Mark A. Richards

Some large igneous provinces (LIPs) are associated with precursory carbonatite eruptions. Models for precursory carbonatites invoking partial melting of a carbonate metasomatized lithosphere appear implausible, given the long time needed for heat conduction from the plume head. Instead, we consider that the plume, not the lithosphere, is the source of carbon. We explore a range of carbon concentrations in the plume source (196 C 440 ppm) and various melting depths of carbonate‐bearing peridotite, depending on the mantle redox state. Our numerical simulations provide a fluid dynamically consistent evolution of the plume‐lithosphere system and enable us to calculate the ascent velocity of carbonate melts. Moreover, we advect millions of passive tracers to follow the 3‐D trajectories of melts. The term “precursory‐C” indicates plume carbon that has reached sub‐lithospheric depths (140 km) before the onset of anhydrous silicate melting. For models with carbonatite melt produced at great depths (200 km), we find that the precursory‐C flux can be up to 6 × mol(C)/yr during a time period of ∼2 Myr preceding the onset of anhydrous silicate melting. The considerable mass of precursory‐C, up to 2 × kg(C), can explain carbonatite complexes predating main phase volcanism in the Paraná province and the Deccan Traps. For some LIPs, environmental and biotic perturbations, indicated by negative carbon isotope excursions and mass extinctions, predate the main phase LIPs eruptions. For the Central Atlantic Magmatic Province (CAMP) we suggest that precursory‐C could represent a non‐negligible carbon source for the earliest volcanism, likely synchronous with the end‐Triassic extinction.

Some hotspot tracks, such as those formed by the Hawai’i and Galápagos mantle plumes, exhibit long-lived cross-track isotopic zonation, thought to reflect the streaking out of heterogeneous material in the plume conduit during upwelling. In lavas associated with the Galápagos mantle plume, three geochemical domains, present for at least 15 Myr, have been identified: northern, southern and central. The most extreme isotopic enrichments are observed in the northern domain of the Cocos Ridge at ~15 Ma, and in the southern domain of the Galápagos Archipelago at the present day. Owing to the northward migration of the Galápagos Spreading Center above the plume at ~5-10 Ma, this relationship suggests that geochemical enrichment in the Galápagos basalts is greatest above the region of the plume furthest from the nearby mid-ocean ridge. We examine the hypothesis that these temporal and spatial variations in geochemical enrichment reflect a ''shallow mantle control'', associated with differences in the mean depth of melting. We conducted forward melting models of a mixed peridotite-pyroxenite mantle to calculate the isotopic composition of the resulting melts formed under two different mantle flow regimes. Our results demonstrate that variations in the average pressure of melt generation, due to the influence of the nearby ridge axis, may explain the range of isotopic compositions across ~15 Ma of Galápagos plume-related volcanism. The patterns of isotopic zonation observed along the hotspot track confirm the paradigm of persistent plume striping, with variations in the degree of geochemical enrichment modulated by shallow mantle processes.

The Galápagos Islands’ unique endemic flora and fauna originated mainly from colonization from South and Central America, including the famous Galápagos iguanas. Genetic analysis suggests that these iguanas arrived from Central America ~5-12 Ma million years ago (Late Miocene) or even earlier, yet the oldest of the present-day islands were formed at ~3.5 Ma. Recent geophysical analysis shows that now-submerged islands along the Cocos Ridge (Galápagos hotspot track) provided terrestrial habitat for colonization and differentiation during the time frame ~6-18 Ma. Remarkably, this was also a time window during which ocean currents and winds were much more favorable for transport from mainland Central America to these ancient islands, prior to the closing of the Isthmus of Panama at ~3-5 Ma due to regional plate tectonic forces. Thus, we can explain both the colonization timing and provenance of Galápagos iguanas in a framework that shows much promise for understanding the origins of other unique Galápagos species.

The existence of a thin, weak asthenospheric layer beneath Earth’s lithospheric plates is consistent with existing geological and geophysical constraints, including Pleistocene glacio-isostatic adjustment, modeling of gravity anomalies, studies of seismic anisotropy, and post-seismic rebound. Mantle convection models suggest that a pronounced weak zone beneath the upper thermal boundary layer (lithosphere) may be essential to the plate tectonic style of convection found on Earth. The asthenosphere is likely related to partial melting and the presence of water in the sub-lithospheric mantle, further implying that the long-term evolution of the Earth, including the apparently early onset and persistence of plate tectonics, may be controlled by thermal regulation and volatile recycling that maintain a geotherm that approaches the wet mantle solidus at asthenospheric depths.

Continental flood basalts intruded and erupted millions of km³of magma over ∼1–5 Ma. Previous work proposed the presence of large (–10⁶ km³) crustal magma reservoirs to feed these eruptions. However, in Paper I, we illustrated that this model is inconsistent with observations, by combining eruptive rate constraints with geochemical and geophysical observations from the Deccan Traps and other Continental flood basalt provinces (CFBs). Here, we use a new mechanical magma reservoir model to calculate the variation of eruptive fluxes (km³/year) and volumes for different magmatic architectures. We find that a single magma reservoir cannot explain the eruptive rate and duration constraints for CFBs. Using a 1D thermal model and characteristic timescales for magma reservoirs, we conclude that CFB eruptions were likely fed by a number of interconnected small‐medium (∼10²–10³ km³) magma reservoirs. It is unlikely that each individual magma reservoir participated in every eruption, thus permitting the occasional formation of large xenocrysts (e.g., megacrystic plagioclase). This magmatic architecture permits (a) large volume eruptive episodes with 10–100s of years duration, and (b) relatively short time‐periods separating eruptive episodes (1000s of years) since multiple mechanisms can trigger eruptions (via magma recharge or volatile exsolution, as opposed to long term (10⁵–10⁶ year) accumulation of buoyancy overpressure), and (c) lack of large upper‐crustal intrusive bodies in various geophysical datasets. Our new proposed magmatic architecture has significant implications for the tempo of CFB volatile release (CO₂and SO₂), potentially helping explain the pre‐K‐Pg warming associated with Deccan Traps.

