Figure 2

Phase boundaries of the water-carbon dioxide binary. Below 1 GPa, two CO₂ hydrates (a sI clathrate and a filled ice) have been reported^1,2,3. Filled triangles demark where the new phase (S3) has been observed to nucleate upon heating. Asterisks indicate pressures below which S3 was seen to decompose. The black dotted line indicates the (unobserved) metastable extension of the CO₂-saturated H₂O(VII) solidus to higher temperatures in the absence of S3, and the red dotted line an approximate metastable extension of the solidus of S3 to lower temperatures. Curves labeled “A” through “D” correspond to the points¹² (one peritectic, two eutectics, and the melting point) presented in Fig. 3. F1 is an aqueous solution, while melting of CO₂ produces a fluid, F2, dominated by that compound. The invariant quadruple point, “Q” (H₂O(VII) + CO₂(I) + S3 + F1), lies at 4.4 GPa and 165 °C.