mineral systems. Other geothermal areas in the north, and many hot springs in Central Chile, ... In Central Chile the evaluation of some hot-spring waters in.
HEAT FLOW AND TEMPERATURE GRADIENTS IN CHILE# MIGUELMu~oz I ANDVALIYA HAMZA2
S u m m a r y : Conventional heat-flow measurements in Chile carried out by other workers are summarized, Between latitudes 26 - 29 ° S heat flow is consistently low (< 42 m Wm -2) excepting a site in the Andes slope (75:3 mWm-2). In Central Chile (33 °S) near Santiago, a value in the Andes (60.7 m W m -2) is lower than the value in the Santiago basin (78.7 mWm-2). Heat flow through the sea bottom around the Chile Ridge (about 44 - 48 ° S; 75 - 80 ° W) ranges between 25 and 414 mWm-2; heat-flow estimates based upon the location in depth of the phase of gas hydrates have also been carried out in this area. In Tierra de/Fuego the only heat-flow value is 96.3 m Wm -2. The present heat-now studies in Chile do not allow any conclusions to be drawn on the genered heat-flow distribution and its description within the frame of new global tectonics. Only some preliminary modal results comparing heat-flow measurements in the area of the Chile Ridge to thermal effects produced by a ridge-trench collision may presently be partially adopted. A general discussion regarding the results from global seismic tomography, maximum depth of seismic coupling and thermal processes in Chile is also presented. The silica geotemperature in the Santiago basin resulting from 257 groundwater analyses is 77.4 +_10.4 °C; the equivalent heat flow is 92.5 ± 16.6 m W m -2 which is in agreement with the conventional heat-flow value in this area. Geochemical thermometry indicates fluid temperature at depth higher than 200 °C in some of the 33 hot-spring areas evaluated using SiO2 , Na-K-Ca and 1Va-Li geothermometers. The evalutation of fluid rock equilibrium and CO2 - fugacities by means of relative Na, K, Mg and Ca contents of thermal waters indicates that only in El 7htio and Puchuldiza in Northern Chile have fluids attained partial equilibrium with both K-Na and K-Mg mineral systems. Other geothermal areas in the north, and many hot springs in Central Chile, correspond to/mmature waters which are generally unsuitable for the evaluation of K/Na and K/Mg equilibrium temperatures. In Central Chile the evaluation of some hot-spring waters in partial equilihrium condition indicate deep temperatures between 80 °C and 245 °C. In the area of El Tatio the combined heat flow (conductive and convective) yields a value of 1465 m Wm -2 with fluid circulating within I km of an underlying magmatic intrusion at 5 - 7 k m depth. The water catchment area may be situated 20 krn to the east of the geothermal area, with the underground fluid moving at a rate of about 1.3 k m y -1 Temperature logs in wells for oil prospection show that temperatures are affected by drilling disturbances. Some preliminary BHT estimates of gradients yield between 26.3°C km -1 and 72.4 °C km-1. Thermal conductivity and diffusivity data from these wells are also shown.
