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.
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
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
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
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.
Ding, S. and Dasgupta, R.
Abstract: The sulfur (S) and copper (Cu) contents of primitive mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) are similar, although the latter are thought to be derived from hotter mantle. To reconcile the sulfur and chalcophile element budgets of OIB, we developed a model to describe the behavior of sulfide and Cu during decompression melting of mantle by combining experimental constraints on decompression melting at different excess mantle potential temperatures, TP, between 1400 and 1650°C, and empirical sulfur content at sulfide saturation (SCSS) models, which take into account the effect of Ni and Cu present in the equilibrium sulfide melt. Model calculations at TP = 1450–1650°C were applied to explain the S and Cu inventory of high-Mg# ‘reference’ OIB. Modeling indicates that partial melts relevant to OIB generation have higher SCSS than those of primitive MORB, because of the positive effect of temperature on SCSS. Therefore, for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle. Calculation of SCSS along melting adiabats at TP = 1450–1550°C, with variable initial S content of peridotite, indicates that sulfide-undersaturated primitive Icelandic basalts with ∼720 ppm S and 74–115 ppm Cu can be generated by 10–25 wt % melting of peridotite containing 100–150 ppm S. The S and Cu budgets of OIB that are thought to represent low-degree melts can be satisfied by (1) peridotite partial melting if a sulfide-saturated partial melt with a Ni content in the sulfide melt ≥25–30 wt % is derived from relatively cold mantle (TP ≤ 1450°C), or (2) if primitive melts parental to OIB are enriched in S (>1500 ppm), or (3) an extremely low (≤1%) degree of melting is applicable. Alternatively, if the Ni content in the equilibrium sulfide in the peridotitic mantle is ≤20–25 wt %, mixing of partial melts, derived from low-degree melting of MORB-eclogite and metapelite, with S-depleted peridotite partial melts may be necessary to reconcile the measured S and Cu contents in the low-F (<10%, where F is melt fraction) basalts from Galapagos spreading center, Lau Basin and Loihi for TP of 1450–1650°C. In this last case, sulfides, equivalent to 50–100 ppm S in the peridotite mantle, can be exhausted by 1–9 wt % partial melting. The total S inventory of the heterogeneous mantle source of these basalts is higher because of the presence of subducted eclogite ± sediments. Our analysis also suggests that compared with peridotite, which is likely to become sulfide-free during partial melting owing to the high SCSS for its partial melts, subducted MORB-eclogite and metapelite probably play important roles in retaining sulfide in the Earth’s shallow mantle, owing to low SCSS in their partial melts and high initial sulfide abundances.
Ding, S. & Dasgupta, R. (2018). Sulfur inventory of ocean island basalt source regions constrained by modeling the fate of sulfide during decompression melting of a heterogeneous mantle. Journal of Petrology 59, 1281–1308. doi:10.1093/petrology/egy061
By: James Eguchi, Rajdeep Dasgupta
Abstract: The redox state of mantle lithologies, based on xenoliths from continental lithospheric mantle, has been shown to decrease with depth and reach oxygen fugacities (fO2) at which graphite/diamond will be the stable form of carbon at pressures greater than about 3-4 GPa (e.g., Frost and McCammon, 2008). On the other hand, the depth-fO2 profile of the convecting mantle remains poorly known. We compare the CO2-Ba and CO2-Nb systematics of natural oceanic basalts to the CO2-trace element concentrations that can be generated via contributions from depleted peridotite partial melts and graphite-saturated partial melts of subducted lithologies. Results suggest that to produce the CO2enrichments relative to the depleted end member observed in natural oceanic basalts, subducted lithologies cannot be graphite-saturated at the onset of melting or must undergo oxidative transformation below the respective volatile-free solidi. Therefore, the oxygen fugacity profile of the continental lithospheric mantle may not be applicable to the deep convecting upper mantle, with the convecting upper mantle to at least 150 km depth being more oxidised than the carbonate vs.graphite/diamond buffer.
A non-van der Waals 2D Material from natural titanium mineral ore Ilmenite
Puthirath Balan, A., Radhakrishnan, S., Kumar, R., Neupane, R., Sinha, S.K., Deng, L., de los Reyes, C.A., Apte, A., Rao, B.M., Paulose, M., Vajtai, R., Chu, C.W., Costin, G., Marti, A.A., Varghese, O.K., Singh, A.K., Tiwary, C.S., Anantharaman, M.R., Ajayan, P.M
Chemistry of Materials https://pubs.acs.org/doi/10.1021/acs.chemmater.8b01935
Two dimensional (2D) materials from naturally occurring minerals are promising and possess interesting physical properties. A new 2D material ‘Ilmenene’ has been exfoliated from the naturally occurring titanate ore ilmenite (FeTiO3) by employing liquid phase exfoliation in dimethylformamide (DMF) solvent by ultrasonic bath sonication. Ilmenene displays  orientation which is confirmed by transmission electron microscopy (TEM). Probable charge transfer excitation from Fe2+Ti4+ to Fe3+Ti3+ results in ferromagnetic (FM) ordering along with the antiferromagnetic (AFM) phase accompanied by enhanced anisotropy due to surface spins. The 2D nature and bandgap states help ilmenene form a heterojunction photocatalyst with titania nanotube arrays, capable of broad spectrum light harvesting and separating/transferring the photo-generated charges effectively for solar photoelectrochemical water splitting.
