Abstract
Earth’s lithospheric mantle is heterogenous in age, composition and rheology. Refractory mantle lithospheres are those that have experienced high levels of melt extraction, resulting in depletion in major and trace elements. These are commonly recognised from Archean cratons, where the cool and buoyant residues underpin and support the longevity of these areas. However, there are increasing occurrences of refractory mantle being documented in post-Archean environments. Here, I document and examine the evolution of some of these apparently Archean mantle lithospheres, from New Zealand, South America and South Africa, with the goal of addressing whether these are relicts of Archean lithospheres or formed in modern tectonic settings.
Peridotite xenoliths from the West Otago refractory mantle domain, which underlies at least a portion of the Southern Alps of New Zealand, have refractory to ultra-refractory olivine Mg# ~91-93, spinel Cr# often > 70, bulk Al2O3 < 1 wt% and depletion in HREE compared to LREE. Despite this lithosphere having compositions similar to those observed in global cratonic peridotite suites, Re-depletion model bulk ages are typically Phanerozoic and do not correspond to melt extraction from the same event. 87Sr/86Sr 24 Ma isotopic values of 0.70282-0.70305 and measured 143Nd/144Nd of 0.51282-0.512911 from West Otago clinopyroxene also point towards Phanerozoic melt extraction and lithosphere stabilisation. It is interpreted that this ~2500 km2 West Otago depleted lithosphere may have formed by accumulated depleted residues experiencing partial melting under hydrous conditions and subsequent lateral accretion of residues at the eastern Gondwana paleo-subduction zone.
The West Otago suite is metasomatised to variable degrees and displays enrichment of LREE in rare clinopyroxene up to two orders of magnitude higher relative to HREE, enrichment in Fe, Ti, K and Al, most notable in the Fish River peridotites, which contain veins of phlogopite, amphibole and clinopyroxene. Amphibole in the sub-continental lithospheric mantle beneath Zealandia and other metasomatically introduced phases have 87Sr/86Sr 24 Ma values between 0.7027 to 0.7031, which are equivalent to other metasomatized peridotites from West Otago. The presence of hydrous metasomes within refractory lithospheric mantle has implications for the source of the intraplate magmas and non-modal batch melting models, which are able to plausibly replicate trace element compositions of existing intraplate magmas in the region. The West Otago refractory mantle appears to have formed post-Archean but also contains fusible zones that may have played an important role in the genesis of intraplate basaltic magmas.
Garnet-facies peridotite xenoliths from the São Francisco Craton in South America have olivine Mg# < 93, low bulk Al2O3 < 1 wt% and 187Os/188Osi = 0.11202–0.11916 (Limeira) and 0.10964–0.11576 (Redondão) that equate to Mesoproterozoic minimum TRD ages. Geothermobarometry indicates that the lithospheric thickness from which these two xenolith suites were exhumed extended to > 150 km at the time of eruption. The occurrence of thick, refractory, mainly Proterozoic refractory lithosphere extending outside the known surface extent of the SFC suggests that the cratonic nucleus is significantly larger than recognised at the surface. I propose that the current limits of the SFC should be revised to consider deep lithosphere to the north and southwest edges, despite these areas having younger rocks at the surface.
The Jagersfontein kimberlite in the Kaapvaal Craton in South Africa has erupted a suite of refractory peridotite xenoliths with anomalous Re-Os isotope systematics and platinum group element (PGE) abundances. TRD ages range between 2.7 to 3.0 Ga and have refractory major element and PGE abundances typical of Archean cratonic nuclei; however, some Jagersfontein peridotites have more radiogenic 187Os/188Os and significant enrichment in Os, Ir, Pd and especially Pt and Ru relative to the Archean-aged samples. Some Ru abundances reach up to ~ 50 × “Primitive Upper Mantle” levels and correspond to samples containing microscopic laurite grains. The enrichment can be modelled via the addition of small volumes of PGE-rich sulphide/alloy contaminant to a refractory Archean peridotite composition. Modelled melts with an 187Os/188Os ratio of ~0.116, equivalent to the convecting mantle at ~1.9 Ga, suggest the Archean lithospheric mantle beneath Jagersfontein has been overprinted by PGE-saturated mantle fluids in the Paleoproterozoic which may have been related to Bushveld-Molopo Farms magmatic event. The Kaapvaal Craton is therefore interpreted to be underlain by a Phanerozoic-overprinted Archean refractory lithosphere.
This thesis has shown that refractory mantle xenolith suites can form in different modes, with some being Archean and overprinted in the Phanerozoic (South Africa), others being Proterozoic in age (South America) and others forming via subduction zones in the modern Earth.