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作者简介:

喻顺,男,1982年生。博士,副研究员,主要从事同位素热年代学研究。E-mail:yushun0722@163.com。

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目录contents

    摘要

    榍石富含U和Th,是(U-Th)/He定年的理想矿物之一。本文以Fish Canyon Tuff榍石为例,开展了榍石He扩散行为和榍石(U-Th)/He定年实验方法研究。榍石分步加热扩散实验结果表明He扩散系数ln(D/a 2)与温度倒数呈负相关,与期望的热活化扩散过程一致。测试Fish Canyon Tuff榍石(U-Th)/He年龄分布在28.3~24.6 Ma之间,平均值为26.7±1.2 Ma (1σ),Th/U分布在4.6~5.5之间,平均值为5.2±0.2,在误差范围内与国际上已出版数据一致,表明建立的榍石(U-Th)/He定年实验方法可靠。本次测试15粒榍石碎片外表层(~20 μm)存在不同程度的磨蚀(即不完整晶体),且榍石表层磨蚀厚度随着等效半径的增加而增大。榍石碎片(U-Th)/He年龄介于完整晶体(U-Th)/He年龄和真实(U-Th)/He年龄之间,且随着榍石等效半径及表层磨蚀厚度(<20 μm)的增大,(U-Th)/He年龄更接近真实年龄,这表明榍石(U-Th)/He年龄不确定度与等效半径大小和表层磨蚀厚度有关。

    Abstract

    Titanite is an ideal mineral for (U-Th)/He dating because of its relatively high U and Th. In this paper, diffusion of helium has been characterized in the Fish Canyon Tuff (FCT) titanite based on high-precision laboratory step heating experiments. We present analyses of FCT titanite (U-Th)/He ages, evaluate data uncertainty, and discuss the influences on (U-Th)/He ages. The ln (D/a 2) in FCT titanites determined from laboratory step heating experiments was negatively correlated with reciprocal temperature, as expected for thermally activated volume diffusion. The titanite (U-Th)/He ages and Th/U ratio are clustered at 28.3~24.6 Ma and 4.6~5.5, respectively, with a mean value of 26.7±1.2 Ma and 5.2±0.2, respectively, which agree within analytical uncertainty with previously reported data, respectively, suggesting that the experimental procedure we established is reliable. The outer 20 μm of the 15 original titanite crystals had been partially or completely abraded and the abrasion thickness increased with increasing equivalent radius. The measured (U-Th)/He ages from FCT titanite fragment are older than the original bulk crystal ages (uncorrected for ejection), and some are approximately equivalent to the true (U-Th)/He ages of the whole crystals. The (U-Th)/He ages are closer to the true age of the original bulk crystal due to the larger size and thicker outer abrasion thickness of original crystals, suggesting that the uncertainty of FCT titanites (U-Th)/He age is correlated with grain size and abrasion thickness of the original crystal face.

  • (U-Th)/He热年代学是基于矿物中U、Th及Sm元素衰变产生4He及晶体内4He热扩散理论而发展起来的一种热年代学方法。Dodson(1973)依据气体热活化扩散过程遵从体扩散的理论提出了封闭温度概念,为理解热年代学的年龄提供了新的视角,He扩散行为成为了解释(U-Th)/He热年代学年龄重要基础。Zeitler et al.(1987)率先通过分步加热的模拟实验研究磷灰石He扩散行为,并指出(U-Th)/He定年有作为一种低温温度计的潜力。Wolf et al.(1996,1998)进一步应用热模拟实验研究了磷灰石He扩散参数及封闭温度,并采用数值模拟方法研究He扩散行为,提出了磷灰石He的敏感温度区间(He部分保留区)。Farley(1996)建立了α射出效应定量校正模型,解决了测试的(U-Th)/He年龄偏小问题,进一步完善了(U-Th)/He定年方法。House et al.(1999)Warnock et al.(1997)通过钻井自然样品(U-Th)/He年龄与温度对比研究,认为热模拟实验获得的磷灰石He扩散动力学参数及封闭温度在自然地质尺度上是可靠的。之后,国际上众多实验室相继开展了锆石(Reiners et al.,2002; Guenthner et al.,2013)、榍石(Reiners et al.,1999; Baughman et al.,2017)、赤铁矿(Evenson et al.,2014; Farley,2018)及萤石(Evans et al.,2005; Wolff et al.,2016)等矿物He扩散动力学参数及封闭温度研究,并建立了这些矿物的(U-Th)/He定年实验方法。目前,(U-Th)/He热年代学已经广泛应用于地球科学研究的各个领域,如火山岩定年(Tagami et al.,2003)、地貌演化(Ehlers et al.,2003; Spencer et al.,2019)、地质体隆升剥蚀(Dai Jingen et al.,2013; Yu Shun et al.,2014; Guenthner et al.,2017)及断层构造活化(Brady,2002; Calzolari et al.,2018)等。

