留言板

尊敬的读者、作者、审稿人, 关于本刊的投稿、审稿、编辑和出版的任何问题, 您可以本页添加留言。我们将尽快给您答复。谢谢您的支持!

姓名
邮箱
手机号码
标题
留言内容
验证码

西南印度洋中脊27洋脊段新火山脊岩浆深部过程研究—来自斜长石斑晶的制约

王聪浩 刘佳 陶春辉 李伟

王聪浩,刘佳,陶春辉,等. 西南印度洋中脊27洋脊段新火山脊岩浆深部过程研究—来自斜长石斑晶的制约[J]. 海洋地质与第四纪地质,2022,42(6): 11-20. doi: 10.16562/j.cnki.0256-1492.2022040101
引用本文: 王聪浩,刘佳,陶春辉,等. 西南印度洋中脊27洋脊段新火山脊岩浆深部过程研究—来自斜长石斑晶的制约[J]. 海洋地质与第四纪地质,2022,42(6): 11-20. doi: 10.16562/j.cnki.0256-1492.2022040101
WANG Conghao,LIU Jia,TAO Chunhui,et al. Deep magmatic process of new volcano ridge in Segment 27, Southwest Indian Ridge: Constraints from plagioclase phenocrysts[J]. Marine Geology & Quaternary Geology,2022,42(6):11-20. doi: 10.16562/j.cnki.0256-1492.2022040101
Citation: WANG Conghao,LIU Jia,TAO Chunhui,et al. Deep magmatic process of new volcano ridge in Segment 27, Southwest Indian Ridge: Constraints from plagioclase phenocrysts[J]. Marine Geology & Quaternary Geology,2022,42(6):11-20. doi: 10.16562/j.cnki.0256-1492.2022040101

西南印度洋中脊27洋脊段新火山脊岩浆深部过程研究—来自斜长石斑晶的制约


doi: 10.16562/j.cnki.0256-1492.2022040101
详细信息
    作者简介:

    王聪浩(1996—),男,硕士,主要从事洋中脊岩浆作用研究,E-mail:2863689642@qq.com

  • 基金项目:  国家重点研发课题“超慢速扩张洋脊局部岩浆供给的深部过程及其成矿效应研究”(2018YFC0309902)
  • 中图分类号: P744

Deep magmatic process of new volcano ridge in Segment 27, Southwest Indian Ridge: Constraints from plagioclase phenocrysts

More Information
  • 摘要: 前人对超慢速扩张西南印度洋脊(SWIR)玄武岩的研究多基于全岩粉末样品,而对能够记录更多岩浆过程的矿物斑晶开展的工作则较为匮乏。本文对西南印度洋脊断桥热液区所在的27段洋脊富含斜长石斑晶的玄武岩进行了全岩和单矿物的地球化学研究。玄武岩样品(34IV-TVG07 和 30III-TVG14)SiO2含量为49.16%和49.50%, MgO含量分别为6.76%和6.52%。全岩微量元素总体上和N-MORB(normal mid-ocean ridge basalts)类似。电子探针测试结果显示,斜长石的An值范围变化较大(76.2~87.9),且绝大部分的斜长石斑晶An值都在80以上,比SWIR 64°E 的Mount Jourdanne火山斜长石超斑玄武岩中的斜长石An值高得多(<70),暗示斜长石的成因不同于Mount Jourdanne,不是由下洋壳辉长岩中的斜长石被后期岩浆直接机械捕获携带上升而形成。运用Petrolg3软件计算模拟也显示斜长石无法由其寄主岩浆直接结晶产生。结合实验岩石学结果以及西南印度洋中脊地幔中存在古老地幔楔熔融残余的多方面地球化学证据推测,断桥区玄武岩中的高An值斜长石斑晶最有可能由软流圈地幔中的古老、亏损的岛弧地幔楔残余熔融形成的岩浆结晶形成。
  • 图  1  研究区水深图

    a. 西南印度洋中脊(SWIR)中心水深图,Crozet群岛在Indomed和Gallieni转换断层以南约900 km(IFZ-GFZ用黑色虚线表示),红色五角星表示Mount Jourdanne火山位置;b. 为a中红色矩形标出的区域,为西南印度洋中脊(SWIR)50.4°E水深图。

    Figure  1.  Bathymetry of the study areas

    a. Bathymetry of the central Southwest Indian Ridge (SWIR). Crozet Archipelago is located ~900 km south of the Indomed and Gallieni FZs (IFZ-GFZ, denoted by the black dashed lines). The red star represents the location of the Mount Jourdanne volcanic; b. bathymetry of SWIR (50.4°E) marked by the red rectangle in figure a.

    图  2  西南印度洋中脊(SWIR)27段岩浆房示意图[5]

    Figure  2.  Schematic diagram of magma chamber in Segment 27 of SWIR[5]

    图  3  玄武岩手标本和镜下照片

    a. 30III-TVG14手标本; b. 34IV-TVG07手标本; c. 30III-TVG14镜下单偏光; d. 30III-TVG14镜下正交偏光; e. 34IV-TVG07镜下单偏光; f. 34IV-TVG07镜下正交偏光; Plag: 斜长石。

    Figure  3.  Pictures of the basalt samples in this study

    a. Photo of sample 30III-TVG14; b. photo of 34IV-TVG07; c. single polarization light photo of 30III-TVG14; d. crossed polarized light photo of 30III-TVG14; e. single polarization light photo of 34IV-TVG07; f. crossed polarized light photo of 34IV-TVG07. plag: plagioclase.

    图  4  西南印度洋中脊27洋脊段玄武岩微量元素图

    a. 稀土元素球粒陨石标准化图; b. 微量元素原始地幔标准化蛛网图灰色阴影区域表示来自IFZ-GFZ段MORB的文献数据[7-8],球粒陨石数据来自文献[11],原始地幔数据来自文献[12],OIB,N-MORB和E-MORB来自文献[11]。

    Figure  4.  Trace element distribution of basalts from segment 27, SWIR

    a. Chondrite normalized rare earth element (REE); b. primitive mantle normalized trace element patterns of the basalts from Segment 27. Grey shaded fields represent MORB data in the literature form the IFZ-GFZ section[7-8], chondrite data are from reference [11], primitive mantle data are from reference [12], OIB, N-MORB, and E-MORB are from reference [11].

    图  5  斑晶电子探针分析点位

    a. 斑晶34IV-TVG07(2)9分析点位; b. 斑晶34IV-TVG07(2)6分析点位。

    Figure  5.  Electron Microprobe Analysis points of phenocrysts

    a. Analysis points of phenocryst 34IV-TVG07(2)9; b. analysis points of phenocryst 34IV-TVG07(2)6.

