海底热液区的磁性特征

Magnetic signature of submarine hydrothermal areas

现代海底热液系统广泛分布于全球大洋中脊、海山、弧后盆地等地.根据基岩环境的不同,海底热液系统主要分为玄武岩型热液系统和超基性岩型热液系统两种,这两种不同类型的热液系统呈现出不同的磁异常特征.玄武岩型热液区主要呈现低磁异常特征,超基性岩型热液区则呈现高磁异常.这些热液区所呈现的磁场异常主要都来源于热液流体对基底岩石的蚀变作用.热液过程既可以破坏玄武岩中的磁性矿物,也可以与超基性岩作用产生磁性矿物.然而,即使是同一类型的海底热液系统,在不同的地质环境下其磁异常特征也有些许差异.如在中快速扩张脊、慢速和超慢速扩张脊、弧后盆地、火山海山等不同地质环境下的玄武岩型热液系统中,热源活动程度和流体通道形态的不同会造成弱磁区在形状和范围上的差异.通常情况下,海底热液区的磁异常区只有几百米的范围甚至更小,它所产生的短波长磁异常通常难以在海面上分辨出来,近底高分辨率磁力测量则是探测热液场磁性结构、寻找热液矿床的有效方法.

Modern submarine hydrothermal systems are widely distributed in the mid-ocean ridges,seamounts,and back-arc basins.According to the different bedrock environment,the submarine hydrothermal system is mainly divided into basalt-hosted hydrothermal system and ultramafic-hosted hydrothermal system,and their magnetic anomaly characteristics are different.The basalt-hosted hydrothermal activity areas are mainly characterized by low magnetic anomaly,while the ultramafic-hosted hydrothermal areas are opposite.The magnetic anomalies in these hydrothermal areas are mainly caused by the alteration of the rock by hydrothermal fluid.The hydrothermal process can not only destroy the titanomagnetite,which is the main magnetic mineral in basalt,but also produce magnetic minerals through serpentinization with peridotite in the ultramafic rock.However,even for the same type of hydrothermal system,the characteristics of magnetic anomalies vary slightly in different geological environments.In basalt-hosted hydrothermal systems in different geological environments such as medium-fast spreading ridges,slow and ultra-slow spreading ridges,back-arc basins,and volcanic seamounts,etc.,differences in heat source intensity and fluid pipes morphology may result in differences in the shape and extent of weak magnetic regions.Usually,the local magnetic anomalies in hydrothermal areas are only a few hundred meters or even smaller,and its short-wave length anomaly is often difficult to distinguish from the sea surface measurements.Therefore,near-bottom high-resolution magnetic measurement is an effective method to detect the magnetic structure of hydrothermal fields and find hydrothermal deposits.