Adrian A. Finch

Magmatism of the mid-Proterozoic Gardar Province, South Greenland: chronology, petrogenesis and geological setting

Abstract A lithospherically weak zone embracing the southern margin of the Greenland Archaean craton and the Ketilidian Julianehab batholith underwent repeated rifting during the interval 1350–1140 Ma, accompanying breakup of Palaeopangaea. The Gardar Province comprises the area affected by the faulting and asssociated alkaline magmatism. While an estimated 2–5 km of Proterozoic cover has been eroded, rift-fill successions have been preserved in early fault-bounded basins. The orientation of dyke swarms changed from WNW–ESE to nearly NE–SW during Gardar times. The principal swarms are inferred to occupy zones of lithospheric thinning and graben development. Central-type intrusive complexes, largely of syenites and nepheline syenites, reached shallow levels. The principal magmatic evolution was from transitional olivine basalt through to phonolites. Accompanying silica-oversaturated magma generation involved greater degrees of crustal assimilation. Anorthositic xenoliths in the Gardar intrusions imply the presence of an extensive anorthositic complex at depth, regarded as an integral part of the North American Proterozoic massif anorthosite association. The most primitive Gardar basalts are themselves relatively evolved, probably as a result of olivine±pyroxene fractionation during crustal underplating. The Gardar basic rocks are troctolitic with elevated Al 2 O 3 /CaO ratios: their incompatible element patterns suggest a significant input from lithospheric sources. The Ca-deficient nature of the Gardar basalts is attributed to an origin involving lithospheric mantle depleted by previous melting events. Trace element and isotopic signatures suggest considerable heterogeneity in the mantle sources which are ascribed to differential metasomatism of clinopyroxene-poor peridotite sources by small-fraction asthenospheric melts. The ultramafic lamprophyre/carbonatite association that recurred throughout the period is inferred to have originated from melting of metasomites deep within the lithospheric mantle. Affinities between the alkaline intrusions over an interval of >100 Ma characterize the Gardar as a coherent magmatic province and support the contention that the magmas are largely of lithospheric origin. The energy required to generate the very large requisite volumes of primitive magmas may have been supplied by successive mantle plumes. The Gardar magmatism pre- and post-dates the ∼1.27 Ga Mackenzie Igneous Events of North America but wholly pre-dates the ∼1.1 Ga Keweenawan magmatism associated with the Midcontinent Rift.

Coordination of Sr and Mg in calcite and aragonite

Abstract Strontium and Mg in calcite and aragonite are widely used as proxies of temperature in palaeo-environmental reconstructions. We use X-ray absorption fine structure (XAFS) to examine Sr and Mg substitution in calcite and aragonite. We have measured the K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of Mg and Sr-bearing calcite and aragonite, plus the carbonates: strontianite, hydromagnesite, magnesite, dolomite and a suite of calcites with differing amounts of Mg. The Sr substitutes ideally for Ca in aragonite but causes a small (2%) dilation of the site. Strontium substitutes for octahedral Ca in calcite but with a 6.5% dilation and distortion. Magnesium in the calcites studied provides a variable XANES indicating that the Mg structural state in calcite is variable. Refinement of EXAFS gives Mg-O bond distances of ~2.12 Å, which are much smaller than the Ca-O bond distance of 2.35 Å but consistent with published amounts of relaxation of the calcite structure. The XANES and EXAFS are consistent with a model whereby some calcites contain nanodomains, e.g. of dolomite and/or huntite structures. The variability in the XANES can be explained by domains of different types and/or sizes. Substitution of Mg into aragonite has 9-fold coordination but relatively short bond distances (2.08 Å) demonstrating either: (1) substantial distortion of the site; or (2) that Mg is accommodated in nanodomains of an unknown phase. Variability in the Mg structural state in calcite may be linked to the variety of temperature dependences observed, e.g. in foraminiferal calcite.