    图  6  斜长石斑晶An值

    Figure  6.  Statistical histogram of An-rich plagioclase

    表  1  西南印度洋中脊27段含长石斑晶玄武岩的主量元素含量

    Table  1.   Major element concentrations of plagioclase-hosted basalt from Segment 27, SWIR

    样品编号SiO2TiO2Al2O3TFe2O3MnOMgOCaONa2OK2OP2O5LOISUM
    30III-TVG1449.501.3017.8310.280.166.5211.982.650.200.12−0.28100.25
    34IV-TVG0749.160.9818.259.430.156.7612.902.530.120.09−0.4599.93
      注:主量元素单位:wt/%。
    下载: 导出CSV

    表  2  西南印度洋中脊27段含长石斑晶玄武岩微量元素含量

    Table  2.   Trace element concentrations of plagioclase-hosted basalt from Segment 27, SWIR

    样品编号LiBeScVCrCoNiCuZnGaRbSrY
    30III-TVG144.320.5037.60264.25204.3039.5469.0060.4379.9117.852.03141.3932.82
    34IV-TVG074.250.3738.90231.06272.0436.6762.9660.6067.2816.161.05134.4924.60
    样品编号ZrNbSnCsBaLaCePrNdSmEuGdTb
    30III-TVG1486.881.530.950.0310.102.518.391.458.603.011.134.340.86
    34IV-TVG0756.770.970.620.036.771.725.591.015.852.290.883.250.64
    样品编号DyHoErTmYbLuHfTaTlPbThU
    30III-TVG145.551.123.350.483.230.492.400.120.080.720.120.11
    34IV-TVG074.240.892.640.392.550.371.580.060.050.500.080.11
      注:微量元素单位:wt/10−6
    下载: 导出CSV

    表  3  斜长石斑晶电子探针分析结果

    Table  3.   Major element concentrations for plagioclase phenocrysts

    分析点号SiO2Na2OK2OFeOAl2O3MgOCaOMnOTiO2Cr2O3NiOTotalAn
    30-III-TVG14(2)1-边45.791.590.020.3433.630.2016.920.020.0598.5485.41
    30-III-TVG14(2)1-幔45.311.380.020.4033.470.1816.980.040.050.010.0197.8487.06
    30-III-TVG14(2)1-核46.211.520.010.2933.010.2116.780.040.0498.1185.87
    30-III-TVG14(2)2-边46.551.730.010.3632.910.2216.810.020.040.0198.6684.26
    30-III-TVG14(2)2-幔46.321.590.010.3933.200.2316.780.010.010.0198.5685.29
    30-III-TVG14(2)2-核46.901.800.030.3633.100.2316.640.010.0499.1183.52
    30-III-TVG14(2)3-边46.571.620.020.3033.390.2116.800.030.0398.9785.05
    30-III-TVG14(2)3-幔47.081.790.000.3233.360.2316.820.010.0299.6383.85
    30-III-TVG14(2)3-核46.641.730.030.3233.130.2216.610.030.040.0098.7384.00
    30-III-TVG14(2)4-边47.311.870.020.3333.120.2216.440.0399.3482.83
    30-III-TVG14(2)4-幔46.071.410.020.3033.900.1617.230.010.010.0199.1187.04
    30-III-TVG14(2)4-核46.861.500.010.3133.810.1717.130.070.020.0299.9186.23
    30-III-TVG14(2)5-边47.341.800.030.3333.390.2516.600.010.010.020.0299.8083.45
    30-III-TVG14(2)5-幔47.401.760.030.3733.040.2016.500.050.010.0499.3983.68
    30-III-TVG14(2)5-核47.291.800.020.3333.340.2216.600.0399.6483.45
    30-III-TVG14(2)6-边47.441.880.010.3433.180.2316.650.010.050.010.0199.8182.95
    30-III-TVG14(2)6-幔47.041.740.030.3433.140.2116.470.0298.9783.83
    30-III-TVG14(2)6-核47.491.880.010.3533.420.2116.420.020.050.0199.8582.76
    30-III-TVG14(2)7-边47.251.800.030.3333.450.2116.660.030.020.0199.7783.54
    30-III-TVG14(2)7-幔47.251.860.020.3433.420.2116.660.010.0099.7783.11
    30-III-TVG14(2)7-核46.781.730.030.2933.560.2216.760.050.070.0799.5484.17
    30-III-TVG14(2)8-边47.051.820.020.3633.260.2216.730.030.0399.5283.47
    30-III-TVG14(2)8-幔46.771.700.030.3833.520.2016.790.010.0299.4284.37
    30-III-TVG14(2)8-核46.831.720.020.3333.420.2016.770.030.020.070.0199.4384.23
    34IV-TVG07(1)13-边47.741.900.010.4133.220.1916.460.030.020.0199.9882.72
    34IV-TVG07(1)13-幔47.341.790.020.4033.430.1616.630.0199.7883.57
    34IV-TVG07(1)13-核47.501.780.020.3833.490.1816.570.010.0699.9983.65
    34IV-TVG07(1)12-边47.281.700.030.3633.840.1916.830.04100.2884.42
    34IV-TVG07(1)12-幔47.411.570.020.3233.700.2116.880.000.06100.1885.53
    34IV-TVG07(1)12-核47.451.740.030.3633.700.2116.840.04100.3684.10
    34IV-TVG07(1)11-边47.851.860.020.3533.550.1716.590.07100.4583.04
    34IV-TVG07(1)11-幔46.531.670.010.3933.600.1616.890.020.000.0199.2984.79
    34IV-TVG07(1)11-核46.441.680.010.3433.750.1716.910.040.0399.3784.74
    34IV-TVG07(1)10-边47.862.120.010.3532.730.2016.140.060.0399.5180.73
    34IV-TVG07(1)10-幔49.052.610.030.3532.120.2615.260.030.0299.7476.23
    34IV-TVG07(1)10-核49.202.420.020.2832.480.2015.590.020.070.04100.3277.99
    34IV-TVG07(1)9-边46.941.710.010.3533.930.1716.980.050.01100.1484.57
    34IV-TVG07(1)9-幔47.081.750.020.3534.080.1816.930.04100.4284.13
    34IV-TVG07(1)9-核46.771.630.010.3533.730.1917.010.0299.7185.17
    34IV-TVG07(1)8-边47.241.630.010.3733.700.1616.830.000.040.020.02100.0185.02
    34IV-TVG07(1)8-幔47.952.150.010.3733.250.2116.200.030.06100.2480.59
    34IV-TVG07(1)8-核47.592.060.020.3533.020.2616.250.020.0599.6281.23
    34IV-TVG07(1)7-边46.971.640.010.3733.190.2016.670.0199.0584.87
    34IV-TVG07(1)7-幔46.271.770.060.4433.540.1816.600.010.0198.8783.54
    34IV-TVG07(1)7-核46.961.710.020.3533.780.1517.010.020.03100.0384.51
    34IV-TVG07(1)5-边48.202.440.020.3732.190.2215.550.030.0499.0577.81
    34IV-TVG07(1)5-幔47.752.220.020.4132.550.1716.020.010.030.070.0199.2679.81
    34IV-TVG07(1)5-核47.691.980.030.3933.010.1516.240.010.0199.5281.76
    34IV-TVG07(1)4-边47.251.960.020.3832.860.1616.360.020.0299.0382.05
    34IV-TVG07(1)4-幔47.962.010.010.3932.710.2015.990.010.010.0199.3181.41
    34IV-TVG07(1)4-核46.511.820.010.3533.300.1416.660.030.060.0198.8983.43
    34IV-TVG07(1)3-边46.321.600.020.3533.780.2016.830.030.020.0299.1685.24
    34IV-TVG07(1)3-幔46.451.520.010.3633.880.1717.150.030.0599.6286.11
    34IV-TVG07(1)3-核46.031.380.020.2934.080.1717.370.050.0299.4087.35
    34IV-TVG07(1)2-边47.261.970.010.3833.260.1816.480.000.020.0699.6282.17
    34IV-TVG07(1)2-幔46.611.770.040.4133.330.1916.690.030.030.0199.0983.73
    34IV-TVG07(1)2-核47.321.730.010.3633.410.1816.760.020.0399.8284.17
    34IV-TVG07(1)1-边46.411.520.020.4033.790.1817.000.010.070.070.0199.4786.01
    34IV-TVG07(1)1-幔47.071.700.030.3633.630.1716.800.030.060.0399.8784.41
    34IV-TVG07(1)1-核47.241.740.030.4333.870.1817.110.02100.6184.36
    34IV-TVG07(2)1-边47.831.970.020.4033.340.1916.480.000.02100.2482.09
    34IV-TVG07(2)1-幔47.141.930.020.4132.850.1816.320.010.0298.8982.24
    34IV-TVG07(2)1-核46.911.620.030.3633.960.1616.940.0199.9985.10
    34IV-TVG07(2)2-边45.901.540.020.3433.260.1316.600.050.0197.8685.55
    34IV-TVG07(2)2-幔45.941.320.020.3633.860.1417.4299.0687.86
    34IV-TVG07(2)2-核44.941.420.030.3732.440.1616.410.030.040.020.0395.8786.32
    34IV-TVG07(2)3-边47.752.210.020.3532.390.1916.010.0398.9479.96
    34IV-TVG07(2)3-幔49.032.600.000.3431.840.2415.250.060.060.030.0599.4976.42
    34IV-TVG07(2)3-核48.172.280.030.4332.220.2115.620.010.040.0199.0178.98
    34IV-TVG07(2)4-边47.101.970.020.3932.750.2016.010.020.0198.4681.72
    34IV-TVG07(2)4-幔46.561.500.040.4133.070.1516.720.040.090.0198.5985.85
    34IV-TVG07(2)4-核47.211.890.020.4533.030.1916.230.040.0199.0782.47
    34IV-TVG07(2)5-边47.021.530.020.3833.760.2017.020.030.010.03100.0085.94
    34IV-TVG07(2)5-幔46.371.620.020.3733.420.2216.880.020.0198.9085.14
    34IV-TVG07(2)5-核46.811.600.020.3833.260.2016.880.010.0199.1685.23
    34IV-TVG07(2)6-边46.901.640.020.4333.440.1916.880.0199.5084.95
    34IV-TVG07(2)6-幔46.891.540.010.3833.640.1616.970.040.0199.6585.81
    34IV-TVG07(2)6-幔46.611.520.020.3933.790.1617.060.020.0399.5986.05
    34IV-TVG07(2)6-幔46.931.670.010.3933.570.1616.830.030.020.0499.6484.74
    34IV-TVG07(2)6-核47.011.510.020.3733.220.1616.850.030.0199.1785.94
    34IV-TVG07(2)7-边47.481.780.020.4833.340.2116.260.020.060.0299.660.83
    34IV-TVG07(2)7-幔48.942.420.040.4532.290.2215.3499.690.78
    34IV-TVG07(2)7-幔47.581.980.010.4533.020.2116.350.010.0599.660.82
    34IV-TVG07(2)7-幔47.651.680.030.4233.030.2016.330.0199.330.84
    34IV-TVG07(2)7-核48.362.100.020.3932.900.1915.860.010.000.0199.840.81
    34IV-TVG07(2)8-边46.241.380.020.3133.650.1517.100.020.0598.930.87
    34IV-TVG07(2)8-幔46.341.660.030.3533.470.2016.620.020.040.0198.740.85
    34IV-TVG07(2)8-核46.871.740.030.3933.580.2016.720.030.0399.590.84
    34IV-TVG07(2)10-边47.061.780.020.3633.400.1716.550.020.010.050.0199.420.84
    34IV-TVG07(2)10-幔46.671.620.020.3533.250.1716.770.010.0298.870.85
    34IV-TVG07(2)10-核45.811.690.030.3733.110.1616.690.0597.910.84
      注:主量元素单位:wt/%。
    下载: 导出CSV

