高温高压下叶蜡石脱水电导率实验

在压力为1.0～4.0 GPa、温度为873～1 223 K下采用Sarltron-1260阻抗/增益-相位阻抗谱分析仪测定了叶蜡石的电导率.实验结果表明,电导率与温度间的关系符合Arrenhius关系式;叶蜡石的脱水引起电导率的突然变化,使电导率急剧上升;脱水前,电导率随着压力增大而减小,其导电机制为电子导电;脱水后,电导率随着压力增大而增大,其导电机制为离子导电.     

A New Report of Serpentinites from Northern Central Indian Ridge(at 6°S)—An Implication for Hydrothermal Activity

Serpentinites from the inside corner high(6°38.5′S/68°19.34′E)from the Northern Central Indian Ridge(NCIR)are comprised mainly of high Mg-rich lizardite and chrysotile pseudomorphs with varying morphologies.‘Mesh rim',‘window',‘hourglass' and‘bastite' are the most common textures displayed by both chrysotile and lizardite.Numerous chrysotile veins in association with cross cutting magnetite veins indicate an advanced stage of serpentinisation.The relatively high abundance of chrysotile and lizardite suggest their close association and formation at a temperature below 250℃. Abundant‘mesh rim'and‘bastite'texture and variegated matrix reveal that the present serpentinites might have formed due to the interaction of harzburgite and seawater.Positive Eu anomaly(Eu/Eu~* up to 3.38),higher La/Sm(up to 4.40)and Nb/La(up to 6.34)ratios suggest substantial hydrothermal influence during the formation of the serpentinites.     

Formation and emplacement of the Dolodrook serpentinite body,Lachlan Orogen,Victoria

Serpentinised peridotite and ultramafic breccia make up an approximately 5 km‐long, 1 km‐wide fault slice within turbidites in the Dolodrook River region of the central Lachlan Orogen. The serpentinite body is surrounded by juvenile, mafic‐ultramafic sedimentary rocks with Cambrian limestone olistoliths representative of locally derived debris flows, and Middle to Upper Ordovician black shale, chert, sandstone and mudstone. The antiformal geometry and nature of the ultramafic breccia and mafic‐ultramafic sedimentary rocks (Garvey Gully Formation) indicate that the serpentinite body may have been either a former oceanic transform fault zone, a Marianas‐style serpentine seamount or a combination of these. Observations of modern‐day forearc regions show that faulting processes have led to the exposure of serpentinised peridotite horst blocks and serpentine mud volcanoes that have intruded along fault conduits (e.g. Marianas and Izu‐Bonin forearcs). At Dolodrook, the structural and metamorphic relationships with the surrounding rocks, and the lithological associations, have much in common with these observations and indicate that Dolodrook may be an ancient, on‐land example of an accreted seamount or oceanic topographic high. Structural relationships, the very low metamorphic grade of all rocks at Dolodrook, and the presence of broken formation developed in not‐fully lithified Middle to Upper Ordovician sandstone and mudstone indicate that the serpentinite body was emplaced at shallow crustal levels within the turbidite wedge (Tabberabbera Zone), possibly as an offscraped topographic high during marginal basin closure. The Dolodrook serpentinite has previously been inferred as part of the Cambrian igneous sequence (‘greenstones’) exposed in the Governor, Mt Wellington and Heathcote Fault Zones, but structural and metamorphic relationships with surrounding rocks, and the Cambrian tectonic setting in which it formed, have remained speculative.     

盐卤硼酸盐化学ⅩⅫ.：天然钠硼解石矿物的热行为研究

通过对小柴旦天然沉积钠硼解石矿物(Na_2O·2CaO·5B_2O_3·16H_2O)的热分析研究确定了热脱水反应的阶段、温度和产物,并由DTA、TG、DTG和DSC确定了钠硼解石脱水、重结晶和熔融的温度,给出了脱水过程的热化学反应方程式及各个脱水阶段产物的X—ray粉末衍射和红外光谱结果。     

The characteristics and significance of pure nitrogen fluid inclusions in Xikuangshan copper deposit, Dongchuan, Yunnan of China

Nitrogen is an important gas-phase element in geological fluid,and normally hosted in mineralizing fluid,in which the content is very low.Nearly 100% mol pure nitrogen inclusion was discovered in copper-bearing quartz vein from Xikuangshan copper deposit,Dongchuan,Yunnan Province in this research.It is between 3 and 5 μm,in shape of ellipse,occurring in group or independence without obvious orientation,and co-existed with primary gas-liquid inclusions.Inclusion analysis by using laser Raman spectroscopy shows its composition is dominated by nitrogen with minor H2O.High concentration of N2 may be the necessary and sufficient condition for forming this kind of inclusion.It is suggested that there are two genesis modes of pure N2 inclusion in the deposit,oxida-tion-devolativation of mantle matter and trap of metasomatic fluid in organic matter decomposition in Luoxue For-mation dolomite.In conclusion,pure N2 inclusion hardly presents in the ore-forming fluids,which providing a new evidence for the genesis of Xikuangshan type copper deposit.     

