EPSL: The influence of plate tectonic style on melt production and CO2 outgassing flux at mid-ocean ridges

The influence of plate tectonic style on melt production and CO2 outgassing flux at mid-ocean ridges

Jocelyn J. Fuentes, John W. Crowly, Rajdeep Dasgupta, and Jerry X. Mitrovica

Ocean ridges are one of the primary connections between Earth’s mantle and surface. Therefore, understanding the evolution of ocean ridge processes through Earth history is important for understanding how Earth’s surface and mantle have evolved. We combine an analytic model of mantle convection with a petrologic model of mantle melting in the presence of CO2 to compute the mantle temperature, plate speed, melt production, and CO2 outgassing flux at ocean ridges for the past 4 billion years. We explore a large suite of realistic mantle and lithospheric parameters to map out a full range of possible thermal histories. Our results show that in order to satisfy thermal constraints, the Earth must have started in a sluggish-lid mode of plate tectonics and transitioned to an active-lid mode. Furthermore, we show that plate speed and CO2 outgassing flux do not necessarily scale with mantle temperature, and that it is possible to reach present-day mantle temperatures and plate speeds with a simple force balance that does not invoke any feedbacks (e.g. grain size evolution, dehydration stiffening) or a fully stagnant-lid mode of convection in the Precambrian. The solutions show a range of evolutionary behaviors depending on the parameters chosen. The transition from sluggish- to active-lid can be smooth, abrupt, or somewhere in between. A smooth transition is difficult to distinguish from a purely active-lid evolutionary path based on temperature, plate speed, melt production, and CO2 outgassing flux. In contrast, an abrupt transition leads to a large increase in the average plate speed with a corresponding increase in melt production and CO2 outgassing flux. As an abrupt transition would have a much larger effect on CO2 outgassing and melt production than a change in ridge length, we investigate the impact of rapidly changing plate speeds and do not consider changes in ridge length. Finally, we show that carbon recycling is required for a large part of Earth history in order to explain present-day CO2 outgassing. Our model highlights the importance of understanding the style of mantle convection when calculating melt production and volatile fluxes through Earth’s history.
Fuentes, J., Crowley, J., Dasgupta, R. & Mitrovica, J. (2019). The influence of plate tectonic style on melt production and CO2 outgassing flux at mid-ocean ridges. Earth and Planetary Science Letters 511, 154-163. doi:10.1016/j.epsl.2019.01.020

Mineralogy and Petrology: Clinopyroxene megacrysts from Marion Island, Antarctic Ocean: evidence for a late stage shallow origin

Clinopyroxene megacrysts from Marion Island, Antarctic Ocean: evidence for a late stage shallow origin

James Roberts, Keabetswe D. Lehong, Andries E. J. Botha, Gelu Costin, Frikkie C. De Beer, Willem J. Hoffman and Callum J. Hetherington

Clinopyroxene megacrysts (up to 5 cm) from a scoria cone on Marion Island, Antarctic Ocean are zoned, with compositionally distinct low (Al + Ti) and high (Al + Ti) patches arranged haphazardly throughout crystals. Inclusions of olivine, pyrrhotite, oxides, sulphides, and rounded inclusions with euhedral micro-crystals interpreted as former melt inclusions are observed. Olivine inclusions have variable compositions, ranging from primary Ti-poor crystals to Ti-rich crystals hosting secondary haematite crystals formed by hydrogenation. The crystals contain voids that are concentrated in the middle of each crystal indicating that the initial crystal growth was skeletal. Subsequent crystallization filled in the skeletal framework creating the patchy zoning in the crystals. The Marion Island megacrysts are not homogenous, but the combination of crustal clinopyroxene compositions, primary and hydrogenated olivine, and the mode of eruption in scoria eruptions indicates that these crystals most likely formed in a shallow magma chamber. Primary olivines crystallized from a mafic magma and secondary altered olivines were incorporated into a rapidly growing megacryst in a super-saturated, fluid-rich environment, prior to being ejected onto surface in a scoria eruption.

