Water in Abyssal Peridotite: Why Are Melt‐Depleted Rocks so Water Rich?

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2018-06, Vol.19 (6), p.1824-1843
• Main Authors: Schmädicke, E., Gose, J., Stalder, R.
• Format: Article
• Language: English
• Subjects: abyssal peridotite; Fertility; Heterogeneity; Infrared radiation; Isotopes; Low temperature; Magma; Mantle; Moisture content; MORB source; nominally anhydrous minerals; orthopyroxene; Peridotite; Removal; Residence time; restitic mantle; Rock; Rocks; Water content; Water loss; Water supply
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2017GC007390

Evidence for the heterogeneity of the mantle's water content is generally acknowledged but it remains unclear if the variability of data is influenced by decompressional water loss. Here based on high water contents in peridotite, we explore if water gain may play a role. The study investigates water in abyssal peridotite by analyzing oriented orthopyroxene with polarized infrared radiation. Orthopyroxene from ODP‐Leg 153 (Mid‐Atlantic Ridge) contains 100 ppm more H2O (range: 220–323 ppm) than Leg 209 samples (range: 121–231 ppm). Possible explanations for high water concentrations are (1) water supply by gabbroic dykes, (2) reequilibration with ancient, H2O‐rich melts or melt‐derived fluid, (3) diffusional exchange between depleted peridotite and fertile rock volumes, or (4) high initial (premelting) water contents. Water in orthopyroxene is unrelated to the proximity of gabbroic dykes, and the inferred low H2O content of the latter renders possibility (1) unlikely. Possibilities (2) and (3) require that the rocks remained at mantle depth for an extended time span. This idea of a prolonged “mantle residence time” is corroborated by (i) literature data on isotopes and (ii) mantle equilibrium temperatures. Peridotite from Leg 153 reequilibrated at lower temperature (950–1,000°C) than Leg 209 peridotite (1,100–1,200°C) implying that more time elapsed between melt removal and exhumation—extending the time span for refertilization and explaining the its higher water content. The alternative that the measured concentrations are residual (4) requires high H2O amounts in original orthopyroxene (1,500 ppm) and peridotite (700 ppm)—values that are typical of an OIB‐source. Key Points Unusually high H2O contents in residual peridotite from the Mid‐Atlantic Ridge are not attributed to secondary enrichment by gabbroic melts The residual water content measured in peridotite from ODP‐Leg 153 is considerably higher compared to that of Leg 209 Estimated initial (premelting) H2O contents are identical for both Legs possibly suggesting water‐rich volumes in the MORB mantle