Standard models for a warm, wet early Mars require a significant CO 2 ‐H 2 O atmosphere in the past. The source for these phases is assumed to be volcanic degassing. However, no consistent, dynamical models exist relating volcanic degassing to evolving mantle temperatures. Here we use a range of thermal, geophysical, geological, and petrological constraints from Mars to constrain mantle convection model simulations of Mars' post‐Noachian stagnant lid evolution. We develop a methodology to self‐consistently calculate melt extraction from the mantle source region. Using a dike‐propagation algorithm, we can calculate the rate of volcanism and rate of volcanic degassing from these simulations and compare them with estimates for Mars. We find that Martian melt production rates are satisfied by a 200‐km thick lithosphere (surface heat flow 25 ± 5 mW/m 3 ) for an intermediate Martian solidus. Core‐mantle temperatures cannot exceed ∼1850°C from geodynamo constraints, and the enrichment of heat‐producing elements into the crust is unlikely to exceed 25–50%. For hotter Martian mantle temperatures in the past, we find an evolution in rates of volcanism from >0.17 km 3 /yr for the early Hesperian to ∼1 × 10 −4 km 3 /yr at present, consistent with geological evidence. During this same interval, CO 2 flux would have declined from 8.8 × 10 7 to 6.7 × 10 6 kg/yr. If the early Hesperian supported a dense (>1 bar) atmosphere, this implies that the average loss rate of CO 2 from the atmosphere was 15 times greater than the maximum influx rate during this time.
Gravity gradiometry has a long legacy, with airborne/marine applications as well as surface applications receiving renewed recent interest. Recent instrumental advances has led to the emergence of downhole gravity gradiometry applications that have the potential for greater resolving power than borehole gravity alone. This has promise in both the petroleum and geosequestration industries; however, the effect of inherent uncertainties in the ability of downhole gravity gradiometry to resolve a subsurface signal is unknown. Here, we utilise the open source modelling package, Fatiando a Terra, to model both the gravity and gravity gradiometry responses of a subsurface body. We use a Monte Carlo approach to vary the geological structure and reference densities of the model within preset distributions. We then perform 100 000 simulations to constrain the mean response of the buried body as well as uncertainties in these results. We varied our modelled borehole to be either centred on the anomaly, adjacent to the anomaly (in the x-direction), and 2500 m distant to the anomaly (also in the x-direction). We demonstrate that gravity gradiometry is able to resolve a reservoir-scale modelled subsurface density variation up to 2500 m away, and that certain gravity gradient components (Gzz, Gxz, and Gxx) are particularly sensitive to this variation in gravity/gradiometry above the level of uncertainty in the model. The responses provided by downhole gravity gradiometry modelling clearly demonstrate a technique that can be utilised in determining a buried density contrast, which will be of particular use in the emerging industry of CO2 geosequestration. The results also provide a strong benchmark for the development of newly emerging prototype downhole gravity gradiometers.
Changes in the solid Earth at the end of the Archean fall into two categories. First are those related to cooling of the mantle and include a decrease in both komatiite abundance and MgO content, a decrease in Ni/Fe ratio in banded iron formation, and increases in incompatible element ratios (such as Nb/Yb, La/Yb, Zr/Y, La/Sm and Gd/Yb) in non-arc type basalts. A second group of changes is related to the extraction of continental crust from the mantle and stabilization of major cratons at 2.7 to 2.5 Ga. These include an increase in Nb/Th ratio and ε~Nd~(T) of non-arc basalts; significant increases in large-ion lithophile and high-field strength elements and a decrease in Sr in continental crust, which reflect a shift in magma types from TTG (tonalite-trondhjemite-granodiorite) to calc-alkaline; a prominent increase in the maximum values of δ^18^O of zircons from granitoids after the end of the Archean; a major peak in gold reserves is found at or near 2.7 Ga; and a peak in Re/Os depletion ages from mantle xenoliths at 2.7 Ga consistent with widespread thickening of the continental lithosphere at this time. All of these changes may be related to the widespread propagation of plate tectonics at the end of the Archean. Subduction produces continental crust in numerous arcs, which rapidly collide to form supercratons. Oceanic slabs sinking into the deep mantle could increase the production rate of mantle plumes, as well as increase the heat flux from the core, which warms the newly arrived slabs. The cooling of the deep mantle would begin after 2.5 Ga and continue until about 2.4 Ga when a 200-My slowdown in plate tectonics begins. This may be the reason for the rapid drop in temperature of the mantle recorded by basalts and komatiites. When plate tectonics comes back on track at about 2.2 Ga, Archean supercratons break up and are dispersed.
A detailed deep 3D geological model is an important basis for many types of exploration and resource modelling. Renewed interest in the structure of the Sydney Basin, driven primarily by sequestration studies, geothermal studies and coal seam gas exploration, has highlighted the need for a model of deep basin geology, structure and thermal state. Here, we combine gravity modelling, seismic reflection surveys, borehole drilling results and other relevant information to develop a deep 3D geological model of the Sydney Basin. The structure of the Sydney Basin is characteristic of a typical intracontinental rift basin, with a deep north–south orientated channel in the Lachlan Fold Belt basement, filled with up to 4 km of rift volcanics, and overlain with Permo-Triassic sediments up to 4 km thick. The deep regional architecture presented in this study will form the framework for more detailed geological, hydrological and geothermal models.
Abstract Samples of ancient Eoarchean volcanism suggest the existence of isotopically distinct mantle domains. 142 Nd evidence suggests that these domains formed within the first several hundred million years of Earth's history and were preserved until circa 3.7 Ga—a span of almost 800 Myr. Here we explore evolving mantle convection models under Hadean conditions to constrain mantle mixing dynamics and in particular focus on the generation, or preservation, of upper mantle hemispheric compositional anomalies. We find that initially radially homogenous models fail to produce hemispheric anomalies spontaneously due to mixing processes and that rheological and buoyancy effects, or asymmetric processes such as plate tectonics or meteorite impacts, do not generate long‐lived heterogeneities in upper mantle composition from homogenous initial conditions. In contrast, models initiated with compositional variations can preserve these hemispheric differences of periods >800 Myr despite vigorous internal convection, as a function of the proclivity of these models to exhibit stagnant‐lid behavior, resulting in isolated convection cells with restricted lateral transport.
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