Roberts, R.J., Lehong, K.D., Botha, A.E.J. et al. (2019). Clinopyroxene megacrysts from Marion Island, Antarctic Ocean: evidence for a late stage shallow origin. Miner Petrol., 1438-1168, 1-13. https://doi.org/10.1007/s00710-018-00651-x

Nature Communications: Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation

Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation

Ming Tang, Cin-Ty A. Lee, Kang Chen, Monica Erdman, Gelu Costin and Hehe Jiang

The surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique in that it also has extensive regions of felsic crust, manifested in the form of continents. Exactly how felsic crust forms when basaltic magmas are the dominant products of melting the mantles of rocky planets is unclear. A fundamental part of the debate is centered on the low Nb/Ta of Earth’s continental crust (11–13) compared to basalts (15–16). Here, we show that during arc magma differentiation, the extent of Nb/Ta fractionation varies with crustal thickness with the lowest Nb/Ta seen in continental arc magmas. Deep arc cumulates (arclogites) are found to have high Nb/Ta (average ~19) due to the presence of high Nb/Ta magmatic rutiles. We show that the crustal thickness control of Nb/Ta can be explained by rutile saturation being favored at higher pressures. Deep-seated magmatic differentiation, such as in continental arcs and other magmatic orogens, is thus necessary for making continents.

Tang, M., Lee, C. A., Chen, K., Erdman, M., Costin, G. & Jiang, H. (2019). Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation. Nature Communications. Springer US 10:235, 1–8. DOI: 10.1038/s41467-018-08198-3

 

Science Advances: Delivery of carbon, nitrogen, and sulfur to the silicate Earth by a giant impact

Delivery of carbon, nitrogen, and sulfur to the silicate Earth by a giant impact

GRL: Nitrogen in the Earth’s outer core

Nitrogen in the Earth’s outer core

Bajgain, S. K., Mookherjee, M., Dasgupta, R., Ghosh, D. & Karki, B. B.

Using first principles molecular dynamic simulations, we explore the effects of nitrogen (N) on the density and sound velocity of liquid iron and evaluate its potential as a light element in the Earth’s outer core. Our results suggest that Fe‐N melt cannot simultaneously explain the density and seismic velocity of the Earth’s outer core. Although ~2.0 wt.% N can explain the bulk sound velocity of the outer core, such N content only lowers the density of liquid Fe by ~3%. Matching both the velocity and density by the other light elements limits the N in the core to ≪2.0 wt.%. Our finding suggests that nitrogen is a minor to trace element in the Earth’s core and is consistent with the geochemical mass balance with terrestrial abundance of N and alloy‐silicate partitioning data, which suggest that there cannot be significant N in the core.

 

Bajgain, S. K., Mookherjee, M., Dasgupta, R., Ghosh, D. & Karki, B. B. (2019). Nitrogen in the Earth’s outer core. Geophysical Research Letters 46, 89-98. doi:10.1029/2018GL080555

SuperComputing (SC'18): Computing Planetary Interior Normal Modes with a Highly Parallel Polynomial Filtering Eigensolver

Abstract: A highly parallel algorithm has been developed and exploited to compute the planetary normal modes of the elastic-gravitational system, which is approximated via the mixed finite element method on unstructured tetrahedral meshes. The eigenmodes of the relevant generalized eigenvalue problem were extracted by a Lanczos approach combined with polynomial filtering. In contrast with the standard shift-and-invert and the full-mode coupling algorithms, the polynomial filtering technique is ideally suited for solving large-scale 3-D interior eigenvalue problems since it significantly enhances the memory and computational efficiency without loss of accuracy. The parallel efficiency and scalability of this approach are demonstrated on Stampede2 at the Texas Advanced Computing Center. To our knowledge, this is the first time that the direct calculation of the normal modes of 3-D strongly heterogeneous planets, in particular, Earth and Mars, is made feasible via a combination of multiple matrix-free methods and a separation of the essential spectra.

Shi, J., Li, R., Xi, Y., Saad, Y. and de Hoop, M.V., 2018, Computing planetary interior normal modes with a highly parallel polynomial filtering eigensolver. In Proceedings of the International Conference for High Performance Computing, Networking, Storage, and Analysis (p. 71). IEEE Press.

