Petrologic and thermal structure of the upper mantle beneath South Africa in the cretaceous

doi:10.1016/0079-1946(75)90033-6

Physics and Chemistry of the Earth

Volume 9, 1975, Pages 455-466

https://doi.org/10.1016/0079-1946(75)90033-6 Get rights and content

Abstract

Assignment of equilibration temperatures and pressures to ultramafic xenoliths from South African kimberlites illustrates that they have been derived from a wide range of depths. The distribution predicates against a cognate hypothesis and indicates that the xenoliths are accidental fragments of the mantle transported to the surface by the kimberlite magma. Suites of xenoliths from single localities show well-defined trends marking Cretaceous geothermal gradients. In general the gradients are comparable to the steady-state model of Clark and Ringwood (1964). The gradients are steepest in the South West Africa region and decrease successively in the Lesotho and Kimberley regions. In all regions there is a sudden steepeing of the interpreted geothermal gradient correlating with presence of intensively sheared xenoliths. It appears that the associated change of slope and textural type marks the top of the low-velocity zone under the shield, giving lithospheric thicknesses which increase from 140 km to 180 and 195 km in the South West Africa, Lesotho and Kimberley regions, respectively. The maximum depth of xenoliths in any locality suggests that kimberlites are derived from minimum depths of 150, 190 and 200 km in the South West Africa, Lesotho and Kimberley regions, respectively.

In general, spinel-bearing xenoliths give place to spinel-plus-garnet- and garnet-bearing xenoliths with increasing depth. The boundaries are diffuse and dependent on compositional variations. Major textural breaks correlate with the disappearance of phlogopite and a peridotite $H_{2} O CO_{2}$ saturated solidus. The phlogopite dehydration and melting reactions mark boundaries of increasing deformation, suggesting that these reactions are critical in defining the rheid properties of the mantle.

