西南印度洋中脊63.9°E斜長石超斑狀玄武巖對超慢速擴張洋脊巖漿過程的指示*

程石 周懷陽

同濟大學海洋與地球科學學院，上海 200092

西南印度洋中脊的全擴張速率約14～16mm/yr(Dicketal., 2003)，是全球最長的超慢速擴張洋中脊之一。對西南印度洋中脊的調(diào)查發(fā)現(xiàn)這一區(qū)域洋殼厚度很薄但巖石圈地幔較厚(Dicketal., 2003; Bach and Früh-Green, 2010)。有的洋脊段火山巖非常薄甚至缺失，大量出露地幔橄欖巖(Zhou and Dick, 2013)。直到近年， Lietal. (2015)才在西南印度洋中脊50°28′E深約9.5km的下地殼探測到潛在的巖漿房。這些現(xiàn)象反映了超慢速擴張洋中脊巖漿起源深度大、上升到淺部地殼的巖漿量少、巖漿房小的特點。

斜長石斑晶體積分數(shù)>10%且占比高于橄欖石的玄武巖被稱為斜長石超斑狀玄武巖(Plagioclase Ultra-phyric Basalts, PUBs)(Cullenetal., 1989)，在匯聚或擴散板塊邊緣以及熱點處均有出露(Karstenetal., 1986; Hekinian and Walker, 1987; Batiza and Niu, 1992; Weinsteigeretal., 2010)。在已經(jīng)發(fā)現(xiàn)的斜長石超斑狀玄武巖中，斑晶一般由斜長石、橄欖石和/或單斜輝石(Cullenetal., 1989)構(gòu)成，斜長石與橄欖石體積分數(shù)的比值從1到30不等(O’Hara, 1968; Longhi, 1987; Groveetal., 1992)。作為斜長石超斑狀玄武巖中最重要的礦物，斜長石斑晶的An值范圍從60至90不等，同一樣品中不同的斜長石斑晶也可以有截然不同的結(jié)構(gòu)和成分分帶(Hansen and Gr?nvold, 2000; Cordieretal., 2007; Hellevang and Pedersen, 2008)。前人提出了三種觀點解釋斜長石超斑狀玄武巖成因：斜長石超斑狀玄武巖是高Al2O3含量母熔體結(jié)晶分異的產(chǎn)物(Panjasawatwongetal., 1995)；或者是多種斑晶或俘虜晶的混合產(chǎn)物(Hansen and Gr?nvold, 2000)；也有人認為是斜長石后期聚集的產(chǎn)物(Weinsteigeretal., 2010; Neaveetal., 2013; Valeretal., 2017)。Bennettetal. (2019)統(tǒng)計了采自Gakkel洋脊(北冰洋中脊)的玄武巖中(斜長石模態(tài)含量大于50%)超過1800個的斜長石晶體，根據(jù)其結(jié)構(gòu)和成分，恢復晶體在超慢速擴張洋中脊深部形成過程中的物理、化學環(huán)境。本研究對西南印度洋中脊63.9°E采集到的斜長石超斑狀玄武巖中斑晶與基質(zhì)以及不同類型斜長石之間的關系進行了分析，并對比研究了斜長石超斑狀玄武巖與同一洋脊段非斑狀玄武巖成分之間的關系，探討西南印度洋中脊63.9°E 斜長石超斑狀玄武巖的形成過程，揭示超慢速擴張洋中脊深部巖漿過程的特征。

1 地質(zhì)背景

西南印度洋中脊(SWIR)西起南大西洋的布維三聯(lián)點(Bouvet Triple Junction, BTJ)(54°50′S、00°40′W)，東至印度洋的羅德里格斯三聯(lián)點(Rodrigues Triple Junction, RTJ)(25°30′S、70°00′E)(Fontetal., 2007)，全長約7700km(Georgenetal., 2001)。西南印度洋中脊是超慢速擴張洋中脊，全擴張速率為14～16mm/yr(Dicketal., 2003)，洋脊的西南端，擴張速率稍有增加為16～18mm/yr，至東北端，擴張速率略有降低為12～13mm/yr(Chu and Gordon, 1999)。

圖1 研究區(qū)位置(a，據(jù)楊陽, 2013修改)和樣品采集點地形圖(b,據(jù)DY43航次多波束數(shù)據(jù)作圖)CIR-Central Indian Ridge，中印度洋中脊；SWIR-Southwest Indian Ridge，西南印度洋中脊；SEIR-Southeast Indian Ridge，東南印度洋中脊Fig.1 Location of study area (a, modified after Yang, 2013) and topographic map of sampling position (b, multibeam data source in DY43 scientific cruise)

