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How precisely can the temperature of a fluid event be constrained using fluid inclusions?

Andras Fall and Robert J. Bodnar
How precisely can the temperature of a fluid event be constrained using fluid inclusions?
Economic Geology and the Bulletin of the Society of Economic Geologists (December 2018) 113 (8): 1817-1843

Abstract

Fluid inclusions in clearly defined fluid inclusion assemblages (FIAs) from various geologic environments were examined to assess the uncertainty associated with determining the temperature of a fluid event based on fluid inclusion homogenization temperatures (Th). A fluid event is defined as a physical or chemical process such as the healing of a microfracture or the formation of a growth zone in a crystal that occurs in the presence of a fluid phase and results in trapping of fluid inclusions to form an FIA. Examination of data from a large number of fluid events collected within a rigorous temporal and spatial (paragenetic) framework forms the basis for developing a complete fluid pressure-temperature-composition (PTX) history. The range in homogenization temperatures of fluid inclusions within well-constrained FIAs was determined, and the minimum (smallest) range in Th, the median range in Th, and the first quartile (Q1 at 25%) and third quartile (Q3 at 75%) of the median Th ranges were calculated for different fluid environments, including the following: 1. Low-permeability sedimentary environments: 49 out of 144 FIAs show a range in T (sub h) of < or =1 degrees C; the median range=2 degrees C (from Q1=1 degrees C to Q3=3.7 degrees C). 2. Mississippi Valley-type deposits: 11 out of 137 FIAs show a range in T (sub h) of < or =1 degrees C; the median range=4.1 degrees C (from Q1=2.3 degrees C to Q3=8.3 degrees C). 3. Epithermal deposits: 102 out of 923 FIAs show a range in T (sub h) of < or =1 degrees C; the median range=9 degrees C (from Q1=3.8 degrees C to Q3=19 degrees C). 4. Porphyry-type deposits: 24 out of 271 FIAs show a range in T (sub h) of < or =1 degrees C; the median range=15 degrees C (from Q1=8 degrees C to Q3=30 degrees C). 5. Orogenic Au deposits: 21 out of 231 FIAs show a range in T (sub h) of < or =1 degrees C; the median range=8.7 degrees C (from Q1=4 degrees C to Q3=20 degrees C). While all environments show some FIAs in which all the fluid inclusions homogenize at essentially the same temperature (range=< or =1 degrees C), we propose that the median range in T (sub h) reported here represents a reasonable and achievable constraint on the uncertainty associated with the temperature of a fluid event in the environments examined. In summary, the temperature of a fluid event, as represented by the range in T (sub h) of the fluid inclusions within an individual FIA, can be constrained to better than 15 degrees C in all environments examined, and in Mississippi Valley-type and low-permeability (deep) sedimentary basin environments, the range in T (sub h) can be constrained to better than 2 degrees C. The processes that produce variability in Th of fluid inclusions within an FIA are many and include natural variations in temperature, pressure, or fluid composition during trapping of the FIA, trapping of immiscible fluids, various forms of reequilibration in nature such as necking, stretching, and leakage, and modification of the inclusions during sample preparation and data collection. If the range in homogenization temperature for an individual FIA is found to be greater than the median range determined here for that environment, then assessment of the cause of the variability might provide useful information concerning the trapping and posttrapping history of the sample.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 113
Serial Issue: 8
Title: How precisely can the temperature of a fluid event be constrained using fluid inclusions?
Affiliation: Bureau of Economic Geology, Austin, TX, United States
Pages: 1817-1843
Published: 20181201
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 135
Accession Number: 2019-014721
Categories: Economic geology, geology of ore depositsIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
N32°30'00" - N42°00'00", W124°30'00" - W114°15'00"
N37°00'00" - N42°30'00", W91°30'00" - W87°30'00"
N43°30'00" - N47°00'00", W66°15'00" - W59°45'00"
N38°30'00" - N40°25'00", W109°00'00" - W106°45'00"
N31°15'00" - N37°00'00", W115°00'00" - W109°00'00"
Secondary Affiliation: Virginia Polytechnic Institute and State University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201910
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