Zr、Hf、Ti 等高场强元素,以及轻稀土相对富集（LREE/HREE=6.06～7.14）、铕负异常较明显（δEu=0.18～0.56）等。通过锆石成因类型及定年研究,获得超高压变质作用时间为254 Ma和退变质作用时间为217 Ma,老核（继承锆石）岩浆碎屑锆石是异地多时代（元古代-古生代）,其寄主岩的原岩为沉积岩,时代不会早于古生代。 相似文献
4.
The varying geochemical and petrogenetic nature of A-type granites is a controversial issue. The oxidized, magnetite-series A-type granites, defined by Anderson and Bender [Anderson, J.L., Bender, E.E., 1989. Nature and origin of Proterozoic A-type granitic magmatism in the southwestern United States of America. Lithos 23, 19–52.], are the most problematic as they do not strictly follow the original definition of A-type granites, and approach calc-alkaline and I-type granites in some aspects. The oxidized Jamon suite A-type granites of the Carajás province of the Amazonian craton are compared with the magnetite-series granites of Laurentia, and other representative A-type granites, including Finnish rapakivi and Lachlan Fold Belt A-type granites, as well as with calc-alkaline, I-type orogenic granites. The geochemistry and petrogenesis of different groups of A-types granites are discussed with an emphasis on oxidized A-type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis. Oxidized A-type granites are clearly distinguished from calc-alkaline Cordilleran granites not only regarding trace element composition, as previously demonstrated, but also in their major element geochemistry. Oxidized A-type granites have high whole-rock FeO t/(FeO t + MgO), TiO 2/MgO, and K 2O/Na 2O and low Al 2O 3 and CaO compared to calc-alkaline granites. The contrast of Al 2O 3 contents in these two granite groups is remarkable. The CaO/(FeO t + MgO + TiO 2) vs. CaO + Al 2O 3 and CaO/(FeO t + MgO + TiO 2) vs. Al 2O 3 diagrams are proposed to distinguish A-type and calc-alkaline granites. Whole-rock FeO t/(FeO t + MgO) and the FeO t/(FeO t + MgO) vs. Al 2O 3 and FeO t/(FeO t + MgO) vs. Al 2O 3/(K 2O/Na 2O) diagrams are suggested for discrimination of oxidized and reduced A-type granites. Experimental data indicate that, besides pressure, the nature of A-type granites is dependent of ƒO 2 conditions and the water content of magma sources. Oxidized A-type magmas are considered to be derived from melts with appreciable water contents (≥ 4 wt.%), originating from lower crustal quartz-feldspathic igneous sources under oxidizing conditions, and which had clinopyroxene as an important residual phase. Reduced A-type granites may be derived from quartz-feldspathic igneous sources with a metasedimentary component or, alternatively, from differentiated tholeiitic sources. The imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct A-type granite groups. Assuming conditions near the NNO buffer as a minimum for oxidized granites, magnetite-bearing granites formed near FMQ buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced A-type granites and, respectively, magnetite-series and ilmenite-series granites is not always observed. 相似文献
5.
林西县朝阳沟地区位于华北板块与西伯利亚板块之间的兴蒙造山带的东部.朝阳沟花岗岩体的LA-ICP-MS锆石U-Pb测年结果表明,岩体的侵位时间为早白垩世（126.2±2.5 Ma）.地球化学分析显示,该花岗岩体具有富硅（SiO 2=68.5%~72.1%）、铝（Al 2O 3=14.05%~14.95%）、碱（K 2O+Na 2O=8.34%~10.03%）和Fe 2O（3 2.27%~4.15%）的特点,属于准铝质碱性花岗岩系列;A/CNK=0.97~1.09,Mg #值较高,为12.66~15.02;稀土配分曲线呈明显的右倾型,轻、重稀土元素分馏明显;具有显著的Eu负异常和Ba、Ta、Nb、Sr、P、Ti亏损,富集Rb、Th、U、K等大离子亲石元素.在（Yb+Nb）-Rb判别图解和R1-R2判别图解中,样品全部落于后碰撞花岗岩区和造山晚期花岗岩区.结合区域地质背景,认为该岩体形成于后碰撞伸展构造环境中. 相似文献
6.