# 1 2
Presented at the International Meeting on Terrestrial Heat Flow and the Structure of Lithosphere, Bechyn~ Castle, Czech Republic, September 2 - 7, 1991. Address: Departamento de Geoffsica, Universidad de Chile, Casilla 2777, Santiago, Chile Address: Institute Astron6mico e Geoffsico, Universidade de S~o Paulo, Caixa Postal 9638, 01050 S~o Paulo, SP, Brasil
Studiagooph, etgeod.37 (1993)
3 15
M, Mtr~oz and V. Hamza
1. INTRODUCTION In this paper several results involving fundamental and applied problems of heat flow and thermal processes in Chile are presented. Many of these results were presented at the International Meeting on Geothermics and Geothermal Energy held in Guaruj~ (Sao Paulo) in 1986 [1]. A more detailed account concerning heat-flow studies, carried out by other workers, is given here. The results concerning geochemical thermometry are discussed in more detail in the present paper, including new outputs from the evaluation of fluid-rock equilibrium and CO L . fugacities using Na-K-Mg-Ca geoindicators. Also, some new geothermal data from wells for oil prospection are shown; particularly, thermal conductivity data of deep boreholes in continental Chile were obtained for the first time after the work by Uyeda et al. [2]. Some of the ongoing work concerning radiogenic heat production in Central Chile will not be presented here; a summary of preliminary results can be found in [3]. Also the effects of climatic changes on geothermal data are not discussed - a general approach to this problem in the southern hemisphere is summarized in [4]. 10o
60I °
|
-20 o x ~7;3 33,5 SAINTACLARAIO.O
EL SALVaO,OR~ 3
EUSA311~
BOOUERONG'HAf ~ ? --30O
• #JVALLENAR • 4L9)21.8 (L:'9.3o~LA AFRICANA78.7 • ~ DiSPUTAOAeO.7
-4O °
0
~OIm
-SO o
ISLATIERRAOELFUEGO9G.3
Fig. 1. Heat flow in mWm -2. Values in the Pacific Ocean are from von Herzen [5], values in brackets from Diment et al. [6]. Other values are from Uyeda et ai. [2]. Heat-flow values in the Chile Ridge are presented in Figs. 2 and 3 and in Table 1.
316
Studia geoph, et geod. 37 (1993)
Heat Flow and Temperature Gradients in Chile
The perspective of heat-flow studies in Chile is one which points to the integration of fundamental and applied (energy) problems. This has been tried from the very beginning of heatflow research conducted during the last decade. In this way this perspective can be set in agreement with the statement by Keflis-Borok [7]: "The integral approach to other studies of the lithosphere, besides it dynamics on short time scales, deserves attention. It is a major and, I believe, the major workhorse in the present perestroika of the solid-Earth sciences: their integration and globalization, integration of basic and applied problems, change of theoretical base, etc." As will be shown in this paper no conclusions about the general heat-flow distribution in Chile can be drawn from the present state of studies. Only some remarks concerning small areas and localized thermal processes can be put forward. 2. HEAT-FLOW MEASUREMENTS BY CONVENTIONAL METHOD The first heat-flow density determinations in continental Chile were communicated by Diment et al. [6]; these were the only heat-flow values known in South America till 1969. Also, two values in the Pacific Ocean near the coast of northern Chile were obtained previously by yon Herzen [5]. Other values of temperature gradients and conventional heat-flow density were published later by Uyeda and Watanabe [8], Uyeda et al. [2], Herron et al. [9] and Cande et al. [10]. Two heat-flow values in the Pacific Ocean (Fig. 1 - 37.3 and 33-5 mWrn-2) near the coast of northern Chile are from [5]. The lowest heat flows from the south-eastern Pacific may be due to the small amount of heat generated by radioactivity in the crust, with little or no heat coming from the mantle; no correction has been applied to the heatflow measurements from these sites, and irregular sea-bottom topography may have no effect in these cases [5]. Between latitudes 26°S and 29°S (Fig. 1) heat flow is consistently low (< 42 mWm -2) excepting the area of El Salvador, in the middle slope of the Andes, with a heat-flow value of 75 mWm -2. Among the heat-flow measurements in continental Chile, a