鋯石SIMS原位微區(qū)U-Th不平衡定年: 以云南騰沖火山巖為例

李 玲, 夏小平, 楊 晴, 李如操

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鋯石SIMS原位微區(qū)U-Th不平衡定年: 以云南騰沖火山巖為例

李 玲1,2, 夏小平1*, 楊 晴1, 李如操1,2

(1. 中國(guó)科學(xué)院 廣州地球化學(xué)研究所 同位素地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州 510640; 2. 中國(guó)科學(xué)院大學(xué), 北京 100049)

報(bào)道了利用中國(guó)科學(xué)院廣州地球化學(xué)研究所同位素地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室的CAMECA IMS 1280-HR型二次離子質(zhì)譜儀(SIMS)建立的鋯石原位微區(qū)U-Th不平衡定年方法。一次離子為16O–, 束斑大小約25 μm, 質(zhì)量分辨率設(shè)定為約6000, 使用單接收系統(tǒng)EM跳峰方式接收各相關(guān)質(zhì)量峰。每個(gè)點(diǎn)經(jīng)35次循環(huán)測(cè)量, 共耗時(shí)約27 min。使用Plesovice為外標(biāo), 對(duì)處于久期平衡狀態(tài)的U-Pb定年標(biāo)準(zhǔn)鋯石91500和Qinghu進(jìn)行(230Th)/(238U)分析測(cè)試, 得到兩個(gè)標(biāo)準(zhǔn)鋯石(230Th)/(238U)重現(xiàn)性分別為2.3%和1.3%, 準(zhǔn)確度分別為98.1%和98.6%, 測(cè)試精密度和準(zhǔn)確度都達(dá)到了國(guó)際上其他實(shí)驗(yàn)室的水平。對(duì)云南騰沖地區(qū)已知U-Th年齡的打鷹山、馬鞍山第四紀(jì)火山巖的年輕鋯石進(jìn)行了U-Th定年, 得到馬鞍山鋯石樣品U-Th等時(shí)線(xiàn)年齡為(84.1±9.2) ka (2σ, MSWD = 2.9); 打鷹山鋯石樣品U-Th等時(shí)線(xiàn)年齡為(85.6±7.6) ka (2σ, MSWD = 2.1), 在誤差范圍內(nèi)與前人報(bào)道一致, 表明我們建立的SIMS原位微區(qū)U-Th定年方法準(zhǔn)確可靠。

二次離子質(zhì)譜; U-Th不平衡定年; 原位微區(qū); 騰沖火山巖

0 引 言

鈾系不平衡定年法是第四紀(jì)研究最重要的定年方法之一。運(yùn)用該方法原則上可獲得小于600 ka[1]的珊瑚化石[2–4]、古人類(lèi)、古動(dòng)物骨骼化石[5–9]、洞穴堆積物[10–11]、風(fēng)化產(chǎn)物[12]、巖漿副礦物[13–18]等年輕體系的年代學(xué)信息, 為古海洋重建、古人類(lèi)起源與演化、古氣候環(huán)境變化以及地下巖漿演化等提供時(shí)間制約。傳統(tǒng)的U-Th定年方法如ID-TIMS可給出非常高的測(cè)試精度, 但空間分辨率差, 只適用于取樣部位內(nèi)部微觀年齡較一致的樣品。而對(duì)于內(nèi)部本身不均勻, 可能存在多個(gè)年齡信息的樣品, 如經(jīng)歷了多期巖漿演化的鋯石等副礦物來(lái)說(shuō), 溶解單顆或多顆鋯石得到的年齡有可能是多期或多成因組分混合的結(jié)果, 不具有明確的地質(zhì)意義。對(duì)于此類(lèi)樣品, 原位微區(qū)分析成為解析其復(fù)雜演化歷史的必要手段[19]。

