Experiments on melt–rock reaction in the shallow mantle wedge

This experimental study simulates the interaction of hotter, deeper hydrous mantle melts with shallower, cooler depleted mantle, a process that is expected to occur in the upper part of the mantle wedge. Hydrous reaction experiments (~6 wt% H₂O in the melt) were conducted on three different ratios of a 1.6 GPa mantle melt and an overlying 1.2 GPa harzburgite from 1060 to 1260 °C. Reaction coefficients were calculated for each experiment to determine the effect of temperature and starting bulk composition on final melt compositions and crystallizing assemblages. The experiments used to construct the melt–wall rock model closely approached equilibrium and experienced <5% Fe loss or gain. Experiments that experienced higher extents of Fe loss were used to critically evaluate the practice of “correcting” for Fe loss by adding iron. At low ratios of melt/mantle (20:80 and 5:95), the crystallizing assemblages are dunites, harzburgites, and lherzolites (as a function of temperature). When the ratio of deeper melt to overlying mantle is 70:30, the crystallizing assemblage is a wehrlite. This shows that wehrlites, which are observed in ophiolites and mantle xenoliths, can be formed by large amounts of deeper melt fluxing though the mantle wedge during ascent. In all cases, orthopyroxene dissolves in the melt, and olivine crystallizes along with pyroxenes and spinel. The amount of reaction between deeper melts and overlying mantle, simulated here by the three starting compositions, imposes a strong influence on final melt compositions, particularly in terms of depletion. At the lowest melt/mantle ratios, the resulting melt is an extremely depleted Al-poor, high-Si andesite. As the fraction of melt to mantle increases, final melts resemble primitive basaltic andesites found in arcs globally. An important element ratio in mantle lherzolite composition, the Ca/Al ratio, can be significantly elevated through shallow mantle melt–wall rock reaction. Wall rock temperature is a key variable; over a span of <80 °C, reaction with deeper melt creates the entire range of mantle lithologies from a depleted dunite to a harzburgite to a refertilized lherzolite. Together, the experimental phase equilibria, melt compositions, and reaction coefficients provide a framework for understanding how melt–wall rock reaction occurs in the natural system during melt ascent in the mantle wedge.     

Diamond precipitation from ascending reduced fluids in the Kaapvaal lithosphere: Thermodynamic constraints

Previous research has shown that the Kaapvaal lithospheric mantle is generally reduced and characterised by a decreasing redox state with increasing depth. As a consequence, C-O-H fluids in the Kaapvaal lithospheric mantle are dominated by H₂O, CH₄, and C₂H₆. Thermodynamic calculations demonstrate that diamond precipitation from such a fluid during ascend is possible as it is exposed to a more oxidised environment and both CH₄ and C₂H₆ are oxidised. However, the calculations also demonstrate that the diamond precipitation potential from such a fluid decreases when: (1) the mantle is either more reduced or oxidised compared to the Kaapvaal mantle, or (2) the change in temperature with pressure is smaller compared to that of the Kaapvaal mantle. Therefore, the presence of reduced mantle fluid species and a generally decreasing oxygen fugacity with increasing depth do not necessarily warrant diamond precipitation from a rising reduced fluid.     

Testing variations within the Tagish Lake meteorite—II: Whole‐rock geochemistry of pristine samples

Four pristine specimens of the Tagish Lake C2 chondrite meteorite were previously determined through mineralogy, petrology, and organic chemistry to have been affected by aqueous alteration in the order (from least to most altered) TL5b < TL11h < TL11i, and TL11v as a mixture of the other specimens (Herd et al. 2011 ; Blinova et al. 2014 ). Here, we report the whole‐rock data for a total of 65 elements for the same four Tagish Lake samples as determined by ICP‐MS and ICP‐AES (utilizing the Parr bomb digestion method on small samples, approximately 50 mg), and by INAA. Our data demonstrate that the determined aqueous alteration sequence has a positive correlation with trace elements, such as K and Br that are mobile during aqueous alteration, which appear to be controlled by an increase of phyllosilicates from least to most altered samples. Yet, the homogeneity of other elements suggests that elemental mass transfer occurred on a localized scale and aqueous alteration was isochemical for these elements, similar to other primitive carbonaceous chondrites. By plotting data from three samples (TL5b, TL11h, and TL11i) on a Zn/Mn versus Sc/Mn diagram, we also confirm that the Tagish Lake meteorite is not a simple mixture of CI and CM material.     

What drives national adaptation? A global assessment

That the climate is changing and societies will have to adapt is now unequivocal, with adaptation becoming a core focus of climate policy. Our understanding of the challenges, needs, and opportunities for climate change adaptation has advanced significantly in recent years yet remains limited. Research has identified and theorized key determinants of adaptive capacity and barriers to adaptation, and more recently begun to track adaptation in practice. Despite this, there is negligible research investigating whether and indeed if adaptive capacity is translating into actual adaptation action. Here we test whether theorized determinants of adaptive capacity are associated with adaptation policy outcomes at the national level for 117 nations. We show that institutional capacity, in particular measures of good governance, are the strongest predictors of national adaptation policy. Adaptation at the national level is limited in countries with poor governance, and in the absence of good governance other presumed determinants of adaptive capacity show limited effect on adaptation. Our results highlight the critical importance of institutional good governance as a prerequisite for national adaptation. Other elements of theorized adaptive capacity are unlikely to be sufficient, effective, or present at the national level where national institutions and governance are poor.     

Transition zone structure under a stationary hot spot: Cape Verde

We report on a two-year seismic deployment in the Cape Verde Islands, one goal of which was to study the upper mantle to determine its structure under a hot spot that is stationary in the hot spot reference frame. We find from analysis of P-to-S receiver functions estimated from broadband seismic recordings that, within uncertainty, the time separation between the 410 and 660 km discontinuities is normal compared to radial earth models. Thus, to exist, even stationary hot spots do not require vertical thermal anomalies from deep melting sources anchored in the lower mantle or at the core–mantle boundary or their anomalies are narrower than ～ 250 km in the upper mantle.     

An upwelling filament off southwest Iberia: Effect on the chlorophyll a and nutrient export

The present paper describes the chemical and biological characteristics of an upwelling filament off southern Iberia in October 2004, based on a total of 42 CTD casts, along with ADCP velocity measurements distributed by an almost regular grid of 15 km mean spacing. Stations were sampled from the surface to a maximum depth of 400 dbar, for nutrients (nitrate, phosphate and silicate) and chlorophyll a determination. The effect of cross-shelf exchange of nutrients and chlorophyll a between the coastal and oceanic waters was also investigated. Data revealed that, during the cruise conducted under relaxed winds, a relatively small filament was formed down to 75 m. However, an amount of 180 t of chlorophyll a was estimated there accompanied by low nutrient concentrations. The observed water properties reveal that SW Iberia is similar to NW Iberia, showing the continuity of the Canary Upwelling System along the Iberian Peninsula, one of poorest upwelling systems of the oceanic eastern boundaries. Nevertheless, the amounts of chlorophyll and nutrients transported through the filament are significant, revealing that those play a key role in the seaward export of matter with an important impact on the regional oceanography of this region. Considering the periods of strong upwelling events and the extent of their duration along the year, the amounts of exported matter must be hugely increased and responsible for the high productivity of these waters.