topography correction was felt to be needed only for this site [2] - the uncorrected value is 59.9 mWrn-2. The site of E1 Salvador is near the southern limit of the active volcanic chain of northern Chile. Low heat-flow values between latitudes 28 °S and 33 °S are encountered in a region where no active volcanism is observed. Also, it is worth noticing that no Andean thermal springs are found in Chile in zones with no active volcanos. The La Africana Mine in the Santiago basin is characterized by a high heat flow (79 m W m -2) - estimates based upon geochemical thermometry corroborate this high value. At the same latitude (nearly 33 °S) the measurement in the La Disputada Mine in the western slope of the Andes yielded a "normal" value of 61 mWm -2. The only measurement in Tierra del Fuego - carried out in the area of an oil field - yielded a high value of 96.3 rnWm-2. As stated by Uyeda et al. [2], the foregoing beat-flow data from Chile are neither sufficient to draw any solid conclusions on the heat-flow distribution of the South American continent, nor do they allow any definitive description of the distribution of heat flow in active arcs areas. Also, preliminary models used to study thermal processes s~dia g~o~. et g~d. 37 (~993)
3 17
M, M t ~ o z and V. Hamza
in subduction zones and to understand the volcanism in Chile - proposed by Honda and Uyeda [11], and based on the simple fluid dynamical model of McKenzie [12] - show only small differences in the thermal structure of the region with volcanic activity when compared with the region where volcanic activity is not observed. This suggests that more sophisticated models and additional heat-flow data are needed to explain the volcanism in Chile. Data are too scarce as to establish the trend of heat flow from the trench-arc zone to the volcanic line and to the back arc region. The contribution from crustal radiogenic sources to heat flow in back arc regions may be sustantial if the back arc region is continental [ 11]. A further preliminary model for the thermal structure of the Central Andes, proposed by Honda [13], shows a strong cooling of the upper continental mantle due to interaction with the subsiding oceanic plate; nevertheless, the model parameters assumed by Honda [13] do not correspond to the Chile subduction zone. Other studies on the thermal state of western South America are contained in [14,15]. A general vertical distribution of temperature in different regions of South America as well as the evolution of the thermal regime of the Archean crust are described in [16]. In the area of the Chile Ridge (44 °S - 48 °S; 75 °W - 79 °W) - including a ridgetrench collision zone - heat flow is very variable with values between 25 and 414 mWm-2. In this case a mean heat-flow value cannot be estimated-complex processes envolving hydrothermal circulation and sedimentation in tectonic areas such as the Chile Ridge may be linked to anomalous dispersion in heat-flow values [17,18]. Values in the area of the Chile Ridge are shown in Figs. 2 and 3 and in Table 1. Cande et al. [10] have found that the value of 212 mWm-2 on the seawardmost end of line A-A'
,,,o
lo9
-4Ei°
~
4
e
Fig. 2.
318
c'
PE~l.s
Heat flow [mWm-2] in the Chile Ridge from Herron et al. [9] and Cande et al. [10]. Heat-flow values in cross-sections A-A' to D-D' are presented in Fig. 3 and Table 1. Studia geoph, et geodo37 (1993)
Heat Flow and Temperature Gradients in Chile
Table 1.
Heat flow in the area of the Chile Ridge [10]
Latitude (S)
Longitude (W)
Water Depth [m]
Heat flow [mWrn-2]
47°23.6' 47022.8 ' 47019.4 ' 47019.0 ' 45054.4 ' 45055.0 ' 45054.5 ' 45°54.2' 45054.2 ' 45°54.1 ' 47044"0' 47°44"0' 47044"0' 47044"5, 47°44"1 ' 45003"9, 45004"3' 45003"8' 45004.6 '
76°24.7 ' 76022.2' 76010.5 ' 76008.2 ' 75044.9 ' 75042.4' 75052.9 ' 75052.0' 75051.9' 75051.6 ' 75051"7' 75047"9' 75047"8' 75°45"8' 75042'4 ' 75047"2' 75049"8' 75°53"1 ' 75055.8 '
3656 3579 3510 3403 1701 1626 3153 2775 2625 2441 1879 1710 1110 1260 842 2017 2122 2358 3225
129 116 148 112 116 162 90 343 316 194 79 I10 92 116 132 72 80 62 132