二次離子質(zhì)譜儀(Secondary Ion Mass Spectrometer, SIMS)也叫離子探針(Ion Microprobe), 具有高靈敏度高空間分辨率, 非常適合原位微區(qū)鈾系不平衡定年。SIMS原位微區(qū)鈾系不平衡定年技術(shù)在國(guó)外有的實(shí)驗(yàn)室已有研究。美國(guó)加利福尼亞大學(xué)洛杉磯分校離子探針實(shí)驗(yàn)室于1997年率先利用CAMECA IMS 1270型離子探針建立了該方法, 并于2010年首次成功將淺層剖面法應(yīng)用于第四紀(jì)鋯石U-Th不平衡年齡研究[20]。美國(guó)地質(zhì)調(diào)查局-斯坦福大學(xué)聯(lián)合實(shí)驗(yàn)室亦利用SHRIMP-RG研究建立了該方法, 并對(duì)新西蘭陶波火山帶成功展開(kāi)了一系列研究[15,16,21–23]。但我國(guó)目前尚未有利用離子探針進(jìn)行鈾系定年的相關(guān)報(bào)道。鑒于此, 我們?cè)谥袊?guó)科學(xué)院廣州地球化學(xué)研究所同位素地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室利用一臺(tái)CAMECA IMS 1280-HR型離子探針建立了U-Th定年方法, 并利用目前國(guó)內(nèi)外廣泛使用的幾個(gè)處于久期平衡狀態(tài)的U-Pb定年鋯石標(biāo)樣Plesovice、Qinghu和91500檢驗(yàn)了該方法(230Th)/(238U)比值(括號(hào)表示放射性比值, 下同)的重現(xiàn)性和準(zhǔn)確度; 對(duì)已知年齡的云南騰沖地區(qū)打鷹山、馬鞍山第四紀(jì)火山巖的年輕鋯石進(jìn)行U-Th定年, 取得了和文獻(xiàn)報(bào)道值一致的分析結(jié)果, 表明方法準(zhǔn)確可靠。

1 U-Th不平衡定年簡(jiǎn)介

238U具有放射性, 在形成最終的穩(wěn)定子體同位素206Pb之前, 會(huì)形成一系列包括230Th在內(nèi)也具有放射性的短壽命同位素子體。一個(gè)含U體系如果在一個(gè)較長(zhǎng)的時(shí)間范圍內(nèi)(一般認(rèn)為大約8倍于230Th半衰期的時(shí)間即600 ka)沒(méi)有地質(zhì)事件擾動(dòng)造成U-Th分餾的話(huà),238U的放射性強(qiáng)度將會(huì)等于230Th的放射性強(qiáng)度, 即238238U =230230Th(為衰變常數(shù),238U、230Th為相應(yīng)的原子數(shù))或(230Th)/(238U)=1。此時(shí)地質(zhì)體處于U-Th平衡狀態(tài), 在此之前的狀態(tài)稱(chēng)為U-Th不平衡狀態(tài)。顯然U-Th不平衡的程度(即230230Th/238238U比值)是時(shí)間的函數(shù)。U-Th不平衡定年就是利用238U和其子體同位素230Th之間的不平衡程度來(lái)確定最近一次導(dǎo)致U-Th分餾的地質(zhì)事件的年代, 也稱(chēng)作230Th定年。

對(duì)本文研究的火成巖中的鋯石樣品來(lái)說(shuō), 當(dāng)其滿(mǎn)足: (1) 火山巖的全巖和其中礦物是同源的; (2) 結(jié)晶后體系保持封閉, 沒(méi)有發(fā)生鈾釷同位素遷出或遷入; 其年代可由以下等時(shí)線(xiàn)方程來(lái)確定[24–26]:

式(1)中: (230Th/232Th)和(238U/232Th)是樣品實(shí)測(cè)的放射性比值; (230Th/232Th)0是樣品形成時(shí)的初始放射性比值;為待測(cè)年齡。用3個(gè)及3個(gè)以上礦物和全巖作以(230Th/232Th)和(238U/232Th)為坐標(biāo)的等時(shí)線(xiàn)圖,則(230Th/232Th)0×e–λ230為等時(shí)線(xiàn)圖的截距, (1–e–λ230)為等時(shí)線(xiàn)圖的斜率。知道了等時(shí)線(xiàn)的斜率就可通過(guò)如下方程得到年齡值。

