2018, Vol. 34
2. 中国地质科学院地质研究所, 北京 100037
2. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
地球碳循环可分为地表碳循环和深部碳循环。“地表碳循环”指固体地球外部大气圈、水圈、生物圈及浅地表之间周期较短(万年尺度内)的碳循环过程；而“深部碳循环”则涉及到地球表层系统与地球壳幔系统之间周期漫长(百万年尺度)的碳的循环历程(张立飞等, 2017)。对地球碳储量的初步估算表明，地球深部蕴藏着地球上超过90%的碳(Dasgupta and Hirschmann, 2010)，是地球上最重要的碳储库。俯冲带在深部碳循环过程中扮演着关键的角色，它可以将地球表层碳(如碳酸盐化沉积物、碳酸盐化洋壳、碳酸盐化地幔岩石等)输送至地球深部；同时，在板块俯冲过程中不同机制所诱发的脱碳效应(如变质反应脱碳、流体溶解脱碳、板片熔融脱碳等)又会导致俯冲板片中的含碳物质通过火山去气作用释放出地表(CO2为主)，从而构成动态的俯冲带碳循环过程。然而，对于该过程中究竟释放出多少碳，又有多少碳进入深部地幔目前争议较大。先前研究学者的估算表明约一半的碳会在弧前或弧下深度迁移出俯冲板片(Gorman et al., 2006; Dasgupta and Hirschmann, 2010; Johnston et al., 2011)，而Kelemen and Manning (2015)通过重新估算则认为板片在俯冲过程中几乎释放了所有的碳，或通过去气作用进入地表，或稳定固存于上覆板块(如地幔楔)中，很少甚至没有碳进入深部地幔。同样地，对西阿尔卑斯利古里亚(Ligurian)高压碳酸盐化超基性岩的研究表明，俯冲带蛇纹岩脱水交代上覆大理岩所形成的COH流体向上迁移并使地幔楔超基性岩发生碳酸盐化是缓冲俯冲板片释放碳质流体的有效途径(Scambelluri et al., 2016)。总体来说，前述研究都重点关注了上覆板块(如地幔楔)的固碳能力，然而却忽视了俯冲带岩石本身对俯冲板片释放的CO2的再吸收作用。事实上，对法国阿尔卑斯山科西嘉(Corsica)地区榴辉岩相大理岩的研究揭示了高压条件下COH流体对俯冲板片-地幔界面处岩石的碳酸盐化效应(Piccoli et al., 2016)，为俯冲带碳质流体复杂的迁移、渗流机制提供了又一启示。
本文结合我国西南天山长阿吾子碳酸盐化云母片岩的研究，探讨俯冲带碳质流体的形成及迁移沉淀机制，并阐述俯冲带岩石的碳酸盐化对深部碳循环的影响，从而为全球碳通量的重新估算提供启示。
1 地质背景西南天山高压-超高压变质带是塔里木板块向伊犁-中天山板块俯冲所形成的一条大洋型变质带，全长约200km(图 1; 张立飞等, 2013)。该变质带出露一套由榴辉岩、蓝片岩、多硅白云母片岩、大理岩和蛇纹岩等为代表的高压-超高压岩石组合(Gao and Klemd, 2001, 2003; Zhang et al., 2002a, b; 张立飞等, 2005; Klemd et al., 2015; Shen et al., 2015, 2016)，并以云母片岩为主体岩石。近年来，云母片岩、榴辉岩中柯石英的陆续发现(Lü et al., 2008, 2009; Yang et al., 2013)以及对含钛粒硅镁石(TiChon)超高压蛇纹岩的报道(Shen et al., 2015)证实了该变质带普遍经历了超高压变质作用。
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图 1 研究区地质背景 (a)中国西天山构造框架简图(据Tian and Wei, 2013修改)；(b)中国西南天山高压-超高压变质带及邻区地质简图(据张立飞等, 2013修改)；(c)采样点分布图(据Shen et al., 2015修改)；(d)中国西南天山由南向北俯冲碰撞构造模式图(据张立飞等, 2013修改) Fig. 1 Geological background of the study area (a) simplified tectonic framework of the western part of Chinese Tianshan (modified after Tian and Wei, 2013); (b) simplified geological map of the Chinese southwestern Tianshan HP-UHP metamorphic belt and adjacent areas (modified after Zhang et al., 2013); (c) detailed geological map of the sample locations (modified after Shen et al., 2015); (d) schematic cross-section of Chinese southwestern Tianshan showing northward subduction of the Tarim plate underneath the Central Tianshan Plate (modified after Zhang et al., 2013) |