    表  4  Petrolog3结晶分异模拟计算结果

    Table  4.   Results of Petrolog3 simulation

    样品编号模拟压力最高An值
    30III-TVG141Kbar78.2
    3Kbar74.8
    5Kbar71.3
    7Kbar67.9
    34IV-TVG071Kbar80.5
    3Kbar77.1
    5Kbar73.7
    7Kbar70.3
    40II-TVG041Kbar75.4
    3Kbar71.9
    5Kbar68.5
    7Kbar65.1
      注:橄榄石、斜长石、单斜辉石模型均来自文献[19]。
    下载: 导出CSV
  • [1] Sauter D, Cannat M. The ultraslow spreading Southwest Indian ridge[M]//Rona P A, DeveyC W, Dyment J, et al. Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges. Washington, D. C.: American Geophysical Union, 2010, 88: 153-173.
    [2] 孙国洪, 田丽艳, 李小虎, 等. 西南印度洋中脊岩石地球化学特征及其岩浆作用研究[J]. 海洋地质与第四纪地质, 2021, 41(5):126-138

    SUN Guohong, TIAN Liyan, LI Xiaohu, et al. A review of studies on the magmatism at Southwest Indian Ridge from petrological and geochemical perspectives [J]. Marine Geology & Quaternary Geology, 2021, 41(5): 126-138.
    [3] Dick H J B, Lin J, Schouten H. An ultraslow-spreading class of ocean ridge [J]. Nature, 2003, 426(6965): 405-412. doi: 10.1038/nature02128
    [4] Li J B, Jian H C, Chen Y J, et al. Seismic observation of an extremely magmatic accretion at the ultraslow spreading Southwest Indian Ridge [J]. Geophysical Research Letters, 2015, 42(8): 2656-2663. doi: 10.1002/2014GL062521
    [5] Jian H C, Singh S C, Chen Y J, et al. Evidence of an axial magma chamber beneath the ultraslow-spreading Southwest Indian Ridge [J]. Geology, 2017, 45(2): 143-146. doi: 10.1130/G38356.1
    [6] Chen J, Cannat M, Tao C H, et al. 780 thousand years of upper - crustal construction at a melt-rich segment of the ultraslow spreading southwest Indian Ridge 50°28′E [J]. Journal of Geophysical Research:Solid Earth, 2021, 126(10): e2021JB022152.
    [7] Yang A Y, Zhao T P, Zhou M F, et al. Isotopically enriched N-MORB: A new geochemical signature of off - axis plume - ridge interaction–A case study at 50°28′E, Southwest Indian Ridge [J]. Journal of Geophysical Research:Solid Earth, 2017, 122(1): 191-213. doi: 10.1002/2016JB013284
    [8] Yu X, Dick H J B. Plate-driven micro-hotspots and the evolution of the Dragon Flag melting anomaly, Southwest Indian Ridge [J]. Earth and Planetary Science Letters, 2020, 531: 116002. doi: 10.1016/j.jpgl.2019.116002
    [9] 李伟. 西南印度洋中脊玄武岩岩石地球化学特征: 对超慢速扩张的启示[D]. 中国地质大学博士学位论文, 2017

    LI Wei. Petrogeochemical characteristics of basalts from Southwest Indian Ridge: Implications for magmatic processes at ultra-slow spreading ridge[D]. Doctor Dissertation of China University of Geosciences (Beijing), 2017.
    [10] 初凤友, 陈建林, 马维林, 等. 中太平洋海山玄武岩的岩石学特征与年代[J]. 海洋地质与第四纪地质, 2005, 25(4):55-59

