The 1994–2001 eruptive period at Rabaul, Papua New Guinea: Petrological and geochemical evidence for basalt injections into a shallow dacite magma reservoir, and significant SO 2 flux

The eruptions that began at Rabaul Caldera on 19 September 1994 had two focal points, the vents Tavurvur and Vulcan, located 6 km apart on opposing sides of the caldera. Vulcan eruptives define a tight cluster of dacite compositions, whereas Tavurvur eruptives span an array from equivalent dacite compositions to mafic andesites. The eruption of geochemically and mineralogically identical dacites from both vents indicates sourcing from the same magma reservoir. This, together with previously reported H₂O-CO₂ volatile contents of dacite melt inclusions, a caldera-wide seismic low-velocity zone, and a seismically active caldera ring fault structure are consistent with the presence at 3–6 km depth of an extensive, tabular dacitic magma body having volume of about 15–150 km³. The Tavurvur andesites form a linear compositional array and have strongly bimodal phenocryst assemblages that reflect dacite hybridisation with a mafic basalt. The moderately large volume SO₂ flux documented in the Tavurvur volcanic plume (and negligible SO₂ flux in the Vulcan plume) combined with high dissolved S contents of basaltic melt inclusions trapped in olivine of Tavurvur eruptives, indicate that the amount of degassed basaltic magma was ~ 0.1 km³ and suggest that the injection of this magma was confined to the Tavurvur-side (eastern to northeastern sector) of the caldera. Circumstantial evidence suggests that the eruption was triggered and evolved in response to a series of basaltic magma injections that may have commenced in 1971 and continued up until at least the start of the 1994 eruptions. The presence of zoned plagioclase phenocrysts reflecting older basalt-dacite interaction events (i.e. anorthite cores overgrown with thick andesine rims), evaluation of limited available data for the products of previous eruptions in 1878 and 1937–1943, and the episodic occurrence of major intra-caldera seismo-deformational events indicates that the shallow magma system at Rabaul Caldera is subjected to repeated mafic magma injections at intervals of several years to several decades.We thank Shane Nancarrow, formerly of Geoscience Australia, for making many aspects of this project possible and AusAID for providing financial support to HP to undertake research at the Australian National University (ANU) into the 1994 and historical eruptions at Rabaul

Planktic foraminifera form their shells via metastable carbonate phases

The calcium carbonate shells of planktic foraminifera provide our most valuable geochemical archive of ocean surface conditions and climate spanning the last 100 million years, and play an important role in the ocean carbon cycle. These shells are preserved in marine sediments as calcite, the stable polymorph of calcium carbonate. Here, we show that shells of living planktic foraminifers Orbulina universa and Neogloboquadrina dutertrei originally form from the unstable calcium carbonate polymorph vaterite, implying a non-classical crystallisation pathway involving metastable phases that transform ultimately to calcite. The current understanding of how planktic foraminifer shells record climate, and how they will fare in a future high-CO 2 world is underpinned by analogy to the precipitation and dissolution of inorganic calcite. Our findings require a re-evaluation of this paradigm to consider the formation and transformation of metastable phases, which could exert an influence on the geochemistry and solubility of the biomineral calcite

Peridotite xenoliths from Grenada, Lesser Antilles Island Arc

Ultramafic xenoliths comprising harzburgite, lherzolite (reacted harzburgite) and spinel-rich dunite, occur in alkali olivine basalts (M series) of Grenada in the Lesser Antilles island arc. Textures are protogranular, porphyroclastic and granular; the latter are restricted to dunites and areas of the harzburgites/lherzolites where interaction with host magma has occurred. Primary mineralogy comprises olivine, orthopyroxene, clinopyroxene, and spinel. Harzburgites are residual from a fractional partial melting event totaling ~22%. Infiltration of harzburgite by (and reaction with) basalt has produced: a wehrlite, with partial dissolution of primary spinel, an increase in the oxygen fugacity (ƒO2) from primary values 1–2 log ƒO2 units above the fayalite-magnetite-quartz (FMQ) buffer, to 2–2.5 log units above the buffer; reaction of orthopyroxene to form patches of intergrown olivine and clinopyroxene, and bronzite andesite glass (60 wt%, SiO2 18–20 wt% Al2O3 and 3–4 wt% Na2O) with flat to light rare earth element-depleted, chondrite-normalized abundances. Refertilisation of the mantle by reacting melts, producing a clinopyroxene-rich lithology, may form a source of ankaramitic (high-Ca) arc basalts

