Isotope data measured by muli-collector TIMS at UC Berkeley, spring 2001. PLEASE DO NOT take this data from this web site. Email me for more information.

Sample	54/56 ratio	d⁵⁶Fe	2 s	n
The zero	0.0636830	0.00		21
Johnson and Beard (1999) "earth-moon average"

The laboratory standard		2.41	0.10	8
St. Severin Chondrite		1.21	0.06	2
Amphibolite		0.07	0.10	5
Finn Pyrite		-0.32	0.08	3
Finn Chalcopyrite (unknown)		-0.70	0.15	3
Finn Chalcopyrite (massive)		-1.07	0.16	3
Finn Sphalerite (treated)		-1.86	0.12	5
Finn Sphalerite (untreated)		-1.38	0.10	6

Isotope Methods and Data

Iron isotope composition was measured in pyrite, chalcopyrite, and sphalerite from the Finn vent chimney.

Samples were hand-picked from a crushed portion of Finn and dissolved in 15N nitric acid (pyrite and chalcopyrite) or a mixture of 12N hydrochloric and 15N nitric acids (sphalerite). About five to twenty milligrams of sphalerite, pyrite, and chalcopyrite were digested and double-spiked (54-58) and the iron separated from the matrix by ion exchange chromatography. The eluted iron was then suspended in 0.5N HNO3 solution and about 4 µg loaded onto a super-pure rhenium filament with high-purity aluminum oxide, phosphoric acid, and silica gel. Double spike subtraction was accomplished in a similar manner to Johnson et al.'s (1999) technique for selenium isotopes.

Analytical results indicate isotope fractionation between Johnson and Beard's (1999) "earth-moon average" and the minerals precipitated at the interface between hot hydrothermal fluid and seawater. Two separates of sphalerite were isotopically light compared to the "earth-moon average" with d⁵⁶Fe of -1.4 and -1.9‰ while two separates of chalcopyrite were also light at -0.7 and -1.1‰. Pyrite from Finn averaged out to -0.3‰, close to the -0.1‰ determined by Polizotto et al. (2000), relative to an igneous rock standard.

The initial goal of this work was to determine the isotopic composition of iron at deep sea vents. The focus of this particular work, however, is on the possible reasons for the presence of "light" iron at the vent. These possibilties include:

Mantle-derived iron that is overwhelmingly negative
Seawater entrained in hydrothermal convection cells is isotopically negative (due to reactions in the photic zone and/or the water column)
Crustal alteration processes fractionate iron
Precipitation/speciation (or both) fractionate iron at the vent- seawater interface as hydrothermal fluid cools and mixes with cold seawater

This last possibility is the focus of subsequent sections.