Trace element variation in speleothem aragonite: potential for palaeoenvironmental reconstruction

Trace element variation in annually laminated stalagmites has the potential to record long-term, high-resolution palaeoclimatic data provided both (1) the relationship between trace element content in the carbonate and the climate, and (2) the mechanism of stalagmite growth, are understood. Here we report a secondary ion mass spectrometry (SIMS) study of an aragonitic speleothem (T7) from Cold Air Cave, Southern Africa, which achieves small (μm) spatial resolution and ppm precision. T7 has a series of fine visible laminae believed to be annual to subannual. This article establishes (1) the usefulness of SIMS in analyzing speleothem aragonite at the spatial resolution and precision relevant to climatic studies, (2) the distribution of trace elements in aragonite on micron scales and their relationship to models for speleothem growth and (3) the potential of aragonite speleothems to encode climatic data at subannual resolution. Complex reproducible oscillations in Sr/Ca and Ba/Ca are observed parallel to the growth direction of the speleothem. However, in addition to this variability, we observe trace element oscillations perpendicular to the growth direction and infer that aragonite crystallites in the speleothem are laterally, as well as vertically zoned. Stepscans parallel to the growth direction demonstrate complex zoning patterns from which simple climatic data are difficult to extract due to small-scale lateral heterogeneity in the aragonite. However, time-series analysis of the whole data set reveals regular cycles in Sr/Ca and Ba/Ca at a scale that matches the magnitude of the visible laminae. On this basis, it appears that within the complex geochemical variability, there resides a regular environmental control, possibly variation in water throughput, hence rainfall.

Strontium heterogeneity and speciation in coral aragonite: implications for the strontium paleothermometer

Sea surface temperatures (SSTs) have been inferred previously from the Sr/Ca ratios of coral aragonite. However, microanalytical studies have indicated that Sr in some coral skeletons is more heterogeneously distributed than expected from SST data. Strontium may exist in two skeletal phases, as Sr substituted for Ca in aragonite and as separate SrCO3 (strontianite) domains. Variations in the size, quantity, or both of these domains may account for small-scale Sr heterogeneity. Here, we use synchrotron X-ray fluorescence to map Sr/Ca variations in a Porites lobata skeleton at a 5 μm scale. Variations are large and unrelated to changes in local seawater temperature or composition. Selected area extended X-ray absorption fine structure (EXAFS) spectroscopy of low- and high-Sr areas indicates that Sr is present as a substitute ion in aragonite i.e., domains of Sr carbonate (strontianite) are absent or in minor abundance. Variations in strontianite abundance are not responsible for the Sr/Ca fluctuations observed in this sample. The Sr microdistribution is systematic and appears to correlate with the crystalline fabric of the coral skeleton, suggesting Sr heterogeneity may reflect nonequilibrium calcification processes. Nonequilibrium incorporation of Sr complicates the interpretation of Sr/Ca ratios in terms of SST, particularly in attempts to extend the temporal resolution of the technique. The micro-EXAFS technique may prove to be valuable, allowing the selection of coral microvolumes for Sr/Ca measurement where strontium is incorporated in a known structural environment.

Corals concentrate dissolved inorganic carbon to facilitate calcification

The sources of dissolved inorganic carbon (DIC) used to produce scleractinian coral skeletons are not understood. Yet this knowledge is essential for understanding coral biomineralization and assessing the potential impacts of ocean acidification on coral reefs. Here we use skeletal boron geochemistry to reconstruct the DIC chemistry of the fluid used for coral calcification. We show that corals concentrate DIC at the calcification site substantially above seawater values and that bicarbonate contributes a significant amount of the DIC pool used to build the skeleton. Corals actively increase the pH of the calcification fluid, decreasing the proportion of DIC present as CO2 and creating a diffusion gradient favouring the transport of molecular CO2 from the overlying coral tissue into the calcification site. Coupling the increases in calcification fluid pH and [DIC] yields high calcification fluid [CO3(2-)] and induces high aragonite saturation states, favourable to the precipitation of the skeleton.

Corroborated rainfall records from aragonitic stalagmites

Abstract Sr and Ba levels of two proximal aragonite speleothems from Cold Air Cave, South Africa, are examined at μm resolution. A compositional map derived from parallel secondary ion mass spectrometry linescans indicates heterogeneity perpendicular to the stalagmite growth axis, precluding the use of single linescans as a climatic proxy technique. A ∼40-yr averaged trace element record for the cave produced from multiple parallel linescans on both stalagmites is compared with regional and local climatic (rainfall, temperature) data for 1955–1996. This period includes two extensive drought episodes. There is poor correlation between trace elements and annual mean temperature. Droughts correspond to minima in trace element ratios and peaks correspond to annual maxima in rainfall. The onset and termination of droughts are notable inflexions. However, in detail annual rainfall magnitude does not correlate directly with trace element ratios, indicating that kinetic factors, notably variations in speleothem growth rate, play a key role in trace element uptake by aragonite. Averaged long data sets constrained by chronological control have potential in the study of sub-decadal precipitation patterns.

