Geochemistry of Indian Ocean tephra and Afro-Arabian lavas

Widespread silicic pyroclastic eruptions of the Oligocene Afro-Arabian flood volcanic province (ignimbrites and airfall tuffs) produced up to 20% of the total flood volcanic stratigraphy (>6*10**4 km**3). Volumes of individual ignimbrites and tuffs exposed on land range from ~150 to >2000 km**3 and eight major units (15-100 m thick) were erupted in <2 Myr, placing these amongst the largest-magnitude silicic pyroclastic eruptions on Earth. They are compositionally distinctive time-stratigraphic markers which were deposited as co-ignimbrite ashfall deposits on a near-global scale around the time of the Oi2 cooling anomaly at ~30 Ma. Two ignimbrites from the lower part of the flood volcanic succession in Yemen have been correlated to: (a) the conjugate rifted margin of Ethiopia (>500 km distant); and (b) to two deep sea ash layers sampled by ODP Leg 115 in the Indian Ocean ~2700 km to the southeast. This correlation is based on whole rock analyses of silicic units for isotope ratios (Pb, Nd) and rare earth element compositions, in conjunction with novel in situ Pb isotope laser ablation multicollector inductively coupled plasma mass spectroscopy analysis of groundmass and glass shards. Compositional diversity preserved on the scale of individual ash shards in these deep sea tephra layers record chemical heterogeneity present in the silicic magma chambers that is not evident in the welded on-land deposits. Ages of the ash layers can be established by correlation to precisely dated on-land ignimbrites, and current evidence suggests that while these eruptions may have exacerbated already changing climatic conditions, they both marginally post-date the Oi2 global cooling anomaly.

渐新世时期，Afro-Arabian溢流火山省（Afro-Arabian flood volcanic province）广泛发育硅质火山碎屑喷发，产物涵盖熔结凝灰岩（ignimbrites）与降落凝灰岩（airfall tuffs），其生成物质占该区域溢流火山地层总容积的20%左右，而该区域溢流火山地层总容积超过6×10⁴ km³。陆上暴露的单套熔结凝灰岩与凝灰岩单体体积介于约150 km³至2000 km³以上，且8套厚度15~100 m的主要喷发单元在不足2百万年的时间内形成，使此类喷发跻身全球已知规模最大的硅质火山碎屑喷发事件之列。这些喷发产物具备独特的成分特征，可作为标志性时间地层单元，在约30 Ma的Oi2冷却异常（Oi2 cooling anomaly）时期，以共熔结火山灰降落沉积的形式在近全球范围内广泛分布。也门境内溢流火山序列下部的两套熔结凝灰岩已被证实可与以下对象对比关联：(a) 距离超过500 km的埃塞俄比亚共轭裂谷边缘；以及(b) 印度洋东南方向约2700 km处，由大洋钻探计划（Ocean Drilling Program, ODP）第115航次采集的两套深海火山灰层。该对比依据的是硅质单元的全岩同位素比值（Pb、Nd）与稀土元素组成分析结果，同时结合了针对基质与玻璃碎屑的新型原位铅同位素激光剥蚀多接收器电感耦合等离子体质谱法（laser ablation multicollector inductively coupled plasma mass spectroscopy）测试数据。这些深海火山碎屑层中单颗火山灰碎屑所保留的成分多样性，记录了硅质岩浆房内部存在的化学不均一性，而这一特征在陆上熔结堆积物中并不显著。通过与精确定年的陆上熔结凝灰岩进行对比，可确定这些深海火山灰层的形成年代。现有证据表明，尽管此类喷发事件可能加剧了当时已发生变化的气候条件，但它们均略晚于Oi2全球冷却异常事件。