式(2)中:為待測(cè)年齡;230是230Th的衰變常數(shù);是等時(shí)線(xiàn)的斜率。

2 分析方法

野外采集的云南騰沖馬鞍山和打鷹山地區(qū)的新鮮巖石樣品經(jīng)機(jī)械破碎、淘洗、磁選后, 每個(gè)樣分離出約200粒到1000粒鋯石。在雙目鏡下每個(gè)樣隨機(jī)挑選兩百粒, 按從左至右從大到小的順序在載玻片上分別粘成平行長(zhǎng)條狀, 然后注入混合好的環(huán)氧樹(shù)脂與固化劑制成樣品靶, 放入烘箱中干燥后進(jìn)行打磨拋光。為避免Th-O-C結(jié)合形成232Th216O12C2+干擾待測(cè)目標(biāo)信號(hào)峰230ThO+, 樣品靶只能鍍金, 不能鍍碳, 并且注意樣品表面清除待測(cè)位置的樹(shù)脂殘余[27]。鍍完金的樣品靶放入前級(jí)樣品室中靜置一晚再進(jìn)行分析測(cè)試, 以盡可能提高分析室真空, 消除干擾雜質(zhì)。

測(cè)試使用的是最新型的CAMECA IMS 1280-HR型離子探針。其基本工作原理為: 帶有高能量的一次離子束轟擊固體樣品, 通過(guò)一系列復(fù)雜的級(jí)聯(lián)碰撞將部分能量傳導(dǎo)給樣品表面原子, 使其結(jié)構(gòu)發(fā)生破壞并濺射出帶有樣品信息的二次離子。二次離子在高壓加速后進(jìn)入質(zhì)譜儀按照荷質(zhì)比分離, 最后被接收器接收測(cè)量[19,28,29]。我們?cè)诜治鲞^(guò)程中采用帶有高能量的16O–作為一次離子束, 濺射出的帶有樣品信息的二次離子有U+、Th+及其氧化物如UO+、ThO+等。在這些二次離子中, 金屬氧化物離子產(chǎn)率(如230Th16O+)大約為其金屬離子(如230Th+)產(chǎn)率的2~10倍[30], 因此為獲得更高的離子信號(hào)強(qiáng)度, 我們主要分析的二次離子為230Th16O+、232Th16O+、238U16O+以代替230Th+、232Th+、238U+。單點(diǎn)剝蝕直徑約25 μm, 質(zhì)量分辨率為6000, 以單接收系統(tǒng)EM跳峰方式循環(huán)測(cè)量所需信號(hào)(表1)。每個(gè)點(diǎn)經(jīng)35次循環(huán)(cycles)測(cè)量, 共耗時(shí)約27 min。背景信號(hào)值244.038(232Th12C+)主要是為了監(jiān)測(cè)樣品分析區(qū)域是否有樹(shù)脂殘留。當(dāng)一次束流轟擊到了分析區(qū)域的殘留樹(shù)脂時(shí)可能產(chǎn)生干擾離子232Th216O12C2+, 該離子只有在質(zhì)量分辨率達(dá)到40000以上時(shí)才能將其與230Th16O+區(qū)分開(kāi)來(lái)。背景值246.300則主要記錄EM暗噪音和分散在230Th16O+峰附近的其他離子信號(hào)[27]。對(duì)于背景信號(hào)值的處理, 我們采用Schmitt[27]的方法, 取兩者平均值進(jìn)行計(jì)算。由于230Th16O+離子計(jì)數(shù)相對(duì)較低, 難以確定峰位置, 測(cè)試過(guò)程中我們采用先對(duì)90Zr92Zr16O4+(245.789)峰進(jìn)行精確定位, 然后根據(jù)它與230Th16O+(246.028)的固定質(zhì)量差(0.239)計(jì)算出相對(duì)應(yīng)的磁場(chǎng)差進(jìn)行磁場(chǎng)偏轉(zhuǎn)的方式來(lái)定位,204Pb+采用同樣的方法進(jìn)行峰對(duì)中。第四紀(jì)鋯石樣品測(cè)試過(guò)程中每四個(gè)樣品點(diǎn)前后插入兩個(gè)標(biāo)樣點(diǎn)。