A detailed study of Oldoinyo Lengai has led to the recognition of two major cone-building stages. An early, predominantly phonolitic stage, Lengai I, forms the southern cone. The recent nephelinitic Lengai II developed following a major sector collapse event over Lengai I. Petrography of Lengai II lavas show that nephelinite is combeite- and wollastonite-bearing. All Oldoinyo Lengai lavas are peralkaline and highly evolved in terms of low Mg#, Ni and Cr values. Within the unique Lengai II combeite–wollastonite–nephelinite (CWN) peralkalinity increases with time to extreme values (Na + K)/Al = 2.36. Mineralogical expression of peralkalinity is the presence of combeite and Na-rich clinopyroxene. In addition, exceptionally high Fe 2O 3 (up to 10.28 wt.%) in nepheline is an indicator for alumina deficiency. Combeite also shows high Fe 3+. Phonolite and CWN of Lengai I and Lengai II show similarly enriched LILE and LREE values and generally parallel patterns in PM normalized and REE plots. 相似文献
7.
A garnet–pyroxene rock containing abundant Ti-clinohumite (ca. 40 vol.%) occurs along with eclogites as small blocks in quartzo-feldsparthic gneiss in the southern end of the Chinese Su-Lu ultrahigh-pressure (UHP) metamorphic terrane. It consists of three aggregates: (1) Ti-clinohumite-dominated aggregate with interstitial garnet and pyroxene, (2) garnet+pyroxene aggregate with Ti-clinohumite inclusions, and (3) Ti-clinohumite-free aggregate dominated by garnet. Apatite, phlogopite, zircon, hematite, pentlandite, and an unknown Ni-Fe-volatile-Si (NFVS) mineral, which is replaced by Ni-greenalite, occur as accessories. Serpentine is the major secondary mineral. Garnet (Prp 63.9–64.6Alm 25.8–26.9Grs 1.4–7.9Uva 0.5–7.6Sps 1.0) in all three aggregates is pyrope-rich with very low grossular component, with that in the aggregate (2) most enriched in Cr (Cr 2O 3=2.55 wt.%). Orthopyroxene is depleted in Al (Al 2O 3=0.16 wt.% in the cores) and Ca (CaO=0.06–0.09 wt.% in the cores), with X Mg (Mg/(Mg+Fe)) values at ca. 0.900. Clinopyroxene is chromian diopside with Fe 3+≥Fe 2+. Matrix clinopyroxene has a lower X Mg (0.862) than that (0.887) included in Ti-clinohumite. The rock contains modest amount of heavy rare earth elements (HREE) (10 to 12×C1 chondrite), with significant enrichment in Cr, Co, Ni, V, Sr, and light rare earth elements (LREE) (22 to 33×C1 chondrite). The clinopyroxene is very enriched in Cr (Cr 2O 3 is up to 2.09 wt.% in the cores) and Sr (ca. 350 ppm) and LREE (Ce N/Yb N=157.7). Ti-clinohumite is enriched in Ni (1981 ppm), Co (123 ppm), and Nb (85 ppm). While it is possible to enrich ultramafites in incompatible elements in a subducted slab, the high Al, Fe, Ti, and low Si, Ca, and Na contents in the Ti-clinohumite rock are difficult to account for by crustal metasomatism of an ultramafite. On the other hand, the similarity in major and trace element compositions and their systematic variations between the Ti-clinohumite-garnet-pyroxene rock of this study and those of Mg-metasomatised Fe–Ti gabbros reported in the literature suggest that crustal metasomatism occurred in a gabbroic protolith, which resulted in addition of Cr, Co, Ni, and Mg and removal of Si, Ca, Na, Al, and Fe. This implies that the rock was in contact with an ultramafite at low pressure. During subsequent subduction, the metagabbro was thrust into the country gneiss, where gneiss-derived hydrous fluids caused enrichment of Sr and LREE in recrystallised clinopyroxene. P–T estimates for the high-pressure assemblage are ca. 4.2 GPa and ca. 760 °C, compatible with those for the eclogites and gneisses in this terrane. It is possible that the Ti-clinohumite-garnet-pyroxene rock and associated eclogites represent remnants of former oceanic crust that was subducted to a great depth. 相似文献
8.
