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

doi:10.1016/0012-821X(91)90063-N

Earth and Planetary Science Letters

Volume 106, Issues 1–4, September 1991, Pages 64-72

https://doi.org/10.1016/0012-821X(91)90063-N Get rights and content

Abstract

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.

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Cited by (56)

Multi-stage garnet formation and destruction in Kimberley harzburgitic xenoliths, South Africa
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Thirty-nine garnet harzburgites from Kimberley in the Kaapvaal Craton (South Africa) were studied to constrain the origin, age and evolution of sub-cratonic lithospheric mantle (SCLM). In order to avoid chemical overprinting by recent metasomatism, only garnet harzburgites that appeared clinopyroxene-free to the naked eye were sampled. The majority of garnets were, however, in equilibrium with clinopyroxene (24 of 39). Whole rock and mineral major–trace element geochemistry and garnet Sr–Nd–Hf isotope data are presented. Equilibration pressures range from 3.8–6.1 GPa, indicating the harzburgites were derived from a large portion of the SCLM (~115–185 km). High olivine Mg# (~93.4, n = 39) and low whole rock heavy rare earth elements (HREE) contents are consistent with large degrees of partial melting (>45%) and garnet exhaustion leaving a dunitic residue with olivine ≥90%, orthopyroxene ≤10% and HREE <0.01 times chondrite. Mineral modes, whole rock Al₂O₃ (0.5–3.2 wt%) and SiO₂ (43.1–49.1 wt%), however, indicate heterogeneous re-introduction of garnet (≤13%) and orthopyroxene (≤50%). Harzburgites with high garnet and relatively low orthopyroxene modes (mostly ~7–13% and ~ 9–30%; n = 6) are characterised by mildly sinusoidal garnet REE patterns (Tb-Dy minimum and high HREE) and Archaean depleted Hf T_DM ages (2.7–3.3 Ga; ε_Hfe: +190 to +709). In contrast, harzburgites with high orthopyroxene and relatively low garnet and modes (~1.5–7.5% and ~ 25–50%; n = 19) are characterised by highly sinuous REE patterns (Ho-Yb minimum and low HREE) and Proterozoic enriched Hf T_DM ages (0.7–1.6 Ga; ε_Hfe: −16 to +6). It is inferred that Archaean G10 garnet re-introduction caused a significant increase in HREE, making melt depletion models based on HREE inaccurate. Orthopyroxene addition, a few hundred million years later, most likely at ~2.7 Ga and associated with Ventersdorp magmatic activity, caused partial consumption of garnet and olivine, and changed garnet compositions leading to: 1) Cr/Al ratio increase; 2) HREE decrease; 3) more sinusoidal REE patterns; and 4) un-radiogenic ¹⁷⁶Hf/¹⁷⁷Hf. Garnets define a Lu-Hf isochron age of 2702 ± 64 Ma (ε_Hfi = +44, n = 31), which is interpreted as a consequence of partial isotopic equilibrium within the SCLM and mixing of the garnet- and orthopyroxene-rich metasomatic components. The low LILE contents and absence of Nb-Ta anomalies are consistent with modal metasomatism caused by intra-plate magmatism. In addition, the REE signatures of metasomatic agents in equilibrium with the garnets suggest that carbonatitic melts and SiO₂-rich hydrous melts were responsible for re-introduction of garnet and orthopyroxene, respectively. Sr-Nd isotope systematics were disrupted associated with kimberlite magmatism (Nd isochron: 217 ± 58 Ma, ε_Ndi = +4; n = 34), consistent with recent G10 garnet transformation into G9 garnets (Ca + Fe-enriched). This event may have caused garnet addition (up to 1%), suggesting that garnet was formed or destroyed in at least 4 different events: i) initial extensive polybaric melting, ii) asthenospheric melts re-introducing the bulk of the garnet, iii) orthopyroxene addition and garnet loss, all in the Archaean, and iv) minor garnet addition possibly related to recent kimberlite magmatism prior to eruption.
Petrological, mineralogical and geochemical peculiarities of Archaean cratons
2019, Chemical Geology
Citation Excerpt :
A common feature of all samples (and most harzburgites from Kimberley) is the very coarse-grained nature of the orthopyroxene, often exceeding 10–15 mm in diameter. To further test the possibility that garnet in ordinary harzburgite originated by exsolution from formerly more aluminous orthopyroxene (e.g. Canil, 1991), we used automated phase mapping with large format SEM-EDS detectors to test the spatial relationship between garnet and orthopyroxene. The studied sample, 17BSK051, is a typical Kimberley field cratonic granular harzburgite, consisting of olivine, orthopyroxene and minor garnet with late accessory phlogopite.
The most outstanding features of Archaean cratons are their extraordinary thickness and enduring longevity. Seismically, Archaean cratonic fragments are sharply-bounded deep roots of mechanically strong, cold lithospheric mantle, clearly distinguishable from non-cratonic lithosphere. Rhenium-depletion of deep cratonic xenolith whole rocks and sulphide inclusions in diamond indicate that melting was broadly coeval with formation of the overlying proto-cratonic crust, which was of limited mechanical strength.