Flood basalts are some of the largest magmatic events in Earth history, with intrusion and eruption of millions of km³ of basaltic magma over a short time period (∼1–5 Ma). A typical continental flood basalt (CFB) is emplaced in hundreds of individual eruptive episodes lasting decades to centuries with lava flow volumes of 10³–10⁴ km³. These large volumes have logically led to CFB models invoking large magma reservoirs (–10⁵ km³) within the crust or at Moho depth. Since there are currently no active CFB provinces, we must rely on observations of past CFBs with varying degrees of surface exposure to develop and test models. In the last few decades, significant improvements in geochronological, geochemical, paleomagnetic, volcanological, and paleo‐proxy measurements have provided high‐resolution constraints on CFB eruptive tempo ‐ the volume, duration, and frequency of individual eruptive episodes. Using the well‐studied Deccan Traps as an archetype for CFB systems, we compile multiple lines of evidence–geochronology, eruption tempo, dike spatial distribution, intrusive‐extrusive ratio, geochemical variations, and volcanological observations–to assess the viability of previous models. We find that the presence of just a few large crustal magma reservoirs is inconsistent with these constraints. Although observations from the Deccan Traps primarily motivate our model, we discuss constraints from other CFBs to illustrate that this conclusion may be broadly applicable for CFB magmatic systems in general.

Flood basalts are some of the largest magmatic events in Earth history, with intrusion and eruption of millions of km$^3$ of basaltic magma over a short time period ($\sim$ 1-5 Ma). A typical continental flood basalt (CFB) is emplaced in hundreds of individual eruptive episodes lasting decades to centuries with lava flow volumes of 10$^3$- 10$^4$ km$^3$. These large volumes have logically led to CFB models invoking large magma reservoirs ($>$ 10$^5$-10$^6$ km$^3$) within the crust or at Moho depth. Since there are currently no active CFB provinces, we must rely on observations of past CFBs with varying degrees of surface exposure to develop and test models. In the last few decades, significant improvements in geochronological, geochemical, paleomagnetic, volcanological, and paleo-proxy measurements have provided high-resolution constraints on CFB eruptive tempo - the volume, duration, and frequency of individual eruptive episodes. Using the well-studied Deccan Traps as an archetype for CFB systems, we compile multiple lines of evidence - geochronology, eruption tempo, dike spatial distribution, intrusive-extrusive ratio, geochemical variations, and volcanological observations - to assess the viability of previous models. We find that the presence of just a few large crustal magma reservoirs is inconsistent with these constraints. Although observations from the Deccan Traps primarily motivate our model, we discuss constraints from other CFBs to illustrate that this conclusion may be broadly applicable, with important implications for interpreting CFB geochemical datasets as well as the timing and volumes of climate-altering volatile emissions associated with CFBs.

A recent multibeam bathymetry survey in the Galápagos archipelago revealed a novel observation between the islands of Santa Cruz and Santiago: a steep flexural moat formed between two volcanic flows, nearly 50 m deep and about 500 m wide. Submarine observations and elastic plate modeling are consistent with this feature forming after one inflationary flow overlapped another young flow, deflecting the underlying lava flow in what appears to be a classical elastic flexural pattern. We present a flexural analysis of this moat feature using a thin‐plate approximation to determine the effective elastic thickness of the underlying flow. Moreover, we propose that this elastic thickness serves as a proxy for the thickness of the cooling, solidifying upper crust of the lava flow at the time of deflection, a time‐dependent thermal property. This approach offers a chronological tool to put bounds on the elapsed time between two overlapping lava flows. We applied this method to the Galápagos moat to constrain the time history of its formation. Additionally, we identified and modeled seven additional flexural moat candidates in the Hawaiian archipelago and in the Galápagos using multibeam bathymetry. Elastic/thermal history analysis of these features yields relative time intervals between submarine lava flows ranging from a few years to about 600 years, which suggests that there is a critical time interval during which elastic deflection of the underlying lava flow may become “frozen in.” This methodology may provide helpful constraints on the temporal relations between overlapping lava flows wherever they form in relatively rapid succession.

The loss of volatiles from a magma reservoir affects the magmatic overpressure responsible for driving ground deformation and eruptions. Although the high‐temperature metamorphic aureole around a magma chamber is typically considered to have low permeability, recent theoretical, experimental, and field studies have highlighted the role of transient permeability in magmatic systems. Also, direct measurements suggest that passive degassing is a significant component of total volatile loss in both basaltic and silicic volcanoes. Consequently, the effective permeability of the crust when magma is present in the system can be many orders of magnitude larger than that of exhumed rock samples. We develop a fully coupled porothermoelastic framework to account for both the flow of volatiles as well as associated effects on the stress state of the crust and calculate an analytical solution for spherical geometry. We then combine a magma chamber box model with these solutions to analyze eruption dynamics in magmatic systems. We find that in addition to viscous relaxation, magma recharge, and cooling timescales, the pore pressure diffusion timescale exerts a first‐order control on volcanic eruptions with moderately high crustal permeabilities of order 10⁻¹⁷ to 10⁻¹⁹ m². We describe a parameter space to identify which components dominate in different regimes for volcanic eruptions according to these different timescales.