Constraints on the thermal conditions within the cratonic lithospheric mantle through time
Citation:
Hoare, Brendan Colin, Constraints on the thermal conditions within the cratonic lithospheric mantle through time, Trinity College Dublin.School of Natural Sciences, 2022Download Item:
Abstract:
The stabilization of Earth s first continents is inescapably linked to the formation of the ancient cratonic lithospheric mantle. The cratonic lithospheric mantle is widely considered to have formed as a residue of extreme degrees of melting of the early mantle, which is not seen today. Such high degrees of melting inevitably lead to extreme depletion in Al2O3, CaO, FeO, Na2O (i.e., basaltic components) and incompatible trace elements including the major heat-producing elements (HPE), K, Th and U. The depletion of HPE allows for the stabilization of a cool lithosphere whilst the extraction of dense basaltic components promotes a positive buoyancy that has been suggested to offset the negative buoyancy imparted from low temperatures observed in the thick cratonic root; the so-called isopycnic hypothesis (Jordan, 1988). The longevity of Earth s cratons has led to several implicit assumptions about the thermal conditions in the lithosphere. These assumptions (summarized in Chapter 1) are:
1) Geothermal gradients or geotherms within the cratonic lithosphere have remained similar throughout Earth s history,
2) The cratonic lithosphere has remained of similar thickness since formation,
3) That the internal radiogenic heat production in the cratonic mantle either real (i.e., generated by reintroduction of HPE by metasomatism) or apparent' (i.e., resulting from the thermal disequilibrium of the crust and lithospheric mantle) is essentially zero,
Yet, all three of these assumptions fail to consider the effect of the Earth s dynamic thermal evolution or the systemic refertilization of the cratonic lithosphere globally. Secular cooling of the Earth is an inescapable planetary phenomenon resulting from both the decrease in radioactive decay of radioelements through time and the gradual loss of primordial heat left over from accretion. Indeed, most current models of terrestrial mantle evolution propose that 2.7 Ga, the mantle potential temperature (Tp) of the convecting mantle was 150-200 °C warmer than today.
Chapter 2 of this thesis tackles the first two assumptions made of the cratonic lithosphere by modelling the effects of secular cooling on lithospheric geotherms within the Kaapvaal Craton of South Africa. It is found that geotherms in a secularly evolving lithosphere cannot be reconciled with either Proterozoic xenolith or diamond inclusion (DI) pressure-temperature (PT) conditions recorded in the Kaapvaal Craton, the latter finding irrespective of diamond age. Rather, to explain the PT conditions recorded by DIs from the Kaapvaal Craton it is required that the early lithosphere was substantially thicker than what is preserved today and that the geothermal gradients in the lithosphere have changed dramatically through time. Depending on the age of diamond formation and the prevailing mantle Tp at that time the total thickness of Kaapvaal Craton could have reached up to ~350 km.
Chapter 3 of this thesis investigates the timing of metasomatism (that may reintroduce HPE) in the Kaapvaal Craton. The mica-amphibole(K-richterite)-rutile-ilmenite-diopside (MARID) and the presumably related phlogopite-K-richterite-peridotite (PKP) suites are examples of the most extreme modal metasomatism seen in the cratonic mantle with extreme HPE abundances. Due to a paucity of natural mineral chronometers in cratonic mantle xenoliths these rocks are attractive prospects to sense metasomatic events in the lithospheric mantle due to their hosting of zircon, which can be dated by the U-Pb method. Zircon ages recorded in these rocks fail to provide evidence of any metasomatic events that predate the Mesozoic as is demonstrated by previous workers, despite the significantly larger samples sizes of this study (n = > 40). Accordingly, HPE-rich metasomatism in the Kaapvaal Craton appears as a reflection of complex metasomatism at shallow levels of the lithosphere in response to the passage of orangeite and kimberlites magmas during the Cretaceous.
Chapter 4 of this thesis presents thermodynamic modelling of the Kaapvaal and Slave lithospheres using the heat equation to assess the potential for a non-zero internal radiogenic heat production of the cratonic lithospheric mantle. The resulting analysis suggests that both the Slave and Kaapvaal Cratons appear to have a relatively high internal radiogenic heat production (Hm) of between 0.04 and 0.05 µW/m3 near to the present-day (i.e., post Mesozoic time). Similarly, mantle xenoliths from the 1.15 Ga Premier kimberlite suggest comparable high values of Hm (~ 0.04 µW/m3) earlier in the Kaapvaal Craton s history. In the Archean, as such values would not allow for stabilization of an extensive diamond-hosting root earlier than 2.5 Ga. The first possibility is that the high Hm values modelled for the Slave and Kaapvaal Cratons, derives from an apparent heat production resulting from the thermal disequilibrium of the crust and lithospheric mantle. A second scenario is that the Hm reflects real radiogenic heat production in the mantle lithosphere through the later metasomatic refertilization of the cratonic mantle with K, Th and U. The former scenario is favoured because: 1) an apparent radiogenic heat production can be generated even if the lithospheric mantle has zero internal radiogenic heat production, which is required for craton stabilization and consistent with the low abundance of HPE in reconstituted peridotites ; 2) it would be required that both the Slave and Kaapvaal Cratons have similar metasomatic histories, which is unlikely; and 3) high HPE metasomatism of the Kaapvaal is seemingly restricted to the recent past (e.g., Chapter 3).
Finally, Chapter 5 of this thesis provides a new U-Pb methodology for dating pyrope garnet from mantle xenoliths to provide emplacement ages of their host kimberlite. Despite low U-concentrations (mostly < 0.1 ppm), sub-concordant mantle derived garnets can be dated using very large laser ablation spots (130 μm) measured by ICPMS. In both locations of the study, the U-Pb lower-intercept discordia ages of garnet, 84.1 ± 7.1 Ma at Bultfontein and 116.3 ± 15.8 Ma at Roberts Victor, reproduce previous emplacement ages. These ages imply that the garnet U-Pb system was open during mantle residence due to high ambient temperatures, and this inference is reinforced by Ni-thermometry data which indicate high equilibration temperatures (950 - 1075 °C). Yet, a discordia fit through garnets from the Roberts Victor separate also yields an upper-intercept age of 2560 ± 490 Ma, indicating that some ancient Pb was retained. This interpretation may be supported by Ni-thermometry data, which indicate much lower equilibration temperatures for some Roberts Victor garnet (~ 850°C). Our analytical protocol is not complex and uses widely available analytical methods. If applied to pyrope-rich garnet inclusions in diamond, it may be possible to provide ages of garnet-entrapment and thus diamond-growth if the U-Pb system remains undisturbed.
Description:
APPROVED
Author: Hoare, Brendan Colin
Advisor:
Tomlinson, EmmaPublisher:
Trinity College Dublin. School of Natural Sciences. Discipline of GeologyType of material:
ThesisAvailability:
Full text availableKeywords:
Geotherm, Heat production, U-Pb geochronology, Lithosphere, Craton, Metasomatism, KaapvaalMetadata
Show full item recordLicences: