Abstract The Baja California Peninsula is located within a complex geodynamic setting, marked by continental rifting immediately to the east in the Gulf of California, and abundant geothermal manifestations. We report helium and carbon isotopic and relative abundance data sampled from gas and fluid localities (n = 12) across northern Baja California, Mexico. Samples were collected in three distinct regions of Baja: Pacific Borderlands (PB) in northwestern Baja, Peninsula Ranges and Inland Valleys (PRIV) in central Baja, and the Gulf Extensional Province (GEP) in northeastern Baja. Helium isotopes (3He/4He), carbon isotopes (δ13CO2), and CO2/3He values all are highest in GEP samples, and lower in samples from the PB and PRIV. In the GEP, helium isotopes reach moderately mantle-like values (1.74 RA) (where RA = air 3He/4He), while in the PRIV and PB provinces values are more radiogenic (~0.11 RA). 3He/4He values suggest mixing between mantle-derived and crustal-derived radiogenic components, with a maximum mantle contribution of ~21% in the GEP samples. High He isotopes correlate with low mantle velocity zones at a depth of 25–40 km, suggesting a mantle source of He in the GEP, due to extensional crustal thinning. Carbon isotopes (δ13CO2) vary widely between −17.6 and 4.5‰ (vs. PDVB) and CO2/3He values vary over several orders of magnitude (2.0 × 105 to 1.1 × 1013). Variable δ13CO2 and CO2/3He values in the PRIV and PB localities are consistent with calcite precipitation in shallow-level (upper 5 km) hydrothermal systems. The bimodal nature of the He-CO2 data suggests fundamentally different transfer mechanisms in each of the three distinct regions, indicating complex interplay between regional mantle degassing, faulting-assisted migration of crustal fluids, and volatile sequestration via secondary processes (e.g., calcite precipitation). These results, along with literature data, allow for systematic assessment of both mantle sources and secondary processes, providing new insights into how volatiles are transported through the Baja California peninsula.

中文翻译：

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挥发性源、汇和途径：对下加利福尼亚州流体和气体的氦-碳同位素研究

摘要 下加利福尼亚半岛位于复杂的地球动力学环境中，以紧邻加利福尼亚湾东部的大陆裂谷和丰富的地热表现为标志。我们报告了从墨西哥下加利福尼亚州北部的气体和流体位置 (n = 12) 采样的氦和碳同位素和相对丰度数据。在巴哈的三个不同地区收集了样本：巴哈西北部的太平洋边境地区 (PB)、巴哈中部的半岛山脉和内陆山谷 (PRIV) 以及巴哈东北部的海湾扩展省 (GEP)。氦同位素 (3He/4He)、碳同位素 (δ13CO2) 和 CO2/3He 值在 GEP 样品中均最高，而在 PB 和 PRIV 样品中较低。在 GEP 中，氦同位素达到中等地幔值 (1.74 RA)（其中 RA = 空气 3He/4He），而在 PRIV 和 PB 省，值更具辐射性（~0.11 RA）。3He/4He 值表明地幔来源和地壳来源的放射性成分之间存在混合，在 GEP 样品中，最大地幔贡献约为 21%。高 He 同位素与 25-40 公里深处的低地幔速度带相关，表明由于地壳伸展变薄，GEP 中存在 He 的地幔源。碳同位素 (δ13CO2) 在 -17.6 到 4.5‰（相对于 PDVB）之间变化很大，并且 CO2/3He 值在几个数量级（2.0 × 105 到 1.1 × 1013）之间变化。PRIV 和 PB 地区的可变 δ13CO2 和 CO2/3He 值与浅层（上部 5 公里）热液系统中的方解石沉淀一致。He-CO2 数据的双峰性质表明三个不同区域中的每一个都有根本不同的转移机制，表明区域地幔脱气、断层辅助地壳流体迁移和通过二次过程（例如方解石沉淀）的挥发性封存之间复杂的相互作用。这些结果与文献数据一起，可以对地幔源和次生过程进行系统评估，为挥发物如何通过下加利福尼亚半岛运输提供新的见解。