  • 我国(U-Th)/He热年代学研究近几年发展迅速,众多学者相继开展了磷灰石及锆石(U-Th)/He定年实验方法(吴林等,2016; Tian Yuntao et al.,2017; Wu Lin et al.,2019; 王英等,2019; Yu Shun et al.,2020)、封闭温度(Chang Jian et al.,2012; 喻顺等,20142019)及地质应用研究(Qiu Nansheng et al.,2010; Wang Fei et al.,2017; Shen Xiaoming et al.,2019),然而关于榍石He扩散动力学参数及(U-Th)/He定年实验方法研究未见报道。榍石是(U-Th)/He定年的理想矿物之一,与锆石和磷灰石相比,榍石在选矿过程中表层易磨损,多呈不规则的碎片状,很难准确地测量和计算晶体的体积与表面积比值,即很难按照Farley et al.(1996)提出的模型准确地计算α射出效应校正系数(FT),这造成了测试的同一样品的榍石(U-Th)/He年龄分散的问题。为了更好地理解榍石(U-Th)/He热年代学,本文以Fish Canyon Tuff(FCT)榍石为例,采用CL、BSE及电子探针分析榍石内部结构和成分,应用高精度分步加热实验研究榍石He扩散行为; 测试15粒FCT榍石(U-Th)/He年龄平均值为26.7±1.2 Ma(1σ),在误差范围内与国际上已出版数据一致,进一步研究表明FCT榍石(U-Th)/He年龄不确定度与等效半径变化和表层差异磨蚀有关,该研究对推进榍石(U-Th)/He定年在地球科学领域的广泛应用具有重要理论和实际意义。

  • 1 样品及实验方法

  • 1.1 实验样品

  • 实验样品来源于美国科罗拉多州南部三胡安火山区Fish Canyon Tuff渐新世凝灰岩(简称FCT)。FCT凝灰岩中含有丰富的透长石、黑云母、磷灰石及锆石等矿物,被广泛的用于裂变径迹、Ar-Ar及(U-Th)/He定年标准物质,这些矿物同位素测年(如锆石U-Pb,透长石和黑云母Ar-Ar,锆石(U-Th)/He及磷灰石裂变径迹)的年龄分布在29.5~27.3 Ma之间(Daze et al.,2003; Reiners et al.,2005; Jourdan et al.,2007; Gleadow et al.,2015; 喻顺等,2019),具有较好的一致性,可用于多种同位素体系年龄之间的相互校正和监控,受到全世界众多实验室欢迎。将FCT岩石样品破碎和筛分,采用磁场和密度等标准矿物分离方法获取榍石。FCT榍石显微镜下呈蜜黄色(图1),具有玻璃光泽,晶体呈不规则状。

  • 1.2 实验方法

  • 1.2.1 CL、BSE及电子探针分析

  • 由于在显微镜下观察榍石晶体清晰度较低,选择代表性的榍石进行阴极发光(CL)和背散射图(BSE)分析,观察榍石的结构、U和Th分带性及包裹体,利用电子探针分析榍石和包裹体的成分。榍石制靶、CL及BSE图像拍摄在武汉上谱分析科技有限责任公司完成。随机选取榍石碎片,安装在环氧树脂上并抛光。CL图像拍摄使用扫描电子显微镜(JSM-IT100),该仪器配备有GATAN MINICL系统,工作电场电压为10.0~13.0 kV,钨灯丝电流为80~85 μA。