(Figs. 2 and 3) is anomalously high by roughly 100 m W m -2 when compared with the theoretical Parsons and Sclater [19] value, and considering the effects of rapid sedimentation indicated by the model of Langseth et al. [20]; they conclude that this high value m a y be due to the effects of the flow of hot water from deeper levels along thrust faults near the foot of the landward slope. In the case of line C-C', the predicted value of heat flow, corrected for sedimentation, is about 112 m W m -2, which is only slightly less than the average of observed values. See also the caption to Fig. 3 for further comments on these heat-flow sites. Also, in this area the heat-flow estimates based upon the location in depth of the phase of gas hydrates in marine sediments [10] are found to be in good correlation with conventional measurements. The boundary of gas hydrates was determined by means of seismic reflection - this method of estimating heat flow is described in [21]. Cande et al. [10] have also compared the heat-flow measurements and estimates to the numerical model of DeLong et al. [22] which considers the thermal effects of a ridge-trench collision and assumes that heat is transferred conductively. The observed heat-flow values compare favourably with the predicted values at a point near (5 km) the trench axis in the case of line B-B'; along lines A-A' and D-D' the measured heat flow is higher than predicted values - Cande et al. [10] suspect that this difference may be due to the simplicity of the model describing the complex structure of the forearc region. The measurements along C-C, are not included in this comparison as they were made above recently deformed trench sediments and do not record the effects of ridge collision. Studia geoph, et geod. 37 (1993)
3 !9
M. Muffoz and V. Hamza
3. GEOCHEMICAL THERMOMETRY OF HOT SPRINGS AND F L U I D - R O C K EQUILIBRIUM
Fluid temperature at depth has been estimated in 33 areas with hot springs activity [ 1,23]. The geothermometers used were SiO2 [24], Na-K-Ca [25], Na-Li and Li [26]. The complex processes controlling fluid-rock equilibrium, and the evolution of fluid chemical composition along their circulation path in the earth, require a comparative study of temperature estimates provided by different geothermometers. Temperature estimates are shown in Table 2, and in a more detailed analysis in Table 3 and 4 for E1 Tatio - the most studied of the geothermal areas in Chile.
HEAT FLOW (mW/rn2) A
K
c
c' 0
-0
-1 so
-2
-2 -3
OSR
-4 -4 -5 "~
B
B'
.
°
-6
D
D' b32 ~
-0
,oI/4 .o
~
--
. .......
~km ~4
~
!o
_.
-4 -5
Fig. 3. Heat flow in the cross-sections of Fig. 2 (from [10]). The value of 212 mWm -2 [9] on line A-A' corresponds to the 5 m. y. old oceanic crust being subducted here; measurements along A-A' reflect the condition of the trench's landward slope about 3 m. y, before the collision. The heat-flow measurements along B-B' reflect the effect of ridge subduction; sites with heat-flow values over 300 mWm -2 on the lower slope above the oceanic crust are roughly 0.5 m. y. old. Sites along C-C' on the crust 9 m, y. old are above recently deformed sediments and do not reflect the effects of the ridge-trench collision. Measurements along D-D' reflect the effect of the ridge-trench collision that ocurred about 6 m. y. ago.
320
Studia geoph, et geod. 37 (1993)
Heat Flow and Temperature Gradients in Chile
Table 2. Hot springs and geothermal areas in Chile. Ts: Temperature of spring discharges. T(SiO2), T(Na-Li), T(Na-K-Ca): Temperatures estimated by chemical geothermometers. TMg(Na-K-Ca): T(Na-K-Ca), Mg corrected. Temperatures in *C. Locality
Latitude
Ref.
TS
T(SiO2) T(Na-Li) T(Na-K-Ca) TMg(Na-K-Ca)
Longitude Bafios Juras¢
18°12'S 69°32~/ Untupujo 18013'S 69"17'W Suriri 18055'S 68059'W Chinchillani 19"08'S 68°55'W Puchuldiza (1) 19"23'S 68°58'W Chusmiza 19041'S 69012'W Pampa L'Lrima 19°51'S 68o56'W Mamifia 20°15'S 69°10'W E1 "ratio(2) 22°20'S 68o01'W Socos 30043'S 71°35'W Colina 33010'S 70°38'W Apoquindo 33025'S 70°25~/ Bafios de Colina 33048'S 70000'W Bafios Morales 33°50'S 700035V Cauquenes 34°16'S 70°35'W Vegas del Flaco 34°57'S 70o28'W San Pedro 35008'S 70027'W
fl = 1/3 fl = 4/3 a
66
105
218
131
63*
129orn.a.
a
15
13
183
151
81.
n.a.
a
82
164
229
234*
a
29
116
176
193
79*
n.a.
a
85
200
237
205*
245
196
a
46
101
58
133
76*
n.a.
a
69
179
264
195.
