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1. Making Continental Crust on Water-bearing Terrestrial Planets

   Borisova, Anastassia; Bernadet, Justine; Guitreau, Martin; Safonov, Oleg; Asimow, Paul D; Nedelec, Anne; Bohrson, Wendy A; Kosova, Svetlana; de_Parseval, Philippe (December 2024, American Geophysical Union)

   Debate regarding early Earth differentiation focuses on the fate, nature, origin, volume, and processes responsible for protocrust(s) and felsic crust formation. One specific aspect of this debate is how Hadean zircons and their felsic parental magmas fit with an expected ultramafic environment. Based on our new experiments, thermodynamic modeling, and elemental partitioning, we infer that felsic liquids could have been generated by shallow (< 20 km) interaction between primordial hydrated peridotite (serpentinite) and basaltic magmas. Felsic melts (SiO2 ≥ 55 wt%) can be generated at a maximum melt fraction of 0.4 when starting serpentinite:basalt mass ratio is high (i.e., higher than 1.5:1). Here we show that felsic melts obtained in our experimental runs can account for the Hf isotope evolutionary array displayed by Hadean detrital zircons worldwide. We propose that open system interactions between serpentinite and basaltic melts at the end of the magma ocean stage after magma degassing and water ocean precipitation allowed the formation of extensive early Hadean felsic crust (4.4 - 4.5 Gy ago). Our calculations indicate that this felsic crust accounts for up to 50% of present-day continental crust mass. The abundant production of primordial felsic crust throughout the Hadean could be due to the impact-induced melting owing to frequent impacts. A similar process could have also occurred on Mars, and other rocky planets, provided that water was abundant at shallow and surficial levels, which would account for the existence of a thick felsic crust. The serpentinised protocrust had a dual role in the primitive planetary environment: to provide the first and most abundant felsic crust and to facilitate the emergence of life in the shallow hydrothermal environments of water-bearing terrestrial planets. 

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2. Making continental crust on water-bearing terrestrial planets

   https://doi.org/10.1126/sciadv.ads6746  

   Bernadet, Justine; Borisova, Anastassia Y; Guitreau, Martin; Safonov, Oleg G; Asimow, Paul; Nédélec, Anne; Bohrson, Wendy A; Kosova, Svetlana A; de_Parseval, Philippe (March 2025, Science Advances)

   The debate about early Earth differentiation focuses on the processes responsible for the formation of protocrust(s) and continental crust of felsic (SiO₂ ≥ 55 weight %) composition. One aspect of this debate is how Hadean zircons fit into an ultramafic environment. On the basis of experiments, thermodynamic modeling, and elemental partitioning, we show that felsic melts could have been generated by shallow interaction between primordial serpentinized peridotite and basaltic magmas on Earth and Mars. On the basis of the hafnium isotopic evolution of Hadean detrital zircons worldwide, we infer that these interactions allowed for the formation of extensive Hadean felsic crust (4.4 to 4.5 billion years ago), which, in turn, would account for up to 50% of the present continental crustal mass. A similar process may have occurred on Mars. The serpentinized protocrust had a dual role in the primitive planetary environment: to provide ingredients for the continental crust and to enable life to emerge on water-bearing terrestrial planets. 

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3. The causes of continental arc flare ups and drivers of episodic magmatic activity in Cordilleran orogenic systems

   https://doi.org/doi.org/10.1016/j.lithos.2021.106307  

   Chapman, J. (July 2021, Lithos)

   null (Ed.)

   Continental arcs in Cordilleran orogenic systems display episodic changes in magma production rate, alternating between flare ups (70–90 km3 km􀀀 1 Myr􀀀 1) and lulls (< 20 km3 km􀀀 1 Myr􀀀 1) on timescales of tens of millions of years. Arc segments or individual magmatic suites may have even higher rates, up several 100 s of km3 km􀀀 1 Myr􀀀 1, during flare ups. These rates are largely determined by estimating volumes of arc crust, but do not reflect melt production from the mantle. The bulk of mantle-derived magmas are recycled back into the mantle by delamination of arc roots after differentiation in the deep crust. Mantle-derived melt production rates for continental arcs are estimated to be 140–215 km3 km􀀀 1 Myr􀀀 1 during flare ups and ≤ 15 km3 km􀀀 1 Myr􀀀 1 during lulls. Melt production rates averaged over multiple magmatic cycles are consistent with independent estimates for partial melting of the mantle wedge in subduction zones, however, the rates during flare ups and lulls are both anomalously high and anomalously low, respectively. The difference in mantle-derived melt production between flare ups and lulls is larger than predicted by petrologic and numerical models that explore the range of globally observed subduction parameters (e.g., convergence rate, height of the mantle wedge). This suggests that other processes are required to increase magmatism during flare ups and suppress magmatism during lulls. There are many viable explanations, but one possibility is that crystallized melts from the asthenospheric mantle wedge are temporarily stored in the deep lithosphere during lulls and then remobilized during flare ups. Basaltic melts may stall in the mantle lithosphere in inactive parts of the arc system, like the back-arc, refertilizing the mantle lithosphere and suppressing melt delivery to the lower crust. Subsequent landward arc migration (i.e., toward the interior of the continent) may encounter such refertilized mantle lithosphere magma source regions, contributing to magmatic activity during a flare up. A review of continental arcs globally suggests that flare ups commonly coincide with landward arc migration and that this migration may start tens of millions of years before the flare up occurs. The region of magmatic activity, or arc width, can also expand significantly during a flare up. Arc migration or expansion into different mantle source regions and across lithospheric and crustal boundaries can cause temporal shifts in the radiogenic isotopic composition of magmatism. In the absence of arc migration, temporal shifts are more muted. Isotopic studies of mantle xenoliths and exposures of deep arc crust suggest that that primary, mantle-derived magmas generated during flare ups reflect substantial contributions from the subcontinental mantle lithosphere. Arc migration may be caused by a variety of mechanisms, including slab anchoring or slab folding in the mantle transition zone that could generate changes in slab dip. Episodic slab shallowing is associated with many tectonic processes in Cordilleran orogenic systems, like alternations between shortening and extension in the upper plate. Studies of arc migration may help to link irregular magmatic production in continental arcs with geodynamic models for orogenic cyclicity. 