在二次離子激發(fā)過(guò)程中, ThO+與UO+具有不同的離子化效率, 因此需要對(duì)儀器測(cè)得的232Th16O+/238U16O+比值進(jìn)行校正, 即需要確定Th/U相對(duì)靈敏度因子RSF (Relative sensitivity factor)。在U-Th-Pb體系保持封閉的鋯石中, 放射成因的208Pb*/206Pb*(*代表放射成因, 下同)和Th/U之間存在如下關(guān)系[20, 31]:

對(duì)于已知結(jié)晶年齡的標(biāo)準(zhǔn)鋯石, 比如Plesovice (= (337.13±0.37) Ma)[32], 則上式可變?yōu)?

考慮到二次離子質(zhì)譜儀對(duì)Pb同位素(208Pb和206Pb)之間的質(zhì)量分餾可以忽略不計(jì)[19, 33], 我們將實(shí)測(cè)的208Pb*/206Pb*比值與232Th16O+/238U16O+比值作圖(圖1), 得到其斜率如圖1所示為0.3414±0.0047, 將其與理論斜率0.3133做比, 即可得該測(cè)試流程中標(biāo)樣Plesovice的Th/U相對(duì)靈敏度因子為1.090±0.015 (2σ)。該方法的優(yōu)點(diǎn)在于它繞開(kāi)了標(biāo)樣鋯石需要Th/U原子比非常均一的要求。因?yàn)閷?duì)于大部分U-Pb定年標(biāo)準(zhǔn)鋯石來(lái)說(shuō), 其Th/U比都是不均一的。

標(biāo)準(zhǔn)樣品的(230Th)/(238U)加權(quán)平均值計(jì)算與圖表繪制, 以及騰沖第四紀(jì)火山巖樣品的表面等時(shí)線(xiàn)年齡計(jì)算和圖表繪制采用Isoplot 3.75[34]完成。

表1 Cameca IMS1280-HR鋯石U-Th定年信號(hào)測(cè)量順序表

圖1 標(biāo)準(zhǔn)鋯石Plesovice 206Pb*/208Pb*-232Th16O+/238U16O+比值圖

3 測(cè)試結(jié)果

3.1 U-Pb定年標(biāo)準(zhǔn)鋯石(230Th)/(238U)測(cè)試結(jié)果

本文采用的標(biāo)準(zhǔn)鋯石是目前世界上主要的U-Pb定年實(shí)驗(yàn)室采用的幾個(gè)U-Pb定年標(biāo)樣Plesovice、91500和Qinghu。它們都具有諧和的U-Pb年齡, 表明礦物結(jié)晶后U-Th-Pb體系一直處于封閉狀態(tài)。它們所有鈾系同位素都處于久期平衡狀態(tài), 故(230Th)/(238U)理論值為1, 因此是檢驗(yàn)我們分析方法測(cè)試(230Th)/(238U)比值準(zhǔn)確度和精密度的良好對(duì)象。本文利用SIMS在約50 nA一次束流條件下, 以鋯石Plesovice為外標(biāo)獲得儀器Th/U相對(duì)靈敏度因子, 然后對(duì)Qinghu和91500鋯石標(biāo)準(zhǔn)進(jìn)行了分析, 分析結(jié)果如下(詳細(xì)數(shù)據(jù)見(jiàn)表2)。

(1) Qinghu鋯石: 該鋯石為中國(guó)科學(xué)院地質(zhì)與地球物理研究所離子探針實(shí)驗(yàn)室自主研發(fā)的U-Pb年齡、Hf-O同位素標(biāo)準(zhǔn)鋯石, U-Pb年齡為(159.5±0.2) Ma, U、Th含量較高, 且不均一[35]。本研究對(duì)Qinghu鋯石進(jìn)行了3個(gè)測(cè)試流程的分析, 經(jīng)過(guò)校正得到26個(gè)數(shù)據(jù)點(diǎn)(230Th)/(238U)加權(quán)平均值為1.014±0.013 (2σ, MSWD = 1.7) (圖2a), 點(diǎn)到點(diǎn)重現(xiàn)性為1.3%, 準(zhǔn)確度(即測(cè)定的(230Th)/(238U)加權(quán)平均值與理論值1的比值, 下同)為98.6%。

