Thallium concentrations and isotope composition of MORB glasses and altered oceani crust from DSDP/ODP Hole 504B and DSDP Hole 52-417D

Hydrothermal fluids expelled from the seafloor at high and low temperatures play pivotal roles in controlling seawater chemistry. However, the magnitude of the high temperature water flux of mid-ocean ridge axes remains widely disputed and the volume of low temperature vent fluids at ridge flanks is virtually unconstrained. Here, we determine both high and low temperature hydrothermal fluid fluxes using the chemical and isotopic mass balance of the element thallium (Tl) in the ocean crust. Thallium is a unique tracer of ocean floor hydrothermal exchange because of its contrasting behavior during seafloor alteration at low and high temperatures and the distinctive isotopic signatures of fresh and altered MORB and seawater. The calculated high temperature hydrothermal water flux is (0.17-2.93)*10**13 kg/yr with a best estimate of 0.72*10**13 kg/yr. This result suggests that only about 5 to 80% of the heat available at mid-ocean ridge axes from the crystallization and cooling of the freshly formed ocean crust, is released by high temperature black smoker fluids.The residual thermal energy ismost likely lost via conduction and/or through the circulation of intermediate temperature hydrothermal fluids that do not alter the chemical budgets of Tl in the ocean crust. The Tl-based calculations indicate that the low temperature hydrothermal water flux at ridge flanks is (0.2-5.4)*10**17 kg/yr. This implies that the fluids have an average temperature anomaly of only about 0.1 to 3.6 °C relative to ambient seawater. If these low temperatures are correct then both Sr and Mg are expected to be relatively unreactive in ridge-flank hydrothermal systems and this may explain why the extent of basalt alteration that is observed for altered ocean crust appears insufficient to balance the oceanic budgets of 87Sr/86Sr and Mg.

海底在高低温环境下喷涌出的热液流体，在调控海水化学组成方面发挥着关键作用。然而，洋中脊轴部的高温水通量规模仍存在广泛争议，而洋中脊侧翼的低温喷流流体体积则几乎未被限定。本研究借助洋壳中铊（thallium, Tl）元素的化学与同位素质量平衡方法，同时估算了高低温热液流体的通量。铊是海底热液交换的独特示踪剂，这是由于其在高低温海底蚀变过程中表现出截然不同的行为特征，且新鲜与蚀变洋中脊玄武岩（MORB）以及海水均具备独特的同位素特征。经计算得到的高温热液水通量为(0.17~2.93)×10^13 kg/年，最优估算值为0.72×10^13 kg/年。该结果表明，洋中脊轴部新生洋壳结晶冷却过程中释放的可用热量中，仅有约5%~80%通过高温黑烟囱流体释出。剩余的热能大概率通过热传导，或是通过不会改变洋壳中铊化学收支的中温热液流体循环而散失。基于铊的计算结果表明，洋中脊侧翼的低温热液水通量为(0.2~5.4)×10^17 kg/年。这意味着相较于背景海水，该类流体的平均温度异常仅约0.1~3.6℃。若该低温流体结论成立，则锶（Sr）与镁（Mg）在洋中脊侧翼热液系统中均表现出相对惰性，这或可解释为何观测到的蚀变洋壳玄武岩蚀变程度，不足以平衡87Sr/86Sr比值与镁的海洋化学收支。