天然麻粒巖高溫流變實(shí)驗(yàn)研究

張慧婷，周永勝，姚文明，何昌榮，黨嘉祥

中國(guó)地震局地質(zhì)研究所，地震動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室，北京 100029

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天然麻粒巖高溫流變實(shí)驗(yàn)研究

張慧婷，周永勝*，姚文明，何昌榮，黨嘉祥

中國(guó)地震局地質(zhì)研究所，地震動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室，北京 100029

本實(shí)驗(yàn)在氣體介質(zhì)三軸高溫流變儀上，采用懷安瓦窯口麻粒巖，在溫度900～1200 ℃、圍壓300 MPa、應(yīng)變速率10-4～10-6/s條件下，開(kāi)展高溫流變實(shí)驗(yàn).實(shí)驗(yàn)樣品麻粒巖由斜長(zhǎng)石(52%)、單斜輝石和斜方輝石(40%)、石英(3%)、磁鐵礦和鈦鐵礦(5%)組成，礦物平均粒度為：斜長(zhǎng)石294 μm、單斜輝石和斜方輝石282 μm、石英97 μm、磁鐵礦和鈦鐵礦109 μm.利用傅里葉變換紅外光譜儀分析獲得變形后樣品的水含量約為0.17±0.05wt%.實(shí)驗(yàn)樣品的強(qiáng)度隨溫度升高而降低，隨應(yīng)變速率降低而降低.基于力學(xué)數(shù)據(jù)，采用穩(wěn)態(tài)流變方程，獲得實(shí)驗(yàn)樣品在900～1000 ℃時(shí)的應(yīng)力指數(shù)為8.1～12.9，在1050～1150 ℃時(shí)的應(yīng)力指數(shù)為 4.8～5.8,平均值5.2.應(yīng)力指數(shù)隨著溫度升高而降低.顯微結(jié)構(gòu)和成分分析表明，在900 ℃時(shí)麻粒巖出現(xiàn)礦物壓扁與定向拉長(zhǎng)特征，樣品以位錯(cuò)滑移和微破裂變形為主；在950～1000 ℃時(shí)，麻粒巖樣品中顆粒邊界變得圓滑，表現(xiàn)出位錯(cuò)攀移特征，輝石和磁鐵礦邊緣出現(xiàn)微量熔體；在1050～1200 ℃時(shí)麻粒巖出現(xiàn)部分熔融，而且隨著溫度和實(shí)驗(yàn)時(shí)間(應(yīng)變)增加，熔體含量增加，熔體結(jié)晶出微粒斜長(zhǎng)石、輝石和橄欖石，部分輝石通過(guò)固體反應(yīng)生成橄欖石.顆粒邊界熔體和礦物反應(yīng)促進(jìn)了擴(kuò)散作用，導(dǎo)致位錯(cuò)攀移和熔體引起的擴(kuò)散蠕變共同控制了麻粒巖的高溫流變.

麻粒巖；流變；變形機(jī)制；熔體；礦物反應(yīng)

1 引言

大陸下地殼流變是大陸板內(nèi)構(gòu)造變形、強(qiáng)震孕育和發(fā)生的基礎(chǔ)，是大陸動(dòng)力學(xué)研究的前緣課題.麻粒巖流變實(shí)驗(yàn)是研究大陸下地殼流變特性的主要途徑之一，高溫流變實(shí)驗(yàn)結(jié)果為確定大陸下地殼流變強(qiáng)度提供了必要的數(shù)據(jù)(例如，Brace and Kohlstedt,1980;Bürgmann and Dresen,2008).早期的基性巖流變實(shí)驗(yàn)多用斜長(zhǎng)石和輝石，如單斜輝石單晶(Kollé and Blacic,1982,1983;Ingrin et al.,1991;Raterron and Jaoul,1991;Jaoul and Raterron,1994;Raterron et al.,1994)、集合體 (Kirby and Kronenberg,1984;Boland and Tullis,1986)和斜長(zhǎng)石集合體 (Shelton and Tullis,1981;Tullis and Yund,1985,1987,1991,1992;Tullis et al.,1996)；近年來(lái)研究主要集中在單斜輝石(Bystricky and Mackwell,2001;Mauler et al.,2000;Dimanov and Dresen,2005;Dimanov et al.,2003;Chen et al.,2006;Hier-Majumder et al.,2005;Dimanov et al.,2007)、斜方輝石(Ohuchi et al.,2011)、微量水與顆粒粒度對(duì)鈣長(zhǎng)石(Rybacki and Dresen,2000,2004;Dimanov et al.,1998,1999,2000,2003;Spiess et al.,2012；Rybacki et al.,2006,2008,2010)、鈣長(zhǎng)石與透輝石組合(Dimanov and Dresen,2005；Dimanov et al.,2003，2011).熱壓合成兩相基性礦物集合體的流變實(shí)驗(yàn)取得了進(jìn)展，如橄欖石-輝石流變(Hitchings et al.,1989;McDonnell et al.,2000;Ji et al.,2001)、鈣長(zhǎng)石與透輝石流變(Dimanov and Dresen,2005;Dimanov et al.,2007,2011)、橄欖石-長(zhǎng)石及其反應(yīng)產(chǎn)物輝石的流變(de Kloe et al.,2000;de Ronde et al.,2004,2005).Zhou等(2009)討論了三相礦物組合的斜長(zhǎng)角閃巖、角閃輝長(zhǎng)巖、變粒巖、閃長(zhǎng)巖的流變.近年來(lái)，通過(guò)榴輝巖與綠輝石集合體(Jin et al.,2001;金振民等，2002；Zhang et al.,2006;Zhang and Green,2007a,b)、輝綠巖(Mackwell et al.,1998)、輝長(zhǎng)巖(Zhou et al.,2009,2012a,b；He et al.,2003)、熱壓合成麻粒巖(王永鋒等，2008；李麗敏等，2011;Zhang and Green,2007b;Wang et al.,2012)等基性巖樣品的流變實(shí)驗(yàn)，給出了基性巖的流變參數(shù)和變形機(jī)制.