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4. Pyroxenite melting at subduction zones

   https://doi.org/10.1130/G50929.1  

   Bowman, Emilie E.; Ducea, Mihai N. (February 2023, Geology)

   Abstract Arc magmatism is thought to be driven by peridotite melting in the mantle wedge. Yet pyroxenites are ubiquitous in the melting region beneath magmatic arcs. Because they typically have lower solidi temperatures and higher melt productivities compared to peridotite, pyroxenites likely play a significant role in magma generation. Here, we use the Zn/Fe ratios of a global database of Pliocene–Holocene primitive arc magmas to show that, as the crustal thickness of the overlying plate increases, so does the proportion of pyroxenite-derived melts relative to peridotite-derived melts. In fact, at arcs with crustal thicknesses >40 km, the majority of magmas are sourced from pyroxenite. Major and trace element geochemistry of pyroxenite melts is consistent with derivation from mafic magmas frozen in the mantle en route to the surface. We hypothesize that, as the thickness of the continental crust increases, the mantle wedge is displaced toward higher pressures and cooler temperatures, thereby lowering the extent of peridotite melting and allowing magmas sourced from the pyroxeniteveined mantle to dominate the arc budget. 

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5. Mafic intrusions record mantle inputs and crustal thickness in the eastern Sierra Nevada batholith, California, USA

   https://doi.org/10.1130/B36646.1  

   Lewis, Madeline J; Ryan-Davis, Juliet R; Bucholz, Claire E (August 2023, Geological Society of America Bulletin)

   Contributions of heat and/or mass from mafic magmas are commonly invoked in models of voluminous granodiorite and andesite generation in magmatic and volcanic arcs worldwide. However, mafic intrusions are a volumetrically minor component in most arc batholiths. This is the case in the Sierra Nevada batholith, California, USA, where gabbro and diorite plutons are smaller and less abundant than the granitoid suites that make up the bulk of the batholith. Here, we constrain the timing and geochemistry of mafic intrusions in the Sierra Nevada batholith to assess the role of these compositions in arc batholith construction. Previous detailed studies on a limited number of mafic intrusions demonstrate that they formed penecontemporaneously with the felsic batholith, but there is a need for a broader survey of mafic plutons using modern geochronological techniques. New U-Pb zircon ages for 13 gabbro to diorite plutons and geochemistry from 17 mafic intrusions in the eastern Sierra Nevada batholith document two main episodes of mafic magmatism in the eastern Sierra Nevada batholith, from 168 Ma to 145 Ma and from 100 Ma to 89 Ma. These episodes overlap with the ages of granitoid plutons in the eastern Sierra Nevada batholith, including the Late Jurassic Palisade Crest and Late Cretaceous John Muir intrusive suites, in addition to other felsic plutons dated in the eastern Sierra Nevada batholith. Non-primitive mineral compositions in the mafic bodies indicate that their parental magmas are the evolved products of mantle-derived basalts that first differentiated in the lower crust prior to ascent and crystallization in the upper crust. The presence of rocks with cumulate textures, as well as a wide range of bulk-rock compositions (SiO2 wt% 38−64, Mg# 39−74), show that magmatic differentiation continued within each mafic body after intrusion into the upper crust. Sr/Y ratios in melt-like (i.e., non-cumulate) mafic samples suggest that the crustal thickness of the Sierra Nevada batholith was roughly 30 km in the Early Jurassic and increased to ∼44 km by the Late Cretaceous. Concomitant intrusion of mafic melts along with voluminous granitoid plutons supports mantle melting as a major contributor of heat and magmatic volumes to the crust during construction of the eastern Sierra Nevada batholith. 

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