Introduction
Diagenetic oxide minerals are formed as the result of reactions that are associated with fluid flow long after the associated host rock has formed. These diagenetic oxides preserve information pertaining to past fluid flow, which has the potential for a number of applications related to paleo-groundwater flow such as recording tectonic, geomorphic, and climatic changes over time. Radioisotopic dates and chemical compositions that are preserved in these oxides could provide valuable new constraints on the previously mentioned geologic processes. The ability to date and effectively interpret this data could provide insights into the formation or thermal history of diagenetic oxides. Despite the amount of information that these diagenetic oxides could provide, our knowledge of these oxides is very poor due to the difficulty associated with dating diagenetic oxides.
One of the main goals of this study is to explore the use of diagenetic oxides to help constrain fault movement. We use (U-Th)/He thermochronology to date diagenetic Mn- and Fe-oxides in an effort to learn about the composition of the groundwater that passed through faults and the timing of fluid flow found in this area. Depending on the location, Mn- and Fe-oxides are either found precipitated together, separate, or a combination of the two. Although, Mn and Fe are commonly found together we know a lot more about Fe-oxides than we do of Mn-oxides in thermochronologic studies. By examining both oxides and the pattern in which we find them within sandstones, we can learn a lot about the composition of the groundwater that carried these dissolved elements and how the groundwater evolved through time. We also use SEM/EDS and chemical analyses to learn more about mineral compositions and the relationship between He dates and element (minor/trace) compositions.
Research Questions
1) What do the ages represent? Preliminary data suggests that in the case of crystalline material it is possible the ages represent formation ages or recrystallization ages due to a few reproducible He ages. Unfortunately, there is too much scatter between some samples which raises the issue of He loss, addition of U-Th, recrystallization in which the He system was partially or fully reset, and/or later growth of other oxide minerals. We are currently waiting on some Ar ages from K-bearing Mn-oxides that could help support or disprove the possibility of formation ages being represented by the He dates.
2) If ages do not represent formation ages then what do they represent? Ages could represent thermal ages. Apatite samples are currently being separated from pure sandstone samples from this location to help answer this question.
3) What is the precipitation sequence in this study area? Sample 14VGCP10 serves as a perfect example of the precipitation sequence we propose in this study: 1) Fe precipitated first 2) mixture of Fe, Si, Ba, and Mn 3) Ba rich Mn-oxide 4) Ba Mn-oxide 5) Pure Mn-oxide. This same sequence can be observed in the cement of this sample. Sample 14VGCP10 is a great indication of what we believe will be seen in other samples found in this location. It is also possible that this sequence will be observed at a bigger scale at the South Fault where coliform structures are observed. This sequence indicates that the fluid carrying the dissolved minerals had more Fe than Mn which is why Fe precipitated first as oxygenated fluid came in contact with the reduced fluid. This idea leads us to believe that precipitation is determined by oxidation state and pH of the fluids coming in contact with each other. If this is the case we would expect to find Fe-oxides overlain by Mn-oxides as we do find in some veins in the study area.
Diagenetic oxide minerals are formed as the result of reactions that are associated with fluid flow long after the associated host rock has formed. These diagenetic oxides preserve information pertaining to past fluid flow, which has the potential for a number of applications related to paleo-groundwater flow such as recording tectonic, geomorphic, and climatic changes over time. Radioisotopic dates and chemical compositions that are preserved in these oxides could provide valuable new constraints on the previously mentioned geologic processes. The ability to date and effectively interpret this data could provide insights into the formation or thermal history of diagenetic oxides. Despite the amount of information that these diagenetic oxides could provide, our knowledge of these oxides is very poor due to the difficulty associated with dating diagenetic oxides.
One of the main goals of this study is to explore the use of diagenetic oxides to help constrain fault movement. We use (U-Th)/He thermochronology to date diagenetic Mn- and Fe-oxides in an effort to learn about the composition of the groundwater that passed through faults and the timing of fluid flow found in this area. Depending on the location, Mn- and Fe-oxides are either found precipitated together, separate, or a combination of the two. Although, Mn and Fe are commonly found together we know a lot more about Fe-oxides than we do of Mn-oxides in thermochronologic studies. By examining both oxides and the pattern in which we find them within sandstones, we can learn a lot about the composition of the groundwater that carried these dissolved elements and how the groundwater evolved through time. We also use SEM/EDS and chemical analyses to learn more about mineral compositions and the relationship between He dates and element (minor/trace) compositions.