  • 电子探针分析在中国地质科学院矿产资源研究所国土资源部成矿作用与资源评价重点实验室完成,分析仪器型号为JXA-8230。元素定量分析的测试条件为:加速电压20 kV,束流50 nA,束斑大小5 μm。

  • 1.2.2 榍石He扩散实验

  • 榍石He阶段分步加热扩散实验在中国地质科学院地质研究所Alphachron II仪器上进行,实验详细步骤与锆石He扩散实验类似(喻顺等,2019)。采用电热偶控温的灯泡对高真空蓝宝石窗口加热。Diffusion Cell 软件精确控制扩散池加热温度、升温速率、加热持续时间等。仪器的优越性在于可对毫米级样品(Nb管包裹的榍石)准确测温,测量温度的准确度为±2℃,每个加热阶段温度的稳定性优于±1℃。阶段分步加热先升温后降温再升温的模式,每阶段升温或降温时间设置为2 min,这个时间相对于某阶段持续加热的时间很小,对He扩散特征研究的影响很小。每个加热阶段释放的4He量与纯3He稀释剂混合后,经过锆铝泵(Line getter)纯化后,进入Pfeiffer四极质谱仪(Prisma Plus)测量4He/3He值,每次释放3He量根据4He标准气体罐释放量(已知)校正。在加热的各阶段测试4He本底稳定,与温度和时间不相关。由于扩散池加热的最高温度为600℃,在完成阶段分步加热后,将样品从扩散池取出,放入Alphachron II系统激光室,利用970 nm半导体二极管激光加热,完全提取并测量榍石中4He含量(详细过程见下节榍石He提取)。

  • 1.2.3 榍石(U-Th)/He定年实验

  • 与磷灰石和锆石相比,榍石颗粒普遍较大,在矿物分选时多呈不规则碎片状,难以准确的计算α射出效应校正系数(FT; Farley et al.,1996)。在显微镜下挑选包裹体少、无裂缝、形状近似规则(可近似计算榍石体积和表面积)、颗粒稍大(长和宽大于200 μm)的榍石碎片测试,并估测碎片的长、宽及高(图1)和估算校正系数(FT)。

  • 将测量完尺寸的榍石碎片包裹在Nb中,加载到Alphachron II仪器激光室。在超高真空环境下利用970 nm半导体二极管激光加热(温度约为1100~1300℃),持续600 s,提取榍石4He含量。之后,将4He与同位素稀释剂3He充分混合,利用PrismaPlus QMG 220四极杆质谱测量4He/3He同位素比值; 样品中4He含量根据同位素稀释剂3He含量确定。同位素稀释剂3He含量则依据标准罐气体4He校正。每粒榍石至少运行二次He含量测量流程,当最后一次测量的He量接近热本底时,即认为榍石中4He被完全释放。最终,测量的4He含量扣除空白即为榍石4He含量,总体上4He测量精确度小于1%。

  • U、Th含量依据同位素稀释电感耦合等离子体质谱法测定(ID-ICP-MS)。在已完成He测试的样品中加入同位素稀释剂溶液(包含约15 ng/g235U 和5 ng/g230Th)及HF溶液。在U和Th标准溶液(浓度为25 ng/g)和空白中加入等量的稀释剂及HF(与样品同等处理)。将样品、标准溶液及空白放入高压溶样罐中,恒温240℃加热40 h溶解榍石。将溶样瓶从高压溶样罐中取出,在电热板上加热48 h蒸干溶液,再加入300 μL浓盐酸至每个溶样瓶,放入高压溶样罐中,恒温200℃条件下加热24 h溶解氟化物。之后,将溶液蒸发至100 μL,加入超纯水将溶液稀释至~10%酸度,在Thermofisher-X2(中国科学院地质与地球物理研究所)电感耦合等离子体质谱仪上测试U、Th同位素含量。由于榍石与锆石(U-Th)/He定年流程相似,在整个实验过程中加入了FCT锆石及PL1锆石标样(喻顺等,2019; Yu Shun et al.,2020)评估实验流程、实验方法的准确性和实验结果的重现性。

