Organic / inorganic carbon content and isotope analysis of 3.1Ga Cleaverville Formation in Pilbara, Australia: Result of DXCL project

Abstract:

DXCL project was targeted for 3.2-3.1 Ga hydrothermal chert-black shale (Dixon Island Formation) and black shale-banded iron formation (Cleaverville Formation). CL3 core (200m long) was drilled from 1) upper part of Black Shale Member (35m thick) to 2) lower part of BIF Member (165m thick) of the Cleaverville Formation. Here, the BIF Member can be divided into three submembers; Greenish shale-siderite (50m thick), Magnetite-siderite (55m thick) and Black shale-siderite (60m) submembers. In this study, we used bulk samples and samples treated by hot hydrochloric acid in order to extract organic carbon.

　The Black shale Member consists of black carbonaceous　matter and fine grain quartz (< 100μm). Organic carbon content (C_org) of black shale is 1.2% in average and organic carbon isotope ratio (δ¹³C_org) is -31.4 to -28.7‰. On the other hand, inorganic carbon isotope ratio of siderite (δ¹³C_carb) was -5.2 to +12.6‰.

　In the BIF Member, the Greenish shale-siderite submember is composed of well laminated greenish sideritic shale and white chert (<7mm thick), which is gradually increase from black shale of the Black shale Member through about 10m. Magnetite-siderite submember contains very fine magnetite lamination with inter-bedded greenish sideritic shale and siderite lamination. Hematite is identified near fractured part. The Black shale-siderite submember is composed of black shale, siderite and chert bands.

　1) Siderite layers of these three submembers showedδ¹³C_carb value of -14.6 to -3.8‰. C_org and δ¹³C_org content are 0.2% and -18.3 to -0.3‰. 2) Siderite grains within greenish sideritic shales showedδ¹³C_carb value of -12.9 to +15.0‰. 3) Black shale of C_org and δ¹³C_org content in the BIF Member are 0.1% and -36.3 to -17.1‰ respectively.

　We found great difference in values of δ¹³C_carb of siderite. One is C_org-rich shale (up to +15.0‰) and the other is C_org-poor siderite layers (up to -3.8‰). The lighter value of siderite layers may be originated from precursor organic carbon which is strongly affected by biological activity.