Constraining Early Planetary Differentiation: The Link Between Chondrites and Achondrites Revealed From the Study of Aubrite Meteorites

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Platinum-group elements, S, Se and Cu in highly depleted abyssal peridotites from the Mid-Atlantic Ocean Ridge (ODP Hole 1274A): Influence of hydrothermal and magmatic processes

Highly depleted harzburgites and dunites were recovered from ODP Hole 1274A, near the intersection between the Mid-Atlantic Ocean Ridge and the 15°20′N Fracture Zone. In addition to high degrees of partial melting, these peridotites underwent multiple episodes of melt-rock reaction and intense serpentinization and seawater alteration close to the seafloor. Low concentrations of Se, Cu and platinum-group elements (PGE) in harzburgites drilled at around 35-85 m below seafloor are consistent with the consumption of mantle sulfides after high degrees (>15-20 %) of partial melting and redistribution of chalcophile and siderophile elements into PGE-rich residual microphases. Higher concentrations of Cu, Se, Ru, Rh and Pd in harzburgites from the uppermost and lowest cores testify to late reaction with a sulfide melt. Dunites were formed by percolation of silica- and sulfur-undersaturated melts into low-Se harzburgites. Platinum-group and chalcophile elements were not mobilized during dunite formation and mostly preserve the signature of precursor harzburgites, except for higher Ru and lower Pt contents caused by precipitation and removal of platinum-group minerals. During serpentinization at low temperature (<250 °C) and reducing conditions, mantle sulfides experienced desulfurization to S-poor sulfides (mainly heazlewoodite) and awaruite. Contrary to Se and Cu, sulfur does not record the magmatic evolution of peridotites but was mostly added in hydrothermal sulfides and sulfate from seawater. Platinum-group elements were unaffected by post-magmatic low-temperature processes, except Pt and Pd that may have been slightly remobilized during oxidative seawater alteration

An undeformed pyroxenite-peridotite sequence from the External Ligurian ophiolites records multiple events of melt-rock interactions.

The External Ligurian mantle sequences represent deep subcontinental lithosphere exhumed in response to Mesozoic lithospheric thinning and opening of the Jurassic Alpine Tethys. They mainly consist of spinel-plagioclase lherzolites, with tectonite to mylonitic structures, and diffuse pyroxenite layering. The deformed pyroxenites have been related to recycling of old crustal material in Lower Palaeozoic to Triassic times (e.g. Montanini et al., 2015; Borghini et al., 2016). The lherzolites of the present study (Monte Gavi) are undeformed and show evidence of melt infiltration and crystallization of plagioclase (Pl) + orthopyroxene (Cpx) at the expense of spinel (Spl) and clinopyroxene (Cpx). The lherzolites include a pyroxenite body with a thickness of 6-10 m and a length of ~ 50 m. The primary assemblage of the pyroxenite consists of Cpx and Al-Spl. Cpx is resorbed and variably replaced by Opx + Pl aggregates. Spl is extensively transformed into Ca-rich Pl + Fe-rich olivine + Cr-Spl ± ilmenite. Clinopyroxene has low Mg# (81-83) and up to 10 wt% Al2O3. Close to the main pyroxenite body, the lherzolite includes up to 10 cm-thick spinel pyroxenite layers containing Mg-rich Cpx (Mg# = 89-90) and, locally, Mg-rich olivine incorporated from the host lherzolite. REE compositions of melts in equilibrium with the preserved primary Cpx display a slight LREE enrichment and a negative HREE fractionation requiring a garnet-bearing source. The Fe-rich pyroxenite body has “melt-like” patterns of highly siderophile elements (HSE), whereas the Mg-rich pyroxenites are enriched in Os and Ir. Bulk rock 187Os/188Os ratios recalculated at the age of the Alpine Tethys opening (165 Ma) show increasingly radiogenic composition from Mg- to Fe-rich pyroxenites (187Os/188Os = 0.185-0.518). We propose that the pyroxenites formed by crystallization of Al-Fe-rich melts derived from aged pyroxenite/eclogite-rich sources. Whereas the thick pyroxenite body represents a melt-dominated system, the thin pyroxenite layers formed by melt/peridotite hybridization. Extensive replacement of the primary assemblage was most likely produced by reactive migration of depleted MORB-type melts under plagioclase facies conditions (P ~0.6 GPa). The pyroxenites preserve high T (1200-1250 °C) recorded by slowly diffusing elements like REE, presumably in response to the melt infiltration event, followed by a rapid subsolidus cooling until 900 °C during rifting-related exhumation of the mantle sequence. REFERENCES Montanini, A., Tribuzio R. (2015): Evolution of recycled crust within the mantle: Constraints from the garnet pyroxenites of the External Ligurian ophiolites (northern Apennines, Italy). Geology, 43, 911-914, Borghini, G., Rampone, E., Zanetti, A., Class, C., Cipriani, A., Hofmann, A.W., Goldstein, S.L. (2016): Pyroxenite Layers in the Northern Apennines’ Upper Mantle (Italy)—Generation by Pyroxenite Melting and Melt Infiltration. J.Petrol., 57, 625-653

Multiple events of melt-rock interaction recorded by undeformed spinel pyroxenites from the External Ligurian ophiolites