表2 標(biāo)準(zhǔn)鋯石測(cè)試結(jié)果

(續(xù)表2)

(2) 91500鋯石: 該標(biāo)準(zhǔn)鋯石產(chǎn)自加拿大安大略省Renfrew地區(qū), 結(jié)晶年齡1065 Ma, U含量為81.2 μg/g,Th含量為28.61 μg/g[32, 36], U、Th較均一。本研究對(duì)91500進(jìn)行了3個(gè)測(cè)試流程的分析, 經(jīng)過(guò)校正得到28個(gè)數(shù)據(jù)點(diǎn)的(230Th)/(238U)加權(quán)平均值為1.019± 0.023 (2σ, MSWD = 0.68) (圖2b), 點(diǎn)到點(diǎn)重現(xiàn)性為2.3%, 準(zhǔn)確度為98.1%。

3.2 騰沖第四紀(jì)火山巖樣品定年

我們利用上述方法在大約15 nA一次束流條件下, 以Plesovice為外標(biāo)得到相對(duì)靈敏度因子RSF為1.105±0.018, 對(duì)云南騰沖地區(qū)兩個(gè)前人研究過(guò)的第四紀(jì)火山巖進(jìn)行了原位微區(qū)鋯石U-Th定年, 獲得結(jié)果如下(詳細(xì)分析結(jié)果見(jiàn)表3)。

(2) 打鷹山安山巖13TC12: 該樣品采自云南騰沖打鷹山火山東南側(cè)約3 km處(GPS: 25°07.252′N(xiāo), 98°28.279′E), 樣品呈灰白色, 氣孔發(fā)育并被后期填充, 粒度小, 可見(jiàn)1~1.5 cm的斜長(zhǎng)石斑晶。鋯石顆粒長(zhǎng)度變化于60~200 μm之間, 長(zhǎng)寬比為1∶1至3.5∶1, 無(wú)色透明, 少數(shù)呈淡棕色或粉紅色。本研究得到了該樣品17個(gè)點(diǎn)的U-Th分析結(jié)果, 這些數(shù)據(jù)大致沿著一條等時(shí)線(xiàn)分布, 初始230Th/232Th比值為0.124, 等時(shí)線(xiàn)年齡為(85.6±7.6) ka (2σ, MSWD = 2.1) (圖3)。與Tucker[37]報(bào)道的該地區(qū)火山巖樣品鋯石等時(shí)線(xiàn)年齡(87.5±6.5) ka (MSWD = 3.7)在誤差范圍內(nèi)一致。

4 討論和結(jié)論

利用中國(guó)科學(xué)院廣州地球化學(xué)研究所同位素國(guó)家重點(diǎn)實(shí)驗(yàn)室新引進(jìn)的CAMECA IMS 1280-HR離子探針對(duì)U-Pb定年標(biāo)準(zhǔn)鋯石Qinghu和91500以及已知U-Th年齡的騰沖馬鞍山、打鷹山安山巖樣品中的鋯石進(jìn)行了U-Th分析, 得到處于U-Th平衡狀態(tài)的U-Pb年齡標(biāo)準(zhǔn)鋯石Qinghu和91500的(230Th)/(238U)重現(xiàn)性分別為1.3%和2.3%; 準(zhǔn)確度分別為98.6%和98.1%, 與國(guó)外實(shí)驗(yàn)室報(bào)道的該比值重現(xiàn)性和準(zhǔn)確度水平相當(dāng)[20,27,31,36,38–40]。對(duì)已知U-Th年齡的云南騰沖第四紀(jì)火山巖樣品進(jìn)行分析獲得的等時(shí)線(xiàn)年齡跟前人研究結(jié)果在誤差范圍內(nèi)也完全一致, 表明我們成功建立了SIMS原位微區(qū)U-Th定年方法。

