云南腾冲地块北部空树河组一个菱铁矿结核中的C、O同位素变化特征

Carbon and Oxygen Isotope Variation in a Siderite Concretion from Kongshuhe Formation, Western Yunnan

空树河组是滇西腾冲地块北部的石炭 二叠纪冈瓦纳相沉积。菱铁矿结核见于该组顶部的黑色泥岩中。将一个结核通过中心切开 ,在切面上由中心向边缘用牙医钻等间隔提取 10个样品。同位素测试结果显示 ,δ13C自中心向边缘逐渐由 - 8.0 9‰减为 - 16.12‰ ,最外一个样例外 ( - 15 .3 9‰ ) ,而δ18O自中心向边缘逐渐由- 10 .64‰增加为 0‰左右。δ13C的变化情况表明 ,菱铁矿结核可能形成于硫酸盐还原带。在解释δ18O的变化情况时 ,推测在结核开始生长时可能有δ18O亏损的大气降水掺入 ,随着埋藏深度的逐渐加大 ,孔隙水的O同位素逐渐趋近正常海水的值( 0‰ )。

Siderite concretions were found in black lutites near the top of the Kongshuhe Formation, which represents a Permo Carboniferous Gondwana glacial to post glacial sequence in the northern part of the Tengchong block, western Yunnan. A concretion was cut through the center, and nine samples were extracted from the cut surface with the help of a dentist's drill. Isotopic investigation shows that δ 13 C decreases outwards steadily from -8.09‰ to -16.12‰ except for the outermost sample (-15.39 ‰), and that δ 18 O increases from -10.64‰ in the center to about 0‰ on the margin. Such a δ 13 C variation implies that this siderite concretion might have been formed in the sulphate reduction zone. For the interpretation of δ 18 O variation, it is assumed that 18 O depleted meteoric water was probably involved at the time when the concretion started to precipitate, and that the oxygen isotope composition of the pore water approached gradually the value of normal ocean water (0‰) with the increase of the burial depth.