    CHU Fengyou, CHEN Jianlin, MA Weilin, et al. Petrologic characteristics and ages of basalt in Middle Pacific mountains [J]. Marine Geology & Quaternary Geology, 2005, 25(4): 55-59.
    [11] Sun S S, McDonough W F. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes [J]. Geological Society, London, Special Publications, 1989, 42(1): 313-345. doi: 10.1144/GSL.SP.1989.042.01.19
    [12] McDonough W F, Sun S S. The composition of the Earth [J]. Chemical Geology, 1995, 120(3-4): 223-253. doi: 10.1016/0009-2541(94)00140-4
    [13] Li W, Tao C H, Zhang W, et al. Melt inclusions in plagioclase macrocrysts at mount Jourdanne, southwest Indian ridge (~64ºE): implications for an enriched mantle source and shallow magmatic processes [J]. Minerals, 2019, 9(8): 493. doi: 10.3390/min9080493
    [14] Beard J S, Borgia A. Temporal variation of mineralogy and petrology in cognate gabbroic enclaves at Arenal volcano, Costa Rica [J]. Contributions to Mineralogy and Petrology, 1989, 103(1): 110-122. doi: 10.1007/BF00371368
    [15] Crawford A J, Falloon T J, Eggins S. The origin of island arc high-alumina basalts [J]. Contributions to Mineralogy and Petrology, 1987, 97(3): 417-430. doi: 10.1007/BF00372004
    [16] Sinton C W, Christie D M, Coombs V L, et al. Near-primary melt inclusions in anorthite phenocrysts from the Galapagos Platfrom [J]. Earth and Planetary Science Letters, 1993, 119(4): 527-537. doi: 10.1016/0012-821X(93)90060-M
    [17] Stolz A J, Varne R, Wheller G E, et al. The geochemistry and petrogenesis of K-rich alkaline volcanics from the Batu Tara volcano, eastern Sunda arc [J]. Contributions to Mineralogy and Petrology, 1988, 98(3): 374-389. doi: 10.1007/BF00375187
    [18] Kudo A M, Weill D F. An igneous plagioclase thermometer [J]. Contributions to Mineralogy and Petrology, 1970, 25(1): 52-65. doi: 10.1007/BF00383062
    [19] Duncan R A, Green D H. The genesis of refractory melts in the formation of oceanic crust [J]. Contributions to Mineralogy and Petrology, 1987, 96(3): 326-342. doi: 10.1007/BF00371252
    [20] Hirschmann M M. Water, melting, and the deep Earth H2O cycle [J]. Annual Review of Earth and Planetary Sciences, 2006, 34: 629-653. doi: 10.1146/annurev.earth.34.031405.125211
    [21] Wang W, Kelley K A, Li Z G, et al. Volatile element evidence of local MORB mantle heterogeneity beneath the southwest Indian ridge, 48º-51ºE [J]. Geochemistry, Geophysics, Geosystems, 2021, 22(7): e2021GC009647.
    [22] Liu J, Tao C H, Zhou J P, et al. Water enrichment in the mid-ocean ridge by recycling of mantle wedge residue [J]. Earth and Planetary Science Letters, 2022, 584: 117455. doi: 10.1016/j.jpgl.2022.117455
    [23] Panjasawatwong Y, Danyushevsky L V, Crawford A J, et al. An experimental study of the effects of melt composition on plagioclase-melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase [J]. Contributions to Mineralogy and Petrology, 1995, 118(4): 420-432. doi: 10.1007/s004100050024
    [24] Danyushevsky L V. The effect of small amounts of H2O on crystallisation of mid-ocean ridge and backarc basin magmas [J]. Journal of Volcanology and Geothermal Research, 2001, 110(3-4): 265-280. doi: 10.1016/S0377-0273(01)00213-X
    [25] Gao C G, Dick H J B, Liu Y, et al. Melt extraction and mantle source at a Southwest Indian Ridge Dragon Bone amagmatic segment on the Marion Rise [J]. Lithos, 2016, 246-247: 48-60. doi: 10.1016/j.lithos.2015.12.007
    [26] Michael P. Regionally distinctive sources of depleted MORB: Evidence from trace elements and H2O [J]. Earth and Planetary Science Letters, 1995, 131(3-4): 301-320. doi: 10.1016/0012-821X(95)00023-6
  • [1] 刘昆, 宋鹏, 胡雯燕, 李虎, 毛雪莲.  南海北部琼东南盆地烃源岩发育特征与气源综合分析 . 海洋地质与第四纪地质, 2022, 42(6): 173-184. doi: 10.16562/j.cnki.0256-1492.2022060601
    [2] 杜学鑫, 祝文君, 牟明杰, 尚鲁宁, 李攀峰, 尉佳, 虞义勇, 孟元库, 胡刚.  菲律宾海板块俯冲与岛弧演化的钻探靶区研究 . 海洋地质与第四纪地质, 2022, 42(5): 199-210. doi: 10.16562/j.cnki.0256-1492.2022062002
    [3] 张国良.  卡洛琳海山链成因及验证地幔柱成因假说的大洋钻探设想 . 海洋地质与第四纪地质, 2022, 42(5): 172-177. doi: 10.16562/j.cnki.0256-1492.2022072401
    [4] 黄子航, 肖媛媛.  祁连与伊豆-小笠原玻安岩的地球化学特征和成因模型对比 . 海洋地质与第四纪地质, 2022, 42(4): 135-145. doi: 10.16562/j.cnki.0256-1492.2022050401
    [5] 赵晗, 张国良, 张吉, 王帅.  卡洛琳地幔柱活动减弱过程中岩浆成因和源区组成演化 . 海洋地质与第四纪地质, 2022, 42(4): 122-134. doi: 10.16562/j.cnki.0256-1492.2022012202
    [6] 鲁银涛, 杨涛涛, 许小勇, 徐宁, 刘忻蕾, 闫春, 邵大力, 范国章, 吕福亮, 李东.  印度尼西亚库泰盆地下中新统混积序列特征研究 . 海洋地质与第四纪地质, 2022, 42(2): 158-166. doi: 10.16562/j.cnki.0256-1492.2021051403
    [7] 齐文菁, 李小艳, 范德江, 张辉, 殷征欣, 刘升发.  印度洋东经90°海岭现代沉积物稀土元素组成及其物源示踪意义 . 海洋地质与第四纪地质, 2022, 42(2): 92-100. doi: 10.16562/j.cnki.0256-1492.2021050701
    [8] 刘家岐, 兰晓东.  中太平洋莱恩海山富钴结壳元素地球化学特征及成因 . 海洋地质与第四纪地质, 2022, 42(2): 81-91. doi: 10.16562/j.cnki.0256-1492.2021041901
    [9] 李响, 叶俊, 刘希军, 石学法, 李传顺, 闫仕娟.  大西洋中脊赤狐热液区热液产物矿物学特征及其地质意义 . 海洋地质与第四纪地质, 2022, 42(2): 46-58. doi: 10.16562/j.cnki.0256-1492.2021062301
    [10] 朱茂林, 刘震, 张枝焕, 刘畅, 杨鹏程, 李佳阳, 崔凤珍.  西湖凹陷平北地区平湖组下段烃源岩分布地震预测 . 海洋地质与第四纪地质, 2022, 42(1): 170-183. doi: 10.16562/j.cnki.0256-1492.2021072901
    [11] 谢世文, 王宇辰, 舒誉, 吴宇翔, 张丽丽, 刘冬青, 王菲.  珠一坳陷湖盆古环境恢复与优质烃源岩发育模式 . 海洋地质与第四纪地质, 2022, 42(1): 159-169. doi: 10.16562/j.cnki.0256-1492.2021110901
    [12] 樊俊宁, 曾志刚, 朱博文, 齐海燕.  东太平洋海隆13°N附近沉积物中类脂化合物的分布特征及其对热液活动的指示 . 海洋地质与第四纪地质, 2022, 42(1): 26-36. doi: 10.16562/j.cnki.0256-1492.2021010201
    [13] 胡昱洁, 李小艳, 宋召军, 张彬, 殷征欣, 张辉, 胡倩男, 丁旋.  印度洋东经90°海岭表层沉积物浮游有孔虫分布特征及其影响因素 . 海洋地质与第四纪地质, 2022, 42(): 1-13. doi: 10.16562/j.cnki.0256-1492.2022032102
    [14] 周静毅, 杜学斌, 陈茂根, 蒋涔, 周英.  澳大利亚波拿巴盆地N区块岩性圈闭识别探讨 . 海洋地质与第四纪地质, 2021, 41(6): 183-193. doi: 10.16562/j.cnki.0256-1492.2020122302
    [15] 朱晓青, 侯方辉, 刘洪滨, 郭兴伟, 孙天本, 秦亚超, 安郁辉, 李凤春.  山东即墨马山粗面英安岩年代学与地球化学特征及其地质意义 . 海洋地质与第四纪地质, 2021, 41(6): 138-150. doi: 10.16562/j.cnki.0256-1492.2021011801
    [16] 佟宏鹏, 姚凯, 陈琳莹, 胡海明, 崔彩英, 陈多福.  马里亚纳弧前Quaker蛇纹岩泥火山自生烟囱生长模式 . 海洋地质与第四纪地质, 2021, 41(6): 15-26. doi: 10.16562/j.cnki.0256-1492.2021062501
    [17] 王小杰, 颜中辉, 刘俊, 刘欣欣, 杨佳佳.  基于模型优化的广义自由表面多次波压制技术在印度洋深水海域的应用 . 海洋地质与第四纪地质, 2021, 41(5): 221-230. doi: 10.16562/j.cnki.0256-1492.2020101202
    [18] 兰蕾, 李友川, 王一博.  南海南部海陆过渡相烃源岩的两类分布模式 . 海洋地质与第四纪地质, 2021, 41(5): 173-180. doi: 10.16562/j.cnki.0256-1492.2021011802
    [19] 孙国洪, 田丽艳, 李小虎, 张汉羽, 陈凌轩, 刘红玲.  西南印度洋中脊岩石地球化学特征及其岩浆作用研究 . 海洋地质与第四纪地质, 2021, 41(5): 126-138. doi: 10.16562/j.cnki.0256-1492.2021021701
    [20] 肖倩文, 冯秀丽, 苗晓明.  南海北部神狐海域SH37岩芯浊流沉积及其物源分析 . 海洋地质与第四纪地质, 2021, 41(5): 101-111. doi: 10.16562/j.cnki.0256-1492.2021011901
  • 加载中
图(6) / 表(4)
计量
  • 文章访问数:  95
  • HTML全文浏览量:  26
  • PDF下载量:  45
  • 被引次数: 0
出版历程
  • 收稿日期:  2022-04-01
  • 修回日期:  2022-04-27
  • 刊出日期:  2022-12-28