Link: SuperComputing (SC’18): Computing Planetary Interior Normal Modes with a Highly Parallel Polynomial Filtering Eigensolver

Paleoceanography & Paleoclimatology: PRYSM v2.0: A Proxy System Model for Lacustrine Archives

Title: PRYSM v2.0: A Proxy System Model for Lacustrine Archives

Abstract: Reconstructions of temperature and hydrology from lake sedimentary archives have made fundamental contributions to our understanding of past, present, and future climate and help evaluate general circulation models (GCMs). However, because paleoclimate observations are an indirect (proxy) constraint on climatic variables, confounding effects of proxy processes complicate interpretations of these archives. To circumvent these uncertainties inherent to paleoclimate data‐model comparison, proxy system models (PSM) provide transfer functions between climate variables and the proxy. We here present a new PSM for lacustrine sedimentary archives. The model simulates lake energy and water balance, sensors including leaf wax δD and carbonate δ18O, bioturbation, and compaction of sediment to lend insight toward how these processes affect and potentially obfuscate the original climate signal. The final product integrates existing and new models to yield a comprehensive, modular, adaptable, and publicly available PSM for lake systems. Highlighting applications of the PSM, we forward model lake variables with GCM simulations of the last glacial maximum and the modern. The simulations are evaluated with a focus on sensitivity of lake surface temperature and mixing to climate forcing, using Lakes Tanganyika and Malawi as case studies. The PSM highlights the importance of mixing on interpretations of air temperature reconstructions from lake archives, and demonstrates how changes in mixing depth alone may induce non‐stationarity between in‐situ lake and air temperatures. By placing GCM output in the same reference frame as lake paleoclimate archives, we aim to improve inference of past changes in terrestrial temperatures and water cycling.

Link: https://doi.org/10.1029/2018PA003413

GRL: Fluid controls on the heterogeneous seismic characteristics of the Cascadia margin

By Jonathan Delph, Alan Levander, and Fenglin Niu

Abstract: The dehydration of oceanic slabs during subduction is mainly thermally controlled and is often expressed as intermediate‐depth seismicity. In warm subduction zones, shallow dehydration can also lead to the buildup of pore‐fluid pressure near the plate interface, resulting in nonvolcanic tremor. Along the Cascadia margin, tremor density and intermediate‐depth seismicity correlate but vary significantly from south to north despite little variation in the thermal structure of the Juan de Fuca Plate. Along the northern and southern Cascadia margin, intermediate‐depth seismicity likely corresponds to increased fluid flux, while increased tremor density may result from fluid infiltration into thick underthrust metasediments characterized by very slow shear wave velocities (<3.2 km/s). In central Cascadia, low intermediate‐depth seismicity and tremor density may indicate a lower fluid flux, and shear wave velocities indicate that the Siletzia terrane extends to the plate interface. These results indicate that the presence of thick underthrust sediments is associated with increased tremor occurrence.

Rice Press Release: Tiny northwest quakes tied to deep crust structure

Delph JR, Levander A, and Niu F (2018) Fluid controls on the heterogeneous seismic characteristics of the Cascadia margin, Geophys Res Lett, doi:10.1029/2018GL079518

EPSL: Slab-mantle interaction, carbon transport, and kimberlite generation in the deep upper mantle

 

By Chenguang Sun and Rajdeep Dasgupta

 

Abstract: Low-degree partial melts from deeply subducted, carbonated ocean crust are carbonatite liquids with ∼35–47 wt% CO2. Their reactions with the overlying mantle regulate the slab–mantle interaction and carbon transport in the deep upper mantle but have not been investigated systematically. Here we present new multi-anvil experiments and parameterized phase relation models to constrain the fate of slab-derived carbonatite melts in the upper mantle. The experiments were conducted at 7 GPa/1400 °C and 10 GPa/1450 °C, and used starting compositions mimicking the ambient mantle infiltrated by variable carbonatite fluxes (0–45 wt%) from the slab surface. Kimberlitic melts (CO2 = 14–32 wt%, SiO2 = 15–33 wt%, and MgO = 20–29 wt%) were produced from experiments with 5.8–25.6 wt% carbonatite influxes. Experimental phase relations demonstrate a reactive melting process in which the carbonatite influx increases in proportion by dissolution of olivine, orthopyroxene, garnet and precipitation of clinopyroxene. This manifests a feasible mechanism for slab-derived carbonatite melts to efficiently transport in the ambient mantle through high-porosity channels. The melt and mineral fractions from this study and previous phase equilibria experiments in peridotite + CO2 ± H2O systems were empirically parameterized as functions of temperature (900–2000 °C), pressure (3–20 GPa), and bulk compositions (e.g., CO2 = 0.9–17.1 wt% and Na2O + K2O = 0.27–2.51 wt%). Applications of the phase relation models to prescribed melting processes indicate that reactive melting of a carbonatite-fluxed mantle source could produce kimberlitic melts with diverse residual lithologies under various melting conditions. However, reactive melting at the slab–mantle interface can only commence when the slab-released carbonatite melt conquers the carbonation freezing front, i.e., the peridotite solidi suppressed by infiltration of CO2-rich melts in an open system. Depending on temperatures and local influxes, reactive melting and carbonation/redox freezing can occur simultaneously above the slab–mantle interface, yielding heterogeneous lithologies and redox conditions as well as various time-scales of carbon transport in Earth’s mantle.