  • 2 实验结果

  • 2.1 榍石电子探针、CL及BSE分析结果

  • 在11颗FCT榍石(CaTiSiO5)碎片上测量了20个探针数据点,详细的分析结果见表1。榍石主要成分含量相对均质,其中SiO2含量为28.9%~30.5%,平均值为29.9%±0.4%,CaO含量为25.6%~28.0%,平均值为27.1%±0.6%,TiO2含量为35.7%~37.3%,平均值为36.3%±0.6%; Al2O3、V2O3、FeO及MnO含量平均值分别为1.3%±0.1%,0.2%±0.1%,2.0%±0.2%,0.2%±0.06%。FCT榍石CL及BSE图像显示结构相对均匀,内部没见明显的U和Th分带现象(图2)。榍石含有较多的包裹体(图3),包裹体大小约为5~40 μm。随机选取这些包裹体进行电子探针成分分析,结果表明包裹体主要成分CaO含量为54.6%~56.2%(表2),平均值为55.5%±0.5%,P2O5含量为39.7%~41.2%,平均值为40.3%±0.4%,F含量为2.2%~3.6%,平均值为2.8%±0.4%,且包裹体成分也相对均质,为含F磷灰石。此外,CL图像也显示磷灰石包裹体呈浅色,与榍石主体暗色形成鲜明的对比,磷灰石包裹体U和Th含量相对较低,对榍石He定年和扩散实验的影响可以忽略不计。

  • 2.2 榍石He扩散实验结果

  • SP-16和SP-18榍石阶段加热He扩散实验采用先升温后降温再升温的循环加热模式(图4),每个阶段加热持续时间、温度、He的释气量(已扣除空白)见表3。基于榍石He扩散的各向同性(Cherniaket al.,2011),且保持与前人研究成果的一致性和对比性,本次扩散系数及封闭温度的计算假设球形扩散模型(Fechtig et al.,1966)。依据FCT榍石分步加热扩散实验数据,计算累计释放气体分数Ft)和Ln(D/a2)见表3。SP-18榍石阶段分步加热释放的He总量为12.45×10-12L,占样品He释放总量的76.5%; SP-16榍石分步加热阶段释放He的总量为3.67×10-12L,占样品He释放总量的71.0%; 这两个样品阶段加热累计释放分数(fcum)都小于0.86,则Ln(D/a2)由Fulda et al.(2000)的扩散模型公式(3)计算。初始升温加热阶段产生的扩散系数较高,呈非线性的Arrhenius扩散行为,这种现象常见于榍石、锆石和磷灰石矿物He扩散实验中,可能与包裹体、裂缝、U和Th分带效应及辐射损伤等因素有关(如Reiners et al.,19992004; Farley,2000)。在排除初始阶段异常点之后,图5是绘制的SP-18及SP-16榍石Ln(D/a2)与温度倒数(1/T)关系图,显示扩散系数Ln(D/a2)与温度倒数呈负相关,与期望的线性Arrhenius扩散行为一致。

  • 图1 典型FCT榍石显微镜下照片

  • Fig.1 Typical microscopic image of FCT titanites

  • (a)、(b)—颗粒FCT-3不同截面的图像;(c)、(d)—颗粒FCT-5不同截面的图像

  • (a) , (b) —showing images of different sections of FCT-3 titanite; (c) , (d) —showing images of different sections of FCT-5 titanite

  • 表1 FCT榍石电子探针分析结果(%)

  • Table1 Electron probe microanalysis measurement results for FCT titanite (%)

  • 图2 FCT榍石CL(a)及BSE(b)图像

  • Fig.2 CL (a) and BSE (b) images of FCT titanite

  • 图3 BSE图像显示FCT榍石含有磷灰石包裹体(a~c)

  • Fig.3 BSE images showing apatite inclusions in FCT titanite (a~c)

