Annual Review of Earth and Planetary Sciences - Volume 51, 2023

Kimberlite-borne xenolithic eclogites, typically occurring in or near cratons, have long been recognized as remnants of Precambrian subducted oceanic crust that have undergone partial melting to yield granitoids similar to the Archean continental crust. While some eclogitized oceanic crust was emplaced into cratonic lithospheres, the majority was deeply subducted to form lithologic and geochemical heterogeneities in the convecting mantle. If we accept that most xenolithic eclogites originally formed at Earth's surface, then their geodynamic significance encompasses four tectonic environments: (a) spreading ridges, where precursors formed by partial melting of convecting mantle and subsequent melt differentiation; (b) subduction zones, where oceanic crust was metamorphosed and interacted with other slab lithologies; (c) the cratonic mantle lithosphere, where the eclogite source was variably modified subsequent to emplacement in Mesoarchean to Paleoproterozoic time; and (d) the convecting mantle, into which the vast majority of subduction-modified oceanic crust not captured in the cratonic lithosphere was recycled.

• ▪  Xenolithic eclogites are fragments of ca. 3.0–1.8 Ga oceanic crust and signal robust subduction tectonics from the Mesoarchean.
• ▪  Multiple constraints indicate an origin as variably differentiated oceanic crust, followed by subduction metamorphism, and prolonged mantle residence.

• ▪  Xenolithic eclogites thus permit investigation of deep geochemical cycles related to recycling of Precambrian oceanic crust.
• ▪  They help constrain asthenosphere thermal plus redox evolution and contribute to cratonic physical properties and mineral endowments.

Terrestrial plants have transformed Earth's surface environments by altering water, energy, and biogeochemical cycles. Studying vegetation-climate interaction in deep time has necessarily relied on modern-plant analogs to represent paleo-ecosystems—as methods for reconstructing paleo- and, in particular, extinct-plant function were lacking. This approach is potentially compromised given that plant physiology has evolved through time, and some paleo-plants have no clear modern analog. Advancements in the quantitative reconstruction of whole-plant function provide new opportunities to replace modern-plant analogs and capture age-specific vegetation-climate interactions. Here, we review recent investigations of paleo-plant performance through the integration of fossil and geologic data with process-based ecosystem- to Earth system–scale models to explore how early vascular plants responded to and influenced climate. First, we present an argument for characterizing extinct plants in terms of ecological and evolutionary theory to provide a framework for advancing reconstructed vegetation-climate interactions in deep time. We discuss the novel mechanistic understanding provided by applying these approaches to plants of the late Paleozoic ever-wet tropics and at higher latitudes. Finally, we discuss preliminary applications to paleo-plants in a state-of-the-art Earth system model to highlight the potential implications of different plant functional strategies on our understanding of vegetation-climate interactions in deep time.

• ▪  For hundreds of millions of years, plants have been a keystone in maintaining the status of Earth's atmosphere, oceans, and climate.
• ▪  Extinct plants have functioned differently across time, limiting our understanding of how processes on Earth interact to produce climate.
• ▪  New methods, reviewed here, allow quantitative reconstruction of extinct-plant function based on the fossil record.
• ▪  Integrating extinct plants into ecosystem and climate models will expand our understanding of vegetation's role in past environmental change.

The strength of lithospheric plates is a central component of plate tectonics, governed by brittle processes in the shallow portion of the plate and ductile behavior in the deeper portion. We review experimental constraints on ductile deformation of olivine, the main mineral in the upper mantle and thus the lithosphere. Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding (GBS), and diffusion-accommodated grain-boundary sliding (diffusion creep). Deformation in most of the lithosphere is dominated by GBS, except in shear zones—in which diffusion creep dominates—and in the brittle-ductile transition—in which low-temperature plasticity may dominate. We find that observations from naturally deformed rocks are consistent with extrapolation of the experimentally constrained olivine flow laws to geological conditions but that geophysical observations predict a weaker lithosphere. The causes of this discrepancy are unresolved but likely reside in the uncertainty surrounding processes in the brittle-ductile transition, at which the lithosphere is strongest.

• ▪  Ductile deformation of the lithospheric mantle is constrained by experimental data for olivine.
• ▪  Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding, and diffusion creep.
• ▪  Observations of naturally deformed rocks are consistent with extrapolation of olivine flow laws from experimental conditions.
• ▪  Experiments predict stronger lithosphere than geophysical observations, likely due to gaps in constraints on deformation in the brittle-ductile transition.

