Stable strontium and barium isotopes are potential tracers for understanding planetary differentiation and the nature of the building blocks of terrestrial planets. Strontium and barium are fluid-mobile elements, but it remains unclear how terrestrial weathering affects the Sr-Ba isotopes compositions in achondrites, thus hampering the utility of Sr-Ba isotopes in cosmochemistry. In this study, we conducted acetic acid leaching on three eucrites with varying weathering degrees (fall: Qiquanhu, hot desert find: Northwest Africa (NWA) 13583, and Antarctic find: Grove Mountains (GRV) 13001). Combined with detailed petrography observations and major and trace element analyses, we investigated the variations in Sr-Ba isotopes during terrestrial weathering. The degree of weathering follows an order of: NWA 13583 > GRV 13001 > Qiquanhu, evaluated based on several alteration signs, including: the presence of secondary carbonate, the enrichment of large ion lithophile elements (e.g., Sr, Ba, and U), and the Ce and Eu anomalies. The concentrations of Sr and Ba in the leachates of NWA 13583 show a good correlation with Ca, suggesting that the soluble Sr and Ba are derived from secondary carbonate. Differently, the concentrations of Sr and Ba in the leachates of Qiquanhu correlate with Al and Na, suggesting that the soluble Sr and Ba in Qiquanhu are derived from primary plagioclases. This also indicates that silicates dissolution may be inevitable in an acid leaching experiment for achondrites, even when using weak acetic acid. GRV 13001 shows no variation in Sr and Ba isotopes during leaching experiments. The Ba in the leachate (0.26 ± 0.02 ‰) of Qiquanhu is higher than that of the residue (0.04 ± 0.03 ‰), reflecting that aqueous fluids preferentially uptake heavy Ba isotopes during plagioclase dissolution. Conversely, the leachate of NWA 13583 shows lower Ba (-0.19 ± 0.05 ‰) than that of residue (-0.10 ± 0.03 ‰), reflecting the lighter Ba isotope composition in carbonate. Notably, the residue of NWA 13583 has Ba ∼ 0.1 ‰ lower than those of Qiquanhu and GRV 13001. This discrepancy may reflect plagioclase dissolution during hot-desert weathering rather than magmatism on the parent body. Different from Ba isotopes, the Sr of Qiquanhu shows tiny variation in the leaching experiment, suggesting that the dissolution of plagioclase may cause negligible Sr isotope fractionation. For NWA 13583, the Sr of leachate is slightly heavier than that of leaching residue and bulk rock, reflecting the high Sr in terrestrial fluids. Our results suggest that Ba and Sr isotopes of eucrites show different behaviors during terrestrial weathering. Sr isotopes show a smaller fractionation scale and may have greater resistance for terrestrial weathering than Ba isotopes.

中文翻译：

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陆地风化对锂辉石稳定 Sr 和 Ba 同位素组成的影响

稳定的锶和钡同位素是了解行星分化和类地行星组成部分性质的潜在示踪剂。锶和钡是可流动的元素，但目前尚不清楚陆地风化如何影响无球粒陨石中的锶钡同位素组成，从而阻碍了锶钡同位素在宇宙化学中的应用。在本研究中，我们对三种不同风化程度的锂辉石（秋季：七泉湖，炎热沙漠发现：西北非洲（NWA）13583，南极发现：格罗夫山脉（GRV）13001）进行了醋酸浸出。结合详细的岩相学观察和主量、微量元素分析，我们研究了陆地风化过程中Sr-Ba同位素的变化。风化程度依次为：NWA 13583 > GRV 13001 >七泉湖，根据多种蚀变迹象进行评估，包括：次生碳酸盐的存在、大离子亲石元素（如Sr、Ba和U）的富集，以及 Ce 和 Eu 异常。 NWA 13583 渗滤液中 Sr 和 Ba 的浓度与 Ca 表现出良好的相关性，表明可溶性 Sr 和 Ba 来源于二次碳酸盐。不同的是，七泉湖渗滤液中 Sr 和 Ba 的浓度与 Al 和 Na 相关，表明七泉湖中可溶性 Sr 和 Ba 来源于原生斜长石。这也表明，在无球粒陨石的酸浸实验中，即使使用弱乙酸，硅酸盐溶解也可能是不可避免的。 GRV 13001 在浸出实验期间显示 Sr 和 Ba 同位素没有变化。七泉湖渗滤液中的Ba（0.26±0.02‰）高于残渣中的Ba（0.04±0.03‰），反映出斜长石溶解过程中水液优先吸收重Ba同位素。相反，NWA 13583 渗滤液的 Ba 含量（-0.19 ± 0.05 ‰）低于残渣（-0.10 ± 0.03 ‰），反映了碳酸盐中较轻的 Ba 同位素组成。值得注意的是，NWA 13583 残渣的 Ba 比七泉湖和 GRV 13001 低 ∼ 0.1 ‰。这种差异可能反映了热沙漠风化过程中斜长石的溶解，而不是母体的岩浆作用。与Ba同位素不同，七泉湖的Sr在浸出实验中表现出微小的变化，这表明斜长石的溶解可能导致可以忽略不计的Sr同位素分馏。对于NWA 13583，渗滤液的Sr略重于浸出残渣和块石，反映了陆地流体中的高Sr。我们的结果表明，锂辉石的 Ba 和 Sr 同位素在陆地风化过程中表现出不同的行为。锶同位素显示出较小的分馏规模，并且可能比钡同位素具有更大的陆地风化抵抗力。