西南印度洋中脊27洋脊段新火山脊岩浆深部过程研究—来自斜长石斑晶的制约

doi: 10.16562/j.cnki.0256-1492.2022040101
    作者简介:

    王聪浩(1996—),男,硕士,主要从事洋中脊岩浆作用研究,E-mail:2863689642@qq.com

基金项目:  国家重点研发课题“超慢速扩张洋脊局部岩浆供给的深部过程及其成矿效应研究”(2018YFC0309902)
  • 中图分类号: P744

摘要: 前人对超慢速扩张西南印度洋脊(SWIR)玄武岩的研究多基于全岩粉末样品,而对能够记录更多岩浆过程的矿物斑晶开展的工作则较为匮乏。本文对西南印度洋脊断桥热液区所在的27段洋脊富含斜长石斑晶的玄武岩进行了全岩和单矿物的地球化学研究。玄武岩样品(34IV-TVG07 和 30III-TVG14)SiO2含量为49.16%和49.50%, MgO含量分别为6.76%和6.52%。全岩微量元素总体上和N-MORB(normal mid-ocean ridge basalts)类似。电子探针测试结果显示,斜长石的An值范围变化较大(76.2~87.9),且绝大部分的斜长石斑晶An值都在80以上,比SWIR 64°E 的Mount Jourdanne火山斜长石超斑玄武岩中的斜长石An值高得多(<70),暗示斜长石的成因不同于Mount Jourdanne,不是由下洋壳辉长岩中的斜长石被后期岩浆直接机械捕获携带上升而形成。运用Petrolg3软件计算模拟也显示斜长石无法由其寄主岩浆直接结晶产生。结合实验岩石学结果以及西南印度洋中脊地幔中存在古老地幔楔熔融残余的多方面地球化学证据推测,断桥区玄武岩中的高An值斜长石斑晶最有可能由软流圈地幔中的古老、亏损的岛弧地幔楔残余熔融形成的岩浆结晶形成。

English Abstract

  • 西南印度洋脊(SWIR)是南极洲板块和非洲板块的分界线,东起罗德里格斯三联点,西至布维三联点,是世界上扩张速率最缓慢的洋脊之一(扩张速率约为1.2~1.8 cm/a)[1-2]。SWIR整体上具有超慢速洋脊的一般特征:岩浆活动匮乏,平均洋壳厚度较薄,拆离断层等构造活动较为发育[3]。 然而,最近的地球物理观测结果发现,位于Indomed和Gallieni转换断层之间的27洋脊段(约50.6°E)具有超厚的洋壳(>10 km)[4]图1a),并且发育尚未固结的岩浆房[5]图2)。中国大洋航次历年海底热液调查发现该洋脊段存在活动的中高温热液活动和多金属硫化物矿化现象(即断桥热液区)。最近,Chen等[6]通过综合地球物理探测对27洋脊段(50°28′E)的地形构造进行了解译,发现该区域发育多个岩浆单元,总体具有高频岩浆喷发和低构造应变的特征,经历了两次间隔约300 Ka的岩浆旋回。Yang等[7]发现该洋脊段存在一类高铝的玄武岩,具有不相容元素亏损而同位素富集的特征,如高206Pb/204Pb(17.990~18.277)、低143Nd/144Nd(0.512983~0.513002),并且这种富集的同位素组成和洋脊南侧的克洛泽洋岛玄武岩值接近。作者认为该类玄武岩的形成与洋脊-克洛泽热点相互作用有关,且深部熔体与下部洋壳存在强烈相互作用。而Yu和Dick[8]则认为此类同位素组成富集而微量元素配分亏损的地球化学特征可能与冈瓦纳大陆裂解后残留在印度洋上地幔中的古老的富集地幔残块有关。玄武岩中的矿物斑晶(如橄榄石,单斜辉石,斜长石等)能够记录更多的岩浆过程,或可以为讨论岩浆深部过程、地幔源区性质等提供更多的证据。前人的工作多集中于全岩粉末样品,基于斑晶的工作较为匮乏。本文选取SWIR 27洋脊段(50.4°E)中富含长石斑晶的两个玄武岩样品为研究对象,报道了全岩主微量元素和斜长石斑晶的矿物化学特征,据此探讨其对该洋脊段深部岩浆过程以及地幔源区属性等方面的指示意义。

    图  1  研究区水深图

    Figure 1.  Bathymetry of the study areas

    图  2  西南印度洋中脊(SWIR)27段岩浆房示意图[5]

    Figure 2.  Schematic diagram of magma chamber in Segment 27 of SWIR[5]

    • 本文研究的样品是由中国大洋第30和第34航次在西南印度洋脊27段通过电视抓斗采集获取,采样位置如图1b所示。岩石样品呈灰黑色,致密块状构造,斑状结构发育,斑晶多具有新鲜核部,表面因与海水接触而遭受不同程度蚀变。玄武岩样品中斑晶主要为斜长石,大多数斑晶具有自形结构,粒径0.5~7.0 mm。镜下可见长石多具有聚片双晶结构和熔体包裹体,部分长石斑晶发育环带(图3)。基质成分主要为斜长石,辉石次之,此外还有少量玻璃质,橄榄石斑晶更少,镜下观察较为新鲜,无明显蚀变现象。

