原地生成宇宙成因核素测年技术思考(一):稳态侵蚀等式解出的ε是当前地表侵蚀速率

原地生成宇宙成因核素测年技术可以方便地计算出地表的暴露年龄和侵蚀速率,从而成为研究地表演化过程的有力工具。在利用原地生成宇宙成因核素测年方法研究地表的侵蚀过程中,常常将稳态侵蚀状态下浓度与侵蚀速率的关系式所计算出的侵蚀速率当成暴露时间内的地表平均侵蚀速率,这其实是不准确的。本文通过数学计算和理论推导,证明由当前稳态侵蚀关系式解出的侵蚀速率是地表当前侵蚀速率(或称为最后恒定侵蚀速率),显然与地表平均侵蚀速率所代表的地质含义是不同的。     

原地生成宇宙成因核素测年技术思考（五）： 单核素计算地表最大抬升速率方法

在以往研究中,原地生成宇宙成因核素测年技术虽然理论上可以利用两种核素（如10Be和26Al）联立方程组同时解出暴露年龄和侵蚀速率,但在实践中却常采用单种核素（常用10Be）分别解出最小暴露年龄和最大侵蚀速率,26Al数据只用于辅助判断。这主要是因为受目前误差水平限制,10Be和26Al的测试数据结果,常常并未能达到联立两个方程解出暴露年龄和侵蚀速率两个参数的精度（当然也有其他因素影响）,这在投影图上常表现为被认为只有一次性暴露历史的样品,投影结果在稳态侵蚀岛之外,只有加上误差,才能进入或靠近稳态侵蚀岛。同理,在推导出包含生成速率加速率的计算等式后,虽然理论上抬升区长期暴露的样品也能够利用两种核素（如10Be和26Al）同时解出侵蚀速率和抬升速率,但实践上10Be和26Al浓度投影结果可能并未在抬升的正常投影区（稳态侵蚀岛上曲线附近）,从而解出的侵蚀速率和抬升速率合理性存疑。对此,参考计算最大侵蚀速率的原理,提出利用长期暴露样品的10Be浓度来解出抬升区地表最大抬升速率的计算方法（,从而可以对碰撞造山抬升区的抬升速率上限进行限定。     

原地生成宇宙成因核素测年技术思考(二):持续抬升区单样品直接计算抬升速率方法

原地生成宇宙成因核素测年技术在计算暴露年龄和侵蚀速率时,一直默认样品所在位置不随时间发生高程变化,从而核素生成速率不因高程变化而改变。在构造稳定区,这样的假设是合理的。在构造活动区,往往会因构造运动,地表样品高程发生改变。大冰盖地区（如南极）的冰川消融或加厚也会引起地壳均衡反弹,从而导致样品高程变化,而核素的生成速率将随高程变化而改变。本文对连续抬升情况下,地表样品中宇宙成因核素浓度与生成速率加速率间的计算关系进行了三种方法的数学推导,并提出如何利用单块样品中两种宇宙成因核素（以10Be和26Al为例）浓度计算地表抬升速率,最后指出已往利用宇宙成因核素方法对构造活动区的侵蚀速率的计算存在高估。本文首次提出利用地表在接近稳态侵蚀状态下的单块样品的两种宇宙成因核素直接计算地表持续抬升速率的方法,从而将原地生成宇宙成因核素方法和构造运动学研究直接联系起来。     

原地生成宇宙成因核素测年技术思考(四):核素浓度年份化数列累加方法探讨

原地生成宇宙成因核素测年技术中,计算核素浓度的传统通用等式假设了地表侵蚀速率(参数ε)及样品的位置在暴露期间保持不变,从而简化了计算过程。然而这种假设可能并不符合复杂地质背景的实际情况,从而造成计算结果误差较大,与其他测年方法结果也难以相互比较和匹配。本文利用将核素总浓度按年份分解为每年生成的浓度,经衰变和侵蚀的综合影响后至今剩余的浓度(称为年份净剩余浓度),组成数列累加的办法,分别推导了地表样品在高程以及侵蚀速率变化情况下的核素浓度计算等式。在此基础上,给出了地表生成速率和侵蚀速率逐年变化情况下的通用浓度年份化累加等式。最后讨论了传统浓度计算等式中的参数ε(侵蚀速率)的地质意义,明确了ε是一个积分中值,并不代表暴露期间的算术平均侵蚀速率,而是每年的侵蚀速率以自然指数加权的复杂方式(即T年前的那一年的生成浓度将乘以e-(λ+ρεΛ)(T-1))影响到样品的最终浓度值。     

