Laser Ablation ICPMS Projects

Geochemical investigations of hydrothermal alteration in young basaltic crust from the eastern flank of the Juan de Fuca Ridge (ODP Leg 168)

PI: Vanko, D.A., Dept. of Geology, GSU

The mineral products of low-temperature seawater-rock interaction have been described for many years as numerous oceanic examples. Investigators have characterized textural and mineralogical features, and have determined numerous geochemical parameters including mineral and whole-rock geochemistry and isotopic characteristics of rocks and mineral separates. First-order conceptual models of how crustal alteration proceeds in young Layer 2 basalts have resulted. The prevailing view is that crustal alteration is initially dominated by low temperature, high oxygen fugacity conditions during a 1 Ma period of open seawater circulation (oxidative alteration), followed by a period up to around 10 Ma where alteration occurs via closed hydrothermal circulation at warmer temperatures (100 oC), and low oxygen fugacity (non- oxidative alteration). Throughout this process, as the physical and chemical conditions evolve, sequential textural changes take place in basalts. Primary porosity resulting from vesicles and fractures is filled, unstable magmatic phases such as glass and primary minerals (olivine, pyroxene, plagioclase) are partially to completely replaced, and characteristic features such as alteration haloes and veins are developed. Basalts cored during ODP Leg 168 exhibit the whole range of mineralogic and textural features noted previously in young oceanic crust. The exciting thing about these new samples is the exquisite control afforded by the drilling program on this sedimented flank of the Juan de Fuca Ridge. Samples come from settings with well known thermal conditions (present-day temperature and temperature history), well characterized basement fluid compositions, and measured hydrologic parameters. Thus, we are able to pose numerous questions and test hypotheses by determining the alteration styles and detailed mineral chemistry of the samples. Our petrologic studies will combine microscopic, SEM, cathodoluminescence, electron microprobe, and laser ablation ICPMS analysis. This combination will provide unique insights, because cathodoluminescence and, particularly, LA- ICPMS techniques have not been widely used in oceanic alteration studies. With the LA-ICPMS we will determine minor and trace element (including REE) compositions of secondary phases such as clays, zeolites and carbonates . The results will lead to improved trace element distribution coefficients.

A cored basalt sample from the Juan de Fuca Ridge (ODP leg-168) with calcite vein. Five holes in the calcite vein are the ablation pits from rastering 50 mm2 areas. C1-Chondrite normalized REE patterns are shown in the diagram.

Five chondrite-normalized REE patterns for calcite vein in basalt from Juan de Fuca Ridge. Each pattern represents analysis from a laser ablation hole in the photo.

A. Mohamad Ghazi Department of Geology Georgia State University Atlanta, GA 30303 404-463-9556