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This is the preprint of the accepted article: Klöcking, M., et. al. (2023). Community recommendations for geochemical data, services and analytical capabilities in the 21st century. In Geochimica et Cosmochimica Acta. Elsevier BV. https://doi.org/10.1016/j.gca.2023.04.024 Abstract The majority of geochemical and cosmochemical research is based upon observations and, in particular, upon the acquisition, processing and interpretation of analytical data from physical samples. The exponential increase in volumes and rates of data acquisition over the last century, combined with advances in instruments, analytical methods and an increasing variety of data types analysed, has necessitated the development of new ways of data curation, access and sharing. Together with novel data processing methods, these changes have enabled new scientific insights and are driving innovation in Earth and Planetary Science research. Yet, as approaches to data-intensive research develop and evolve, new challenges emerge. As large and often global data compilations increasingly form the basis for new research studies, institutional and methodological differences in data reporting are proving to be significant hurdles in synthesising data from multiple sources. Consistent data formats and descriptions as well as appropriate information on data quality are becoming crucial to enabling reproducibility and integration of results and fostering confidence for data reuse. Here, we explore the key challenges faced by the geo- and cosmochemistry community and, by drawing comparisons from other communities, recommend possible approaches to overcome them. The first challenge is bringing together the numerous sub-disciplines within our community. One key factor for this convergence will be gaining endorsement from the international geochemical, cosmochemical and analytical societies and associations, journals and institutions. Increased education and outreach, spearheaded by ambassadors recruited from leading scientists across disciplines, will further contribute to raising awareness, and to uniting and mobilising the community. Appropriate incentives, recognition and credit for good data management as well as an improved, user-oriented technical infrastructure will be essential for achieving a cultural change towards an environment in which the effective use and real-time interchange of large datasets is common-place. Finally, the development of best practices for standardised data reporting and exchange, driven by expert working groups, will be a crucial step towards making geo- and cosmochemical data more Findable, Accessible, Interoperable and Reusable by both humans and machines (FAIR).

This contribution reviews the effects of source heterogeneities, melt-rock reactions and intracrustal differentiation on magma chemistry across mid-ocean ridges, intraplate settings and subduction zones using experimental studies and natural data. We compare melting behaviors of pyroxenites and peridotites and their relative contributions to magmas as functions of composition, mantle potential temperatures and lithospheric thickness. We also discuss the fate of chemically distinct melts derived from heterogeneities as they travel through a peridotitic mantle. Using nearly 60,000 natural major element compositions of volcanic rocks, melt inclusions, and crystalline cumulates, we assess broad petrogenetic trends in as large of a global dataset as possible. Consistent with previous studies, major element chemistry of mid-ocean ridge basalts (MORBs) and their cumulates favor a first-order control of intracrustal crystal-liquid segregation, while trace element studies emphasize the role of melt-rock reactions, highlighting the decoupling between the two. Ocean island basalts (OIB) show a larger compositional variability than MORB, partly attributed to large variations of pyroxenite proportions in the mantle source. However, the estimated proportions vary considerably with heterogeneity composition, melting model and thermal structure of the mantle. For arcs, we highlight current views on the role of the downgoing slab into the source of primary arc magmas, and the role of the overriding lithosphere as a magmatic chemical filter and as the repository of voluminous arc cumulates. Our approach of simultaneously looking at a large database of volcanic + deep crustal rocks across diverse tectonic settings underscores the challenge of deciphering the source signal versus intracrustal/lithospheric processes., Comment: 42 pages, 9 figures, Accepted chapter to appear in "Mantle Convection and Surface Expressions", H. Marquardt, M. Ballmer, S. Cottar, K. Jasper (eds.), AGU Monograph Series