      图  3  玄武岩手标本和镜下照片

      Figure 3.  Pictures of the basalt samples in this study

    • 首先去除岩石样品表面的蚀变部分,选取新鲜块体进行破碎处理,然后将破碎好的岩石放入盛有去离子水的烧杯中,置入超声波清洗器清洗至去离子水不再浑浊,烘干后用玛瑙研钵磨至小于200目,以供分析测试。样品的主微量元素组成测试通过波长色散X射线荧光光谱仪进行,在武汉上谱分析科技有限责任公司完成,仪器型号为 ZSXPrimusⅡ。测试选取安山岩、玄武岩、花岗闪长岩和辉长岩标样(GSR-2、GSR-3、GSR-9、GSR-10)进行标控,分析结果表明,主量元素组成的精准度>5%。使用安捷伦7700e ICP-MS仪器对溶解的样品溶液进行微量元素分析,测试选取安山岩、玄武岩、花岗闪长岩和辉长岩标样(GSR-2、GSR-3、GSR-9、GSR-10)进行标控,分析结果表明,微量元素组成的精准度高于5%。主微量元素分析结果见表1表2

      表 1  西南印度洋中脊27段含长石斑晶玄武岩的主量元素含量

      Table 1.  Major element concentrations of plagioclase-hosted basalt from Segment 27, SWIR

      样品编号SiO2TiO2Al2O3TFe2O3MnOMgOCaONa2OK2OP2O5LOISUM
      30III-TVG1449.501.3017.8310.280.166.5211.982.650.200.12−0.28100.25
      34IV-TVG0749.160.9818.259.430.156.7612.902.530.120.09−0.4599.93
        注:主量元素单位:wt/%。

      表 2  西南印度洋中脊27段含长石斑晶玄武岩微量元素含量

      Table 2.  Trace element concentrations of plagioclase-hosted basalt from Segment 27, SWIR

      样品编号LiBeScVCrCoNiCuZnGaRbSrY
      30III-TVG144.320.5037.60264.25204.3039.5469.0060.4379.9117.852.03141.3932.82
      34IV-TVG074.250.3738.90231.06272.0436.6762.9660.6067.2816.161.05134.4924.60
      样品编号ZrNbSnCsBaLaCePrNdSmEuGdTb
      30III-TVG1486.881.530.950.0310.102.518.391.458.603.011.134.340.86
      34IV-TVG0756.770.970.620.036.771.725.591.015.852.290.883.250.64
      样品编号DyHoErTmYbLuHfTaTlPbThU
      30III-TVG145.551.123.350.483.230.492.400.120.080.720.120.11
      34IV-TVG074.240.892.640.392.550.371.580.060.050.500.080.11
        注:微量元素单位:wt/10−6

      玄武岩中的斜长石斑晶背散射图像采集和化学组成在自然资源部第二海洋研究所海底科学重点实验室采用JEOL-JXA-8100型电子探针分析获取。分析条件为: 加速电压15 kV、试样电流20 nA和聚焦光束1 μm。分析中采用的天然和合成标准用于指定元素:橄榄石(Si、Mg)、磷灰石(Ca、P)、赤铁矿(Fe)、钠长石(Na、Al)、正长石(K)、红白云石(Mn)、金红石(Ti)和Tugtupite (Cl)。使用ZAF校正对原始数据进行校正。长石探针分析结果见表3