西南天山高压-超高压变质带的榴辉岩和蓝片岩中广泛发育高压脉体(如绿辉石脉)，暗示了强烈的流体活动。前期研究认为这些高压脉体形成于蓝片岩向榴辉岩转变阶段，是进变过程中流体活动的证据(Gao and Klemd, 2001; Gao et al., 2007; John et al., 2008; Beinlich et al., 2010; Li et al., 2013)。然而，近年来对榴辉岩中蓝片岩相退变流体的叠加(van der Straaten et al., 2008, 2012)以及大量退变高压脉体(如石英脉和碳酸盐脉)的识别(Lü et al., 2012b; Zhang et al., 2016; Li et al., 2017)揭示了俯冲板片折返过程中流体所诱发的元素迁移沉淀行为(吕增等, 2013)。
上述研究主要基于非碳酸盐化体系，然而对于俯冲带含碳岩石(碳酸盐化变质沉积岩/片岩、碳酸盐化榴辉岩、碳酸盐化超基性岩等)的探讨则有助于深入了解俯冲带碳质流体的迁移沉淀特性及其诱发的岩石碳酸盐化过程，对于俯冲带深部碳循环具有重要意义。结合作者之前的研究，西南天山长阿吾子出露的与超高压蛇纹岩伴生的高压碳酸盐化蛇纹岩(HP-ophidolomite)较好地揭示了俯冲带蛇纹岩在高压条件下的固碳作用，对俯冲带深部碳循环具有一定的启示意义。本文通过对我国西南天山长阿吾子与高压ophidolomite伴生的碳酸盐化云母片岩的研究，探讨高压条件下俯冲带岩石固碳的普遍性，从而为俯冲带碳通量的重新估算提供启示。
2 野外产状及岩石学本文样品采自西南天山长阿吾子一带(图 1c)，主要为超高压蛇纹岩的围岩云母片岩(C1603-1、C1606-1、C1607、C1608-1、C1633-1和C1633-3)、碳酸盐化云母片岩(C16-7.27-2、C1632-1、C1632-2、C1632-3和C1659)以及与碳酸盐化云母片岩伴生的碳酸盐化透闪石岩、透辉石岩(C1648-1、C1651)。其中，碳酸盐化云母片岩主要出露于云母片岩与UHP蛇纹岩及HP-ophidolomite的接触部位，并与退变榴辉岩及蓝片岩伴生，厚度约为10~20m(图 2)。野外露头上，云母片岩和蛇纹岩呈深色且前者较后者颜色略浅，而碳酸盐化云母片岩呈易于辨认的土黄色(图 2)，可能由于岩石中较高含量的碳酸盐受风化作用所致。
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图 2 西南天山长阿吾子地区岩石组合的野外特征 (a)研究区总体野外特征；(b)碳酸盐化云母片岩位于蛇纹岩和云母片岩之间，与高压碳酸盐化蛇纹岩(HP-ophidolomite)和退变榴辉岩伴生 Fig. 2 The characteristics of lithological assemblages at Changawuzi, Chinese southwestern Tianshan (a) general features of the study area; (b) the carbonated micaschist is located between serpentinite and micaschist, showing intimate association with high-pressure carbonated serpentinite (HP-ophidolomite) and retrograded eclogite |
长阿吾子云母片岩的矿物组成与西南天山科布尔特(Kebuerte)和哈布腾苏(Habutengsu)地区云母片岩的组成类似，主要包括石榴石、多硅白云母、钠长石、石英及少量绿泥石、磷灰石、金红石和碳酸盐矿物(菱镁矿、白云石)等(详见张立飞等, 2003; Lü et al., 2012a; Yang et al., 2013)，碳酸盐矿物的含量相对较低(< 5%)。然而，碳酸盐化云母片岩中石榴石残留较少，绿泥石含量显著升高(~30%)，多硅白云母因受碳酸盐交代而明显减少(~5%; 图 3a-c)，碳酸盐以白云石为主(~25%)，菱镁矿含量相对较低(~5%)，金红石与碳酸盐共生，未发生或略微发生榍石/钛铁矿退变(图 3d, e)，另外还含有石英(~25%)、钠长石(~10%)和微量磷灰石等。
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图 3 西南天山长阿吾子碳酸盐化云母片岩的岩相学特征 (a-c)碳酸盐交代多硅白云母；(d、e)碳酸盐与金红石共生 Fig. 3 Petrographic characteristics of carbonated micaschists from Changawuzi, Chinese southwestern Tianshan (a-c) phengite was metasomatized by carbonates; (d, e) the intergrowth of carbonates and rutile |
硅酸盐及碳酸盐矿物的主量元素分析在北京大学造山带与地壳演化教育部重点实验室JEOL JXA-8230型电子探针仪器上完成。实验设置加速电压为15kV，电子束流为10nA，束斑直径为2μm(白云母为5μm)，采用ZAF自动校准程序进行修正。矿物分子式通过AX软件(Tim Holland; https://www.esc.cam.ac.uk/research/research-groups/holland/ax)计算。文中所有矿物缩写据Whitney and Evans (2010)。
3.2 全岩主、微量元素全岩主量元素在中国地质大学(北京)地学实验中心分析完成，利用等离子体发射光谱仪(ICP-OES)进行检测，所有主量元素的精度均优于2%。全岩微量元素丰度由等离子体质谱仪(ICP-MS)分析测定，样品前处理过程和分析流程见Song et al. (2010)。通过对比标样GSR-1和GSR-12的测量值和参考值来监控微量元素的分析精度，其中大部分微量元素的分析精度优于10%，但Zr、Nb、Gd、Lu和Ta等元素的分析精度介于10%~15%。
