The Systematics of Olivine CaO + Cr-Spinel in High-Mg# Arc Volcanic Rocks: Evidence for in-Situ Mantle Wedge Depletion at the Arc Volcanic Front

We investigated the state of the arc background mantle (i.e. mantle wedge without slab component) by means of olivine CaO and its Cr-spinel inclusions in a series of high-Mg# volcanic rocks from the Quaternary Trans-Mexican Volcanic Belt. Olivine CaO was paired with the Cr# [molar Cr/(Cr + Al) *100] of Cr-spinel inclusions, and 337 olivine+Cr-spinel pairs were obtained from 33 calc-alkaline, high-K and OIB-type arc front volcanic rocks, and three monogenetic rear-arc basalts that lack subduction signatures. Olivine+Cr-spinels display coherent elemental and He–O isotopic systematics that contrast with the compositional diversity of the bulk rocks. All arc front olivines have low CaO (0.135 ± 0.029 wt %) relative to rear-arc olivines which have the higher CaO (0.248 ± 0.028 wt %) of olivines from mid-ocean ridge basalts. Olivine 3He/4He–δ18O isotope systematics confirm that the olivine+Cr-spinels are not, or negligibly, affected by crustal basement contamination, and thus preserve compositional characteristics of primary arc magmas. Variations in melt H2O contents in the arc front series and the decoupling of olivine CaO and Ni are inconsistent with controls on the olivine CaO by melt water and/or secondary mantle pyroxenites. Instead, we propose that low olivine CaO reflects the typical low melt CaO of high-Mg# arc magmas erupting through thick crust. We interpret the inverse correlation of olivine CaO and Cr-spinel Cr# over a broad range of Cr# (~10–70) as co-variations of CaO, Al and Cr of their (near) primary host melts, which derived from a mantle that has been variably depleted by slab-flux driven serial melt extraction. Our results obviate the need for advecting depleted residual mantle from rear- and back-arc region, but do not upset the larger underlying global variations of melt CaO high-Mg# arc magmas worldwide, despite leading to considerable regional variations of melt CaO at the arc front of the Trans-Mexican Volcanic Belt

Deep hydrous mantle reservoir provides evidence for crustal recycling before 3.3 billion years ago

Water strongly influences the physical properties of the mantle and enhances its ability to melt or convect. Its presence can also be used to trace recycling of surface reservoirs down to the deep mantle1, which makes knowledge of the water content in the Earth's interior and its evolution crucial for understanding global geodynamics. Komatiites (MgO-rich ultramafic magmas) result from a high degree of mantle melting at high pressures2 and thus are excellent probes of the chemical composition and water contents of the deep mantle. An excess of water over elements that show similar geochemical behaviour during mantle melting (for example, cerium) was recently found in melt inclusions in the most magnesium-rich olivine in 2.7-billion-year-old komatiites from Canada3 and Zimbabwe4. These data were taken as evidence for a deep hydrated mantle reservoir, probably the transition zone, in the Neoarchaean era (2.8 to 2.5 billion years ago). Here we confirm the mantle source of this water by measuring deuterium-to-hydrogen ratios in these melt inclusions and present similar data for 3.3-billion-year-old komatiites from the Barberton greenstone belt. From the hydrogen isotope ratios, we show that the mantle sources of these melts contained excess water, which implies that a deep hydrous mantle reservoir has been present in the Earth's interior since at least the Palaeoarchaean era (3.6 to 3.2 billion years ago). The reconstructed initial hydrogen isotope composition of komatiites is more depleted in deuterium than surface reservoirs or typical mantle but resembles that of oceanic crust that was initially altered by seawater and then dehydrated during subduction. Together with an excess of chlorine and depletion of lead in the mantle sources of komatiites, these results indicate that seawater-altered lithosphere recycling into the deep mantle, arguably by subduction, started before 3.3 billion years ago

New Olivine Reference Material for In Situ Microanalysis

A new olivine reference material – MongOL Sh11‐2 – for in situ analysis has been prepared from a central portion of a large (20 cm × 20 cm × 10 cm) mantle peridotite xenolith from a ~ 0.5 Ma old basaltic breccia at Shavaryn‐Tsaram, Tariat region, central Mongolia. The xenolith is a fertile mantle lherzolite with minimal signs of alteration. Approximately 10 g of 0.5 to 2 mm gem quality olivine fragments were separated under binocular microscope and analysed by EPMA, LA‐ICP‐MS, SIMS and bulk analytical methods (ID ICP‐MS for Mg and Fe, XRF, ICP‐MS) for major, minor and trace elements at six institutions worldwide. The results show that the olivine fragments are sufficiently homogeneous with respect to major (Mg, Fe, Si) and minor and trace elements. Significant inhomogeneity was revealed only for phosphorus (homogeneity index of 12.4), whereas Li, Na, Al, Sc, Ti and Cr show minor inhomogeneity (homogeneity index of 1–2). The presence of some mineral and fluid‐melt micro‐inclusions may be responsible for the inconsistency in mass fractions obtained by in situ and bulk analytical methods for Al, Cu, Sr, Zr, Ga, Dy and Ho. Here we report reference and information values for twenty‐seven major, minor and trace elements

