10:00 〜 10:15
[SGC37-05] Boron, lithium and chlorine systematics of geothermal fluids of Mexico: mantle and crustal sources
キーワード：Boron, Strontium, Geothermal fluid, Mexico
Boron (B), Lithium (Li), and Chlorine (Cl) are fluid-mobile elements (FMEs) that serve as tracers for volatile cycling in subduction zones. This study presents data on B, Li, and Cl, along with Strontium (Sr), Hydrogen (H), Oxygen (O), and noble gases, collected from geothermal fluids across four major geothermal fields in Mexico. The isotopic data for boron in Mexican geothermal fluids is notably scarce, and there is a lack of corresponding data for sedimentary and magmatic rocks. Thus, the behaviour of boron in central Mexico, specifically in the subduction zone of the Cocos Plate, remains largely unknown.
In this research work, we measured 96 new boron isotopic values. We integrated them with existing isotopic data on H, O, Br, Cl, Sr, and He and elemental data on Cl and Li to identify potential sources of boron in deep fluids. The measured 11B/10B ratios in geothermal fluids range from -6.2 to +14.8‰ relative to NIST SRM951a and exhibit correlations with 87Sr/86Sr, indicating three primary sources: seawater and crustal materials for the Cerro Prieto field fluids, circulating within an ancient paleodeltaic reservoir; and mantle-derived sources (leaching of local basalts-andesites) potentially combined with seawater in the Los Azufres field within the Trans-Mexican Volcanic Belt (TMVB). However, the 18O/16O values do not exhibit a seawater signature, suggesting that boron may originate from evaporites (borates) rather than seawater. The relationship between lithium and strontium sources mirrors that of boron, although crustal and mantle endmembers display different Li/Sr ratios than anticipated, likely due to strontium precipitation in hydrothermal carbonates and subsequent depletion in geothermal fluids.
In this research work, we measured 96 new boron isotopic values. We integrated them with existing isotopic data on H, O, Br, Cl, Sr, and He and elemental data on Cl and Li to identify potential sources of boron in deep fluids. The measured 11B/10B ratios in geothermal fluids range from -6.2 to +14.8‰ relative to NIST SRM951a and exhibit correlations with 87Sr/86Sr, indicating three primary sources: seawater and crustal materials for the Cerro Prieto field fluids, circulating within an ancient paleodeltaic reservoir; and mantle-derived sources (leaching of local basalts-andesites) potentially combined with seawater in the Los Azufres field within the Trans-Mexican Volcanic Belt (TMVB). However, the 18O/16O values do not exhibit a seawater signature, suggesting that boron may originate from evaporites (borates) rather than seawater. The relationship between lithium and strontium sources mirrors that of boron, although crustal and mantle endmembers display different Li/Sr ratios than anticipated, likely due to strontium precipitation in hydrothermal carbonates and subsequent depletion in geothermal fluids.