Preliminary investigations of Nb in melt-fluid systems using

in situ X-ray spectroscopy

Adam J. Layman1

and

Alan J. Anderson2

1 Dalhousie University, Halifax, Nova Scotia, Canada

2 St. Francis Xavier University, Antigonish, Nova Scotia, Canada

Introduction

The partitioning of Nb in hydrothermal fluid-melt systems is important to our understanding of the transport and deposition of Nb in crustal rocks.

Recent experimental work (e.g., Linnen 2004 Lithos v.80) has advanced our knowledge of Nb solubility in granitic melts. However, there is still very little known about Nb transport in hydrothermal fluids.

Evidence for fluid transport Nb

- fenites

- hydrothermal veins

- albitized felsic rocks

The behavior of Nb in melt-fluid systems is investigated using Synchrotron radiation micro-X-ray fluorescence (SR-μXRF)

SR-μXRF allows for in situ, quantitative, measurements of experiments in the Hydrothermal Diamond Anvil Cell (HDAC), at geologically relevant pressures and temperatures

Methodology

Nb-bearing synthetic glasses

- characterized by EMP analysis

Glass WNb

- peraluminous, CNb ~ 445 ppm

Glass ASI06

- peralkaline, CNb ~ 5000 ppm

Four experimental fluids

- aqueous ± sodium carbonate

- varying densities

- CO32- concentrations (0, 0.5, 1 M)

- Trace Br; 500 ppm, 1000ppm

Hydrothermal Diamond Anvil Cell

Tungsten Carbide SeatHeater Wires

Sample Chamber

Thermocouples

Gasket

Synchrotron X-ray Microprobe

Experimental Setup

Detector

Microscope

HDAC

Phase transitions

T = 22°C

T = 635°CT = 475°C

T = 190°C

Spatial resolution of microbeam

T = 22 °C

After experiment

Br X-ray map

T = 22°C

Nb X-ray map

T = 22°C

Distribution of elements in HDAC

Co

un

ts

X-ray Energy (eV)

X-ray Fluorescence Spectra

Re L’s Br Ka

Compton Scattering

Nb Ka

Br Kb

Composition of Fluid Coexisting with MeltC

ou

nts

X-ray Energy (eV)

Nb Ka

Mo KaBr Kb

Br Ka

Fitting XRF Spectrum

Background fitting routine

Br Ka

Br Kb

Nb Ka

Compton scattering

Re L’sMo Ka

Co

un

ts

X-ray Energy (eV)

Peak Areas from Fitting Routine

Peak Energy Area

Fe Ka 6.400 989.6

Ni Ka 7.472 2150.1

Re La 9.637 45766.8

Re Lb 10.179 0.0

Br Ka 11.907 4313.8

Br Kb 13.296 780.1

Nb Ka 16.584 7065.5

Mo Ka 17.443 3114.6

Peak Energy Area

Fe Ka 6.400 989.6

Ni Ka 7.472 2150.1

Re La 9.637 45766.8

Re Lb 10.179 0.0

Br Ka 11.907 4313.8

Br Kb 13.296 780.1

Nb Ka 16.584 7065.5

Mo Ka 17.443 3114.6

Peak Areas from Fitting Routine

Quantification

CNb

CBr PABr

PANb

Fundamental Parameters

- incorporates absorption and matrix effects, pre-sample and pre-detector absorbing layers (diamond)

- uses known Br concentration as an internal standard, to calculate Nb concentration

Experimental Summary

- Initial experiments using glass WNb, CNb ~ 445 ppm, and 1 M sodium carbonate and pure water fluids

- Experiments using ASI06, CNb = 5000 ppm, and 0.5 M sodium carbonate and pure water fluids

- Evaluation of precision and accuracy using standard solutions in the HDAC and capillary tubes

Results

Nb in pure aqueous fluid coexisting with a silicate melt is not detected at all P-T conditions

- at relatively high densities, silico-aqueous single phase fluid

- ≥ 0.95 g/cm3 for carbonate-bearing fluid

- ≥ 0.965 g/cm3 for pure water

- Nb does partition into carbonate-bearing fluid at high to submagmatic T

0 100 200 300 400 500 600 7000.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014

0.0016

0.0018

0.0020

0.0022

0.0024

0.0026

0.0028

0.0030C

Nb

(wt%

)

Temperature (degC)

TwoPhase SinglePhase

Nb concentration in fluidD = 0.95 g/cm3

With 1 M sodium carbonate fluid, Nb partitions strongly into the fluid phase.

At high temperatures, in a single phase fluid, CNb= 25 ppm

During cooling, two phases are apparent, a melt and a aqueous fluid. Nb is partitioned into the fluid, with CNb= 19 ppm at 600°C, 7 kbars, and CNb≤ 3ppm at 400°C, 3.5 kbars

Conclusions

Nb is not detected in pure aqueous fluid coexisting with a silicate melt.

In contrast, significant amounts of Nb were measured in carbonate-bearing fluids at magmatic temperatures.

Carbonate-bearing fluids may provide an alternate/parallel host and transport mechanism for Nb.

Acknowledgements

NSERC

Advanced Photon Source, Argonne National Laboratory, and the United States Department of Energy

Bill Bassett, Bob Mayanovic and I-Ming Chou

GSECARS Beamline 13-ID, Steve Sutton and Matt Newville

PNC-XOR Beamline 20, Robert Gordon, Steve Heald, Dale Brewe and Julie Cross

Barrie Clarke

Dalhousie University

St. Francis Xavier University

Methodology