The solubility of ZrO 2 in rutile is strongly temperature-dependent and has been identified as a potentially powerful thermometer when the rutile coexists with an appropriate buffer assemblage, e.g. zircon + quartz. In combination with experimental data at 10 kbar, previous consideration of data on natural rutile has not identified a pressure dependence for the thermometer. However, the expected volume change as a result of substitution of the larger Zr 4+ cation for Ti 4+ suggests that the Zr content of rutile should decrease with increasing pressure. To investigate the pressure dependence of the thermometer, piston cylinder (at 10, 20 & 30 kbar) and 1 atm furnace experiments were performed in the system ZrO 2-TiO 2-SiO 2. The solubility of ZrO 2 in rutile, in the presence of zircon and quartz was reversed at each pressure value. From these experiments, the thermodynamics of the end-member reaction ZrSiO 4 = SiO 2 + ZrO 2 (in rutile) have been determined. There is a secondary pressure effect accompanying the primary temperature dependence of the Zr content of rutile. New thermometer equations are, in the α -quartz field: in the β -quartz field and in the coesite field in which φ is ppm Zr, P is in kbar and R is the gas constant, 0.0083144 kJ K −1. Thermometric results using these equations are shown for a range of geological settings. 相似文献
9.
西藏东南缘记录的唐加-松多古特提斯增生杂岩带对认识古特提斯洋晚古生代的构造演化提供了新的证据.针对该杂岩带中新发现的长英质凝灰岩开展了全岩主、微量元素,锆石LA-ICP-MS U-Pb定年和锆石Hf同位素组成的研究.结果显示冲尼凝灰岩喷发于278~275 Ma,具有较高的SiO 2含量（63.47%~72.65%）、Al 2O 3含量（14.53%~21.31%）,较低的K 2O含量（1.30%~2.51%）和TiO 2含量（0.50%~1.17%）,MgO含量较低,介于0.92%~2.00%,Mg#范围在19.9~34.2（均低于40）.富集大离子亲石元素（LILE）、亏损高场强元素（HFSE）.锆石具有较高的ε Hf（t）值（+10.2~+14.4）和相对年轻的地壳模式年龄T DMc=351~621 Ma,认为冲尼凝灰岩是唐加-松多古特提斯洋向北俯冲背景下的新生地壳部分熔融的产物,洋盆俯冲消减的开始时代不晚于早二叠世,并且在早二叠世拉萨地体东南缘存在新生地壳生长事件. 相似文献
10.
High-calcium, nepheline-normative ankaramitic basalts (MgO > 10 wt.%, CaO/Al 2O 3 > 1) from Rinjani volcano, Lombok (Sunda arc, Indonesia) contain phenocrysts of clinopyroxene and olivine (Fo 85–92) with inclusions of spinel (Cr# 58–77) and crystallised melt. Olivine crystals have variable but on average low NiO (0.10–0.23 wt.%) and high CaO (0.22–0.35 wt.%) contents for their forsterite number. The CaO content of Fo 89–91 olivine is negatively correlated with the Al 2O 3 content of enclosed spinel (9–15 wt.%) and positively correlated with the CaO/Al 2O 3 ratios of melt inclusions (0.9–1.5). Major and trace element patterns of melt inclusions are similar to that of the host rock, indicating that the magma could have formed by accumulation of small batches of melt, with compositions similar to the melt inclusions. The liquidus temperature of the magma was 1275 °C, and its oxygen fugacity ≤ FMQ + 2.5. Correlations between K 2O, Zr, Th and LREE in the melt inclusions are interpreted to reflect variable degrees of melting of the source; correlations between Al 2O 3, Na 2O, Y and HREE are influenced by variations in the mineralogy of the source. The melts probably formed from a water-poor, clinopyroxene-rich mantle source. 相似文献
11.
