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The origin of salts on Mount Erebus and along the coast of Ross Island, Antarctica
Isotope Geoscience, 1 (1983) 57 -64 Elsevier Science Publishers B.V ., Amsterdam - Printed in The Netherlands
57
THE ORIGIN OF SALTS ON MOUNT EREBUS AND ALONG THE COAST OF ROSS ISLAND, ANTARCTICA*
LOIS M. JONES', GUNTER FAURE' , KAREN S. TAYLOR' CORBATO'
and CHARLES E.
I Research and Development Department, CONOCO, Inc ., Ponca City, OK 74603 (U.S.A.) , Department of Geology and Mineralogy and Institute of Polar S tud ies, The Ohio Statio Un iversity, Columbus, OH 43210 (U .S .A.)
(Received February 9,1982; revised and accepted February 23 , 1982)
ABSTRACT Jones, L.M. , Faure, G., Taylor, K.S . and Corbat6, C.E., 1983 . The origin of salts on Mount Erebus and along the co ast of Ross Island, Antarctica . Isot. Geosci., 1: 57-64 . Salt deposits in the form of crusts a nd efflorescences are a co m m o n feature of rock exposures along the coast of Ross Island and on the summit of Mt. Erebus, an active volcano that dominates the island. The 8'Sr/ 8· Sr ratios of salts from coastal sites decrease with increasing elevation from 0.70861 at Cape Royds to 0.70 344 at the top of Castle Rock on Hut Point penin sula, only 413 m above sea level, bu t may also vary along the coast in response to the seasonal duration of open water. The variation of ·'Sr / 8· Sr ratios ca n be attributed to mixing of marine Sr witb Sr derived from the volcanic rocks and confirms the importance of chemical weathering as a source of Sr in these secondary salts. YeHow salts of complex mineralogical composition from the summit of Mt. Erebus have 8' Sr/ 8· Sr ratios whose average is 0 .70345. This value is consistent with the hypothesis that these salts formed both from the plume of volcanic gases emanating from a convecting lava lake of phonolitic composition and by chemical weathering of the volcanic rocks. On e sample of white salt from ice caves at the summit has an anomalously high 8'Sr/ 8·Sr ratio of 0.70460 perhaps because it contains a component of marine Sr released by melting of large quantities of snow in the ice caves.
INTRODUCTION
Ross Island is part of a large volcanic province located in southern Victoria Land of Antarctica (Kyle and Cole, 1974) and is composed of at least four overlapping volcanic cones, the largest of which is Mount Erebus with an elevation of 3794 m (Fig. 1). The rocks of Ross Island consist primarily of basanite and basanitoid with lesser amounts of hawaiite, phono*Laboratory for Isotope Geology and Geochemistry (Isotopia) Contribution No. 60.
0167·6695/83/$03 .00
©
1983 Elsevier Science Publishers B.V.
58
lite and benmoreite. The chemical composition and petrologic evolution of these rocks have been studied by Goldich et al. (1975), Kyle and Rankin (1976), and by Kyle (1981). Age determinations by the K-Ar method indicate that volcanic activity on Ross Island started less than 5.0 Ma ago (Armstrong, 1978). Fumarolic activity at the summit of Mt. Erebus has occurred continuously at least since 1841 when the volcano was first sighted by Captain James Ross. The presence of a small lava lake within the Inner Crater inside the Main Crater was first reported by Giggenbach et al. (1973) and was described in detail by Kyle (1977). Salt efflorescences and crusts occur widely along the coast of Ross Island and in the summit area of Mt. Erebus. The most common minerals are halite (NaCI), mirabilite (Na2S04·10H20), thenardite (Na2S04), gypsum (CaS04·2H20), alunite (K 2AI6(OH)dS04)4), and calcite (CaC0 3 ) . Keys and Williams (1981) collected samples of salt from the summit of Mt. Erebus and tentatively identified several additional salts, including malladrite (NazSiF6), alunogen (AI2(S04h·18H20) and jarosite «K,Na)(Fe,AI)(S04h(OH)6). Many of the salt deposits on Mt. Erebus are yellow in color and superficially resemble native sulfur which has not been found on Mt. Erebus. Yellow salts composed of natrojarosite, alunite and fibroferrite (Fe(S04)OH· 5H zO) were also reported by Vennum (1979) from rock exposures in the area of the Orville Coast and Ellsworth Land. He attributed the occurrence of these minerals to the oxidation of pyrite in shale, slate and granodiorite. The salts on Ross Island are derived both from marine and volcanic sources. Salts of marine origin dominate along the coast where they form primarily from marine aerosols, by melting of snow containing marine salts, and by evaporation of the resulting meltwater ponds (Dort and Dort, 1970; Bowser et al., 1970; Keys and Williams, 1981). In the summit area of Mt. Erebus salts may be deposited directly from the plume of volcanic gases and may form by acid-rock interactions. Salt at locations on Ross Island that are intermediate in elevation between the coast and the summit of Mt. Erebus should be composed of material derived both from marine and volcanic sources. The samples for this study were collected at Cape Royds, Cape Evans and on Hut Point peninsula along the coast of Ross Island and from the summit of Mt. Erebus (Fig. 1). The primary objective of this study is to determine the origin of Sr in the salts on Ross Island by means of measurements of its isotopic composition which is an effective parameter for this purpose because the 87Srj86Sr ratios in the oceans and in young volcanic rocks have distinctly different values (Faure and Powell, 1972; Faure, 1977). The 87Srj86Sr ratio of seawater from the Ross Sea is 0.7090 (Jones and Faure, 1967), in good agreement with the average for modem marine Sr which is 0.70906 ± 0.0003 (20) (Faure, 1982) . The average 87Srj86Sr ratio of 38 volcanic rocks on Ross Island and in southern Victoria Land is 0.70366 ± 0.00025 (2a) (Jones and Walker , 1972). Therefore, the 87Srj86Sr ratios of salts collected along the coast of Ross Island and from the summit of Mt.