The opening of the North Atlantic about 56 My ago was associated with the emplacement of the North Atlantic Igneous Province, including the deposition of voluminous extrusive basaltic successions and intrusion of magma into the surrounding sedimentary basins. The mid-Norwegian Margin is a global type example of such volcanic rifted margins and is well suited for scientific drilling with its thin sediment cover and good data coverage. During International Ocean Discovery Program Expedition 396, 21 boreholes were drilled at 10 sites in five different geological settings on this volcanic margin. The boreholes sampled a multitude of igneous and sedimentary settings ranging from lava flow fields to hydrothermal vent complexes, along with thick successions of upper Paleocene and lower Eocene strata. A comprehensive suite of wireline logs was collected in eight boreholes. The main goals of the expedition were to provide constraints for geodynamic models to test different hypotheses that can explain the rapid emplacement of large igneous provinces and the hypothesis that the associated Paleocene/Eocene Thermal Maximum was caused by hydrothermal release of carbon in response to magmatic intrusions. Successful drilling, combined with high core recovery of target intervals of all nine primary sites and one additional alternate site, should allow us to achieve these goals during postcruise work.

The Earth’s mantle is heterogeneous as a result of early planetary differentiation and subsequent crustal recycling during plate tectonics. Radiogenic isotope signatures of mid-ocean ridge basalts have been used for decades to map mantle composition, defining the depleted mantle endmember. These lavas, however, homogenize via magma mixing and may not capture the full chemical variability of their mantle source. Here, we show that the depleted mantle is significantly more heterogeneous than previously inferred from the compositions of lavas at the surface, extending to highly enriched compositions. We perform high-spatial-resolution isotopic analyses on clinopyroxene and plagioclase from lower crustal gabbros drilled on a depleted ridge segment of the northern Mid-Atlantic Ridge. These primitive cumulate minerals record nearly the full heterogeneity observed along the northern Mid-Atlantic Ridge, including hotspots. Our results demonstrate that substantial mantle heterogeneity is concealed in the lower oceanic crust and that melts derived from distinct mantle components can be delivered to the lower crust on a centimetre scale. These findings provide a starting point for re-evaluation of models of plate recycling, mantle convection and melt transport in the mantle and the crust.

Geochemically enriched signatures in global oceanic basalts have long indicated a heterogeneous mantle source, but the role of lithologic heterogeneity in producing mantle partial melts, particularly fertile pyroxenite rocks, remains unclear. Uranium-series disequilibria in basalts are particularly sensitive to the increased garnet mode and melting rates of pyroxenite rocks, making the system a useful indicator of mantle lithologic heterogeneity in the melt region for oceanic basalts. Here we summarize evidence for the presence and importance of pyroxenite rocks in the upper mantle and their role in melt generation of mid-ocean ridge basalts and ocean island basalts, with a synthesis of U-series disequilibrium systematics in oceanic basalts and implications for global lithologic heterogeneity of the upper mantle. We further synthesize the melt modeling approaches for the interpretation of U-series disequilibria in basalts and demonstrate the use of numerical solution models for time-dependent reactive porous flow and dynamic melting during decompression of a two-lithology mantle in thermal equilibrium. Our model outcomes corroborate prior interpretations in favor of reactive porous flow and two-porosity transport for relatively homogeneous, peridotite-dominated mantle regimes, and further support contributions of pyroxenite partial melts to aggregated melts in order to reproduce the heterogeneous global basalt data. To most accurately predict the conditions of melting by comparison with measured data, two-lithology melting calculations should carefully consider the role of thermal equilibrium, mineral/melt partitioning, non-linear variations in mineral modes, and degree of melting during the melting process.

Natural forsterite(Fo)-rich olivines represent the major constituent of the Earth's upper mantle. Collected at the surface in mantle xenoliths, they are commonly used in scientific research. Many electron microprobe labs use the San Carlos standard USNM111312/44 [1], provided by the Smithonian Institution, for calibrations. Non-USNMdistributed crystals of San Carlos olivine are also often used as starting material in experimental studies [e.g., 2, 3]. However, the potential inherent chemical variability of starting materials can affect results and their scientific interpretations. Hence, it is important to characterize the full chemical variability of the San Carlos olivine. Fournelle [4] showed that the USNM material shows limited variability (Fo89.6 to Fo90.5), but that non-standard San Carlos olivines can be significantly more variable, with Fo contents ranging from 87 to 92%. Following these results, we report new major and trace element analyses on grains (0.5 mm - >5mm) of non-USNM San Carlos olivine and compare them with analyses on USNM San Carlos standard compositions. We also investigate the presence of potential grain-scale chemical variations by looking at composition profiles on large (> 5mm) grains. Observed major-element variations (Fo88.4 to Fo91.4) are consistent with Fournelle’s results [4]. Additionally, we show that minor and trace element concentrations present significant and contrasted variations between grains (e.g., 17 % Ni, 28 % Mn, 44% V, 69 % Al, 285 % P, relative). At the scale of the individual grain, however, San Carlos olivines appear relatively homogeneous with no systematic core-rim variations. Results and implications for the use of this material in experimental studies and for interpretations of the petrogenetic processes will be discussed.

