Abstract Published (U-Th)/He ages indicate that an oolitic Miocene channel iron deposit (CID) on Mesa J in the Hamersley province of Western Australia (∼22 °S latitude) contains three predominant generations of Fe (III) oxides – two generations of older hematite and one of younger goethite. Selective dissolution and sequential atom balance calculations were combined with chemical and oxygen isotope analyses to determine the δ18O values of hematite, goethite, and quartz in eight samples from a core drilled in that CID. For the quartz (Q), δ18OQ = +21.7(±0.9)‰. For the younger hematite (Hm1), δ18OHm1 ≥ +3.6(±0.6)‰, whereas for the older hematite (Hm2), δ18OHm2 ≤ +2.1(±0.8)‰. The oolitic CID goethite (G) crystallized in the late Miocene [7(±1) Ma] and is a solid solution represented as Fe(1−Y)AlY(CO3)XO(1−X)OH with values of δ18OG that range from −0.4‰ to +0.7‰. A mixing model for the solid solution expresses the value of δ18OG in terms of Y, X, relevant fractionation factors, and the δ18O of the endmember FeOOH (δ18Ogt). Mixing model-derived values of δ18Ogt range from −2.4‰ to −0.7‰. Paired values of δDgt and δ18Ogt in seven of eight late Miocene oolitic goethites from Mesa J indicate crystallization from meteoric waters at an average temperature of 20(±5) °C. In the late Miocene, Mesa J was in the subtropics at a paleolatitude of ∼29 °S, which invites comparison with the climate of the near-coastal community of Mingenew in Western Australia (presently at 29 °S latitude). The modern MAT at Mingenew is 20 °C – i.e., indistinguishable from the late Miocene Mesa J average of 20 °C. However, this similarity is not matched by the mean annual precipitation (MAP). The modern MAP at Mingenew is about 400 mm, but the lateritic weathering that produces abundant pedogenic Fe (III) oxides appears to require a seasonally contrasting climate with rainfall totals of at least 1300 mm/yr. In the late Miocene, southward-shifted summer season tropical cyclones might have delivered such large amounts of rain (with relatively low δD and δ18O values). If so, the δDgt and δ18Ogt of the oolitic goethites of Mesa J support the idea of a rainfall-driven, possibly microbially mediated, summer season bias in the timing of crystallization. Thermal buffering in the regolith can reconcile summer-dominated goethite crystallization with a temperature that would be analytically indistinguishable from the MAT. Comparison of the δ18Ogt values of the Mesa J goethites with published δ18O values of goethites of similar age from two other sites in the Hamersley province suggest a possible increase in δ18O of late Miocene rainfall with increasing distance from the coast.

中文翻译：

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来自西澳大利亚河道铁矿床的多代 Fe (III) 氧化物混合物中中新世鲕粒铁矿的 18O/16O 的回收和解释

摘要 已发表的 (U-Th)/He 年龄表明西澳大利亚哈默斯利省（纬度~22°S）的 Mesa J 上的鲕粒中新世通道铁矿床 (CID) 包含三个主要世代的 Fe (III) 氧化物——两个几代较老的赤铁矿和一种较年轻的针铁矿。选择性溶解和连续原子平衡计算与化学和氧同位素分析相结合，以确定来自在该 CID 中钻孔的岩心的八个样品中赤铁矿、针铁矿和石英的 δ18O 值。对于石英 (Q)，δ18OQ = +21.7(±0.9)‰。对于年轻的赤铁矿（Hm1），δ18OHm1 ≥ +3.6（±0.6）‰，而对于较老的赤铁矿（Hm2），δ18OHm2 ≤ +2.1（±0.8）‰。鲕粒 CID 针铁矿 (G) 在晚中新世 [7(±1) Ma] 结晶，是一种固溶体，表示为 Fe(1-Y)AlY(CO3)XO(1-X)OH，δ18OG 值在该范围内从-0。4‰至+0.7‰。固溶体的混合模型用 Y、X、相关分馏因子和端元 FeOOH (δ18Ogt) 的 δ18O 表示 δ18OG 的值。δ18Ogt 的混合模型衍生值范围从 -2.4‰ 到 -0.7‰。来自 Mesa J 的 8 个晚中新世鲕粒铁矿中的 7 个的 δDgt 和 δ18Ogt 成对值表明在平均温度为 20(±5)°C 时从大气水结晶。在晚中新世，Mesa J 位于~29°S 的古纬度，这可以与西澳大利亚明格纽近海岸群落的气候（目前在 29°S 纬度）进行比较。Mingenew 的现代 MAT 为 20 °C - 即与晚中新世 Mesa J 平均 20 °C 无法区分。然而，这种相似性与年平均降水量 (MAP) 不匹配。Mingenew 的现代 MAP 约为 400 毫米，但产生丰富的成土 Fe (III) 氧化物的红土风化似乎需要季节性对比的气候，总降雨量至少为 1300 毫米/年。在中新世晚期，南移的夏季热带气旋可能带来了如此大量的降雨（δD 和 δ18O 值相对较低）。如果是这样，Mesa J 鲕粒铁矿的 δDgt 和 δ18Ogt 支持降雨驱动的想法，可能是微生物介导的夏季结晶时间偏差。风化层中的热缓冲可以使夏季主导的针铁矿结晶与在分析上与 MAT 无法区分的温度相协调。