Harzburgite Protoliths Research Articles

Platinum‐group element signature of the primitive mantle rejuvenated by melt‐rock reactions: evidence from Sumail peridotites (Oman Ophiolite)

AbstractPlatinum‐group element (PGE) abundances in mantle rocks are generally considered to result from a late meteorite addition to the early Earth, post‐dating the core separation event. As such, PGEs are key tracers for the Earth accretion history. For decades, the PGEs systematics of undepleted mantle peridotites has been used to constrain the composition of meteorite impactors involved in the late veneer material. Despite multiple evidence of considerable modifications by partial melting, harzburgites from the Sumail ophiolite (Oman) display a mean PGE composition very akin to recently refined estimates for the Primitive Upper Mantle (PUM) of the Earth. These rocks document a resetting of the PUM signature by percolating basaltic melts, which precipitated Pd‐enriched Cu–Ni sulphides within a strongly Pd‐depleted residual harzburgitic protolith. Such a resetting casts doubt on both the reliability of any PUM estimates and relevance of the PUM concept itself, at least for PGEs.

Trace Element Residence and Partitioning in Mantle Xenoliths Metasomatized by Highly Alkaline, Silicate- and Carbonate-rich Melts (Kerguelen Islands, Indian Ocean)

Mantle xenoliths in alkaline lavas of the Kerguelen Islands consist of: (1) protogranular, Cr-diopside-bearing harzburgite; (2) poikilitic, Mg-augite-bearing harzburgite and cpx-poor lherzolite; (3) dunite that contains clinopyroxene, spinel phlogopite, and rarely amphibole. Trace element data for rocks and minerals identify distinctive signatures for the different rock types and record upper-mantle processes. The harzburgites reflect an initial partial melting event followed by metasomatism by mafic alkaline to carbonatitic melts. The dunites were first formed by reaction of a harzburgite protolith with tholeiitic to transitional basaltic melts, and subsequently developed metasomatic assemblages of clinopyroxene + phlogopite ± amphibole by reaction with lamprophyric or carbonatitic melts. We measured two-mineral partition coefficients and calculated mineral–melt partition coefficients for 27 trace elements. In most samples, calculated budgets indicate that trace elements reside in the constituent minerals. Clinopyroxene is the major host for REE, Sr, Y, Zr and Th; spinel is important for V and Ti; orthopyroxene for Ti, Zr, HREE, Y, Sc and V; and olivine for Ni, Co and Sc.

Peridotites from the Izu-Bonin-Mariana Forearc (ODP Leg 125): Evidence for Mantle Melting and Melt-Mantle Interaction in a Supra-Subduction Zone Setting

Ocean Drilling Program Leg 125 recovered serpentinized harzburgites and dunites from a total of five sites on the crests and flanks of two serpentinite seamounts, Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu–Bonin forearc. These are some of the first extant forearc peridotites reported in the literature and they provide a window into oceanic, supra-subduction zone (SSZ) mantle processes. Harzburgites from both seamounts are very refractory with low modal clinopyroxene (<4%), chrome-rich spinels (cr-number = 0.40–0.80), very low incompatible element contents, and (with the exception of amphibole-bearing samples) U-shaped rare earth element (REE) profiles with positive Eu anomalies. Both sets of peridotites have olivine–spinel equilibration temperatures that are low compared with abyssal peridotites, possibly because of water-assisted diffusional equilibration in the SSZ environment. However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite–magnetite–quartz) – 1.1 (log units) and FMQ + 0.4 which overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunites from Conical Seamount contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover, they are considerably more oxidized than the harzburgites to which they are spatially related, with calculated oxygen fugacities of FMQ – 0.2 to FMQ + 1.2. Using textural and geochemical evidence, we interpret these harzburgites as residual MORB mantle (from 15 to 20% fractional melting) which has subsequently been modified by interaction with boninitic melt within the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissolved from the harzburgite protolith. In contrast, harzburgites from Torishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0.8 and FMQ + 1.6, similar to those calculated for other subduction-zone related peridotites and similar to those calculated for the dunites (FMQ + 1.2 to FMQ + 1.8) from the same seamount. In this case, we interpret both the harzburgites and dunites as linked to mantle melting (20–25% fractional melting) in a supra-subduction zone environment. The results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. They also demonstrate that both types of mantle lithosphere may have undergone extensive interaction with subduction-derived magmas.