西南印度洋中脊的東段(45°～70°E)被兩個重要的斷裂帶即Gallieni斷裂帶(52°20′E)和Melville斷裂帶(60°45′E)劃分為三個區(qū)域，這段洋脊從東到西表現(xiàn)出從構(gòu)造主導擴張到巖漿主導擴張的變化(Mendeletal., 1997; Cannatetal., 1999)，這與自東向西洋脊平均水深減小、軸部的火山數(shù)量增多一致(Mendeletal., 1997; Cannatetal., 1999)，表明從東向西地殼厚度增加和/或地幔密度減小(Cannatetal., 1999; Sauteretal., 2001)。這些現(xiàn)象可能是因為洋脊軸部的熔體經(jīng)歷了更長期的巖石圈地幔結(jié)晶(Lizarraldeetal., 2004)或者是自東北到西南，洋脊軸部的巖漿供應量增多(Mendeletal., 1997; Cannatetal., 1999)造成的。

研究區(qū)位于Melville斷裂帶(60°45′E)以東，是西南印度洋中脊東段的三個區(qū)域中，平均水深較深(大于4000m)(Mendeletal., 2003)、洋殼厚度最薄、上升到地表的熔體量最少的一段洋脊。樣品采集點位于靠近洋脊段中心，水深較淺(2958m)。值得注意的是，Lietal. (2017)和Ciprianietal. (2011)在樣品采集點周邊遠離軸部、水深較深的區(qū)域(63.5°E)采集到了橄欖巖(圖1)。

2 樣品采集與測試方法

本文研究的玄武巖樣品來自于西南印度洋洋中脊東段東北端(63.9°E)的電視抓斗地質(zhì)取樣，共1件。樣品為斑狀結(jié)構(gòu)，斑晶主要由斜長石和橄欖石組成，主要為斜長石(>95%)，粒徑從1～5mm不等，僅有少量橄欖石斑晶(<5%)，粒徑小于1mm，斜長石與橄欖石的體積比大致為20:1，基質(zhì)為玄武質(zhì)玻璃(圖2a)。樣品中的斜長石斑晶主要為板狀，部分斜長石有環(huán)帶、篩狀、溶蝕現(xiàn)象(圖2b, c)。

表1SWIR63.9°E洋中脊玄武巖全巖、玻璃基質(zhì)主量元素分析結(jié)果(wt%)

Table 1 Contents of major elements in whole rock and glass matrix of Plagioclase Ultra-phyric Basalt collected at SWIR63.9°E (wt%)

CommentSiO2Al2O3Na2OMgOK2OTiO2CaONiOFeOTMnOCr2O3TotalMg#Wholerock50.7719.313.865.850.191.0111.3—7.570.130.03100.02—Glass152.0115.753.916.740.301.3510.350.007.790.190.0398.4263.2Glass251.8215.783.836.760.261.3910.450.037.860.160.0098.3363.0Glass351.8315.654.056.690.291.5010.430.017.820.200.0598.5062.9Glass452.0415.893.916.680.251.2910.440.047.870.150.0698.6162.7Glass552.0215.834.006.850.251.3110.800.007.800.170.0999.1163.5Glass652.4315.634.056.670.271.4010.550.007.970.160.0999.2162.4Glass752.0616.003.887.000.241.4010.570.067.930.180.0099.3263.6Glass852.2015.714.116.970.231.3710.310.007.890.160.0999.0463.6AverageGlass52.0515.783.976.800.261.3810.490.027.870.170.0598.8263.1

圖2 SWIR63.9°E斜長石超斑狀玄武巖薄片照片(a)及具有溶蝕結(jié)構(gòu)的斜長石斑晶(b)和具有篩狀結(jié)構(gòu)的斜長石斑晶(c)Fig.2 Photo of thin section (a) and plagioclase phenocryst with resorbed texture (b), plagioclase phenocryst with sieve texture (c) of Plagioclase Ultra-phyric Basalt collected at SWIR63.9°E

圖3 本文樣品與全球洋中脊系統(tǒng)的玄武質(zhì)熔體成分對比全球洋中脊系統(tǒng)中玄武質(zhì)熔體的數(shù)據(jù)來源于PetDB: http://www.earthchem.org/petdbFig.3 Comparing Al content of the sample with basaltic melts in global ridge system

樣品全巖的主量元素含量分析在澳實分析測試(廣州)有限公司完成。利用PANalytical PW2424 X射線熒光光譜儀(XRF)對全巖常量元素SiO2、TiO2、Al2O3、FeOT、MgO、CaO、Na2O、K2O、MnO2、P2O5、Cr2O3、SO3進行測試。測試過程中采用GBM908-10和MRGeo08作為標準樣品，相對誤差和相對標準差均小于5%。

橄欖石、斜長石和玻璃的主量元素含量原位分析在同濟大學海洋科學技術研究中心完成，儀器型號為JEOL JXA-8230電子探針分析儀。選擇薄片中粒徑>1mm的斜長石的核部和邊緣區(qū)域，共計37個；以及粒徑<0.1mm的斜長石進行分析。分析時間為2min。分析時采用15kV加速電壓，10nA探針電子束流，3～5μm的束斑直徑，分析時間為2min。標準樣品采用SPI國際標準，樣品修正方法采用ZAF法，相對標準差低于小于0.05%。