上述這些流變實(shí)驗(yàn)使用的樣品成分、粒度、含水量等有很大差別，而且與真實(shí)大陸下地殼成分有一定差別.由于成分、粒度、含水等都顯著地影響流變實(shí)驗(yàn)結(jié)果，利用這些基性巖流變實(shí)驗(yàn)數(shù)據(jù)給出的大陸下地殼流變結(jié)構(gòu)比較復(fù)雜(Jackson,2002;Burov and Watts,2006;Bürgmann and Dresen,2008;Zhou et al.,2009)，其中，一些模型顯示出大陸具有弱的下地殼(Wang et al.,2012)，而另一些模型給出相對(duì)強(qiáng)的下地殼(Mackwell et al.,1998)，仍然不能解決大陸下地殼流變強(qiáng)度(周永勝,2013)問(wèn)題.

麻粒巖作為大陸下地殼的主要巖石，其流變特性是大陸動(dòng)力學(xué)中最為關(guān)注的前沿課題之一，開(kāi)展天然麻粒巖流變實(shí)驗(yàn)，可以為揭示大陸下地殼的真實(shí)流變提供最直接的實(shí)驗(yàn)證據(jù).然而，有關(guān)麻粒巖流變實(shí)驗(yàn)研究非常少，而且都采用熱壓合成麻粒巖(王永鋒等，2008；李麗敏等，2011;Zhang and Green,2007b;Wang et al.,2012)，缺少天然麻粒巖的流變實(shí)驗(yàn)研究.

本論文采用天然麻粒巖在氣體介質(zhì)三軸高溫流變儀開(kāi)展高溫流變實(shí)驗(yàn)，用傅里葉變換紅外光譜儀(FTIR)分析實(shí)驗(yàn)樣品中的水含量，利用電子探針、掃描電鏡與能譜等分析實(shí)驗(yàn)變形樣品的微觀結(jié)構(gòu)與成分，研究天熱麻粒巖高溫流變特征.

2 實(shí)驗(yàn)樣品與實(shí)驗(yàn)條件

2.1 實(shí)驗(yàn)樣品麻粒巖成分、微量水分析

實(shí)驗(yàn)樣品麻粒巖采集自河北懷安瓦窯口.樣品中礦物隨機(jī)分布，沒(méi)有定向組構(gòu)，顆粒邊界平直，表明沒(méi)有經(jīng)過(guò)顯著的構(gòu)造變形(圖1).在掃描電鏡下，利用線測(cè)法(Zhou et al.,2012)統(tǒng)計(jì)的礦物平均粒度為：斜長(zhǎng)石294±233 μm、單斜輝石和斜方輝石282±193 μm、石英97±75 μm、磁鐵礦和鈦鐵礦109±96 μm(圖2).利用掃描電鏡圖像統(tǒng)計(jì)出的主要礦物含量為：斜長(zhǎng)石(52%)、單斜輝石和斜方輝石(40%)，次要礦物有石英(3%)、磁鐵礦和鈦鐵礦(5%).根據(jù)電子探針數(shù)據(jù)分析(JXA-8100電子探針?lè)治鰞x)計(jì)算出實(shí)驗(yàn)用麻粒巖的標(biāo)準(zhǔn)礦物組成為(圖3)：斜長(zhǎng)石An45-47Ab45-48Or1-2，單斜輝石En29-33Fs17-19Wo42-45和斜方輝石En49-50Fs47Wo0-1.X射線熒光光譜分析(AxiosmAX X射線熒光光譜儀)，得到樣品全巖化學(xué)成分為SiO2(48.96wt%)、AlO3(13.52wt%)、CaO(10.21wt%)、FeO(15.95wt%)、MgO(6.26wt%)、Na2O(2.64wt%)、K2O(0.271wt%)、TiO2(1.38wt%)、MnO(0.224wt%)、P2O5(0.126wt%).

