Isotopic and chemical compositions of lower crust rocks and minerals from ODP Hole 176-735B

The composition of gabbroic rocks from the drill core of Hole 735B (ODP Leg 176) at the 11 Ma Atlantis II bank close to the slow spreading Southwest Indian Ridge (SWIR) has been analyzed for major and trace elements and Sr, Nd and Pb isotopic composition. The samples are thought to represent much of the mineralogical and geochemical variation in a vertical 1-km section (500-1500 m below the sea floor) of the lower ocean crust. Primitive troctolitic gabbros, olivine gabbros and gabbros that have Mg#=84-70, Ca#>61 and low Na# (Na/(Na+Al)) (8-17) are intruded by patches or veins of more evolved FeTi-oxide rich gabbroic and dioritic rocks with Mg# to 20, Ca# to 32, Na#=14-23, TiO2<7 wt.% and FeOtotal<18 wt.%. All rocks are acdcumulates, and incompatible element concentrations are low, e.g. Pb=0.1-0.7 ppm and U</=0.005 ppm in the primitive rocks and up to 2 ppm Pb and 0.2 ppm U in the evolved. The range of isotopic compositions of the unleached rocks is: 87Sr/86Sr=0.70280-0.70299, average 0.70287+/-0.00005 (1 S.D., N=30 samples) (except one felsic vein with 87Sr/86Sr=0.7045), 143Nd/144Nd=0.51304-0.51314, average 0.51310+/-0.00002 (1 S.D., N=28), 206Pb/204Pb=17.43-18.55, 207Pb/204Pb=15.40-15.61 and 208Pb/204Pb=37.19-38.28. The range of Sr and the almost constant Nd isotopic composition resemble that found in the upper 500 m of Hole 735B, while Pb ranges to more radiogenic compositions. In general, there is a decrease in isotopic variation of Sr and Pb as well as ? (238U/204Pb), U and Pb with depth, with a trend towards relatively unradiogenic compositions. This correlates with a decrease in alteration and frequency of evolved rock-types in the core. Leached samples generally have less radiogenic Pb with values trending towards 206Pb/204Pb=17.35, 207Pb/204Pb=15.35 and 208Pb/204Pb=37.0, while their 87Sr/86Sr ratios deviate less systematically from unleached rocks and reach both higher, 0.70307, and lower values, 0.70276. Separated clinopyroxene has elevated 87Sr/86Sr up to 0.7035, while plagioclase generally has close to whole rock Sr. Leaching reduced 87Sr/86Sr in clinopyroxene and in two (out of nine) cases leached separates and whole rock display isotopic equilibrium. Relatively minor hydrothermal seawater alteration is thought to have increased 87Sr/86Sr in the rocks, while a secondary high temperature percolation of a mantle-derived agent is thought to be the cause for the trend towards radiogenic Pb. This material had intermediate 87Sr/86Sr and may have