3 測試結(jié)果

SWIR63.9°E斜長石超斑狀玄武巖全巖的SiO2含量為50.77%，CaO含量為11.30%，Na2O含量為3.86%，Al2O3含量為19.31%，MgO含量為5.85%，F(xiàn)eOT含量為7.57%(表1)，其Al2O3含量高于99%的全球洋中脊系統(tǒng)中已經(jīng)發(fā)現(xiàn)的玄武質(zhì)熔體成分，其中包括洋中脊玄武巖、玻璃以及熔體包裹體(圖3)。

通過數(shù)點法對樣品薄片中各組分的體積分數(shù)進行統(tǒng)計，發(fā)現(xiàn)斜長石斑晶的體積分數(shù)約25%，橄欖石斑晶的體積分數(shù)約1%，玻璃基質(zhì)體積分數(shù)約74%。

樣品中玻璃基質(zhì)的SiO2含量為52.05%，CaO含量為10.49%，Na2O含量為3.97%，Al2O3含量為15.78%，MgO含量為6.80%，F(xiàn)eOT含量為7.87%(表1)。

表2SWIR63.9°E洋中脊玄武巖中晶體的主量元素分析結(jié)果(wt%)

Table 2 Contents of major elements in olivine and plagioclase of Plagioclase Ultra-phyric Basalt collected at SWIR63.9°E (wt%)

CommentSiO2Al2O3Na2OMgOK2OTiO2CaONiOFeOTMnOCr2O3TotalAnFoOl0440.620.040.0146.810.000.010.30—13.040.200.03101.06—87.8Ol0540.610.050.0146.840.010.000.31—12.900.210.06101.01—87.9Ol0640.420.060.0046.410.000.000.27—13.110.230.05100.56—87.6Ol0740.650.060.0146.680.000.010.32—12.950.230.08100.99—87.8Ol3640.380.040.0045.700.010.020.320.0913.170.200.0199.95—87.4Ol3740.220.010.0046.370.010.000.320.2012.620.230.08100.05—88.0Ol5640.420.050.0246.300.000.000.290.1712.920.210.04100.42—87.8Ol6040.110.050.0046.060.000.000.280.1412.960.210.0799.88—87.7Ol6140.540.060.0245.890.010.000.350.1712.590.270.0399.93—87.9Ol6240.120.080.0245.670.000.000.260.1812.700.190.0499.25—87.8Ol7240.450.030.0046.800.030.040.320.1312.840.230.04100.90—87.9pl01-core152.0629.983.640.190.030.0413.330.000.310.000.0099.5967.0—pl01-core251.2030.403.530.170.040.0613.640.050.370.000.0099.4768.1—pl01-core352.4329.433.730.190.040.1113.100.000.330.000.0199.3766.0—pl01-rim151.4830.113.680.190.050.0913.450.000.440.020.0299.5266.9—pl01-rim252.1330.073.510.200.020.0813.540.010.360.000.0099.9268.1—pl01-rim351.7830.363.550.180.040.0913.750.060.430.020.01100.2768.2—pl03-core152.1230.103.510.160.080.0313.390.010.330.000.0199.7267.9—pl03-core252.2030.893.410.200.020.0314.080.040.330.000.01101.2169.5—pl03-core352.0030.193.460.180.040.0513.930.010.310.020.00100.1869.0—pl03-rim52.5630.343.590.240.010.1113.610.010.380.000.05100.9067.8—pl08-core153.1128.214.380.160.030.1011.940.000.310.020.0098.2660.1—pl08-core254.8928.694.390.190.060.0311.830.000.290.000.00100.3559.9—pl08-rim151.1830.473.260.200.010.0614.050.060.330.010.0099.6370.5—pl08-rim252.4330.723.420.170.020.0514.050.000.350.030.00101.2369.4—pl08-rim351.4729.753.440.190.040.0313.800.000.480.000.0099.2168.9—pl09-core51.9229.313.780.180.060.0313.390.040.370.000.0099.0766.2—pl09-rim151.5729.443.620.190.040.1013.660.000.350.000.0098.9767.6—pl09-rim252.1030.193.740.200.040.0113.400.050.540.010.00100.2966.5—pl09-rim351.8130.213.400.170.030.0913.830.000.450.020.0099.9969.2—pl11-core151.5429.613.560.190.050.0013.450.010.360.010.0098.7567.7—pl11-core252.7730.163.710.160.050.0013.080.000.320.000.02100.2566.1—pl11-rim151.6930.403.610.160.000.0614.010.060.500.020.00100.5268.2—pl11-rim251.0929.963.420.210.060.0413.800.000.510.000.0199.0869.1—pl11-rim351.6129.923.390.170.070.0113.960.030.440.000.0199.6169.5—pl13-core152.3930.083.750.160.070.0513.330.070.290.000.01100.2166.3—pl13-core251.4129.083.760.170.070.0813.370.000.320.000.0098.2566.3—pl13-core352.6629.293.830.210.060.0112.800.000.280.020.0499.1964.9—pl13-rim151.6030.363.380.190.040.0313.910.020.370.000.0099.9169.5—pl13-rim251.6629.423.610.190.040.1113.570.010.360.020.0299.0067.5—pl13-rim352.2230.313.330.200.040.0713.790.000.460.000.04100.4569.6—pl14-core152.2629.683.660.200.020.0413.300.000.280.020.0099.4666.8—pl14-core252.3429.493.830.130.040.0513.270.030.310.020.0099.5065.7—pl14-core353.2429.923.880.210.040.0513.190.010.310.000.01100.8565.3—pl14-rim151.4729.413.680.180.030.0713.280.000.450.020.0098.5866.6—pl14-rim250.8829.623.190.200.030.0014.150.000.470.040.0098.5971.0—pl16-core152.7428.944.050.180.040.0312.570.050.270.000.0398.8863.2—