原地生成宇宙成因核素在地学研究中的应用

传统测年方法(14C、热释光、光释光等)无法直接测量地貌面或基岩面的形成年代,利用宇宙生成核素定出的年代可以直接计算地质、地貌体的暴露年代和埋藏时代。随着测量仪器的长足进步,特别是加速器质谱(AMS)检出限(可测至106原子)的大幅度提高,原地生成宇宙成因核素定年技术给地貌学带来了革命性的变化,因此宇宙生成核素被广泛应用于古气候学、构造地质学、火山年代学及古地磁学等。本文阐释了原地生成宇宙核素定年方法的基本原理,并在地学领域应用的现有基础上,从冰川、断层、阶地等研究对象出发,以沉积物埋藏年龄、地表侵蚀速率、断层滑动速率等为研究内容,具体描述该定年技术在冰川地貌、构造地貌、地貌过程及地貌演化研究中的国内外研究现状,以及应用中尚待解决的诸如核素产生速率与空间、时间关系;样品地质、地貌条件对结果造成的不确定性等问题。     

原地生成宇宙成因核素暴露和埋藏年龄计算方法的统一

原地生成宇宙成因核素测年技术计算暴露和埋藏年龄以往分属于不同的计算方法体系,采用的计算等式不同。实际上,无论是暴露过程还是埋藏过程,都涉及了核素的新生和已生成核素的减少,其浓度计算方法原理上有相通之处。考察被研究样品历史上某时点的状态,侵蚀使样品距地表深度不断减小,埋藏使样品距地表深度不断增加,刚好是一种相反的过程,而原地生成宇宙成因核素的生成速率与距地表深度呈指数函数形式相关。本文按照原地生成宇宙成因核素浓度与暴露时间、侵蚀速率间关系的计算原理,推导出在匀速埋藏过程中,计算被测试样品的浓度随时间的变化时,以样品目前生成速率为标准,可将埋藏速率(β)视为负侵蚀速率(-ε),从而将暴露年龄和埋藏年龄的计算等式统一起来。非匀速埋藏情况,通过求导计算,仍可视埋藏为负侵蚀。在埋藏速率和侵蚀速率用同一参数指代,埋藏速率可视为负侵蚀速率的基础上,推导出不同埋藏(侵蚀)模式下样品的核素浓度计算等式。理论上根据不同埋藏(侵蚀)模式,可计算复杂地表演化过程样品的埋藏(暴露)年龄。

     

原地生成宇宙成因核素^10Be和^26Al样品采集及处理

着重介绍原地生成宇宙成因核素^10Be和^26Al理想样品的特点、野外采集注意事项、实验室分离与纯化以及加速器质谱测量用靶样制备等。     

宇宙成因核素埋藏年龄法: 原理及应用

宇宙成因核素埋藏年龄法是继宇宙成因核素暴露年龄法之后发展起来的又一同位素定年法,这一方法主要应用于沉积物定年。宇宙成因核素埋藏年龄法的原理是: 具有不同半衰期的成对宇宙成因核素浓度及比值会随时间而发生变化,这些变化可以表示成时间的函数。因此,通过测定石英中成对宇宙成因核素的含量,可以定量化沉积物的沉积时间。宇宙成因核素埋藏年龄法在实际应用中还存在几方面不确定性: 测量误差、参数引入的误差以及与地质模型的差异所引入的误差,本文对这些不确定因素进行了讨论。在实际采样过程中,应注意采样点及地质背景尽可能符合地质模型; 在样品处理过程中,应保证石英纯度以及选用 10Be含量尽可能低的铍载体。在最后,本文例举了宇宙成因核素埋藏年龄法在昔格达湖相沉积和大邑砾岩定年中的应用,以及 26Al- 10Be-21 Ne联合定年。可以期待,这一方法在中国将被广泛地用于研究大型河流演化、中国早期人类的演化历史、第四纪早期冰川发育以及与青藏高原演化相关的构造和沉积问题。     