originated from non-MORB off axis mantle. The main primary igneous isotopic variation of the gabbros is suggested to have been derived from the MORB-mantle and is defined mainly by leached samples from both ODP Leg 176 and Leg 118 and can be explained by two-component mixing of an end-member with composition like Central Indian Ridge basalts and an end-member with composition unlike any MORB. The latter is characterized by very unradiogenic Pb, in particular 207Pb/204Pb, and may have an origin with affinity to old depleted mantle (DM). The isotopic composition of the magmas parental to the FeTi-oxide rich rocks cannot be distinguished from the magmas parental to the primitive gabbros and an intimate relationship is indicated. The small-scale inhomogeneity indicated for the SWIR MORB-mantle at the Atlantis II Fracture Zone was probably inherited by the lower crustal rocks due to small-scale melting and monogenetic magma chambers at this slow spreading ridge.对位于慢速扩张西南印度洋中脊（Southwest Indian Ridge, SWIR）附近11 Ma亚特兰蒂斯II海台的735B钻孔（大洋钻探计划Leg 176航次，Ocean Drilling Program, ODP）岩心的辉长岩类岩石开展了组成分析，测试了主量元素、微量元素以及Sr、Nd、Pb同位素组成。所采集样品代表了洋壳下部垂直1 km段（海底以下500~1500 m）内绝大多数矿物学与地球化学变化特征。 原始的橄长岩、橄榄辉长岩以及Mg#（Mg/(Mg+Fe²+)）为84~70、Ca#（Ca/(Ca+Al)）>61且低Na#（Na/(Na+Al)）为8~17的辉长岩，被富铁钛氧化物的更演化辉长岩类与闪长岩类的斑块或脉体侵入；这类演化岩石的Mg#可达20、Ca#可达32、Na#为14~23、TiO₂含量<7 wt.%、全铁（以FeO计）FeO_total<18 wt.%。所有岩石均为堆积岩（cumulates），不相容元素含量较低：原始岩石中Pb含量为0.1~0.7 ppm，U≤0.005 ppm；演化岩石中Pb含量最高可达2 ppm，U可达0.2 ppm。 未淋滤样品的同位素组成范围为：⁸⁷Sr/⁸⁶Sr=0.70280~0.70299，平均值为0.70287±0.00005（1倍标准偏差，样本量N=30，仅1条长英质脉体例外，其⁸⁷Sr/⁸⁶Sr=0.7045）；¹⁴³Nd/¹⁴⁴Nd=0.51304~0.51314，平均值为0.51310±0.00002（1倍标准偏差，N=28）；²⁰⁶Pb/²⁰⁴Pb=17.43~18.55，²⁰⁷Pb/²⁰⁴Pb=15.40~15.61，²⁰⁸Pb/²⁰⁴Pb=37.19~38.28。 Sr同位素的变化范围与近乎恒定的Nd同位素组成，与735B钻孔上部500 m的特征相似，但Pb同位素组成更具放射成因特征。总体而言，随着深度增加，Sr、Pb同位素以及μ（²³⁸U/²⁰⁴Pb）、U和Pb的同位素变异程度降低，呈现出向相对非放射成因组成演化的趋势。该特征与岩心中蚀变作用强度降低、演化岩石类型出现频率减少具有相关性。 淋滤后的样品通常具有更低放射成因的Pb同位素组成，其值趋近于²⁰⁶Pb/²⁰⁴Pb=17.35、²⁰⁷Pb/²⁰⁴Pb=15.35和²⁰⁸Pb/²⁰⁴Pb=37.0；而其⁸⁷Sr/⁸⁶Sr比值与未淋滤样品的偏离规律性较弱，既可达到0.70307的较高值，也可低至0.70276。分离出的单斜辉石的⁸⁷Sr/⁸⁶Sr比值可达0.7035，而斜长石的Sr同位素组成通常与全岩接近。淋滤作用降低了单斜辉石中的⁸⁷Sr/⁸⁶Sr比值，且在9个单矿物分离样品中的2个案例中，淋滤后的单矿物与全岩达到了同位素平衡。 普遍认为，较弱的热液海水蚀变作用提升了岩石中的⁸⁷Sr/⁸⁶Sr比值；而幔源介质的次生高温渗滤作用，则是Pb同位素向放射成因方向演化的原因。该介质具有中等的⁸⁷Sr/⁸⁶Sr比值，可能源自非洋中脊玄武岩（Mid-Ocean Ridge Basalt, MORB）的轴外幔源。 辉长岩的主要原生火成同位素变异特征被认为源自洋中脊玄武岩幔源，主要可通过735B钻孔所在的大洋钻探计划Leg 176与Leg 118航次的淋滤样品进行限定，其成因可通过两个端元的混合作用解释：一个端元的组成类似于中印度洋脊玄武岩，另一个端元的组成则与已知洋中脊玄武岩均不相同。后者以极低放射成因的Pb同位素为特征，尤其是²⁰⁷Pb/²⁰⁴Pb，其成因可能与古老亏损地幔（Depleted Mantle, DM）具有亲缘性。 富铁钛氧化物岩石的母岩浆同位素组成，与原始辉长岩的母岩浆无法区分，指示二者存在密切成因联系。西南印度洋中脊亚特兰蒂斯断裂带处的洋中脊玄武岩幔源所表现出的小规模不均一性，可能因该慢速扩张脊的小规模熔融作用与单成因岩浆房，而被下地壳岩石继承下来。