  • 图4 样品SP-16(a)和SP-18(b)榍石扩散实验阶段加热温度及时间

  • Fig.4 Step-heating schedules for sample SP-16 (a) and SP-18 (b) titanite diffusion experiments

  • 表2 FCT榍石晶体中的包裹体成分分析结果(%)

  • Table2 Electron probe microanalysis measurement results for mineral inclusions in FCT titanite (%)

  • 表3 SP-16和SP-18榍石阶段加热He扩散实验结果

  • Table3 Results of cycled step-heating He diffusion experiments for samples SP-16 and SP-18

  • 图5 SP-16(a)和SP-18(b)榍石阶段加热He扩散实验Arrhenius曲线图

  • Fig.5 Arrhenius plot of the cycled step-heating diffusion experiments with SP-16 (a) and SP-18 (b)

  • 2.3 榍石(U-Th)/He定年实验结果

  • 15颗FCT榍石碎片(U-Th)/He定年结果见表4。单颗粒榍石Th/U较集中,分布在4.6~5.5之间,平均值为5.2±0.2。(U-Th)/He年龄为28.3~24.6 Ma,平均值为26.7±1.2 Ma(1σ),相对不确定度为4%。图6为HelioPlot软件(Vermeesch,2010)绘制的(U-Th)/He年龄分布,显示FCT榍石(U-Th)/He年龄分布比较集中,其几何平均值为26.4±0.3 Ma(S.E.),且(U-Th)/He年龄与Th/U不相关。15颗榍石碎片等效半径估算值分布在108~157 μm之间,对应的α射出效应校正(FT)值分布在0.86~0.91之间,估算FT校正(U-Th)/He年龄分布在31.8~28.3 Ma之间,平均值为30.3±1.1 Ma。FCT榍石有效U浓度(eU=U+Th×0.235)分布在82~183 μg/g之间,平均值为119±28 μg/g。

  • 表4 FCT榍石(U-Th)/He年龄表

  • Table4 (U-Th) /He dating results for FCT titanite

  • 注:有效U浓度eU=U+0.235×Th; α剂量代表辐射损伤程度; 由于榍石为不规则碎片状,颗粒的半径为估算值,因此,FTFT校正年龄、碎片质量、有效U浓度(eU)及α 剂量皆为估算值。

  • 图6 FCT榍石未校正(U-Th)/He定年结果

  • Fig.6 The result of FCT titanite (U-Th) /He ages (uncorrected for ejection)

  • 白色的椭圆为FCT整个数据集的几何平均年龄组成,紫色至绿色的椭圆反映Th/U变化

  • The geometric mean composition is shown as a white ellipse and the Th/U ratios are color-coded

  • 3 讨论

  • 3.1 榍石He扩散参数及封闭温度

  • 热活化扩散过程服从Arrhenius方程DT)/a2=D0/a2×e-Ea/RTD为扩散系数,a为扩散域(即颗粒等效半径),Ea为活化能,T为温度,R为气体常数),其中活化能EaD0/a2可依据该方程线性回归求取。SP-16和SP-18榍石Ln(D/a2)与温度倒数(1/T)关系见图5。SP-16样品直线的斜率-Ea/R为-1.607,y轴的截距Ln(D0/a2)为4.76,线性拟合度R2为0.95; SP-18样品直线的斜率-Ea/R为-1.766,y轴的截距Ln(D0/a2)为7.63,线性拟合度R2为0.96。依据拟合直线的斜率计算样品SP-16和SP-18活化能分别为133.9 kJ/mol和147.2 kJ/mol,稍小于前人FCT榍石实验结果(161.1~189.5 kJ/mol)(Reiners et al.,1999; Shuster et al.,2003; Baughman et al.,2017),统计19个已出版的榍石He扩散活化能(不包括高辐射损伤的榍石)数据主要分布在161.1~197.9 kJ/mol之间,平均值为184.2±9.4 kJ/mol。