原地宇宙成因核素暴露测年方法中石英的提取

原地宇宙成因核素10Be和26Al的暴露年龄测定是近年来发展较快的测年技术,已在地学研究中得到广泛应用。该方法需要选用经一系列前处理过程获得的纯净石英作为待测样品,制备成BeO和Al2O3以供加速器质谱仪测量。因此获得高纯度的石英样品,是该测年方法的关键环节之一。本研究在已有报道的石英提纯化学流程的基础上,尝试对流程进行部分优化,通过实验对比不同粒径组分、不同固液比水浴振荡器和滚筒HF-HNO3蚀刻效果,确定使用HF-HNO3(1%或2%,固液比15.0 g/L)滚筒加热法刻蚀样品以去除铝硅酸盐,多钨酸钠重液分离样品中的石英和其他组分。优化的分离纯化流程应用于处理采自祁连山北缘河流阶地含石英的岩石样品,经纯化的石英纯度可达98%以上,Al的含量小于200μg/g,表明采用优化的提取流程获得了高纯度的石英样品,可以满足10Be和26Al暴露测年所需样品要求。     

宇宙成因核素~(10)Be揭示的北祁连山侵蚀速率特征

山脉侵蚀速率的大小和时空分布信息是研究山脉构造—气候相互作用和地貌演化的关键切入点,其大小是受气候还是构造控制争论已久。宇宙成因核素10Be方法为从千年至万年尺度上定量研究流域平均侵蚀速率提供了一种先进和快捷的技术手段,为揭示侵蚀速率与现代气候和构造地貌因子的关系并进行相关分析提供了基础。利用该方法对北祁连山近现代侵蚀速率进行了研究。所采集的9个流域现代河沙样品,结合前人数据进行共同分析,结果显示该区侵蚀速率的变化范围为18.7~833 mm/ka,北祁连山中段的侵蚀速率约为323 mm/ka,该区侵蚀速率与降雨量没有明显的对应关系,但与流域平均坡度呈现很好的非线性关系,揭示坡度是该区侵蚀速率的最主要控制因素。通过对比北祁连山地表平均侵蚀速率和该区域的断层垂直滑动速率发现整体上该区域地表侵蚀速率要低于祁连山北缘断层的垂直滑动速率,反映了北祁连山正处于地形抬升和生长的过程之中。     

Stone run (block stream) formation in the Falkland Islands over several cold stages,deduced from cosmogenic isotope (10Be and 26Al) surface exposure dating

Cosmogenic isotope (¹⁰Be and ²⁶Al) surface exposure dating has been applied to valley‐axis and hillslope stone runs (relict periglacial block streams) and their source outcrops in the Falkland Islands, South Atlantic. The data indicate that stone runs are considerably older landforms than previously envisaged and afford no evidence that they are a product of the Last Glacial Maximum; the samples range in apparent ¹⁰Be age from 42k to 731k yr BP, but some of these are minima. The results indicate that valley‐axis stone runs may be up to 700–800k yr old, have simple exposure histories and are composite landforms that developed over several cold stages. Analyses of some hillslope and outcrop samples also demonstrate simple exposure histories with ¹⁰Be ages from 42k to 658k yr BP. In contrast, isotopic ratios from other hillslope and outcrop samples reveal they have had a complex exposure history involving periods of burial or shielding; the samples range in ¹⁰Be age from 59k to 569k yr BP and these are regarded as minimum age estimates. Larger stone runs may be older than smaller runs and there is a possibility that stone runs older than 800k yr exist in other parts of the Falklands. The assertion that glaciation in the Falklands was restricted to the highest uplands is supported by the data, and the potential for age determination of other boulder‐strewn and bedrock landforms, using cosmogenic isotope analysis, in order to extend the geochronology of Quaternary events and processes is noted. Copyright © 2008 John Wiley & Sons, Ltd.     

