Whole-rock and mineral geochemistry of ODP Hole 176-735B gabbros

We report mineral chemistry, whole-rock major element compositions, and trace element analyses on Hole 735B samples drilled and selected during Leg 176. We discuss these data, together with Leg 176 shipboard data and Leg 118 sample data from the literature, in terms of primary igneous petrogenesis. Despite mineral compositional variation in a given sample, major constituent minerals in Hole 735B gabbroic rocks display good chemical equilibrium as shown by significant correlations among Mg# (= Mg/[Mg + Fe2+]) of olivine, clinopyroxene, and orthopyroxene and An (=Ca/[Ca + Na]) of plagioclase. This indicates that the mineral assemblages olivine + plagioclase in troctolite, plagioclase + clinopyroxene in gabbro, plagioclases + clinopyroxene + olivine in olivine gabbro, and plagioclase + clinopyroxene + olivine + orthopyroxene in gabbronorite, and so on, have all coprecipitated from their respective parental melts. Fe-Ti oxides (ilmenite and titanomagnetite), which are ubiquitous in most of these rocks, are not in chemical equilibrium with olivine, clinopyroxene, and plagioclase, but precipitated later at lower temperatures. Disseminated oxides in some samples may have precipitated from trapped Fe-Ti-rich melts. Oxides that concentrate along shear bands/zones may mark zones of melt coalescence/transport expelled from the cumulate sequence as a result of compaction or filter pressing. Bulk Hole 735B is of cumulate composition. The most primitive olivine, with Fo = 0.842, in Hole 735B suggests that the most primitive melt parental to Hole 735B lithologies must have Mg# 0.637, which is significantly less than Mg# = 0.714 of bulk Hole 735B. This suggests that a significant mass fraction of more evolved products is needed to balance the high Mg# of the bulk hole. Calculations show that 25%-45% of average Eastern Atlantis II Fracture Zone basalt is needed to combine with 55%-75% of bulk Hole 735B rocks to give a melt of Mg# 0.637, parental to the most primitive Hole 735B cumulate. On the other hand, the parental melt with Mg# 0.637 is far too evolved to be in equilibrium with residual mantle olivine of Fo > 0.89. Therefore, a significant mass fraction of more primitive cumulate (e.g., high Mg# dunite and troctolite) is yet to be sampled. This hidden cumulate could well be deep in the lower crust or simply in the mantle section. We favor the latter because of the thickened cold thermal boundary layer atop the mantle beneath slow-spreading ridges, where cooling and crystallization of ascending mantle melts is inevitable. These observations and data interpretation require reconsideration of the popular concept of primary mantle melts and relationships among the extent of mantle melting, melt production, and the composition and thickness of igneous crust.本研究报道了大洋钻探176航次钻探采集并筛选的735B钻孔样品的矿物化学、全岩主量元素组成及微量元素分析数据。本研究结合176航次船载分析数据及文献刊载的118航次样品数据，围绕原生火成岩岩石成因展开讨论。 尽管单一样品内矿物组成存在一定差异，但735B钻孔辉长岩类中的主要造岩矿物整体呈现出良好的化学平衡状态：橄榄石、单斜辉石与斜方辉石的镁指数（Mg#，即Mg/[Mg + Fe²+]），以及斜长石的钙长石分子占比（An，即Ca/[Ca + Na]）之间均存在显著相关性，佐证了这一化学平衡状态。 这表明各类岩性的矿物组合均从各自的母岩浆中协同结晶析出：例如橄长岩troctolite的矿物组合为橄榄石+斜长石，辉长岩gabbro为斜长石+单斜辉石，橄榄辉长岩olivine gabbro为斜长石+单斜辉石+橄榄石，辉长苏长岩gabbronorite为斜长石+单斜辉石+橄榄石+斜方辉石等。 多数岩石中普遍发育的铁钛氧化物（钛铁矿ilmenite与钛磁铁矿titanomagnetite）与橄榄石、单斜辉石及斜长石并未达到化学平衡，而是在更低温度下后期结晶析出的产物。 部分样品中浸染状分布的铁钛氧化物，可能源自被捕获的富铁钛熔体的结晶析出。而沿剪切带/剪切区域富集的铁钛氧化物，则可能指示了因压实或压滤作用从堆晶岩cumulate序列中挤出的熔体汇聚及运移区域。 735B钻孔的全岩样品整体具有堆晶岩组成特征。735B钻孔中最原始的橄榄石（Fo=0.842，Fo为Mg/(Mg+Fe²+)的摩尔占比）表明，对应735B钻孔岩性的最原始母岩浆的镁指数（Mg#）应为0.637，该数值显著低于735B钻孔全岩样品的Mg#（0.714）。 这意味着需掺入占比可观的更高演化程度的产物，才能平衡全岩样品偏高的Mg#。计算结果显示，若要得到对应735B钻孔最原始堆晶岩的母岩浆（Mg#=0.637），需将25%~45%的东亚特兰蒂斯II断裂带平均玄武岩与55%~75%的735B钻孔全岩样品进行混合。 另一方面，Mg#为0.637的母岩浆演化程度过高，无法与Fo值大于0.89的残留地幔橄榄石达到化学平衡。因此，仍有占比可观的更原始的堆晶岩（如高Mg#纯橄岩dunite与橄长岩）尚未被采样获取。 这些未被采样的隐伏堆晶岩可能位于下地壳深部，也可能直接赋存于地幔剖面中。我们更倾向于后者，因为在慢速扩张洋脊下方的地幔顶部，存在增厚的冷热边界层，上升的地幔熔体在此处不可避免地发生冷却与结晶作用。 上述观测结果与数据解读，要求我们重新审视当前主流的原生地幔熔体概念，以及地幔熔融程度、熔体产量与火成地壳组成及厚度之间的关联机制。