Carbonate minerals contain stable isotopes of carbon and oxygen with different masses whose abundances and bond arrangement are governed by thermodynamics. The clumped isotopic value Δi is a measure of the temperature-dependent preference of heavy C and O isotopes to clump, or bond with or near each other, rather than with light isotopes in the carbonate phase. Carbonate clumped isotope thermometry uses Δi values measured by mass spectrometry (Δ47, Δ48) or laser spectroscopy (Δ638) to reconstruct mineral growth temperature in surface and subsurface environments independent of parent water isotopic composition. Two decades of analytical and theoretical development have produced a mature temperature proxy that can estimate carbonate formation temperatures from 0.5 to 1,100°C, with up to 1–2°C external precision (2 standard error of the mean). Alteration of primary environmental temperatures by fluid-mediated and solid-state reactions and/or Δi values that reflect nonequilibrium isotopic fractionations reveal diagenetic history and/or mineralization processes. Carbonate clumped isotope thermometry has contributed significantly to geological and biological sciences, and it is poised to advance understanding of Earth's climate system, crustal processes, and growth environments of carbonate minerals.

• ▪  Clumped heavy isotopes in carbonate minerals record robust temperatures and fluid compositions of ancient Earth surface and subsurface environments.
• ▪  Mature analytical methods enable carbonate clumped Δ47, Δ48, and Δ638 measurements to address diverse questions in geological and biological sciences.
• ▪  These methods are poised to advance marine and terrestrial paleoenvironment and paleoclimate, tectonics, deformation, hydrothermal, and mineralization studies.

For the first time, from early 2019 to the end of 2022, Mars’ shallow and deep interiors have been explored by seismology with the InSight mission. Thanks to the performances of its seismometers and the quality of their robotic installation on the ground, 1,319 seismic events have been detected, including about 90 marsquakes at teleseismic distances, with Mw from 2.5 to 4.7 and at least 6 impacts, the largest ones with craters larger than 130 m. A large fraction of these marsquakes occur in Cerberus Fossae, demonstrating active regional tectonics. Records of pressure-induced seismic noise and signals from the penetration of a heat flow probe have provided subsurface models below the lander. Deeper direct and secondary body wave phase travel time, receiver function, and surface wave analysis have provided the first interior models of Mars, including crustal thickness and crustal layering, mantle structure, thermal lithospheric thickness, and core radius and state.

• ▪  With InSight's SEIS (Seismic Experiment for Interior Structure of Mars) experiment and for the first time in planetary exploration, Mars’ internal structure and seismicity are constrained.
• ▪  More than 1,300 seismic events and seismic noise records enable the first comparative seismology studies together with Earth and lunar seismic data.
• ▪  Inversion of seismic travel times and waveforms provided the first interior model of another terrestrial planet, down to the core.
• ▪  Several impacts were also seismically recorded with their craters imaged from orbit, providing the first data on impact dynamic on Mars.

Planets are expected to conclude their growth through a series of giant impacts: energetic, global events that significantly alter planetary composition and evolution. Computer models and theory have elucidated the diverse outcomes of giant impacts in detail, improving our ability to interpret collision conditions from observations of their remnants. However, many open questions remain, as even the formation of the Moon—a widely suspected giant-impact product for which we have the most information—is still debated. We review giant-impact theory, the diverse nature of giant-impact outcomes, and the governing physical processes. We discuss the importance of computer simulations, informed by experiments, for accurately modeling the impact process. Finally, we outline how the application of probability theory and computational advancements can assist in inferring collision histories from observations, and we identify promising opportunities for advancing giant-impact theory in the future.

• ▪  Giant impacts exhibit diverse possible outcomes leading to changes in planetary mass, composition, and thermal history depending on the conditions.
• ▪  Improvements to computer simulation methodologies and new laboratory experiments provide critical insights into the detailed outcomes of giant impacts.
• ▪  When colliding planets are similar in size, they can merge or escape one another with roughly equal probability, but with different effects on their resulting masses, densities, and orbits.
• ▪  Different sequences of giant impacts can produce similar planets, encouraging the use of probability theory to evaluate distinct formation hypothesis.