Rotten Rocks at the Heart of the Atlantis Massif – A dive into reaction porosity in the Lost City Hydrothermal Field

The Atlantis Massif is a well-studied oceanic core complex in the Atlantic Ocean that hosts the Lost City Hydrothermal Field (LCHF). The LCHF is a low-moderate temperature, high pH vent system. In gabbroic intrusions within the serpentinite-dominated substrate of the LCHF, a variety of hydrothermal alteration reactions occur, including replacement, dissolution creating macroscopic (mm scale) reaction porosity, and precipitation of secondary minerals including chlorite, amphibole, prehnite and clays.

Many samples recovered from Expedition 399 and earlier expeditions contain  zones of reaction porosity. This work presents SEM, EMPA and other analysis of sample: U1601C 18R2 75-78 and U1309D-310R1 92-95 from Exp. 399, as well as several other samples analyzed concurrently, used only for example purposes.

Reaction porosity filled with actinolite is present at several levels in the gabbroic hole U1309D, including in areas that were newly deepened by Expedition 399. We highlight sample U1309D-310R1 92-95, collected at a depth of 1495 meters below seafloor (mbsf), which contains porosity partially filled with amphiboles zoned from edenitic hornblende cores to actinolite rims, suggesting dissolution by relatively higher temperature fluids.

Hole U1601C is dominated by serpentinised peridotite; porosity is widespread in gabbroic intrusions with a wide range of fills including chlorite, tremolite, diopside, serpentine, prehnite and saponite. Sample U1601C 18R2 75-78 consists of a 1 cm wide gabbroic vein (domain 1) within serpentinised peridotite (domain 2). Along the boundary with domain 2,  domain 1 contains a  ~5 mm zone of porosity partially filled by secondary diopside and serpentine. Relict porosity up to 200 µm in size is common. Domain 2 also contains porosity filled with diopside and serpentine, as well as zoned rosettes, of various stages of hydrogarnet solid solution, moving from pyrope-rich in the inner core, to more definitively hydro-andradite (identified by Raman spectroscopy (Frezzotti et al. 2012) and EPMA) in the rosette rim. The rosettes here may be replacing pyroxene.

We suggest that gabbroic veins acted as conduits for fluid flow during hydrothermal alteration, probably at temperatures of 300-400 °C, and contributed to the intense serpentinisation of the mantle rocks.  Magnetite is not observed in this sample, but hydrous andradite rich in Fe³⁺ offers another potential H₂ generating reaction.

Work in progress includes XCT analysis of the porosity. Further work will involve characterising the geological sequence of events, and in some cases their subsequent deformation (through sequence mapping), investigating the arguments for dissolution versus fill reactions (through extensive SEM, EMPA and X-Ray Tomography) and characterising the extent of reaction porosity in the Atlantis Massif.

References

Frezzotti, M.L., Tecce, F. and Casagli, A. (2012) ‘Raman spectroscopy for fluid inclusion analysis’, Journal of Geochemical Exploration, 112, pp. 1–20. Available at: https://doi.org/https://doi.org/10.1016/j.gexplo.2011.09.009.#