实验室合成石盐包裹体的均一温度以及古气候意义

石盐是表生环境下形成蒸发岩系的主要矿物,在形成过程中捕获大量的原生包裹体。通过低温冷冻测温技术,可以得到石盐中原生包裹体的一系列均一温度,如何正确理解这些数据是应用石盐原生包裹体恢复古气候的关键。石盐沉积可以发生在气水界面以及水体底部,其均一温度记录了卤水结晶的温度。Lowenstein et al. (1998) 曾经用水浴法在恒定水温下合成了人工合成石盐,进行原生包裹体的均一温度研究,然而如何用水温来恢复古气温是古环境解释的关键。本次实验通过40℃下(气温,烘箱中蒸发)人工合成石盐,在岩相学观察基础上,对形成于气水界面漏斗晶中的包裹体和在底水沉积人字晶中的包裹体的均一温度进行低温冷冻测温法测定。测温过程中石盐包裹体可以得到一系列均一温度(10.6~39.9℃),而只有最大均一温度才能反映卤水结晶时的温度,两种类型的包裹体也显示了相似的最大均一温度。因此在浅水环境下,两种包裹体都可以用来反映古气温。     

罗布泊卤水室内蒸发及天然石盐包裹体均一温度分析探讨

罗布泊卤水经室内恒温（30℃和52℃）蒸发结晶析出石盐，其内含有丰富的液相包裹体。测试这些石盐液相包裹体均一温度，发现仅有少量包裹体均一温度与石盐结晶卤水温度一致，大多数均一温度明显偏离结晶温度，大多数高出卤水温度约20℃～90℃，少数低于卤水温度约6℃～30℃，罗布泊盐湖自然沉积的石盐液相包裹体均一温度也存在类似情况，除少部分数据与当地卤水温度相近，大多也存在明显的差异。说明石盐包裹体均一温度可能受制于多个因数影响。因此，用石盐包裹体均一温度恢复沉积时期古水温或流体介质温度时，不能简单应用，需要综合分析才能从中提取可靠的地质信息。     

柴达木盆地西部中更新世气候转型期的古水温:来自SG-1钻孔石盐流体包裹体的证据

中更新世气候转型事件(MPT)是全球性冷气候事件,在柴达木盆地也有记录,但关于该事件形成时的古温度数据较少。石盐原生流体包裹体形成于浅水环境,其均一温度可直接反映晶体形成时的卤水温度,是恢复古温度常用的指标之一。本文选择柴达木盆地西部钻孔SG-1中1.22~0.88 Ma期间的石盐晶体进行流体包裹体均一温度测试,共获得390个石盐流体包裹体数据。其均一温度最高为50 ℃,最低为6.8 ℃,90%以上温差值在10 ℃以内,且石盐流体包裹体大小与温度没有明显线性相关关系,这说明SG-1钻孔石盐流体包裹体被捕获后没有受到后期热液的改造。均一温度数据反映了石盐沉积时的古水温特征。石盐晶体主要在暖季析出,原生流体包裹体恢复的古水温可能是暖季节的温度。均一温度的最高值可能受到热液和气候的共同作用。MPT时期,石盐流体包裹体均一温度(中位值T_h,med)接近于现代盆地7月份大气温度的平均值,高于盆地的全年温度及MPT时期的全球气温,与MPT时期地中海的海水表面温度相当,均一温度的平均值(T_h,avg)高于以上温度。SG-1钻孔记录的柴达木盆地MPT事件最冷期出现在约1.165~1.0 Ma。     

