Magnetic Mineral Populations in Lower Oceanic Crustal Gabbros (Atlantis Bank, SW Indian Ridge): Implications for Marine Magnetic Anomalies
Magnetic Mineral Populations in Lower Oceanic Crustal Gabbros (Atlantis Bank, SW Indian Ridge): Implications for Marine Magnetic Anomalies
复制标题
DOI：
10.1029/2019gc008847
复制
发表时间：
2020-03
期刊：
影响因子：
3.7
通讯作者：
J. Bowles;A. Morris;M. Tivey;I. Lascu
中科院分区：
文献类型：
--
作者：
J. Bowles;A. Morris;M. Tivey;I. Lascu
为了进一步了解下大洋地壳的磁性及其对海洋磁异常的贡献，从西南印度洋脊亚特兰蒂斯银行的国际海洋发现计划孔U1473 A收集了辉长岩样品。详细的磁性工作将某些磁性行为和磁畴状态与特定的磁性矿物种群联系起来。对整个岩石和矿物分离物的测量包括磁滞、一阶反转曲线、低温剩磁测量、热磁分析和磁力显微镜。热磁数据特征表明，孔上部~500 m经历了热液蚀变。热磁和自然剩磁磁化数据与735 B孔的早期观测结果一致，表明剩磁来自低钛磁铁矿，在演化的铁钛氧化物辉长岩中，自然剩磁磁化高达25 A m−1，但大多数小于1 A m−1。磁铁矿至少以三种形式存在。原生磁铁矿与粗粒氧化物有关，粗粒氧化物在孔的上部更常见。这种磁性群体与主要的“伪单畴”行为有关，这种行为是由大氧化物内的细尺度层状共生体引起的。在孔的更深处，磁信号更常见地由相互作用的单畴组合主导，最有可能在橄榄石和/或辉石中沿沿着晶体不连续性发现。第三个贡献来自斜长石中的非相互作用单畴包裹体。由于富含氧化物的高磁性辉长岩的浓度在表面最大，因此来自粗氧化物的信号可能会主导亚特兰蒂斯银行近底部的磁异常信号。
To learn more about magnetic properties of the lower ocean crust and its contributions to marine magnetic anomalies, gabbro samples were collected from International Ocean Discovery Program Hole U1473A at Atlantis Bank on the Southwest Indian Ridge. Detailed magnetic property work links certain magnetic behaviors and domain states to specific magnetic mineral populations. Measurements on whole rocks and mineral separates included magnetic hysteresis, first‐order reversal curves, low‐temperature remanence measurements, thermomagnetic analysis, and magnetic force microscopy. Characteristics of the thermomagnetic data indicate that the upper ~500 m of the hole has undergone hydrothermal alteration. The thermomagnetic and natural remanent magnetization data are consistent with earlier observations from Hole 735B that show remanence arises from low‐Ti magnetite and that natural remanent magnetizations are up to 25 A m−1 in evolved Fe‐Ti oxide gabbros, but are mostly <1 A m−1. Magnetite is present in at least three forms. Primary magnetite is associated with coarse‐grained oxides that are more frequent in the upper part of the hole. This magnetic population is linked to dominantly “pseudo‐single‐domain” behavior that arises from fine‐scale lamellar intergrowths within the large oxides. Deeper in the hole the magnetic signal is more commonly dominated by an interacting single‐domain assemblage most likely found along crystal discontinuities in olivine and/or pyroxene. A third contribution is from noninteracting single‐domain inclusions within plagioclase. Because the concentration of the highly magnetic, oxide‐rich gabbros is greatest toward the surface, the signal from coarse oxides will likely dominate the near‐bottom magnetic anomaly signal at Atlantis Bank.