Large igneous provinces on Earth result from anomalously enormous volcanic eruptions at high melt production rates. These eruptions are often linked to catastrophic events such as mass extinctions, global climate changes, or continental break-up. Decoding their petrogenesis is therefore of great importance for our comprehensive understanding of the evolution and geodynamics of our planet. The ~260 Ma Emeishan large igneous province is an important geological feature of SW China with world-class ore deposits and is also suggested to be linked with the Capitanian mass extinction. However, fundamental aspects of the genesis of Emeishan province's most primitive lavas (picrites), such as the source lithology (pyroxenite or peridotite), the origin of compositional variations of olivines and the melting temperature and pressure conditions, remain poorly constrained. Here, we compile information on melt inclusion and host olivine, and whole-rock compositions from the ELIP picrites and show that these data are consistent with decompression melting of a relatively homogeneous peridotitic mantle plume, with a potential temperature higher than 1560 °C. The compositional variability of the olivines and picrites can be explained by varying the equilibrium depth of primary magma segregation and does not require the contribution of a pyroxenite component as previously suggested. Our results favor a scenario for the origin of the Emeishan large igneous province in which the decompression melting during upwelling of a hot hydrous and oxidized mantle plume is accompanied by catastrophic lithospheric thinning. In combination with the now extensive multi-element geochemical data, our findings provide a starting point for re-evaluation of the petrogenesis models for large igneous provinces.

Geochemical and isotopic data suggest that the source regions of oceanic basalts may contain\ud pyroxenite in addition to peridotite. In order to incorporate the wide range of compositions and melting\ud behaviors of pyroxenites into mantle melting models, we have developed a new parameterization, Melt-PX,\ud which predicts near-solidus temperatures and extents of melting as a function of temperature and pressure\ud for mantle pyroxenites. We used 183 high-pressure experiments (25 compositions; 0.9–5 GPa; 1150–1675°C)\ud to constrain a model of melt fraction versus temperature from 5% melting up to the disappearance of\ud clinopyroxene for pyroxenites as a function of pressure, temperature, and bulk composition. When applied to\ud the global set of experimental data, our model reproduces the experimental F values with a standard error of\ud estimate of 13% absolute; temperatures at which the pyroxenite is 5% molten are reproduced with a\ud standard error of estimate of 30°C over a temperature range of ~500°C and a pressure range of ~4 GPa. In\ud conjunction with parameterizations of peridotite melting, Melt-PX can be used to model the partial melting\ud of multilithologic mantle sources—including the effects of varying the composition and the modal\ud proportion of pyroxenite in such source regions. Examples of such applications include calculations of\ud isentropic decompression melting of a mixed peridotite + pyroxenite mantle; these show that although the\ud potential temperature of the upwelling mantle plays an important role in defining the extent of magma\ud production, the composition and mass fraction of the pyroxenite also exert strong controls.