石盐中流体包裹体在古气候研究中的应用进展

蒸发岩沉积在地质历史上分布十分广泛。其中的石盐属于广温相矿物,其内部捕获的流体包裹体记录了大量石盐沉积时的古温度信息。石盐中流体包裹体的冷冻测温法已经广泛应用于包裹体均一温度的测定。通过该方法,目前已经初步建立了包裹体记录的均一温度与古气温的关系,即石盐中纯液相原生流体包裹体均一温度最大值与卤水结晶温度符合,与最大平均气温关系密切,且形成于卤水底层"人"字型晶石盐中的流体包裹体记录的温度信息更能代表较长时间尺度的古气温变化等。除此以外,成岩作用对于石盐中流体包裹体的温度记录的影响也被广泛讨论,尽管石盐矿物中流体包裹体可能发生变形以及热再平衡等一系列的变化,但"人"字型结构的原生石盐晶体特征以及石盐颗粒云雾状环带中呈负立方体形状的流体包裹体为原生包裹体等一系列识别标志的建立,为我们利用石盐中流体包裹体进行古温度的研究提供了很好的样品基础。通过对石盐中原生流体包裹体最大均一温度测试,已经得到了一些现代盐湖以及古代盐类沉积时的环境温度信息,并在古气温波动方面也展示了一定前景。尽管石盐中流体包裹体在古气候研究中取得了初步成果,但仍存在一些亟待解决的问题:如除最大均一温度的其它温度数据代表的环境温度意义问题;通过对盐类地质剖面进行连续的包裹体测温建立高分辨率定量古温度变化记录等,也尚未有大量文章报道。我国盐类资源分布广泛,石盐包裹体测温技术的应用,对于盐类发育地区古气候环境定量化信息提取具有十分重要的意义。     

蒸发岩盆地古环境的直接记录:来自石盐流体包裹体的证据

蒸发岩是保存古环境信息的宝库,而石盐是蒸发岩盆地完全干涸后最主要的矿物。石盐矿物具有很好的封闭性,在浅埋深状态就会固结成岩,没有孔隙并且不可压缩。石盐、石膏、钙芒硝、芒硝中的包裹体,是潟湖或陆地盐湖环境下,通过蒸发而结晶析出过程中所捕获的流体(液体和/或气体)。因此,在石盐矿物的内部可以保存下来良好的原生石盐流体包裹体,它们记录了原始海洋/盐湖的温度、化学组分和大气成分的信息,为古环境研究提供了绝妙的直接记录。其中石盐原生流体包裹体的均一温度,可记录卤水沉积时的温度;浅水环境石盐流体包裹体记录的卤水温度近似等于气温。石盐原生流体包裹体中的卤水成分代表了海水/盐湖蒸发浓缩过程中的卤水,可通过其来推断当时的海水/盐湖水体的成分。石盐漏斗晶形成时是漂浮在卤水表面的,其原生流体包裹体可捕获当时的大气,而石盐是一种稳定的无机矿物,其原生流体包裹体可保存原始的大气信息。石盐原生流体包裹体可以提供其他传统地球化学手段无法提供的直接、精确、定量的地质记录,因此在未来的古环境研究中将成为焦点。     

四川盆地东部三叠系嘉陵江组成盐期浓缩海水古温度及其意义

四川盆地东部嘉陵江组四段石盐岩中发育大量原生的单一液相包裹体,为揭示石盐结晶时古海水的温度,恢复早三叠世古气候提供了有利条件。本文在蒸发岩岩相学研究的基础上,对流体包裹体均一温度进行了测试。测温结果表明,石盐结晶时卤水的温度在17.7~63.5℃之间,与该地区碳酸盐岩氧同位素数据所反映的温度特征基本一致,说明早三叠世时海水具有较高的温度。这有利于古海水的强烈蒸发浓缩,为成钾奠定了良好的气候条件。     

