Cl/Br of scapolite as a fluid tracer in the earth's crust: insights into fluid sources in the Mary Kathleen Fold Belt, Mt. Isa Inlier, Australia

Abstract

A combination of analytical methods, including trace element analysis of Br in scapolite by LA-ICP-MS, was employed to unravel the fluid-rock interaction history of the Mary Kathleen Fold Belt of northern Australia. Halogen ratios in the metamorphic and hydrothermally derived scapolite from a range of rock-types record interaction between the host rocks and magmatic-hydrothermal fluids derived from granite plutons and regional metamorphism. The results show that halite-dissolution supplied at best only minor chlorine to fluids in the Fold Belt. Chlorine/bromine ratios in metamorphic scapolite indicate that fluids were dominantly derived from basinal brines formed from sub-aerial evaporation of seawater beyond the point of halite saturation. This bittern fluid infiltrated the underlying sedimentary sequences prior to regional metamorphism. Zoned scapolite in a major late metamorphic mineralized shear-zone records three discrete pulses of magmatic and metamorphic fluid, and it is suggested that fluid mixing may have assisted mineralization along and around this shearzone. As a crucial prerequisite for halogen fluid tracer studies using scapolite, we find in our samples that Cl and Br do not fractionate when incorporated in scapolite. Furthermore, unaltered rims of heavily retrogressed scapolite show indistinguishable Cl/Br signatures compared with fresh grains from the same sample indicating retrograde metamorphism did not significantly affect Cl and Br signatures in scapolite group minerals.

Sulfur isotope signatures in the lower crust: A SIMS study on S-rich scapolite of granulites

“…This strongly suggests preferential HREE partitioning into garnet at the time of scapolite formation, resulting in a more prominent HREE depletion in the coexisting scapolite (cf. Hammerli et al, 2014). Based on this, together with textural relationships and polygonal grain boundaries, we infer that scapolite in the studied granulites formed at peak metamorphic conditions, when the major mineral phases crystallized together.…”

Basinal Brines at the Origin of the Imiter Ag-Hg Deposit (Anti-Atlas, Morocco): Evidence from LA-ICP-MS Data on Fluid Inclusions, Halogen Signatures, and Stable Isotopes (H, C, O)

“…The geochemical features of the Imiter Ag brines are similar to those of deep basinal brines (Kendrick et al, 2002;Hammerli et al, 2013). The Cl/Br molar ratios (330 for Ag quartz, 384 for dolomite) are interpreted as typical of residual bittern brines after halite extraction (Holser, 1979;McCaffrey et al, 1987;Hammerli et al, 2014) that was diluted later. The high Na/Ca molar ratios (QIII and QIV brines) as well as the high K and Ba-Sr concentrations further support the presence of bittern brines, which could have reached sylvite and epsomite saturation (Fontes and Matray, 1993).…”

The F, Cl, Br and I Contents of Reference Glasses BHVO‐2G, BIR‐1G, BCR‐2G, GSD‐1G, GSE‐1G, NIST SRM 610 and NIST SRM 612

“…The halogen content of metamorphic rocks and minerals provides information about the salinity and origin of metamorphic fluids (Markl and Bucher , Pan and Dong , Hammerli et al . , Kusebauch et al . ) and the potential for halogens to be subducted into the mantle (Sharp and Barnes , Kendrick et al .…”