圖1 實(shí)驗(yàn)樣品天然麻粒巖在偏光顯微鏡下的微觀結(jié)構(gòu)(a) 單偏光；(b) 正交偏光.Fig.1 Fabrics of starting sample of graunlite under optical microscope(a) Polarized light;(b) Crossed light.

圖2 實(shí)驗(yàn)用麻粒巖中主要礦物的粒度統(tǒng)計(jì)Fig.2 Grain size distribution of main minerals in starting sample of granulite

利用地震動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室的傅里葉變換紅外吸收光譜儀(FTIR,Bruker VERTEX 70V,Hyperion 1000)對(duì)實(shí)驗(yàn)后的樣品進(jìn)行了水含量分析，得到實(shí)驗(yàn)樣品水含量約為0.17wt%，其中斜長(zhǎng)石顆粒的含水量0.02%～0.25%，平均值0.12%，而輝石顆粒的含水量0.05%～0.6%，平均值0.22%.

2.2 實(shí)驗(yàn)設(shè)備與實(shí)驗(yàn)條件

實(shí)驗(yàn)設(shè)備為氣體介質(zhì)三軸高溫流變儀，該實(shí)驗(yàn)設(shè)備采用了內(nèi)置和外置應(yīng)力傳感器，可以準(zhǔn)確獲得應(yīng)力與摩擦力，保證實(shí)驗(yàn)數(shù)據(jù)有更高的精度.加溫爐采用三段電阻絲加溫，通過(guò)控制三段的輸出功率控制溫度分布，確保樣品腔內(nèi)部溫度均勻.在設(shè)備投入使用前在300 ℃到1200 ℃范圍內(nèi)進(jìn)行了溫度標(biāo)定.軸壓采用伺服控制，可實(shí)現(xiàn)在應(yīng)力與位移控制之間實(shí)現(xiàn)平穩(wěn)切換.

圖3 實(shí)驗(yàn)樣品麻粒巖中斜長(zhǎng)石(a)和輝石成分(b)Fig.3 The main mineral composition of the starting natural granulite,measured by microprobe analysis.(a) Plagioclase and (b) Pyroxene

實(shí)驗(yàn)在溫度900～1200 ℃、圍壓300 MPa、應(yīng)變速率10-4～10-6/s條件下進(jìn)行.實(shí)驗(yàn)采用等應(yīng)變速率控制加載.為了求得應(yīng)力指數(shù)，在固定圍壓、溫度條件下進(jìn)行多個(gè)等應(yīng)變速率臺(tái)階.部分實(shí)驗(yàn)在應(yīng)變速率臺(tái)階的基礎(chǔ)上開(kāi)展了改變溫度的流變實(shí)驗(yàn).

3 實(shí)驗(yàn)力學(xué)數(shù)據(jù)

表1給出了全部實(shí)驗(yàn)樣品、實(shí)驗(yàn)條件、實(shí)驗(yàn)力學(xué)數(shù)據(jù)和樣品含水量.從應(yīng)力-應(yīng)變曲線看(圖4)，隨著溫度升高，麻粒巖強(qiáng)度逐漸降低，麻粒巖樣品的強(qiáng)度從900 ℃(14WYK-06-01)時(shí)的764 MPa下降到了1140 ℃(14WYK-06-11)時(shí)的112 MPa.樣品強(qiáng)度隨著應(yīng)變速率降低而降低.在900～1000 ℃，部分樣品屈服后呈現(xiàn)出應(yīng)變強(qiáng)化的現(xiàn)象，例如，在溫度1000 ℃(14WYK-06-03)、應(yīng)變速率3.125×10-6/s條件下，麻粒巖樣品在應(yīng)變量接近15%時(shí)顯示出了應(yīng)變強(qiáng)化的趨勢(shì).與此相反，1050 ℃以上，樣品屈服后呈現(xiàn)出應(yīng)變?nèi)趸默F(xiàn)象，例如，在1050 ℃(14WYK-06-04)和1100 ℃(14WYK-06-05)溫度下，樣品在各應(yīng)變速率條件下出現(xiàn)了比較明顯的應(yīng)變?nèi)趸F(xiàn)象.由此推測(cè)，在1000 ℃之下，樣品的位錯(cuò)過(guò)程大于重結(jié)晶過(guò)程，而在1050 ℃以上，重結(jié)晶過(guò)程大于位錯(cuò)過(guò)程，甚至可能出現(xiàn)了部分熔融.

