Nature: Scientific Reports: A 5‑km‑thick reservoir with > 380,000 km3 of magma within the ancient Earth’s crust

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Rais Latypov, Sofya Chistyakova, Richard A. Hornsey, Gelu Costin & Mauritz van der Merwe

 

Abstract

Several recent studies have argued that large, long-lived and molten magma chambers may not occur in the shallow Earth’s crust. Here we present, however, field-based observations from the Bushveld Complex that provide evidence to the contrary. In the eastern part of the complex, the magmatic layering continuously drapes across a ~ 4-km-high sloping step in the chamber floor. Such deposition of magmatic layering implies that the resident melt column was thicker than the stepped relief of the chamber floor. Prolonged internal differentiation within this thick magma column is further supported by evolutionary trends in crystallization sequence and mineral compositions through the sequence. The resident melt column in the Bushveld chamber during this period is estimated at > 5-km in thickness and > 380,000 km3 in volume. This volume of magma is three orders of magnitude larger than any known super-eruption in the Earth’s history and is only comparable to the extrusive volumes of some of Earth’s large igneous provinces. This suggests that super-large, entirely molten, and long-lived magma chambers occur, at least occasionally, in the geological history of our planet. Therefore, the classical view of magma chambers as ‘big magma tanks’ remains a viable research concept for some of Earth’s magmatic provinces.

Latypov, R., Chistyakova, S., Hornsey, R. A. & Costin, G. (2022). A 5 ‑ km ‑ thick reservoir km 3 of magma within the ancient Earth ’ s crust. Scientific Reports. Nature Publishing Group UK 1–12. https://doi.org/10.1038/s41598-022-19915-w

Lithos: Correlations between olivine composition and groundmass mineralogy in Sierra Leone kimberlites provide constraints on craton-specific melt-lithosphere interactions

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Anton Viljoen, Geoffrey H. Howarth, Andrea Giuliani, Angus Fitzpayne, Gelu Costin

 

Abstract

Sierra Leone contains two Jurassic-aged diamondiferous kimberlite clusters, namely Koidu and Tongo-Tonguma (hereafter referred to as Tongo), consisting of eruptive pipes and NE-SW trending dikes. In this study, a combination of detailed petrography, and phlogopite, spinel, and olivine compositions in hypabyssal samples is presented to classify and constrain the petrogenesis of these kimberlites. Both the Koidu and Tongo rocks are predominantly macrocrystic and highly micaceous with phlogopite abundances, normalised to olivine-free, of 36–65 vol% in the groundmass. These phlogopite contents are comparable to those of some cratonic lamproites and significantly higher than any other kimberlites. Other groundmass minerals include spinel, perovskite, and apatite set in a base of carbonate and serpentine. Phlogopite and spinel have similar compositions in the Koidu and Tongo samples, displaying evolutionary trends consistent with those observed in worldwide kimberlites. Olivine macrocrysts and microcrysts display complex zoning with distinct cores, internal zones, and rims. The core compositions display a range in Mg# (81–95) and are interpreted to be derived from the disaggregation of lithospheric mantle xenoliths and proto-kimberlite-related megacrysts. The Tongo olivine rims, interpreted to be primary magmatic crystallisation products, have similar compositions from various locations within the cluster whereas the Koidu samples display a range in rim compositions (Mg# 87–89) from different locations within the cluster. The average Koidu rim compositions display a strong positive correlation with the average core compositions, consistent with the trend formed by kimberlites worldwide and indicative of a strong control by melt-lithosphere interaction on melt compositions. Previously, it has been shown that the Mg# of olivine rim’s negatively correlates with the abundance of groundmass phlogopite (± oxide minerals) in kimberlites. However, the Koidu and Tongo kimberlites are exceptionally phlogopite-rich given their olivine Mg#, fall outside of the worldwide kimberlite array and have olivine compositions and phlogopite abundances like some cratonic lamproites. This leads us to suggest that Koidu and Tongo represent a rare style of highly micaceous kimberlite magmatism, not previously reported in other cratonic regions, and are genetically linked to the assimilation of K2O-rich, metasomatic mantle lithologies. We further suggest that the K2O content, reflected by groundmass phlogopite abundances, of worldwide kimberlite and cratonic lamproite parent magmas ascending to the surface may be related to assimilation of craton-specific styles of metasomatic lithologies in the SCLM.

