Polybaric fractional crystallisation of arc magmas: an experimental study simulating trans-crustal magmatic systems

Crystallisation-driven differentiation is one fundamental mechanism proposed to control the compositional evolution of magmas. In this experimental study, we simulated polybaric fractional crystallisation of mantle-derived arc magmas. Various pressure–temperature trajectories were explored to cover a range of potential magma ascent paths and to investigate the role of decompression on phase equilibria and liquid lines of descent (LLD). Fractional crystallisation was approached in a step-wise manner by repetitively synthesising new starting materials chemically corresponding to liquids formed in previous runs. Experiments were performed at temperatures ranging from 1140 to 870 °C with 30 °C steps, and pressure was varied between 0.8 and 0.2 GPa with 0.2 GPa steps. For most fractionation paths, oxygen fugacity (fO2) was buffered close to the Ni-NiO equilibrium (NNO). An additional fractionation series was conducted at fO2 corresponding to the Re-ReO2 buffer (RRO ≈ NNO+2). High-pressure experiments (0.4–0.8 GPa) were run in piston cylinder apparatus while 0.2 GPa runs were conducted in externally heated pressure vessels. Resulting liquid lines of descent follow calc-alkaline differentiation trends where the onset of pronounced silica enrichment coincides with the saturation of amphibole and/or Fe–Ti–oxide. Both pressure and fO2 exert crucial control on the stability fields of olivine, pyroxene, amphibole, plagioclase, and Fe–Ti–oxide phases and on the differentiation behaviour of arc magmas. Key observations are a shift of the olivine–clinopyroxene cotectic towards more clinopyroxene-rich liquid composition, an expansion of the plagioclase stability field and a decrease of amphibole stability with decreasing pressure. Decompression-dominated ascent trajectories result in liquid lines of descent approaching the metaluminous compositional range observed for typical arc volcanic rocks, while differentiation trends obtained for cooling-dominated trajectories evolve to peraluminous compositions, similar to isobaric liquid lines of descent at elevated pressures. Experiments buffered at RRO provide a closer match with natural calc-alkaline differentiation trends compared to fO2 conditions close to NNO. We conclude that decompression-dominated fractionation at oxidising conditions represents one possible scenario for arc magma differentiation.

A mantle-source solution to the enigma of bimodal arc volcanism

Silicate melts in arc environments are dominated by mafic (low-silica) and silicic (high-silica) compositions, often generating a characteristic bimodal pattern. We investigate the whole arc crust and show that the plutonic lower crust shares the bimodal pattern of melts from volcanoes. This key observation reveals that, contrary to some explanations of bimodal volcanism, variation in mantle source and mantle processes must fundamentally control bimodalism. We also recognise bimodalism in Th/La composition of the whole arc crust and suggest a new working hypothesis: bimodalism originates by melting of distinct sub-arc mantle sources, one dominated by relatively dry peridotite and the other by hydrous pyroxenite. The two groups of primary melts fractionate along distinct liquid lines of descent that lead to relatively dry mafic melts (Th/La~0.1) versus hydrous silicic melts (Th/La>0.2) by 65–80% fractional crystallisation. Common crustal processes such as crystal fractionation, assimilation, reactive flow and/or magma mixing may also lead to differentiation of both groups.

MushPEC: Correcting Post-entrapment Processes Affecting Melt Inclusions Hosted in Olivine Antecrysts