Reconstruction of deglacial sea surface temperatures in the tropical Pacific from selective analysis of a fossil coral

The Sr/Ca of coral skeletons demonstrates potential as an indicator of sea surface temperatures (SSTs). However, the glacial-interglacial SST ranges predicted from Sr/Ca of fossil corals are usually higher than from other marine proxies. We observed infilling of secondary aragonite, characterized by high Sr/Ca ratios, along intraskeletal pores of a fossil coral from Papua New Guinea that grew during the penultimate deglaciation (130 {+-} 2 ka). Selective microanalysis of unaltered areas of the fossil coral indicates that SSTs at {approx}130 ka were {le} 1 C cooler than at present in contrast with bulk measurements (combining infilled and unaltered areas) which indicate a difference of 6-7 C. The analysis of unaltered areas of fossil skeletons by microprobe techniques may offer a route to more accurate reconstruction of past SSTs.

Strontium in coral aragonite: 2. Sr coordination and the long-term stability of coral environmental records

We have used X-ray Diffraction (XRD) and Sr K-edge Extended X-ray Absorption Fine Structure (EXAFS) to determine the structural state of Sr in a suite of coral aragonite samples. Our samples encompassed a selection of coral species (Porites lobata, Porites lutea, Pocillopora eydouxi, Montastrea annularis, Pavona gigantea and Pavona clavus) including some commonly used for palaeoenvironmental reconstruction. Aragonite was the only carbonate observed by XRD. We refined the isolated EXAFS against structural models for Sr in aragonite and two-phase strontianite/aragonite mixes. Our data are indistinguishable from Sr ideally substituted in aragonite and strontianite was present below detection levels (estimated at <5% of Sr present). Comparisons of recent and ancient coral aragonite show no sign of exsolution, either by spinodal decomposition or by the direct nucleation of strontianite domains. Published diffusion rates of Sr in ionic solids support the view that exsolution would occur prohibitively slowly. Coral aragonites are metastable materials with slow diffusion kinetics that have the potential to encode environments over timescales of millions of years.

Ionoluminescence of zircon: rare earth emissions and radiation damage

The luminescence of synthetic rare earth (RE)-doped zircons and three natural zircons has been studied by ion beam excitation (ion beam luminescence or ionoluminescence (IL)). Luminescence results from electronic transitions between energy levels of the RE ions, giving emissions with characteristic energies. Studies of co-doped Ho : Dy zircon show that electronic cascades from Ho3+ to Dy3+ ions result in intense Dy3+ luminescence even when Ho is far more abundant. Dy3+ ions have the greatest luminescence cross-section of common RE3+ ions. Implantation of heavier N+ ions into zircon, chosen to mimic accelerated radiation-induced structural modification, causes significant sample degradation. Frenkel-type defects and groups in the zircon are associated with a broad luminescence band of ~600 nm, and these defects modify energy cascades between RE3+ ions, thereby reducing the efficiency of energy deposition via the Dy3+ luminescence decay pathway. The broad band of ~600 nm created by implantation is common in natural zircons and is attributed primarily to intrinsic Frenkel defects caused by natural radiation. Dy3+ luminescence is apparent from natural zircons even though Dy is one of the least abundant RE ions in zircon, and this behaviour is explained by the high luminescence cross-section of Dy3+ in zircon. Radiation-induced structural damage reduces the efficiency of energy transport between different RE3+ ions. IL provides key insights into both the interactions between defects and the nature of luminescence centres in zircon.

Strontium in coral aragonite: 1. Characterization of Sr coordination by extended absorption X-ray fine structure

Abstract Aragonite was analyzed from Porites lobata, Pavona gigantea, Pavona clavus, and Montastrea annularis corals by Sr K-edge extended absorption X-ray fine structure (EXAFS) and compared with aragonite, strontianite, and mechanically mixed standards. Bulk analyses were performed and data compared with equivalent micro-EXAFS analyses on small (∼400 μm 3 ) analytical volumes with a microfocused X-ray beam. As a result of the architecture of the coral skeleton, the crystals within the microanalytical volume are not randomly oriented, and the microanalytical X-ray absorption spectra show orientational dependence. However, refinement of bulk and microanalytical data provided indistinguishable interatomic distances and thermal vibration parameters in the third shell (indicative of Sr speciation). The Sr K-edge EXAFS of all the coral samples refine, within error, to an ideally substituted Sr in aragonite, in contrast to previous studies, in which significant strontianite was reported. Some samples from that study were also analyzed here. Strontianite may be less widely distributed in corals than previously thought.