 

Viljoen, A., Howarth, G. H., Giuliani, A., Fitzpayne, A. & Costin, G. (2022). Correlations between olivine composition and groundmass mineralogy in Sierra Leone kimberlites provide constraints on craton-specific melt-lithosphere interactions. LITHOS. Elsevier B.V. 430–431, 106846. https://doi.org/10.1016/j.lithos.2022.106846

Geophysical Research Letters: Autocorrelation R2 on Mars

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Sizhuang Deng and Alan Levander

 

Plain Language Summary

The subsurface structures of Earth are imaged by the detailed analysis of seismic data recorded by thousands of stations deployed on Earth’s surface. The InSight mission landed a seismograph on Mars which was deployed at the end of 2018 to investigate the planet’s interior structure and dynamic evolution. In this study, we preprocessed the continuous vertical-component seismic data, and by autocorrelation retrieved a Rayleigh wave, one class of seismic surface wave, that orbits Mars. Rayleigh wave group velocities between 115 and 200s period were measured from the observed Mars orbiting Rayleigh waves. Synthetic seismograms were calculated using current estimates of the velocity structure of Mars for comparisons to the observation. The spherically symmetric model was updated with a Monte Carlo algorithm, an inversion method that randomly perturbs the velocity model and determines the model that best matches the Mars orbiting surface waves through trial and error. An S-wave low-velocity zone is observed to the depth of ~400km beneath the Martian surface, consistent with other InSight seismic observations and velocity models measured from geophysical modeling and high-pressure laboratory experiments.

 

Link 

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2022GL099580

 

AGU Advances: Clumped-isotope constraint on upper-tropospheric cooling during the last glacial maximum

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Asmita Banerjee, Laurence Y. Yeung, Lee T. Murray, Xin Tie, Jessica E. Tierney, and Allegra N. Legrande

Abstract

Ice cores and other paleotemperature proxies, together with general circulation models, have provided information on past surface temperatures and the atmosphere’s composition in different climates. Little is known, however, about past temperatures at high altitudes, which play a crucial role in Earth’s radiative energy budget. Paleoclimate records at high-altitude sites are sparse, and the few that are available show poor agreement with climate model predictions. These disagreements could be due to insufficient spatial coverage, spatiotemporal biases, or model physics; new records that can mitigate or avoid these uncertainties are needed. Here, we constrain the change in upper-tropospheric temperature at the global scale during the Last Glacial Maximum (LGM) using the clumped-isotope composition of molecular oxygen trapped in polar ice cores. Aided by global three-dimensional chemical transport modeling, we exploit the intrinsic temperature sensitivity of the clumped-isotope composition of atmospheric oxygen to infer that the upper troposphere (effective mean altitude 10–11 km) was 6–9°C cooler during the LGM than during the late preindustrial Holocene. A complementary energy balance approach supports a minor or negligible steepening of atmospheric lapse rates during the LGM, which is consistent with a range of climate model simulations. Proxy-model disagreements with other high-altitude records may stem from inaccuracies in regional hydroclimate simulation, possibly related to land-atmosphere feedbacks.

Plain Language Summary

Atmospheric temperatures at high altitudes determine the fate of montane glaciers and the energy balance of the planet. They change with Earth’s climate, but our knowledge of this relationship is poor: the few available temperature records for high-altitude cooling at the most recent ice age, which are limited to the tropics, disagree with model predictions for unknown reasons. Here we report a global-scale constraint for high-altitude temperature, applied to the last ice age, which yields results consistent with global climate model predictions. This new proxy―based on the isotopic variants of molecular oxygen trapped in polar ice cores―can be applied deeper in the past to understand the relationship between surface and high-altitude temperatures in different climates.

doi: 10.1029/2022AV000688

Companion piece from Seltzer and Tyne: Retrieving a “weather balloon” from the last ice age

Journal of Scientific Computing: A Non-perturbative Approach to Computing Seismic Normal Modes in Rotating Planets

