中文摘要

歐淑芳

海底洋殼磁性異常圖譜記錄地球磁場由過去到現在所發生的反轉,同時也記錄海洋地殼的形成與演化,成為板塊地體構造學說最重要的證據之一。但是對於洋底蓆狀岩牆中主要攜磁礦物的特性與成因一直是被爭論著。本論文針對深海鑽探計畫(DSDP)/海洋鑽探計畫(ODP)504B鑽井,第83、111、137、140和148等五個航次所鑽探之蓆狀岩牆岩心樣品,首度結合岩石磁學方法與高解析岩象學(穿透式電子顯微分析,TEM)與礦物學分析技術來研究磁性礦物。研究結果顯示這些蓆狀岩牆玄武岩均受到不等程度之熱水蝕變作用,次生礦物群反映出綠色片岩相/角閃岩相變質度。整個蓆狀岩牆玄武岩中之原生鈦磁鐵礦,已經受到高溫氧化、偏析作用,以及蝕變作用後轉變成磁鐵礦,成為蓆狀岩牆中主要攜磁礦物。電子顯微鏡觀察之次生磁鐵礦條紋寬度與岩石磁學性質推測之結果吻合,表示蓆狀岩牆磁鐵礦顆粒度是落在於假單磁域的範圍,並隨深度的增加而有增大的趨勢。本研究顯示 TEM觀察結果可以代表蓆狀岩牆玄武岩中磁性礦物之特徵。根據此磁鐵礦的成因模式,可以推論蓆狀岩牆玄武岩約在500℃即可獲得熱化學殘磁(高溫氧化或偏析作用),而在350℃左右再獲得一部份化學殘磁(熱水蝕變),其磁化年代和洋脊玄武岩形成的年代稍微有一個落後的差距。蓆狀岩牆中自然殘磁是噴出岩的一半,乃是因為不具磁性之鈦鐵礦與具磁性之磁鐵礦在鈦磁鐵礦假晶中各佔有約1/2之體積。由於蓆狀岩牆的厚度大約是噴出岩的三倍,其整體的磁化量對海底磁性異常應該有一定的貢獻。

Abstract

The pattern of seafloor magnetic anomalies is a record for the self-reversals of the Earth magnetic field from the long past to the present. It has preserved crucial data for the formation and evolution of oceanic crusts and is one of the most important evidences for the theory of plate tectonics. However, the features and origins of magnetic carriers in the sheeted dikes of oceanic crusts have not been completely understood and are still in debate. In the present study, magnetic minerals in the core samples, which were drilled from the sheeted dikes at the DSDP/ODP 504B drill hole during Legs 83, 111, 137, 140, and 148, have been studied by using methods of rock magnetism and mineralogy with high-resolution petrographic tools (transmission electron microscopy, TEM). Our results indicate that the sheeted dike basalts have been subjected to different degrees of hydrothermal alterations, which are equivalent to greenschist facies to amphibolite facies metamorphism on the basis of the secondary mineral assemblages. The primary titanomagnetite in all the sheeted dike basalts has suffered high-temperature oxidation, exsolution, and hydrothermal alteration, and transformed into magnetite, which becomes the main magnetic mineral in the sheeted dikes. The lamellar widths of the secondary magnetite, as observed with electron microscopy, are consistent with the grain sizes inferred form the rock magnetic properties. The grain sizes of the magnetite are within the pseudo-single-domain field and increase with depths of the sheeted dikes. The consistent results of the whole-rock magnetic properties and the TEM observations have proved that the secondary magnetite and its textural features are representative of the features of magnetic mineral in the sheeted dikes. Therefore, on the basis of the formation model of the magnetite, it is inferred that the sheeted dike basalts obtained thermal chemical remanent magnetization (TCRM) at ~500aC (high-temperature oxidation, or exsolution), and then obtained chemical remanent magnetization (CRM) at ~350aC (hydrothermal alteration). The timing for the magnetization of the sheeted dike basalts thus lags slightly behind their formation. The primary titanomagnetite in the sheeted dikes has been completely transformed into pseudomorphs that consist of approximately half magnetite and half ilmenite or other phases. Thus, the natural remanent magnetization (NRM) of the sheeted dikes is only about half of that for the extrusive pillow basalts. However, the total thickness of the sheeted dikes is about three times of that for the pillow basalts. The sheeted dikes should have contributed to the seafloor magnetic anomalies to some extents.