A very important process of proto-cratonic development was vertical crustal reorganisation that eventually yielded a thermally stable, cratonised crust with a highly K-U-Th-rich uppermost crust and much more depleted deeper crust. Clastic sedimentary rocks available for geochemical study are predominantly found in the youngest parts of supracrustal stratigraphies and over-represent the highly evolved rocks that appeared during cratonisation. Vertical crustal reorganisation was driven by crustal radiogenic heat and emplacement of proto-craton-wide, incubating and dense supracrustal mafic and ultramafic volcanic rocks. Statistical analysis of these cover sequences shows a preponderance of basalt and a high abundance of ultramafic lavas with a dearth of picrite. The ultramafic lavas can be grouped into Ti-enriched and Ti-depleted types and high pressure and temperature experimental data indicate that the latter formed from previously depleted mantle at temperatures in excess of 1700 °C.
Most mantle harzburgite xenoliths from cratonic roots are highly refractory, containing very magnesian olivine and many have a high modal abundance of orthopyroxene. High orthopyroxene mode is commonly attributed to metasomatic silica-enrichment or a non-pyrolitic mantle source but much of the excess silica requirement disappears if melting occurred at high pressures of 4–6 GPa. Analysis of experimental data demonstrates that melting of previously depleted harzburgite can yield liquids with highly variable Si/Mg ratios and low Al₂O₃ and FeO contents, as found in komatiites, and complementary high Cr/Al residues. In many harzburgites, there is an intimate spatial association of garnet and spinel with orthopyroxene, which indicates formation of the Al-phase by exsolution upon cooling and decompression. New and published rare earth element (REE) data for garnet and orthopyroxene show that garnet has inherited its sinusoidal REE pattern from the orthopyroxene. The lack of middle-REE depletion in these refractory residues is consistent with the lack of middle- over heavy-REE fractionation in most komatiites. This suggests that such pyroxene or garnet (or precursor phases) were present during komatiite melting. In the Kaapvaal craton, garnet exsolution upon significant cooling occurred as early as 3.2 Ga and geobarometry of diamond inclusions from ancient kimberlites also supports cool Archaean cratonic geotherms. This requires that some mantle roots have extended to 300 to possibly 400 km and that early cratons must have been much larger than 500 km in diameter.
We maintain that the Archaean-Proterozoic boundary continues to be of geological significance, despite the recognition that upper crustal chemistry, as sampled by sedimentary rocks, became more evolved from ca. 3 Ga onwards. The boundary coincides with the disappearance of widespread komatiite and marks the end of formation of typical refractory cratonic lithosphere. This may signify a fundamental change in the thermal structure of the mantle after which upwellings no longer resulted in very high temperature perturbations. One school of thought is that the thermal re-ordering occurred at the core-mantle boundary whereas others envisage Archaean plumes to have originated at the base of the upper mantle. Here we speculate that Archaean cratonic roots may contain remnants of older domains of non-convecting mantle. These domains are potential carriers of isotope anomalies and their base could have constituted a mechanical and thermal boundary layer. Above laterally extensive barriers, emerging proto-cratons were protected from the main mantle heat loss. The eventual collapse of these mechanical barriers terminated very high temperature upwellings and dismembered portions of the barrier were incorporated into the cratonic mantle during the final Neoarchaean ‘superplume’ event. The surviving cratons may therefore preserve biased evidence of geological processes that operated during the Archaean.
Plume impingement on the Siberian SCLM: Evidence from Re-Os isotope systematics
2015, Lithos