柴达木盆地大浪滩梁ZK02孔早—中更新世石盐纯液相流体包裹体均一温度及其对钾盐成矿的约束

运用低温冷冻成核方法,对柴达木盆地大浪滩地区梁ZK02孔早—中更新世石盐样品的纯液相流体包裹体进行均一温度测试,获得158个均一温度数据。所测石盐样品的镜下形貌显示有石盐“人”字晶特征,其内部流体包裹体呈负立方体晶形,且测试均一温度数据与包裹体粒度相关性不明显,指示了测试石盐及包裹体原生性良好,其最大均一温度可代表古代夏季气温。最大均一温度测试结果表明:(1)早更新世最大均一温度主要分布于25~30 ℃温度区间,高泥质、粉砂岩性特征指示早期气候较为温和,淡水补充相对丰富;晚期盐类矿物含量升高,反映干旱程度升高;个别时间段夏季温度高于40 ℃,呈高温干旱特征。(2)中更新世温度位于30~35 ℃温度区间,出现了更新世以来最高温度50.6 ℃,包裹体最大均一温度与高含量盐类矿物共同指示了高温干旱环境特征。与早更新世相比,中更新世环境温度总体呈升高趋势,蒸发盐含量升高,干旱程度增加,温度波动加剧,显示出中更新世更加不稳定的气候特点,与世界范围内中更新世总体气候特征一致,且盐类分布显示这种气候不稳定性在大浪滩地区自早更新世晚期就开始显现了。结合石盐包裹体古温度研究成果,对阿尔金山前砂砾岩孔隙含钾卤水矿成矿模式进行补充:早更新世较为温和湿润的环境下,风化盐分和N22古盐发生溶滤,汇聚形成含盐卤水,赋存于阿尔金山前冲洪积扇砂砾岩孔隙中,历经中更新世的高温蒸发,最终形成KCl砂砾岩孔隙储层卤水矿床。     

云南勐野井钾盐矿床石盐包裹体特征及古环境意义

云南江城勐野井钾盐矿床是我国唯一的一个古代氯化物型固体钾盐矿床,主要盐类矿物是石盐,其次是钾石盐和光卤石,其中钾石盐是该矿床主要的钾盐矿物。本文采用流体包裹体冷冻加热法,对该矿床含矿层中的原生单一液相石盐包裹体进行了均一温度测定,以期获得该矿床钾盐成矿和盐湖古水温方面的信息。实验结果表明,从白色石盐层→青灰色钾石盐层→含光卤石的青灰色钾石盐层,均一温度的均值呈现逐渐升高的趋势,即由23.8℃→27.7℃→38.6℃,这与盐类矿物晶出的顺序一致,即由石盐→钾石盐→光卤石。可见,上部含光卤石的钾石盐层的蒸发强度明显高于其下部的钾石盐层和底部的石盐层,说明钾盐矿层的形成是湖盆卤水不断浓缩咸化的结果,这种咸化是缘于湖盆卤水温度的不断升高。湖盆卤水温度的变化是环境气候变化的结果。因此,原生包裹体均一温度可直接反映矿床形成时湖盆的古水温,同时可推测当时的气温。     

江陵凹陷早始新世石盐流体包裹体均一温度研究及其古气候意义

近年来江陵凹陷因深层富钾卤水及钾盐矿物的发现受到各界的关注。而钾盐的形成与古环境有着密切的联系。本文通过石盐包裹体均一温度的研究,对江陵凹陷早始新世古气候进行了初步探讨,力求为确定江陵凹陷成钾有利期提供一定依据。通过对石盐样品中的单一液相流体包裹体采用冷冻法进行均一温度测定,测温结果显示其均一温度范围是14.9~32.3℃,均值23.7℃,且各样品间温度数据波动不大。由此说明,江陵凹陷早始新世新沟嘴组中段的古气温较高,且比较稳定,属于温暖干旱的气候。此外,还通过分析比对前人研究成果,发现该区域古气温自沙市组至新沟嘴组下段有明显降低的趋势,这可能与古新世—始新世极热事件(PETM)之后的降温期有关。     