3.3 碳酸盐C、O同位素碳酸盐C、O同位素的测定在中国科学院地质与地球物理研究所稳定同位素实验室完成，利用美国Thermo Fisher公司GasBench Ⅱ仪器进行测定。首先将称量好的样品加入反应瓶，置于恒温(72±0.1℃)样品盘中，通过自动进样器充气针向反应瓶中充入高纯(99.999%)氦气从而排出瓶中空气(5min)。取下充气针，换上酸针和采样针，一边加磷酸一边进行采样分析。为保证反应进行完全，加酸与进样分析之间的时间间隔为1h。反应生成的CO2由氦气带入MAT 253同位素比值质谱仪进行C、O同位素的测定。测试过程中使用的标准物质有NBS19、GBW04405、GBW04406、GBW04416、GBW04417。测量精度以实验室内部标准物质CaCO3的长期精度为准，δ13C和δ18O的精度分别为0.15‰和0.20‰。测定的δ13C和δ18O值都以VPDB(Vienna Pee Dee Belemnite)为参考标准，其中，δ18OVPDB转化为δ18OVSMOW的公式为：δ18OVSMOW=1.03091×δ18OVPDB+30.91‰(Coplen et al., 1983)。
3.4 全岩Sr同位素全岩Rb和Sr的分离与提纯以及Sr同位素的测定均在北京大学造山带与地壳演化教育部重点实验室完成。其中，在超净实验室利用传统的离子交换程序对Rb和Sr进行分离与提纯，而Sr同位素则通过多接收等离子体质谱仪(MC-ICP-MS)进行测定。实验中，玄武岩标样BCR-2用于检测Rb和Sr的分离与提纯过程，其87Sr/86Sr测定值为0.705018±0.000004(2σ)；美国国家标准局987样品用于监控实验分析过程中的数据质量，其87Sr/86Sr的测定值为0.710245±0.000005(2σ)。
4 分析结果 4.1 矿物主量元素西南天山长阿吾子碳酸盐化云母片岩中主要矿物(Chl、Ab、Ph、Dol和Mgs)的代表性探针成分数据见表 1。其中，多硅白云母的Si(a.p.f.u.)为3.58~3.73，高于西南天山超高压云母片岩中多硅白云母的Si(图 4a, Zhang et al., 2003; Wei et al., 2009; Lü et al., 2012a; Yang et al., 2013)；碳酸盐以富铁的白云石和菱镁矿为主(图 4b)，前者XMg[Mg/(Mg+Fe)]值(0.79~0.82)略高于后者(XMg=0.63~0.79)。
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表 1 西南天山长阿吾子碳酸盐化云母片岩(C1632-2)中矿物的代表性探针成分数据(wt%) Table 1 Representative EMPA data of minerals from the carbonated micaschist (C1632-2) from Changawuzi, Chinese southwestern Tianshan (wt%) |
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图 4 西南天山长阿吾子碳酸盐化云母片岩中多硅白云母和碳酸盐的成分特征 (a)多硅白云母K2O-Si图解，西南天山其他地区超高压云母片岩中多硅白云母的成分来自Zhang et al., 2003; Wei et al., 2009; Lü et al., 2012a; Yang et al., 2013；(b)碳酸盐CaCO3-MgCO3-FeCO3图解 Fig. 4 The compositional features of phengites and carbonates from carbonated micaschists from Changawuzi, Chinese southwestern Tianshan (a) the diagram of K2O and Si contents in phengites from carbonated micaschist; those of UHP micaschists from Chinese southwestern Tianshan (Zhang et al., 2003; Wei et al., 2009; Lü et al., 2012a; Yang et al., 2013) are plotted for comparison; (b) carbonate compositions are plotted in the ternary CaCO3-MgCO3-FeCO3 diagram |