Evolution of the West Andean Escarpment at 18°S (N. Chile) during the last 25 Ma: uplift, erosion and collapse through time

The geological record of the Western Andean Escarpment (WARP) reveals episodes of uplift, erosion, volcanism and sedimentation. The lithological sequence at 18°S comprises a thick pile of Azapa Conglomerates (25–19 Ma), an overlying series of widespread rhyodacitic Oxaya Ignimbrites (up to 900 m thick, ca. 19 Ma), which are in turn covered by a series of mafic andesite shield volcanoes. Between 19 and 12 Ma, the surface of the Oxaya Ignimbrites evolved into a large monocline on the western slope of the Andes. A giant antithetically rotated block (Oxaya Block, 80 km×20 km) formed on this slope at about 10–12 Ma and resulted in an easterly dip and a reversed drainage on the block's surface. Morphology, topography and stratigraphic observations argue for a gravitational cause of this rotation. A “secondary” gravitational collapse (50 km³), extending 25 km to the west occurred on the steep western front of the Oxaya Block. Alluvial and fluvial sediments (11–2.7 Ma) accumulated in a half graben to the east of the tilted block and were later thrust over by the rocks of the escarpment wall, indicating further shortening between 8 and 6 Ma. Flatlying Upper Miocene sediments (<5.5 Ma) and the 2.7 Ma Lauca–Peréz Ignimbrite have not been significantly shortened since 6 Ma, suggesting that recent uplift is at least partly caused by regional tilting of the Western Andean slope.     

Cosmogenic isotope (Cl) surface exposure dating of the Norber erratics, Yorkshire Dales: Further constraints on the timing of the LGM deglaciation in Britain

Cosmogenic isotope (³⁶Cl) surface exposure dating of four of the erratic boulders at Norber in the Yorkshire Dales National Park, northwest England, yielded mean ages of ∼22.2 ± 2.0 ka BP and ∼18.0 ± 1.6 ka BP for their emplacement. These two mean values derive from different ³⁶Cl production rates used for exposure age calculation. The ages are uncorrected for temporal variations in production rates and may underestimate the true ages by 5-7%. The former age, although implying early deglaciation for this area of the British ice sheet, is not incompatible with minimum deglaciation ages from other contexts and locations in northwest England. However, the latter age is more consistent with the same minimum deglaciation ages and geochronological evidence for ice-free conditions in parts of the northern sector of the Irish Sea. Within uncertainties, the younger of the mean ages from Norber may indicate that boulder emplacement was associated with North Atlantic Heinrich event 1. The limited spatial (downvalley) extent of the Norber boulders implies that at the time of their deposition the ice margin was coincident with the distal margin of the erratic train. Loss of ice cover at Norber was followed by persistent stadial conditions until the abrupt opening of the Lateglacial Interstadial when large carnivorous mammals colonised the area. The ³⁶Cl ages are between ∼3.0 ka and ∼13.0 ka older than previous estimates based on rates of limestone dissolution derived from the heights of pedestals beneath the erratics.     

Pressure effect on the electronic structure of iron in (Mg,Fe)(Si,Al)O3 perovskite: a combined synchrotron Mössbauer and X-ray emission spectroscopy study up to 100 GPa

We investigated the valence state and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample with the composition (Mg_0.88Fe_0.09)(Si_0.94Al_0.10)O₃ between 1 bar and 100 GPa and at 300 K, using diamond cells and synchrotron Mössbauer spectroscopy techniques. At pressures below 12 GPa, our Mössbauer spectra can be sufficiently fitted by a “two-doublet” model, which assumes one ferrous Fe²⁺-like site and one ferric Fe³⁺-like site with distinct hyperfine parameters. The simplest interpretation that is consistent with both the Mössbauer data and previous X-ray emission data on the same sample is that the Fe²⁺-like site is high-spin Fe²⁺, and the Fe³⁺-like site is high-spin Fe³⁺. At 12 GPa and higher pressures, a “three-doublet” model is necessary and sufficient to fit the Mössbauer spectra. This model assumes two Fe²⁺-like sites and one Fe³⁺-like site distinguished by their hyperfine parameters. Between 12 and 20 GPa, the fraction of the Fe³⁺-like site, Fe³⁺/∑Fe, changes abruptly from about 50 to 70%, possibly due to a spin crossover in six-coordinate Fe²⁺. At pressures above 20 GPa, the fractions of all three sites remain unchanged to the highest pressure, indicating a fixed valence state of iron within this pressure range. From 20 to 100 GPa, the isomer shift between the Fe³⁺-like and Fe²⁺-like sites increases slightly, while the values and widths of the quadruple splitting of all three sites remain essentially constant. In conjunction with the previous X-ray emission data, the Mössbauer data suggest that Fe²⁺ alone, or concurrently with Fe³⁺, undergoes pressure-induced spin crossover between 20 and 100 GPa.