A thermal and mechanical framework is presented for analysis of pressure‐temperature (P‐T) data and structural observations from high‐pressure‐low‐temperature (HPLT) terrains. P‐T data from 281 HPLT ...rocks exhibit two regimes separated at a pressure of ∼1.5 GPa, which corresponds to the modal maximum depth of thrust faulting in subduction zones. At pressures ≲1.5 GPa, interpreted as recording conditions on the plate interface, temperatures increase at about 350°C/GPa and are consistent with conditions calculated for shear stresses of ∼30–100 MPa on the interface. Such shear stresses are high enough to carry several kilometers' thickness of sediment at least to the base of the plate interface. Burial of material on plate interfaces occurs predominantly during large‐to‐great earthquakes; the exhumation phase involves contrasts in ascent rates of adjacent units, because of their differing buoyancies and strengths. In consequence, juxtaposition of unrelated rock types is expected to be ubiquitous, during both descent and ascent. The scarcity of temperatures higher than ∼650°C at pressures ≳1.5 GPa may reflect loss of material from the wedge‐slab interface by buoyant ascent. Exhumation of rocks in the subduction interface requires substantial reduction in shear stress, most plausibly by (near‐)cessation of subduction. During prograde metamorphism temperatures increase smoothly with depth in the plate interface, with almost isothermal compression in the wedge‐slab interface. Following cessation of subduction, rocks rising along the wedge‐slab interface are likely to heat slightly during decompression. Within the plate interface, temperatures drop following the cessation of shear heating, and rocks follow counter‐clockwise hairpin PT paths.
Key Points
A simple thermal and mechanical framework is presented for the interpretation of P‐T data from high‐pressure‐low‐temperature (HPLT) terrains
PT data from HPLT terrains are consistent with thermal regimes of present‐day plate interfaces (PI), with shear stresses of ∼30–100 MPa
Stresses are great enough that earthquakes can carry sediments to base of PI. Exhumation requires (near‐)cessation of subduction
The compositional range of ∼2,000 marine sediments and ∼19,000 oceanic igneous rocks is encapsulated by a set of 12 sedimentary and 10 mafic rock compositions, allowing computation of phase ...relationships on P‐T paths along subduction interfaces. These are described economically by a partitioning analysis, which connects the mineral assemblages to different parts of the subduction P‐T space and facilitates assessment of prograde dehydration, melting, densification, and rheological systematics. Dehydration and densification occur at shallower depths than in studies that neglect shear heating. Lawsonite stability is limited to interfaces where convergence is slower than 20 mm/yr; such rates also favor transport of volatiles beyond the arc. Terrigenous sediments and mafic rocks reach their solidi close to the top of the wedge‐slab interface; melt fractions are enhanced by fluid from the dehydrating slab interior. Rheological calculations show that the most abundant sediment types have interface capacities of hundreds of meters to kilometers, and that the strengths of mafic rocks comfortably exceed their buoyancy stresses. Above ∼650°C sediments are weak enough to rise as diapirs into the mantle wedge. Carbonate‐ and serpentinite‐rich lithologies are weaker than other interface rocks, and ascend most rapidly at the cessation of subduction. Ascent rates drop abruptly as rocks enter the plate interface, probably leading to retrograde equilibrium at P ∼ 1–1.5 GPa. The seismic‐aseismic transition is expected at about 500°C in mafics, and 400°C in metasediments. Seamounts are weaker than most other interface rocks, and unlikely to form asperities. Slow slip and tremor may be associated with the blueschist‐eclogite transition.