圖4 高溫流變實(shí)驗(yàn)的應(yīng)力-應(yīng)變曲線Fig.4 The stress-strain curves of deformed granulite

編號(hào)圍壓(MPa)溫度(℃)應(yīng)力(MPa)應(yīng)變速率(s-1)總應(yīng)變量(%)含水量(wt%)SEMEDS14WYK-06-013009007645×10-57.4Bulk:0.20±0.127061×10-58.7Pl:0.12±0.056415×10-69.7Py:0.29±0.16YY14WYK-06-023009506395×10-59.26142.5×10-510.65901.25×10-511.85736.25×10-612.54723.125×10-613.5Bulk:0.20±0.10Pl:0.14±0.04Py:0.26±0.12YY14WYK-06-0330010005965×10-59.85462.5×10-511.24941.25×10-512.44596.25×10-613.44253.125×10-615.1Bulk:0.10±0.04Pl:0.09±0.03Py:0.12±0.04YY14WYK-06-0430010504525×10-58.94072.5×10-510.23601.25×10-511.23136.25×10-612.2Bulk:0.26±0.14Pl:0.24±0.13Py:0.28±0.15YY14WYK-06-0530011003215×10-59.42822.5×10-510.42511.25×10-511.42246.25×10-612.4Bulk:0.14±0.06Pl:0.11±0.04Py:0.17±0.09YY14WYK-06-063001150965×10-53.7892.5×10-54.7791.25×10-55.7596.25×10-66.8423.125×10-67.8Bulk:0.12±0.04Pl:0.10±0.04Py:0.13±0.04YY14WYK-06-0730011251415×10-56.51202.5×10-57.51021.25×10-58.5846.25×10-69.5Bulk:0.23±0.14Pl:0.12±0.04Py:0.34±0.13YY14WYK-06-093001200125×10-5-Bulk:0.11±0.05Pl:0.09±0.03Py:0.13±0.06YY14WYK-06-103001140無(wú)無(wú)無(wú)Bulk:0.11±0.03Pl:0.10±0.02Py:0.12±0.04YY14WYK-06-1130011401125×10-54.51002.5×10-56.2851.25×10-57.4716.25×10-68.4663.125×10-69.5Bulk:0.17±0.14Pl:0.08±0.02Py:0.27±0.22YY

續(xù)表1

注：所有樣品的應(yīng)力(強(qiáng)度)均為應(yīng)變量7.8～15.1%時(shí)的平均強(qiáng)度精確到MPa；SEM和EDS兩欄中Y表示進(jìn)行了該項(xiàng)測(cè)試、N表示沒(méi)有進(jìn)行該項(xiàng)測(cè)試.

本研究采用常用的穩(wěn)態(tài)流變方程(1)來(lái)描述實(shí)驗(yàn)結(jié)果：

(1)

根據(jù)實(shí)驗(yàn)的差應(yīng)力和應(yīng)變速率(表1)可以求得應(yīng)力指數(shù)(圖5).應(yīng)力指數(shù)隨著溫度的升高而降低，在900～1000 ℃，應(yīng)力指數(shù)為8.1～12.9，在溫度1050～1150 ℃時(shí)降到4.8～5.8，表現(xiàn)出了位錯(cuò)蠕變特征.

圖5 根據(jù)應(yīng)力-應(yīng)變速率關(guān)系獲得的應(yīng)力指數(shù)Fig.5 Stress exponents based on data of stress-strain rate

4 實(shí)驗(yàn)變形樣品微觀結(jié)構(gòu)

隨著實(shí)驗(yàn)溫度從900 ℃上升到1200 ℃，麻粒巖樣品變形微觀結(jié)構(gòu)出現(xiàn)了顯著的變化.微觀結(jié)構(gòu)與成分分析表明，在900 ℃條件下斜長(zhǎng)石和輝石都明顯地出現(xiàn)壓扁和拉長(zhǎng)現(xiàn)象，表現(xiàn)出位錯(cuò)滑移特征；樣品中普遍存在晶內(nèi)破裂，礦物顆粒之間以棱角接觸，顯示出半脆性破裂特征(圖6a).950～1000 ℃溫度條件下，在樣品上下端部區(qū)域出現(xiàn)礦物壓扁，沿壓縮方向45°出現(xiàn)形態(tài)定向，晶內(nèi)破裂現(xiàn)象逐漸減少甚至消失，顆粒邊界亞顆粒化，樣品中部部分礦物顆粒邊界變得圓滑，顯示出位錯(cuò)攀移特征；輝石和磁鐵礦邊緣局部出現(xiàn)微量熔體，成分分析顯示為富鐵熔體，而且缺少鈉與鈣，顯示斜長(zhǎng)石沒(méi)有參與熔融(圖6b).在變形過(guò)程中，熔體沿斜長(zhǎng)石顆粒邊界擴(kuò)散和遷移，導(dǎo)致在斜長(zhǎng)石顆粒之間出現(xiàn)了富鐵熔體.表明在950～1000 ℃條件下，位錯(cuò)蠕變逐漸取代位錯(cuò)滑移和半脆性破裂，成為樣品變形的主要機(jī)制.1050～1200 ℃溫度條件下，礦物變形不顯著，但出現(xiàn)顯著的部分熔融與礦物反應(yīng).隨著溫度和實(shí)驗(yàn)時(shí)間(應(yīng)變)增加，熔體含量增加，熔體分布于多數(shù)礦物邊緣(圖6c)，而且熔體擴(kuò)散遷移特征顯著(圖6e).成分分析顯示，熔體成分強(qiáng)烈依賴(lài)于參與熔融的礦物成分，雖然大部分熔體中普遍富含鐵，但斜長(zhǎng)石邊緣的熔體成分比輝石邊緣熔體成分更富鈉與鈣，顯示在高溫條件下，斜長(zhǎng)石開(kāi)始參與熔融.在1100～1200 ℃條件下，熔體結(jié)晶出細(xì)粒針狀斜長(zhǎng)石和長(zhǎng)柱狀輝石，以及等粒狀橄欖石(圖6d)，部分輝石通過(guò)固體反應(yīng)生成橄欖石(圖6f).因此，在高溫條件下，顆粒邊界熔體和礦物反應(yīng)促進(jìn)了擴(kuò)散作用，導(dǎo)致位錯(cuò)攀移和熔體引起的擴(kuò)散蠕變共同控制了麻粒巖的流變.

