This project is supported by the Petrology & Geochemistry and the Education & Human Resources programs in the Division of Earth Sciences and by the Africa, Near East and South Asia program of the Office of International Science and Engineering.
The Bushveld Complex of South Africa is a fossil magma chamber that solidified in the shallow crust about 2 billion years ago. The body is well-known to geologists because it holds the world's largest reserves of platinum and related elements and is a major source of chromium and vanadium. The Bushveld is also notable for its size, which has been estimated to be up to 600,000 cubic kilometers, and for the fact that its structure reflects a long and complex solidification history. Despite its enormity, geochemical characteristics of the rocks indicate that some amount of the original magma had escaped from the chamber and is thus missing from the present body. This raises the questions of what was the nature of this additional magma and where could it have gone. These questions need to be answered if geologists are ever to understand how and under what the conditions the Bushveld Complex solidified. Research to date by the team has led them to hypothesize that the missing magma is now represented by a sequence of lavas known as the Rooiberg Group (plus their intrusive equivalents, the Rashoop granophyres), the ages of which are identical to the Bushveld age within uncertainty estimates. The present research is directed at testing this hypothesis. As part of the project, Mathez and colleagues will also organize a week-long workshop for 20-25 South African earth science teachers on the mineral resources of South Africa. The workshop will be taught by Mathez, VanTongeren, and two African-American New York City school teachers, who will thus gain knowledge of both mineral deposits and unique experience in teaching across cultural divides.
More specifically, the research seeks to test the hypothesis that the Rooiberg lavas and associated rocks originated from the Bushveld Upper Zone. This will be accomplished by a detailed study of the trace element geochemistry of the Upper Zone to determine the compositions of the liquids in equilibrium with the cumulate minerals. The derived liquid compositions will then be compared with Rooiberg and Rashoop rock compositions. The top members of the Rooiberg Group are dominated by rhyolite, whereas the Bushveld Upper Zone, which is the most chemically evolved part of the intrusion, is dominated by cumulate rocks consisting of assemblages with most or all of the minerals plagioclase, Ca-poor pyroxene, Ca-rich pyroxene, magnetite, ilmenite, Fe-rich olivine, and apatite. Therefore, the possibility that a rhyolite could be in equilibrium with these phases bears on one of the most fundamental and debated questions in igneous petrology, namely whether mafic magmas differentiate toward Fe-rich or Si-rich compositions. In addition, if the Rooiberg and Rashoop rocks indeed originated from the Bushveld magma chamber, this implies that the Bushveld intruded into a much shallower level of the crust than has been generally appreciated, thus helping to define the geodynamic setting of the intrusion.
Intellectual merit The goals of the research were to understand the origin and evolution of the Bushveld Complex, South Africa, and its relations to overlying rocks. The Bushveld Complex is a gigantic fossil magma body that solidified about 2 billion years ago into a layered sequence of igneous rocks more than 8 kilometers thick. It has great economic importance to both South Africa and the world because it provides 73% of the world’s platinum, 33% of its palladium, 37% of its vanadium, and about 50% of its chromium (as of 2008). The research supported by this grant bears indirectly on why these ore deposits exist and how they formed. More specifically, the research sought to understand (1) the genetic relationships between the so-called Upper Zone of the Bushveld Complex and overlying lavas and associated rocks, (2) the conditions under which the Bushveld magmas intruded and solidified, and (3) how composition of a mafic magma changes as it undergoes extreme fractional crystallization. The latter is an especially important academic question because it concerns a process that likely explains the diversity of igneous rocks on Earth and other planets. Three scholarly works have been created with support of the grant. The first hypothesized that extreme fractional crystallization of the Bushveld magma resulted in the formation of two immiscible silicate liquids at the top of the intrusion. One of these was rhyolitic in composition, and this material partly escaped to form a series of sills we now see exposed immediately above the Bushveld Complex. A second study hypothesized that the Upper Zone formed by gradual and periodic injections of hotter, more primitive magma into the resident chamber. A third found that the Rooiberg Group consists of distinctive magnesian and ferroan lavas and hypothesized that the latter represent differentiation products of the Bushveld magmas. These hypotheses represent significant scientific advances in our understanding of the Bushveld Complex. Broader impacts The primary purpose of the grant was to support the research of Dr. Jill VanTongeren, who, when the grant was awarded, was a graduate student at Columbia University and the American Museum of Natural History. She was awarded the Ph.D. by Columbia in 2011. The grant also supported a week-long, teacher-training workshop in South Africa for South African Earth science/geography teachers. The workshop was organized in response to the desire of Mathez and VanTongeren to give back something to the communities with which they had come in contact in the course of their fieldwork and to address the significant need for trained k-12 teachers in South Africa. The workshop was made possible through the collaboration of the South African Agency for Science and Technology Advancement (SAASTA), and with the help of Dr. Maritza MacDonald, a senior director in AMNH’s Department of Education. The earth science content part of the workshop was taught by Mathez and VanTongeren and three experienced teachers who had been recruited to teach pedagogical methods. SAASTA’s role in the collaboration was to recruit local teachers; fund their travel, lodging, and subsistence; supply curriculum guidance; and provide the local venue. Nineteen secondary and high school teachers and curriculum advisors attended the workshop. Each day included content and pedagogical follow-up lessons. The workshop was entitled Geography Institute on the Mineral Resources of South Africa, reflecting our desire to teach broad lessons of Earth science (e.g., antiquity of Earth, rocks and minerals and the rock cycle, plate tectonics) through the lens of the mineral resources of South Africa. The point was to make the abstract elements of the science as relevant as possible to the lives of the teachers (and their students). The teachers responded to this approach, displaying both curiosity about South Africa’s natural resources and pride in their country’s global prominence as a supplier of essential metals. The content was kept at an elementary level. Although a challenge to teach, it turned out to be the right approach because, while backgrounds varied, a number of the teachers had little understanding of the science. One day was devoted to a fieldtrip to the Premier (now Cullinan) diamond mine. Although most of the teachers had taken their students on trips, the Cullinan mine provided far more real content than they had experienced elsewhere. The experience and the pedagogical discussions that ensued seemed to energize teachers, and most expressed a desire to incorporate more fieldtrips in their teaching. Certificates of Participation, signed by the Director of SAASTA and Mathez, were awarded to the participants.
