Open AccessAssessment of Factors Controlling Clumped Isotopes and δ 18 O Values of Hydrothermal Vent Calcites
Author(s) -
Kluge Tobias,
John Cédric M.,
Boch Ronny,
Kele Sándor
Publication year - 2018
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2017gc006969
Subject(s) - carbonate , isotopes of oxygen , geothermal gradient , geology , stable isotope ratio , hydrothermal circulation , isotopes of carbon , mineralogy , isotope , oxygen isotope ratio cycle , carbonate minerals , calcite , environmental chemistry , geochemistry , chemistry , total organic carbon , paleontology , physics , organic chemistry , quantum mechanics
The clumped isotope composition of CaCO 3 (Δ 47 ) is a geochemical proxy that can provide mineral formation temperatures and, together with measured carbonate δ 18 O, inferred fluid δ 18 O values. Under natural conditions, carbonates form within a relatively wide pH range and varying growth rates which are typically not reflected in laboratory‐based calibrations (mostly ∼pH 8, moderate growth rates). A pH and growth‐rate dependence is known for oxygen isotopes and was also postulated for clumped isotopes. Theoretical predictions suggest that Δ 47 values could lie between the carbonate equilibrium value and the value inherited from the dissolved inorganic carbon (predicted offset: +0.04‰ pH < 4 and −0.025‰ at high pH > 12). Here we test whether pH (in addition to temperature) is recorded in the carbonate clumped isotope composition using modern calcites from natural travertine‐forming streams and scales precipitated in pipes of deep geothermal wells from Italy, Hungary, and Turkey (pH: 6.1–7.5, T: 33–100°C). Although a comparison of all samples with expected equilibrium values in this pH range and known formation temperatures reveals only an insignificant Δ 47 offset (0.006 ± 0.004‰, 1SE, n = 9), the clumped isotope values of samples with the highest growth rates (0.014 ± 0.007‰, 1SE, n = 5) are consistent with the theoretical prediction attributable to pH of 0.01‰. Similarly, deviations in δ 18 O of up to −2‰ follow a growth‐rate dependence. This field‐based study shows that pH‐related effects are mostly small for Δ 47 in the subsurface environment at lower pH and that high mineral growth rates control the magnitude of this disequilibrium.