      表 3  斜长石斑晶电子探针分析结果

      Table 3.  Major element concentrations for plagioclase phenocrysts

      分析点号SiO2Na2OK2OFeOAl2O3MgOCaOMnOTiO2Cr2O3NiOTotalAn
      30-III-TVG14(2)1-边45.791.590.020.3433.630.2016.920.020.0598.5485.41
      30-III-TVG14(2)1-幔45.311.380.020.4033.470.1816.980.040.050.010.0197.8487.06
      30-III-TVG14(2)1-核46.211.520.010.2933.010.2116.780.040.0498.1185.87
      30-III-TVG14(2)2-边46.551.730.010.3632.910.2216.810.020.040.0198.6684.26
      30-III-TVG14(2)2-幔46.321.590.010.3933.200.2316.780.010.010.0198.5685.29
      30-III-TVG14(2)2-核46.901.800.030.3633.100.2316.640.010.0499.1183.52
      30-III-TVG14(2)3-边46.571.620.020.3033.390.2116.800.030.0398.9785.05
      30-III-TVG14(2)3-幔47.081.790.000.3233.360.2316.820.010.0299.6383.85
      30-III-TVG14(2)3-核46.641.730.030.3233.130.2216.610.030.040.0098.7384.00
      30-III-TVG14(2)4-边47.311.870.020.3333.120.2216.440.0399.3482.83
      30-III-TVG14(2)4-幔46.071.410.020.3033.900.1617.230.010.010.0199.1187.04
      30-III-TVG14(2)4-核46.861.500.010.3133.810.1717.130.070.020.0299.9186.23
      30-III-TVG14(2)5-边47.341.800.030.3333.390.2516.600.010.010.020.0299.8083.45
      30-III-TVG14(2)5-幔47.401.760.030.3733.040.2016.500.050.010.0499.3983.68
      30-III-TVG14(2)5-核47.291.800.020.3333.340.2216.600.0399.6483.45
      30-III-TVG14(2)6-边47.441.880.010.3433.180.2316.650.010.050.010.0199.8182.95
      30-III-TVG14(2)6-幔47.041.740.030.3433.140.2116.470.0298.9783.83
      30-III-TVG14(2)6-核47.491.880.010.3533.420.2116.420.020.050.0199.8582.76
      30-III-TVG14(2)7-边47.251.800.030.3333.450.2116.660.030.020.0199.7783.54
      30-III-TVG14(2)7-幔47.251.860.020.3433.420.2116.660.010.0099.7783.11
      30-III-TVG14(2)7-核46.781.730.030.2933.560.2216.760.050.070.0799.5484.17
      30-III-TVG14(2)8-边47.051.820.020.3633.260.2216.730.030.0399.5283.47
      30-III-TVG14(2)8-幔46.771.700.030.3833.520.2016.790.010.0299.4284.37
      30-III-TVG14(2)8-核46.831.720.020.3333.420.2016.770.030.020.070.0199.4384.23
      34IV-TVG07(1)13-边47.741.900.010.4133.220.1916.460.030.020.0199.9882.72
      34IV-TVG07(1)13-幔47.341.790.020.4033.430.1616.630.0199.7883.57
      34IV-TVG07(1)13-核47.501.780.020.3833.490.1816.570.010.0699.9983.65
      34IV-TVG07(1)12-边47.281.700.030.3633.840.1916.830.04100.2884.42
      34IV-TVG07(1)12-幔47.411.570.020.3233.700.2116.880.000.06100.1885.53
      34IV-TVG07(1)12-核47.451.740.030.3633.700.2116.840.04100.3684.10
      34IV-TVG07(1)11-边47.851.860.020.3533.550.1716.590.07100.4583.04
      34IV-TVG07(1)11-幔46.531.670.010.3933.600.1616.890.020.000.0199.2984.79
      34IV-TVG07(1)11-核46.441.680.010.3433.750.1716.910.040.0399.3784.74
      34IV-TVG07(1)10-边47.862.120.010.3532.730.2016.140.060.0399.5180.73
      34IV-TVG07(1)10-幔49.052.610.030.3532.120.2615.260.030.0299.7476.23
      34IV-TVG07(1)10-核49.202.420.020.2832.480.2015.590.020.070.04100.3277.99
      34IV-TVG07(1)9-边46.941.710.010.3533.930.1716.980.050.01100.1484.57
      34IV-TVG07(1)9-幔47.081.750.020.3534.080.1816.930.04100.4284.13
      34IV-TVG07(1)9-核46.771.630.010.3533.730.1917.010.0299.7185.17
      34IV-TVG07(1)8-边47.241.630.010.3733.700.1616.830.000.040.020.02100.0185.02
      34IV-TVG07(1)8-幔47.952.150.010.3733.250.2116.200.030.06100.2480.59
      34IV-TVG07(1)8-核47.592.060.020.3533.020.2616.250.020.0599.6281.23
      34IV-TVG07(1)7-边46.971.640.010.3733.190.2016.670.0199.0584.87
      34IV-TVG07(1)7-幔46.271.770.060.4433.540.1816.600.010.0198.8783.54
      34IV-TVG07(1)7-核46.961.710.020.3533.780.1517.010.020.03100.0384.51
      34IV-TVG07(1)5-边48.202.440.020.3732.190.2215.550.030.0499.0577.81
      34IV-TVG07(1)5-幔47.752.220.020.4132.550.1716.020.010.030.070.0199.2679.81
      34IV-TVG07(1)5-核47.691.980.030.3933.010.1516.240.010.0199.5281.76
      34IV-TVG07(1)4-边47.251.960.020.3832.860.1616.360.020.0299.0382.05
      34IV-TVG07(1)4-幔47.962.010.010.3932.710.2015.990.010.010.0199.3181.41
      34IV-TVG07(1)4-核46.511.820.010.3533.300.1416.660.030.060.0198.8983.43
      34IV-TVG07(1)3-边46.321.600.020.3533.780.2016.830.030.020.0299.1685.24
      34IV-TVG07(1)3-幔46.451.520.010.3633.880.1717.150.030.0599.6286.11
      34IV-TVG07(1)3-核46.031.380.020.2934.080.1717.370.050.0299.4087.35
      34IV-TVG07(1)2-边47.261.970.010.3833.260.1816.480.000.020.0699.6282.17
      34IV-TVG07(1)2-幔46.611.770.040.4133.330.1916.690.030.030.0199.0983.73
      34IV-TVG07(1)2-核47.321.730.010.3633.410.1816.760.020.0399.8284.17
      34IV-TVG07(1)1-边46.411.520.020.4033.790.1817.000.010.070.070.0199.4786.01
      34IV-TVG07(1)1-幔47.071.700.030.3633.630.1716.800.030.060.0399.8784.41
      34IV-TVG07(1)1-核47.241.740.030.4333.870.1817.110.02100.6184.36
      34IV-TVG07(2)1-边47.831.970.020.4033.340.1916.480.000.02100.2482.09
      34IV-TVG07(2)1-幔47.141.930.020.4132.850.1816.320.010.0298.8982.24
      34IV-TVG07(2)1-核46.911.620.030.3633.960.1616.940.0199.9985.10
      34IV-TVG07(2)2-边45.901.540.020.3433.260.1316.600.050.0197.8685.55
      34IV-TVG07(2)2-幔45.941.320.020.3633.860.1417.4299.0687.86
      34IV-TVG07(2)2-核44.941.420.030.3732.440.1616.410.030.040.020.0395.8786.32
      34IV-TVG07(2)3-边47.752.210.020.3532.390.1916.010.0398.9479.96
      34IV-TVG07(2)3-幔49.032.600.000.3431.840.2415.250.060.060.030.0599.4976.42
      34IV-TVG07(2)3-核48.172.280.030.4332.220.2115.620.010.040.0199.0178.98
      34IV-TVG07(2)4-边47.101.970.020.3932.750.2016.010.020.0198.4681.72
      34IV-TVG07(2)4-幔46.561.500.040.4133.070.1516.720.040.090.0198.5985.85
      34IV-TVG07(2)4-核47.211.890.020.4533.030.1916.230.040.0199.0782.47
      34IV-TVG07(2)5-边47.021.530.020.3833.760.2017.020.030.010.03100.0085.94
      34IV-TVG07(2)5-幔46.371.620.020.3733.420.2216.880.020.0198.9085.14
      34IV-TVG07(2)5-核46.811.600.020.3833.260.2016.880.010.0199.1685.23
      34IV-TVG07(2)6-边46.901.640.020.4333.440.1916.880.0199.5084.95
      34IV-TVG07(2)6-幔46.891.540.010.3833.640.1616.970.040.0199.6585.81
      34IV-TVG07(2)6-幔46.611.520.020.3933.790.1617.060.020.0399.5986.05
      34IV-TVG07(2)6-幔46.931.670.010.3933.570.1616.830.030.020.0499.6484.74
      34IV-TVG07(2)6-核47.011.510.020.3733.220.1616.850.030.0199.1785.94
      34IV-TVG07(2)7-边47.481.780.020.4833.340.2116.260.020.060.0299.660.83
      34IV-TVG07(2)7-幔48.942.420.040.4532.290.2215.3499.690.78
      34IV-TVG07(2)7-幔47.581.980.010.4533.020.2116.350.010.0599.660.82
      34IV-TVG07(2)7-幔47.651.680.030.4233.030.2016.330.0199.330.84
      34IV-TVG07(2)7-核48.362.100.020.3932.900.1915.860.010.000.0199.840.81
      34IV-TVG07(2)8-边46.241.380.020.3133.650.1517.100.020.0598.930.87
      34IV-TVG07(2)8-幔46.341.660.030.3533.470.2016.620.020.040.0198.740.85
      34IV-TVG07(2)8-核46.871.740.030.3933.580.2016.720.030.0399.590.84
      34IV-TVG07(2)10-边47.061.780.020.3633.400.1716.550.020.010.050.0199.420.84
      34IV-TVG07(2)10-幔46.671.620.020.3533.250.1716.770.010.0298.870.85
      34IV-TVG07(2)10-核45.811.690.030.3733.110.1616.690.0597.910.84
        注:主量元素单位:wt/%。
    • 此次测试的两块玄武岩(34IV-TVG07和30III-TVG14)样品SiO2含量分别为49.16%和49.50%,MgO含量分别为6.76%和6.52%。样品烧失量为负,表明分析样品非常新鲜。相比该区其它玄武岩来说[8],两个样品具有高Al的特征,Al2O3含量分别为18.25%和17.83%(表1)。34IV-TVG07和30III-TVG14的全岩Mg# (Mg#=Mg/(Mg+Fe)×100)分别为61.4和58.5,低于原生岩浆含量(约72)[9-10],说明样品经历了显著的结晶分异作用。玄武岩的球粒陨石标准化稀土元素在图4中给出,且全部落在前人的数据范围之内(图4)。样品的REE具有轻稀土亏损的特征,34IV-TVG07和30III-TVG14的(La/Sm)N值分别为0.49和0.54,(Ce/Yb) N值分别为0.61和0.72,(Sm/Yb)N比值分别为0.99和1.04(N代表球粒陨石标准化,球粒陨石数据来自Sun和McDonough[11]),与典型的N-MORB类似,表明LREE和HREE之间的分异程度较低。两个样品的Eu*(Eu*=2EuCH/(SmCH+GdCH),CH代表球粒陨石标准化)值分别为0.98和0.95。