Metasomatic processes in lherzolitic and harzburgitic domains of diamondiferous lithospheric mantle: REE in garnets from xenoliths and inclusions in diamonds

Garnets from diamondiferous and non-diamondiferous peridotite nodules from the Roberts Victor kimberlite were analysed for trace elements by secondary ion mass spectrometry (SIMS). Comparison with inclusions in diamonds from Akwatia (Ghana) shows that there are no significant differences between garnets in peridotite xenoliths and those included in diamonds. High chromium contents (Cr 2O 3 generally >5 wt%) indicate that both harzburgitic and lherzolitic garnets from Roberts Victor (nodules) and Akwatia (diamonds) were crystallised from precursor rocks that had suffered depletion by partial melting in the spinel stability field which resulted in high Cr/Al in the residue. Harzburgitic and lherzolitic garnets, despite their common spinel harzburgite protoliths, show distinctive trace element chemistries that are not transitional to each other. This contrasts with the observation of concordant overlapping trends for a number of major elements and some trace elements for southern African garnet harzburgite and lherzolite xenoliths [F.R. Boyd, Earth Planet. Sci. Lett. 96 (1989) 15-26]. The REE patterns of harzburgitic garnets, when normalised to a `primitive' garnet composition, are depleted in MREE and HREE and enriched in LREE. A positive slope within the HREE, particularly prominent in inclusions in diamonds, points to depletion (partial melting) originating in the garnet stability field. We suggest that the protolith had undergone polybaric fractional melting (as envisaged for modern MORB extraction) before being subducted to the depth of the diamond stability field. The growing garnets acquired a depleted REE pattern which was subsequently overprinted by a metasomatising fluid with very high LREE/HREE. The lherzolitic garnets, despite their high chromium contents, have flat REE patterns when normalised to a primitive garnet, a paradox which requires a secondary enrichment event to produce the unfractionated REE abundances. Lower LREE in lherzolitic relative to harzburgitic garnets may be attributed to re-equilibration with clinopyroxene. Differences in MREE and particularly in HREE cannot be explained by such a process, due to strongly increasing D Grt/Cpx from LREE to HREE. Therefore, lherzolitic garnets indicate an additional metasomatic process not seen by their harzburgitic counterparts, which introduced major elements (such as Ca, Fe and Si) sufficiently to stabilise clinopyroxene and which re-enriched the previously depleted MREE and HREE.

Subduction of ophicarbonates and recycling of CO2 and H2O

Because subducted serpentinites may release significant quantities of volatiles, high-pressure phase equilibria were computed for two end-member hydrothermally altered mantle harzburgite protoliths (ophicarbonates): calcite + antigorite + brucite and calcite + antigorite + talc. For both bulk compositions, most of the H 2 O released by metamorphic dehydration occurs at subarc depths; thus dehydration of serpentinites could be a major source for H 2 O in arc magmas. In contrast, for both model compositions a significant fraction of the original carbonate is retained to depths exceeding 200 km. Consequently, deep subduction of ophicarbonate rocks of the oceanic lithosphere and/or downward drag of mantle wedge ophicarbonates provide a mechanism for carbonating the mantle and thus a potentially significant CO 2 source for deep mantle melts. The probable CO 2 sources for arc magmas are metamorphic decarbonation of marine sediments and/or carbonated mafic volcanics in the subducted slab.

Geochemistry of polymetamorphic ultramafics (Major, Trace, Noble and Rare Earth Elements): An example from the Helvetic basement, Central Alps, Switzerland

Polymetamorphic ultramafic rocks in orogenic terranes rarely preserve relic structures or minerals from their former mantle stages. The determination of their protoliths and their tectonic evolution by chemical discrimination methods is often difficult due to possible metasomatic processes. Ultramafics of the pre-Variscan Helvetic basement (Central Alps, Switzerland) have been investigated geochemically to address these problems. These ultramafics are partially to completely serpentinised. According to field observations several ultramafic lenses were part of an ophiolite suite, but distinct cumulate ultramafic lenses were also recognized. CIPW norms indicate that large parts of the ultramafics are harzburgites, but metasomatic CaO depletion may have produced an overestimation of the importance of the harzburgite protoliths. Major element distributions suggest a depleted mantle protolith. Close to chondritic or slightly depleted REE patterns are characteristics of the studied samples. The REE normalized patterns confirm the presence of harzburgites, lherzolites und cumulates. In some samples light REE enrichment processes have occurred. The noble metal concentrations are both affected (Pt-Pd-Au) und unaffected (Ir-Os-Ru) by melt infiltration processes. They suggest the presence of undepleted or slightly enriched harzburgites und more differentiated, probably cumulate ultramafics. Information obtained by different chemical elements leads to contrasting results. REE and noble metals show enrichment inconsistent with the major element depletion. Refertilization of depleted ultramafics is proposed.

