A comparison of SNARF-1 and skeletal δ11B estimates of calcification media pH in tropical coral

Funding: SIMS analyses were supported by the Natural Environment Research Council, UK (IMF689/0519).Coral skeletal boron geochemistry offers opportunities to probe the pH of the calcification media (pHCM) of modern and fossil specimens, to estimate past changes in seawater pH and to explore the biomineralisation response to future ocean acidification. In this research we grew 2 Stylophora pistillata coral microcolonies over glass coverslips to allow analysis of the pH sensitive dye SNARF-1, in the extracellular calcification medium at the growing edge of colonies where the first aragonite crystals are formed, under both light and dark conditions. We use secondary ion mass spectrometry (SIMS) to measure the boron isotopic composition (δ11B) of the skeleton close to the growth edge after 2 to 3 days of additional calcification had enlarged the crystals until they joined, generating a continuous sheet of aragonite. Mean skeletal δ11B-pHCM estimates are higher than those of by SNARF-1 by 0.35 to 0.44 pH units. These differences either reflect real variations in the pH of the calcification media associated with each measurement technique or indicate other changes in the biomineralisation process which influence skeletal δ11B. SNARF-1 measures directly the pH of the extracellular calcification medium while skeletal δ11B analyses aragonite potentially formed via both extracellular and intracellular biomineralisation pathways. Analysis of a third coral specimen, also growing on a glass slide but with a 5 cm long branch, indicated good agreement between the δ11B value of the apex of the branch and the skeletal growth edge. The tissues overlying both these regions were transparent indicating they had low symbiont densities. This suggests that the biomineralisation process is broadly comparable between these sites and that studies growing corals over glass slides/coverslips provide representative data for the colony apex.Publisher PDFPeer reviewe

Brachiopods recording seawater temperature — a matter of class or maturation?

Calcite fibres of the innermost secondary layer of low magnesium-calcite brachiopod shells are in oxygen isotope equilibrium with ambient seawater. Previous work on Terebratalia transversa indicates that the first formed calcite fibres of the secondary layer are not in isotopic equilibrium while the later fibres of mature valves are in oxygen isotope equilibrium with seawater. Here, secondary ion mass spectrometry (SIMS) analyses reveal that this trend of oxygen isotope equilibrium, being attained towards the innermost shell rather than being a feature throughout the secondary layer, occurs in brachiopods from two Orders and two sub-orders. This suggests a general pattern among brachiopods with low magnesium-calcite shells and leads to the recommendation that only the innermost fibres of mature valves be included in proxy calculations of seawater temperature. Although the trend is common in the three species studied here, the extent of the isotopic range is much less marked in the species that lacks punctae

Detrital zircon age, oxygen and hafnium isotope systematics record rigid continents after 2.5 Ga

The Neoarchean-Paleoproterozoic boundary at 2.5 Ga is marked by fundamental changes in the composition of the mantle, crust and atmosphere-hydrosphere. These changes show that the evolution of Earth's deep interior and its exterior are linked, but the causes of the global transitions are cryptic. The isotopic signatures of detrital zircon enable the nature of felsic magma sources before and after 2.5 Ga to be compared, providing insight into the processes driving secular change. For this purpose, we present new oxygen and Hf isotope data from detrital zircon grains hosted by Paleoproterozoic metasedimentary rocks of the North Australian Craton, which record three magmatic events at 2.7 Ga, 2.5 Ga and 1.87 Ga. Scattered zircon εHf (+6 to −10) coupled with mantle-like δ18O at 2.7 Ga indicates both new crustal addition and the reworking of older materials. At 2.5 Ga, a wide range in zircon εHf (+7 to −12) and δ18O (5 to 7‰) reflects reworking of infracrustal and (subordinate) supracrustal components of various crustal residence age. The dominance of subchondritic zircon εHf suggests that depleted mantle inputs were limited. The εHf array contracts markedly (+3 to −8) at 1.87 Ga and is coupled with isotopically heavy oxygen (δ18O from 7 to 9.5‰), indicating a substantial contribution from clay-rich supracrustal sources. We attribute the contraction of the zircon εHf array at ca. 1.87 Ga to the melting of a range of Neoarchean crustal components, where the disparate Hf isotope signatures of these were partially homogenised by sedimentary processes. The shift in felsic magma sources after 2.5 Ga, from dominantly infracrustal to supracrustal, implies a change in the mechanical behaviour of the lithosphere, from soft to rigid. This may have contributed to the transition in the composition of the continents at the Neoarchean-Paleoproterozoic boundary

Apatite evidence for a fluid-saturated, crystal-rich magma reservoir forming the Quellaveco porphyry copper deposit (Southern Peru)