NaCl-X-H2O三元体系25℃等温蒸发石盐流体包裹体均一温度实验研究

复杂卤水组分对于石盐流体包裹体均一温度的影响尚不明确,文章基于NaCl-X-H_2O(X=KCl, MgCl_2,CaCl_2, Na_2SO_4)三元卤水体系,尝试探讨K~+、Mg~(2+)、Ca~(2+)、SO_4~(2-)对石盐流体包裹体均一温度测试结果的影响。不同卤水体系最大均一温度分析结果表明,K~+、Mg~(2+)、Ca~(2+)的存在总体上会导致石盐流体包裹体均一温度偏大,SO_4~(2-)的存在对均一温度的影响很小。以NaCl-H_2O体系为参照,NaCl-Na_2SO_4-H_2O体系平均均一温度较之要低,而NaCl-KCl-H_2O、NaCl-MgCl_2-H_2O和NaCl-CaCl_2-H_2O体系与其相反。NaCl-KCl-H_2O体系中的KCl浓度与平均均一温度呈现负相关关系,NaCl-MgCl_2-H_2O、NaCl-CaCl_2-H_2O、NaCl-Na_2SO_4-H_2O体系中的w(MgCl_2)、w(CaCl_2)和w(Na_2SO_4)与平均均一温度则呈现正相关关系。平均和最大均一温度分析结果都显示出复杂卤水体系中不同离子及其浓度对石盐流体包裹体均一温度会产生影响。本研究对于具有复杂化学组分卤水结晶析出石盐均一温度的研究具有重要的参考价值。     

Cambrian skeletal halite crystals and experimental analogues

A suite of unusual highly skeletal halite pseudomorphs is described from the lower Middle Cambrian rocks that crop out at Ardmore in the Georgina Basin, northern Australia. The pseudomorphs are preserved as both moulds and casts within a dark and light colour-banded chert. They are orientated parallel to bedding and represent halite crystal growth within a brine pool. Laterally equivalent rocks are characterized by sedimentary textures and structures indicative of periodic emergence and desiccation. Laboratory grown halite crystals are also described and compared with the pseudomorphs from Ardmore. Experimentally produced halite crystals formed from either: (1) brine solutions obtained as residues after organic matter extraction from phosphate rocks, or (2) solutions of NaCI and distilled water. Both pyramidal rafted hoppers and floor nucleated cuboids formed in solutions containing only NaCI and distilled water; whereas in the residue solutions, that contained humic acids, rafted pyramidal forms were absent and crystal nucleation was restricted to the floor of the evaporative dish. As brine depth decreased the halite precipitation rate increased and resulted in a suite of excrescent, highly skeletal crystals that formed as a result of brine evaporation to dryness. The variable crystal morphology depended upon both the brine depth and slope of the evaporative dish floor. Horizontally orientated chevron halite crystals formed where the evaporative dish was inclined and precipitation preferentially occurred on cube faces. Pagoda, reticulate ridge and dendritic forms represent an increasingly skeletal crystal suite characterized by the preferential precipitation of NaCI on cube edges and corners rather than faces. Using the experimentally grown crystals as analogues the pseudomorphs at Ardmore are interpreted as forming in very shallow brine pools that evaporated to dryness.     

Paleotemperatures from fluid inclusions in halite: method verification and a 100,000 year paleotemperature record, Death Valley, CA