The External Ligurian mantle sequences are interpreted as deep subcontinental lithosphere exhumed to the ocean floor in response to Mesozoic lithospheric thinning and opening of the Jurassic Ligurian-Piedmontese basin. The sequences consist of spinel-plagioclase lherzolites with diffuse pyroxenite layers which have been related to recycling of crustal material (Montanini et al., 2012) or to eclogite-bearing peridotite sources (Borghini et al., 2016). The mantle lherzolite body considered in the present study (Monte Gavi) includes an undeformed, irregularly shaped body of spinel pyroxenites. This body has a thickness of 6-10 m, a length of about 50 m and encloses several meter-sized lherzolite lenses. Close to the main pyroxenite body, the host lherzolite frequently includes up to 10 cm thick spinel pyroxenite layers. The pyroxenites are coarse-grained and consist of clinopyroxene- and Al-spinel-rich domains. Clinopyroxene is resorbed, variably replaced by orthopyroxene + plagioclase aggregates, and locally rimmed by titanian pargasite. Spinel-rich domains are largely transformed into Ca-rich plagioclase + Fe-rich olivine + Cr-spinel ± ilmenite. Clinopyroxene locally has relatively low Mg# (≈ 83) and Cr2O3 (≤ 0.3 wt%), and up to 10 wt% Al2O3 and 1.8 wt% Na2O. The thin pyroxenite layers are characterized by Mg-rich clinopyroxene (Mg# = 0.89-0.90) and in places include forsterite-rich deformed olivine, which is interpreted to be a relic of the host lherzolite. The Fe-rich pyroxenites have basaltic, “melt-like” patterns of highly siderophile elements (HSE), whereas the Mg-rich pyroxenites are significantly enriched in Os and Ir. Bulk rock 187Os/188Os ratios recalculated for the age of the Ligurian-Piedmontese basin opening (165 Ma) vary from slightly to moderately radiogenic (0.185-0.518). We propose that the pyroxenites formed by crystallization of Al-rich melts derived by an aged pyroxenite/eclogite-rich source. In this view, the thick pyroxenite body represents a melt-dominated system, whereas the thin pyroxenite layers are hybrid rocks derived from melt/peridotite reactions. Resorption and extensive replacement of the primary clinopyroxene-spinel assemblage was most likely related to reactive migration of ultra-depleted melts under plagioclase facies conditions, during exhumation of the mantle sequence. The host lherzolites also show textural, mineralogical and geochemical evidence of melt infiltration and crystallization of plagioclase + orthopyroxene at the expense of spinel and clinopyroxene

Sulfide chemistry of pyroxenites from the External Liguride Peridotite Massif, Italy - Implications for melt-rock interactions in the mantle.

The ophiolites of the Northern Apennine thrust and fold belt represent samples of deep subcontinental lithosphere from an oceancontinent transition in Mesozoic times. The External Liguride (EL) units consist of fertile lherzolites with MOR-type isotopic signatures and diffuse pyroxenite layers. Pyroxenites are a main contributor to mantle heterogeneity, yet their origin and petrogenesis is still disputed. Highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) and the Re-Os isotopic system, which are predominantly controlled by base metal sulfides (BMS), are considered powerful tools to study mantle processes offering a different perspective than conventional lithophile trace elements. To better understand the melt-rock interactions in the lithospheric mantle on an individual grain scale, 22 BMS from two EL pyroxenite samples from Monte Gavi and Rio Strega were analyzed for HSE and 187Os/188Os signatures. The Monte Gavi pyroxenite consists of a coarse-grained matrix of Cpx, Opx, Ol and secondary mica. Base metal sulfides occur as abundant (n=30-40 grains per thin section) large grains (up to 400 μm), systematically associated to mica and adjacent chlorite-filled fractures crosscutting the entire sample. They are Py, Pn and Cpy in close to equal proportions (40%:35%:25%, respectively) frequently intergrown with ilmenite and magnetite. The Rio Strega garnet clinopyroxenite has a fine-grained matrix of Opx, Cpx, Grt, Spl and Plag. Base metal sulfides are as abundant as in the Monte Gavi pyroxenite, smaller in size (50-100 μm) and show a different sulfide assemblage (Po:Pn:Cpy = 55%:25%:20%). They are located at Cpx-Plag grain boundaries showing no association with oxides. In the BMS, HSE concentrations span over two orders of magnitude, ranging from 1 ppb to 5 ppm, showing an overall broad, positive CI-chondrite normalized HSE pattern (Pd/Ir = 29-59; Re/Os = 2-17). Their 187Os/188Os signatures vary from unradiogenic to highly radiogenic (0.1081-0.3745) with one outlier at 1.0062. The pyroxenites, which formed by melt-rock reaction between a silicate melt and a peridotitic protolith, have thus inherited their 187Os/188Os signatures at the grain scale from both the mantle residue and metasomatic agent. Additionally, the BMS are comparatively less radiogenic than the respective bulk pyroxenite (187Os/188Os: 0.6906 for Monte Gavi, 1.0544 for Rio Strega), suggesting that radiogenic Os, and potentially other HSE, are also significantly hosted by platinum group minerals or silicates. Ultimately, this study highlights the necessity of considering not only bulk rock isotopic signatures, but also the mineral scale ones, when studying the heterogeneity of the terrestrial mantl