2003-2004 USAP Field Season

Geology & Geophysics

Dr. Rama K. Kotra Program Manager

G-067-E	NSF/OPP 02-28842
Station: Special Project
RPSC POC: John Evans
Research Site(s): Australia's Davis Station
Dates in Antarctica: Mid November to early February

Boron in antarctic granulite-facies rocks: Under what conditions is boron retained in the middle crust?

	Dr. Edward S. Grew The University of Maine Department of Geological Sciences esgrew@maine.edu

At the joint Geological Association of Canada-Mineralogical Association of Canada meeting in Vancouver (May '03), principle investigators Ed Grew (left) and Chris Carson plan for upcoming fieldwork on borosilicate minerals in the Larsemann Hills, Prydz Ba

Deploying Team Members:

Chris Carson . Edward S. Grew

Research Objectives: Trace elements provide valuable information on the changes sedimentary rocks undergo as temperature and pressure increase during burial. One such element, boron, is particularly sensitive to increasing temperature because of its affinity for aqueous fluids, which are lost as rocks are buried. The boron content of unmetamorphosed pelitic sediments ranges from 20 to over 200 parts per million, but rarely exceeds 5 parts per million in rocks subjected to the conditions of the middle and lower crust. Devolatization with loss of aqueous fluid and partial melting with removal of melt have been cited as primary causes for boron depletion in granulite-facies rocks. Despite the pervasiveness of both these processes, rocks rich in boron are locally found in granulite-facies in the Larsemann Hills along Prydz Bay. More than 20 lenses and layered bodies containing four borosilicate mineral species crop out over a 50-square-kilometer area. While most investigators have focused on the causes of boron loss, we will use field observations and mapping, chemical analyses of minerals and their host rocks, and microprobe age-dating to investigate how boron is retained during high-grade metamorphism. Our working hypothesis is that a high initial content facilitates retention of boron during metamorphism. For example, in a rock with large amounts of the borosilicate tourmaline (such as strata-bound tourmalinite), the breakdown of tourmaline to melt could result in the formation of prismatine and grandidierite, two borosilicates found in the Larsemann Hills. This situation is rarely observed in rocks with a modest boron content, in which tourmaline breakdown releases boron into partial melts, which in turn remove it when they leave the system. Strata-bound tourmalinite is associated with manganese-rich quartzite, phosphorus-rich rocks, and sulfide concentrations that could be indicative of a tourmalinite protolith in a highly metamorphosed complex where sedimentary features have been destroyed by deformation. Because partial melting plays an important role in the fate of boron, our research will focus on the relationship between borosilicate units, granite pegmatites, and other granitic intrusives. Our results will provide information on boron cycling at deeper levels in the Earth's crust and on possible sources of boron for granites originating from deep-seated rocks.