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Abstracts


Laser ablation-ICP-MS compositional profiling of chamber walls in planktonic Foraminifera; implications for Mg/Ca thermometry

S. EGGINS, P. DE DECKKER# AND J. MARSHALL
Research School of Earth Sciences, Australian National University, Canberra, 0200, Australia
#Department of Geology, Australian National University, Canberra, 0200, Australia

We have profiled compositional variation through the chamber walls of individual Foraminifera using a new, high resolution, laser ablation ICPMS technique. Mg/Ca ratios are able to be measured accurately and precisely in specific shell parts (wall layers and chambers) along with simultaneous analysis of other trace elements (e.g. Mn, Zn, Sr, Ba, Cd, U) and their isotopes with detection limits to low ng/g levels in <60 seconds. The mass of material consumed by each analysis is ~20-30ng (cf. typical test mass of ~10-30µg) and replicate analyses can be made on individual chambers. The technique readily distinguishes outer crust and inner primary wall layer compositions, as well as the presence of an outermost zone characterised by Mg, Mn, Ba and Zn concentrations increasing strongly towards the surface. Exploratory studies on planktonic Foraminifera (Globigerina and Neogloboquadrina sp.) define the capabilities of our technique, highlight the inherent limitations of conventional bulk analysis methods, and provide new insights into fundamental biomineralisation processes. Mg/Ca ratios can be reproducibly measured within particular chamber wall layers to (2-5% and more uniformly distributed Sr/Ca ratios to (1%). Inner primary wall layers of different chambers in the same test are commonly found to have distinct Mg/Ca compositions, whereas the inner portions of the outer crust on all chambers have similar and lower Mg/Ca. The strong enrichment in Mg, Mn, Ba, and Zn toward the outermost crust surface in both modern and fossil shells, suggests a significant vital effect change during the biomineralisation process. The ability to measure Mg/Ca in shell parts grown under specific seawater conditions, rather than bulk compositions integrated over the life-cycles of entire shells, may enable calibration of more accurate and precise Mg/Ca thermometers than is currently possible. The tiny amount of sample consumed also allows for subsequent d18O microanalysis of the same shell material, and the simultaneous acquisition of other trace elements presents an opportunity to further develop their potential as proxies for seawater temperature, composition, and nutrient levels.

(submitted May 2002 to Goldschmitt Conference, Davos, Switzerland, 2002)

Compositional heterogeneity in NIST610-617 glasses
S.M. EGGINS and J.M.G. SHELLEY
Research School of Earth Sciences, Australian National University, Canberra, 0200, Australia

Extensive compositional heterogeneity affecting more than 1/3 of the doped trace elements in the NIST610-617 glasses is revealed by profiling and mapping using laser ablation ICPMS. Every NIST610-617 wafer probably contains domains that are significantly depleted in Ag, As, Au, B, Bi, Cd, Cr, Cs, Mo, Pb, Re, (Rh), Sb, Se, Te, Tl, W and possibly also Rb, and enriched in Cu (and Pt). Large enrichments in Fe, Mn and Cd are also encountered in some wafers. These compositionally heterogeneous domains are visible in doubly polished wafers by unaided visual inspection and by transmitted light and schlieren microscopy. They occur in close proximity to the wafer perimeters and also as stretched and complexly folded forms within wafer interiors. The chemical and optical properties of these depleted domains are consistent with those of compositional cords, a phenomenon of glass manufacture where glass bulk composition and physical properties are modified by loss of volatile components from the molten glass surface. The refractory oxide forming elements (e.g. REE, Sc, Th, U, Zr, Hf, Nb, Ta, Be, Ba, Sr, Ca, etc.) show no evidence of heterogeneity, and indicate that the glass constituents and any residues remaining in the furnace from preceding glass batches have been well homogenised. The NIST610-617 glasses may be considered suitable reference materials for micro or bulk analysis of only between 1/2 and 2/3 of the trace elements with which they were doped.

(submitted to Geostandards Newsletter, June, 2002)

The 1994 to 2001 eruption at Rabaul, Papua New Giunea: evidence of repeated basaltic magma influx into a sub-caldera dacite magma reservoir.

H. Patia#, S.M. Eggins*, C.O. McKee+, R.J. Arculus#, and R.W. Johnson*
#Department of Geology, Australian National University, ACT 0200, Australia
Research School of Earth Science, Australian National University, ACT 0200, Australia
+Geophysical Observatory, P O Box 323, Port Moresby, Papua New Guinea
*Geoscience Australia, Canberra, ACT 2601, Australia

The latest eruption at Rabaul Caldera consisted of two phases: Phase 1 began on 19 September 1994 when Vulcan and Tavurvur volcanoes erupted simultaneously from opposite sides of the caldera. Phase 1 activity ended on 16 April 1995. The total volume of pyroclastic material erupted was 300x106m3. Phase 2 commenced in November 1995 when eruptive activity resumed at Tavurvur and continued intermittently until mid-2001. Seven Strombolian eruptions, of which four produced lava flows, occurred between May 1996 and July 1997. The total volume of pyroclastic deposits erupted during Phase 2 was about 25x106m3.
Magma mixing and mingling arising from probable new influxes of basaltic magma into a dacite magma reservoir is a feature in both Phases 1 and 2. The nature of the basaltic and dacitic magma interactions has been assessed using petrological and geochemical methods. Magma interaction and mixing between dacite magma and basaltic magma in Phase 1 and 2 is revealed by XRF whole-rock data. The dacite/basalt mixing trajectory projects towards a composition that intersects the estimated liquid line of descent at >7 wt% MgO.
The dacite magmas erupted from Vulcan and Tavurvur during Phase 1 and from Tavurvur during Phase 2 are indistinguishable (about 1.9 wt% MgO, 63 wt% SiO2), and similar in composition and mineralogy to those rocks erupted during the 1878 and 1937 eruptions at Rabaul. They are not as evolved as the Rabaul Ignimbrite (65-67 wt% SiO2) erupted during the latest major caldera-forming eruption about 1400 years BP (Wood et al., 1995).
The Phase 1 and 2 dacites are characterized by a dominant phenocryst assemblage comprising plagioclase (An50-60) and lesser clinopyroxene (Mg# 70-75), orthopyroxene, titanomagnetite, sulfide, and apatite. Mixed magmas of andesitic composition have strongly bimodal phenocryst assemblages and well-developed sub-populations crystallized from basaltic and dacitic parent magmas. The basaltic phenocryst sub-populations are dominated by relatively magnesian clinopyroxene (Mg# strongly centered upon Mg#84), magnesian olivine (Fo84), calcic plagioclase (An85-95), and titanomagnetite. There is limited or no development of inverse compositional zoning in the dacite phenocryst populations, as indicated by core and rim microprobe analyses and detailed compositional profiles. In contrast, the basaltic plagioclase phenocryst population comprises both unzoned (no sodic rim development) and normally zoned An90 cores and An50-60 rims) sub-populations that are attributable to fresh and previous basaltic magma inputs to the dacite magma reservoir.

Presented at the Australian Geological Conference, Adelaide, 2002.