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International audienceAlthough lawsonite-bearing rocks are rare in exhumed high-pressure (HP) terranes, they are considered to exert a primary role in subduction dynamics. Recent observations in natural settings have shown that fluid–rock interaction at HP conditions, including metasomatism, may lead to unusually high lawsonite amounts even in rocks that originally contained little or no lawsonite. This process may therefore bear important implications for element recycling in subduction zones. A detailed characterization of the geochemical fingerprints associated with lawsonite metasomatism is presented in this contribution. The studied rocks belong to the HP terranes of Alpine Corsica (France), which is the largest documented exposure for lawsonite metasomatism. Metasomatic lawsonite displays complex compositional zoning, including high trace element, Cr and Ti content. The trace element content is much higher compared with the average of non-metasomatic lawsonite, and is in line with the re-incorporation of large amounts of trace elements (e.g. REE, Sr, Pb, Th) in the rock during metasomatism, as shown by mass transfer calculations. Our data suggest that serpentinites represented the main fluid source for the metasomatism, with concurrent contribution of other, possibly Ca-rich lithologies, such as mafic or meta-sedimentary rocks. We propose that the breakdown of metasomatic lawsonite may contribute to the genesis of magmas and their characteristic geochemical signatures

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This contribution reviews the existing data on lawsonite stability and trace element geochemistry, and provides new data for metabasaltic and metasedimentary (quartzite) rocks from New Caledonia, Turkey and California. Lawsonite is a major host of REE, Sr, U, Th and Pb in basaltic compositions. Trace element-rich lawsonite also occurs in metasedimentary rocks, in which comparatively fewer phases compete for trace elements than in metabasaltic rocks. Trace element patterns in lawsonite are influenced by the coexistence or breakdown of allanite, titanite, apatite and garnet that compete for these elements in high-P metamorphic rocks. Lawsonite is restricted to cool geotherms and therefore is an indicator mineral for subduction-zone metamorphism. The lawsonite stability field shows a strong dependence on temperature and composition and it is largest in rocks with a high normative anorthite content and, in basaltic systems, carbon content. Along cold geotherms, lawsonite can transport water and trace elements to great depths, providing a source for these elements in the deep mantle. Along warmer geotherms, lawsonite disappears on a continuous reaction, gradually releasing water over a temperature interval of several tens of degrees. During lawsonite breakdown in complex systems, Th and LREE remain trapped in newly formed accessory allanite. However, owing to extreme LREE content, allanite has lower Pb/Ce and Sr/Nd than lawsonite, resulting in a relative enrichment of Sr and Pb compared with Ce and Nd in the fluids produced during lawsonite breakdown. Existing experimental data on the solidus of altered oceanic crust suggest that the lawsonitebreakdown reaction is within 50°C of the solidus at sub-arc pressures of 3-4 GPa.

Combining U–Pb ages with Lu–Hf data in zircon provides insights into the magmatic history of rocky planets. The Northwest Africa (NWA) 7034/7533 meteorites are samples of the southern highlands of Mars containing zircon with ages as old as 4476.3 ± 0.9 Ma, interpreted to reflect reworking of the primordial Martian crust by impacts. We extracted a statistically significant zircon population (n = 57) from NWA 7533 that defines a temporal record spanning 4.2 Gyr. Ancient zircons record ages from 4485.5 ± 2.2 Ma to 4331.0 ± 1.4 Ma, defining a bimodal distribution with groupings at 4474 ± 10 Ma and 4442 ± 17 Ma. We interpret these to represent intense bombardment episodes at the planet’s surface, possibly triggered by the early migration of gas giant planets. The unradiogenic initial Hf-isotope composition of these zircons establishes that Mars’s igneous activity prior to ∼4.3 Ga was limited to impact-related reworking of a chemically enriched, primordial crust. A group of younger detrital zircons record ages from 1548.0 ± 8.8 Ma to 299.5 ± 0.6 Ma. The only plausible sources for these grains are the temporally associated Elysium and Tharsis volcanic provinces that are the expressions of deep-seated mantle plumes. The chondritic-like Hf-isotope compositions of these zircons require the existence of a primitive and convecting mantle reservoir, indicating that Mars has been in a stagnant-lid tectonic regime for most of its history. Our results imply that zircon is ubiquitous on the Martian surface, providing a faithful record of the planet’s magmatic history.

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