Timescales of petrogenesis, in an active caldera, Rabaul, Papua New Guinea

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Modulation and daily banding of Mg/Ca in Orbulina universa tests by symbiont photosynthesis and respiration: A complication for seawater thermometry?

The Mg/Ca composition of calcium carbonate tests (shells) secreted by planktonic foraminifera is increasingly being employed to estimate past seawater temperatures and reconstruct paleocean and climate records spanning hundreds of thousands of years. We show, using two high-resolution microanalysis techniques, that the final chamber of the planktonic foraminifera Orbulina universa typically comprises between three and six paired, low and high Mg, growth bands. The number and spacing of these bands is consistent with a diurnal origin, modulated by changing pH within the foraminiferal microenvironment due to the day-night, photosynthesis-respiration cycle of algal symbionts. The amplitude of Mg/Ca variation within individual tests and across many daily growth bands cannot be accounted for by seawater temperature in the shallow, euphotic zone habitat of O. universa. Our results indicate the Mg/Ca composition of calcite precipitated by O. universa in nature is strongly influenced by diurnal changes in the biological activity of algal symbionts and the host foraminifer. This brings into question the fundamental premise often made in applying Mg/Ca palaeoseawater thermometry, that the Mg/Ca composition of foraminiferal calcite is determined by seawater temperature, and whether the Mg/Ca composition of other planktonic species that are more widely used for palaeoseawater thermometry are subject to similar influences. © 2004 Elsevier B.V. All rights reserved

Longevity in maternal transmission of isotopic marks in a tropical freshwater rainbowfish and the implications for offspring morphology

Transgenerational marking is increasingly being used to study the early life history, biology and ecology of fishes. However, the timeframe over which the injected enriched stable isotopes remain in the mother and are passed onto her offspring is largely unknown. Similarly, we have relatively little knowledge of the effects of isotope labelling on the morphology of offspring. In this study, we injected adult female eastern rainbowfish (Melanotaenia splendida) with two doses (20 μgg -1 and 40 μgg -1) of enriched 137Ba or 87Sr stable isotopes to mark the otoliths of their larvae and examine the effects of isotope labelling on larvae morphology. Isotope ratios in larval otoliths were significantly different from controls in larvae hatched up to 174 days post-injection, indicating that enriched isotopes can mark the larvae of this daily spawning species up to 6 months after a single injection. Isotope-marked larvae displayed variable, but generally increased physical size, indicating that enriched stable isotope labelling may have some unintended effects on larvae morphology. Consequently, transgenerational marking provides a long-term tool for marking the offspring of M. splendida to disentangle their patterns of survivorship and dispersal, with the caveat that such studies should be interpreted in light of potential isotope-related changes in offspring morphology

Characterization of Mg/Ca distributions in planktonic foraminifera species by electron microprobe mapping

The distribution of Mg/Ca within the tests of eight modern planktonic foraminifer species has been characterized using electron microprobe mapping. Species include several that are commonly used for estimation of past seawater temperatures (Globigerinoides ruber, G. sacculifer, Neogloboquadrina incompta (synonym of N. pachyderma dex.), N. dutertrei, and G. truncatulinoides). Each of the investigated species displays large variations in Mg/Ca composition within individual tests. However, the pattern of Mg/Ca variation is notably different between symbiont-bearing and symbiont-free species. In symbiotic species, cyclic Mg/Ca compositional banding occurs that is characterized by narrow (<1-3 μm), high-Mg/Ca (typically 8-11 mmol/mol) bands, intercalated between broader low Mg/Ca (typically 1-5 mmol/mol) bands. This factor of 2-3 difference equates to an apparent calcification temperature change of 10°C or more. Such temperature changes are considered highly improbable and suggest vital effects significantly modify the incorporation of Mg/Ca into the tests of symbiont-bearing species. These vital effects remain poorly understood and demand further careful evaluation as they may need to be accounted for when making reliable reconstructions of past oceanic temperatures. Symbiont-free species typically have fewer and broader compositional bands that may reflect more closely changes in calcification temperature as these species migrate within a water column. Copyright 2005 by the American Geophysical Union