5 討論

與成分類(lèi)似的輝長(zhǎng)巖(Zhou et al.,2012a,b)和熱壓合成麻粒巖(Wang et al.,2012)流變實(shí)驗(yàn)結(jié)果對(duì)比，本實(shí)驗(yàn)麻粒巖具有很高的強(qiáng)度與應(yīng)力指數(shù).圖7顯示，在相同溫度條件下輝長(zhǎng)巖(Zhou et al.,2012b)的應(yīng)力遠(yuǎn)小于天然麻粒巖相應(yīng)條件下的應(yīng)力；而輝長(zhǎng)巖(Zhou et al.,2012b)在800～1050 ℃的應(yīng)力指數(shù)約為3.0，本實(shí)驗(yàn)麻粒巖在相同溫度條件下的應(yīng)力指數(shù)5.6～12.9遠(yuǎn)大于輝長(zhǎng)巖的應(yīng)力指數(shù).在727～977 ℃溫度條件下，熱壓合成基性麻粒巖(Wang et al.,2012)的應(yīng)力范圍與本實(shí)驗(yàn)中性麻粒巖的應(yīng)力范圍相當(dāng)，但熱壓合成基性麻粒巖(Wang et al.,2012)的應(yīng)力指數(shù)約2.7，遠(yuǎn)小于本實(shí)驗(yàn)的應(yīng)力指數(shù).出現(xiàn)這種現(xiàn)象的原因可能與礦物粒度、實(shí)驗(yàn)樣品是否達(dá)到穩(wěn)態(tài)流變、以及礦物反應(yīng)與部分熔融相關(guān).

圖6 變形樣品微觀結(jié)構(gòu)(a) 900 ℃時(shí)，箭頭指示礦物被壓縮方向，方框區(qū)域?yàn)榫?nèi)微破裂密集區(qū)；(b) 1000 ℃時(shí)，方框區(qū)域?yàn)榫?nèi)微破裂密集區(qū)，箭頭指示礦物間出現(xiàn)的微量熔體；(c) 1140 ℃時(shí)，橢圓區(qū)域?yàn)槿垠w條帶；(d) 1140 ℃時(shí)，箭頭指示熔體結(jié)晶出細(xì)粒針狀斜長(zhǎng)石和長(zhǎng)柱狀輝石與等粒橄欖石；(e) 1140 ℃時(shí)，箭頭指示輝石邊緣的富鐵熔體被擠壓遷移到長(zhǎng)石邊緣；(f) 1140 ℃時(shí)，橢圓區(qū)域?yàn)椴糠州x石反應(yīng)生成橄欖石.pl為斜長(zhǎng)石，cpx為單斜輝石，opx為斜方輝石，mag為磁鐵礦，ilm為鈦鐵礦，qtz為石英，ol為橄欖石.Fig.6 Micro-structures of experimental deformed granulite under SEM(a) The arrows refer to the compressed direction of deformed sample,and the rectangle shows area where fractures develop in crystals at 900 ℃;(b) The rectangle shows area where fractures develop in crystals and the arrows show trace melts appeared at the grain boundaries at 1000 ℃;(c) The ellipse shows melts at 1140 ℃;(d) The arrows show new minerals of fine-grained plagioclase,pyroxene and olivine crystallized from melt at 1140 ℃;(e) The arrows show iron-rich melts around grains of pyroxene immigrated into grain boundaries of plagioclase at 1140 ℃;(f) The ellipse shows new grains of olivine reaction from pyroxene at 1140 ℃.

