FLUORINE AND CHLORINE BEHAVIOR IN ALKALINE ROCKS OF THE BIG BEND REGION, TEXAS

Fluorine and chlorine contents of apatites have been studied in layered igneous intrusions, rhyolites and granites, and meteorites, but little work has been done on halogen behavior in apatites in alkaline igneous rocks, despite their generally high halogen contents. I have collected and investigated samples of the analcime-bearing syenodiorite of the Rattlesnake Mountain sill and the analcime-bearing alkali gabbro of the Bone Springs sill from Big Bend National Park, Texas. Rattlesnake Mountain sill has an age of 28.6 ± 0.4 Ma, which puts it into a regional context of early extension — i.e., post-compression, pre-Basin and Range faulting. Bone Springs sill has been grouped into this same period based on field relations, but a date is pending.

All natural rock samples contain fluoroapatite (X_F 0.4-0.8; X_Cl 0.02-0.03), and in Bone Springs sill apatite always coexists with Ti-rich biotite (6.5 to >9.5 wt.% TiO₂) with 0.1-0.4 wt.% F. In Rattlesnake Mountain sill, fluoroapatite is sometimes accompanied by halogen-bearing amphibole instead.

I have run piston-cylinder melting experiments on a Rattlesnake Mountain sill sample at conditions from 8-20 kb, 925-1160º C, and with 0, 2, 4, and 8 wt.% added H₂O. Halogen-free secondary (or reequilibrated) phases include some or all of the following: Fe-Ti oxides, clinopyroxene, K-feldspar, plagioclase, and garnet. All run products contain some melt (glass) and almost all contain apatite and another halogen-bearing phase, either biotite or amphibole. Initial analyses of experimental runs indicate that F/Cl in secondary and reequilibrated apatite correlates positively with wt.% H₂O added to the charge. Fluorine is only rarely present above detection limits in glass, but wt.% Cl in glass correlates positively with H₂O wt.% in the glass.

These and future data, in conjunction with thermodynamic calculations, allow for the investigation of variation within and between alkaline igneous rocks in the Trans-Pecos magmatic province, including F and Cl fugacity in the magmas and partitioning behavior of F and Cl between apatite, biotite, glass, and amphiblole.