Mantle peridotite in newly discovered far-inland subduction complex,southwest Arizona: initial report

The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe³⁺/Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al₂O₃, Cr₂O₃, and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.     

Interactions between serpentinite devolatilization, metasomatism and strike-slip strain localization during deep-crustal shearing in the Eastern Alps

The Greiner shear zone in the Tauern Window, Eastern Alps, changes from a zone of distributed (dominantly sinistral) shear in supracrustal rocks to a series of narrow, gully forming dextral splays where it enters basement gneisses. Within these splays, granodiorite is transformed into quartz‐poor biotite and/or chlorite schists, reflecting hydration, removal of Si, Ca and Na, and concentration of Fe, Mg and Al. Stable isotope analyses show a prominent increase in δD and a decrease in δ¹⁸O from granodiorite into the shear zones. These changes indicate significant channelized flow of an externally derived, low‐δ¹⁸O, high‐δD fluid through the shear zones. The shear zone schists are chemically similar to blackwall zones developed around serpentinite bodies elsewhere in the Greiner zone and the stable isotope data support alteration via serpentinite‐derived fluid. Monazite in schist from one shear zone yields spot dates of 29–20 Ma, indicating that the fluid influx and switch from sinistral to dextral shear occurred at or shortly after the thermal peak of the Alpine orogeny (c. 30 Ma). We suggest that Alpine metamorphism of serpentinites released large amounts of high‐δD, low‐δ¹⁸O, Si‐undersaturated, Fe + Mg‐saturated fluids that became channelized along prior zones of weakness in the granodiorite. Infiltration of this fluid facilitated growth of chlorite and biotite, which in turn localized later dextral strain in the narrow splays via cleavage‐parallel slip. This dextral strain event can be linked to other structures that accommodated tectonic escape of major crustal blocks during dextral transpression in the Eastern Alps. This study shows that serpentinite devolatilization can play an important role in modifying both the chemistry and rheology of surrounding rocks during orogenesis.     

A revised petrological model for subducted oceanic crust: Insights from phase equilibrium modelling

Many geological and geodynamical studies of metamorphism in subduction zones have relied upon worldwide compilations of modelled slab‐top pressure–temperature (P–T) conditions, although recent evaluation of such data sets suggests that these predictions are ~100–300°C colder at any given pressure than the conditions recorded by exhumed metamorphic rocks. As such, geochemical, petrological and geophysical interpretations formulated using such ‘cold’ assumptions may be subject to error and uncertainty. Here, we apply thermodynamic phase equilibrium calculations to forward‐model how phase assemblages, the P–T conditions of key devolatilization reactions and the effect of densification with depth vary for typical mid‐ocean ridge basalt (MORB) along these newly defined ‘hotter’ subduction zone geotherms for cold, warm and average environments. The depth and extent of devolatilization of MORB is strongly dependent on the geotherm along which the oceanic crust subducts. At the onset of subduction along a warm geotherm, metabasites contain ~3   wt% H₂O and release ~45% of this fluid in a single pulse at ~20 km, correlating with chlorite and epidote breakdown. Below these depths, metamorphosed MORB will dehydrate incrementally due to gradual amphibole breakdown, becoming almost completely dehydrated at ~70 km. Oceanic crust subducting along an average geotherm will contain ~3.5 wt% of H₂O at the onset of subduction and will release ~40% of the bulk‐rock H₂O in two fluid pluses occurring at ~30 and 50 km, correlating with chlorite breakdown. Below these depths, gradual dehydration of ~50% of the bulk‐rock H₂O due to amphibole breakdown leads to near‐complete dehydration at ~80 km. By contrast, in cold subduction zones, metamorphosed MORB will typically be H₂O‐undersaturated and will dehydrate gradually at different depths, transporting ~0.6 wt% H₂O to sub‐arc depths. As the volume of fluid released via these dehydration reactions differs strongly between cold, average and warm scenarios, different degrees of serpentinization of the mantle forearc are expected worldwide and thus, the efficacy of buoyancy‐driven exhumation should vary strongly in space and time. Metabasites subducting along a warm and average geotherm will liberate most of the fluids at shallower depths, suggesting that these lithologies might preferentially exhume, yet MORB subducting along cold geotherms will not dehydrate until greater depths, inhibiting its return to the surface. Critically, while we show that metabasites formed along warmer geotherms are denser than metabasites from colder geotherms at any equivalent depth, buoyancy‐driven exhumation provoked by fluids plays a notably more important role in exhumation potential than the overall bulk‐rock ‘metamorphic’ density. Furthermore, we show that lawsonite does not stabilize in average and warmer subduction zones, which provides a simple but important solution to the mismatch between its predicted abundance in experiments and its rarity in nature and argues against its use as a reliable petrogenetic indicator of subduction throughout deep geological time, as has been suggested by some recent studies.     