西南天山长阿吾子云母片岩及碳酸盐化云母片岩的主量和微量元素分析结果见表 2。由于白云母及碳酸盐含量的差异性，云母片岩具有变化的烧失量(LOI=3.08%~6.86%)。去除烧失量后，对主量元素进行重新标准化，其主要组成为：SiO2(60.00%~76.44%)、Al2O3(12.31%~18.44%)、Fe2O3T(1.58%~6.56%)、MgO(0.81%~4.24%)、CaO(2.51%~6.01%)、Na2O(1.61%~3.16%)、K2O(0.74%~4.54%)；与西南天山其他采样点的高压-超高压云母片岩具有相似的化学组成(去除烧失量后重新标准化)：SiO2(53.06%~73.48%)、Al2O3(12.06%~20.07%)、Fe2O3T(4.41%~10.48%)、MgO(0.98%~4.68%)、CaO(1.08%~4.64%)、Na2O(0.80%~6.86%)、K2O(0.07%~3.92%)(图 5a, b; Wei et al., 2009; Lü et al., 2012a; Yang et al., 2013; Liu et al., 2014)。然而，长阿吾子碳酸盐化云母片岩去除烧失量后重新标准化的化学组成为：SiO2(50.38%~52.75%)、Al2O3(9.98%~10.68%)、Fe2O3T(10.13%~11.09%)、MgO(15.42%~17.64%)、CaO(6.37%~9.93%)、Na2O(1.27%~1.80%)、K2O(0.05%~0.66%)，较云母片岩具有显著的Si、Al降低、Mg升高的特征(图 5a, b)。
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表 2 西南天山长阿吾子云母片岩和碳酸盐化云母片岩全岩主量元素(wt%)及微量元素(×10-6)成分 Table 2 Whole-rock major (wt%) and trace element (×10-6) contents of micaschists and carbonated micaschists from Changawuzi, Chinese southwestern Tianshan |
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图 5 西南天山长阿吾子云母片岩、碳酸盐化云母片岩的全岩主量及微量成分特征 对比的西南天山高压-超高压云母片岩的主量成分来自Wei et al., 2009; Lü et al., 2012a; Yang et al., 2013; Liu et al., 2014, 微量成分来自Liu et al., 2014；长阿吾子蛇纹岩的微量成分来自彭卫刚, 未发表数据；申婷婷, 未发表数据；全球俯冲沉积物(GLOSS)平均组分来自Plank and Langmuir, 1998；西南天山榴辉岩平均组分来自van der Straaten et al., 2008 Fig. 5 Whole-rock major and trace element compositions of micaschists and carbonated micaschists from Changawuzi, Chinese southwestern Tianshan For comparison, the bulk-rock major element contents of HP-UHP micaschists (Wei et al., 2009; Lü et al., 2012a; Yang et al., 2013; Liu et al., 2014), the trace element abundances of micaschists (Liu et al., 2014) and serpentinites (Peng, unpublished data; Shen, unpublished data), and the average composition of the global subducting sediment (GLOSS, Plank and Langmuir, 1998) and of eclogites (van der Straaten et al., 2008) from Chinese southwestern Tianshan are also shown |
微量元素上，相比于云母片岩(Co=1.53×10-6~17.0×10-6、Ni=5.53×10-6~58.7×10-6、Rb/Sr=0.06~1.21、∑REE=157×10-6~225×10-6)，碳酸盐化云母片岩具有较高的Co(45.8×10-6~56.6×10-6)、Ni(206×10-6~308×10-6)含量以及较低的Rb/Sr比值(0.01~0.07)和稀土总量(20.9×10-6~30.8×10-6，图 5c, d)。稀土元素配分图中，长阿吾子云母片岩与西南天山云母片岩(Liu et al., 2014)特征基本一致，然而碳酸盐化云母片岩具有相对较低的稀土总量，但总体继承了云母片岩的稀土配分模式(图 6)。
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图 6 西南天山长阿吾子云母片岩、碳酸盐化云母片岩球粒陨石标准化稀土元素配分图(标准化值据Sun and McDonough, 1989) 西南天山云母片岩(Liu et al., 2014)、全球俯冲沉积物(GLOSS)平均组分(Plank and Langmuir, 1998)、西南天山榴辉岩平均组分(van der Straaten et al., 2008)及长阿吾子蛇纹岩(彭卫刚, 未发表数据；申婷婷, 未发表数据)的稀土特征作为对比 Fig. 6 Chondrite-normalized REE patterns for micaschists and carbonated micaschists from Changawuzi, Chinese southwestern Tianshan (normalized values after Sun and McDonough, 1989) The