Metasomatism and Hydration of the Oceanic Lithosphere: a Case Study of Peridotite Xenoliths from Samoa

Abstract

Water influences geodynamic processes such as melting, deformation and rheology. Yet its distribution in the oceanic upper mantle is primarily known indirectly from melt inclusions and glasses of erupted mantle melts (i.e. mid-ocean ridge and ocean island basalts). To better constrain the mechanisms influencing the distribution of H2O in the mantle, particularly regarding the role of metasomatism, we analyzed 15 peridotite xenoliths from Savaiʻi and 2 dunite xenoliths from Taʻū (Samoa) for structural H2O (by polarized Fourier transform infrared spectroscopy), major and trace element concentrations. Clinopyroxenes from the Taʻū dunites show trace element concentrations consistent with equilibration with their host lavas, but lower H2O contents than expected. Savaiʻi peridotites are highly depleted harzburgites (melt depletion ≥17%). They show strong evidence of transient metasomatism by both carbonatite and silicate melts, with highly variable Ti and Zr depletions and light rare earth element (LREE) enrichments. However, despite metasomatism the H2O concentrations in olivines (0 to 4 ppm H2O) and orthopyroxenes (17 to 89 ppm H2O) are among the lowest reported in oceanic xenoliths, but higher than expected for the estimated degree of depletion. In general, H2O concentrations vary less than other incompatible trace elements in these samples. Transects across mineral grains show generally homogenous distributions of H2O, indicating no significant H2O loss or gain during ascent. Raman spectroscopy on inclusions in minerals shows the presence of CO2 but an absence of molecular H2O. This agrees with the absence of H2O concentration variations between inclusion-rich and -poor domains in minerals. The above data can be explained by transient metasomatism along grain boundaries, now recorded as planes of inclusions within annealed grains. Fast diffusion of hydrogen (but not lithophile elements) from the inclusions into the host mineral phase will simultaneously enrich H2O contents across the grain and lower them in the inclusion-rich domains. The result is highly variable metasomatism recorded in lithophile elements, with smaller magnitude H2O variations that are decoupled from lithophile element metasomatism. Comparison with xenoliths from Hawai‘i shows that evidence for metasomatism from lithophile elements alone does not imply rehydration of the oceanic lithosphere. Instead, H2O concentrations depend on the overall amount of H2O added to the lithosphere through metasomatism, and the proximity of sampled material to areas of melt infiltration in the lithosphere.

Postmelting hydrogen enrichment in the oceanic lithosphere

“…S1). This behavior is opposite to experimental predictions after partial melting ( 11 ) and to elemental trends observed in some xenoliths ( 28 ). Further, H 2 O negatively correlates with Ti and Na in both pyroxenes ( Fig.…”

“…Depending on the mechanism of hydrogen incorporation, xenoliths can show positive ( 56 ) or negative ( 54 ) correlations between H 2 O and ƒ o 2 so that oxidizing conditions under which xenoliths evolve are not a prediction for their water contents. Although mantle xenoliths are prone to melt-rock reactions ( 5 ), some retain broad positive correlations between H 2 O and other incompatible elements ( 28 , 61 ), opposite to the VLS time series. It is possible that, in xenoliths, a loss of Na + during melting would not necessarily be followed by H + addition from a percolating melt, as samples may re-equilibrate in the lithospheric mantle for millions of years before metasomatism and/or eruption occurs.…”

The potential for aqueous fluid-rock and silicate melt-rock interactions to re-equilibrate hydrogen in peridotite nominally anhydrous minerals

“…These models also show that decoupling of NAM water contents from their REE abundances (e.g., Warren and Hauri 2014;Peslier et al 2017;Ashley et al 2020 Unraveling the origin of the water contents measured in peridotite NAMs requires additional constraints on the history of the sample, including petrographic analysis and measurement of other trace elements.…”

Trachytic Melt Inclusions Hosted in Clinopyroxene Offer a Glimpse Into Samoan EM2‐Endmember Melts

“…Lastly, there is evidence that the EM2 source has been enriched not only by addition of sediments, but also carbonatitic material. Carbonatitic metasomatism beneath Samoa has been proposed by many authors (Ashley, 2019; Ashley et al., 2020; Hauri et al., 1993; Hauri & Hart, 1994). Evidence for carbonatite metasomatism is clearest in Samoan mantle peridotite xenoliths that show evidence of interaction with carbonatitic liquids based on their LREE enrichment and Zr and Ti depletions in clinopyroxene (Hauri & Hart, 1994; Hauri et al., 1993).…”

“…Evidence for carbonatite metasomatism is clearest in Samoan mantle peridotite xenoliths that show evidence of interaction with carbonatitic liquids based on their LREE enrichment and Zr and Ti depletions in clinopyroxene (Hauri & Hart, 1994;Hauri et al, 1993). More recently, Ashley et al (2020) performed an analysis of water systematics and major and trace element characteristics in Samoa peridotite xenoliths (focusing on orthopyroxenes) revealing variable depletions in water contents of nominally anhydrous minerals, and in addition to their trace element characteristics, indicate metasomatism of carbonatitic and silicate liquids. Further, previous studies have suggested that partial melting of mantle enriched with carbonatitic liquids can result in phonolitic to trachytic liquids (Ashwal et al, 2016;Hauri et al, 1993;Loges et al, 2019;Tsuno and Dasgupta, 2011).…”