Most of our direct knowledge of how hot the Earth can be comes from the study of such modern lavas such as those erupted in Hawaii and Iceland. But the Earth has an older record of melting and crystallization, locked in rocks that we can see and sample. One such occurrence on the coast of Labrador represents an intrusion of magma into the crust of the Earth from the mantle 1.3-billion years ago. This magma crystallized into olivine-bearing plagioclase rocks called troctolite. A recent experimental study by a UMass graduate student in the Five-College Experimental Petrology Laboratory housed at Smith College has shown high melting temperatures for these rocks at crustal pressures. Extrapolation to higher pressure suggests that the melted material may have separated from a mantle source more than a hundred degrees hotter than our familiar ocean basalts. We will test this inference through further melting experiments. Whatever the outcome, the results can help us understand the thermal structure of the Precambrian Earth at this location. In turn it will illuminate the origin of valuable nickel-cobalt ore deposits from the nearby Labrador coast.

This study will capitalize on previous NSF support for the study of this remarkable occurrence of well-preserved rocks called the Kiglapait Intrusion and its associated anorthosites. Its results will lead to a better understanding of the origin of the troctolite-related Ni-Co sulfide ores at the Voisey's Bay mine some 70 km south of the Kiglapait intrusion. Using the model bulk composition of the intrusion, they have traced the plagioclase - olivine equilibrium to 15 kbar and found multiple saturation with spinel and two Al-pyroxenes at 13 kb, 1296 degrees C. It is proposed to extend the study by adding appropriate olivine to the mix until it saturates. It is expected that the experiments will lead to the appropriate conditions where a multi-saturated source temperature is in equilibrium with olivine ~Fo84, which may exceed 1300 degrees C. If proven correct, it would indicate that the source regions of the Labrador massif anorthosites, and their world-wide companions that are associated with troctolites, were typically at much high temperatures than currently thought. It is also planned to perform additional petrologic and geochemical work using a Cameca Ultra-Chron electron microprobe, including exploring the partitioning of Sr in plagioclase and apatite in the Kiglapait and Skaergaard intrusions, determing the Ba content in plagioclase, a study of phosphorus in olivine as an indicator of crystallization history, and the completion of the study of the Upper Border Zone of the Kiglapait Intrusion, and its record of liquid compositions throughout the 1-million-year crystallization history. Finally, a full sample catalog will be prepared for inclusion with the Kiglapait sample collection as it is eventually transferred to the American Museum of Natural History for curation and posting on its web site.

Project Report

Scientific progress related to Earth history The rocks being investigated in this experimental and analytical study were formed 1307 million years ago deep within the Earth’s crust, and there are no known modern counterparts to these rocks. They are rocks called troctolites to ferrosyenites, crystallized over a period of a million years, unsaturated in silica. and preserving a uniquely complete record of magmatic fractionation. The primary goal of this research was to explore the high-temperature and high-pressure origin of the parent magma. This has been accomplished to first order and the result is that we can model an origin in the Earth’s mantle at ~15 kbar and 1400 ?C at ~1308 Ma. This is a very hot source temperature and that probably accounts for the absence of modern volcanic rocks of this nature. Hence the primary goal of this research effort reveals to us a piece of Earth history that is unusual and not well understood in terms of the bulk Earth evolution. In the course of this enterprise several unusual papers have emerged. These contain newly discovered principles of physical chemistry that inform our understanding of crystallization and melting in the Earth. One is the 2011 paper on The fractional latent heat of crystallizing magmas. Even slowly cooled igneous rocks commonly have immature (metastable) grain-to-grain mineral textures involving the mineral plagioclase feldspar, and the degree to which these rocks have become annealed and their approach to being texturally stable has been the subject of a revolutionary scientific advance by the Cambridge University group led by Professor Marian Holness. Her studies show that details of the cooling history of long-buried rocks can be illuminated by careful measurements of a specific class of dihedral angles measured under the microscope. These measurements tend to go in waves when referred to stratigraphic height and hence reaction progress in magma. The quantitative identification waves of fractional latent heat, associated with the periodic saturation of new crystal phases in the evolving magma, starkly reveals the periodicity of the textural history and links the microscopic evidence to the physics of magmatic cooling. In addition, a fortuitous experiment with a euhedral olivine crystal containing a negative-crystal shaped melt inclusion showed that the inclusion was strongly fractionated and therefore not at all representative of the bulk liquid surrounding the olivine. This result is a warning to a cottage industry of melt-inclusion studies that assumes the capture of parental melts in crystals. It is not likely to be true. These studies will belong to the textbooks of the future. Social Scientific History The source rocks of this experimental enterprise are from the Kiglapait Layered Intrusion on the coast of Labrador in the Province of Newfoundland and Labrador, Canada. This body of well-exposed, glaciated rocks is rimmed by a fringe of steep, 1-km high mountains rising from the sea, except that the seaward part of the fringe is missing. The intrusion is thus surrounded on three sides by salt water; but to the north and south the water is an inland passageway that freezes over in winter. The sea also freezes over, but where open it furnishes ice that is prone to tidal instability. The result is that for eons of human history the back country of the Kiglapait Intrusion has been the highway for coastwise postal travel by dogsled and now by snowmobile for hunting and fishing by native people. This highway provides security from ocean storms, but is famous for its treeless exposure to blizzard conditions that have trapped many close to death over the years. Accordingly, the Kiglapait Intrusion and its shoreline has been a special place related to sustenance and travel for all human time. In the view of the local population, it is a special place. It is also a special place to the work of science, a unique feature of the Earth’s geologic past, and a resource for study since 1942 and far into the future. We have barely scratched the surface. New field studies are needed, new sampling programs are being contemplated. Paleomagnetic studies are now under way for the first time. Mineralogical specimens from the intrusion have been used in Lunar and Martian studies, in the newly invented technique of atom probe tomography, and in the study of near-infrared reflectance (C. Jackson Letter 18 Apr 2014). The intrusion is a resource of unlimited scope in its value to science. There is a whiff in the air of the idea of establishing sites of something like World Heritage Scientific Recognition. The unique and gigantic Bushveld Complex of South Africa might be the first to be considered for such designation. The Kiglapait Intrusion could perhaps be the next. This PI would welcome such a designation if were to be thoughtfully designed for the pursuit of scientific and human history and environmental care.

Agency
National Science Foundation (NSF)
Institute
Division of Earth Sciences (EAR)
Type
Standard Grant (Standard)
Application #
0948095
Program Officer
Sonia Esperanca
Project Start
Project End
Budget Start
2010-05-01
Budget End
2014-04-30
Support Year
Fiscal Year
2009
Total Cost
$88,127
Indirect Cost
Name
University of Massachusetts Amherst
Department
Type
DUNS #
City
Amherst
State
MA
Country
United States
Zip Code
01003