Table 1 Mg isotope values of the LTH fluids.

From: New isotope constraints on the Mg oceanic budget point to cryptic modern dolomite formation

Sample	Temp. at SBI^a (°C)	Mg^a (mmol kg⁻¹)	f_Mg^b	δ²⁶Mg (‰)	2SD (‰)	δ²⁵Mg (‰)	2SD (‰)	n	ε_solid-fluid^c (‰)	ε-error^c (‰)
Basement (formation) fluids
1362A-MVBS-13	64	1.3	0.02	−1.77	0.09	−0.94	0.11	4	0.25	0.02
1362B-sled #5	64	1.3	0.02	−1.84	0.12	−1.01	0.14	4	0.27	0.03
1024–46	23	43.2	0.82	−0.94	0.21	−0.53	0.22	4	0.52	1.04
1024-90	23	43.2	0.82	−0.91	0.10	−0.48	0.11	3	0.41	0.51
1025-#3-acidified	41	22.0	0.42	−1.40	0.07	−0.75	0.07	3	0.65	0.09
1025-#2-unacidified	41	43.4	NA	−0.95	0.05	−0.49	0.07	4	NA	NA
Corr.−1025-#2-unacidified^d	41	22.0	0.42	−1.57	0.05	−0.83	0.06	NA	0.84	0.09
1025-3608-blue	41	26.9	0.51	−1.52	0.01	−0.78	0.03	4	1.03	0.11
1025-3608-red	41	27.7	NA	−1.58	0.09	−0.79	0.02	4	NA	NA
Corr.−1025-3608-red^d	41	26.9	0.51	−1.63	0.06	−0.82	0.03	NA	1.19	0.15
Springs fluids
4775-6-Dorado	15	52.5	0.99	−0.93	0.08	−0.54	0.14	6	7	33
4777-7-Dorado	15	52.4	0.98	−0.86	0.12	−0.45	0.16	5	1.5	10
4775-9-dorado	15	52.3	0.98	−0.95	0.12	−0.51	0.10	4	6	23
4777-6-Dorado	15	53.3	1.00	−0.94	0.17	−0.47	0.18	4	NA	NA
Pore water
1363G-3H1	6.1^e	36	0.68	−1.07	0.08	−0.62	0.11	6	0.63	0.25
1363D-4X1	ND	33.3	0.63	−1.17	0.13	−0.60	0.12	8	NA	NA
1363D-4X2	ND	33.3	0.63	−1.24	0.06	−0.66	0.04	6	NA	NA
1363D-4X3	ND	34.7	0.66	−1.27	0.06	−0.70	0.04	4	NA	NA
1363D-5X1	ND	31.3	0.59	−1.36	0.12	−0.72	0.10	8	NA	NA
1363D SBI-extrapolated^f	32.7	28.6	0.54	−1.54	0.14	−0.83	0.21	4	1.16	0.26
1363B-4H5	ND	50.6	0.96	−1.01	0.07	−0.52	0.03	4	NA	NA
1363B-7X1	ND	39.5	0.75	−1.01	0.06	−0.51	0.02	4	NA	NA
1363B-8X2	ND	36.8	0.70	−0.92	0.02	−0.48	0.05	4	NA	NA
1363B-8X3	ND	38.2	0.72	−0.91	0.04	−0.49	0.04	6	NA	NA
1363B SBI-extrapolated^f	12.2^e	35	0.66	NA	NA	NA	NA	NA	NA	NA
1363F 4H1	ND	37.0	0.70	−0.87	0.07	−0.38	0.07	4	NA	NA
1363F 4H2	ND	37.8	0.71	−0.93	0.08	−0.46	0.08	4	NA	NA
1363F 4H3 IW29	ND	36.7	0.69	−1.00	0.17	−0.54	0.20	3	NA	NA
1363F SBI-extrapolated^f	7.1^e	35	0.66	−1.18	0.24	−0.75	0.27	3	0.85	0.60

NA not applicable, ND not determined
^aSBI temperatures and Mg concentration data from Wheat et al.^34,36,37,39, Fisher et al.³⁸, and this study (see Supplementary Table 1)
^bf_Mg is the remaining fraction of Mg relative to bottom seawater, which were taken as 52.9 mmol kg⁻¹ in most cases. For Dorado samples, bottom seawater was taken as 53.3 mmol kg⁻¹, the highest concentration measured there
^cε_solid-fluid was calculated assuming a Rayleigh distillation (see Methods), initial seawater δ²⁶Mg = −0.83‰ and the measured f_Mg and δ²⁶Mg of each sample. The error on the ε was propagated
^dCorrected values for mixing with seawater are indicated by Corr
^eMg concentrations suggest that the temperature at SBI is 30–40 °C
^fPore water values were extrapolated to the SBI depth (see Supplementary Table 1 for temperature and Mg concentration data and references). δ²⁶Mg values of pore water from 1363B show small variations and opposite correlation with depth and therefore an extrapolated value was not calculated

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Table 1 Mg isotope values of the LTH fluids.

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