Large volume, intermediate-felsic magma reservoirs are the source of melt and mineralising fluids which generate porphyry copper deposits. Cooling and crystallisation of hydrous magmas drives the exsolution and expulsion of a magmatic volatile phase—a process which remains challenging to constrain in porphyry Cu systems where the record of magma volatile compositions is rarely preserved. Here, we use the halogen compositions of apatite inclusions shielded as inclusions within zircon to constrain volatile evolution in magma reservoirs which pre-date and are synchronous with porphyry Cu mineralisation at Quellaveco, Southern Peru. Geochemical and textural data confirm that the zircon-included apatites escaped re-equilibration with hydrothermal fluids, unlike apatites found in the groundmass of the same rocks. We, therefore, recommend that future studies attempting to reconcile magmatic volatile budgets using apatite in porphyry Cu systems should focus on apatite inclusions in zircon. By combining the apatite inclusion data with numerical modelling, we find evidence that the magma reservoir sourcing porphyry Cu mineralisation remained fluid-saturated for the entire period recorded by apatite crystallisation. By contrast, the pre-mineralisation batholith shows more variable, potentially fluid-undersaturated behaviour. Our modelling suggests that in order to attain the porphyry melt volatile compositions inferred from apatite, the magma reservoir must have exsolved a large proportion of its volatile budget, consistent with having been held at high crystallinity (40–60% crystals). This crystallisation interval coincides with peak chlorine and copper extraction from intermediate-felsic magmas, and would have permitted efficient fluid migration and accumulation at the roof of the system. We suggest that the storage of large-volume, long-lived, crystal-rich magma reservoirs in magmatic arcs may be a critical step in generating world-class porphyry copper deposits.ISSN:0010-7999ISSN:1432-096

Sulfur from the subducted slab dominates the sulfur budget of the mantle wedge under volcanic arcs

Sulfur is of a crucial importance in the Earth system influencing biological, climate, ore-forming, and redox processes. Subduction zones play a key role in the global sulfur cycle. Arc magmas have higher sulfur contents and are more oxidised than mid-ocean ridge basalts (MORBs) due to either an oxidised mantle source or magma differentiation. Melt oxidation state and sulfur content may interrelate, as sulfur is a potential oxidising agent during slab-mantle interaction. Here, we use melt inclusions (MIs) to determine the sulfur isotopic composition (δ 34S) of primary arc magmas from three volcanic centres along the Central American Volcanic Arc (CAVA): Fuego (Guatemala), Cerro Negro (Nicaragua), and Turrialba (Costa Rica). These three locations sample much of the global arc magma trace element variability: Ba/La ratios range from 22 (Turrialba) to 118 (Cerro Negro). Melt δ 34S values are between -0.5h and +4.9h. Sulfur contents and δ 34S values of homogenised and naturally quenched MIs overlap, indicating post-entrapment processes do not affect sulfur contents and sulfur isotope ratios in the studied MIs. Degassing causes limited sulfur isotope fractionation; calculated gas-melt isotope fractionation factors are between 0.998-1.001. Our model calculations predict that most volcanic gases along the CAVA have δ 34S between -1h and +6h, becoming enriched in 34S as degassing progresses. We estimate initial melt δ 34S values for Fuego, Cerro Negro, and Turrialba to be +0.7±1.4h, +2.2±1.0h, and +1.6±0.8h (two standard errors), respectively. All these values are elevated compared to MORBs (-0.9h). Addition of oxidised slab material enriched in 34S to the mantle wedge can explain elevated arc primary melt δ 34S and the oxidising conditions observed in arc magmas globally. Based on mass balance, a slab component with δ 34S between +2h to +5h is present in the mantle wedge under the CAVA, elevating local arc mantle S contents to 360±30 ppm at Fuego, 462±11 ppm at Cerro Negro. Modelling suggests that 40-70% of sulfur in the mantle wedge originates from a slab-derived component. Slab subduction is expected to have major control on the evolution of Earth’s sulfur cycle and mantle oxidation state over its geological history

The effect of variations in cooling rates on mineral compositions in mid-ocean ridge basalts

We present textural and chemical analyses of minerals from a 150 m thick sequence of submarine mid-ocean ridge basalts from the South China Sea in order to showcase the effect of variations in magma cooling rates on mineral texture and mineral composition. Groundmass plagioclase and clinopyroxene show gradual changes in texture and composition as flows grade from slowly cooled, meter-thick massive flows to more rapidly cooled dm-thick pillow lobes with quenched glassy margins. The corresponding change in cooling-rate is estimated to vary from <1 to ≥100 °C/h. With increasing cooling rate, plagioclase forms elongated, sector-zoned swallow-tail crystals. Sector zoning is associated with increasing FeO (up to 1.5 wt%) and MgO (up to 0.6 wt%) abundances at near-constant anorthite (An), related to a two orders of magnitude increase in growth rate. Sr, Ba, Li and Ti abundances remain constant and appear unaffected by kinetic factors at such growth rates. With increasing cooling rate clinopyroxene becomes dendritic, and its composition is sensitive to changes in crystallization conditions. Increasing cooling rate (<1 to ≥100 °C/h) leads to increasing Al2O3 (average of 3.2 to 4.3 wt%), TiO2 (1.3 to 2.8 wt%) and Na2O (0.37 to 0.44 wt%) and a decrease in SiO2 (50.1 to 46.4 wt%) and Mg#Fetot, i.e., molar MgO/(MgO + FeOtot), from 71.3 to 53.1. Trace element abundances (Y, Zr, Ce, V, Sr) in clinopyroxene increase by up to an order of magnitude at cooling rates ≥100 °C/h and become more heterogeneous spatially. These results support experimental evidence that rapid crystal growth leads to significant departure of mineral compositions from equilibrium, in particular for clinopyroxene. Although plagioclase composition remains relatively insensitive to changes in growth conditions at the studied cooling rates, the sensitivity of clinopyroxene composition to growth rates imply that it should be used with caution as a tool to infer magmatic crystallization conditions.ISSN:0009-2541ISSN:1872-683