Quantitative Relationships Between Basalt Geochemistry, Shear Wave Velocity, and Asthenospheric Temperature Beneath Western North America

©2018. American Geophysical Union. All Rights Reserved. Western North America has an average elevation that is ∼2 km higher than cratonic North America. This difference coincides with a westward decrease in average lithospheric thickness from ∼240 to 260 basaltic samples. Forward and inverse modeling of carefully selected major, trace, and rare earth elements were used to determine melt fraction as a function of depth. Basaltic melt appears to have been generated by adiabatic decompression of dry peridotite with asthenospheric potential temperatures of 1340 ± 20 °C. Potential temperatures as high as 1365 °C were obtained for the Snake River Plain. For the youngest (i.e., <5 Ma) basalts with a subplate geochemical signature, there is a positive correlation between shear wave velocities and trace element ratios such as La/Yb. The significance of this correlation is explored by converting shear wave velocity into temperature using a global empirical parameterization. Calculated temperatures agree with those determined by inverse modeling of rare earth elements. We propose that regional epeirogenic uplift of western North America is principally maintained by widespread asthenospheric temperature anomalies lying beneath a lithospheric plate, which is considerably thinner than it was in Late Cretaceous times. Our proposal accounts for the distribution and composition of basaltic magmatism and is consistent with regional heat flow anomalies

Thermal Structure of Eastern Australia's Upper Mantle and Its Relationship to Cenozoic Volcanic Activity and Dynamic Topography

Funder: Geoscience AustraliaFunder: Shell Global, Shell Exploration and Production Company; Id: http://dx.doi.org/10.13039/100011092Abstract: Spatio‐temporal changes of upper mantle structure play a significant role in generating and maintaining surface topography. Although geophysical models of upper mantle structure have become increasingly refined, there is a paucity of geologic constraints with respect to its present‐day state and temporal evolution. Cenozoic intraplate volcanic rocks that crop out across eastern Australia provide a significant opportunity to quantify mantle conditions at the time of emplacement and to independently validate geophysical estimates. This volcanic activity is divided into two categories: age‐progressive provinces that are generated by the sub‐plate passage of mantle plumes and age‐independent provinces that could be generated by convective upwelling at lithospheric steps. In this study, we acquired and analyzed 78 samples from both types of provinces across Queensland. These samples were incorporated into a comprehensive database of Australian Cenozoic volcanism assembled from legacy analyses. We use geochemical modeling techniques to estimate mantle temperature and lithospheric thickness beneath each province. Our results suggest that melting occurred at depths ≤80 km across eastern Australia. Prior to, or coincident with, onset of volcanism, lithospheric thinning as well as dynamic support from shallow convective processes could have triggered uplift of the Eastern Highlands. Mantle temperatures are inferred to be ∼50–100°C hotter beneath age‐progressive provinces that demarcate passage of the Cosgrove mantle plume than beneath age‐independent provinces. Even though this plume initiated as one of the hottest recorded during Cenozoic times, it appears to have thermally waned with time. These results are consistent with xenolith thermobarometric and geophysical studies

A tale of two domes: Neogene to recent volcanism and dynamic uplift of northeast Brazil and southwest Africa

Topographic domes that are located far from active plate boundaries are often characterised by rapid, youthful uplift, contemporaneous mafic volcanism, radial drainage patterns, and positive long-wavelength gravity anomalies. There is increasing evidence that they are underlain by anomalously low sub-plate seismic velocities. Despite their well-known geomorphological expression, the origin of these epeirogenic features remains enigmatic and is much debated. Here, we investigate potential mechanisms of uplift by combining disparate observations from the Borborema and Angolan plateaux that straddle the Brazilian and southwest African margins, respectively. Oceanic residual depth measurements, drainage analysis, stratigraphic architecture, emergent marine terraces and basement denudation are used to constrain their regional uplift histories. In both cases, the bulk of topographic growth occurred within the last 30 Ma in the absence of significant tectonic deformation. We derive present-day mantle temperature and lithospheric thickness from Neogene to recent volcanic trace element compositions and upper mantle shear wave velocities. Volcanic geochemistry in northeast Brazil is compatible with decompression melting of warm asthenosphere and potentially a minor contribution from metasomatised lithospheric mantle. In Angola, melting of metasomatised lithosphere is most likely triggered by injection of small-degree asthenospheric-derived melts. We find no evidence for an asthenospheric thermal anomaly > 50 ◦ C above ambient beneath either region. Present-day lithospheric thickness is ∼ 100 km beneath Angola and could be as thin as 60 km in the Borborema Province. For Angola, thermobarometry on mantle xenocrysts from Cretaceous kimberlites is used to estimate palaeogeothermal gradients. Results indicate a pre- existing gradient in lithospheric thickness between the edge of the Congo craton and the centre of the Angolan dome at ∼ 120 Ma. This gradient likely steepened as a result of additional Neogene thinning by ∼ 30 ± 10 km beneath the centre of the dome. We conclude that the mechanism for Neogene epeirogenic uplift of the Borborema and Angolan domes is introduction of a small positive temperature anomaly into the asthenosphere that causes thermomechanical thinning of the overlying lithospheric mantle