Maximum homogenization temperatures of fluid inclusions (Th_max) in halite (laboratory-grown crystals and modern samples, Death Valley, CA) match maximum brine temperatures during halite precipitation. Maximum brine temperatures during halite precipitation in Death Valley, late April, 1993 (34.4°C) agree with Th_max (34°C) and correlate well with average maximum air temperatures in April (31.3°C) and May (37.6°C). Th_max may be used for paleoclimate interpretations based on the close relationship between saline lake temperatures and average air temperatures from modern settings. Lower homogenization temperatures, demonstrably below the temperatures at which halite grew, are interpreted to reflect collapse of some fluid inclusion walls due to the pressure difference between the inside and outside of inclusions. By only using Th_max, the problems of anomalously low homogenization temperatures due to possible collapse of fluid inclusions are avoided. Halite samples from 30 stratigraphic intervals, 90 to 0 m (100 to 0 ka), Core DV93-1, Death Valley, CA, were used to measure homogenization temperatures of fluid inclusions. Virtually all homogenization temperatures from Core DV93-1 are below the modern Th_max of 34°C (halite precipitation late April, 1993). Lacustrine halites, deposited in a perennial saline lake 35 to 10 ka, have Th_max between 19°C and 30°C, which suggests brine temperatures approximately 4°C to 15°C below modern late April values. Ephemeral saline lake halites precipitated 60 to 35 ka have Th_max between 23°C and 28°C, 6 to 11°C below modern values. The highest Th_max value in the 100 ka record (up to 35°C) is from a halite sample formed approximately 100 ka in a climate regime somewhat colder than the modern.     

Degree of Brine Evaporation and Origin of the Mengyejing Potash Deposit: Evidence from Fluid Inclusions in Halite

The Mengyejing potash deposit is located in the southern port of the Simao Basin, Yunnan Province, and is hosted in mid-Cretaceous strata. The chemical compositions of fluid inclusions in halite crystals, collected from the level-610 adit in the deposit, were analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The results show that the brine is of the Na-K-Mg-Ca-Cl type and has K concentrations that are distinctly higher than those of Mg and Ca, unlike normal brines associated with Cretaceous halite. The high K concentrations indicate that the degree of evaporation of the ancient Mengyejing saline lake was very high, reaching the sylvite deposition stage but rarely reaching the carnallite deposition stage. The trajectory of the H and O isotopic compositions of the brines in the halite-hosted fluid inclusions corresponds to intense evaporation, indicating that the net evaporation exceeded the net inflow of brines. These brine compositions in halite-hosted fluid inclusions were likely formed by the dissolution of previously deposited K-bearing minerals by fresh continental and/or seawater, forming a type of modified seawater, with deep hydrothermal fluids potentially supplying additional potassium. The basin likely experienced multiple seawater incursion, dissolution and redeposition events in a high-temperature environment with high evaporation rates.     

青藏高原末次冰期盛冰阶的时限与干盐湖地质事件

本文根据高原盐湖原生石盐矿物包裹体流质的氢、氧稳定同位素分析以及包裹体流质的Ｎａ^＋、Ｍｇ^2＋测定讨论了青藏高原北部近五万年来的气候环境演变，论述了末次冰期盛冰阶在高原地区的时限问题，初步确定盛冰阶的时限为２１０００－１５０００ａＢ．Ｐ．，该期的平均温度比现今低约６－７℃。由于高寒的气候环境，促使高原盐湖补给水锐减，在其盛冰阶的晚期普遍形成干盐湖地质事件，这从各不同时间形成的原生石盐包裹体的流质中ｖｇ^2＋／Ｎａ^＋值的分布得到充分的证实。     

老挝他曲地区石盐流体包裹体特征、氢氧同位素组成及成盐物质补给方式

对老挝他曲地区钻孔中石盐流体包裹体特征、类型以及氢氧同位素组成进行了研究,用以重塑盆地成盐过程中的古环境,并在此基础上对盆地成盐物质的补给方式进行了探讨。结果显示,钻孔中塔贡组下盐段原生石盐岩主要包括人字形和漏斗形晶体两种类型。其中人字形石盐晶体在下盐段中广泛发育,漏斗形晶体数量较少,但通常与人字形石盐晶体产于同一层位,指示了下盐段沉积时盆地处于一种极浅水环境。石盐原生流体包裹体氢氧同位素组成明显偏离全球大气降水线,集中分布于其右下方,反映盆地成盐过程中处于强烈蒸发条件之下。氢氧同位素组成在纵向上的变化,可能揭示了在盐湖演化至钾盐沉积阶段大气温度具有升高趋势。白垩纪中期全球海平面达到极大值,而在呵叻盆地内部,持续的坳陷和强烈的蒸发作用将导致湖平面快速下降。这样,外海与盐盆之间则会产生水位差,由此产生的水力梯度将为外海海水通过障壁向盆地方向进行渗透提供潜在的动力。此外,早白垩世呵叻高原广泛发育的沙漠沉积,则为外海海水大规模的侧向渗透进入盐盆提供了可能的通道。因此,结合现有的资料,本文提出,除海侵补给外,外海通过障壁侧向渗透补给对老挝钾盐盆地成盐物质的供给也具有重要作用。     