Effects of seafloor and laboratory dissolution on the Mg/Ca composition of Globigerinoides sacculifer and Orbulina universa tests - A laser ablation ICPMS microanalysis perspective

Partial or selective dissolution of planktonic foraminiferal tests on the seafloor has been shown to alter original test Mg/Ca compositions and thus may limit the accuracy of Mg/Ca-based thermometry for reconstructions of past sea surface temperatures. We have employed laser ablation ICPMS to determine the extent of dissolution-caused changes in Mg/Ca distribution across individual chamber walls of the planktonic foraminifera Globigerinoides sacculifer and Orbulina universa. G. sacculifer samples collected from a core-top depth transect in the NE Indian Ocean and laboratory dissolution experiments show little if any evidence of preferential removal of Mg-rich calcite layers by progressive dissolution of the tests. We attribute the absence of selective dissolution to the banded distribution of Mg across the chamber walls of these foraminiferal species and to the minimal presence of calcite crusts with relatively low-Mg composition on the outer surfaces of tests. Mg/Ca microanalyses of G. sacculifer from core-top samples further indicate that for samples collected above the calcite lysocline the effect of postdepositional dissolution on Mg/Ca sample mean values is minimal and within the uncertainty of Mg/Ca thermometry (i.e. ±0.4mmol/mol; ±0.8°C at ~28°C). Comparison with previously published results for G. sacculifer supports these observations. Simple modelling of G. sacculifer test dissolution indicates that selective removal of calcite with high-Mg/Ca values from within the final chamber of G. sacculifer test appears insufficient to cause the ~10% decrease in Mg/Ca values observed above calcite lysocline. These changes in test composition might be related to development/removal as a function of Δ[CO32-] of a thin diagenetic surface coating which has a relatively high-Mg/Ca composition (i.e. 20-25mmol/mol). © 2010 Elsevier B.V

Calcification rate and shell chemistry response of the planktic foraminifer Orbulina universa to changes in microenvironment seawater carbonate chemistry

We use LA-ICP-MS depth profiling to explore the sensitivity of shell chemistry of the symbiotic planktic foraminifer Orbulina universa to diurnal changes in the holobiont physiology, over a wide range of seawater pH and DIC compositions. B/Ca and U/Ca vary in concert with diurnal Mg/Ca banding, forming compositional bands that are qualitatively consistent with physiological modification of seawater carbonate chemistry (pH, [] and []) within the foraminiferal microenvironment by the net effects of photosynthesis, respiration and calcification. The amplitude of B/Ca banding broadly conforms to banding predicted using the bulk-shell B/Ca sensitivity to the carbonate chemistry changes in the foraminiferal microenvironment. U/Ca banding tends to be greater than predicted using the published bulk-shell sensitivity of this proxy to carbonate chemistry. This either suggests that carbonate chemistry changes in the foraminiferal microenvironment are greater than predicted by modeling and/or the published bulk shell calibration does not accurately reflect the U/Ca sensitivity at the micro-scale. A fourfold increase in seawater DIC composition (1026 to 4019 μmol kg−1) is associated with significant increases in Sr/Ca and Mg/Ca partitioning, and a decrease in Mn/Ca partitioning into shell calcite. The accompanying fourfold increase in calcite saturation produces only a twofold increase in calcification rate (0.14 to 0.28, ±0.02 μm hr−1), suggesting that seawater carbonate chemistry exerts only a small effect on foraminiferal calcification rates, but does have a significant influence on trace element incorporation at both the inter-shell and bulk-shell scale