Eclogite xenoliths from Orapa: Ocean crust recycling, mantle metasomatism and carbon cycling at the western Zimbabwe craton margin

Aulbach, Sonja; Jacob, Dorrit E.; Cartigny, Pierre; Stern, Richard A.; Simonetti, Stefanie S.; Viljoen, K.S.

doi:10.1016/j.gca.2017.06.038

Cited by 36 publications

(15 citation statements)

References 122 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…We note that the resetting temperature and pressure cluster around a mantle geotherm associated with surface heat flow of 45 mW/m 2 , appropriate for ambient conditions at the time of the kimberlite eruption, so they may indicate that the xenolith was incorporated at the time of eruption (Litasov et al, 2003). On the other hand, the measured remanent inclusion pressures definitely exclude the possibility that the studied mantle eclogite xenoliths are the product of direct high-pressure magma crystallization at mantle depths, as already supported by geochemical evidence for other xenoliths (Griffin and O'Reilly, 2007;Aulbach et al, 2017). Direct crystallization at depth would result in either coesite inclusions, or inclusions whose remanent pressures would indicate entrapment within the stability field of quartz.…”

Section: Eclogite Xenoliths: Magmatic or Metamorphic?supporting

confidence: 65%

Fossil subduction recorded by quartz from the coesite stability field

Alvaro

Mazzucchelli

Angel

et al. 2019

Geology

View full text Add to dashboard Cite

Metamorphic rocks are the records of plate tectonic processes whose reconstruction relies on correct estimates of the pressures and temperatures (P-T) experienced by these rocks through time. Unlike chemical geothermobarometry, elastic geobarometry does not rely on chemical equilibrium between minerals, so it has the potential to provide information on overstepping of reaction boundaries and to identify other examples of non-equilibrium behavior in rocks. Here we introduce a method that exploits the anisotropy in elastic properties of minerals to determine the unique P and T of entrapment from a single inclusion in a mineral host. We apply it to preserved quartz inclusions in garnet from eclogite xenoliths hosted in Yakutian kimberlites (Russia). Our results demonstrate that quartz trapped in garnet can be preserved when the rock reaches the stability field of coesite (the high-pressure and high-temperature polymorph of quartz) at 3 GPa and 850 °C. This supports a metamorphic origin for these xenoliths and sheds light on the mechanisms of craton accretion from a subducted crustal protolith. Furthermore, we show that interpreting P and T conditions reached by a rock from the simple phase identification of key inclusion minerals can be misleading.

show abstract

Section: Eclogite Xenoliths: Magmatic or Metamorphic?supporting

confidence: 65%

Fossil subduction recorded by quartz from the coesite stability field

Alvaro

Mazzucchelli

Angel

et al. 2019

Geology

View full text Add to dashboard Cite

show abstract

“…A websteritic inclusion suite was first recognized in diamonds from the Orapa kimberlite (Gurney et al 1984) and documents the presence of an unusually wide spectrum of "mafic" diamond substrates in the local lithospheric mantle. This is also evident from the compositions of a large suite of eclogite and pyroxenite xenoliths from the Orapa (Aulbach et al 2017). In Karowe, the presence of orthopyroxene-bearing (websteritic) substrates is evidenced by a single enstatite inclusion with low Mg# (57).…”

Section: Discussionmentioning

confidence: 90%

“…Mineralogical variability in the eclogitic substrates is also highlighted by the occasional presence of kyanite and coesite inclusions. Kyanite occurs in Group I diamondiferous eclogites from Orapa (Shee and Gurney 1979) and was recently described in Group II diamond-free eclogites by Aulbach et al (2017). It is a characteristic phase of aluminous eclogites in general (Spetsius 2004;Shu et al2016).…”

Section: Discussionmentioning

confidence: 95%

Mineral inclusions in diamonds from Karowe Mine, Botswana: super-deep sources for super-sized diamonds?

Motsamai¹,

Harris²,

Stachel³

et al. 2018

Miner Petrol

View full text Add to dashboard Cite

Mineral inclusions in diamonds play a critical role in constraining the relationship between diamonds and mantle lithologies. Here we report the first major and trace element study of mineral inclusions in diamonds from the Karowe Mine in northeast Botswana, along the western edge of the Zimbabwe Craton. From a total of 107 diamonds, 134 silicate, 15 oxide, and 22 sulphide inclusions were recovered. The results reveal that 53 % of Karowe inclusion-bearing diamonds derived from eclogitic sources, 44 % are peridotitic, 2 % have a sublithospheric origin, and 1 % are websteritic. The dominant eclogitic diamond substrates sampled at Karowe are compositionally heterogeneous, as reflected in wide ranges in the CaO contents (4-16 wt%) of garnets and the Mg# (69-92) and jadeite contents (14-48 mol%) of clinopyroxenes. Calculated bulk rock REEN patterns indicate that both shallow and deep levels of the subducted slab(s) were sampled, including cumulate-like protoliths. Peridotitic garnet compositions largely derive from harzburgite/dunite substrates (~90 %), with almost half the garnets having CaO contents <1.8 wt %, consistent with pyroxene-free (dunitic) sources. The highly depleted character of the peridotitic diamond substrates is further documented by the high mean and median Mg# (93.1) of olivine inclusions. One low-Ca garnet records a very high Cr2O3 content (14.7 wt%), implying that highly depleted cratonic lithosphere at the time of diamond formation extended to at least 220 km depth. Inclusion geothermobarometry indicates that the formation of peridotitic diamonds occurred along a 39-40 mW/m 2 model geotherm. A sublithospheric inclusion suite is established by three eclogitic garnets containing a majorite component, a feature so far unique within the Orapa cluster. These low-and high-Ca majoritic garnets follow pyroxenitic and eclogitic trends of majoritic substitution, respectively. The origin of the majorite-bearing diamonds is estimated to be between 330 to 420 km depth, straddling the asthenosphere-transition zone boundary. This new observation of superdeep mineral inclusions in Karowe diamonds is consistent with a sublithospheric origin for the exceptionally large diamonds from this mine.

