Fluid inclusion studies on M t. Capanne pegmatites, Isola d’Elba, Tuscany, Italy

Abstract

Swarms of aplitic-pegmatic dykes are associated with the peripheral parts of the Tertiary monzogranitic stock of Mt. Capanne (western Isola d'Elba, Tuscany, Italy). Fluid inclusions in quartz, beryl, and tourmaline crystals from miarolitic cavities and in quartz crystals from massive pegmaties, aplites and monzogranites at San Piero and Seccheto were examined. By textural and microthermometric criteria, four main types of fluid inclusions were recognized: I) The earliest recorded fluid is represented by liquid-rich, mostly biphasic inclusions, in places with one or more solid phases, including a few crystal-rich inclusions. Homogenization temperatures (L+V -L) are between 323 o and 388 o C, and salinities (from Tm-ice) range from 0.7 to 7.7 wt.% NaCl equiv. Crushing tests point to the presence of a gas phase, whose average molar composition (micro-Raman analysis) is 89% CO 2 , 4% CH 4 , 6% N 2 and 1% H 2 S. II) Some early' inclusions in one quartz sample have similar characteristics to the above, but in addition they show the appearance at low temperatures of a separate non-aqueous fluid. The overall composition of this non-aqueous fluid averages 83 mol% CH 2 , 10 mol% CH 4 , 6 mol% N 2 , and 1 mol% H 2 S. III) Inclusions of ambiguous textural relationships in quartz are recognized from the above types by distinctly lower Th (247 o to 320 o C). IV) Low-salinity, low-Th (189 o to 223 o C) inclusions occur in all minerals as clearly secondary trails, except in the tip of a pink tourmaline, where they appear to be primary. Inclusion populations I and II are interpreted to represent essentially the same fluid, which is suggested to be dominantly magmatic. By assuming a pressure of 1.5 kbar (compatible with the geological setting and with the stability of the assemblage petalite + quartz), the calculated trapping temperatures are ≥500 o C. The crystal-rich inclusions might suggest the presence of a coexisting aluminosilicate hydrous melt. Populations III and IV record later stages of fluid evolution toward lower salinities and temperatures (about 400 o and eventually about 300 o C assuming a pressure of 1.5 kbar), and possibly indicate an increasing contribution of meteoric fluids