Olivine-hosted melt inclusions (MIs) are widely used as a tool to study the early stages of magmatic evolution. There are a series of processes that affect MI compositions after trapping, including post-entrapment crystallization (PEC) of the host mineral at the MI boundaries, exsolution of volatile phases into a “shrinkage bubble” and diffusive exchange between a MI and its host. Classical correction schemes applied to olivine-hosted MIs include PEC correction through addition of olivine back to the melt until it reaches equilibrium with the host composition and “Fe-loss” correction due to Fe-Mg diffusive exchange. These corrections rely on the assumption that the original host composition is preserved. However, for many volcanic samples the crystal cargo is thought to be antecrystic, and the olivine composition may thus have been completely re-equilibrated during long crystal storage times. Here, we develop a novel MI correction scheme that is applicable when the original host crystal composition has not been preserved and the initial MI composition variability can be represented by simple fractional crystallization (FC). The new scheme allows correction of MI compositions in antecrystic hosts with long and varied temperature histories. The correction fits a set of MI compositions to modelled liquid lines of descent generated by FC. A MATLAB® script (called MushPEC) iterates FC simulations using the rhyolite-MELTS algorithm. In addition to obtaining the corrected MI compositions, the application of this methodology provides estimations of magmatic conditions during MI entrapment. A set of MIs hosted in olivine crystals of homogeneous composition (Fo77–78) from a basaltic tephra of Akita-Komagatake volcano was used to test the methodology. The tephra sample shows evidence of re-equilibration of the MIs to a narrow Mg# range equivalent to the carrier melt composition. The correction shows that olivine hosts were stored in the upper crust (c. 125 – 150 MPa) at undersaturated H2O contents of c. 1 – 2 wt% H2O).

Skeletal stochastic differential equations for superprocesses

It is well understood that a supercritical superprocess is equal in law to a discrete Markov branching process whose genealogy is dressed in a Poissonian way with immigration which initiates subcritical superprocesses. The Markov branching process corresponds to the genealogical description of prolific individuals, that is, individuals who produce eternal genealogical lines of descent, and is often referred to as the skeleton or backbone of the original superprocess. The Poissonian dressing along the skeleton may be considered to be the remaining non-prolific genealogical mass in the superprocess. Such skeletal decompositions are equally well understood for continuous-state branching processes (CSBP).In a previous article [16] we developed an SDE approach to study the skeletal representation of CSBPs, which provided a common framework for the skeletal decompositions of supercritical and (sub)critical CSBPs. It also helped us to understand how the skeleton thins down onto one infinite line of descent when conditioning on survival until larger and larger times, and eventually forever.Here our main motivation is to show the robustness of the SDE approach by expanding it to the spatial setting of superprocesses. The current article only considers supercritical superprocesses, leaving the subcritical case open.

Mineral compositions and thermobarometry of basalts and boninites recovered during IODP Expedition 352 to the Bonin forearc

Central aims of IODP Expedition 352 were to delineate and characterize the magmatic stratigraphy in the Bonin forearc to define key magmatic processes associated with subduction initiation and their potential links to ophiolites. Expedition 352 penetrated 1.2 km of magmatic basement at four sites and recovered three principal lithologies: tholeiitic forearc basalt (FAB), high-Mg andesite, and boninite, with subordinate andesite. Boninites are subdivided into basaltic, low-Si, and high-Si varieties. The purpose of this study is to determine conditions of crystal growth and differentiation for Expedition 352 lavas and compare and contrast these conditions with those recorded in lavas from mid-ocean ridges, forearcs, and ophiolites. Cr# (cationic Cr/Cr+Al) vs. TiO2 relations in spinel and clinopyroxene demonstrate a trend of source depletion with time for the Expedition 352 forearc basalt to boninite sequence that is similar to sequences in the Oman and other suprasubduction zone ophiolites. Clinopyroxene thermobarometry results indicate that FAB crystallized at temperatures (1142–1190 °C) within the range of MORB (1133–1240 °C). When taking into consideration liquid lines of descent of boninite, orthopyroxene barometry and olivine thermometry of Expedition 352 boninites demonstrate that they crystallized at temperatures marginally lower than those of FAB, between ~1119 and ~1202 °C and at relatively lower pressure (~0.2–0.4 vs. 0.5–4.6 kbar for FAB). Elevated temperatures of boninite orthopyroxene (~1214 °C for low-Si boninite and 1231–1264 °C for high-Si boninite) may suggest latent heat produced by the rapid crystallization of orthopyroxene. The lower pressure of crystallization of the boninite may be explained by their lower density and hence higher ascent rate, and shorter distance of travel from place of magma formation to site of crystallization, which allowed the more buoyant and faster ascending boninites to rise to shallower levels before crystallizing, thus preserving their high temperatures.