  • 依据扩散参数EaD0/a2,由Dodson(1973)公式计算SP-16和SP-18榍石封闭温度分别为131.2℃、142℃(冷却速率为10℃/Ma),稍小于前人研究FCT榍石封闭温度为155~201℃(Reiners et al.,1999; Shuster et al.,2003; Baughman et al.,2017),可能与测试的榍石等效半径较小(小于160 μm)有关。Reiners et al.(1999)开展榍石He扩散实验认为He封闭温度为175~210℃(冷却速率为10℃/Ma,半径为100~400 μm),且与颗粒半径呈正相关。KTB钻井剖面(U-Th)/He年龄-温度关系进一步验证榍石He封闭温度为200±10℃(半径为300 μm; Stockli et al.,2004)。Baughman et al.(2017)研究认为榍石He扩散性受辐射损伤程度影响,当α剂量小于阀值~50×1016 α/g,榍石He封闭温度约为150~210℃,当α剂量大于这个阀值时,He扩散性增加,封闭温度随辐射损伤增大而急剧减小。这与锆石辐射损伤模型类似(Guenthner et al.,2013),当α剂量小于阀值2×1018 α/g,锆石He封闭温度(约140~220℃)随辐射损伤程度增加而增大,当α剂量大于阀值时,锆石He封闭温度随损伤程度增加而急剧减小。然而,由于榍石颗粒半径和α剂量难以精确的确定,低α剂量(<50×1016 α/g)时He封闭温度是否与辐射损伤程度正相关仍然需要进一步研究(Baughman et al.,2017)。图7为低辐射剂量的锆石和榍石封闭温度与冷却速率关系,显示榍石和锆石He封闭温度随冷却速率的增大而增加; 缓慢冷却时(冷却速率为0.1℃/Ma),榍石He封闭温度分布在139~168℃,平均值为155.6±9.3℃,快速冷却时(冷却速率为100℃/Ma),封闭温度分布在195~235℃之间,平均值为218.3±11.7℃。若同一样品中的榍石和锆石都经历了高剂量辐射损伤,一般情况下锆石具有较高的eU浓度,更容易聚集较高剂量的辐射损伤(或到达损伤剂量阀值),He的保存性下降速度比榍石更快,则榍石仍记录相对较高的温度的热史(Baughman et al.,2017)。总体上,样品经历相同的热演化史,榍石He封闭温度一般略高于锆石He封闭温度。

  • 3.2 FCT榍石(U-Th)/He年龄及不确定度

  • FCT锆石U-Pb年龄为28.48±0.06 Ma(Schmitz et al.,2001),透长石和黑云母Ar-Ar年龄分别为28.03±0.18 Ma(Jourdan et al.,2007)、28.13±0.47 Ma(Dazé et al.,2003),锆石和磷灰石裂变径迹年龄分别为27.9±2.2 Ma(Carpéna et al.,1987)、28.8~27.4 Ma(Gleadow et al.,2015),锆石(U-Th)/He年龄28.3~27.3 Ma(Tagami et al.,2003; Reiners,2005; Gleadow et al.,2015; 喻顺等,2019)。因此,根据Ar-Ar、裂变径迹及(U-Th)/He同位素定年体系的一致性,FCT榍石(U-Th)/He年龄预期值为28.8~27.3 Ma。事实上,前人测试了FCT榍石(U-Th)/He年龄平均值分别为27.9±2.2 Ma(2σ)(House et al.,2000)、30.1± 2(2σ)(Reiners et al.,1999)、28.1±1.4 Ma(2σ)(Horne et al.,2016),稍大于本次测试榍石碎片(U-Th)/He年龄(26.7±1.2 Ma(1σ))(图8)。统计63个FCT榍石Th/U数据服从正态分布(图9),平均值为5.2±0.3(1σ),与本次测试Th/U平均值为5.2±0.2(1σ)相吻合。总体上,测试FCT榍石(U-Th)/He年龄及Th/U与已出版数据在误差范围内一致,(U-Th)/He年龄与Th/U不相关(图8)。统计所有榍石(U-Th)/He年龄的平均值为28.1±1.3 Ma,相对不确定度为4.7%; 而FCT锆石(U-Th)/He年龄平均值为28.3±3.1 Ma(Dobson et al.,2008),相对不确定度达10.9%。相对于锆石,榍石(U-Th)/He年龄分布更加稳定,主要原因FCT锆石U和Th分带较为严重(Schmitz et al.,2001),锆石颗粒之间的U和Th分布变化较大,不正确的α射出效应校正(FT)(假定锆石晶体U和Th均匀分布)增加了锆石(U-Th)/He年龄误差(Dobson et al.,2008)。