續(xù)表2ContinuedTable2CommentSiO2Al2O3Na2OMgOK2OTiO2CaONiOFeOTMnOCr2O3TotalAnFopl16-core253.5628.554.340.170.020.0512.030.000.300.000.0699.1060.5—pl16-core354.2928.444.500.170.090.0911.710.040.320.000.0299.6759.0—pl16-rim152.7029.654.050.250.050.1013.010.000.490.000.00100.3064.0—pl16-rim252.4229.063.960.190.040.0212.820.040.270.000.0598.8564.2—pl16-rim353.5828.174.370.230.060.1012.020.000.290.040.0098.8660.3—pl16-rim252.0429.663.570.240.050.0213.370.140.440.000.0099.5167.4—pl17-core152.4930.353.510.190.020.0513.830.000.270.020.02100.7468.6—pl17-core252.6930.353.660.180.030.0513.590.030.310.000.02100.9067.3—pl17-core352.9329.763.900.210.060.0213.110.000.300.000.00100.2965.1—pl17-rim151.1730.433.400.150.060.0614.110.000.410.000.0299.8069.7—pl17-rim252.5430.723.530.190.060.0213.770.050.370.000.04101.3068.3—pl17-rim352.1230.413.430.190.020.0513.840.000.390.040.00100.4869.1—pl18-core153.1629.123.970.220.020.0012.680.000.320.040.0399.5463.9—pl18-core252.6629.124.080.200.060.0312.710.000.450.020.0099.3163.3—pl18-core352.5728.674.080.180.040.0612.700.000.330.000.0198.6463.3—pl18-rim151.7530.273.500.200.020.0813.550.000.400.020.0099.7968.2—pl18-rim252.8529.294.050.200.040.0112.740.040.340.000.0399.5963.5—pl18-rim353.1629.204.070.190.060.0512.730.000.360.030.0099.8463.4—pl18-rim451.4029.593.620.250.030.0713.550.080.430.000.0299.0467.5—pl20-core151.0030.763.180.160.020.0414.140.000.280.000.0099.5871.1—pl20-core251.8329.873.610.180.040.0013.350.000.350.000.0499.2667.2—pl20-core351.1430.483.260.140.090.0014.290.000.200.000.0399.6270.8—pl20-core451.5130.383.270.150.030.0513.930.000.320.000.0099.6470.2—pl20-rim152.6229.953.650.220.040.0413.550.020.430.000.02100.5467.3—pl20-rim252.9429.553.980.200.030.0113.260.000.310.000.01100.2964.9—pl21-core152.4929.883.740.170.030.0512.910.050.360.030.0099.7065.6—pl21-core252.7330.133.710.170.040.0313.190.000.300.020.00100.3266.3—pl21-core350.9530.313.290.160.020.0513.970.060.290.000.0199.1170.2—pl21-rim150.9930.113.410.200.050.1013.780.000.450.000.0099.0869.1—pl21-rim251.4430.103.460.190.040.0213.850.030.530.000.0299.6768.9—pl22-core152.7229.953.540.160.030.0213.200.020.300.000.0099.9367.4—pl22-core253.2030.393.700.160.000.0113.220.020.310.020.02101.0666.4—pl22-core351.2829.353.830.160.050.0813.390.000.340.030.0298.5165.9—pl22-rim151.5430.283.420.190.050.0014.000.000.360.040.0099.8869.4—pl22-rim252.0430.483.590.170.040.0813.450.000.330.020.00100.2067.5—pl24-core152.7229.943.930.200.080.0313.090.020.290.000.00100.3064.8—pl24-core253.4929.803.910.190.040.0212.950.000.320.000.00100.7164.7—pl24-core352.8729.473.920.190.030.0013.070.050.310.000.0199.9164.9—pl24-rim152.7529.733.860.210.040.1113.030.000.430.030.01100.1965.1—pl24-rim252.2730.063.620.220.040.0713.870.030.400.050.00100.6367.9—pl24-rim351.9929.833.750.240.030.0613.250.000.440.000.0499.6366.1—pl23-core150.7929.743.520.170.050.0313.510.040.320.010.0098.1868.0—pl23-core250.8729.763.540.180.040.0013.600.020.300.000.0098.3168.0—pl23-rim151.5229.103.820.170.070.0513.170.000.420.030.0098.3465.7—pl23-rim250.7529.903.370.200.040.0013.780.000.420.020.0298.4869.4—pl25-core51.7729.463.920.190.040.0712.870.000.330.000.0498.6764.5—

