Thermo‐Mechanical State of Ultraslow‐Spreading Ridges With a Transient Magma Supply

Abstract

The thermal structure of mid‐ocean ridge axes is a critical control on the mechanical properties of young lithosphere and modulates the faulting styles that shape the Earth's seafloor. Models that balance a steady input of magmatic heat with a vigorous hydrothermal output are generally successful at explaining the thermal state of magmatically robust mid‐ocean ridges. However, the magma supply of ultraslow spreading ridges is often subdued and highly episodic, and the depth‐extent and vigor of hydrothermal circulation in these settings remain poorly known, requiring modifications to the standard magmatic‐hydrothermal paradigm. We develop models that couple repeated magmatic intrusions with hydrothermal convection in a permeable domain whose boundaries are controlled by the extent of grain‐scale thermal cracking. Our simulations reveal decreasing trends of venting temperatures with increasing intrusion periodicity, as well as with increasing permeability. Our model explains the seismically inferred depth (∼10 km) to the brittle‐ductile transition and the depth of MORB crystallization at the Mid‐Cayman Spreading Center by invoking repeated intrusion of sills every 20–50 kyrs, depending on their size. Doubling this periodicity predicts a thicker (∼15 km) seismogenic layer and lower‐temperature venting, as observed at the Southwest Indian Ridge (SWIR) at 64˚E. However, if fluid convection is not possible below ∼6 km, our model requires that high‐temperature venting at ultraslow ridges (e.g., Longqi at SWIR 49.7˚E) be fueled by shallow episodic magma intrusions into the long‐term convective region, which maintain high‐output circulation for at most a few tens of kyrs.

Thermal Regime of Slow and Ultraslow Spreading Ridges Controlled by Melt Supply and Modes of Emplacement

“…We used a 2D numerical thermal model (Fan et al., 2021) that couples repeated melt intrusions and hydrothermal convection to constrain the thermal regimes inferred from two magma supply endmembers at ultraslow spreading ridges: the most magmatically robust SWIR 50°28'E and the nearly amagmatic SWIR 64°30'E. The variability of the thermal regimes at slow and ultraslow spreading ridges was explored by varying parameters associated with melt supply, modes of magma emplacement (i.e., the frequencies and depths of melt injections), and hydrothermal circulation (i.e., the maximum hydrothermal domain depth and permeability).…”

“…4 of 19 numerical approach of Fan et al (2021) and Olive and Crone (2018), who used a finite volume method to solve for conservation of mass and energy in fluid-saturated, non-deforming porous medium obeying Darcy's law:…”

Detachment-parallel recharge explains high discharge fluxes at the TAG hydrothermal field

“…Unfortunately, constraining the timing of intrusive events remains a challenge. Yet, intrusion frequencies of several thousand to ten thousands of years appear plausible 48 . A reasonable number for the total heat required to "make" the active TAG mound is 2 × 10 19 J based on the massive sulfide accumulation size and volume of hot fluids needed to form it 8 .…”

Microseismicity and lithosphere thickness at a nearly amagmatic mid-ocean ridge

“…Intriguingly, the more magmatically-robust Dragon Flag has a seismogenic lithosphere that is equivalent or greater in thickness to that determined here for the nearly-amagmatic SWIR 64°30'E with the same spreading rate. This questions current numerical models of the links between spreading rate, melt supply, and the thermal regime of the MOR 18,20 . We propose that hydrothermal removal of magmatic heat at Dragon Flag may be more efficient than modelled, as evidenced by numerous black smokers 36,37 .…”

“…Seismicity provides a means to study magmatic, tectonic, and hydrothermal processes within the lithosphere of mid-ocean ridges [11][12][13][14][15][16][17] (MORs), and is an indirect proxy for the thermal regime by constraining the depth to the base of the seismogenic lithosphere 18,19 . The thickness of the axial seismogenic lithosphere is numerically predicted to increase as spreading rate decreases 18 and/or as melt supply decreases 20 . The ultraslow spreading SWIR 64°30'E, being nearly amagmatic, may be regarded as a geothermal calibration for the MOR system.…”