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Diamondiferous Paleoproterozoic mantle roots beneath Arctic Canada: A study of mantle xenoliths from Parry Peninsula and Central Victoria Island
2018, Geochimica et Cosmochimica Acta
Citation Excerpt :
Identifying corresponding melts produced from the high degrees of melting is, however, problematic due to the overall paucity of exposed ultramafic magmatic rocks. While it has been widely accepted that there are thermal differences between cratonic and circum-cratonic lithosphere in the Kaapvaal craton and surrounding mobile belts (MacGregor, 1975; Boyd and Gurney, 1986), more recent work has shown that in fact the paleogeotherms derived from mantle xenoliths erupted through Paleoproterozoic terrains at the time of kimberlite eruption were broadly similar within the interior of the craton and around the margin of the craton (e.g., Boyd et al., 2004; Janney et al., 2010; Mather et al., 2011). The main difference in the xenolith-derived paleogeotherms in southern Africa seems to be recent evidence for thinning associated with a localized basal thermal disturbance that did not have time to re-equilibrate the shallower portions of the mantle geotherm.
While the mantle roots directly beneath Archean cratons have been relatively well studied because of their economic importance, much less is known about the genesis, age, composition and thickness of the mantle lithosphere beneath the regions that surround the cratons. Despite this knowledge gap, it is fundamentally important to establish the nature of relationships between this circum-cratonic mantle and that beneath the cratons, including the diamond potential of circum-cratonic regions. Here we present mineral and bulk elemental and isotopic compositions for kimberlite-borne mantle xenoliths from the Parry Peninsula and Central Victoria Island, Arctic Canada. These xenoliths provide key windows into the lithospheric mantle underpinning regions to the North and Northwest of the Archean Slave craton, where the presence of cratonic material has been proposed. The mantle xenolith data are supplemented by mineral concentrate data obtained during diamond exploration. The mineral and whole rock chemistry of peridotites from both localities is indistinguishable from that of typical cratonic mantle lithosphere. The cool mantle paleogeotherms defined by mineral thermobarometry reveal that the lithospheric mantle beneath the Parry Peninsula and Central Victoria Island terranes extended well into the diamond stability field at the time of kimberlite eruption, and this is consistent with the recovery of diamonds from both kimberlite fields. Bulk xenolith Se and Te contents, and highly siderophile element (including Os, Ir, Pt, Pd and Re) abundance systematics, plus corresponding depletion ages derived from Re-Os isotope data suggest that the mantle beneath these parts of Arctic Canada formed in the Paleoproterozoic Era, at ∼2 Ga, rather than in the Archean. The presence of a diamondiferous Paleoproterozoic mantle root is part of the growing body of global evidence for diamond generation in mantle roots that stabilized well after the Archean. In the context of regional tectonics, we interpret the highly depleted mantle compositions beneath both studied regions as formed by mantle melting associated with hydrous metasomatism in the major Paleoproterozoic Wopmay-Great Bear-Hottah arc systems. These ∼2 Ga arc systems were subsequently accreted along the margin of the Slave craton to form a craton-like thick lithosphere with diamond potential thereby demonstrating the importance of subduction accretion in building up Earth’s long-lived continental terranes.
Contrasting types of metasomatism in dunite, wehrlite and websterite xenoliths from Kimberley, South Africa
2008, Geochimica et Cosmochimica Acta
Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal Craton in southern Africa. All three types were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment. The melt depletion event, dated by Re–Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# (Mg/(Mg + Fe)) of silicate minerals (olivine (0.89–0.93); pyroxene (0.88–0.93); garnet (0.72–0.85)), high Cr# (Cr/(Cr + Al)) of spinel (0.53–0.84) and mostly low whole-rock SiO₂, CaO and Al₂O₃ contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt (>60 wt% SiO₂) derived by partial melting of eclogite before or during cratonization. The melt–peridotite interaction converted olivine into orthopyroxene.
All three xenolith types have secondary metasomatic clinopyroxene and garnet, which occur along olivine grain boundaries and have an amoeboid texture. As indicated by the preservation of oxygen isotope disequilibrium in the minerals and trace-element concentrations in clinopyroxene and garnet, this metasomatic event is probably of Mesozoic age and was caused by percolating alkaline basaltic melts. This melt metasomatism enriched the xenoliths in CaO, Al₂O₃, FeO and high-field-strength-elements, and might correspond to the Karoo magmatism at 200 Ma. The websterite xenoliths experienced both the orthoyproxene-enrichment and clinopyroxene–garnet metasomatic events, whereas dunite and wehrlite xenoliths only saw the later basaltic melt event, and may have been situated further away from the source of melt migration channels.
Garnet lherzolites from Louwrensia, Namibia: Bulk composition and P/T relations
2004, Lithos
Bulk, mineral and trace element analyses of garnet lherzolite xenoliths from the Louwrensia kimberlite pipe, south-central Namibia, together with previously published Re–Os isotopic data [Chem. Geol. (2004)], form the most extensive set of chemical data for off-craton suites from southern Africa. The Louwrensia suite is similar to those from the Kaapvaal craton in that it includes both predominantly coarse-grained, equant-textured peridotites characterised by equilibration temperatures <1100 °C and variably deformed peridotites, frequently sheared, with higher equilibration temperatures >1200 °C. Re-depletion ages range back to 2.1 Gy, concordant with the age of the crustal basement and about 1 Gy younger than the older peridotites of the adjacent Kaapvaal craton root. The coarse, low-temperature Louwrensia peridotites have an average Mg number for olivine of 91.6 in comparison to 92.6 for low-temperature peridotites from the craton. Orthopyroxene content averages 24 wt.% with a range of 11–40 wt.% for Louwrensia low-temperature peridotites, in comparison to a mean of 31.5 wt.% and a range of 11–44 wt.% for low-temperature peridotites from the Kaapvaal craton. Other major, minor and trace element concentrations in minerals forming Louwrensia lherzolites are more similar to values in corresponding Kaapvaal peridotite minerals than to those in lithospheric peridotites of Phanerozoic age as represented by off-craton basalt-hosted xenoliths and orogenic peridotites. Proportions of clinopyroxene and garnet in both the Louwrensia and Kaapvaal lherzolites overlap in the range up to 10 wt.% forming a trend extending towards pyrolite composition. Disequilibrium element partitioning between clinopyroxene and garnet for some incompatible trace elements is evidence that some of the trend is caused by enrichment following depletion. The disequilibrium is interpreted to have been caused by relatively recent growth of diopside, as previously suggested for cratonic peridotites. Attempts to constrain the depth of melting required to produce the Louwrensia peridotites suggests formation at pressures <3 GPa. Estimates of temperature and depth of equilibration for low-temperature Louwrensia lherzolites yield a trend that is in approximate agreement with the average geotherm for the Kaapvaal craton and may indicate that the lithospheric mantle beneath this region was at one time as thick as that beneath the craton (>200 km). Temperature–depth plots for the high-temperature Louwrensia rocks, however, form pronounced, apparent higher-temperature thermal anomalies at depths of 140 km and above. These anomalies are believed to reflect regional igneous activity, perhaps associated with thermal erosion of an originally thicker lithosphere, a short time prior to eruption.
Mesozoic thermal evolution of the southern African mantle lithosphere
2003, Lithos
The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150–200 °C isobaric temperature range at 5–6 GPa confirms original interpretations of a conductive geotherm, which is perturbed at depth, and therefore does not record steady state lithospheric mantle structure.
Xenoliths from both Archean and Proterozoic terranes record conductive limb temperatures characteristic of a “cratonic” geotherm (∼40 mW m⁻²), indicating cooling of Proterozoic mantle following the last major tectonothermal event in the region at ∼1 Ga and the probability of thick off-craton lithosphere capable of hosting diamond. This inference is supported by U–Pb thermochronology of lower crustal xenoliths [Schmitz and Bowring, 2003. Contrib. Mineral. Petrol. 144, 592–618].
The entire region then suffered a protracted regional heating event in the Mesozoic, affecting both mantle and lower crust. In the mantle, the event is recorded at ∼150 Ma to the southeast of the craton, propagating to the west by 108–74 Ma, the craton interior by 85–90 Ma and the far southwest and northwest by 65–70 Ma. The heating penetrated to shallower levels in the off-craton areas than on the craton, and is more apparent on the southern margin of the craton than in its western interior. The focus and spatial progression mimic inferred patterns of plume activity and supercontinent breakup 30–100 Ma earlier and are probably connected.
Contrasting thermal profiles from Archean and Proterozoic mantle result from penetration to shallower levels of the Proterozoic lithosphere by heat transporting magmas. Extent of penetration is related not to original lithospheric thickness, but to its more fertile character and the presence of structurally weak zones of old tectonism. The present day distribution of surface heat flow in southern Africa is related to this dynamic event and is not a direct reflection of the pre-existing lithospheric architecture.
The earth's thermal profile: Is there a mid-mantle thermal boundary layer?
1984, Journal of Geodynamics
Shock observations on melting of iron by Brown and McQueen with the inner core boundary (ICB) density contrast estimated by Masters are used with the assumption that the light ingredient of the outer core is oxygen to calculate the boundary temperature T_ICB = (5000 ± 900) K. Adiabatic extrapolation to the core-mantle boundary (CMB) gives T_ICB = (3800 ± 800) K. The temperature increment across the D″ layer is not well constrained, but is estimated to be T_D″ = (800 ± 400) K and a slightly superadiabatic extrapolation to 670 km gives T_{670 +} = (2300 ± 950) K. This is only about 300 K higher than the extrapolation to the same level from the upper mantle, T₆₇₀₋ = (1970 ± 150) K. The difference is far too small to make a viable mid-mantle boundary layer. Remaining unceertainties are too large to discount such a boundary layer with certainty, but agreement of our new temperature profile with temperatures deduced from equation of state studies on the lower mantle and core encourages the view that we are converging to a well-determined temperature profile for the Earth.
Geotherms, evolution of the lithosphere and plate tectonics
1981, Tectonophysics
For the purpose of calculating Precambrian geotherms, modern “average” geotherms need to be defined. Curves calculated from estimated plate sections are close to those of Clark and Ringwood (1964). Thicknesses of lithospheric plates at any time in the past can be estimated from “fossil” geotherms. Because subduction of oceanic plates depends upon their density, average densities of Precambrian plates should place important constraints on the beginning of subduction. This method, however, only indicates that in all probability subduction did not begin before 1000 Ma ago. Geological data suggest a date of about 800 or 900 Ma ago. Formation of Archean continents would have proceeded from partial melting of basaltic crust, and the end of that phase of earth's evolution would be a thermally controlled feedback mechanism lowering geothermal gradients in sub-continental mantle. The beginning of subduction was also thermally controlled, and probably accompanied the increasing stability of eclogite in the upper mantle.