The dissolution and precipitation rates of boehmite, AlOOH, at 100.3 °C and limited precipitation kinetics of gibbsite, Al(OH) 3, at 50.0 °C were measured in neutral to basic solutions at 0.1 molal ionic strength (NaCl + NaOH + NaAl(OH) 4) near-equilibrium using a pH-jump technique with a hydrogen-electrode concentration cell. This approach allowed relatively rapid reactions to be studied from under- and over-saturation by continuous in situ pH monitoring after addition of basic or acidic titrant, respectively, to a pre-equilibrated, well-stirred suspension of the solid powder. The magnitude of each perturbation was kept small to maintain near-equilibrium conditions. For the case of boehmite, multiple pH-jumps at different starting pHs from over- and under-saturated solutions gave the same observed, first order rate constant consistent with the simple or elementary reaction: . This relaxation technique allowed us to apply a steady-state approximation to the change in aluminum concentration within the overall principle of detailed balancing and gave a resulting mean rate constant, (2.2 ± 0.3) × 10−5 kg m−2 s−1, corresponding to a 1σ uncertainty of 15%, in good agreement with those obtained from the traditional approach of considering the rate of reaction as a function of saturation index. Using the more traditional treatment, all dissolution and precipitation data for boehmite at 100.3 °C were found to follow closely the simple rate expression: Rnet,boehmite=10-5.485{mOH-}{1-exp(ΔGr/RT)}, with Rnet in units of mol m−2 s−1. This is consistent with Transition State Theory for a reversible elementary reaction that is first order in OH− concentration involving a single critical activated complex. The relationship applies over the experimental ΔGr range of 0.4–5.5 kJ mol−1 for precipitation and −0.1 to −1.9 kJ mol−1 for dissolution, and the pHm ≡ −log(mH+) range of 6–9.6. The gibbsite precipitation data at 50 °C could also be treated adequately with the same model:Rnet,gibbsite=10-5.86{mOH-}{1-exp(ΔGr/RT)}, over a more limited experimental range of ΔGr (0.7–3.7 kJ mol−1) and pHm (8.2–9.7). 相似文献
12.
对东天山大白石头南片麻状花岗岩的研究,可以为新元古代早期Rodinia超大陆汇聚事件提供约束.在野外调查和岩相学研究的基础上,对该片麻状花岗岩开展了LA-ICP-MS锆石U-Pb定年、全岩地球化学和锆石原位Hf同位素分析.研究显示,片麻状花岗岩LA-ICP-MS锆石U-Pb年龄为922.7±7.9 Ma.岩石富SiO 2（70.04%~71.60%）、碱（Na 2O+K 2O=5.93%~6.58%）、Al 2O 3（13.88%~14.91%）和低MgO（1.13%~1.29%）.岩石Al 2O 3/TiO 2（25~27）小于100,CaO/Na 2O（0.7）大于0.3,K 2O/Na 2O（1.6~2.2）大于1.0,富集大离子亲石元素Rb、Th、K及La等,亏损Ba、Ta、Nb和Sr等,（La/Yb） N（7.29~8.11）小于10,δEu小于0.5,均显示出S型花岗岩特征.锆石ε Hf（t）均为正值（3.226 78~13.727 46）,二阶段模式年龄为920~1 598 Ma,表明新元古代花岗岩形成于大陆边缘构造环境.综合已有研究结果,可以推断天山地区出露的该期构造岩浆事件可能对应于新元古代的罗迪尼亚（Rodinia）超大陆汇聚事件. 相似文献
13.
