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Anthropogenic activities increase the level of dissolved heavy metals in some tropical near‐shore environments threatening reef ecosystems. The skeleton of stony corals like Porites species potentially provides a high‐resolution geochemical archive for past heavy metal concentrations, with potentially century long records revealing baseline values before large‐scale human disturbance. However, few data exist for heavy metal partitioning into coral skeleton aragonite. To address this, culturing experiments exposing Porites lobata and Porites lichen to a mixture of dissolved Cr, Mn, Ni, Cu, Zn, Ag, Cd, Sn, Hg, and Pb over a wide concentration range have been performed. Water samples were taken frequently to monitor changes in the heavy metal concentration. Laser ablation ICP‐MS measurements of the coral aragonite revealed metal concentrations that were positively correlated with Cr, Mn, Ni, Zn, Ag, Cd, and Pb concentrations in seawater. The DTE values for most metals appear dependent on the seawater metal content, approximating a power law, and therefore stabilize at higher seawater metal/Ca ratios. The partitioning of Pb into the coral skeleton is a notable exception, with DPb being stable around 2 to 1 across a large range of "natural" to highly polluted seawater Pb concentrations. This and the general agreement with partition coefficients estimated by previous work suggests that the reconstruction of the heavy metal concentration in seawater for ecosystem monitoring is possible. However, the high variability within and between coral colonies requires further study and suggests that multiple records from multiple coral colonies should be combined to obtain robust reconstructions. Key Points: Porites corals grow normally with increased exposure to multiple metals over >1 yearSkeletal partitioning variable within and between colonies and with seawater metal contentGood agreement with previous work, especially for Pb across a large range of metal content [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) variability in the south-eastern tropical Indian Ocean is crucial for rainfall variability in Indian Ocean rim countries. A large body of literature has focused on zonal variability associated with the Indian Ocean Dipole (IOD), but it is unclear whether meridional shifts in the position of the Intertropical Convergence Zone (ITCZ), which at present co-vary with the IOD, may also occur independently. We have developed a new, monthly resolved Sr/Ca record from a sub-fossil coral cored at Enggano Island (Indonesia, 5° S, 102° E). Core sections containing diagenetic phases are omitted from the SST reconstruction. U/Th dating shows that the Sr/Ca-based SST record extends from 1917–1868 and from 1861–1823 with a relative age uncertainty of ±2.4 years (2σ). At Enggano Island, coastal upwelling and cooling in austral spring is coupled to the position of the ITCZ, and impacts SST seasonality. The sub-fossil coral indicates an increase in SST seasonality due to enhanced austral spring cooling between 1917 and 1855, which we attribute to stronger SE winds and a northward shift in the position of the ITCZ in austral spring. A nearby sediment core indicates SST cooling and a shallowing of the thermocline prior to ~1930. These results are consistent with an increase in the North-South SST gradient in the eastern Indian Ocean calculated from historical temperature data, that is not seen in the zonal SST gradient. We conclude that the relationship between meridional and zonal variability in the eastern Indian Ocean is non-stationary and influenced by long-term temperature trends. [ABSTRACT FROM AUTHOR]

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 mantle.sup.1, 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 pressures.sup.2 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 Canada.sup.3 and Zimbabwe.sup.4. 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. Hydrogen isotopes and compositions of melt inclusions in olivine in komatiites indicate a hydrous source produced by recycling of seawater-altered crust into the deep mantle over 3.3 billion years ago., Author(s): Alexander V. Sobolev [sup.1] [sup.2] , Evgeny V. Asafov [sup.2] , Andrey A. Gurenko [sup.3] , Nicholas T. Arndt [sup.1] , Valentina G. Batanova [sup.1] [sup.2] , Maxim V. [...]

The recent development of LA‐ICP‐MS/MS techniques enables in situ Rb‐Sr dating of micas, but there is not yet a matrix‐matched reference material (RM). In this contribution we optimised laser conditions and developed a new data reduction approach for Rb‐Sr dating of micas with non‐matrix‐matched RMs Mica‐Mg‐NP and NIST SRM 612. This was achieved by systematically investigating the effects of laser ablation settings, data reduction methods and the selection of the RMs on the Rb‐Sr dating quality of a 306 ± 1 Ma hydrothermal muscovite. We then tested this approach using twelve mica samples with known ages from 306 to 1.4 Ma. The results show that this technique can achieve accuracy of 2.5% or better, and precision of ~1–3% for micas > 15 Ma. The dating accuracy (> 9%) and precision (> 21%) decrease for micas younger than 5 Ma due to the short radiogenic accumulation time. This technique provides a way to date many types of mineral deposits and rocks, with a relatively high spatial resolution (down to 38 μm), a fast turnaround time and a higher closure temperature for muscovite than the Ar‐Ar system, which makes it a powerful tool in geochronological studies. [ABSTRACT FROM AUTHOR]

Post-collisional volcanism contains important clues for understanding the processes that prevail in orogenic belts, including those in the mantle and the uplift and collapse of continents. Here we report new geochronological and geochemical data for a suite of post-collisional Miocene to Pleistocene volcanic rocks from northwest Iran. Four groups of volcanic rocks can be distinguished according to their geochemical and isotopic signatures, including: (1) Miocene depleted lavas with high Nd and Hf but low Pb and Sr isotopic ratios, (2) less depleted lavas with quite variable Pb isotopic composition, (3) lavas with non-radiogenic Nd and Hf isotopic values, but highly radiogenic Sr and Pb isotopic composition, and (4) Pleistocene adakitic rocks with depleted isotopic signatures. The isotopic data reveal that the Miocene rocks are derived from asthenospheric and highly heterogeneous sub-continental lithospheric mantle sources. Evidence suggests that the lithospheric mantle contains recycled upper continental material and is isotopically similar to the enriched mantle two (EMII) end-member. Analysis of Sr-Nd-Pb-Hf-O isotopes in both mineral and rock groundmass, in conjunction with energy-constrained assimilation and fractional crystallization (EC-AFC) numerical modeling, demonstrates that the incorporation of continental crust during magma fractionation via AFC had an insignificant impact on the isotopic composition of the Miocene lavas. Moreover, adakites are the youngest rocks and show a geochemical signature consistent with the partial melting of a young and mafic continental lower crust. Both seismological data and geochemical signatures on these Miocene to Pleistocene volcanic rocks indicate the initiation of asthenospheric upwelling and orogen uplift in the Arabia-Eurasia collision zone, which occurred after slab break-off, following the Neotethyan closure. [ABSTRACT FROM AUTHOR]