  • 图7 榍石及锆石He封闭温度(Tc)与冷却速率关系

  • Fig.7 The relationship between closure temperatures and cooling rates for zircon and titanite

  • 计算封闭温度的参数(EaD0)来源于Reiners et al.(1999,2002,2004); Stockli et al.(2004); Wolfe et al.(2010)

  • The closure temperature calculated using parameters (Ea and D0 data) obtained from Reiners et al. (1999, 2002, 2004) ; Stockli et al. (2004) ; Wolfe et al. (2010)

  • 已有的研究表明影响榍石(U-Th)/He年龄不确定度主要因素有:① 岩体缓慢冷却,榍石长时间处于He部分保留区,在晶体内部形成He不均匀分布剖面(如核部老,边部年轻)(Reiners et al.,1999),选择了其中的部分碎片(如核部或边部)测试,其年龄与整个晶体年龄不同; ② 榍石颗粒半径大小引起的He封闭温度变化,如半径分别为400 μm和100 μm的颗粒经历相同热史,封闭温度相差约24~27℃; 若样品缓慢冷却通过He部分保留区,则(U-Th)/He年龄存在差异较大; 若快速通过He部分保留区,则年龄差异较小; ③ 榍石晶体辐射损伤程度的影响,当α剂量大于阀值50×1016 α/g,He封闭温度急剧减小(Baughman et al.,2017),(U-Th)/He年龄与eU呈负相关; ④ 强烈的U和Th的分带效应引起He不均匀分布,在α射出效应校正或表层磨蚀后,部分He并非来自母体U和Th; ⑤ 榍石表层α射出效应导致He丢失。多种热年代学方法(如锆石U-Pb、Ar-Ar、(U-Th)/He和裂变径迹等)测试结果显示FCT榍石经历了快速冷却过程,CL和BSE图像显示榍石U和Th分带效应较弱,α剂量(0.7×1016~1.6×1016 α/g; 表4)远小于阀值50×1016 α/g,这些研究表明FCT榍石(U-Th)/He年龄受①~④项影响较小。

  • FCT榍石(U-Th)/He年龄不确定度主要来源于表层He丢失程度。由于榍石在矿物分选阶段易磨损,呈不规则碎片状,造成表层~20 μm可能部分或全部磨蚀,磨蚀后的碎片与完整晶体相比较,表层He丢失量不同程度的减少。图10绘制了FCT榍石(U-Th)/He年龄与等效半径及表层剥蚀厚度的关系图。随着榍石等效半径的增大,(U-Th)/He年龄逐渐增大,若榍石颗粒半径自100 μm 增大到400 μm,完整晶体(即表层剥蚀0 μm)(U-Th)/He年龄则自24.6 Ma增大到27.5 Ma,这表明半径大的榍石He丢失比例相对较小,(U-Th)/He年龄更接近真实年龄。随着榍石表层磨蚀厚度增大,(U-Th)/He年龄增大,直到表层磨蚀厚度大于或等于20 μm,(U-Th)/He年龄接近真实年龄(图10a)。此外,若榍石表层磨蚀相同的厚度(< 20 μm),半径大的榍石(U-Th)/He年龄大,如表层磨蚀5 μm,半径为100 μm碎片(U-Th)/He年龄比半径为400 μm碎片(U-Th)/He年龄小5.4%。若榍石碎片半径为400 μm,碎片表层磨蚀程度引起(U-Th)/He年龄的差异最大为4%,即(U-Th)/He年龄至多比真实年龄小4%。本次测试的榍石颗粒外表层~20 μm存在部分或者全部磨蚀,且随着半径的增大,榍石表层磨蚀厚度具有增大的趋势。FCT榍石(U-Th)/He年龄接近或大于完整晶体年龄,部分年龄接近榍石真实年龄; FT校正的(U-Th)/He年龄为31.8~28.3 Ma,平均值为30.3±1.1 Ma(表4),大于FCT榍石(U-Th)/He真实年龄,即存在过度校正现象。FCT榍石(U-Th)/He年龄随等效半径和表层磨蚀厚度的增加而增大(图10b),部分榍石半径和表层剥蚀程度较小,导致测试(U-Th)/He年龄相对较小。因此,对于快速冷却的榍石(U-Th)/He定年,二种方法可减少表层He丢失引起的年龄不确定度:① 磨蚀表层20 μm,测试榍石碎片内部,减少α射出效应影响; ② 挑选颗粒较大的榍石测试,如挑选半径大于400 μm榍石,(U-Th)/He年龄至多比真实年龄小4%,即表层磨蚀程度引起的年龄误差至多4%。此外,随着碎片半径增大,表层磨蚀程度增大的概率增加(图10b),从而增加了碎片(U-Th)/He年龄接近真实年龄的概率。