Evaporation Stage of Paleo-saline Lake in the Sichuan Basin, China: Insight from Fluid inclusions in Halite

Sichuan Basin is one of the most important marine–salt forming basins in China. The Simian and Triassic have a large number of evaporites. The Triassic strata have found a large amount of polyhalite and potassium-rich brine. However, no soluble potassium salt deposit were found. In this study, the halite in well Changping 3 which is located at the eastern part of the Sichuan basin was studied using the characteristics, hydrogen and oxygen isotopes of the fluid inclusion in halite to reconstruct the paleoenvironment. The salt rocks in well Changping 3 can be divided into two types: grey salt rock and orange salt rock. The result shows that the isotopic composition of the halite fluid inclusion is distinct from the global precipitation line reflecting that the salt formation process is under strong evaporation conditions and the climate is extremely dry. At the same time, compared with the hydrogen and oxygen isotopes of brine in the Sichuan Basin and the hydrous isotope composition of the inclusions in the salt inclusions of other areas in China, it is shown that the evaporation depth of the ancient seawater in the Sichuan Basin was high and reached the precipitation of potassium and magnesium stage.     

Experimentally produced halite compared with Triassic layered halite-rock from Cheshire, England

The experimental evaporation of brine in shallow vessels shows that layered halite can form by the overgrowth of crystals in mats that have foundered from their initial position of growth at the brine-air interface; by the upward and lateral growth of crystals that have nucleated on the floor; and by the overgrowth on detrital halite. Each of these processes, and its resultant crystal fabric, is described in relation to the environmental factors which influence it. The fabrics of primary halite crystals in layered halite-rock and associated deposits from the Lower Keuper Saliferous Beds of the Cheshire Basin are described and compared with those of the experimentally produced halite. Some of the ancient halite-rock may be matched with floor-nucleated crystals, and some with detrital halite; none can be shown to have grown from foundered mats. The halite-rock is regarded as having formed in shallow brine bodies, perhaps only a few centimetres deep, but of unknown extent, whilst the presence of planar solution or deflation surfaces at the top of the layered units, and disruptions in bedding similar to buckled salt-crust structures, point to periodic and perhaps prolonged emergence.     

Hydrogen and Oxygen Isotope Reference Materials for the Analysis of Water Inclusions in Halite

Fluid inclusions represent a unique opportunity for a straightforward determination of the chemical and isotopic composition of fluids that composed the hydrosphere and atmosphere over Earth’s history. The production of reference materials in the laboratory is needed to monitor and to validate the determination of hydrogen and oxygen isotope compositions of water inclusions. We propose a protocol leading to the experimental synthesis of halite crystals that contain water inclusions whose δD and δ¹⁸O values can be related to those of surrounding evaporating waters where the crystals grew. Corrections to isotopic measurements were performed by applying an orthogonal projection of the raw data to the water evaporation trajectory line whose slope can be predicted by taking into account the parameters developed in the linear resistance model of Craig and Gordon (1965). Several hundreds of grams of halite reference material can be produced rapidly (within 2 d) at a low cost and can be stored within a vacuum desiccator at ambient temperature over several months or years. The described method is especially useful for the analysis of anhydrous salts and the interpretation of isotopic fractionations that operate within the surficial water cycle.