Key Points
We compute phase relations for subduction of 22 rock types that span 20,000 ocean‐floor samples, grouping the results parsimoniously
Shear heating controls metamorphic grade, release of fluid, and melting; strength of interface calculated from P, T, and phase relations
Calculations explain transport of interface rocks to depth, whether they rise into mantle wedge, and speed of ascent at end of subduction
Models of thermal evolution, crustal production, and CO
cycling are used to constrain the prospects for habitability of rocky planets, with Earth-like size and composition, in the stagnant lid ...regime. Specifically, we determine the conditions under which such planets can maintain rates of CO
degassing large enough to prevent global surface glaciation but small enough so as not to exceed the upper limit on weathering rates provided by the supply of fresh rock, a situation which would lead to runaway atmospheric CO
accumulation and an inhospitably hot climate. The models show that stagnant lid planets with initial radiogenic heating rates of 100-250 TW, and with total CO
budgets ranging from ∼10
to 1 times Earth's estimated CO
budget, can maintain volcanic outgassing rates suitable for habitability for ≈1-5 Gyr; larger CO
budgets result in uninhabitably hot climates, while smaller budgets result in global glaciation. High radiogenic heat production rates favor habitability by sustaining volcanism and CO
outgassing longer. Thus, the results suggest that plate tectonics may not be required for establishing a long-term carbon cycle and maintaining a stable, habitable climate. The model is necessarily highly simplified, as the uncertainties with exoplanet thermal evolution and outgassing are large. Nevertheless, the results provide some first-order guidance for future exoplanet missions, by predicting the age at which habitability becomes unlikely for a stagnant lid planet as a function of initial radiogenic heat budget. This prediction is powerful because both planet heat budget and age can potentially be constrained from stellar observations. Key Words: Exoplanets-Habitability-Stagnant lid tectonics-Carbon cycle-Volcanism. Astrobiology 18, 873-896.
Studies of oceanic crust, which covers a large proportion of the Earth's surface, have provided significant insight into the dynamics of crustal accretion processes at mid‐ocean ridges. It is now ...recognized that the nature of oceanic crust varies fundamentally as a function of spreading rate. Ocean Drilling Program (ODP) Hole 1256D (eastern Pacific Ocean) was drilled into the crust formed at a superfast spreading rate, and hence represents a crustal end member. Drilling recovered a section through lava and sheeted dykes and into the plutonic sequence, the study of which has yielded abundant insight into magmatic and hydrothermal processes operating at high spreading rates. Here, we present zircon U‐Pb dates for Hole 1256D, which constrain the age of the section, as well as the duration of crustal accretion. We find that the main pulse of zircon crystallization within plutonic rocks occurred at 15.19 Ma, consistent with magnetic anomalies, and lasted tens of thousands of years. During this episode, the main plutonic body intruded, and partial melts of the base of the sheeted dykes crystallized. One sample appears to postdate this episode by up to 0.25 Myr, and may be an off‐axis intrusion. Overall, the duration of crustal accretion was tens to several hundreds of thousands of years, similar to that found at the fast‐spreading East Pacific Rise and the slow‐spreading Mid‐Atlantic Ridge. This indicates that crustal accretion along slow‐ to superfast‐spreading ridges occurs over similar time scales, with substantially longer periods of accretion occurring at ultraslow‐spreading ridges characterized by thick lithosphere.
Plain Language Summary
The oceanic crust paves approximately 2/3 of the Earth's surface. It is formed at mid‐ocean ridges, where tectonic plates separate and new crust is formed by the solidification of magma. This magma is formed by partial melting of the upper mantle beneath the ridge axis. Plates spread at different rates at different mid‐ocean ridges, and the fastest‐known spreading occurred some 11–18 million years ago in the eastern Equatorial Pacific. A section of the crust formed during this episode of superfast‐spreading was recovered by scientific drilling in the framework of the Integrated Ocean Drilling Program (IODP). This study presents age data that determine when this section of superfast‐spreading crust formed, and how long it took to build the crust. We find that the age of the section is 15.19 Ma, and that crustal formation lasted between tens and several hundreds of thousands of years. This duration is similar to that found at mid‐ocean ridges with slow‐ to fast‐spreading rates, such as the Mid‐Atlantic Ridge and East Pacific Rise. However, it is much shorter than the formation of crust at ultraslow‐spreading ridges, where the cool and thick nature of the lithosphere leads to prolonged episodes of crustal formation.