Role of basaltic magmatism within the Parnaíba cratonic basin, NE Brazil

The sedimentary fill of the Parnaíba basin in NE Brazil is punctuated by two episodes of basaltic magmatism in Jurassic and Cretaceous times, known as the Mosquito and Sardinha formations, respectively. A quantitative understanding of the depth and degree of melting that generated these basalts should provide useful constraints on the history of vertical motions in this cratonic basin. We carried out forward and inverse modelling of major, trace and rare earth element concentrations for primitive melts from both igneous provinces. We calculated the melt fraction as a function of depth to determine the melt volumes and mantle potential temperatures. Our results suggest that both episodes of magmatism resulted from shallow decompression melting within the asthenospheric mantle. The Mosquito basalts record an excess temperature of 55–75°C and are probably related to the widespread Central Atlantic Magmatic Province associated with the initial break-up of Gondwanaland. By contrast, the Sardinha basalts record localized lithospheric thinning to less than half its original thickness with excess temperatures of 15–25°C. This younger activity is probably part of the Paraná–Etendeka large igneous province, which is linked to rifting between South America and Africa. We suspect that these magmatic episodes are associated with phases of regional epeirogenic uplift affecting the Parnaíba basin.This study is part of the BP funded and supported Parnaíba Basin Analysis Project

Continental lithospheric temperatures: A review

Thermal structure of the lithosphere exerts a primary control on its strength and density and thereby its dynamic evolution as the outer thermal and mechanic boundary layer of the convecting mantle. This contribution focuses on continental lithosphere. We review constraints on thermal conductivity and heat production, geophysical and geochemical/petrological constraints on thermal structure of the continental lithosphere, as well as steady-state and non-steady state 1D thermal models and their applicability. Commonly used geotherm families that assume that crustal heat production contributes an approximately constant fraction of 25–40% to surface heat flow reproduce the global spread of temperatures and thermal thicknesses of the lithosphere below continents. However, we find that global variations in seismic thickness of continental lithosphere and seismically estimated variations in Moho temperature below the US are more compatible with models where upper crustal heat production is 2–3 times higher than lower crustal heat production (consistent with rock estimates) and the contribution of effective crustal heat production to thermal structure (i.e. estimated by describing thermal structure with steady-state geotherms) varies systematically from 40 to 60% in tectonically stable low surface heat flow regions to 20% or lower in higher heat flow tectonically active regions. The low effective heat production in tectonically active regions is likely partly the expression of a non-steady thermal state and advective heat transport

Development of an innovative peat lipstick based on the UV-B protective effect of humic substances

Humic acids (HA) are known for their antiviral and UV-B protecting effects, and are considered promising as ingredients for a UV-protective lipstick which is being developed to minimise or even prevent recurrences of UV- induced herpes. In this study, the UV/Vis spectra of three natural HA and three synthetic HA-like substances are analysed to determine the appropriateness of their UV-absorbing characteristics for the product under development. The contribution of a matrix component (castor oil) to the total UV absorption of the lipstick is also assessed. The results confirm the expected high UV-B absorption of the individual test substances, but reveal considerable differences in the UV-A wavelength range. Castor oil absorbs only UV-B radiation; and when mixed with HA it enhances total absorption in the UV-B range, but reduces it in the UV-A range. This is probably due to molecular interactions between castor oil and HA. Preliminary results from cultures of human U937 cells assayed for survival 24 hours after exposure to UV-B radiation show that both HA and castor oil exert a significant concentration-dependent UV-B protective filter effect similar to that of the UV-B absorbing reference substance p-aminobenzoic acid (PABA)

Cenozoic Dynamic Topography of Madagascar

Abstract: It has been proposed that Oligo‐Miocene regional uplift of Madagascar was generated and is maintained by mantle dynamical processes. Expressions of regional uplift include flat‐lying Upper Cretaceous‐Paleogene marine limestones that crop out at elevations of hundreds of meters along the western seaboard and emergent Quaternary coral‐rich terraces that rim the coastline. Here, we explore the history of subcrustal topographic support through a combined analysis of four sets of observational constraints. First, we exploit published receiver function estimates of crustal thickness and spectral admittance between gravity and topography. An admittance value of ∼+40 ± 10 mGal km−1 at wavelengths >500 km implies that ∼1 km of topography is supported by subcrustal processes. Secondly, new apatite fission‐track and helium measurements from 18 basement samples are inverted, constraining temperature and denudation histories. Results suggest that 0.5–1.6 km of regional uplift occurred after ∼30 Ma. Thirdly, we calculate a history of regional uplift by minimizing the misfit between observed and calculated longitudinal river profiles. Results suggest that topography was generated during Neogene times. Finally, inverse modeling of rare earth element concentrations in Neogene mafic rocks indicates that melting of the asthenospheric source occurred at depths of ≤65 km with potential temperatures of 1300–1370 °C. Melting occurred at higher temperatures beneath Réunion Island and northern Madagascar and at lower temperatures beneath the Comores and southern Madagascar. These inferences are consistent with shear wave velocities obtained from tomographic models. We conclude that Madagascar is underlain by thinned lithospheric mantle and that a thermal anomaly lies within an asthenospheric layer beneath northern Madagascar