To better assess the potential role of pyroxenites in basalt generation at mid-ocean ridges, we performed partial melting experiments on two natural websterites and one clinopyroxenite representative of worldwide pyroxenites. The experiments were conducted at 1 and 1.5 GPa in a piston-cylinder apparatus; the microdike technique was used to separate the liquid from the solid phases and to obtain reliable glass analyses even at low degrees of melting. Contrasted melting behaviors were observed depending on the phase proportions at the solidus, especially the abundance of orthopyroxene. (1) If orthopyroxene is abundant, the main melting reaction is similar to the melting reaction in peridotites (clinopyroxene + orthopyroxene +/- spinel = liquid + olivine), and the liquids are similar to peridotite-derived melts for most major elements. (2) In the absence of orthopyroxene, the main melting reaction is clinopyroxene + spinel = liquid + olivine, yielding liquids that are strongly depleted in SiO2 in comparison to peridotite-derived melts. This low-SiO2 content can be associated with a high FeO content, a combination usually ascribed to a high average pressure of melting (of a peridotitic source). Because of their higher melt productivities and lower solidus temperatures, 5 wt.% of pyroxenites in a heterogeneous mantle may contribute up to 40 wt.% of the total melt production. (1) In some cases, pyroxenite-derived melts differ strongly from peridotite partial melts, leading to a distinct pyroxenite signature in the average melt (lower alkali and TiO2 contents, lower SiO2, higher FeO and/or lower Mg#). The classical criteria used to select primitive mantle-derived magmas (melt inclusions hosted into high Mg# olivine or MORB glasses with Mg# >= 67) or to track down enriched mantle sources (MORB glasses with high incompatible element contents) must be considered with caution, otherwise melts carrying a pyroxenite signature may be eliminated. (2) In general, however, the major-element signature of pyroxenites should be hardly detectable in the average melt because of the similarity of most pyroxenite-derived melts with peridotite partial melts. This similarity may explain why MORB have relatively uniform major-element compositions, but may have variable trace element and/or isotopic compositions.

International audience; Piston-cylinder experiments were performed to characterize the composition of liquids formed at very low degrees of melting of two fertile lherzolite compositions with 430 ppm and 910 ppm K2O at 1 and 1.3 GPa. We used the microdike technique (Laporte et al., 2004) to extract the liquid phase from the partially molten peridotite, allowing us to analyze liquid compositions at degrees of melting F down to 0.9%. At 1.3 GPa, the liquid is in equilibrium with olivine + orthopyroxene + clinopyroxene + spinel in all the experiments; at 1 GPa, plagioclase is present in addition to these four mineral phases up to about 5% of melting (View the MathML sourceT≈1240°C). Important variations of liquid compositions are observed with decreasing temperature, including strong increases in SiO2, Na2O, K2O, and Al2O3 concentrations, and decreases in MgO, FeO, and CaO concentrations. The most extreme liquid compositions are phonolites with 57% SiO2, 20–22% Al2O3, Na2O + K2O up to 14%, and concentrations of MgO, FeO, and CaO as low as 2–3%. Reversal experiments confirm that low-degree melts of a fertile lherzolite have phonolitic compositions, and pMELTS calculations show that the amount of phonolite liquid generated at 1.2 GPa increases from 0.3% in a source with 100 ppm K2O to 3% in a source with 2000 ppm K2O. The enrichment in silica and alkalis with decreasing melt fraction is coupled with an increase of the degree of melt polymerization, which has important consequences for the partitioning of minor and trace elements. Thus Ti4+ in our experiments and, by analogy with Ti4+, other highly charged cations, and rare earth elements become less incompatible near the peridotite solidus.Our study brings a strong support to the hypothesis that phonolitic lavas or their plutonic equivalents (nepheline syenites) may be produced directly by partial melting of upper mantle rock-types at moderate pressures (1–1.5 GPa), especially where large domains of the subcontinental lithospheric mantle has been enriched in potassium by metasomatism. The circulation of low-degree partial melts of peridotites into the upper mantle may be responsible for a special kind of metasomatism characterized by Si- and alkali-enrichment. When they are unable to escape by porous flow, low-degree melts will ultimately be trapped inside neighboring olivine grains and give rise to the silica- and alkali-rich glass inclusions found in peridotite xenoliths.