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Petrologic and thermal structure of the upper mantle beneath South Africa in the cretaceous

Abstract

Geochim. Cosmochim. Acta

Earth Sci. Rev.

Hydrolitic weakening of quartz and olivine

Phase relationships in natural and synthetic peridotite-H2O and peridotite-H2OCO2 systems at high pressures

Phys. Chem. Earth

Texture and fabric of peridotite nodules from kimberlite at Mothae, Thaba Putsoa and Kimberley

Garnet peridotites and the system CaSiO3Al2O3

Mineral. Soc. Amer. Spec. Pap.

Structure of the upper mantle beneath Lesotho

Carnegie Inst. Yearb.

Diamond-graphite equilibrium line from growth and graphitization of diamond

J. Chem. Phys.

Density distribution and constitution of the mantle

Rev. Geophys.

On diamondiferous diatremes

Econ. Geol.

The so-called “cognate xenoliths” of kimberlite

The join Mg2Si2O6CaMgSi2O6 at 30 kb pressure and its application to pyroxenes from kimberlite

J. Geophys. Res.

The tectonic setting of African kimberlite magmatism

Phase relationships in natural and synthetic peridotite-H₂O and peridotite-H₂OCO₂ systems at high pressures

Garnet peridotites and the system CaSiO₃Al₂O₃

The join Mg₂Si₂O₆CaMgSi₂O₆ at 30 kb pressure and its application to pyroxenes from kimberlite