show abstract

“…Since δ 18 O in garnet from unenriched eclogites from the northern Slave, Lace, and Koidu kimberlites are largely within the canonical mantle range, they cannot be used to assess the effect of mantle metasomatism. There is, however, some evidence that this has occurred in the sample suite from Orapa, where mantle‐like δ 18 O is associated with radiogenic Sr ascribed to overprint by a small‐volume melt derived from an aged lithospheric mantle metasome (Aulbach et al, 2017). Figure 3a shows that UM carbonated melt metasomatism is accompanied by an increase in Cr 2 O 3 , suggested to reflect stealth addition of Cr 2 O 3 ‐rich pyroxene.…”

Section: Discussionmentioning

confidence: 99%

“…In the Slave craton, interaction with a UM carbonated melt produced extremely diamond‐rich high‐Mg eclogites or pyroxenites (Smart et al, 2009), and it has been suggested based on experimental observations that both monocrystalline and fibrous lithospheric diamonds form by redox reactions from carbonated fluids or melts (Bureau et al, 2018). Conversely, in mantle eclogites from Orapa and Koidu, the diamond‐destructive nature of UM carbonated melt metasomatism is indicated by the association of diamond with high‐Ca and low‐Mg eclogites, but absence in LREE‐enriched high‐Mg eclogites and pyroxenites (Aulbach, Jacob, et al, 2017; Aulbach, Woodland, et al, 2019). Indeed, it has been shown experimentally that interaction with oxidizing small‐volume carbonated silicate melts, such as kimberlite, eventually causes diamond resorption and associated carbon remobilization at mantle depth (Fedortchouk et al, 2019).…”

Section: Discussionmentioning

confidence: 99%

Ultramafic Carbonated Melt‐ and Auto‐Metasomatism in Mantle Eclogites: Compositional Effects and Geophysical Consequences

Aulbach

Massuyeau

Garber

et al. 2020

Geochem Geophys Geosyst

Self Cite

View full text Add to dashboard Cite

The mineralogy, chemical composition, and physical properties of cratonic mantle eclogites with oceanic crustal protoliths can be modified by secondary processes involving interaction with fluids and melts, generated in various slab lithologies upon subduction (auto-metasomatism) or mantle metasomatism after emplacement into the cratonic lithosphere. Here we combine new and published data to isolate these signatures and evaluate their effects on the chemical and physical properties of eclogite. Mantle metasomatism involving kimberlite-like, ultramafic carbonated melts (UM carbonated melts) is ubiquitous though not pervasive, and affected between~20% and 40% of the eclogite population at the various localities investigated here, predominantly at~60-150 km depth, overlapping cratonic midlithospheric seismic discontinuities. Its hallmarks include lower jadeite component in clinopyroxene and grossular component in garnet, an increase in bulk-rock MgO ± SiO 2 , and decrease in FeO and Al 2 O 3 contents, and LREE-enrichment accompanied by higher Sr, Pb, Th, U, and in part Zr and Nb, as well as lower Li, Cu ± Zn. This is mediated by addition of a high-temperature pyroxene from a UM carbonated melt, followed by redistribution of this component into garnet and clinopyroxene. As clinopyroxene-garnet trace-element distribution coefficients increase with decreasing garnet grossular component, clinopyroxene is the main carrier of the metasomatic signatures. UM carbonated melt-metasomatism at >130-150 km has destroyed the diamond inventory at some localities. These mineralogical and chemical changes contribute to low densities, with implications for eclogite gravitational stability, but negligible changes in shear-wave velocities, and, if accompanied by H 2 O-enrichment, will enhance electrical conductivities compared to unenriched eclogites. Plain Language SummaryOceanic crust formed at spreading ridges is recycled in subduction zones and undergoes metamorphism to eclogite. Some of this material is captured in the overlying lithospheric mantle, where it is exhumed by passing magmas. Having formed in spreading ridges, these eclogites have proven invaluable archives for the onset of plate tectonics, for the construction of cratons during subduction/collision, as probes of the convecting mantle from which their precursors formed, and as generators of heterogeneity upon recycling into Earth's convecting mantle. During subduction and until exhumation, interaction with fluids and melts (called metasomatism) can change the mineralogy, chemical composition, and physical properties of mantle eclogites, complicating their interpretation, but a comprehensive study of these effects is lacking so far. We investigated mantle eclogites from ancient continents (cratons) around the globe in order to define hallmarks of metasomatism by subduction-related fluids and small-volume ultramafic carbonated mantle melts. We find that the latter is pervasive and occurs predominantly at midlithospheric depths where seismic discontinuities are detected, ...

show abstract

Eclogite xenoliths from Orapa: Ocean crust recycling, mantle metasomatism and carbon cycling at the western Zimbabwe craton margin

Cited by 36 publications

References 122 publications

Fossil subduction recorded by quartz from the coesite stability field

Fossil subduction recorded by quartz from the coesite stability field

Mineral inclusions in diamonds from Karowe Mine, Botswana: super-deep sources for super-sized diamonds?

Ultramafic Carbonated Melt‐ and Auto‐Metasomatism in Mantle Eclogites: Compositional Effects and Geophysical Consequences

Contact Info

Product

Resources

About