 

Sun, C. & Dasgupta, R. (2019) Slab-mantle interaction, carbon transport, and kimberlite generation in the deep upper mantle. Earth and Planetary Science Letters 506: 38-52. doi:10.1016/j.epsl.2018.10.028

SEG Special Publications, Chapter 9: Anhydrite assimilation by ultramafic melts of the Bushveld Complex, and its consequences to petrology and mineralization

Anhydrite assimilation by ultramafic melts of the Bushveld Complex, and its consequences to petrology and mineralization

Yudovskaya, M.A., Sluzhenikin, S.F., Costin, G., Shatagin, K.N., Dubinina, E.O., Grobler, D.F., Ueckermann, H., and Kinnaird1, J.A.

in “Metals, Minerals and Society”, Special Publication no. 21, Society of Economic Geologists, Chapter 9,  pp. 177-206

Abstract

Sulfate assimilation by mafic to ultramafic melt is thought to be an important process in the genesis of magmatic PGE-Ni-Cu deposits. We consider petrological indicators and possible mechanisms of anhydrite assimilation by ultramafic melts of the northern limb of the Bushveld Complex. On farm Turfspruit, an anhydrite-bearing sedimentary raft of the Duitschland Formation separates the Platreef from underlying Lower zone peridotites. The proportion of anhydrite across the raft increases from negligible in corundum-sillimanite-magnetite hornfels at the base to 95 to 100% in anhydrite marble at the top. Underlying Lower zone peridotites lack anhydrite, whereas overlying Platreef pyroxenites contain both widespread interstitial to euhedral anhydrite as well as spherical to irregularly shaped anhydrite inclusions in association with olivine chadacrysts inside oikocrystic orthopyroxene. Olivine chadacryst compositions (Mg# 79–81 and 0.33–0.46 wt % NiO) support their pristine liquidus origin, although an association of Al-enriched orthopyroxene and interstitial anorthite indicates exchange reactions involving anhydrite and aluminosilicates from hornfels. Plagioclase from the anhydrite-contaminated rocks has an Sr isotope initial ratio (Sri) of 0.7047 to 0.7063, similar to the compositions of Bushveld early primitive magmas, in agreement with a relatively non-radiogenic signature of the anhydrite-bearing contaminant with Sri of 0.7057 to 0.7094. The range of Sri of plagioclase from the underlying Lower zone peridotites (0.7040– 0.7067) and from the Turfspruit platinum reefs just below the Main zone contact (0.7068–0.7084) supports their correlation and synchronous emplacement with the Lower zone and the top of the Upper Critical zone in the western and eastern limbs of the Bushveld. The δ34S values of anhydrite (12.2–14.5‰) and a coexisting pyrrhotite-millerite-chalcopyrite sulfide assemblage (6.2–7.8‰) in a hornfelsed raft and overlying pyroxenites are interpreted to have resulted from open system isotopic exchange, indicating closure temperatures of 750° to 820°C. The assimilation of sedimentary anhydrite is interpreted to be an important component of contact-style mineralization of the Platreef at Turfspruit that took place through the erosion and disintegration of footwall rocks by dynamic pulses of hot magmas. Chemical dissolution, thermal decomposition, and melting of sulfate-bearing rafts or xenoliths are viable assimilation processes that result in the saturation of silicate melt with sulfate, exsolution of immiscible sulfate melts, crystallization of cumulus and interstitial anhydrite, and precipitation of contact-style sulfide mineralization at the base of the intrusion. Reef-style mineralization at the top of the Platreef shows contrastingly negligible compositional and isotopic evidence of sulfate assimilation.

Yudovskaya, M.A., Sluzhenikin, S.F., Costin, G., Shatagin, K.N., Dubinina, E.O., Grobler, D.F., Ueckermann, H., and Kinnaird1, J.A. (2018): Anhydrite assimilation by ultramafic melts of the Bushveld Complex, and its consequences to petrology and mineralization. In “Metals, Minerals and Society”, Editors Antonio M. Arribas and Jeffrey L. Mauk. SEG Special publications no. 21, pp. 177-206.

https://www.segweb.org/store_info/SP/SP-21-Additional-Product-Info.pdf