續(xù)表2ContinuedTable2CommentSiO2Al2O3Na2OMgOK2OTiO2CaONiOFeOTMnOCr2O3TotalAnFopl46-rim252.0730.943.400.180.050.0014.320.040.280.040.01101.3370.0—pl46-rim352.0429.713.730.160.060.0913.500.020.370.000.0499.7166.7—pl47-core150.7829.873.200.190.040.0513.890.000.290.010.0098.3270.6—pl47-core250.3030.063.320.200.040.0313.900.010.270.010.0098.1269.9—pl47-core352.7530.193.720.180.050.0613.300.000.330.000.01100.5866.4—pl47-rim151.6530.383.390.180.040.0113.860.000.420.000.0099.9469.3—pl47-rim251.6130.833.410.170.070.0013.880.000.430.020.02100.4169.3—pl47-rim351.3515.864.166.920.231.3910.600.007.850.140.1098.6058.5—pl48-core53.1528.294.110.210.060.0712.380.000.340.000.0098.6162.5—pl48-rim150.7329.693.470.200.020.0113.750.000.330.020.0098.2268.7—pl48-rim251.6030.083.610.180.050.0913.390.000.480.000.0099.4767.2—pl49-core152.4730.553.670.210.060.0013.130.000.330.030.07100.5266.4—pl49-core251.9530.683.480.210.020.0913.760.000.320.000.04100.5468.7—pl49-core352.6930.163.460.220.070.0513.470.050.280.040.00100.4868.3—pl49-rim152.4730.443.450.190.030.0713.890.020.400.020.00100.9969.0—pl49-rim251.8931.113.300.190.040.0814.050.000.430.000.00101.0970.2—pl49-rim351.7230.513.200.220.020.0813.850.040.410.040.00100.0970.5—pl50-core152.5329.244.080.210.040.0412.900.000.310.060.0199.4163.6—pl50-core251.9630.303.440.150.000.0113.590.020.280.030.0099.7768.6—pl50-core351.7130.453.270.180.030.0213.980.020.310.010.0099.9870.3—pl50-rim151.6130.123.590.190.030.0013.630.000.460.030.0399.6967.7—pl50-rim251.6130.473.420.210.080.0814.120.020.470.010.03100.5169.5—pl50-rim352.0930.503.440.170.050.0113.800.010.420.000.00100.4869.0—pl51-core152.5929.554.080.190.070.0112.700.020.370.000.0299.6063.3—pl51-core252.6129.663.970.200.050.0112.990.000.270.000.0199.7764.4—pl51-core351.9929.473.890.200.030.0813.100.000.270.010.0299.0465.1—pl51-rim151.3129.913.300.190.050.0813.660.000.460.010.0098.9769.6—pl51-rim252.3929.453.880.180.040.1513.050.000.400.000.0299.5665.1—pl54-core153.2230.043.780.160.040.0013.400.000.230.030.02100.9366.2—pl54-core252.1430.553.520.210.050.0313.870.010.260.000.04100.6868.6—pl54-core352.3429.813.630.160.080.0613.420.020.310.000.0199.8367.2—pl54-rim153.1130.593.520.190.050.0213.640.020.380.050.00101.5768.2—pl54-rim252.2515.843.856.770.271.3810.160.028.140.130.0598.8559.4—pl54-rim351.6730.133.510.200.040.0013.670.020.390.030.0099.6668.3—fine-grainedpl152.4330.214.000.230.040.0713.250.000.520.010.00100.7564.7—fine-grainedpl253.9829.293.990.230.060.0512.560.030.560.000.00100.7563.5—fine-grainedpl354.0129.324.060.220.070.1112.740.010.520.030.00101.0763.5—fine-grainedpl452.9429.073.860.190.060.1112.600.000.530.000.0099.3664.4—fine-grainedpl552.5829.063.920.180.040.0012.810.000.560.000.0099.1664.4—fine-grainedpl653.1530.083.850.190.060.0412.990.000.450.030.00100.8365.1—fine-grainedpl752.9928.574.090.560.090.1512.250.041.160.000.0099.9062.4—fine-grainedpl852.3729.534.030.160.030.0613.160.000.550.000.0299.9064.4—fine-grainedpl952.0129.753.780.190.050.0712.990.010.550.000.0099.4065.6—fine-grainedpl1053.0028.674.120.190.060.0312.360.000.600.040.0499.1162.4—fine-grainedpl1151.9829.703.780.210.030.0613.150.020.570.040.0099.5465.8—

斜長石斑晶的SiO2含量為50.30%～54.89%；CaO含量為10.16%～14.50%；Na2O含量為2.87%～4.52%；An值范圍58.5～72.3(表2)，低于大多數(shù)已經(jīng)發(fā)現(xiàn)的斜長石超斑狀玄武巖中斜長石斑晶的An值。橄欖石斑晶的SiO2含量為40.11%～40.65%；MgO含量為45.67%～46.84%；FeOT含量為12.59%～13.17%；Fo值范圍87.7～87.9(表2)。