Zircon and xenotime, from two mineralogically and chemically contrasting granite suites occurring in the Kru?né Hory/Erzgebirge Mts., display extended compositional variability with respect to abundances of Zr, Hf, REE, Y, P, Th, Ca, Al, Fe and As. According to their geochemical signatures, P-rich (S-type) and P-poor (A-type) granites could be distinguished here. Both granite suites display high Ga/Al ratios (>2.6) and according to FeO tot./(FeO tot. + MgO) ratio can be classified as ferrous granites. Consequently, the both ratios cannot be used for discrimination S- and A-type granites. Both minerals are characterized by a variety of complex zircon-xenotime textures. They are usually strong hydrated and enriched in F. Zircon from P-rich granites displays a significant enrichment in P (up 0.24 apfu P), whereas zircon from P-poor granites has lower P and higher Y (up to 0.15 apfu Y). The xenotime-type substitution is the most important mechanism of isomorphic substitution in zircon in both granite suites. Zircon from both granite suites is typically enriched in Hf, especially unaltered zircon from P-rich granites (up to 8.2 wt. % HfO 2). However in altered zircons the Hf/Zr ratio is higher in the P-poor granites. The Hf-rich zircon from unaltered P-rich granite crystallised from low temperature granite melt, whereas altered zircons crystallised during post-magmatic hydrothermal alteration (greisenization). Xenotime from P-poor granites displays a considerable enrichment in HREE (up to 40 mol. % HREEPO 4) compared to xenotime from P-rich granites (up to 20 mol. % HREEPO 4). Xenotime compositions from P-rich granites are influenced by brabantite-type substitution, whereas for xenotime from P-poor granites the huttonite-type substitution is dominant. Unusual enrichments in HREE is significant for xenotime from P-poor granites, especially in Yb (up to 0.17 apfu Yb) and Dy (up to 0.11 apfu). 相似文献
14.
祁漫塔格构造带是东昆仑造山带的重要组成部分,在青藏高原北部早古生代汇聚板块边缘地质研究中具有重要意义.祁漫塔格构造带在早古生代通常被认为是弧或弧后盆地,但其早期形成阶段的地质记录尚未报道,初始俯冲的时代也有待进一步限定.本文选择了祁漫塔格东段拉陵高里河地区祁漫塔格群火山岩和闪长岩进行岩石学、地球化学、锆石U-Pb年龄学... 相似文献
15.
The Fe 2+–Mg distribution coefficients between sapphirine and spinel: were experimentally determined at pressures of 9–13 kbar and temperatures of 950–1150 °C using a natural ultrahigh-temperature (UHT) granulite with paragenesis of these minerals from the Napier Complex in East Antarctica [ XMg = Mg / (Fe + Mg); XFe = Fe / (Fe + Mg)]. A new sapphirine–spinel geothermometer has been obtained as: We applied the exchange thermometer to UHT or high-grade metamorphic rocks that were reported from various complexes in the world. If the KD values of 2.63–4.34 obtained from low-Cr mineral pairs such as XCrSpr < 0.016 and XCrSpl < 0.047 were substituted into the equation, their temperature conditions would be estimated as 806–1050 °C at 11 kbar. The XCr means Cr / (Al + Cr(+ Fe3+)). These temperatures are reasonable retrograde or near peak metamorphic condition. 相似文献
16.
中国东部花岗岩类141个Mg-Fe云母的化学成分将近90%的变化属于八面体层内的类质同象置换,置换矢量Mg 1Fe +2和Fe -3+2(R Ⅵ+3) -2□ Ⅵ组成了天然黑云母平面,大约80%的变化应当解释为基本置换8Mg 1Fe +2+Fe -3+2(R Ⅵ+3) 2□ Ⅵ.这些是Mg-Fe云母在广泛的自然条件下表现出来的最主要的晶体化学关系。文中还提出了置换矢量的长度、分量和以及电价和三个参数,用以识别矿物化学成分变化的类质同象置换特征。 相似文献
17.
对内蒙古大沟井钒矿床的矿石样品进行了系统的常量和微量元素、X射线粉晶衍射以及显微能谱分析,探讨矿石中钒的赋存形式、形成环境以及保存条件等.研究结果表明,矿石样品中P2 O5含量变化范围为0.81％～6.14％,显示其磷含量普遍较高,反映地层中较为富集磷;样品烧失量的范围为7.75％～41.24％,比较大,反映云母及泥质... 相似文献
18.