Research Questions
1) What do the ages represent? Preliminary data suggests that in the case of crystalline material it is possible the ages represent formation ages or recrystallization ages due to a few reproducible He ages. Unfortunately, there is too much scatter between some samples which raises the issue of He loss, addition of U-Th, recrystallization in which the He system was partially or fully reset, and/or later growth of other oxide minerals. We are currently waiting on some Ar ages from K-bearing Mn-oxides that could help support or disprove the possibility of formation ages being represented by the He dates.
2) If ages do not represent formation ages then what do they represent? Ages could represent thermal ages. Apatite samples are currently being separated from pure sandstone samples from this location to help answer this question.
3) What is the precipitation sequence in this study area? Sample 14VGCP10 serves as a perfect example of the precipitation sequence we propose in this study: 1) Fe precipitated first 2) mixture of Fe, Si, Ba, and Mn 3) Ba rich Mn-oxide 4) Ba Mn-oxide 5) Pure Mn-oxide. This same sequence can be observed in the cement of this sample. Sample 14VGCP10 is a great indication of what we believe will be seen in other samples found in this location. It is also possible that this sequence will be observed at a bigger scale at the South Fault where coliform structures are observed. This sequence indicates that the fluid carrying the dissolved minerals had more Fe than Mn which is why Fe precipitated first as oxygenated fluid came in contact with the reduced fluid. This idea leads us to believe that precipitation is determined by oxidation state and pH of the fluids coming in contact with each other. If this is the case we would expect to find Fe-oxides overlain by Mn-oxides as we do find in some veins in the study area.
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4) What do the different cement textures between samples tell us about these types of deposits and how they form? For example, sample 14VGCP04 shows very odd cement textures that are possibly due to preferential fluid flows between the sand grains. One idea is that once the Fe precipitated, a reduced fluid carrying Mn in solution dissolved the original cement causing those bleached areas and once it came in contact with oxygenated water the Mn precipitated. This sample shows dendritic Mn-cement in the core, a bleached area with no cement, and finally an Fe cement rim.
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Probe section image of sample 14VGCP04 showing dendritic Mn-oxide cement with zones of bleached grains along rims that are surrounded by Fe-oxide cement.
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Previous Studies/Mn Deposits
Intro to Fe and Mn deposits: Click link to learn more about Fe and Mn.
Mn deposits worldwide: Click link to learn about Mn deposits across the world.
Microbial Mn deposits: Click link to learn about microbes and their influence on Mn deposits.
Mn deposits in Argentina: Click link to learn about a Mn deposit with a similar precipitation sequence to the one in this study.
*Don't forget to check out Moki Marbles Matinee to learn more about diagenetic oxides! Follow the link or check it out in my home page*
Intro to Fe and Mn deposits: Click link to learn more about Fe and Mn.
Mn deposits worldwide: Click link to learn about Mn deposits across the world.
Microbial Mn deposits: Click link to learn about microbes and their influence on Mn deposits.
Mn deposits in Argentina: Click link to learn about a Mn deposit with a similar precipitation sequence to the one in this study.
*Don't forget to check out Moki Marbles Matinee to learn more about diagenetic oxides! Follow the link or check it out in my home page*
Study Area
Figure 1. Map of study area showing sample sites where diagenetic oxides were collected from for analyses. The faults are referred to as the North Fault, South Fault, and East Fault in this study.
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Flat Iron MesaThe study area (Figure 1.) is located 30 km south of Moab, Utah where diagenetic Mn and Fe oxides are located northwest of the Lisbon Vally Fault. This area is underlain by the Paradox Basin, which was an intracratonic basin filled with carbonates, clastics, and evaporites during the late Paleozoic. The Navajo Sandstone, which makes up almost the entirety of the study area, is a well-sorted, fine-grained quartz dominated Cretaceous sandstone that was the result of a huge eolian sand dune deposit.
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North Fault
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Sample: 14vgcp10The North Fault offers great views of oxide material due to diggins made by miners prospecting for manganese ore in the early 1940's. Two samples were collected from here: one being sample 14VGCP10 and the other 13PRCP8. Both of these samples show great fracture fill pure oxide crystals as well as oxide cement.
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South Fault
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SAmples 14VGCP01-09The South Fault consists of two small faults from which reduced waters rich in Mn and Fe flowed from left to right coming in contact with oxygenated water as it crossed the faults causing Mn and Fe to precipitate out of solution. Growth stages can be observed in the colliform structures on the lower right of the first image. White tape shows locations where samples were collected from (numbered 01-09 from left to right).
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East Fault
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samples 14vgcp11-13bThe East Fault sample site is located roughly 800 feet west of U.S. Route 191 and cuts a tributary of the Muleshoe Wash. Most of the samples collected came from outcrops and consists mostly of oxide cement. Botryoidal Fe-oxide samples were found mostly as float material but one sample was collected from an outcrop.
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