> isn't Earth's H2O mainly circulating and, hence, 'old' in the sense that all molecules are?
A lot of it may be, but some of it isn't.
You can buy quartz, for example that contain water or petroleum sloshing around inside [1] although some think the water could seep in and out over time.
But quartz with fluid inclusions (small bubbles containing water) are completely sealed and have the same composition as they were up to 3 billion years ago. "the fluid in the inclusion is an actual sample of fluid in which the mineral was growing" [2]
A lot of it may be, but some of it isn't.
You can buy quartz, for example that contain water or petroleum sloshing around inside [1] although some think the water could seep in and out over time.
But quartz with fluid inclusions (small bubbles containing water) are completely sealed and have the same composition as they were up to 3 billion years ago. "the fluid in the inclusion is an actual sample of fluid in which the mineral was growing" [2]
[1] https://en.m.wikipedia.org/wiki/Enhydro_agate
[2] https://www.gamineral.org/writings/enhydros-gray.html