Palaeointensities of Oligocene and Miocene volcanic sections from Ethiopia: field behaviour during the Cainozoic
Palaeointensities of Oligocene and Miocene volcanic sections from Ethiopia: field behaviour during the Cainozoic
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DOI：
10.1093/gji/ggy491
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发表时间：
2019-02
影响因子：
2.8
通讯作者：
F. Lhuillier;V. Shcherbakov;V. Shcherbakova;S. Ostner;G. Hervé;N. Petersen
中科院分区：
文献类型：
--
作者：
F. Lhuillier;V. Shcherbakov;V. Shcherbakova;S. Ostner;G. Hervé;N. Petersen
对埃塞俄比亚早渐新世（Belessa剖面）和中中新世（Debre Sina剖面）的火山岩进行了绝对古强度（API）测量。根据样品的高场热磁曲线的可逆性进行仔细选择，得出明确的古方向后，我们将分析限制在五个（分别为5个）。6)来自Belessa（负责）的冷却装置。Debre Sina)，提交给Thellier和wilson风格的实验。x射线衍射测量和反射光下的显微观察使我们对Belessa玄武岩剩余物载体的可靠性有了很好的信心，在钛磁铁矿（TM）颗粒中有伪板岩析出，这是高温（即氘）氧化的高级阶段的代名词。相比之下，Debre Sina粗面玄武岩可能经历了低温（即后固结）蚀变，其特征是TM颗粒的粒化，导致API估计的分散性更高。与“世界古强度数据库”相比，埃塞俄比亚早渐新世的偶极矩（30±8 ZAm2）低于中中新世（65±20 ZAm2），这与全球从30 Ma到现在的偶极子强度增加相一致，也表明渐新世期间偶极子强度较低。然而，这一观测结果必须谨慎对待，因为全球数据库中上新世-更新世测定的代表性过高，而且偶极子强度与古强度测定的数量之间存在部分相关性。
We conducted an absolute palaeointensity (API) survey on Ethiopian volcanics from Early Oligocene (Belessa section) and Middle Miocene (Debre Sina section). After a careful selection based on the reversibility of high-field thermomagnetic curves from samples yielding unambiguous palaeodirections, we restricted our analysis to five (resp. six) cooling units from Belessa (resp. Debre Sina), submitted to both Thellier and Wilson-style experiments. X-ray diffraction measurements and microscopical observations under reflected light gave us good confidence in the reliability of the remanence carriers of the Belessa basalts, with pseudo-brookite exsolutions in titanomagnetite (TM) grains, synonymous of an advanced stage of high-temperature (i.e. deuteric) oxidation. In contrast, the Debre Sina trachybasalts may have been submitted to low-temperature (i.e. post-consolidation) alteration featured by granulation of TM grains and leading to a higher dispersion of API estimates. Compared to the ‘world palaeointensity database’, the lower dipole moment found in Ethiopia for the Early Oligocene (30 ± 8 ZAm2) than the Middle Miocene (65 ± 20 ZAm2) is compatible with a global increase of dipole strength from 30 Ma to present, and also suggestive of a dipole low during the Oligocene. This observation must however be treated with caution because of the over-representation of Plio-Pleistocene determinations in the global database and the partial correlation between dipole strength and number of palaeointensity determinations.