Geochemical Constraints on Mantle Melting and Magma Genesis at Pohnpei Island, Micronesia

The lithospheric mantle is of paramount importance in controlling the chemical composition of ocean island basalts (OIBs), influencing partial melting and magma evolution processes. To improve the understanding of these processes, the pressure–temperature conditions of mantle melting were investigated, and liquid lines of descent were modelled for OIBs on Pohnpei Island. The studied basaltic samples are alkalic, and can be classified as SiO2-undersaturated or SiO2-saturated series rocks, with the former having higher TiO2 and FeOT contents but with no distinct trace-element composition, suggesting melting of a compositionally homogenous mantle source at varying depths. Both series underwent sequential crystallization of olivine, clinopyroxene, Fe–Ti oxides, and minor plagioclase and alkali feldspar. Early magnetite crystallization resulted from initially high FeOT contents and oxygen fugacity, and late feldspar crystallization was due to initially low Al2O3 contents and alkali enrichment of the evolved magma. The Pohnpei lavas formed at estimated mantle-melting temperatures of 1486–1626 °C (average 1557 ± 43 °C, 1σ), and pressures of 2.9–5.1 GPa (average 3.8 ± 0.7 GPa), with the SiO2-undersaturated series forming at higher melting temperatures and pressures. Trace-element compositions further suggest that garnet rather than spinel was a residual phase in the mantle source during the melting process. Compared with the Hawaiian and Louisville seamount chains, Pohnpei Island underwent much lower degrees of mantle melting at greater depth, possibly due to a thicker lithosphere.

Magmatic Evolution Following Damp Tholeiitic and Wet Calc-alkaline Liquid Lines of Descent: An Eastern Pontides (NE Turkey) Example

Associations between tholeiitic and calc-alkaline arc magmatism with close spatial and temporal relationships can provide critical constraints on magma genesis and allow the reconstruction of subduction polarity at convergent margins. This study identifies two compositionally distinct intrusive series from the Yusufeli region in the Eastern Pontides arc, NE Turkey. The intrusive rocks from the Yusufeli intrusive complex were emplaced at 179–170 Ma and are dominated by the low- to medium-K tholeiitic series, with depleted Hf isotopic compositions. In contrast, the intrusive rocks from the Camlikaya intrusive complex were emplaced at 151–147 Ma and are characterized by the medium- to high-K calc-alkaline series, with relatively enriched Hf isotopic compositions. The Al-in-hornblende geobarometer reveals that the magmas of both two intrusive complexes crystallized at upper crustal levels (∼150–250 MPa, ∼5–8 km). The presence of patchy-textured plagioclase and the widespread occurrence of coeval dykes and magmatic mafic enclaves (MMEs) indicate that two intrusive complexes are derived from multiple magma pulses in open magmatic systems. The mineral crystallization order of amphibole and plagioclase, the trace elemental signatures (e.g. Sr/Y and Y), and rare-earth element modeling collectively suggest that the Yusufeli intrusive complex was dominated by plagioclase and clinopyroxene fractionation with earlier plagioclase crystallization than amphibole, whereas the Camlikaya intrusive complex was dominated by the fractionation of amphibole accompanied by co-crystallization of plagioclase. Such significant differences in the fractionating mineral assemblages at comparable intrusion pressures can be attributed to different initial H2O contents of the Yusufeli and Camlikaya parental magmas, which ultimately control their distinct liquid lines of descent (LLD). In accord with thermodynamic modeling results derived using the Rhyolite-MELTS software, we propose that the Yusufeli intrusive rocks are derived from damp (∼1–2 wt.% H2O) parental magmas formed dominantly by decompression melting of mantle wedge in a back-arc setting. In contrast, the wet parental magmas (>∼2 wt.% H2O) of Camlikaya intrusive rocks are more hydrous and formed through flux melting of supra subduction zone mantle wedge. Combined with the back-arc basin related Jurassic sedimentary and structural records previously determined in the Southern Zone of the Eastern Pontides, the geochemical compositions and spatial relationship of the Yusufeli and Camlikaya intrusive complexes are preferred to be explained by the southward subduction of the Paleotethys oceanic lithosphere in the Early to Late Jurassic.