147 48
b
41-52
92
112
111
53*
a,c,d,e
78
171
262
207*
b
26
64
11
74
29*
f
30
58
189
112
38* 112 or n.a.
f
22
67
101
26* n.a.
g
50
-
257
170.
208 77
f
22
69
75
126.
116 126
f
48
90
134
226*
121 226
f
77
145
150
205*
193 194
f
34
69
235
234*
313 209
233* 197
(see Table 3)
to be continued Studia geoph,et geod, 37 (1993)
321
M. Muftoz and V. Hamza
Table 2.
continuation
LocMi~
Latitude
Ref.
TS
T(SiO2) T(Na-Li)
Longitude Bafios de Azufre 35°16'S 70o38'W Panim~ivida 35045'S 71°25~V Campanario(3) 35°56'S 70033'W Catillo 36°16'S 71°34'W ChillOn 36°57'S 71033'W Pemehue 38°03'S 71o44'W Tolguaca 38°14'S 71o44'W Manzanar 38°27'S 71043'W Rfo Blanco 38°35'S 71o42'W AguasdelaVaca 38°37'S 71o37'W Minettie 39°19'S 71o44'W San Luis 39o2rs 71o33,w Palgufn 39%3'S 71 °47'W Llif6n 40°12'S 72°17a~V AguasC~ientes 40°37'S 72°23"~V Puyehue 40°39'S 72o21'W
T(Na-K-Ca) TMg(Na-K-Ca) ]]= 1/3 /l= 4/3
f
39
150
141
193
92* n.a.
f
32
77
124
98
38* 98 or n.a.
f
32
120
90(259)
172.
172
153
f
35
97
185
103
64*
102or n.a.
f
89
204
242
191
71.
n.a.
f
37
128
148
152.
126
152
f
90
145
266
150
93*
81
f
48
110
196
-
f
90
173
-
192
46*
n.a.
f
60
141
280
212.
125
173
f
36-46
110
124
138.
100
f
36-46
100
158
140
73*
f
44-46
102
157
129
70*
94 or n.a.
f
17
81
287
165
36*
n.a.
f
50-75
131
164
154.
109
147
f
50-60
124
156.
142.
108
112
to be continued
322
Studiageoph, et geod. 37 (1993)
Heat b'low and Temperature Gradients in Chile
Table 2. Ref.:
continuation
Chemical composition taken from: a - Lahsen [27]; b - Merino [28]; c - Cusicanqui et al. [29]; d - Lahsen and Trujillo [30]; e - Ellis [31] as reported in Giggenbach [32]; f - De Grys [33]; g - Benado and Maffn [34]
Notes:
(1) (2)
(*)
temperatures are the mean calculated temperatures of three springs. temperatures are the mean calculated temperatures of the thirty-three springs sampled (see Table 3). T(Na-Li) has been calculated for C1- < 0.2 M and (CI- > 0.3 M) [26] as C1- in Campanario is 0.29 M. indicates the temperature to be chosen according to the rule by Fournier and Truesdell
(-)
lack of data.
(3)
[25].
The Mg correction of the calculated T(Na-K-Ca) was carried out by referring to the graphs of Fournier and Potter [35]. n.a. an Mg correction according to [35] was not applied; in some particular cases an Mg corrected temperature is given for the 13= 113 branch of the Na-K-Ca temperature [25]. An Mg correction to T(Na-K-Ca) was not applied (n.a.) if T(Na-K-Ca) was below 70 *(2, or if R = {Mg/(/vlg + Ca + K)} x 100 (in equivalent units of concentration) was less than 0.5, or if the ATMg correction was negative.