4 討論

4.1 SWIR63.9°E斜長石超斑狀玄武巖的基質(zhì)成分特征

一般認為，斑晶是在熔體上升過程中通過結(jié)晶分異作用形成的，如果熔體與在其結(jié)晶產(chǎn)生的斑晶一起噴發(fā)出地表形成火山巖，那么斑晶與玻璃基質(zhì)的成分應該是平衡的。

根據(jù)元素在熔體和礦物之間的分配系數(shù)，可以計算與熔體達到平衡時的礦物成分。取KdOl-LiqMg-Fe=0.3(Fordetal., 1983)，計算與玻璃基質(zhì)成分平衡的橄欖石Fo值，得到與玻璃基質(zhì)平衡的橄欖石Fo值為74.2±0.4，這與樣品中的橄欖石斑晶(Fo 87.7～87.9)相比存在較大的差異，即橄欖石斑晶與玻璃基質(zhì)之間存在明顯的不平衡，因此它們應該來源于不同期次的熔體。利用Namuretal. (2012)的模型計算與玻璃基質(zhì)成分平衡的斜長石An值，得到的平衡斜長石An值為66.0±0.8。平衡斜長石An值在樣品中斜長石的An值(58.5～72.3)范圍之內(nèi)，發(fā)現(xiàn)斜長石斑晶發(fā)育溶蝕、篩狀這種受到后期熔體作用形成的特殊結(jié)構(gòu)(圖2c)，說明斜長石斑晶與玻璃基質(zhì)并非同一期次熔體的產(chǎn)物，這與Bennetetal. (2019)的研究成果相符。

Fontetal. (2007)結(jié)合Meyzenetal. (2003)的數(shù)據(jù)，對比了西南印度洋洋中脊東段的玄武巖玻璃基質(zhì)的主量元素含量，發(fā)現(xiàn)該段洋脊中采自同一位置樣品的K2O、P2O5含量變化范圍較大，據(jù)此推測該區(qū)域可能受到多期次熔體的作用。這與本文的觀點一致。

4.2 SWIR63.9°E斜長石超斑狀玄武巖中不同類型斜長石之間的關系

利用Rudge (2008)的方法對不同類型斜長石晶體的An值進行核密度估算(Kernel density estimations, KDEs)(圖4)，在這一分析中將斜長石分為斑晶(>1mm)核心、斑晶(>1mm)邊緣以及微晶(<100μm)。通過分析中可以發(fā)現(xiàn)，斑晶核心An值分布范圍較分散(59～72)，但斑晶邊緣以及微晶的An值呈雙峰分布且范圍較集中，直觀地表現(xiàn)出SWIR63.9°E斜長石超斑狀玄武巖中不同類型斜長石之間存在成分差異。

圖4 樣品中三類斜長石An值的KDEsFig.4 Kernel density estimations (KDEs) of plagioclase An in sample

一般情況下隨著巖漿的結(jié)晶分異，斜長石從核心到邊緣An值連續(xù)降低。具有反環(huán)帶結(jié)構(gòu)的斜長石常見于島弧巖石中，通常是由于巖漿房內(nèi)部熔體發(fā)生再富集成分改變引起的(Tepley Ⅲetal., 1999)，洋中脊玄武巖中比較少見(Hellevang and Pedersen, 2008)。但是SWIR63.9°E斜長石超斑狀玄武巖中的斜長石斑晶出現(xiàn)了類似反環(huán)帶結(jié)構(gòu)的現(xiàn)象(圖5)。根據(jù)斜長石BSE圖中不同區(qū)域的明暗變化和沿長軸的測線，發(fā)現(xiàn)斜長石的邊緣和核心周邊的區(qū)域(BSE圖中較淺色區(qū)域)An值明顯偏高。產(chǎn)生這種現(xiàn)象的原因有兩種，一種是熔體成分發(fā)生改變，另一種是熔體的減壓結(jié)晶作用。雖然減壓導致結(jié)晶產(chǎn)出的斜長石具有較高的An值，但是與早期的結(jié)晶的斜長石相比其TiO2含量卻不會發(fā)生變化(Bennettetal., 2019)，這與樣品中的實際現(xiàn)象不符，因此可以確定樣品中斜長石成分的改變是由熔體成分的變化引起的。

圖6 斜長石超斑狀玄武巖基質(zhì)與非斑狀玄武巖成分對比Galapagos Island-加拉帕戈斯群島；SEIR-Southeast Indian Ridge，東南印度洋中脊；Gorda Ridge-戈爾達脊；SWIR-Southwest Indian Ridge，西南印度洋中脊；Arctic Ridge-北冰洋中脊.數(shù)據(jù)來源于：楊陽, 2013; Christie, 2004; Cullen et al., 1989; Hellevang and Pedersen, 2008; Weinsteiger et al., 2010; PetDB: http://www.earthchem.org/petdbFig.6 Comparing the composition of PUB’s matrix and adjacent aphyric basalts