圖7 本實(shí)驗(yàn)結(jié)果與輝長(zhǎng)巖和熱壓合成麻粒巖對(duì)比圖Fig.7 Stress versus strain rate of this study and compared with data of deformed gabbro and hot-pressed granulite

5.1 礦物粒度對(duì)麻粒巖流變強(qiáng)度的影響

本次流變實(shí)驗(yàn)樣品的礦物平均粒度在300 μm左右，而輝長(zhǎng)巖的粒度在100～150 μm(Zhou et al.,2012a,b)和熱壓合成麻粒巖的粒度在60～100 μm(Wang et al.,2012).麻粒巖粒度大，樣品粒度和分布非均勻?qū)е逻_(dá)到穩(wěn)態(tài)流變難度增加.在輝石-橄欖石集合體的變形機(jī)制圖中(圖8)，存在擴(kuò)散蠕變-位錯(cuò)蠕變過(guò)渡帶.在該過(guò)渡帶，由于顆粒邊界滑移型位錯(cuò)主導(dǎo)了變形，樣品粒度對(duì)流變的影響比較顯著.在圖8中，本次實(shí)驗(yàn)樣品的應(yīng)力-粒度范圍如圖中方框所示，正處于擴(kuò)散-位錯(cuò)蠕變過(guò)渡帶，在該過(guò)渡區(qū)，粒度與應(yīng)力具有一定的正線性相關(guān)性.本研究中樣品礦物顆粒粒度較大，對(duì)應(yīng)的應(yīng)力也較大，而且粒度引起非均勻流變，出現(xiàn)高應(yīng)力指數(shù)特征.

圖8 輝石和橄欖石集合體的樣品變形機(jī)制(Hansen and Warren ,2015).其中，方框范圍是本實(shí)驗(yàn)結(jié)果，矩形范圍是輝石-橄欖石集合體的結(jié)果(Hansen and Warren,2015)Fig.8 Deformation mechanism for aggregates of pyroxene and olivine.Square shows the results of this study,and rectangle shows results of aggregate of pyroxene and olivine by Hansen and Warren (2015)

5.2 實(shí)驗(yàn)加載方式、變形機(jī)制和應(yīng)力取值對(duì)應(yīng)力指數(shù)的影響

由于在高溫條件下樣品出現(xiàn)應(yīng)變?nèi)趸?，因此，在?yīng)力取值時(shí)，取峰值和弱化段的穩(wěn)態(tài)值對(duì)求取應(yīng)力指數(shù)有很大的影響(圖9)，特別是在改變應(yīng)變速率的實(shí)驗(yàn)中，樣品變形是否達(dá)到穩(wěn)態(tài)，對(duì)應(yīng)力指數(shù)的影響很大.在圖9a中，應(yīng)力取峰值時(shí)(圖9a點(diǎn)線)，為樣品恒定變形微觀結(jié)構(gòu)狀態(tài)，而取弱化段的穩(wěn)態(tài)值(圖9a短斷線)則為樣品穩(wěn)態(tài)流變微觀結(jié)構(gòu).通常指的穩(wěn)態(tài)流變即為應(yīng)力穩(wěn)態(tài)值對(duì)應(yīng)的流變狀態(tài).在圖9b中，取峰值應(yīng)力時(shí)，對(duì)應(yīng)的應(yīng)力指數(shù)相對(duì)比較小(圖9b點(diǎn)線)；取弱化段的穩(wěn)態(tài)應(yīng)力值，對(duì)應(yīng)的應(yīng)力指數(shù)相對(duì)比較大(圖9b實(shí)線).本研究中，加載方式采用改變應(yīng)變速率的加載方式，在每一個(gè)應(yīng)變速率條件下，應(yīng)力值都取穩(wěn)態(tài)值，而不是峰值，因此，對(duì)應(yīng)的應(yīng)力指數(shù)比較大.另外，從應(yīng)力-應(yīng)變曲線中發(fā)現(xiàn)，并不是所有實(shí)驗(yàn)的應(yīng)力都出現(xiàn)峰值，然后轉(zhuǎn)變到弱化和穩(wěn)態(tài)，部分實(shí)驗(yàn)沒(méi)有出現(xiàn)應(yīng)力弱化段.部分實(shí)驗(yàn)在切換應(yīng)變速率后，有可能沒(méi)有達(dá)到穩(wěn)態(tài)流變狀態(tài)，引起應(yīng)力取值不統(tǒng)一，影響了求取應(yīng)力指數(shù)的準(zhǔn)確性.

圖9 樣品變形過(guò)程對(duì)應(yīng)力指數(shù)的影響(Hansen et al.,2012)Fig.9 The effect of experimental process to the stress exponent

5.3 礦物反應(yīng)與部分熔融對(duì)麻粒巖流變的影響

微觀結(jié)構(gòu)和成分分析顯示(圖10)，950 ℃時(shí)輝石和磁鐵礦邊界出現(xiàn)微量熔體.1000 ℃時(shí)輝石邊界發(fā)生反應(yīng)生成細(xì)粒橄欖石；1050 ℃時(shí)，局部長(zhǎng)石邊界和輝石的內(nèi)部出現(xiàn)淺色小顆粒橄欖石；1100～1150 ℃時(shí)，輝石普遍轉(zhuǎn)變?yōu)榱铋蠙焓?圖 10(a,b，f))，同時(shí)出現(xiàn)部分熔融，熔體結(jié)晶出細(xì)粒針柱狀礦物斜長(zhǎng)石和長(zhǎng)柱狀輝石以及等粒橄欖石(圖10(c,d,e)).這些礦物反應(yīng)與部分熔融增加了麻粒巖流變的復(fù)雜性.