REE patterns of micaschists from southwestern Tianshan (Liu et al., 2014), the average composition of the global subducting sediment (GLOSS, Plank and Langmuir, 1998), the average composition of eclogites (van der Straaten et al., 2008) from southwestern Tianshan, and serpentinites (Peng, unpublished data; Shen, unpublished data) from Changawuzi are shown for comparison |
西南天山长阿吾子云母片岩及碳酸盐化云母片岩中碳酸盐的C、O同位素分析结果见表 3。云母片岩中碳酸盐的δ13C值为-1.9‰~-1.5‰，与海水的C同位素组成接近(图 7; Veizer et al., 1999)。然而碳酸盐化云母片岩中碳酸盐具有显著低于海水的δ13C值(-5.6‰~-4.6‰)；类似地，与碳酸盐化云母片岩伴生的碳酸盐化钙镁质岩石(透闪石岩)亦具有较低的δ13C值(-5.5‰)。此外，碳酸盐化云母片岩及碳酸盐化钙镁质岩石中碳酸盐的O同位素值较云母片岩中碳酸盐的O同位素值略低，前者δ18O=+13.3‰~+14.5‰，后者δ18O=+14.7‰~+16.2‰(图 7)。
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表 3 西南天山长阿吾子云母片岩、碳酸盐化云母片岩和碳酸盐化钙镁质岩中碳酸盐的C、O同位素 Table 3 Carbon and oxygen isotopes of carbonates in micaschists, carbonated micaschists, and carbonated Ca-Mg-rocks from Changawuzi, Chinese southwestern Tianshan |
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图 7 西南天山长阿吾子云母片岩、碳酸盐化云母片岩和碳酸盐化钙镁质岩石中碳酸盐的C、O同位素特征 西南天山榴辉岩中碳酸盐的C、O同位素值来自Collins et al., 2015和Zhu et al., 2017 Fig. 7 Carbon and oxygen stable isotopic characteristics for carbonates from micaschists, carbonated micaschists, and carbonated Ca-Mg-rich rocks from Changawuzi, Chinese southwestern Tianshan The values of C and O isotopes for carbonates in eclogites (Collins et al., 2015; Zhu et al., 2017) are also presented for comparison |
西南天山长阿吾子云母片岩、碳酸盐化云母片岩及伴生的碳酸盐化钙镁质岩石的全岩Sr同位素分析结果见表 4。云母片岩、碳酸盐化云母片岩和碳酸盐化钙镁质岩石的87Sr/86Sr比值分别为0.711735~0.726265、0.706847~0.707388和0.706947~0.706996。其中，长阿吾子云母片岩(6件样品)全岩的87Rb/86Sr-87Sr/86Sr表现出较好的线性相关性(R2=0.978)，利用ISOPLOT软件计算出的初始87Sr/86Sr(Isr)值为0.7110±0.0017；若结合西南天山其他采样点云母片岩全岩的Sr同位素(Liu et al., 2014)，则可得到0.7098±0.0013的Isr值(R2=0.961，图 8a)。同样地，长阿吾子碳酸盐化云母片岩(5件样品)亦具有较好的线性相关性(R2=0.968)，并给出0.7068±0.0001的Isr值(图 8a)。
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表 4 西南天山长阿吾子云母片岩、碳酸盐化云母片岩和碳酸盐化钙镁质岩全岩Sr同位素 Table 4 Strontium isotopes of micaschists, carbonated micaschists, and carbonated Ca-Mg-rocks from Changawuzi, Chinese southwestern Tianshan |