班公湖-怒江缝合带西段洞错石榴石麻粒岩岩石学特征及其构造意义

班公湖-怒江缝合带是青藏高原中部一条重要的构造边界,将北部的羌塘地块与南部的拉萨地块分隔开,当前对班公湖-怒江洋壳的俯冲过程知之甚少,本文报道班公湖-怒江缝合带西段洞错地区的石榴石麻粒岩,将有助于提升班公湖-怒江缝合带构造演化过程的认识。石榴石麻粒岩呈5.0~_10mm的透镜体产出于斜长角闪岩内,后者呈构造岩片产出于班公湖-怒江缝合带内,与超基性岩片呈断层接触。石榴石麻粒岩的石榴石呈变斑晶产出,具有不规则的成分环带,石榴石核部成分较均匀,以富Ca和Mg,贫Fe和Mn为特征,石榴石边缘及内部裂隙成分则与之相反,富Fe和Mn,与绿泥石和黝帘石组成的细脉共生,反映了石榴石遭受后期流体的交代作用。单斜辉石为透辉石,在其内部发现早期进变质阶段的细小石榴石与角闪石颗粒,石榴石粒径_10μm,该石榴石相对富Fe和Mn,贫Ca和Mg。角闪石均为钙质系列的角闪石,早期角闪石以相对富Na和Fe~(3+),含有微量Ba,而与晚期角闪石区分开。早期角闪石常产出于石榴石和透辉石变斑晶内部,脱水熔融生成富Ca、贫Fe~(3+)和Na的透辉石和以钠长石为主的熔融囊体,囊体内部发育斜长石和富Ba冰长石的包体/出溶体。晚期角闪石常与斜长石呈细小的后成合晶产出于石榴石边部或呈变斑晶产出于基质之中,表明斜长角闪岩系石榴石麻粒岩退变质的产物。此外,金红石斑晶常变成细粒、不规则的贫Al榍石和钛铁矿,基质中有相对富Al榍石产出。因此该麻粒岩进变质阶段矿物组合由早期石榴石(Grt__1)、早期角闪石(Amp_1)、相对富Al的榍石(Spn_1)等矿物组成。峰期矿物组合包括:核部石榴石(Grt2)+透辉石+斜长石(Pl_1)+石英+金红石+富Na和Ba的熔体,采用传统温/压计估算此峰期矿物组合形成条件为870℃、11.7kbar。退变质矿物组合有:边部石榴石(Grt3)+晚期贫Na和Fe~(3+)的角闪石(Amp_1)+黝帘石+绿泥石+斜长石(Pl_2)+白云母+贫Al榍石(Spn2)+钛铁矿。岩相学观察与地球化学示踪、变质温压估计表明洞错石榴石麻粒岩系热洋壳俯冲的产物,为前人提出的"班公-怒江洋盆包含次级小洋盆"之观点提供了变质岩石学的证据。从峰期到退变质阶段,角闪石的Fe~(3+)含量明显下降,铁铝榴石含量明显升高,指示该麻粒岩经历了还原反应,早期角闪石脱水熔融时被还原,生成了高氧逸度的埃达克质岩浆,促进了Cu、Au等成矿元素从俯冲热洋壳的活化与迁移至弧岩浆之中,因此本文的研究提供了一个探讨俯冲热洋壳与上覆地幔楔之间化学反应的研究案例,这有助于理解班公-怒江成矿带内众多斑岩型铜、金矿床的形成。     

Dehydration and anatexis of UHP metagranite during continental collision in the Sulu orogen

Dehydration and anatexis of ultrahigh‐pressure (UHP) metamorphic rocks during continental collision are two key processes that have great bearing on the physicochemical properties of deeply subducted continental crust at mantle depths. Determining the time and P–T conditions at which such events take place is needed to understand subduction‐zone tectonism. A combined petrological and zirconological study of UHP metagranite from the Sulu orogen reveals differential behaviours of dehydration and anatexis between two samples from the same UHP slice. The zircon mantle domains in one sample record eclogite facies dehydration metamorphism at 236 ± 5 Ma during subduction, exhibiting low REE contents, steep MREE–HREE patterns without negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 651–750 °C and inclusions of quartz, apatite and jadeite. A second mantle domain records high‐T anatexis at 223 ± 3 Ma during exhumation, showing high REE contents, steeper MREE–HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, high temperatures of 698–879 °C and multiphase solid inclusions of albite + muscovite + quartz. In contrast, in a second sample, one zircon mantle domain records limited hydration anatexis at 237 ± 3 Ma during subduction, exhibiting high REE contents, steep MREE–HREE patterns with marked negative Eu anomalies, high Hf, Nb, Ta, Th and U contents, medium temperatures of 601–717 °C and multiphase solid inclusions of albite + muscovite + hydrohalite. A second mantle domain in this sample records a low‐T dehydration metamorphism throughout the whole continental collision in the Triassic, showing low REE contents, steep MREE–HREE patterns with weakly negative Eu anomalies, low Th, Nb and Ta contents, low temperatures of 524–669 °C and anhydrite + gas inclusions. Garnet, phengite and allanite/epidote in these two samples also exhibit different variations in texture and major‐trace element compositions, in accordance with the zircon records. The distinct P–T–t paths for these two samples suggest separate processes of dehydration and anatexis, which are ascribed to the different geothermal gradients at different positions inside the same crustal slice during continental subduction‐zone metamorphism. Therefore, the subducting continental crust underwent variable extents of dehydration and anatexis in response to the change in subduction‐zone P–T conditions.