Key Points
The main pulse of crystallization of superfast‐spreading crust at Hole 1256D occurred at 15.19 Ma
Crustal accretion lasted between tens and several hundreds of thousands of years
Crustal accretion along slow‐ to superfast‐spreading ridges occurs over similar time scales
Understanding of the thermal and geophysical evolution of the lower continental crust is limited by the resolution of conventional thermochronology. Intracrystalline daughter nuclide distribution ...profiles preserve a rich and underutilized record of thermal history. Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry, we outline here a method to simultaneously acquire 206Pb/238U age and trace element profiles from U-bearing accessory phases. Inversion of 206Pb/238U age depth profiles yields thermal history information from an extended temperature range compared to inversion of age versus grain size relationships. Thermally-activated volume diffusion of Pb and Zr in rutile is sensitive to the thermal evolution of the mid- to lower-lithosphere. We document the ability of Laser Ablation depth-profiling to simultaneously resolve 206Pb/238U age and Zr diffusion profiles in the outer ∼35 μm of lower-crustal rutile euhedra from the Ivrea Zone, Southern Alps, with <1.2 μm depth resolution. Inversion of the age profiles reveals a continuous cooling history characterized by initially rapid cooling from >600°C at ∼180 Ma followed by a period of slower cooling from ∼525°C to ∼450°C. Combined with the topology of Zr diffusion profiles, these data indicate that the Ivrea Zone underwent a brief thermal pulse in the early Jurassic, plausibly associated with hyperextension of the Adriatic margin. Inversion of near-surface 206Pb/238U age distributions can be employed to resolve otherwise inaccessible thermal history information from the lower lithosphere.
•Inversion of intragrain U–Pb age profiles yields high resolution thermal histories.•Rutile U–Pb and Zr systematics are dominated by volume diffusion.•LA-ICPMS depth-profiling resolves 206Pb/238U age and Zr closure profiles.•Method reveals that rutile from Ivrea Zone records thermal pulse at 180 Ma.
High‐temperature–low‐pressure metamorphism is commonly associated with intermediate to felsic magmatism in continental orogenic belts. The heat budgets and transfer mechanisms responsible for such ...elevated temperatures and partial melting of the upper crust are uncertain. The Trois Seigneurs massif, French Pyrenees, preserves a structurally continuous record of Variscan high‐temperature–low‐pressure metamorphism through a sequence of upper‐to‐mid‐crustal Paleozoic metasedimentary rocks. Conventional thermobarometry and phase equilibria calculations show that metamorphic conditions span ~2.5 kbar, 575°C to suprasolidus conditions of ~6 kbar, 700°C. Peak temperatures depend strongly on depth: temperature gradients of 50–60°C/km are present through the uppermost 12 km of the section; deeper portions (12–20 km) define restricted temperature conditions of ~650–700°C. The lowest‐grade metamorphic rocks preserve the largest spread in monazite 206Pb*/238U dates, from c. 325–285 Ma, while the spread in dates is restricted to c. 305–290 Ma in the highest‐grade rocks. Within this spread, each sample yields a well‐defined population of monazite 206Pb*/238U dates with peaks at c. 305 Ma in the andalusite schists, 295 Ma in the sillimanite schists, and 300 Ma in the migmatite sample. Monazite trace‐element compositions capture a systematic change with decreasing date and increasing metamorphic grade, including a more negative Eu‐anomaly and decreasing Sr concentrations, consistent with co‐crystallizing feldspar; increasing HREE and Y contents, consistent with xenotime breakdown; and decreasing Th/U, reflecting increasing U content during breakdown of inherited zircon. Zircon rims from a granite unit that formed via partial melting of the Paleozoic sedimentary package yields a 206Pb/238U‐207Pb/235U concordia age of 304.1 ± 3.73 Ma. These rims have trace‐element compositions reflecting cogenetic apatite and zircon growth during granite formation. Zircon from a calc‐alkaline granodiorite intrusion preserves a 40 Ma record of melt‐related activity in the lower crust that preceded the regional thermal climax. We interpret these petrochronological data to show that the Trois Seigneurs field gradient including andalusite schist and biotite granite samples represents a genuine geotherm through Variscan orogenic crust during the regional thermal climax at 305 Ma. When combined with constraints from other Pyrenean massifs, the form of the geotherm is consistent with a thermal scenario in which heat is advected to the upper crust by intermediate‐composition magmas generated in the lower crust. A simple thermal model for this process indicates that anatexis in the upper crust may plausibly occur within 10 Ma of the initiation of the lower‐crustal melting. Such a thermal scenario, however, requires focusing of melt through a fertile lower crust and an elevated Moho heat flux. We suggest that this process may have controlled the attainment of high‐temperature–low‐pressure metamorphic conditions along the Variscan belt and may currently be operating in zones of post‐orogenic continental extension.
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