Open AccessStable isotope and chemical systematics of pseudotachylyte and wall rock, Homestake shear zone, Colorado, USA: Meteoric fluid or rock‐buffered conditions during coseismic fusion?
Author(s) -
Moecher David P.,
Sharp Zachary D.
Publication year - 2004
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2004jb003045
Subject(s) - gneiss , geology , biotite , quartz , geochemistry , metamorphic rock , protolith , shear zone , meteoric water , mineralogy , groundwater , paleontology , geotechnical engineering , tectonics
A hydrous fluid phase is critical in controlling effective stress and fault mechanics, and influencing the mineralogy and strength of materials within fault zones. Oxygen and hydrogen isotope and chemical analysis of wall rock gneiss, pseudotachylyte, and selected minerals in gneiss and pseudotachylyte from the Homestake shear zone was used to assess whether melting occurred in the presence of meteoric water or involved only minor amounts of H 2 O derived from micas in wall rock gneiss. Bulk pseudotachylyte has slightly lower δ 18 O SMOW than the whole rock protolith. δD for one bulk pseudotachylyte is essentially identical to biotite in gneiss; δD for two samples is lower by ∼20‰. Bulk pseudotachylyte has lower SiO 2 and K 2 O, and higher Al 2 O 3 , FeO, MgO, CaO, and H 2 O, than gneiss. The lower SiO 2 of pseudotachylyte compared to gneiss is explained by physical segregation of 25 to 72 volume % of quartz clasts from the mobile melt phase. Samples of gneiss and pseudotachylyte define a SiO 2 ‐δ 18 O mixing line between quartz and the most SiO 2 ‐ and 18 O‐depleted pseudotachylyte. Physical segregation of quartz (highest oxygen isotope composition in the pseudotachylyte‐gneiss system) accounts for the slightly lower oxygen isotope composition of bulk pseudotachylyte relative to gneiss. The similar δD of pseudotachylyte and biotite from gneiss in one sample is consistent with dehydration melting of biotite during frictional heating and dissolution of biotite‐derived H 2 O in the melt. Late devitrification of glass and formation of greater amounts of fine‐grained muscovite, accompanied by 10–30% loss of hydrogen as H 2 O, results in lower δD values in other samples. In general, melt generation occurred in a fault zone closed to infiltration of meteoric water. There was no free, H 2 O‐rich pore fluid present at the time of slip to potentially influence the behavior of the fault.