Supplementary materials for PhD thesis 'The Origin and Petrogenesis of the Ethiopian Flood Basalts': Supplement 1 - Microprobe analyses

This dataset comprises the files contained on a CD-ROM which was attached to the thesis when it was submitted in 2009. It was uploaded to ORDO in 2025 for preservation purposes. For more information, please refer to the thesis “The Origin and Petrogenesis of the Ethiopian Flood Basalts” via ORO.AbstractThe volcanic rocks of the Afro-Arabian volcanic province chart the nature and evolution of the mantle plume source responsible for a major continental flood basalt event and the manner in which it interacted with the lithosphere over the course of continental separation. The flood basalts of the Ethiopian Plateau constitute the larger part of this province and are associated with melting in the head of the Afar Plume. High-Ti picrites from the eastern part of the NW Plateau are among the earliest uncontaminated outpourings of flood volcanism; their compositions are therefore considered to be close to that of the primary melt. These are exposed in a stratigraphically coherent sequence with variably porphyritic high-Ti olivine-rich basalts and ankaramites, which together provide a continuous record of volcanism reflecting a transition from high- to low-flux magmatism. Phase-relations differ fromthose in other tholeiitic flood basalts in that clinopyroxene and magnetite crystallisebefore plagioclase. This is typical of ferropicrites and reflects a high-Fe-Ti-, and low AI-source. The olivine phenocrysts exhibit a range of morphologies and compositions which reflect fractionation and magma-mixing at varying depths prior to eruption, and the introduction of water into the magmas at shallow levels is evident from pervasive iddingsitization. The lavas have similar major element chemistry to MORB but show higher concentrations of incompatible elements inherited from a HIMU-type lithospheric component. and enrichments in LREE relative to HREE which reflect the presence of residual garnet in the source. The picrites and ankaramites have lower AI2O3 and higher K2O, P2O5, TiO2 and Fe2O3(tot) contents than any reported lavas with equivalent MgO content from other flood basalt provinces. These least evolved lavas were derived from oxidised (QFM+1), deep-seated magmas generated by different degrees of partial melting of the primary melt, whereas the olivine basalt magmas evolved by crystal fractionation at a shallow level from a melt similar in composition to the ankaramites. The MgO content of the primary melt calculated from the most Mg-rich olivines (Fo90) is between 17 and 18 %, and it is likely that this ponded at the base of the crust where it evolved at pressures near to 1.2 GPa. Primary melt fractions of 3 - 6 % were generated in the plume at pressures of 4 - 5 GPa and temperatures in excess of 1600 °C, implying that melting occurred beneath thick (120- 150 km) lithosphere prior to extension. Sub-chondritic initial 187Os/188Os ratios (0.125 - 0.126) indicate derivation from a depleted mantle source, and high 3He/4He (18.6 Ra) and solar-like Ne isotopic ratios imply an origin in undegassed mantle.本数据集包含2009年提交学位论文时随附的只读光盘（CD-ROM）内的全部文件，于2025年上传至ORDO进行长期保存。如需更多信息，请通过ORO查阅学位论文《埃塞俄比亚溢流玄武岩的成因与岩石成因》（"The Origin and Petrogenesis of the Ethiopian Flood Basalts"）。 Abstract：Afro-Arabian火山省（Afro-Arabian volcanic province）的火山岩完整记录了引发大型大陆溢流玄武岩事件的地幔柱源区的属性与演化历程，以及该源区在大陆裂解进程中与岩石圈的相互作用模式。埃塞俄比亚高原溢流玄武岩是该火山省的主体组成部分，其成因与阿法尔地幔柱（Afar Plume）头部的熔融作用密切相关。 埃塞俄比亚高原西北部东部地带产出的高钛苦橄岩（High-Ti picrites）属于溢流火山作用最早的未受污染喷发产物之一，其地球化学组成被认为与原始岩浆高度近似。该类苦橄岩与具不等斑结构的高钛富橄榄石玄武岩及安卡玄武岩（ankaramite）形成层序连贯的地层序列，共同构成了一套完整的火山作用记录，反映了岩浆作用从高通量向低通量的转变过程。 该套岩石的相变关系与其他拉斑质溢流玄武岩存在显著差异：单斜辉石（clinopyroxene）与磁铁矿（magnetite）的结晶顺序早于斜长石（plagioclase）。这一特征是富铁苦橄岩（ferropicrites）的典型标志，反映其源区具有高钛高铁、低铝的地球化学属性。 橄榄石斑晶具有多种形貌与成分特征，反映了喷发前不同深度下的岩浆分异与岩浆混合作用；而普遍发育的伊丁石化（iddingsitization）现象则证明岩浆在浅部阶段曾有水介质加入。 该套熔岩的主量元素化学组成与洋中脊玄武岩（MORB）相近，但继承了HIMU型岩石圈组分的特征，不相容元素含量更高；同时轻稀土元素（LREE）相对于重稀土元素（HREE）的富集特征，反映其源区残留有石榴石。 相较于其他溢流玄武岩省中报道的同等氧化镁（MgO）含量的熔岩，本研究中的苦橄岩与安卡玄武岩具有更低的氧化铝（Al₂O₃）含量，以及更高的氧化钾（K₂O）、五氧化二磷（P₂O₅）、二氧化钛（TiO₂）和全铁（Fe₂O₃(tot)）含量。 这些演化程度最低的熔岩源自氧化状态为QFM+1的深部岩浆，该岩浆由原始岩浆经不同程度的部分熔融形成；而橄榄玄武岩岩浆则由成分近似安卡玄武岩的熔体在浅部经晶体分异作用演化而来。由最富镁的橄榄石（Fo₉₀）计算得到的原始岩浆氧化镁含量为17%~18%，该岩浆可能在地壳底部发生岩浆储集，并在约1.2 GPa的压力条件下完成演化。 地幔柱中形成的原始熔体占比为3%~6%，形成压力为4~5 GPa，温度超过1600 ℃，这表明熔融作用发生在大陆裂解前的厚达120~150 km的岩石圈之下。初始187Os/188Os比值（0.125~0.126）低于球粒陨石值，表明其源区为亏损地幔；而高的3He/4He比值（18.6 Ra）以及类太阳的氖同位素组成，则暗示其源自未脱气的地幔源区。