We report Re–Os and platinum group element (PGE) systematics for a suite of 16 mantle peridotites from the Udachnaya (360 Ma) and Obnazhennaya (160 Ma) kimberlite pipes, Siberia. Xenoliths from these pipes bracket the thermal climax of the Siberian plume, which is represented by the emplacement of the ~ 250 Ma Siberian Flood Basalts (SFBs). Thus, these xenoliths represent snapshots of the sub-continental lithospheric mantle (SCLM) before and after plume modification. Pre-plume Udachnaya peridotite xenoliths generally display unradiogenic Os-isotopes with respect to CI-chondrite (expressed by γOs, the percentage difference between the Os-isotope composition of a sample and the average chondrite composition; ¹⁸⁷Os/¹⁸⁸Os — 0.127), coupled with low [Pd/Ir]_N, for both whole-rock and olivine mineral-fraction analyses. Such signatures are typical of an ancient depleted cratonic mantle that underwent melt extraction. The preservation of unradiogenic Os-isotope compositions (γOs − 5 to − 14), coupled with low (< 0.4) ¹⁸⁷Re/¹⁸⁸Os ratios, provides robust melt extraction age estimates, ranging from ~ 3 Ga to ~ 1.2 Ga. This indicates that craton stabilization/growth events not only occurred during the Archean, but also extended into the Proterozoic. A number (4) of post-plume Obnazhennaya peridotites display ¹⁸⁷Os/¹⁸⁸Os ratios (> 0.1292), which overlap the convecting mantle range. At first glance, these observations are in agreement with garnet chemistry data, which indicate that high-degrees of silicate-melt percolated through the lithosphere during the emplacement of the SFB. However, Obnazhennaya olivine mineral-separates display ‘depleted’ systematics (> Fo 92 and low [Pd/Ir]_N), consistent with ‘pristine’ melt residues. We suggest that these Obnazhennaya xenoliths represent ‘newly formed’ residues associated with partial melts extracted from the impinging Siberian plume on the SCLM. During plume impingement, thermo-chemical erosion of the lithosphere is thought to be an important process, acting to remove the original depleted material. In the space created, new refractory residues may be able to infill this void; i.e., plume-subcretion. Importantly, two Obnazhennaya peridotites display very unradiogenic Os-isotope compositions (γOs − 9 and − 10) indicating that ancient depleted lithosphere is preserved after the emplacement of the SFB. Overall, we suggest that plume impingement may not be ubiquitous within the SCLM, leaving a lithosphere that is characterized by both its original depleted residues and newly formed, plume related residues.
In addition, two samples from Obnazhennaya and one from Udachnaya, contain highly radiogenic ¹⁸⁷Os/¹⁸⁸Os ratios (γOs > + 85). Such radiogenic values cannot be accounted for by kimberlitic or plume-related metasomatism. Instead, these samples reflect metasomatism from fluids that derived from a source characterized by long-term high ¹⁸⁷Re/¹⁸⁸Os, such as recycled subducted material (i.e., oceanic crust and sediments). These lithologies were introduced into the SCLM during craton growth events from ~ 1.8 to 3.0 Ga. The identification of such samples reflects an important metasomatic source within the SCLM. Overall, these two suites of mantle peridotites reveal the complex evolution of the Siberian cratonic lithosphere, from birth in the Archean, followed by major depletion events during the Proterozoic, and ending with major tectonothermal perturbation during the impingement of the Siberian plume.
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Fragments of the Earth’s mantle are frequently transported to the surface via volcanic rocks that are dominantly alkaline in nature. These fragments range up to sizes in excess of 1 m across. The term “mantle xenoliths” or “mantle nodules” is applied to all rock and mineral inclusions of presumed mantle derivation that are found within host rocks of volcanic origin. The purpose of this contribution is to review the geochemistry of mantle xenoliths.
The Formation and Evolution of Cratonic Mantle Lithosphere - Evidence from Mantle Xenoliths
2013, Treatise on Geochemistry: Second Edition
The fragments of Earth's mantle (xenoliths) that erupted in deeply derived volcanic rocks provide unique windows into the chemistry of the deep Earth and the formation of the earliest continents. They also provide one of the most direct ways of estimating the primitive composition of the Earth's mantle. The geochemical measurements of mantle xenoliths beneath the oldest parts of the continents’ ‘cratons’ can be used to constrain the age, depth of formation, and hence tectonic environment that led to the formation of deep, stable lithospheric keels extending to over 200 km. The processes that obscure the primary geochemical signatures in mantle xenoliths in the context of new stable isotopic variations are reviewed. New compositional versus depth data are compiled that are used along with recent parameterizations of polybaric mantle melting to evaluate models for the formation of deep cratonic keels.

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

Abstract

Earth Planet. Sci. Lett.

Chem. Geol.

Geochim. Cosmochim. Acta

Tectonophysics

Geochim. Cosmochim. Acta

Phys. Chem. Earth

Majorite fractionation recorded in the geochemistry of peridotites from South Africa

Nature

Speculations on the Archean mantle: missing link between komatiite and depleted garnet peridotite

J. Geophys. Res.

Textural studies of garnet lherzolites: evidence of exsolution from high temperature harzburgites