Element concentrations and different index analysis of the spheroidally-weathered diabase and the un-weathered rocks sampled near Yeragumballi, southern India

Major, trace element concentrations and Nd, Sr isotope ratios were measured in micro-drilled samples of a 2.37 Ga-old, hand-specimen sized spheroidally weathered diabase from southern India. A sample of the un-weathered diabase dike was also analyzed. X-ray micro-CT imaging of the weathered sample shows three dominant mineral phases which are plagioclase, pyroxene, and a Fe-bearing phase (possibly hematite and/or ilmenite). This imaging documents the pervasive nature of two generations of ribbon-like, cross-cutting fractures. The older fracture is sealed while the more recent fracture is open without any in-filling. The values of the Chemical Index of Alteration (CIA) of the samples show a wide range but are less than 50. Despite being a relatively less weathered rock, we observe that concentrations of major, minor and trace elements vary significantly with the percentage relative standard deviation (%RSD) for the elements ranging from 10.2?41.8. The CIA of the samples do not show any trend with the position of the sample in the hand-specimen. Barring Ca and Li, whose concentrations decrease from the core to the rim of the sample, there is no significant spatial trend in the concentrations of the elements. Concentrations of Na, Al, and Sr increase with increasing CIA values while concentrations of Mg, Fe, and Sc decrease with increasing CIA. The strong positive correlations of Na and Al, as well as Na and Sr indicates preferential weathering of plagioclase in the diabase. Na/Ca increases while Mg/Al, Mg/Na, Mg/Ca, Fe/Al and Sc/Sr decrease with increasing CIA values and the un-weathered rock plots in the middle of these trends. Such variations are explained in terms of differential weathering of plagioclase (in samples with lower CIA than the un-weathered rock, W1-type) and pyroxene (in samples with higher CIA than the un-weathered rock, W2-type) which have varying resistance to weathering. At the hand-specimen scale, the variability in the weathering indices like CIA are controlled by differential weathering of minerals and might not accurately reflect the intensity of weathering. Chondrite-normalized La/Sm and Gd/Lu co-vary with CIA values indicating mobility of the REEs during spheroidal weathering even at the hand-specimen scale. The Eu anomaly also increases with increasing CIA values which is explained by differential weathering of pyroxene and plagioclase. We observe large percentage deviations of the Nb-normalized concentrations of elements from the un-weathered rock in specific samples but no spatial trend is observed. Overall, the variations in element concentrations can be explained by varying fluid mobility of the elements, selective weathering of the minerals in the diabase, and ambient environmental conditions. Considerable Nd and Sr isotopic variability is observed at the hand-specimen scale and is explained in terms of weathering-related fractionation of parent/daughter ratios. This elemental fractionation must have happened long time ago to allow for radiogenic decay of the long-lived isotopes of 87Rb and 147Sm. The spread (%RSD) in the initial Sr and Nd isotope compositions of the weathered samples reach a minimum value around 1.2?1.3 Ga which we interpret as the timing of the peak weathering event which led to fractionation of the parent/daughter ratios. For Nd isotopes, the average epsilon-Nd (1.2 Ga) of the weathered samples coincides with the epsilon-Nd (1.2 Ga) of the un-weathered rock. The timing of the weathering event coincides with the timing of the breakup of the Columbia supercontinent and follows wide-spread alkaline volcanism in the Indian subcontinent. This is the first such attempt to determine the timing of a weathering event in rocks using long-lived radioactive isotopes本次研究对采自印度南部、形成于23.7亿年（2.37 Ga）的手标本级球状风化辉绿岩的微钻取样品，开展了主量、次要及痕量元素浓度与钕（Nd）、锶（Sr）同位素比值测定，同时还分析了一块未风化辉绿岩脉样品。对风化样品的X射线显微CT（X-ray micro-CT）成像结果显示，其存在三种主要矿物相：斜长石（plagioclase）、辉石（pyroxene）以及含铁相（可能为赤铁矿（hematite）和/或钛铁矿（ilmenite））。该成像揭示了两期带状穿插裂隙的普遍发育特征：早期裂隙已被充填封闭，而晚期裂隙呈开放状态且无任何充填物。样品的化学风化指数（Chemical Index of Alteration，CIA）取值跨度较大，但均低于50。尽管该岩石整体风化程度相对较低，但主量、次要及痕量元素的浓度存在显著差异，各元素的相对标准偏差（%RSD）介于10.2至41.8之间。样品的CIA值与手标本内的采样位置无明显相关趋势。除钙（Ca）与锂（Li）的浓度从样品核部向边缘逐渐降低外，其余元素浓度未表现出显著空间分布趋势。 钠（Na）、铝（Al）与锶（Sr）的浓度随CIA值升高而增加，而镁（Mg）、铁（Fe）与钪（Sc）的浓度则随CIA值升高而降低。钠与铝、钠与锶之间的强正相关关系，表明辉绿岩中的斜长石发生了选择性风化。钠钙比（Na/Ca）随CIA值升高而升高，而镁铝比（Mg/Al）、镁钠比（Mg/Na）、镁钙比（Mg/Ca）、铁铝比（Fe/Al）以及钪锶比（Sc/Sr）则随CIA值升高而降低；未风化岩石的各项比值均位于这些趋势的中间位置。上述变化可通过斜长石与辉石的差异性风化解释：斜长石抗风化能力较弱（对应CIA值低于未风化岩石的W1型样品），辉石抗风化能力较强（对应CIA值高于未风化岩石的W2型样品）。在手标本尺度下，诸如CIA这类风化指数的变化受矿物差异性风化控制，可能无法准确反映岩石的实际风化强度。 球粒陨石标准化的镧钐比（La/Sm）与钆镥比（Gd/Lu）随CIA值呈协同变化，表明即使在手标本尺度下，球状风化过程中稀土元素（Rare Earth Elements，REEs）也发生了迁移。铕异常（Eu anomaly）也随CIA值升高而增强，这一现象同样可通过斜长石与辉石的差异性风化解释。我们观察到，部分样品中以铌（Nb）标准化的元素浓度与未风化岩石相比存在较大偏差，但未发现显著空间分布趋势。总体而言，元素浓度的变化可通过元素流体迁移能力的差异、辉绿岩中矿物的选择性风化以及周围环境条件予以解释。 在手标本尺度下，Nd与Sr同位素也存在显著分异，这可归因于与风化作用相关的母/子体同位素比值分馏。这种元素分馏事件必然发生在足够久远的时期，以使长寿命放射性同位素87铷（87Rb）与147钐（147Sm）发生放射衰变。风化样品的初始Sr与Nd同位素组成的离散程度（%RSD）在约12亿至13亿年（1.2–1.3 Ga）时达到最小值，我们将该时期解释为导致母/子体同位素比值分馏的风化作用峰值事件发生时间。对于Nd同位素而言，风化样品在12亿年时的平均ε钕（epsilon-Nd，εNd）值与未风化岩石在同时期的εNd值一致。该风化事件的发生时间与哥伦比亚超大陆（Columbia supercontinent）裂解时间相吻合，且紧随印度次大陆广泛的碱性火山活动之后。本研究首次尝试利用长寿命放射性同位素，确定岩石中风化事件的发生时间。