Rajdeep Dasgupta

Melting in the Earth's deep upper mantle caused by carbon dioxide

The onset of partial melting beneath mid-ocean ridges governs the cycling of highly incompatible elements from the mantle to the crust, the flux of key volatiles (such as CO2, He and Ar) and the rheological properties of the upper mantle. Geophysical observations indicate that melting beneath ridges begins at depths approaching 300 km, but the cause of this melting has remained unclear. Here we determine the solidus of carbonated peridotite from 3 to 10 GPa and demonstrate that melting beneath ridges may occur at depths up to 330 km, producing 0.03–0.3% carbonatite liquid. We argue that these melts promote recrystallization and realignment of the mineral matrix, which may explain the geophysical observations. Extraction of incipient carbonatite melts from deep within the oceanic mantle produces an abundant source of metasomatic fluids and a vast mantle residue depleted in highly incompatible elements and fractionated in key parent-daughter elements. We infer that carbon, helium, argon and highly incompatible heat-producing elements (such as uranium, thorium and potassium) are efficiently scavenged from depths of ∼200–330 km in the upper mantle.

Copper Systematics in Arc Magmas and Implications for Crust-Mantle Differentiation

Copper-Bottomed Crust The formation of volcanic arc chains near subduction zones brings large amounts of magma from the upper mantle to the crust, contributing to the formation of island chains in the ocean and adding material to continents. Over time, arc magmas also contribute indirectly to the composition of the oceans and atmosphere through outgassing and weathering of volcanic minerals; however, it is unclear what determines the oxidized nature of arc magmas themselves. Lee et al. (p. 64) measured Cu contents in a range of arc-derived volcanic rocks as a proxy for arc magma redox states. An overall depletion of Cu, which is sensitive to reduced sulfur contents, in global continental crust suggests that there is a hidden reservoir of copper-rich sulfides deep in Earths interior. The copper contents of magmas imply that the formation of sulfide-bearing cumulates under reducing conditions is a critical step in the formation of continental crust. Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper’s (Cu’s) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.

Carbon-dioxide-rich silicate melt in the Earth's upper mantle.

The onset of melting in the Earth’s upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic melt could be stable 250 km or less beneath mid-oceanic ridges, owing to the small fraction (∼0.03 wt%) its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present at ∼200 km (refs 3–5) but large melt fractions are thought to be restricted to shallower depths. Here we present experiments on carbonated peridotites over 2–5 GPa that constrain the location and the slope of the onset of silicate melting in the mantle. We find that the pressure–temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and that silicate melting of dry peridotite + CO2 beneath ridges commences at ∼180 km. Accounting for the effect of 50–200 p.p.m. H2O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ∼100 p.p.m. CO2 becomes as deep as ∼220–300 km. We suggest that, on a global scale, carbonated silicate melt generation at a redox front ∼250–200 km deep, with destabilization of metal and majorite in the upwelling mantle, explains the oceanic low-velocity zone and the electrical conductivity structure of the mantle. In locally oxidized domains, deeper carbonated silicate melt may contribute to the seismic X-discontinuity. Furthermore, our results, along with the electrical conductivity of molten carbonated peridotite and that of the oceanic upper mantle, suggest that mantle at depth is CO2-rich but H2O-poor. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth’s deep upper mantle, and the inventory of carbon, H2O and other highly incompatible elements at ridges becomes controlled by the flux of the former.

The redox state of arc mantle using Zn/Fe systematics

Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle. As a consequence, the sub-arc mantle wedge is widely believed to be oxidized. The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source. Here we show that Zn/FeT (where FeT = Fe2+ + Fe3+) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, Fe2+ and Zn behave similarly, but because Fe3+ is more incompatible than Fe2+, melts generated in oxidized environments have low Zn/FeT. Primitive arc magmas have identical Zn/FeT to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/FeT during early differentiation involving olivine requires that Fe3+/FeT remains low in the magma. Only after progressive fractionation does Fe3+/FeT increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood.

Upside-down differentiation and generation of a primordial lower mantle

Except for the first 50–100 million years or so of the Earth’s history, when most of the mantle may have been subjected to melting, the differentiation of Earth’s silicate mantle has been controlled by solid-state convection. As the mantle upwells and decompresses across its solidus, it partially melts. These low-density melts rise to the surface and form the continental and oceanic crusts, driving the differentiation of the silicate part of the Earth. Because many trace elements, such as heat-producing U, Th and K, as well as the noble gases, preferentially partition into melts (here referred to as incompatible elements), melt extraction concentrates these elements into the crust (or atmosphere in the case of noble gases), where nearly half of the Earth’s budget of these elements now resides. In contrast, the upper mantle, as sampled by mid-ocean ridge basalts, is highly depleted in incompatible elements, suggesting a complementary relationship with the crust. Mass balance arguments require that the other half of these incompatible elements be hidden in the Earth’s interior. Hypotheses abound for the origin of this hidden reservoir. The most widely held view has been that this hidden reservoir represents primordial material never processed by melting or degassing. Here, we suggest that a necessary by-product of whole-mantle convection during the Earth’s first billion years is deep and hot melting, resulting in the generation of dense liquids that crystallized and sank into the lower mantle. These sunken lithologies would have ‘primordial’ chemical signatures despite a non-primordial origin.