A decrease in temperature (Δ T up to 45.5 °C) and chloride concentration (ΔCl up to 4.65 mol/l) characterises the brine–seawater boundary in the Atlantis-II, Discovery, and Kebrit Deeps of the Red Sea, where redox conditions change from anoxic to oxic over a boundary layer several meters thick. High-resolution (100 cm) profiles of the methane concentration, stable carbon isotope ratio of methane, and redox-sensitive tracers (O 2, Mn 4+/Mn 2+, Fe 3+/Fe 2+, and SO 42−) were measured across the brine–seawater boundary layer to investigate methane fluxes and secondary methane oxidation processes. Substantial amounts of thermogenic hydrocarbons are found in the deep brines (mostly methane, with a maximum concentration up to 4.8×105 nmol/l), and steep methane concentration gradients mainly controlled by diffusive flow characterize the brine–seawater boundary (maximum of 2×105 nmol/l/m in Kebrit Deep). However, locally the actual methane concentration profiles deviate from theoretical diffusion-controlled concentration profiles and extremely positive δ13C–CH4 values can be found (up to +49‰ PDB in the Discovery Deep). Both, the actual CH4 concentration profiles and the carbon-13 enrichment in the residual CH4 of the Atlantis-II and Discovery Deeps indicate consumption (oxidation) of 12C-rich CH4 under suboxic conditions (probably utilizing readily available—up to 2000 μmol/l—Mn(IV)-oxihydroxides as electron acceptor). Thus, a combined diffusion–oxidation model was used to calculate methane fluxes of 0.3–393 kg/year across the brine–seawater boundary layer. Assuming steady-state conditions, this slow loss of methane from the brines into the Red Sea bottom water reflects a low thermogenic hydrocarbon input into the deep brines. 相似文献
19.
位于南秦岭武当地区的天宝铌矿床是我国典型的碱性火山岩型铌矿床,但其岩浆成因、演化及成矿机理等方面的研究十分薄弱.天宝碱性火山岩主要分为粗面质岩石和碱性玄武岩两大类,两者在空间上紧密共生.在详细的岩(矿)相学及榍石显微结构研究的基础上,利用LA-ICP-MS分析技术对天宝粗面质火山岩中的榍石进行原位U-Pb定年,并利用E... 相似文献
20.
The igneous rocks of the Katzenbuckel, Southwest Germany, represent a unique and unusual alkaline to peralkaline association within the European Volcanic Province. The magmatic activity can be subdivided into two main phases. Phase I comprises the main rock bodies of phonolite and nepheline syenite, which were later intruded by different peralkaline dyke rocks (tinguaites and alkali feldspar syenite dykes) of phase II. The dyke assemblage was accompanied by magnetite and apatite veins and was followed by a late-stage pneumatolytic activity causing autometasomatic alterations. As is typical for alkaline to peralkaline igneous rocks, early mafic minerals of phase I rocks comprise olivine, augite and Fe–Ti oxides, which are substituted in the course of fractionation by Na-amphibole and Na-pyroxene. For the early magmatic stage, calculated temperatures range between 880 and 780 °C with low silica activities (0.4 to 0.6) but high relative oxygen fugacities between 0.5 and 1.9 log units above the FMQ buffer. Even higher oxygen fugacities (above the HM buffer) are indicated for the autometasomatic alteration, which occurred at temperatures between 585 and 780 °C and resulted in the formation of pseudobrookite and hematite. The unusually high oxygen fugacities (even during the early magmatic stage) are recorded by the major element compositions of the mafic minerals (forsterite content in olivine between 68 and 78 mol%, up to 6.2 wt.% ZrO2 and 8.5 wt.% TiO2 in clinopyroxene), the unusual mineral assemblages (pseudobrookite, freudenbergite) and by the enrichment of Fe3+ in the felsic minerals (up to 2.8 wt.% Fe2O3 in alkali feldspar and up to 2.6 wt.% Fe2O3 in nepheline). These observations point to a metasomatically enriched and highly oxidized lithospheric mantle as a major source for the Katzenbuckel melts. 相似文献
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