圖2 鋯石U-Pb定年標(biāo)樣的(230Th)/(238U)測(cè)試(數(shù)據(jù)誤差棒為1σ)

表3 騰沖火山鋯石U-Th同位素比值、含量與年齡測(cè)試結(jié)果

注: (1) 本研究采用計(jì)算一次實(shí)驗(yàn)中所有標(biāo)準(zhǔn)鋯石測(cè)試點(diǎn)的U計(jì)數(shù)平均值對(duì)應(yīng)一個(gè)統(tǒng)計(jì)的平均U含量(本研究中對(duì)標(biāo)準(zhǔn)樣品Plesovice采用800 μg/g U含量)的方法來(lái)計(jì)算未知樣品每一點(diǎn)的U含量;(2) 表中年齡為鋯石-全巖兩點(diǎn)模式年齡, 誤差1σ, 全巖數(shù)據(jù)分別引自文獻(xiàn)Zou.[20]和 Tucker[37]。

圖3 騰沖火山鋯石U-Th等時(shí)線(xiàn)年齡圖

全巖數(shù)據(jù)分別引自文獻(xiàn)Zou.[20]和Tucker[37]。

Whole rock data sources: Zou.[20]and Tucker[37]

野外和實(shí)驗(yàn)過(guò)程中得到了中國(guó)科學(xué)院廣州地球化學(xué)研究所張兆峰研究員、張樂(lè)工程師、劉芳同學(xué)的支持和幫助, 在此一并表示感謝。同時(shí)感謝審稿人對(duì)本文提出的寶貴意見(jiàn)和建議。本研究受到廣州市科技計(jì)劃項(xiàng)目二次離子質(zhì)譜原位微區(qū)分析技術(shù)開(kāi)發(fā)與應(yīng)用(201607020029)以及國(guó)家自然科學(xué)基金(41173007)資助。

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U-Th dating of zircons by secondary ion mass spectrometry: A case study of the volcanic rocks from Tengchong, Yunnan Province

LI Ling1’2, XIA Xiao-ping1*, YANG Qing1and LI Ru-cao1,2

1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China

This paper reports a method ofU-Th dating of Quaternary zircons by a CAMECA IMS 1280-HR Secondary Ion Mass Spectrometer installed at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. Analytical settings included mass-filtered16O–beam, ~25 μm diameter spot size, and ~6000 mass resolution. A single electron multiplier collector was used to collect the related mass peaks with peak-jumping mode. Each analysis was consisted of 35 cycles and a total analytical time was about 27 minutes. Using zircon Plesovice as an external standard, the measured ratios of (230Th)/(238U) for standard zircon 91500 and Qinghu in this study showed a reproducibility of 2.3% and 1.3%and an accuracy of 98.1% and 98.6%, respectively, which stand at a level of what was reported previously. Quaternary zircon crystals separated from the Dayingshan and Maanshan volcanic rocks at Tengchong, Southwest China, were analyzed in this study and yielded the isochron ages of (84.1±9.2) ka (2σ, MSWD = 2.9) and (85.6±7.6) ka (2σ, MSWD = 2.1), respectively. These results show an excellent agreement with previous data, and indicate that the method is successfully established.

SIMS; U-Th disequilibrium dating;micro-analysis; Tengchong volcanic rock

P597

A

0379-1726(2016)04-0398-09

2016-04-03;

2016-05-27;

2016-06-16

廣州市科技計(jì)劃項(xiàng)目(二次離子質(zhì)譜原位微區(qū)分析技術(shù)開(kāi)發(fā)與應(yīng)用) (201607020029); 國(guó)家自然科學(xué)基金(41173007)

李玲(1990–), 女, 碩士研究生, 地球化學(xué)專(zhuān)業(yè)。E-mail: liling19900602@163.com

XIA Xiao-ping, E-mail: xpxia@gig.ac.cn, Tel: +86-20-85292137