Consecuencias de la transferencia horizontal de genes en la evolución genómica eucarionte [Consequences of horizontal gene transfer in eukaryotic genomic evolution]

Gene exchange between prokaryotes has long been recognized as an important biological process, to the extent that it has been proposed that a more suitable metaphor to describe the evolutionary process, at least in prokaryotes, should be that of "the web of life", where lines of descent not only diverge but also communicate and even fuse with each other, resulting in a reticulated pattern. This pattern partially describes and outlines the actual complexity because, with the exception of some ancestral horizontal gene transfer (HGT) events, it relegates most eukaryotes.In this work, I summarize the evidence on how such lines of descent diverge, communicate, and merge with each other, in both prokaryotes and eukaryotes, involving deep branches and recent divergences. In fact, both direct and indirect evidence shows the existence of active HGT mechanisms in eukaryotes, indicating that horizontal transfer of genetic material is an ongoing evolutionary process in these organisms, as important as it is in the case of prokaryotes. Some of the consequences in eukaryotic genomic evolution that this phenomenon implies include, as in prokaryotes, the acquisition of novel functions, reacquisition of lost genes, and incorporation of adaptive advantages.HGT involves not only phagocytic single-celled eukaryotes but all eukaryotes —protists, plants, fungi, and animals, including humans. This lateral transfer of genetic material occurs through homologous HGT mechanisms that also exist in bacteria: conjugation, transduction, endogenization, and transformation. Given the evidence, I suggest a global economy of genetic material among all current organisms: viruses, prokaryotes, and eukaryotes; fading the notion of barriers between species, as a result of the continuous horizontal circulation and use of available genetic material.Finally, I propose an ecological classification focused on the various interactions with which foreign genetic material is obtained, to facilitate the understanding of HGT as an ecological and evolutionary phenomenon, natural and global, that affects all organisms and involves all reservoirs of genetic material. **Resumen**Desde hace tiempo, se reconoce al intercambio de genes entre procariontes como un proceso biológico importante, al grado de que se ha propuesto que una metáfora más adecuada para describir el proceso evolutivo, al menos procarionte, debería ser la de “la red de la vida” en donde las líneas de la descendencia no sólo divergen sino también se comunican e incluso se fusionan unas con otras, dando como resultado un patrón reticulado. Este patrón describe y esquematiza parcialmente la complejidad real debido a que, con excepción de algunos eventos de transferencia horizontal de genes (THG) ancestrales, relega a la mayoría de los eucariontes.En este trabajo sintetizo la evidencia sobre cómo dichas líneas de descendencia divergen, se comunican y fusionan unas con otras, tanto en procariontes como en eucariontes, involucrando ramas profundas y divergencias recientes. De hecho, diversas evidencias tanto directas como indirectas, muestran la existencia de mecanismos de THG activos en los eucariontes, indicando que la transferencia horizontal de material genético es un proceso evolutivo actual en estos organismos, tan importante como lo es en el caso de los procariontes. Algunas de las consecuencias en la evolución genómica eucarionte que este fenómeno implica incluyen, al igual que en los procariontes, la adquisición de funciones novedosas, readquisición de genes perdidos e incorporación de ventajas adaptativas.La THG involucra no sólo a eucariontes unicelulares principalmente fagocíticos sino a todos los eucariontes —protistas, plantas, hongos y animales, incluyendo a los humanos. Esta transferencia lateral de material genético occure mediante mecanismos homólogos de THG que también existen en bacterias: conjugación, transducción, endogenización y transformación. Dada la evidencia, planteo una economía global del material genético entre todos los organismos actuales —virus, procariontes y eucariontes; desvaneciendo la noción de barreras entre las especies, resultado de la continua circulación horizontal y aprovechamiento del material genético disponible. Finalmente, propongo una clasificación ecológica centrada en las diversas interacciones con las que se obtiene el material genético foráneo, para facilitar el entendimiento de la THG como un fenómeno ecológico y evolutivo, natural y global, que afecta a todos los organismos e involucra a todos los reservorios de material genético.