Neither the SiO2 nor the Na-K-Ca geothermometers are reliable for acid sulfate-rich waters that contain little chloride [25]. This is the case of the Chillfm thermal waters (pH: 2.4 - 5-87; SO4/C1:300-500 - [36]) - a much lower SO4/CL ratio of 4.274 is given by De Grys [33]; a Na-Li temperature estimate for these fluids is 242 °C. Following the classification of White et ai. [37], the characteristics of this system seem to be those of a dry vapor system. These systems may provide a mechanism to separate volatile Fig from other volatile elements; Hg deposits could then be encountered in their surroundings and - this being more speculative - porphyritic copper below their phreatic level [37]. Rio Blanco (Table 2) is one of the moderate acid sulfate waters (pH: 4.33; SO4/C1: 5.0); no data on the Li content are available to calculate a T(Na-Li) estimate. For Baftos Jurase, Chinchillani and Chusmiza, the SO4/C1 ratio ranges between 4.3 and 5.0 (pH is about 7-5); the content of these waters corresponds approximately to the acid HCO3-SO4 type waters of White et al. [38]. Also, the composition of the Tolguaca springs is similar to those of acid sulfate waters, but their pH = 6.43 and the HCO3/C1 ratio is lower than that of acid HCO3-SO4 type waters. The analyses and classification of waters of central Chile were reported by De Grys [33]. A magnesium correction to the Na-K-Ca geothermometer [35] has also been carried out, and the TMg(Na-K-Ca) estimates are shown in Table 2. Magnesium corrections are important in the cases of Suriri, San Pedro, Campanario, Aguas de la Vaca and Puyehue.
Studia geoph, et geod. 37 (1993)
323
M. Mufloz and V. Hamza Table 3.
El Tatio. Analysis of 33 hot springs.
Number of sampled springs: 33 Mean surface temperature: 78 °C
Standard deviation: 13 °C
mean T(SiO2): mean T(Na-K-Ca): mean T(Na-Li):
st. dev: st. dev: st. dev:
171 °C 207 *C 262 °C
170C 20°C 9°C
ATMg: Mg correction to T(Na-K-Ca) - Temperature va,lues in *C • Number of springs with available Mg analysis: 12 Spring
T(Na-K-Ca)
R%
ATMg
65 80 109 149 181 186 202 218 227 238 244 339
173 175 202 196 211 208 218 229 227 230 231 229
3.8 0,6 0.3 0.8 3.3 0.5 1-7 0-8 0.1 0.2 0-1 0-2
6.2 7.2 n.a. 1-1 11.7 8.8 0.6 0.3 n.a. n.a. n.a. n.a.
= { Mg/(Mg + Ca + K)} x 100, using equivalent units of concentration; R% ,aTMg to be subtracted from the calculated T(Na-K-Ca): ATMg = - 1.03 + 59.971 log R + 145.05 (log R)2 - 36711 (log R)21T- 1.67 xl07 log R/T 2 the calculated Na-K-Ca temperature in K; T an Mg correction to T(Na-K-Ca) according to [35] was not applied n.a.
For many hot waters a Mg correction is not applied according to [35] - see last note in Table 2. The cases of Pampa Lirima and Bafios de Colina will be considered later. The results of geochemical thermometry applied to 33 hot spring-waters of the E1 Tatio geothermal area are shown in Table 3. The tectonic features and location of springs and wells of El Tatio are shown in Fig. 4. As can be seen in Table 3, the Mg correction to T(Na-K-Ca) is generally not relevant in E1 Tatio. The highest temperature estimate is given by the Na-Li geothermometer (262 °C) showing the smallest standard deviation (9 °C). The temperature estimates from well discharges are given in Table 4.
324
Studia geoph, et geod. 37 (1993)
H~at F/ow and Temperature Gradients in Chilo
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