Formation of podiform chromitite deposits : implications from PGE abundances and Os isotopic compositions of chromites from the Troodos complex, Cyprus

Podiform chromitite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone settings (SSZ). We have measured PGE abundances and Os isotopic compositions of three major chromitite deposits (Kannoures, Hadji Pavlou, Kokkinorostos) and associated mantle peridotites from the Troodos Ophiolite Complex in order to investigate the petrogenesis of these rocks, and their genetic relationships and to examine the geochemical behaviour of the PGE. Spinels from the chromitite deposits have flat chondrite-normalized PGE patterns, but have distinct negative Pt anomalies. Thus, Pd, Os, Ru and Ir concentrations are very high compared to the Pt concentrations (Os: 13.7-104 ng/g, Ir 11.3-19.0 ng/g, Ru 34.3-83.6 ng/g, Pt 0.41-9.07 ng/g, Pd 11.1-76.8 ng/g). With the exception of Pd, this appears to be a general feature of chromitites from ophiolites worldwide. However, Pd concentrations determined in this study are high compared to other studies where whole rock samples were analysed. There is no simple explanation for this difference because mass balance constraints would not allow that this is solely due to Pd-depletion in the interstitial component. Rather, it implies that chromitites display large variations of relative PGE abundances, even on a local scale. Podiform chromitite deposits form as a result of the interaction of fluid-rich, percolating melts with surrounding mantle peridotites. Osmium, Ir, Ru and Cr concentrations decrease systematically from harzburgite to dunite surrounding the deposits. In addition, dunites and chromites have complementary PGE distribution patterns. Thus, the mantle peridotite is the source of these metals in chromitites. This also indicates that these elements behave incompatibly and are mobilized during continuous melt percolation. However, the low Pt concentrations in the chromitites suggest that Pt is not as effectively mobilized during melt percolation. Uniformly high Pt concentrations in harzburgite and dunite (ca. 11 ppb) also imply that most Pt remains in the mantle peridotite. This can be explained if residual Pt-rich phases, such as PtFe alloys, limit the mobility of Pt. PGE and Cr become concentrated when chromite and sulfide liquids precipitate as a result of the mixing of percolating melts in magma pools near the crust-mantle boundary. The Os-187/Os-188 ratios of the chromite separates (0.1265-0.1301) are less variable than those of the associated peridotites (0.1235-0.1546). The average isotopic composition of the chromites (Os-187/O-188: 0.1284 +/- 0.0021) is superchondritic compared with the carbonaceous chondrite value (Os-187/Os-188: 0.1260 +/- 0.0013 after Geochim. Cosmochim. Acta 66 (2002) 329; Geochim. Cosmochim. Acta 66 (2002) 4187) and similar to the average value measured for podiform chromitites worldwide (0.12809 +/- 0.00085 after Geochim. Cosmochim. Acta 66 (2002) 329; Geochim. Cosmochim. Acta 66 (2002) 4187). Radiogenic melts/fluids derived from the subducting slab trigger partial melting in the overlying mantle wedge and add significant amounts of radiogenic Os to the peridotites. Mass balance calculations suggest that a melt/rock ratio of approximately 17:1 (melt:Os-187/Os-188: 0.163. Os: 0.01 ng/g, mantle peridotite: Os-187/Os-188: 0.127, Os 4.2 ng/g) is necessary in order to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a surprisingly low average melt/rock ratio during the formation of chromitite deposits. The percolating melts likely have variable isotopic composition and PGE concentration. However, in the chromitite pods the Os from many melts is pooled and homogenized, which is the reason why the chromitite deposits show such a small variation in their Os isotopic composition. The results of this study suggest that the Os-187/Os-188 ratio of chromitites is not representative for the DMM, but only reflects an upper limit

Conditions of chondrule formation in ordinary chondrites

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Multiple sources for tephra from AD 1259 volcanic signal in Antarctic ice cores

International audienceStrong volcanic signals simultaneously recorded in polar ice sheets are commonly assigned to major low-latitude eruptions that dispersed large quantities of aerosols in the global atmosphere with the potential of inducing climate perturbations. Parent eruptions responsible for specific events are typically deduced from matching to a known volcanic eruption having coincidental date. However, more robust source linkage can be achieved only through geochemical characterisation of the airborne volcanic glass products (tephra) sometimes preserved in the polar strata. We analysed fine-grained tephra particles extracted from layers of the AD 1259 major bipolar volcanic signal in four East Antarctic ice cores drilled in different widely-spaced locations of the Plateau. The very large database of glassshard geochemistry combined with grain size analyses consistently indicate that the material was sourced from multiple distinct eruptions. These are the AD 1257 mega-eruption of Samalas volcano in Indonesia, recently proposed to be the single event responsible for the polar signal, as well as a newly-identified Antarctic eruption occurred in AD 1259. Finally, a further eruption that took place somewhere outside Antarctica has contributed to tephra deposition. Our high-resolution, multiple-site approach was critical to reveal spatial heterogeneity of tephra at the continental scale. Evidence from ice-core tephra indicates recurrent explosive activity at the Antarctic volcanoes and could have implications for improved reconstruction of post-volcanic effects on climate from proxy polar records