International audience; Based on previous and new results on partial melting experiments of pyroxenites at high pressure, we attempt to identify the major element signature of pyroxenite partial melts and to evaluate to what extent this signature can be transmitted to the basalts erupted at oceanic islands and mid-ocean ridges. Although peridotite is the dominant source lithology in the Earth's upper mantle, the ubiquity of pyroxenites in mantle xenoliths and in ultramafic massifs, and the isotopic and trace elements variability of oceanic basalts suggest that these lithologies could significantly contribute to the generation of basaltic magmas. The question is how and to what degree the melting of pyroxenites can impact the major-element composition of oceanic basalts. The review of experimental phase equilibria of pyroxenites shows that the thermal divide, defined by the aluminous pyroxene plane, separates silica-excess pyroxenites (SE pyroxenites) on the right side and silica-deficient pyroxenites (SD pyroxenites) on the left side. It therefore controls the melting phase relations of pyroxenites at high pressure but, the pressure at which the thermal divide becomes effective, depends on the bulk composition; partial melt compositions of pyroxenites are strongly influenced by non-CMAS elements (especially FeO, TiO2, Na2O and K2O) and show a progressive transition from the liquids derived from the most silica-deficient compositions to those derived from the most silica-excess compositions. Another important aspect for the identification of source lithology is that, at identical pressure and temperature conditions, many pyroxenites produce melts that are quite similar to peridotite-derived melts, making the determination of the presence of pyroxenite in the source regions of oceanic basalts difficult; only pyroxenites able to produce melts with low SiO2 and high FeO contents can be identified on the basis of the major-element compositions of basalts. In the case of oceanic island basalts, high CaO/Al2O3 ratios can also reveal the presence of pyroxenite in the source-regions. Experimental and thermodynamical observations also suggest that the interactions between pyroxenite-derived melts and host peridotites play a crucial role in the genesis of oceanic basalts by generating a wide range of pyroxenites in the upper mantle: partial melting of such secondary pyroxenites is able to reproduce the features of primitive basalts, especially their high MgO contents, and to impart, at least in some cases, the major-element signature of the original pyroxenite melt to the oceanic basalts. Finally, we highlight that the fact the very silica depleted compositions (SiO2 < 42 wt.%) and high TiO2 contents of some ocean island basalts seem to require the contribution of fluids (CO2 or H2O) through melting of either carbonated lithologies (peridotite or pyroxenite) or amphibole-rich veins.

We performed a thermodynamic and experimental study to investigate the fate of pyroxenite-derived melts during their migration through the peridotitic mantle. We used a simplified model of interaction in which peridotite is impregnated by and then equilibrated with a finite amount of pyroxenite-derived liquid. We considered two pyroxenite compositions and three contexts of pyroxenitic melt impregnation: (1) in a subsolidus lithospheric mantle; (2) beneath a mid-ocean ridge (MOR) in a subsolidus asthenospheric mantle at high pressure; (3) beneath a MOR in a partially molten asthenospheric mantle. Calculations were performed with pMELTS at constant pressure and temperature with a melt–rock ratio varying in the range 0–1. Concurrently, a series of impregnation experiments was performed at 1 and 1·5 GPa to reproduce the final stages of the calculations where the melt–rock ratio is 1. Incoming melt and host-rocks react differently according to the melt composition and the physical state of the surrounding mantle. Whereas clinopyroxene (Cpx) is systematically a reaction product, the role of olivine (Ol) and orthopyroxene (Opx) depends on the incoming melt silica activity a^0_(SiO2): if it is lower than the silica activity Formula of a melt saturated in Ol and Opx at the same pressure P and temperature T, Opx is dissolved and Ol precipitates, and conversely if a_(SiO2) > a^0_(SiO2). Such contrasted reactions between pyroxenitic melts and peridotitic mantle may generate a large range of new lithological heterogeneities (wehrlite, websterite, clinopyroxenite) in the upper mantle. Also, our study shows that the ability of pyroxenite-derived melts to migrate through the mantle depends on the melting degree of the surrounding peridotite. The reaction of these melts with a subsolidus mantle results in strong melt consumption (40–100%) and substantial Cpx production (with some spinel or garnet, depending on P). This is expected to drastically decrease the system permeability and the capacity of pyroxenite-derived melts to infiltrate neighbouring rocks. In contrast, melt migration to the surface should be possible if the surrounding mantle is partially melted; although liquid reactivity varies with composition, melt consumption is restricted to less than 20%. Hence, magma–rock interactions can have a significant impact on the dynamics of melting and magma migration and should not be neglected when modelling the partial melting of heterogeneous mantle.