Effect of variable carbonate concentration on the solidus of mantle peridotite

Abstract To explore the effect of variable CO2 concentrations on the solidus of natural carbonated peridotite, we determined near-solidus phase relations of three different nominally anhydrous, carbonated lherzolite bulk compositions at 6.6 GPa. Starting mixes (PERC, PERC2, and PERC3) were prepared by adding variable proportions of a carbonate mixture that has the same Ca:Mg:Fe:Na:K ratio as the base silicate peridotite [MixKLB-1: Mg no. = 89.7; Ca no. = molar Ca/(Ca + Mg + Fe*) = 0.05]. For all three bulk compositions, the subsolidus assemblage includes olivine, orthopyroxene, clinopyroxene, garnet, and magnesite solid solutions. Above the solidus, crystalline carbonate disappears and quenched Fe, Na-bearing dolomitic carbonatite melts were observed. For PERC3 (1.0 wt% bulk CO2; Na2O/CO2 weight ratio = 0.30), the observed solidus is between 1190 and 1220 °C; for PERC (2.5 wt% bulk CO2; Na2O/CO2 = 0.12), it is between 1250 and 1275 °C; and for PERC2 (5.0 wt% bulk CO2; Na2O/CO2 = 0.06), it is between 1300 and 1330 °C. At 6.6 GPa, experimental solidi of natural magnesite peridotites are 100.200 °C lower than the CMAS-CO2 solidus, chiefly owing to the fluxing effect of alkalis, and solidus temperatures increase with increasing bulk CO2 (i.e., decreasing bulk Na2O/CO2), owing to dilution of Na2O in near-solidus melt. The effects of Mg no. and Ca no. on carbonated peridotite solidi appear to be less significant than that of Na2O/CO2. Trends of decreasing solidus temperature with increasing Na2O/CO2 and with decreasing CO2 indicate that natural mantle peridotite with ~100.1000 ppm bulk CO2 will have solidus temperatures ~20° to ~100° lower than that determined experimentally. The solidus of peridotite drops discontinuously by ~600 °C (at 6.6 GPa) at the CO2 bulk concentration (~5 ppm) at which carbonate is stabilized, but then varies little with increasing bulk CO2. This result contrasts with the effect of H2O, which lowers the solidus continuously with increasing concentration.

Water follows carbon: CO2 incites deep silicate melting and dehydration beneath mid-ocean ridges

Hydrous defects in nominally anhydrous minerals influence the physical properties and reduce the melting point of mantle rocks. Consequently, it is believed that extraction of H 2 O beneath mid-ocean ridges by dehydration melting enhances the creep strength of the upper mantle and produces a chemical lithosphere. However, recent studies show that trace water induces melting at shallower depths beneath ridges than previously believed. Here we explore the hypothesis that incipient melting, dehydration, and strengthening of the subridge mantle is promoted by trace quantities of carbon. Experiments at 3 GPa on carbonated peridotite with 1 and 2.5 wt% CO 2 produce partial melts that change from carbonatites with 2 to carbonated silicate melts with >25 wt% SiO 2 between 1325 and 1350 °C. Enhanced melting of carbonated peridotite relative to volatile free peridotite is parameterized as a simple function of the concentration of CO 2 in the melt. Application of this model to mantle with 100 ppm bulk CO 2 suggests that the subridge mantle undergoes silicate melting ∼130–140 °C below the CO 2 -free solidus, corresponding to a depth of ∼110 km beneath ridges, and that 0.2 wt% partial melting will be attained at a depth of ∼75 km. The combined effects of H 2 O and CO 2 further enhance deep silicate melting, as H 2 O partitions from nominally anhydrous silicates to carbonated silicate melts. Thus, the deepest silicate melting beneath ridges will be induced by the combination of H 2 O and CO 2 , rather than simply by dehydration melting of nominally anhydrous peridotite, and this combination is likely responsible for dehydration strengthening of the oceanic lithosphere and for geochemical signatures of deep melting in mid-oceanic ridge basalt.