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A continuous Galerkin method based approach is presented to compute the seismic normal modes of rotating planets. Special care is taken to separate out the essential spectrum in the presence of a fluid outer core using a polynomial filtering eigensolver. The relevant elastic-gravitational system of equations, including the Coriolis force, is subjected to a mixed finite-element method, while self-gravitation is accounted for with the fast multipole method. Our discretization utilizes fully unstructured tetrahedral meshes for both solid and fluid regions. The relevant eigenvalue problem is solved by a combination of several highly parallel and computationally efficient methods. We validate our three-dimensional results in the non-rotating case using analytical results for constant elastic balls, as well as numerical results for an isotropic Earth model from standard “radial” algorithms. We also validate the computations in the rotating case, but only in the slowly-rotating regime where perturbation theory applies, because no other independent algorithms are available in the general case. The algorithm and code are used to compute the point spectra of eigenfrequencies in several Earth and Mars models studying the effects of heterogeneity on a large range of scales.

 

Shi, J., Li, R., Xi, Y., Saad, Y., and de Hoop, M. V. A Non-perturbative Approach to Computing Seismic Normal Modes in Rotating Planets. J Sci Comput 91, 67 (2022). https://doi.org/10.1007/s10915-022-01836-5

 

EPSL: Carbon recycling efficiency in subduction zones constrained by the effects of H2O-CO2 fluids on partial melt compositions in the mantle wedge

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Michael Lara, Rajdeep Dasgupta

The extent of CO2 transfer from subducting lithologies to the overlying mantle wedge in general and to the arc magma source regions in particular remains debated. The limit of CO2 transfer to the sub- arc mantle could be estimated if the effects of CO2 on the primary hydrous melt compositions of mantle wedge can be assessed in relation to the observed compositions of primitive arc magmas. Here we present new piston cylinder and multi-anvil experiments using Au75Pd25 and Au capsules on four (CO2 +H2O)] of 0–0.17. Experiments were performed at 2–4 GPa and 1200 ◦C to constrain how the depleted peridotite + H2O ± CO2 starting compositions with 3.5 wt.% H2O and XCO2 [=molar CO2 / presence of variable CO2 in slab-derived aqueous fluids affects the composition of peridotite partial melts. ± clinopyroxene. Melts at 2–4 GPa are basaltic for XCO2 of 0–0.10 and become SiO2-poor and CaO-rich All experiments consisted of low degree melts (<10 wt.%) in equilibrium with olivine + orthopyroxene at XCO2 > 0.10. Comparison between our experimental partial melt compositions with a global dataset of the most primitive arc magmas suggests that the upper limit of XCO2 in fluids inducing melting in model for subduction zones and estimate that at least 34–86% of CO2 entering subduction zones bypasses mantle wedges is ∼0.10 at 2–4 GPa. We apply these new constrains to an H2O and CO2 mass balance the sub-arc melt generation zone and is subducted to the convecting mantle, either carried by the slab or by the down-dragged limb of the mantle wedge directly above the slab.

Precambrian Research: Origin of the J-M Reef and lower banded series, Stillwater Complex, Montana, USA

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Christopher Jenkins, James E. Mungall, Michael L. Zientek, Gelu Costin, and Zhuo-Sen Yao