We performed experiments in a piston-cylinder apparatus to determine the effects of focused magma transport into highly permeable channels beneath mid-ocean ridges on: (1) the chemical composition of the ascending basalt; and (2) the proportions and compositions of solid phases in the surrounding mantle. In our experiments, magma focusing was supposed to occur instantaneously at a pressure of 1.25 GPa. We first determined the equilibrium melt composition of a fertile mantle (FM) at 1.25 GPa-1,310°C; this composition was then synthesised as a gel and added in various proportions to peridotite FM to simulate focusing factors Ω equal to 3 and 6 (Ω = 3 means that the total mass of liquid in the system increased by a factor of 3 due to focusing). Peridotite FM and the two basalt-enriched compositions were equilibrated at 1 GPa-1,290°C; 0.75 GPa-1,270°C; 0.5 GPa-1,250°C, to monitor the evolution of phase proportions and compositions during adiabatic decompression melting. Our main results may be summarised as follows: (1) magma focusing induces major changes of the coefficients of the decompression melting reaction, in particular, a major increase of the rate of opx consumption, which lead to complete exhaustion of orthopyroxene (and clinopyroxene) and the formation of a dunitic residue. A focusing factor of ≈4—that is, a magma/rock ratios equal to ≈0.26—is sufficient to produce a dunite at 0.5 GPa. (2) Liquids in equilibrium with olivine (±spinel) at low pressure (0.5 GPa) have lower SiO2 concentrations, and higher concentrations in MgO, FeO, and incompatible elements (Na2O, K2O, TiO2) than liquids produced by decompression melting of the fertile mantle, and plot in the primitive MORB field in the olivine–silica–diopside–plagioclase tetrahedron. Our study confirms that there is a genetic relationship between focused magma transport, dunite bodies in the upper mantle, and the generation of primitive MORBs.

We present a method that can be used to estimate the amount of recycled material present in the source region of mid-ocean ridge basalts by combining three key constraints: (1) the melting behaviour of the lithologies identified to be present in a mantle source, (2) the overall volume of melt production, and (3) the proportion of melt production attributable to melting of each lithology. These constraints are unified in a three-lithology melting model containing lherzolite, pyroxenite and harzburgite, representative products of mantle differentiation, to quantify their abundance in igneous source regions.\ud \ud As a case study we apply this method to Iceland, a location with sufficient geochemical and geophysical data to meet the required observational constraints. We find that to generate the 20 km of igneous crustal thickness at Iceland's coasts, with 30±10%30±10% of the crust produced from melting a pyroxenitic lithology, requires an excess mantle potential temperature (ΔTp) of ⩾130 °C (View the MathML sourceTp⩾1460°C) and a source consisting of at least 5% recycled basalt. Therefore, the mantle beneath Iceland requires a significant excess temperature to match geophysical and geochemical observations: lithological variation alone cannot account for the high crustal thickness. Determining a unique source solution is only possible if mantle potential temperature is known precisely and independently, otherwise a family of possible lithology mixtures is obtained across the range of viable ΔTp. For Iceland this uncertainty in ΔTp means that the mantle could be >20% harzburgitic if View the MathML sourceΔTp>150°C (View the MathML sourceTp>1480°C).\ud \ud The consequences of lithological heterogeneity for plume dynamics in various geological contexts are also explored through thermodynamic modelling of the densities of lherzolite, basalt, and harzburgite mixtures in the mantle. All lithology solutions for Iceland are buoyant in the shallow mantle at the ΔTp for which they are valid, however only lithology mixtures incorporating a significant harzburgite component are able to reproduce recent estimates of the Iceland plume's volume flux. Using the literature estimates of the amount of recycled basalt in the sources of Hawaiian and Siberian volcanism, we found that they are negatively buoyant in the upper mantle, even at the extremes of their expected ΔTp. One solution to this problem is that low density refractory harzburgite is a more ubiquitous component in mantle plumes than previously acknowledged.