OXYBIOTITES AS A SENSOR OF ERUPTIVE CONDITIONS

Oxybiotites in igneous rocks have considerable potential for constraining both pre and post eruptive volcanic processes.  We are investigating the hydrogen isotope and oxidation behavior of biotites experimentally and in samples from the Jemez volcanic field.  This will allow us to gain insight into the formation of oxybiotites in a large silicic volcanic sequence.  Thermal dehydration experiments at temperatures between 500-800°C have been performed under oxidizing conditions at 1 atm which produce oxybiotites by the process of dehydrogenation.  dD values for experimental samples and Jemez samples have been measured.

The oxybiotite products of two different processes, near-surface oxidation and in-situ dehydrogenation, are chemically indistinguishable with the exception of dD values.

Dehydration/Oxidation:  2 KFe₃²⁺AlSi₃O₁₀(OH)₂ +  ½ O₂ = 2 KFe₂²⁺ Fe³⁺AlSi₃O₁₀O(OH)  + H₂O;

Dehydrogenation: 2 KFe₃²⁺AlSi₃O₁₀(OH)₂ = 2 KFe₂²⁺ Fe³⁺AlSi₃O₁₀O(OH) + H₂;

Hydrogen isotope variations in oxybiotite allow the two processes to be distinguished because DD (biotite-H₂) values are large and positive (high biotite dD) whereas DD (biotite-H₂O) values are negative (low biotite dD).  dD values of the Jemez samples range from -117‰ to +12‰, suggesting that both dehydrogenation and dehydration/oxidation processes were active.  However, our experimental results show that even under highly oxidizing conditions, dehydrogenation occurs.  The biotites lose light hydrogen, decreasing their water content and increasing their dD value to as high as ~108‰.  In-situ conditions which might produce the low dD values found in the Jemez samples have not been replicated in the laboratory.  Therefore, the low dD values are probably related to surficial hydrothermal processes. 

Experimental biotite dehydrogenation data follow a Rayleigh fractionation curve with a constant a(biotite-H₂) of 0.874 ± 0.002, irrespective of temperature, a maximum dD of ~108‰, and an inverse correlation between biotite water content and dD values.  EELS will be used to measure biotite Fe²⁺/Fe³⁺ ratios on both experimental and Jemez samples and examine biotite microstructures.  This will allow us to determine if there are differences in mechanism between the two oxybiotite forming reactions.