      图  4  西南印度洋中脊27洋脊段玄武岩微量元素图

      Figure 4.  Trace element distribution of basalts from segment 27, SWIR

      电子探针测试结果显示斜长石的An值范围变化较大(76.2~87.9)(表3)。斜长石种类为培长石,与典型的大洋拉斑玄武岩特征相符合。Na2O含量为1.32%~2.61%(平均1.80%);Al2O3含量为31.84%~33.96%(平均33.26%);CaO含量为15.25%~17.42%(平均16.57%)。绝大部分的斜长石斑晶An值基本都在80以上,根据斜长石An值的核-边成分变化,可以将斜长石斑晶分为3类:第1类,从核到边An值逐渐增加,如斑晶34IV-TVG07(2)9(图5 a),第2类,从核到边An值逐渐减少,如斑晶34IV-TVG07(2)6(图5 b),第3类,斜长石斑晶 An 值从核部到边部呈现震荡现象,如斑晶30-III-TVG14(2)4,这些环带特征可能与岩浆多期次的注入和混合有关。

      图  5  斑晶电子探针分析点位

      Figure 5.  Electron Microprobe Analysis points of phenocrysts

    • 前人对超慢速扩张西南印度洋脊玄武岩长石斑晶成分的系统分析较少。Li 等[13]报道了西南印度洋洋中脊64°E处的Mount Jourdanne火山斜长石超斑状玄武岩中的长石组成。此火山玄武岩中的长石斑晶粒径可达1 cm以上,其An值为60~69。根据长石微量元素和熔体包裹体成分,Li等[13]认为Mount Jourdanne超斑玄武岩中的长石是由前期岩浆在下部洋壳深度结晶,然后由后期的岩浆机械分离捕获并喷发携带上来。本文所报道的50.4°E玄武岩中的长石An最高值达87.9(图6),平均值可达83.6,显著高于Mount Jourdanne超斑玄武岩斜长石斑晶的An范围,暗示了与其不同的成因模式。

      图  6  斜长石斑晶An值

      Figure 6.  Statistical histogram of An-rich plagioclase

    • 高An值斜长石可以在岛弧高铝玄武岩和某些靠近热液影响区域的大洋中脊玄武岩中出现[14-17]。形成高An值斜长石斑晶的可能原因通常包括:① 岩浆富水[18];② 岩浆结晶压力较高;③ 岩浆具有高Al2O3的特征[14];④ 母岩浆具有异常高的CaO/Na2O [19]

      高水含量可以显著提高斜长石的An值,这可能是岛弧玄武岩中存在高An值斜长石的原因,但是大洋中脊环境下水含量很难达到岛弧玄武岩的水平[20]。Wang等[21]和Liu等[22]报道了西南印度洋48°~52°E玄武岩淬火玻璃的水含量,其最高值也并未超过0.5%。因此,原始岩浆高含水量不是造成本文样品高An值的原因。考虑到50.4°E洋脊段具有比相邻洋脊段更厚的洋壳厚度,较高的结晶压力是潜在的成因。然而,根据Panjasawatwong等[23]的长石结晶实验结果,增加熔体的Ca#(Ca/(Ca+Na)×100)和Al#(Al/(Al+Si)×100)会增加斜长石的含量,但在更高压下的结晶实际上会起到相反的作用,即在更高压力下结晶出的长石反而比在低压下结晶的长石An值更低。

      为了更好地了解高An值斜长石结晶过程,我们用 Petrolog3 软件进行了结晶模拟计算[24]表4展示了分别采用本文报道的两个样品和本洋脊段具有最高MgO含量的样品(40II-TVG04,MgO为10.49%)的成分作为岩浆初始成分进行模拟计算获得的最高长石An值。可以看到,这几种成分的岩浆在不同压力下都无法直接结晶形成An值大于81的斜长石斑晶。因此,这些斜长石并非由其寄主岩浆直接结晶产生。

      表 4  Petrolog3结晶分异模拟计算结果

      Table 4.  Results of Petrolog3 simulation

      样品编号模拟压力最高An值
      30III-TVG141Kbar78.2
      3Kbar74.8
      5Kbar71.3
      7Kbar67.9
      34IV-TVG071Kbar80.5
      3Kbar77.1
      5Kbar73.7
      7Kbar70.3
      40II-TVG041Kbar75.4
      3Kbar71.9
      5Kbar68.5
      7Kbar65.1
        注:橄榄石、斜长石、单斜辉石模型均来自文献[19]。

      通过上述分析,初步排除了高水含量和高压结晶造成50.4°E玄武岩长石高An值的可能性,唯一的可能解释是形成高An值长石的原始母岩浆本身具有特殊性。Panjasawatwong 等[23]的实验结果表明,如果原始熔体具有高的CaO/Na2O比值(>10),在低水含量的情况下也可以结晶出高An值的斜长石。而这类熔体可以由亏损的或者难熔的地幔橄榄岩部分熔融产生。一般来说,经历过早期熔体提取的地幔可具有这种亏损岩浆组成(低的Na2O含量,高的CaO/Na2O)[22]。实际上,无论是Yang等[7]还是Yu和Dick[8]都认为西南印度洋洋中脊48°~52°E地幔中存在过古老的地幔熔体抽取事件,不同之处在于前者认为该熔体抽取是克洛泽地幔柱的部分熔融,而后者认为是残存于冈瓦纳大陆裂解之前的大陆岩石圈地幔中的古老亏损地幔。Gao等[25]对西南印度洋53°E处直接出露在洋底的橄揽岩进行的元素模拟计算显示,龙骨区地幔经历过古老、富水岛弧环境下的部分熔融。这些古老的亏损型地幔在印度洋打开后,零星分布于上地幔软流圈中,之后因地幔对流被输运到洋中脊下方[8]。最近,Liu等[22]对SWIR龙旂超级洋脊段(48°~52°E)的玄武岩玻璃进行了水含量、微量元素和H-B等多同位素分析,结果表明,本研究的27洋脊段玄武岩具有异常高的H2O/Ce比值(>1000), 而一般的洋中脊玄武岩H2O/Ce比值最高仅为200[26],此外,27洋脊段的岩浆具有较高的δD和较低的Ce/Pb比值特征[22],均指向了富水的岛弧地幔成因。也就是说,多个方面的地球化学证据显示了西南印度洋中脊下存在古老的岛弧环境相关的亏损地幔楔残余,这种岛弧特征属性在断桥热液区所在的27洋脊段尤其显著[22]。本文所报道的高An值长石斑晶推测正是由此类地幔发生部分熔融产生的岩浆结晶形成,从而为SWIR地幔源区的属性提供了间接佐证。

    • 超慢速扩张西南印度洋超慢速洋脊断桥热液区所在的洋脊段(50.4°E)发育有含大量长石斑晶的玄武岩。大多数斜长石斑晶具有自形结构,An值较高(76.2~87.9),斜长石斑晶存在正、反和韵律成分环带结构。相比西南印度洋64°E 的Mount Jourdanne火山中的斜长石超斑玄武岩来说,断桥区的样品斜长石斑晶具有高An特征,表明了其斜长石不是由类似的下洋壳辉长岩中的斜长石被后期岩浆直接机械捕获携带上升而形成。Petrolg3软件的模拟也显示了这些斜长石无法由其寄主岩浆直接结晶产生。结合实验岩石学结果,以及西南印度洋中脊地幔中存在古老地幔楔熔融残余的多方面地球化学证据,认为断桥区玄武岩中的高An值斜长石斑晶成因是由于地幔源区存在古老亏损的岛弧地幔楔残余,这些不均一的亏损地幔物质熔融形成的原始岩浆随后发生结晶分异,形成了断桥区的高An值斜长石。

参考文献 (26)

目录

    /

    返回文章
    返回