輝長(zhǎng)巖高溫流變實(shí)驗(yàn)(Zhou et al.,2012a)表明，熔體擴(kuò)散和熔體引起的顆粒邊界弱化，導(dǎo)致實(shí)驗(yàn)樣品的應(yīng)力隨實(shí)驗(yàn)時(shí)間(樣品應(yīng)變)變小，熔體弱化了樣品強(qiáng)度.另一方面，在實(shí)驗(yàn)變形過(guò)程中，輝石轉(zhuǎn)變?yōu)殚蠙焓?，由于橄欖石的流變?qiáng)度高于輝石，導(dǎo)致實(shí)驗(yàn)樣品的應(yīng)力隨實(shí)驗(yàn)時(shí)間(樣品應(yīng)變)變大，礦物反應(yīng)強(qiáng)化了樣品強(qiáng)度(Zhou et al.,2012b).這兩種過(guò)程同時(shí)出現(xiàn)在天然麻粒巖高溫流變實(shí)驗(yàn)中，導(dǎo)致其變形機(jī)制非常復(fù)雜，應(yīng)力指數(shù)對(duì)比(圖7)中清楚地反應(yīng)出這種復(fù)雜性.

5.4 水對(duì)麻粒巖流變的影響

以基性麻粒巖為主的下地殼也存在一定的結(jié)構(gòu)水.對(duì)漢諾壩和女山捕虜體麻粒巖和地體麻粒巖中主要組成礦物單斜輝石、斜方輝石、斜長(zhǎng)石、石榴石的結(jié)構(gòu)水測(cè)定，得到的水含量最高分別可以達(dá)到0.23 wt%、0.18 wt%、0.10 wt%、0.11 wt%，而麻粒巖平均水含量約為0.04 wt%，最高約0.10 wt% (Xia et al.,2006).本研究使用的懷安瓦窯口麻粒巖的平均含水為0.17 wt%，其中斜長(zhǎng)石含水量0.02%～0.25%，平均值大約0.12%，輝石含水量0.05%～0.6%，平均值0.22%.

微量水對(duì)流變?nèi)趸饔玫玫奖姸鄬?shí)驗(yàn)證實(shí)，隨著水含量增加，樣品的蠕變速率增加，而應(yīng)力指數(shù)、激活能、流變強(qiáng)度都減小(參見(jiàn)Zhou et al.,2009)，這是因?yàn)榫w內(nèi)部的水促進(jìn)了位錯(cuò)攀移、恢復(fù)，晶體邊界的水加速了邊界遷移與擴(kuò)散.在基性巖流變實(shí)驗(yàn)中，含水鈣長(zhǎng)石(Rybacki and Dresen,2000,2004)、透輝石(Dimanov et al.,2003,2005)、普通輝石(Chen et al.,2006)、輝長(zhǎng)巖(Zhou et al.,2012b)比對(duì)應(yīng)的干樣品的流變強(qiáng)度都顯著降低(周永勝，2013).

此外，樣品中的微量水有利于麻粒巖在流變過(guò)程中發(fā)生礦物反應(yīng)形成新的反應(yīng)產(chǎn)物橄欖石.在輝長(zhǎng)巖高溫流變實(shí)驗(yàn)中，干樣品輝長(zhǎng)巖中出現(xiàn)了部分熔融，但基本沒(méi)有出現(xiàn)顯著的礦物反應(yīng)(Zhou et al.,2012a)，但在相同的輝長(zhǎng)巖樣品中，如果含有0.15 wt%的微量水，在相同實(shí)驗(yàn)條件下，出現(xiàn)了輝石向橄欖石轉(zhuǎn)變的礦物反應(yīng)(Zhou et al.,2012b)，與本研究結(jié)果基本一致.

6 結(jié)論

本論文在氣體介質(zhì)三軸高溫流變儀上，采用天然麻粒巖，在溫度900 ℃～1200 ℃、圍壓300 MPa、應(yīng)變速率1×10-6/s-1×10-4/s條件下，進(jìn)行了11次實(shí)驗(yàn)，對(duì)實(shí)驗(yàn)樣品進(jìn)行了微觀結(jié)構(gòu)和成分分析，得到以下結(jié)論：

(1) 900～1000 ℃時(shí)，應(yīng)力指數(shù)n為8.1～12.9，在1050～1150 ℃時(shí)，應(yīng)力指數(shù)為 4.8～5.8,平均值5.2.應(yīng)力指數(shù)隨著溫度升高而降低.麻粒巖樣品的應(yīng)力指數(shù)顯著高于輝長(zhǎng)巖和熱壓合成麻粒巖.