The origin and parental magma for layered cumulates in the Lower Banded series (LBS) and the J-M Reef Pd-Pt deposit of the Stillwater Complex remains poorly constrained. We present whole-rock lithogeochemistry and mineral chemistry from LBS rocks collected from drill holes and surface samples from the Mountain View area of the complex that in total span nearly the entirety of the LBS stratigraphy. Excess S, Pt, and Pd in the noritic and gabbronoritic cumulates of the LBS indicate that small amounts of high tenor sulfide liquid generated at very low degrees of sulfide oversaturation were ubiquitous parts of the cumulate assemblage. We show that a simple two-stage thermodynamic model of assimilation-batch crystallization of a komatiitic parental magma in the lower crust produces a close match to a common suite of fine-grained gabbronorite dikes and sills that intrude both the complex and its footwall. After fractionating ultramafic cumulates in the lower crust, the model contaminated komatiitic liquid produces upper crustal cumulates by batch crystallization en route to or at the level of the
intrusion. The modeled rocks have compositions and mineral assemblages closely resembling pyroxenite of the Bronzitite zone and both norite and gabbronorite cumulates in the lower LBS. The trends from the Bronzitite zone through Norite zone I and Gabbronorite zone I can be understood as the result of deposition of crystals from successive batches of the same contaminated parental magma, with an upward trend toward greater amounts of cooling before the separation of crystals from liquid. The olivine-bearing suite of Olivine-bearing zone I, which includes the J-M Reef, can be modeled by partial remelting of the same norite and gabbronorite cumulates due to a temporarily increased flux of hot, moderately less contaminated LBS parental magma that infiltrated partially molten cumulates because its density exceeded that of the interstitial liquid. This model suggests that infiltration of hot Mg-rich parental liquid into moderately PGE-enriched footwall cumulates may be fundamental to the formation of the extremely high tenor sulfide mineralization in the J-M Reef ore zone, and perhaps other reef type deposits worldwide. The same metal tenors that would require silicate/sulfide mass ratios (i.e., R-factors)
of 105 to 106 in a single stage of equilibration would be attained during this second stage of interaction by the incremental infiltration and passage of LBS parental magma through previously sulfide saturated cumulate mush.

Jenkins CM, Mungall JE, Zientek ML, Costin G, Yao ZS (2021): Origin of the J-M Reef and lower banded series, Stillwater Complex, Montana, USA. Precambrian Research, Volume 367, 106457, ISSN 0301-9268, https://doi.org/10.1016/j.precamres.2021.106457

GRL: Resolving long-term variations in North Atlantic tropical cyclone activity using a pseudo proxy paleotempestology network approach

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Elizabeth J. Wallace, Sylvia G. Dee, and Kerry A. Emanuel

Geophysical Research Letters (2021) e2021GL094891.

doi: 10.1029/2021GL094891

Abstract

Paleohurricane reconstructions extend the observational record of tropical cyclones back thousands of years. However, these records are subject to biases – capturing only close-moving intense storms at varying resolutions. We devise two pseudo proxy networks drawing from the full suite of published paleohurricane studies in the North Atlantic. We run synthetic storms forced with two global climate model simulations of the past millennium through each pseudo network to assess the theoretical skill of paleohurricane proxies at capturing low frequency variability in North Atlantic basin-wide and intrabasin tropical cyclones. We find that basin-wide and paleohurricane compiled tropical cyclone counts are significantly correlated with one another for the past millennium on annual to multi-decadal timescales, but compilation skill is limited by proxy temporal resolution. Current paleohurricane proxy networks predominantly capture storms moving in the Caribbean/Gulf of Mexico. Increasing the quantity of paleohurricane records from the North American coastline substantially improves reconstruction skill.

 

Earth-Science Reviews: Cognate versus xenocrystic olivines in kimberlites – A review

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Authors: Andy Moore, Gelu Costin, Alexander Proyer

Abstract

Models for a xenocryst origin for kimberlite olivines emphasise the similarity between their core compositions and those in mantle peridotites. While this permits a xenocryst origin, it does not provide proof, as magmas generated in equilibrium with mantle olivines could, in principle, crystallize initial olivines matching those in the source region. Further, in several kimberlites, there is a striking disparity between the compositional range of olivine cores and that in associated mantle peridotite xenoliths from the same locality. Olivine-liquid Mg-Fe exchange coefficients and Ni partition coefficients permit equilibrium between Mg-rich mantle olivines (Mg# ~ 94–93) and magmas matching kimberlite bulk rock compositions. Glass inclusions in olivine megacrysts from the Monastery kimberlite, with compositions which overlap the range of archetypal Group I kimberlites, were interpreted to represent original liquids trapped at pressures of 4.5–6 GPa. These glass inclusions provide direct petrographic support for primitive melts matching kimberlite bulk chemistry in the lower SCLM.

