Reconstructing Earths Oxygen History from Iron-Rich Rock Samples
Start Date
15-4-2021 2:45 PM
End Date
15-4-2021 3:00 PM
Student's Major
Chemistry and Geology
Student's College
Science, Engineering and Technology
Mentor's Name
Chad Wittkop
Mentor's Department
Chemistry and Geology
Mentor's College
Science, Engineering and Technology
Description
The Freedom Formation is a sedimentary rock unit that was deposited in the middle Proterozoic Eon between 1.7 and 1.5 billion years ago. This rock formation is difficult to study because it is located only in the subsurface of southern Wisconsin, resulting in little being known about the mineral and chemical composition of the unit. The Freedom Formation is unique in that it contains both iron and manganese minerals, and formed during a time when the history of Earth’s atmospheric oxygen levels is not well known. The sensitivity of iron and manganese to changes in oxygen levels allow iron- and manganese-enriched rocks to serve as records for the past history of Earth’s oxygenation. Data from X-ray fluorescence (XRF) analysis indicates the weight percent of various chemical compounds within the core samples. This data indicates that there are Freedom Formation intervals that are rich in both iron and manganese (greater than 20 weight percent). Using X-ray Diffraction (XRD) analysis the crystalline (mineral) phases within these samples are identified. Mineral phases that have been identified include the carbonates rhodochrosite, siderite, and ankerite, which indicates mineral substitution of iron and manganese within the samples. Additionally, XRD data shows the presence of iron oxide minerals including hematite and magnetite. Manganese appears to be concentrated in reduced-phase carbonate minerals, while iron occurs in both reduced and oxidized mineral forms. This assemblage of iron and manganese minerals suggest that the Freedom Formation formed in an environment that contained enough oxygen to oxidize iron, but not enough to fully oxidize manganese, consistent with an environment hosting lower levels of atmospheric oxygen relative to today. These chemical and mineralogical results are being compared with sample textures using microscopy to further constrain the relationships between mineral phases and sample chemistry.
Reconstructing Earths Oxygen History from Iron-Rich Rock Samples
The Freedom Formation is a sedimentary rock unit that was deposited in the middle Proterozoic Eon between 1.7 and 1.5 billion years ago. This rock formation is difficult to study because it is located only in the subsurface of southern Wisconsin, resulting in little being known about the mineral and chemical composition of the unit. The Freedom Formation is unique in that it contains both iron and manganese minerals, and formed during a time when the history of Earth’s atmospheric oxygen levels is not well known. The sensitivity of iron and manganese to changes in oxygen levels allow iron- and manganese-enriched rocks to serve as records for the past history of Earth’s oxygenation. Data from X-ray fluorescence (XRF) analysis indicates the weight percent of various chemical compounds within the core samples. This data indicates that there are Freedom Formation intervals that are rich in both iron and manganese (greater than 20 weight percent). Using X-ray Diffraction (XRD) analysis the crystalline (mineral) phases within these samples are identified. Mineral phases that have been identified include the carbonates rhodochrosite, siderite, and ankerite, which indicates mineral substitution of iron and manganese within the samples. Additionally, XRD data shows the presence of iron oxide minerals including hematite and magnetite. Manganese appears to be concentrated in reduced-phase carbonate minerals, while iron occurs in both reduced and oxidized mineral forms. This assemblage of iron and manganese minerals suggest that the Freedom Formation formed in an environment that contained enough oxygen to oxidize iron, but not enough to fully oxidize manganese, consistent with an environment hosting lower levels of atmospheric oxygen relative to today. These chemical and mineralogical results are being compared with sample textures using microscopy to further constrain the relationships between mineral phases and sample chemistry.
