The Hekpoort paleosols comprise a regional paleoweathering horizon developed on 2.224 +/- 0.021 Ga basaltic andesite lavas at the top of the Hekpoort Formation of the Pretoria Group, Transvaal Supergroup, South Africa. In five separate profiles, from outcrops along road cuts near Waterval Onder and the Daspoort Tunnel and in three drill cores from the Bank Break Area (BB3, BB8, and BB14), the top of the paleosol is a sericite-rich zone. The sericite zone grades downward into a chlorite-rich zone. In core BB8 and in the road cut at the Daspoort Tunnel, we sampled the underlying or parent basaltic andesite into which the chlorite zone grades. We did not obtain samples of the parent material at Waterval Onder and in cores BB3 and BB14, but chemical analyses indicate that the chlorite and sericite zones in these profiles derive from underlying lavas similar to the ones we sampled in core BB8 and at the Daspoort Tunnel. The presence of apparent rip-up clasts of the paleosol in the overlying ironstones of the Strubenkop Formation in the cores from Bank Break makes it very unlikely that most of the alteration was a result of interactions with hydrothermal fluids. Desiccation cracks at the top of the paleosol that were filled with sand during the deposition of the overlying sediments at Waterval Onder point to a subaerial weathering origin. Very little, if any, Al, Ti, Zr, V, or Cr moved a discernible distance during weathering of any of the five profiles. The vertical distribution of Fe, Mg, Mn, Ni, and Co indicates that these elements were largely removed from the top of the soil during weathering. The overall abundance of these elements in each of the profiles indicates that a significant fraction of the complement lost from the top subsequently reprecipitated in the lower portion of the soil as constituents of an Fe2(+) -rich smectite. The loss of Fe from the top of the soil during weathering of the Hekpoort paleosols indicates that atmospheric PO2 was less than 8 x 10(-4) atm about 2.22 Ga. Fe2(+) -rich smectite should only precipitate during soil formation if atmospheric PCO2 is less than or equal to 2 x 10(-2) atm (Rye, Kuo, and Holland, 1995). Ca and Na were largely lost during weathering. Some Na was apparently added to the sericite zone in cores BB3, BB8, and BB14 after weathering. All five profiles are enriched in K and Rb, and most are enriched in Ba. The distribution of these elements indicates that they all were added during post-weathering hydrothermal metasomatism. Rb-Sr analysis of the paleosol at the Daspoort Tunnel indicates that metasomatism last affected that profile 1.925 +/- 0.032 Ga (Macfarlane and Holland, 1991).

中文翻译：

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南非比勒陀利亚群 Hekpoort 玄武岩上发育的古土壤的地质和地球化学

Hekpoort 古土壤包括在南非德兰士瓦超群比勒陀利亚群 Hekpoort 组顶部的 2.224 +/- 0.021 Ga 玄武质安山岩熔岩上形成的区域性古风化层。在五个单独的剖面中，从 Waterval Onder 和 Daspoort 隧道附近道路切割的露头以及 Bank Break 区域（BB3、BB8 和 BB14）的三个钻芯中，古土壤的顶部是富含绢云母的区域。绢云母带向下分级为富含绿泥石的带。在 BB8 核心和 Daspoort 隧道的道路切割中，我们对绿泥石带分级的下伏或母玄武安山岩进行了采样。我们没有在 Waterval Onder 以及 BB3 和 BB14 岩心中获得母材样品，但化学分析表明，这些剖面中的绿泥石和绢云母带源自与我们在 BB8 核心和 Daspoort 隧道采样的熔岩相似的下伏熔岩。在 Bank Break 的岩心中，Strubenkop 组上覆的铁岩中存在明显的古土壤破裂碎屑，这使得大部分蚀变不太可能是与热液流体相互作用的结果。在 Waterval Onder 上覆沉积物沉积期间，充满沙子的古土壤顶部的干燥裂缝表明地下风化起源。在五个轮廓中的任何一个的风化过程中，Al、Ti、Zr、V 或 Cr 很少（如果有的话）移动了可辨别的距离。Fe、Mg、Mn、Ni 的垂直分布，和 Co 表明这些元素在风化过程中大部分从土壤顶部去除。每个剖面中这些元素的总体丰度表明，从顶部损失的大量补充随后在土壤下部重新沉淀，作为富含 Fe2(+) 的蒙脱石的成分。Hekpoort 古土壤风化过程中土壤顶部的 Fe 损失表明大气中的 PO2 小于 8 x 10(-4) atm 约 2.22 Ga。如果大气，富含 Fe2(+) 的蒙脱石应该只在土壤形成过程中沉淀PCO2 小于或等于 2 x 10(-2) atm（Rye、Kuo 和 Holland，1995）。Ca和Na在风化过程中大量流失。风化后，一些 Na 明显添加到岩心 BB3、BB8 和 BB14 的绢云母带中。所有五个配置文件都富含 K 和 Rb，大多数都富含巴。这些元素的分布表明它们都是在风化后热液交代过程中添加的。对 Daspoort 隧道古土壤的 Rb-Sr 分析表明交代作用最后影响了该剖面 1.925 +/- 0.032 Ga（Macfarlane 和 Holland，1991）。