Puthirath Balan, A., Radhakrishnan, S., Kumar, R., Neupane, R., Sinha, S.K., Deng, L., de los Reyes, C.A., Apte, A., Rao, B.M., Paulose, M., Vajtai, R., Chu, C.W., Costin, G., Marti, A.A., Varghese, O.K., Singh, A.K., Tiwary, C.S., Anantharaman, M.R., Ajayan, P.M., 2018. A non-van der Waals 2D Material from natural titanium mineral ore Ilmenite. Chem. Mater. acs.chemmater.8b01935. https://doi.org/10.1021/acs.chemmater.8b01935
Impurity controlled crystal growth in low dimensional bismuth telluride
Sharifi, T., Yazdi, S., Costin, G., Apte, A., Coulter, G., Tiwary, C., and Ajayan, P.M.
Chemistry of Materials, https://pubs.acs.org/doi/10.1021/acs.chemmater.8b02548
Topological insulators, such as layered Bi2Te3, exhibit extraordinary properties, manifesting profoundly only
at nanoscale thicknesses. However, it has been challenging to synthesize these structures with controlled thickness. Here,
control over the thickness of solvothermally grown Bi2Te3 nanosheets is demonstrated by manipulating the crystal growth
through selected and controlled impurity atom addition. By a comprehensive analysis of growth mechanism and the intentional
addition of Fe impurity, it is demonstrated that the nucleation and growth of few-layer nanosheets of Bi2Te3 can be
stabilized in solution. By optimizing the Fe concentration, nanosheets thinner than 6 nm, and as thin as 2 nm, can be
synthesized. Such thicknesses are smaller than the anticipated critical thickness for the transition of topological
insulators to the quantum spin Hall regime.
Sharifi, T., Yazdi, S., Costin, G., Apte, A., Coulter, G., Tiwary, C., and Ajayan, P.M. (2018). Impurity controlled crystal growth in low dimensional bismuth telluride. Chemistry of Materials, p. acs.chemmater.8b02548. doi: 10.1021/acs.chemmater.8b02548
Laurence Y. Yeung, Justin A. Hayles, Huanting Hu, Jeanine L. Ash, and Tao Sun
Rapid Comm. Mass Spectrom. (2018)
Rationale: Isotope ratio measurements have become extremely precise in recent years, with many approaching parts‐per‐million (ppm) levels of precision. However, seemingly innocuous errors in signal baselines, which exist only when gas enters the instrument, might lead to significant errors. These “pressure‐baseline” (PBL) offsets may have a variety of origins, such as incoherent scattering of the analyte, isobaric interferences, or electron ablation from the walls of the flight tube. They are likely present in all but ultra‐high‐resolution instruments, but their importance for high‐precision measurements has not been investigated.
Methods: We derive the governing equations for the PBL effect. We compare the oxygen triple‐isotope composition of gases on three different mass spectrometers before and after applying a correction for PBLs to determine their effects. We also compare the composition of atmospheric O2 with that of several standard minerals (San‐Carlos Olivine and UWG‐2) on two high‐precision mass spectrometers and compare those results with the differences reported in the literature.
Results: We find that PBLs lead to stretching or compression of isotopic variations. The scale distortion is non‐mass‐dependent, affecting the accuracy of triple‐isotope covariations. The governing equations suggest that linear stretching corrections using traditional isotopic delta values (e.g., δ18O) are rigorous for PBL‐induced errors in pure gases. When the reference and sample gases are not comparable in composition or purity, however, a different correction scheme may be required. These non‐mass‐dependent errors are systematic and may have influenced previous measurements of triple‐isotope covariations in natural materials.
Conclusions: Accurate measurements of isotopic variations are essential to biogeochemistry and for testing theoretical models of isotope effects. PBLs are probably ubiquitous, contributing to the interlaboratory disagreements in triple‐isotope compositions of materials differing greatly in δ18O values. Moreover, they may lead to inaccurate determination of triple‐isotope compositions and fractionation factors, which has implications for isotopic studies in hydrology and biogeochemistry.
Department of Earth, Environmental and Planetary Sciences
6100 Main Street
Houston, TX 77005 USA