  • 图8 FCT未校正榍石(U-Th)/He年龄与Th/U关系

  • Fig.8 The relationship between uncorrected (U-Th) /He ages and Th/U ratios for FCT titanite

  • 图9 FCT榍石Th/U分布(曲线表示正态分布)

  • Fig.9 Histogram showing the distribution of Th/U ratio (the line represents a normal distribution fit)

  • 图10 FCT榍石(U-Th)/He年龄与等效半径关系

  • Fig.10 The relationship between (U-Th) /He ages and equivalent radiuses for FCT titanite

  • 等效半径为估值,假设误差约为5%; 蓝色、橙色、绿色及红色虚线分别为磨蚀榍石表层0 μm(完整晶体)、 5 μm、10 μm及20 μm;(b)为放大图(a),显示(U-Th)/He年龄与等效半径的关系

  • The equivalent sphere radius were estimated, assuming an error of 5%; the colored dotted lines show outer thickness of original crystals was abraded for 0 μm, 5 μm, 10 μm and 20 μm, respectively; figure (b) is a larged view of a portion of figure (a) , showing relationship between (U-Th) /He ages and equivalent radiuses

  • 4 结论

  • (1)FCT榍石He扩散实验显示扩散系数ln(D/a2)与温度倒数呈负相关,与期望的热活化扩散过程一致。SP-16和SP-18榍石活化能(Ea)分别为133.9 kJ/mol和147.2 kJ/mol,封闭温度分别为131.2℃、142℃(冷却速率为10℃/Ma),稍小于前人封闭温度研究结果(155~201℃)。榍石和锆石He封闭温度随冷却速率的增大而增加,若样品经历相同的热演化史,榍石He封闭温度一般略高于锆石。

  • (2)测试FCT榍石Th/U平均值为5.2±0.2,与已出版的数据(5.2±0.3(1σ))相吻合。测试FCT榍石(U-Th)/He年龄为28.3~24.6 Ma,平均值为26.7± 1.2 Ma(1σ),稍小于前人测试年龄结果(28.5±1.1 Ma(1σ)),主要原因是部分榍石半径和表层磨蚀程度相对较小,表层He丢失程度相对较大; 总体上,(U-Th)/He年龄在误差范围内与已出版数据一致。

  • (3)FCT榍石(U-Th)/He年龄不确定度主要来源于榍石表层He丢失程度,与榍石半径变化和表层差异磨蚀有关。FCT榍石碎片榍石表层(~20 μm)存在不同程度的磨蚀,随着榍石颗粒等效半径的增大,榍石表层磨蚀程度具有增大的趋势。FCT榍石碎片(U-Th)/He年龄介于完整晶体(U-Th)/He年龄和真实(U-Th)/He年龄之间,且随着等效半径和表层磨蚀厚度(<20 μm)的增大,(U-Th)/He年龄更接近真实年龄。

  • 致谢:感谢中国地质科学院地质研究所孙敬博和沈泽及中国科学院地质与地球物理研究所吴林在He及U、Th测试过程中提供的帮助; 感谢两位匿名审稿专家宝贵的意见和建议。

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