Continental arc–island arc fluctuations, growth of crustal carbonates, and long-term climate change

The Cretaceous to early Paleogene (ca. 140–50 Ma) was characterized by a greenhouse baseline climate, driven by elevated concentrations of atmospheric CO_2. Hypotheses for the elevated CO_2 concentrations invoke an increase in volcanic CO_2 production due to higher oceanic crust production rates, higher frequency of large igneous provinces, or increases in pelagic carbonate deposition, the last leading to enhanced carbonate subduction into the mantle source regions of arc volcanoes. However, these are not the only volcanic sources of CO_2 during this time interval. We show here that ocean-continent subduction zones, manifested as a global chain of continental arc volcanoes, were as much as 200% longer in the Cretaceous and early Paleogene than in the late Paleogene to present, when a cooler climate prevailed. In particular, many of these continental arcs, unlike island arcs, intersected ancient continental platform carbonates stored on the continental upper plate. We show that the greater length of Cretaceous–Paleogene continental arcs, specifically carbonate-intersecting arcs, could have increased global production of CO_2 by at least 3.7–5.5 times that of the present day. This magmatically driven crustal decarbonation flux of CO_2 through continental arcs exceeds that delivered by Cretaceous oceanic crust production, and was sufficient to drive Cretaceous–Paleogene greenhouse conditions. Thus, carbonate-intersecting continental arc volcanoes likely played an important role in driving greenhouse conditions in the Cretaceous–Paleogene. If so, the waning of North American and Eurasian continental arcs in the Late Cretaceous to early Paleogene, followed by a fundamental shift in western Pacific subduction zones ca. 52 Ma to an island arc–dominated regime, would have been manifested as a decline in global volcanic CO_2 production, prompting a return to an icehouse baseline in the Neogene. Our analysis leads us to speculate that long-term (>50 m.y.) greenhouse-icehouse oscillations may be linked to fluctuations between continental- and island arc–dominated states. These tectonic fluctuations may result from large-scale changes in the nature of subduction zones, changes we speculate may be tied to the assembly and dispersal of continents. Specifically, dispersal of continents may drive the leading edge of continents to override subduction zones, resulting in continental arc volcanism, whereas assembly of continents or closing of large ocean basins may be manifested as large-scale slab rollback, resulting in the development of intraoceanic volcanic arcs. We suggest that greenhouse-icehouse oscillations are a natural consequence of plate tectonics operating in the presence of continental masses, serving as a large capacitor of carbonates that can be episodically purged during global flare-ups in continental arcs. Importantly, if the global crustal carbonate reservoir has grown with time, as might be expected because platform carbonates on continents do not generally subduct, the greenhouse-driving potential of continental arcs would have been small during the Archean, but would have increased in the Neoproterozoic and Phanerozoic after a significant reservoir of crustal carbonates had formed in response to the evolution of life and the growth of continents.

Effect of variable CO2 on eclogite-derived andesite and lherzolite reaction at 3 GPa—Implications for mantle source characteristics of alkalic ocean island basalts

We have performed reaction experiments between 1, 4, and 5 wt % CO2-bearing MORB-eclogite (recycled oceanic crust)-derived low-degree andesitic partial melt and fertile peridotite at 1375°C, 3 GPa for infiltrating melt fractions of 25% and 33% by weight. We observe that the reacted melts are alkalic with degree of alkalinity or Si undersaturation increasing with increasing CO2 content in reacting melt. Consequently, an andesite evolves through basanite to nephelinite owing to greater drawdown of SiO2 from melt and enhanced precipitation of orthopyroxene in residue. We have developed an empirical model to predict reacted melt composition as a function of reacting andesite fraction and source CO2 concentration. Using our model, we have quantified the mutual proportions of equilibrated melt from andesite-peridotite (+ CO2) hybridization and subsequent peridotite (± CO2)-derived melt required to produce the major element composition of various ocean island basalts. Our model can thus be applied to characterize the source of ocean islands from primary alkalic lava composition. Accordingly, we determined that average HIMU source requires 24 wt % of MORB-eclogite-derived melt relative to peridotite containing 2 wt % CO2 and subsequent contribution of 45% of volatile-free peridotite partial melt. We demonstrate that mantle hybridization by eclogite melt-peridotite (± CO2) reaction in the system can produce high MgO (>15 wt %) basaltic melts at mantle potential temperature (TP) of 1350°C. Therefore, currently used thermometers to estimate TP using MgO content of primary alkalic melts need to be revised, with corrections for melt-rock reaction in a heterogeneous mantle as well as presence of CO2.

High pressure, near‐liquidus phase equilibria of the Home Plate basalt Fastball and melting in the Martian mantle

Near-liquidus phase equilibria experiments have been conducted on a synthetic Fastball basalt composition, as analyzed at Home Plate plateau of Mars (Gusev Crater), to test if it represents a primitive mantle derived melt and place constraints on the temperature of the ancient mantle and on the lithosphere-asthenosphere boundary of Mars. The Fastball basalt is multiply saturated with olivine and orthopyroxene at ∼1.2 GPa and 1430°C. Based on melting models, we predict that the Fastball composition could be produced by 13–23% equilibrium melting of the Martian mantle, with extraction of melt from the base of ∼105 km thick lithosphere. The multiple saturation for Fastball also constrains the potential temperature of the Martian mantle to be approximately 1480–1530°C with an initial melting pressure of 4.0–4.7 GPa. This potential temperature is much lower than that of the terrestrial mantle derived from similarly ancient magmas, i.e., komatiites.