U- and Th- Decay Series Dating of Hydrothermal Mineral Deposits: A Preliminary Study

Susan Humphris, Geology and Geophysics
Meg Tivey, Marine Chemistry and Geochemistry


2012 DOEI Ocean Ridge Initiative Project


Seafloor sulfide deposits form on time-scales of minutes to thousands of years, are products of significant heat and mass transfer from the lithosphere to the oceans, and form in conjunction with activity that supports significant microbial and animal communities in the deep sea.  Understanding time-scales over which the deposits form, and whether they form continuously or episodically, can thus provide important information both about energy and mass transfer and feedback within the ocean crust and lithosphere, and about environments, energy, and nutrient sources for hydrothermal biological communities.

 We currently have both qualitative and quantitative models of formation of sulfide-rich chimneys, which form on time-scales of days to years, and qualitative to semi-quantitative models of formation of large deposits that indicate episodicity in activity over time-scales of thousands of years.  While these studies have shown the potential of using U- and Th- decay series methods for determining chimney growth rates and for dating relict deposits, numerous questions of open-system behavior persist (both during chimney growth and post-depositional alteration) and thus the determined ages are often questioned.

 We propose to conduct a preliminary study of U and Th concentrations and isotope ratios in mineral separates from young, active chimneys (sulfides and sulfates) and from relict deposits (sulfides) in collaboration with Ken Sims (U.  Wyoming) to demonstrate the feasibility of investigating the time-scales over which hydrothermal processes are operating.  These data will provide the basis for resubmission of an NSF proposal to (i) assess the efficacy of using multiple isotope systems to age date hydrothermal mineral deposits, and (ii) apply our techniques to various seafloor hydrothermal sites representing a range of ages.  The different parent-daughter nuclides we will measure (233U-234U-230Th-226Ra-210Pb- 210Po; 232Th-228Ra-228Th) have starkly contrasting chemistries resulting in significant elemental fractionations during a variety of hydrothermal processes, and their different half-lives (138 days to 75 kyrs) will allow the investigation of time scales ranging from ~1 to 105 years.