The response of ridge-crest hydrothermal systems to segmented, episodic magma supply

Segmented, episodic magma supply and cracking models, advanced to explain ridge-axis discontinuities and along-strike variability, have provoked a recent shift in mid-ocean ridge crest hydrothermal models. Segment-scale and time-series observations of hydrothermal vents and plumes along fast and intermediate spreading centres reveal a close coupling between magma supply and hydrothermal fluxes. Based on the premise that magma supply exerts a primary control on hydrothermal processes, a new paradigm of segmented hydrothermal circulation has arisen featuring independent temporal evolution of hydrothermal systems along individual ridge segments. This paradigm explains observed segment-to-segment variations in the chemical composition of vent fluids and plumes (‘chemical segmentation’). Hydrothermally active segments can be classified, independently of spreading rate, into two types that represent end-members for the coupling of hydrothermal and magmatic processes: magma-rich segments and magma-starved segments. Along magma-rich segments, magma intrudes to shallow depths where water:rock ratios are high. Seafloor vent distribution tends to be magmatically controlled along dyke-induced fissures. Along magma-starved segments, intrusion is at greater depth, and deep cracks (usually faults) are required to transport water to and from the heat source. Water:rock ratios are low and vent locations are influenced by crustal permeability structure, as well as heat source location. Although both segment types can occur at any spreading rate, magma-rich segments are more common at faster spreading rates, and magma-starved segments are more typical at slow spreading rates.