Abstract
The Heinitang hot spring is located in the western Yunnan of China and
lies in the Tibet-Yunnan geothermal zone and the Tengchong Block. The
hot water is of HCO3-Ca•Na type and the F concentration
is relatively high (ranging from 3.8 to 5 mg/L). Fossil travertine
mounds and cones and new travertine are found at the spring vents.
Hydrochemical analyses indicate that the primary factor affecting the
deposition of travertine is the concentration of ion pair
CaCO30 in the Heinitang hot springs.
The Heinitang hot spring shows a positive correlation between the
concentration of CaCO30 and the
saturation index with respect to aragonite, calcite and dolomite. They
are not linearly related, but have an exponential relationship. When the
aragonite, calcite and dolomite are in the state of dissolved
equilibrium, the corresponding concentrations of
CaCO30 are 0.01 mmol/L, 0.008 mmol/L
and 0.012 mmol/L, respectively, indicating that the precipitation of
travertine can occur when the concentration of
CaCO30 is higher than 0.008 mmol/L. In
addition, the concentrations of CaCO30in the Heinitang hot springs decrease gradually with the deposition of
travertine. The precipitation of travertine cannot occur when the
concentration of CaCO30 decreases to
lower than 0.008 mmol/L.
Keywords: hot springs; travertine; hydrochemistry;
CaCO30; Yunnan;
Introduction
The terms “travertine” and “tufa”, in its broadest sense, refer to
non-marine carbonate precipitates near springs, rivers, lakes and caves,
which consist of calcite or aragonite (Viles and Goudie, 1990;
Pentecost, 2005). Tufa is the precipitation of calcium carbonate under
cool water (near ambient temperature) and travertines are calcareous
deposits that precipitate where thermal groundwaters emerge at springs
(Ford and Pedley, 1996; Li and Robert, 1998; Fedoseev et al., 2017).
Tufa or travertine can provide valuable information to study the
paleo-climate and paleo-environment (Chafetz et al., 1991; Fouke et al.,
2000) and have implications for hydrogeological evolution history and
present or past fluid dynamic regimes (Andreo et al., 1999; Naroa et
al., 2018). Travertine may reflect changes in temperature and rainfall
by changing isotopic compositions, minerals, the rate of growth and
texture. In addition, travertine have various applications in
construction, agriculture, decorative arts, etc. (Erdoğan, 2011).
Travertine deposits form by important precipitation of carbonates and
are distributed over the world (Pentecost, 1995a). However, the
precipitation of travertine only occurs near a small portion of hot
springs (Zhou et al., 2017). Travertine deposits are formed when thermal
groundwater, rich in Ca, HCO3 and CO2,
flows to the surface and outgases CO2 to the atmosphere.
Re-equilibration of CO2 in groundwater can result in
saturation of CaCO3 and travertine deposits downstream
from the hot spring (Pentecost, 1999; Acikel and Ekmekci, 2016).
Therefore, it is significant to study the hydrochemical characteristics
of hot springs to better understand the factors affecting the
precipitation of travertines. Interest in the travertine-depositing hot
springs has increased and the studies of hydrochemical characteristics
of hot springs have attracted attention since the 19th century. In the
travertine-depositing hot springs, the concentrations of major and trace
elements were determined by numerous researchers. The characteristics of
hydrochemical compositions in travertine-depositing hot springs in Italy
are high concentrations of CO2, Ca, Mg and
SO4 (Pentecost, 1995b). The Pamukkale hot springs in
Turkey contain high concentrations of HCO3, Ca and
SO4 in the ranges from 1100 to 1300 mg/L, from 400 to
500 mg/L and from 600 to 750 mg/L, respectively (Yesertener and Elhatip,
1997). Kawai et al. (2009) proposed
that the precipitation rate of CaCO3 is mainly
controlled by Ca and alkalinity, which is relatively low when Ca
concentrations are less than 65 mg/L. However, the precipitations of
travertine from the Jifei hot spring in western Yunnan of China took
place on a cliff face when the concentration of Ca is 58.1 mg/L (Liu et
al., 2012; Jones and Peng, 2016). The high concentrations of Ca and Mg
of hot springs can arise apparently saturation with respect to calcite
and aragonite when the thermal groundwater rises to the surface with the
CO2-outgassing. In addition, other hydrochemical factors
proposed as influential in the precipitation of travertine from hot
springs include the saturation index with respect to carbonate
(Dandurand et al., 1982), hydrochemical type (Dilsiz et al., 2004),
temperature and pH (Veysey et al., 2008) and the relationship between
major elements (Kele et al., 2008). The hydrochemical type and
saturation index with respect to calcite are common indications in the
analyses of the travertine-depositing hot springs. The Pamukkale
travertine-depositing hot springs exhibited a
HCO3•SO4-Ca type and positive values of
saturation index calculated with Phreeqc-2 (Dilsiz, 2006). Fouke (2011)
established a quantitative correlation among the temperature, pH and
flux and classically-defined travertine depositional facies at the
Mammoth hot springs in the Yellowstone National Park. Kele et al. (2011)
suggested that the Jandarma hot spring in Turkey show a positive
correlation between the equivalent concentrations of Ca and
HCO3 and the values of\(\mathrm{\gamma}\)Ca/\(\mathrm{\gamma}\)HCO3 range from
1.5 to 1.8. Wang et al. (2015) noted that the concentrations of Ca and
HCO3 decrease gradually along the path of travertine
deposition at the Lianchangping hot springs in western Yunnan of China
and the values of\(\mathrm{\gamma}\)Ca/\(\mathrm{\gamma}\)HCO3 are almost
constant (nearly 1). Frery et al. (2017) noted that the precipitation of
travertine takes place near the outlet of the hot springs that represent
the chemical characteristics of high PCO2 values and
saturation index (SI) values with respect to carbonate minerals.
However, there is limited data on the influence of
CaCO30 ions pairs on travertine
deposition from hot springs.
It seems that further research is needed to interpret the hydrochemical
factors affecting the deposition of travertine. So far, substantial
papers to clarify this matter have been published. In the present paper,
we examine 8 water samples collected at the Heinitang hot springs in
western Yunnan of China in 2013 and 2018 during field investigations
where travertines are depositing from the hot springs. This paper places
its emphasis on a new hydrochemical factor affecting the deposition of
travertine from the Heinitang hot springs by analyzing the
concentrations of CaCO30 and establish
the relationship between the concentrations of
CaCO30 and the saturation index with
respect to aragonite, calcite and dolomite that other researchers have
not done.