根據(jù)兩種類型斜長石的分布形態(tài)和位置，認為初始斜長石結(jié)晶后受到后期熔體作用，其成分發(fā)生了改變。與核心區(qū)域相比，受后期熔體作用的部位An值偏高，推測早期結(jié)晶形成的低An值斜長石可能受到后期高Ca/Na熔體溶蝕作用的影響，成分發(fā)生改變。因此樣品中斜長石斑晶的結(jié)構(gòu)和成分變化記錄了多期次熔體作用。

4.3 SWIR63.9°E斜長石超斑狀玄武巖與同一洋脊段MORB成分的一致性

SWIR63.9°E斜長石超斑狀玄武巖全巖的Al2O3含量高于全球洋中脊系統(tǒng)中已發(fā)現(xiàn)的99%的熔體成分(圖3)。Langeetal. (2013)將前人測試過的全球斜長石超斑狀玄武巖玻璃基質(zhì)的主量元素含量，與同一洋脊或構(gòu)造單元的非斑狀玄武巖的主量元素含量進對比，發(fā)現(xiàn)兩者的范圍相似，認為產(chǎn)生兩種類型玄武巖的母熔體之間不存在差異。但是以整個洋脊為背景對兩種類型的巖石進行對比，可能會因為同一洋脊中不同洋脊段之間玄武巖的成分差異巨大使得玄武巖的主量元素含量的范圍過大，掩蓋了兩者之間真正的差異。本文對此方法進行了改進，即按照已有斜長石超斑狀玄武巖的經(jīng)緯度和水深，選取至少采自相同洋脊段的玄武巖(優(yōu)先選擇玻璃)進行對比。參與對比的所有數(shù)據(jù)均來自于前人的研究成果，其中北冰洋中脊(Arctic Ridge)的斜長石超斑狀玄武巖采自Mohns和Knipovich脊。為了更加直觀地展現(xiàn)兩種類型玄武巖的成分，利用MgO+FeOT用來對比兩者母熔體之間是否存在演化程度或地幔源區(qū)的明顯差異；由于斜長石超斑狀玄武巖中超高的斜長石含量使得全巖成分富Al，因此選擇Al2O3+SiO2這兩種斜長石中的主要元素來對比兩者的母熔體是否存在成分上的差異。根據(jù)對比結(jié)果(圖6)，我們可以發(fā)現(xiàn)全球各洋中脊或構(gòu)造單元的斜長石超斑狀玄武巖主量元素含量，幾乎都位于同一洋脊段的玄武巖主量元素含量范圍以內(nèi)。尤其是與西南印度洋中脊同屬于超慢速擴張洋中脊的北冰洋中脊，樣品量較多，兩種類型玄武巖的基質(zhì)成分也表現(xiàn)出很高的相似性。因此認為形成斜長石超斑狀玄武巖的熔體，與同一洋脊段玄武巖的母熔體并無成分上的差異，這與Langeetal. (2013)的觀點一致。

圖7 SWIR63.9°E斜長石超斑狀玄武巖成因機制模式圖(背景圖據(jù)Cannat et al., 1995)紅色箭頭代表斜長石超斑狀玄武巖的母熔體，在熔體到達巖漿房之前斜長石和橄欖石可能已經(jīng)在密度的作用下完成了分選，到達巖漿房后斜長石憑借較熔體小的密度懸在巖漿房頂部漂浮并聚集，最后被噴發(fā)的巖漿攜帶到海底形成斜長石超斑狀玄武巖Fig.7 Hypothetical model for the occurrence of SWIR63.9°E PUBs (background map after Cannat et al., 2015)Arrow in red represents parent melts of plagioclase ultra-phyric basalts. Because of the difference in density, olivine separat from melts and deposit, plagioclase is carried by melts to magma chamber. Plagioclase float and accumulate at the top of magma chamber because of the less density than melts before eruption

4.4 SWIR63.9°E斜長石超斑狀玄武巖的成因機制

SWIR63.9°E斜長石超斑狀玄武巖全巖的Al2O3含量高于絕大多數(shù)的洋中脊玄武巖及其中的玻璃和熔體包裹體(圖3)。但是其基質(zhì)成分與同一洋脊段的非斑狀玄武巖相似，樣品中除斜長石以外缺乏其他富Al的礦物，斜長石斑晶的An值也低于其他洋中脊區(qū)域發(fā)現(xiàn)的同類樣品中斜長石的An值(76～92)(數(shù)據(jù)來源于Langeetal., 2013)。因此認為SWIR63.9°E斜長石超斑狀玄武巖超高的Al2O3含量是高斜長石體積分數(shù)的結(jié)果，而如此高體積分數(shù)的斜長石不可能僅僅是結(jié)晶分異作用的產(chǎn)物。與此同時，SWIR63.9°E斜長石超斑狀玄武巖中的斑晶種類比較單一，絕大多數(shù)為自形程度較高且成分相近的斜長石，其篩狀、溶蝕等指示礦物在結(jié)晶后受熔體作用的結(jié)構(gòu)廣泛發(fā)育。所以，高體積分數(shù)的斜長石更有可能是斜長石在深部結(jié)晶、反應、增生后被熔體帶出地表，而不是熔體上升過程中從圍巖中捕獲而來。由于Gakkel洋脊玄武巖樣品中的斜長石模態(tài)含量極高(>50%)，且沒有明確證據(jù)能夠證明北冰洋中脊下方存在巖漿房，因此Bennettetal. (2019)認為斜長石晶體來自于晶體主導的環(huán)境(如晶粥帶)。但是，本文樣品的晶體含量明顯低于Gakkel洋脊玄武巖，且西南印度洋下方存在具有小規(guī)模巖漿房的可能，因此本文樣品中的斜長石來源于熔體主導環(huán)境(如巖脈或巖漿房)(Bennettetal., 2019)。