A majority of kimberlitic olivines show normal (decreasing Mg#) core to rim zonation. Cores of normal-zoned kimberlitic olivines are typically homogeneous, but collectively define a field with a range in Mg # and invariant or slightly decreasing Ni towards more Fe-rich compositions. The most Mg-rich cores of normal-zoned olivines typically have Mg# in the range 94–93, but there are marked differences in the Fe-rich extreme of the normal-zoned population between different kimberlite clusters. Olivine rims typically define a field characterized by steeply decreasing Ni, coupled with invariant or slightly increasing or decreasing Mg#, which invariably overlaps the Fe-extreme of core compositions of the relatively Mg-rich, normal-zoned olivines. Consequently, while there is a sharp inflection in chemical gradient between the respective fields of cores and rims, they nevertheless define a continuous compositional field. Trace element modelling demonstrates that these zonation patterns can be explained in terms of a Raleigh crystallization model.

Most, if not all kimberlites are characterized by a subordinate group of olivine macrocrysts with cores that are Fe-rich relative to the field for rims, and thus show reverse zonation, which are interpreted to be linked to the Cr-poor megacryst suite. Rare Mg-rich olivines (relative to rims), have high-pressure inclusions of garnet, clinopyroxene and orthopyroxene. When present, such inclusions often show disequilibrium features such as internal chemical zonation. This points to a very short mantle residence time prior to entrainment by the host kimberlite, indicating a link to the Cr-rich megacryst suite rather than mantle peridotites. In addition to a variable, but generally subordinate proportion of olivines derived from Cr-poor and Cr-rich megacrysts, xenocrysts derived from disaggregated mantle peridotites will undoubtedly be present. While their proportions are difficult to quantify, the collective evidence points to a cognate origin for a majority of kimberlitic olivines. A kimberlite magma ascent model is proposed which provides a framework for understanding both olivine compositional variation and apparently enigmatic internal and external olivine morphology.

Moore, A.,  Costin, G., and Proyer, A. (2021): Cognate versus xenocrystic olivines in kimberlites – A review. Earth-Science Reviews, 103771, ISSN 0012-8252, https://doi.org/10.1016/j.earscirev.2021.103771

EPSL: The effect of carbon concentration on its core-mantle partitioning behavior in inner Solar System rocky bodies

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Authors: Damanveer S. Grewal, Rajdeep Dasgupta, Sanath Aithala

Abstract:

Partitioning of carbon (C) into the cores of rocky protoplanets and planets is one of the primary causes of its depletion in their bulk silicate reservoirs. Most of the experimental studies that determined the alloy to silicate melt partition coefficient of carbon (DCalloy/silicate) have been conducted in graphite-saturated conditions. Because carbon is a minor element in all known protoplanetary and planetary cores, it is not known whether graphite-saturated DCalloy/silicate values are applicable to core-mantle differentiation in rocky bodies which likely occurred in C-poor conditions. In this study we experimentally determined DCalloy/silicate in MgO capsules with variable bulk C contents between oxygen fugacity (fO2) of IW–6.35 and IW–2.59 at a fixed P (3 GPa)-T (1700 °C). A mafic-ultramafic (NBO/T = 1.23-1.72) and mildly hydrous (bulk H = 44-161 ppm) nature of the silicate melts caused anhydrous C species (CO32− + CO) to dominate over a wider fO2 range (>IW–4.2) in comparison to previous studies. This resulted in an increase in DCalloy/silicate with decreasing fO2 from IW–2.6 to IW–4.2 followed by a drop at more reduced conditions due to the formation of C-H species. Importantly, DCalloy/silicate increases with increasing bulk C content of the system at a given fO2. Partitioning of C between alloy and silicate melts follows non-Henrian behavior (i.e., it depends on bulk C content) because the activity coefficient of C in the alloy melt (γCalloymelt) varies with C content in the alloy. Therefore, in addition to other intensive (P, T, fO2) and extensive variables (alloy and silicate melt compositions), DCalloy/silicate also depends on the bulk C content available during core-mantle differentiation. Consequently, previously determined DCalloy/silicate for C-rich alloys are not directly applicable for core-mantle differentiation in relatively C-poor magma oceans (MOs). Because the experiments from the present study more realistically simulate C-poor cores and mildly hydrous, mafic-ultramafic silicate MOs, our data can be used to more accurately predict C fractionation between MOs and cores in inner Solar System rocky bodies. Our study suggests that closed system MO-core equilibration should have led to less severe depletion of C in the silicate reservoirs of inner Solar System rocky bodies than previously predicted.