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图 8 西南天山长阿吾子云母片岩、碳酸盐化云母片岩和碳酸盐化钙镁质岩石的全岩Sr同位素特征 (a)云母片岩和碳酸盐化云母片岩的全岩87Rb/86Sr-87Sr/86Sr特征，参考的西南天山云母片岩的Sr同位素来自Liu et al., 2014；(b)以320Ma计算的长阿吾子云母片岩、碳酸盐化云母片岩和碳酸盐化钙镁质岩石的87Sr/86Sr特征，参考的长阿吾子蛇纹岩的Sr同位素来自彭卫刚，未发表数据；参考的西南天山云母片岩及榴辉岩的Sr同位素分别来自Liu et al., 2014和Huang et al., 2005; Van Der Straaten et al., 2012 Fig. 8 Whole-rock Sr isotopic characteristics for micaschists, carbonated micaschists, and carbonated Ca-Mg-rich rocks from Changawuzi, Chinese southwestern Tianshan (a) whole-rock 87Rb/86Sr-87Sr/86Sr characteristics for micaschists and carbonated micaschists; the Sr isotopes of micaschists from Chinese southwestern Tianshan are from Liu et al., 2014; (b) Sr isotopic ratios for micaschists, carbonated micaschists, and carbonated Ca-Mg-rich rocks were back calculated to 320Ma; the referenced (87Sr/86Sr)320Ma values of serpentinites from Changawuzi are from Peng, unpublished data while those of micaschists and eclogites from Chinese southwestern Tianshan are from Liu et al., 2014 and Huang et al., 2005; van der Straaten et al., 2012, respectively |
此外，以西南天山超高压变质作用年龄(~320Ma, Li et al., 2011; Yang et al., 2013; Du et al., 2014)计算出的长阿吾子云母片岩、碳酸盐化云母片岩和碳酸盐化钙镁质岩石的(87Sr/86Sr)320Ma值分别为0.709869~0.711831、0.706417~0.706781和0.706924~0.706974(图 8b)。
5 讨论 5.1 COH流体的迁移及云母片岩的碳酸盐化俯冲带碳质流体的形成机制及其活动性近年来受到越来越广泛的关注。热动力学模拟(Kerrick and Connolly, 1998, 2001a, b; Connolly and Kerrick, 2002; Connolly, 2005; Gorman et al., 2006; Collins et al., 2015)及实验岩石学研究(Molina and Poli, 2000; Thomsen and Schmidt, 2008; Poli et al., 2009; Tsuno and Dasgupta, 2011)表明沿低-中等地热梯度俯冲的大洋板片在弧前释放的CO2有限，绝大多数C将被携带至深部地幔。然而，上述研究主要基于温压升高所导致的变质反应脱碳效应，而富水流体溶解俯冲板片中的碳酸盐目前则被认为是产生俯冲带碳质流体的重要机制(Frezzotti et al., 2011, 2014; Ague and Nicolesco, 2014)，也是使俯冲带岩石发生碳酸盐化的潜在“C源”(Piccoli et al., 2016)。事实上，俯冲带蛇纹岩的分解会释放出大量富水流体，既为弧岩浆的形成提供重要的“水源”(Scambelluri et al., 1995; Ulmer and Trommsdorff, 1995; Tatsumi and Kogiso, 1997)，也是深源地震的主要诱因(Peacock, 2001; Hacker et al., 2003; Ranero et al., 2003)。尽管叶蛇纹石完全脱水相变为橄榄石所需温度大于650℃(Ulmer and Trommsdorff, 1995; Wunder and Schreyer, 1997; Bromiley and Pawley, 2003)，但该过程实际上是渐进的，尤其当温度超过550℃时，蛇纹岩的脱水即可释放出大量流体(Barnes et al., 2004)。
西南天山长阿吾子碳酸盐化云母片岩较云母片岩偏高的Co、Ni含量(图 5c)及偏低的Rb/Sr和稀土总量(图 5d、图 6)揭示了交代流体的元素丰度特征。由于Co、Ni等元素的相对不活动性(Kodolányi et al., 2012; Bjerga et al., 2015)，俯冲带蓝片岩向榴辉岩脱水相变过程中Co、Ni含量不会发生太大变化。但实际上，西南天山榴辉岩的低Co、Ni含量及高稀土总量(图 5c, d; van der Straaten et al., 2008)表明蓝片岩转变为榴辉岩过程中释放的流体的元素特征无法与碳酸盐化云母片岩较高的Co、Ni含量及较低的稀土总量相耦合(图 5c, d、图 6)。相反，与碳酸盐化云母片岩伴生的蛇纹岩(图 2)的高Co(>84.7×10-6)、Ni(>1560×10-6)含量及低稀土总量(∑REE<3.04×10-6)(图 5c, d、图 6; 彭卫刚, 未发表数据; 申婷婷, 未发表数据)表明其释放的流体具有使云母片岩发生碳酸盐化所需流体的元素特征。实际上，俯冲带蛇纹岩脱水释放的流体还具有高Mg、低Al、亏损Rb/Sr的特征(van der Straaten et al., 2008)，与碳酸盐化云母片岩中白云石较菱镁矿略高的XMg值(图 4b)及全岩特征(图 5a, b)耦合，进一步证实了蛇纹岩起源的流体对云母片岩的交代作用。