Neaveetal. (2013)認為熔體在從深部運移到巖漿房的過程中，密度的差異可能就已經(jīng)使得熔體中攜帶的鎂橄欖石與鈣質(zhì)斜長石分離。在Lange and Carmichael (1990)以及Lange (1997)對熔體摩爾體積的研究基礎上對熔體密度進行計算，發(fā)現(xiàn)與高Fo值橄欖石和高An值斜長石平衡的Haleyjabunga玻璃(Gurenko and Chaussidon, 1995)密度為2.699g/mm3。根據(jù)Smyth and McCormick (1995)的數(shù)據(jù)，F(xiàn)o值88的橄欖石密度為3.573g/mm3，而An值88的斜長石密度為2.746g/mm3。因此在熔體上升過程中橄欖石就會沉入熔體底部，而斜長石會被上升的熔體帶入巖漿房(Neaveetal., 2013)，只有在熔體上升速度足夠快的情況下才會有少量橄欖石斑晶被熔體攜帶至巖漿房與斜長石發(fā)生混合。這與斜長石超斑狀玄武巖中，高且不穩(wěn)定的斜長石/橄欖石比值及兩者間成分的不平衡特征相符。Weinsteigeretal. (2010)計算了不同條件下斜長石超斑狀玄武巖中斜長石、寄主熔體和熔體包裹體的密度，發(fā)現(xiàn)斜長石的密度與其An值呈正相關：An值70的斜長石密度為2.66g/mm3，An值94的斜長石密度為2.71g/mm3。寄主熔體和包裹體的密度隨著演化程度的升高而增大，在H2O含量小于0.5%的條件下，寄主熔體的密度為2.69～2.73g/mm3，熔體包裹體的密度為2.69～2.72g/mm3，這意味著鈣質(zhì)斜長石在儲存無水巖漿的巖漿房中呈懸浮狀態(tài)，聚集在巖漿房頂部。因此，SWIR63.9°E低An值(An<70)的斜長石超斑狀玄武巖成因可以歸結(jié)為各種礦物及熔體間的密度差異：在熔體上升過程中共結(jié)晶的斜長石斑晶和橄欖石斑晶由于密度差異發(fā)生分離，橄欖石斑晶沉降而斜長石斑晶懸浮于熔體中向上運移到達巖漿房；巖漿房內(nèi)，斜長石斑晶與熔體之間的密度差使其能夠漂浮在熔體上部，并在巖漿房頂部形成斜長石聚集體(圖7)。最終在上升熔體的作用下，斜長石聚集體被攜帶并噴發(fā)出地表形成斜長石超斑狀玄武巖。

SWIR63.9°E斜長石超斑狀玄武巖的形成也對我們認識超慢速擴張洋中脊深部巖漿過程具有一定的意義。西南印度洋中脊洋中脊作為超慢速擴張洋脊，巖漿擴張中心和非巖漿擴張中心在脊軸上相間出現(xiàn)，巖漿活動呈點狀分布。但是超慢速擴張洋中脊周期性供應熔體且數(shù)量較少的特點，為斜長石在巖漿房內(nèi)的聚集提供了足夠的時間，也為后期熔體的作用提供了時間。與此同時，雖然西南印度洋洋中脊深部沒有較大規(guī)模的巖漿房，但是較小規(guī)模的巖漿匯聚中心(Lietal., 2015)，也為斜長石在深部的聚集提供了充足的空間。斜長石超斑狀玄武巖與同一洋脊段玄武巖表現(xiàn)形式的差異和基質(zhì)成分的一致性證明了超慢速擴張洋中脊深部可能存在多種形式的巖漿過程。

5 結(jié)論

SWIR63.9°E斜長石超斑狀玄武巖雖與同一洋脊段玄武巖的母熔體無成分上的差異。但橄欖石斑晶與基質(zhì)的不平衡，斜長石斑晶化學成分的多樣性及其斑晶中不平衡結(jié)構(gòu)的發(fā)育說明這些斑晶并不是單一玄武質(zhì)熔體結(jié)晶分異的產(chǎn)物，而可能是早期結(jié)晶的斑晶受隨后多期熔體作用的結(jié)果。此外，斜長石超斑狀玄武巖中超高含量的斜長石斑晶是由于礦物及熔體間存在密度差異，并通過浮力分選在巖漿房頂部聚集的結(jié)果。斜長石超斑狀玄武巖的出露從側(cè)面反映了西南印度洋中脊作為超慢速擴張洋脊具有熔體供應量少、供應周期長的特點。