Paleomagnetic of basalts from ODP Site 145-883 on Detroit Seamount

Paleomagnetic data were measured from basaltic flows cored by the Ocean Drilling Program (ODP) at Site 883 on the summit of Detroit Seamount, located in the northernmost Emperor seamounts. These data are important because they reflect the paleolatitude of Hawaiian volcanism for the Late Cretaceous and bear upon geodynamic models of hotspot drift. A total of 143 samples were measured, from cores acquired at two ~20-30 m apart. Most samples gave apparently reliable magnetic directions that were analyzed in a tiered fashion to compute a composite inclination vs. depth curve. One hole gave 13 distinct inclination groups, the other 10, and the two were combined into nine groups thought to represent independent measurements of paleofield direction. These data indicate normal magnetic polarity and give a mean inclination of 61.5+10.6°/-6.4° and paleolatitude of 42.8+13.2°/-7.6° (95% confidence limits). This paleolatitude is 6.2° higher than results from another ODP site (884) drilled on the lower flank of the same seamount. The difference is thought to result partly from an age difference (1-3 Myr) and partly from incomplete averaging of paleosecular variation at both drill sites. Together, the data from the two sites reinforce the conclusion that the northern Emperor seamounts were formed far north of the present-day hotspot latitude (~19.5°N) and suggest prior estimates of the amount and rate of southward drift may have been low. This analysis also illustrates uncertainties in determining paleolatitude from a small number of lava flow units from a single drill site.研究针对位于最北天皇海山（Emperor seamounts）链的底特律海山（Detroit Seamount）山顶883站位，对大洋钻探计划（Ocean Drilling Program, ODP）采集的玄武岩岩芯开展了古地磁数据（paleomagnetic data）测量。该数据集具备重要科学价值：其既反映了晚白垩世夏威夷火山活动的古纬度，也为热点漂移的地球动力学模型提供了关键约束。本次研究共获取并测量143个样品，样品取自两处间距约20~30米的岩芯。多数样品获得了可靠的磁方向数据，研究人员采用分层分析方法计算得到了综合倾角-深度曲线。其中一个钻孔得到13个独立倾角组，另一钻孔得到10个，最终将两组数据合并为9个代表古地磁场方向独立测量结果的分组。数据显示为正常磁极性，得到的平均倾角为61.5°（+10.6°/-6.4°），古纬度为42.8°（+13.2°/-7.6°，95%置信限）。该古纬度较同一海山下部侧翼钻取的884站位的测量结果偏高6.2°。这种差异被认为部分源于年代差（1~3百万年），部分源于两个钻孔站位均未完全平均古地磁场长期变化。两处站位的数据共同佐证了一项结论：北部天皇海山形成于远高于现今热点纬度（约19.5°N）的区域，且此前对热点南向漂移的幅度与速率的估算可能偏低。本次分析同时也揭示了利用单个钻孔站位的少量熔岩流单元估算古纬度时存在的不确定性。