西南天山长阿吾子云母片岩中少量原生碳酸盐(< 5%)的C同位素特征指示其海相成因(图 7)，且云母片岩限定的初始Sr同位素比值(ISr=0.7098±0.0013)与奥陶纪海水的Sr同位素组成(87Sr/86Sr=0.7078~0.7092, Veizer et al., 1999)类似，表明西南天山云母片岩具有典型的海相成因。而碳酸盐化云母片岩较低的C同位素特征表明其受到了具有低δ13C值的COH流体的交代作用；类似地，与碳酸盐化云母片岩伴生的碳酸盐化钙镁质岩石也反映了低δ13C流体-岩石的相互作用(图 7)。此外，碳酸盐化云母片岩的初始87Sr/86Sr比值为0.7068±0.0001(图 8a)，明显低于海水的Sr同位素组成；再结合以320Ma计算的碳酸盐化云母片岩的87Sr/86Sr比值明显低于云母片岩(图 8b)，我们认为交代碳酸盐化云母片岩的碳质流体不仅具有低δ13C值，也具有较低的Sr同位素组成。事实上，西南天山碳酸盐化榴辉岩中碳酸盐(δ13C=-14.1‰~+2.6‰, Collins et al., 2015; Zhu et al., 2017)具有向低C同位素演化的特征(图 7)，即流体溶解碳酸盐化榴辉岩所产生的COH流体交代云母片岩可以使其具有低C同位素的印迹。此外，以320Ma计算的碳酸盐化云母片岩的87Sr/86Sr比值与榴辉岩的低87Sr/86Sr比值(0.7053~0.7078, Huang et al., 2005; Van Der Straaten et al., 2012)相似(图 8b)，表明交代云母片岩的COH流体可能是由蛇纹岩释放的富水流体溶解俯冲洋壳中的碳酸盐(碳酸盐化榴辉岩)所形成。van der Straaten et al.(2008, 2012)的研究也表明在高压折返阶段(P=~15kbar，T=~580℃)，来源于蛇纹岩的富水流体会水化西南天山高压-超高压榴辉岩，从而形成退变蓝片岩。结合碳酸盐化云母片岩与其伴生的高压碳酸盐化蛇纹岩(HP-ophidolomite)类似的Sr同位素特征(以320Ma计算的87Sr/86Sr=0.7064~0.7073，彭卫刚，未发表数据)，我们推测使云母片岩发生碳酸盐化的COH流体可能主要来自于蛇纹岩释放的富水流体对俯冲洋壳中碳酸盐的溶解。事实上，富水流体对俯冲板片中碳酸盐的溶解具有一定的选择性，具体来说，高压条件下Ca质碳酸盐在纯水中溶解度较高，且随温度升高而增大(Manning et al., 2013)。尽管目前缺乏高压下镁质碳酸盐的溶解实验，但对低压实验结果的外推表明菱镁矿在富水流体中的溶解度较低，且随温度升高而降低(Saldi et al., 2010; Bénézeth et al., 2011)。换句话说，相对于菱镁矿(镁质碳酸盐)，俯冲带蛇纹岩释放的富水流体在较高温压条件下可能更偏向于溶解碳酸盐化榴辉岩中的白云石(钙镁质碳酸盐)。
对云母片岩发生碳酸盐化的温压条件的探讨有助于进一步了解俯冲带碳质流体的活动性及其沉淀机制。长阿吾子碳酸盐化云母片岩中被碳酸盐交代的多硅白云母(图 3a, b, c)的Si含量高于已报道的西南天山超高压云母片岩中多硅白云母的Si含量(图 4a; Zhang et al., 2003; Wei et al., 2009; Lü et al., 2012a; Yang et al., 2013)，且对NCKMnFMASHO体系下西南天山云母片岩的相平衡模拟表明多硅白云母的Si含量确实随压力的升高而增加(Wei et al., 2009)，暗示了碳酸盐化作用发生之前寄主岩石(云母片岩)可能已经经历了超高压变质。Zhang et al. (2003)在长阿吾子云母片岩中发现的白云石分解反应(Dol→Mgs+Arg)也表明该地区云母片岩确实经历了超高压变质作用。结合碳酸盐化云母片岩中碳酸盐对多硅白云母的交代(图 3a-c)、相对高压的白云石和菱镁矿作为主要的碳酸盐矿物相以及碳酸盐与高压矿物(如金红石，图 3d, e)共生的岩石学特征，我们认为云母片岩的碳酸盐化作用可能发生在超高压变质阶段稍后的高压折返阶段，这与西南天山高压折返碳酸盐化蓝片岩(P=~15kbar, T=~580℃; van der Straaten et al., 2012)及区内伴生的高压折返碳酸盐化蛇纹岩(HP-ophidolomite)的形成条件吻合。此外，西南天山超高压榴辉岩内发育的切穿早期面理的高压(P=13~21kbar, T=540~580℃)脉体(如绿辉石脉、含金红石石英脉)也表明西南天山高压-超高压变质带在高压折返阶段流体活动十分强烈(吕增等, 2013)，包括COH流体的迁移沉淀(van der Straaten et al., 2008, 2012; Li et al., 2017)。事实上，这些温压条件所反映的西南天山俯冲板片经历的“热弛豫”过程(温度较压力峰期对应的温度升高约100℃, 张立飞等, 2013)恰恰也是蛇纹岩脱水释放大量流体的阶段(Barnes et al., 2004)，从而有利于俯冲板片中碳酸盐的溶解以及俯冲带中COH流体的迁移。