(2) 微觀結(jié)構(gòu)與成分分析表明，在900 ℃時(shí)，麻粒巖以位錯(cuò)滑移和微破裂變形為主，在950～1000 ℃時(shí)，麻粒巖以位錯(cuò)滑攀移為主.在1050～1200 ℃條件下，顆粒邊界出現(xiàn)部分熔融，熔體結(jié)晶出斜長(zhǎng)石、輝石和橄欖石，部分輝石反應(yīng)生成橄欖石.熔體和礦物反應(yīng)促進(jìn)了擴(kuò)散作用，導(dǎo)致位錯(cuò)攀移與熔體、反應(yīng)引起的擴(kuò)散蠕變共同控制了麻粒巖的高溫流變.

致謝 本實(shí)驗(yàn)數(shù)據(jù)是地震動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室氣體介質(zhì)三軸高溫流變儀的首批高溫實(shí)驗(yàn)數(shù)據(jù).在氣體介質(zhì)三軸高溫流變儀的改造過(guò)程中，得到劉樹(shù)山、趙樹(shù)清、白武明、楊偉的大力支持，對(duì)他們的辛勤付出致以誠(chéng)摯的謝意.張?jiān)ズ暝谠撛O(shè)備上首次開(kāi)展的低-中溫大理巖變形實(shí)驗(yàn)(400～700 ℃)積累了實(shí)驗(yàn)經(jīng)驗(yàn)，為本實(shí)驗(yàn)順利進(jìn)行打下了良好基礎(chǔ).劉貴在BSD分析上給予了幫助；白武明和劉俊來(lái)對(duì)實(shí)驗(yàn)結(jié)果給出了很好的建議.

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(本文編輯 胡素芳)

Experimental study on the rheology of natural granulite at high temperature

ZHANG Hui-Ting,ZHOU Yong-Sheng*,YAO Wen-Ming,HE Chang-Rong,DANG Jia-Xiang

State Key Laboratory of Earthquake Dynamics,Institute of Geology,China Earthquake Administration,Beijing 100029,China

Samples of natural granulite were deformed in a gas medium (Paterson) apparatus to evaluate the flow strength of the lower crust.We performed 40 creep tests with 11 samples at 300 MPa confining pressure,temperatures of 900～1200 ℃,and strain rates between 10-6～10-4/s.The samples were collected at Wayaokou village,located in Huai′an,Hebei province,China.Composition of the sample was about ～52 vol % plagioclase,～40 vol % pyroxene,～3 vol % quartz,～5 vol % magnetite and ilmenite,with mean grain sizes of 294 μm,282 μm,97 μm and 109 μm for plagioclase,pyroxene,quartz,magnetite and ilmenite,separately.Water content of samples was ～0.17±0.05wt% measured by a Fourier transform infrared spectrometer on the deformed samples.

Granulite;Rheology;Deformation mechanism;Melt;Mineral reaction

張慧婷，周永勝，姚文明等.2016.天然麻粒巖高溫流變實(shí)驗(yàn)研究.地球物理學(xué)報(bào)，59(11):4188-4199,

10.6038/cjg20161121.

Zhang H T,Zhou Y S,Yao W M,et al.2016.Experimental study on the rheology of natural granulite at high temperature.Chinese J.Geophys.(in Chinese),59(11):4188-4199,doi:10.6038/cjg20161121.

國(guó)家自然科學(xué)基金課題(41374184)和地震動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室自主課題(LED2013A05，LED2015A04)資助.

張慧婷，女，1990年5月出生，碩士研究生，固體地球物理專(zhuān)業(yè).E-mail:Nancy9@yeah.net

*通訊作者 周永勝，男，1969年1月出生，研究員，主要從事高溫高壓巖石流變學(xué)實(shí)驗(yàn)研究.E-mail:zhouysh@ies.ac.cn

10.6038/cjg20161121

P313

2016-01-13，2016-06-22收修定稿

The sample strength decreased with increasing of temperature and decreasing of strain rate under experimental conditions.Based on creep data of samples,the stress exponent n was calculated,and the value of n is between 8.1～12.9 at 900～1000 ℃,and 4.8～5.8 with a mean value of 5.2 at 1050～1150 ℃.The stress exponents decrease with temperature increasing.Microstructural observations on thin sections parallel to the sample axis using optical microscopy and SEM show that deformed samples are different from the starting materials.At temperature of 900 ℃,grains are elongated and a shape preferred orientation developed perpendicular to the compression direction,which imply that the sample deformed as dislocation slip with intra-granular micro-cracks.At temperatures between 950～1000 ℃,grain boundaries of most minerals in deformed sample became round,showing dislocation climb.Trace melt films appeared mainly at some of grain boundaries of pyroxene and magnetite.At temperatures between 1050～1200 ℃,partial melting occurred at most grain boundaries of deformed samples,the content of melt increases with increasing of temperature and the duration of the experiment.The fine-grained new plagioclase,pyroxene and olivine crystallized from melt,and the solid phase reaction from pyroxene to olivine happened.So,dislocation climb is one of major deformation mechanism,but partial melt and mineral reaction induced diffusion process,and melt and reaction assisted diffusion and dislocation climb controlled the rheology of granulite at high temperature.