Experimental evidence for the exsolution of cratonic peridotite from high-temperature harzburgite

Phase equilibrium experiments were performed on typical ‘oceanic’ and ‘cratonic’ peridotite compositions and a Ca, Al-rich orthopyroxene composition, to test the proposal that garnet lherzolites exsolved from high-temperature harzburgites, and to further our understanding of the origin of ancient cratonic lithospheres. ‘Oceanic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1450–1600°C, but at 5 GPa and temperatures less than 1450°C, crystallize clinopyroxene to become true lherzolites. ‘Cratonic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1300–1600°C. Garnet-free harzburgite crystallizes from both ‘cratonic’ and ‘oceanic’ peridotite at temperatures above 1450°C and pressures below 4.5–5 GPa. Phase relations for the high Ca, Al-rich orthopyroxene composition essentially mirror those for ‘oceanic’ peridotite. The complete solution of garnet and clinopyroxene into orthopyroxene observed in all three starting compositions at temperatures near or above the mantle solidus at pressures less than 6 GPa supports the hypothesis that garnet lherzolite could have exsolved from harzburgite. The inferred cooling path for the original high-temperature harzburgite protoliths of garnet lherzolites differs depending on bulk composition. The precursor harzburgite protoliths of garnet lherzolites and harzburgites with ‘cratonic’ bulk compositions apparently experienced simple isobaric cooling from formation temperatures near the peridotite solidus to those at which most of these peridotites were sampled in the mantle (< 1200°C). The cooling histories for harzburgite protoliths of sheared garnet lherzolites with ‘oceanic’ compositional affinity are speculated to have involved convective circulation of mantle material to depths deeper than those at which it was originally formed. Phase equilibria and compositional relationships for orthopyroxenes produced in phase equilibrium experiments on peridotite and komatiite are consistent with an origin for ‘cratonic’ peridotite as a residue of Archean komatiite extraction, which has since cooled and exsolved clinopyroxene and garnet to become the common low-temperature, coarse-grained peridotite thought to comprise the bulk of the mantle lithosphere beneath the Archean Kaapvaal craton.

Emplacement of deep crustal and mantle rocks on the west median valley wall of the MARK area (MAR, 23°N)

Exposures of deep crustal and mantle rocks within the axial rift valley characterize accretion processes in the slow-spreading Mid-Atlantic Ridge. Nine dives of the submersible Nautile explored the western axial valley wall in the northern cell of the MARK area (Mid-Atlantic Ridge/Kane fracture zone), where peridotite and gabbro outcrops had been previously reported, in order to constrain the structure and determine emplacement mechanisms. The ridge/transform intersection massif on the western wall shows a section of gabbros from 6000 to 2500 mbsl, locally overlain by metabasalts and metadolerites, capped by slightly weathered basalts. The morphology is controlled by ridge-parallel faults, dipping moderately (40–65°) to the east. Transform-parallel scarps, present in the northernmost dives, become rare toward the south. Brittle deformation, along moderately dipping fault scarps, produced a dense microcrack network filled with greenschist facies minerals (chlorite, actinolite, epidote, quartz), locally overprinting high-temperature ductile deformation fabrics. The small hill located at 23°20′ in the western wall shows good exposures of serpentinized peridotite between 3700 and 3100 mbsl. Above 3100 mbsl, the summit of the hill is composed of pillow-basalts and sediments. The peridotite outerops display a strong schistosity dipping 20–40° to the east, parallel to striated normal fault planes. Some steeper east-facing fault scarps truncate the lower-dipping fault surfaces. The serpentinites, deriving from a harzburgitic protolith, are cut by rare dikelets of highly differentiated gabbro. These new data, combined with previous results of Alvin dives, are used to draw a generalized geological map of the western axial valley wall. This map suggests a variation in the thickness of crustal units and composition along strike in the northern cell of the MARK area: the gabbro body disappears toward the south where basalts appear to directly overlie the mantle peridotites which are cut by isolated gabbroic dikelets. Low-angle stretching affecting this heterogeneous lithosphere is probably responsible for the exposure of gabbros in the intersection massif and of peridotites further south.