5.2 俯冲带岩石的碳酸盐化对深部碳循环的启示基于矿物体积分数(Chl=~28vol%, Ab=~13vol%, Ph=~5vol%, Q=~23vol%, Rt < 0.5vol%, Dol=~26vol%, Mgs=~5vol%)及探针成分计算的西南天山长阿吾子代表性碳酸盐化云母片岩(C1632-2)的有效全岩成分在误差范围内与实验测得的全岩成分接近，并给出了全岩15.55%含量的CO2(表 5)。结合已发表的数据(Wei et al., 2009; Lü et al., 2012a; Yang et al., 2013; Liu et al., 2014)，西南天山云母片岩中变化的白云母及海相碳酸盐含量导致其较大变化范围的烧失量(LOI=1.21%~6.86%，平均值为3.23%)。然而对全球俯冲沉积物(GLOSS)平均组分的估算表明初始俯冲沉积物中约含有(3.01±1.44)% CO2和(7.29±0.41)% H2O(Plank and Langmuir, 1998)，其烧失量总和(10.30±1.85)%显著高于上述西南天山超高压云母片岩的烧失量，我们认为这是由于一方面沉积物在俯冲过程中经压实作用会发生脱水，另一方面温压的升高也会导致变质脱挥发分反应的发生。事实上，热动力学模拟表明GLOSS平均组分沿西南天山这样的冷俯冲带(~5℃/km, Zhang et al., 2003; Wei et al., 2009; Lü et al., 2012a; Shen et al., 2015)在弧前会发生一定的脱水作用但几乎不发生变质脱碳作用(Kerrick and Connolly, 2001b)。尽管富水流体溶解俯冲板片中的碳酸盐可能是产生俯冲带碳质流体的重要机制(Frezzotti et al., 2011, 2014; Ague and Nicolesco, 2014)，但该过程中含碳相尤其是碳酸盐的被溶解量目前仍没有一致的定量结论。因此，我们假设在俯冲过程中沉积物很少甚至没有释放CO2，以此来探讨俯冲带岩石的高压碳酸盐化所能固存CO2的最小值。基于上述西南天山长阿吾子碳酸盐化云母片岩及全球俯冲沉积物(GLOSS)平均组分(Plank and Langmuir, 1998)中CO2的含量，我们估算出云母片岩高压碳酸盐化过程中吸收的CO2约为11.10%~13.98%。计算过程中我们采用的矿物密度值如下：Chl(2.60~3.30g/cm3)，Ab(2.62g/cm3)，Ph(2.77~2.88g/cm3)，Q(2.65g/cm3)，Rt(4.23~5.50g/cm3)，Dol(2.86g/cm3)，Mgs(2.98g/cm3)(Deer et al., 1992)，据此估算的西南天山长阿吾子碳酸盐化云母片岩的密度值为2.71~2.92g/cm3。根据野外观察，俯冲板片中发生碳酸盐化的云母片岩层厚度约为10~20m(图 2)，与法国阿尔卑斯山科西嘉(Corsica)地区具有相似成因的碳酸盐化钙质片岩层厚度(1~10m)类似(Piccoli et al., 2016)；再结合俯冲板片的汇聚速率约为3km2/yr(Reymer and Schubert, 1984; Dasgupta and Hirschmann, 2010; Alt et al., 2013)，我们估算出俯冲带高压碳酸盐化云母片岩所能固存的C含量为2.46~6.68Mt/yr。假设俯冲板片对俯冲带碳通量的贡献约为40~66Mt C/yr(Kelemen and Manning, 2015)，那么，俯冲带岩石的高压碳酸盐化每年可固存俯冲板片中约4%~17%的C。上述也已提及，由于我们并未考虑沉积物在俯冲过程中的脱碳效应，所以该值实际上是俯冲带岩石高压碳酸盐化所能吸收C的最小值；然而，俯冲带中其他岩石类型(如基性岩、超基性岩)是否具有与俯冲板片-地幔楔之间的云母片岩类似的固碳能力还值得进一步定量研究。但结合van der Straaten et al.(2008, 2012)对西南天山碳酸盐化蓝片岩的探讨以及作者前期对与碳酸盐化云母片岩伴生的高压碳酸盐化蛇纹岩(HP-ophidolomite)的研究，我们认为俯冲板片来源的COH流体并未完全迁移进入地幔楔，或者说地幔楔并非俯冲带CO2的唯一“归宿”；相反，俯冲板片中不同岩石类型对于碳质流体也具有不可忽视的吸收和固存能力。
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表 5 西南天山长阿吾子代表性碳酸盐化云母片岩(C1632-2)中CO2含量(wt%) Table 5 Estimated CO2 content (wt%) in a representative sample of carbonated micaschist (C1632-2) from Changawuzi, Chinese southwestern Tianshan |
(1) 西南天山长阿吾子碳酸盐化云母片岩记录了俯冲板片来源的COH流体对俯冲带云母片岩的交代作用，地球化学特征表明蛇纹岩起源的富水流体溶解俯冲洋壳中的碳酸盐可能是产生碳质流体的重要机制；
(2) 西南天山长阿吾子碳酸盐化云母片岩的岩石学特征及与高压折返碳酸盐化蛇纹岩(HP-ophidolomite)伴生的野外地质产状特征指示碳酸盐化作用可能发生在俯冲板片压力峰期稍后的高压折返阶段；
(3) 俯冲带云母片岩对俯冲板片释放的CO2的固存表明上覆板块(如：地幔楔)并非俯冲板片来源COH流体的唯一“归宿”，而俯冲带岩石本身对于碳质流体也具有很好的吸收能力。初步估算表明俯冲带云母片岩的高压碳酸盐化每年可固存至少2.46~6.68Mt C，约占俯冲板片每年进碳量的4%~17%。
致谢 感谢课题组博士研究生王杨对野外工作的帮助；感谢北京大学李小犁工程师、马芳博士、黄宝玲博士、中国地质大学(北京)秦红工程师和中国科学院地质与地球物理研究所李洪伟工程师在实验过程中给予的帮助。
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