Table 1 Mixing model parameters.

	Cl⁻	Na⁺	K⁺	DSi	Ca²⁺	Mg²⁺	δ¹⁸O	δD	⁸⁷Sr/⁸⁶Sr
SW	544 mM	470 mM	10 mM	24 µM	10 mM	54 mM	−0.159 ‰	−1.7 ‰	0.7092
GT	18.5 ± 12.4 mM	16 ± 8.23 mM	0.523 ± 0.311 mM	1420 ± 276 µM	2.96 ± 1.78 mM	0.23 ± 0.229 mM	−9.17 ± 0.87 ‰	−65.9 ± 7.01 ‰	0.7043

Endmember values for seawater and geothermal groundwater used in the pore water mixing model, with the latter derived as the average of geothermal spring data from Negri et al. (ref. ¹³). For these, the error represents the 95% confidence interval based on the range of reported values, which are used to estimate the window of possible mixing lines shown in Fig. 4 and Supplementary Fig. 1. The geothermal groundwater ⁸⁷Sr/⁸⁶Sr and Sr²⁺ endmembers are from Held et al. (ref. ¹²). The mudline elemental and isotopic composition is assumed to represent the seawater endmembers.

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