59 Cape B! rd
Cape Royds
Cape Crozier
169'E
78'45'S
Contour Intervol: 1000 meters!
ROSS ISLAN D
T
Scale
N
6 5 166'E
161'E
168'E
10 15 2'0 2'5 kilometers
j
Fig. 1. Topographic map of Ross Island, Antarctica, showing sample localities.
Erebus should indicate the relative importance of Sr of marine and of volcanic origin at each of these sites. SALTS FROM COASTAL LOCATIONS ON ROSS ISLAND
Five samples of salts were collected from rock surfaces at Cape Royds, Cape Evans and Hut Point peninsula (Treves, 1962; Kyle and Treves, 1974) shown in Fig. 1. The salts were dissolved in demineralized water whereas rock and mineral samples were treated with a mixture of hydrofluoric, nitric and perchloric acid. Sr was separated from the solutions by cationexchange chromatography in HCI medium. The 87Sr/86Sr ratios were measured on a Varian MAT® 260 mass spectrometer, using double filaments (Re) and double Faraday collectors. The salts from sites along the coast are composed of thenardite, halite and calcite based on X-ray diffraction (XRD) scans, using a Cu-target X-ray source (Table I). The 87Sr/86Sr ratios of the salts from the coastal sites vary from 0.70861 at Cape Royds to 0.70344 at the summit of Castle Rock and thus encompass virtually the entire range from nearly pure marine to dominantly volcanic sources. Sr of marine origin prevails in the salts from Cape Royds and Cape Evans close to sea level, but its abundance in the salts appears to decline with increasing elevation as shown in Fig. 2. The salt on
60 T ABLE I Is o topic co m p ositio n o f salt ef fl oresce nc es and volcanic rocks, Ross ISland, Antarctica Sample
Location
Ele vation (m)
Description
" Sr,8·Sr ± 2a
thenardite and halite anorthoclase phonolite thenardite and halite thenardite calcit e
0 .70861 0 .70363 0.70849 0 .70729 0 .70344
± ± ± ± ±
0.00002 0.00004 0.00005 0.00003 0 .00002
yell o w salt yellow salt carbonates*3 tan-colored salt anorthoclase
0 .70353 0 .70341 0 .70460 0.70342 0 .70330
± ± ± ± ±
0 .00003 0.00002 0.00003 0.00002 0.00003
SrC0 3
0.70811 ± 0.00002
Coastal loea tions: 24 25 * ' 42 26 29
Cap e R o y ds Cap e Royds Cap e Ev ans Hut Point p eninsula Castle Rock
15 15 15 302 413
Su m m it of M t. Ere bus :
8 1 0 06 8 1 0 07 8 1008 8 1 0 0 10 G4 *'
summit summit ice cave summit summit
- 3, 6 0 0 - 3 ,6 0 0 - 3,60 0 -3,600 -3,6 0 0
Is otop e s tan dard : Elmer and Amend®
* I Rb ; 108 ± 1.5 ppm; Sr > 1260 ± 22 ppm. * 2 Rb > 40 ± 0.8 ppm; Sr > 2302 ± 12 ppm . * 3Thls sample effe rv esced strongly wh en a n aqueous solu t ion of it was acidified. An XRD pattern is not d efinitive bu t suggests that the sample con t ain s a mixture o f Na-bicarbonate s and N a-c arbon a te .
Cast le Roe l< E
400
.-----
-
Hu t Poin t Penin sul a
OJ
>
OJ ...J
~
•
300
Cinder Cone "302"
Vl
~ o
.0
'" c 200 .... o
'" > OJ
L:j 100
..
s;
Vl
Vl
U
OJ C
C
u '"
s,
'"
:E:
'0 z>
!
Ca pe Evans+Royds
~
0.706
0. 704
••
!
0.708
87Sri 86 Sr 8
Fi g. 2. Plo t of " Sr/ · Sr ratios of salts, collected near t he coast of Ross Island, vs. t he ele vat ion o f the collecting site.
the summit of Castle Ro ck at an elevation of only 413 m above sea level (a.s.l.) contains Sr that was derived en tirely from the hyaloclastite of which that volcanic feature is composed . Fig. 2 also illustrates the point that the
61
87Sr/86Sr ratios of the salts collected from rock exposures at Cape Royds and Cape Evans (~15 m a.s.l.) are significantly less than the marine value, presumably because they contain Sr derived from the volcanic substrate by water-rock interactions. Salt from a cinder cone on Hut Point peninsula (302 m a.s.l.) contains still more volcanic Sr, whereas the salt on Castle Rock contains Sr that appears to be entirely of volcanic origin. These results suggest that the effective transport of marine Sr decreases with increasing elevation above sea level and that chemical weathering of silicate minerals is a significant source of Sr under the climatic conditions prevailing in this region of Antarctica. These conclusions regarding the possible sources of Sr in salts on Ross Island are consistent with the results of earlier studies by Jones and Faure (1967, 1978) on soil salts and brines in the Wright and Taylor valleys of southern Victoria Land. The effect of ice-free conditions along the coast of Ross Island on the 87Sr/86Sr ratio of salt efflorescences has not yet been studied. Both Cape Royds and Cape Evans experience ice-free conditions during the summer. Open water probably exists longer at Cape Royds than at Cape Evans which may account for the higher 87SrjB6Sr ratio of salts at Cape Royds. SALTS FROM THE SUMMIT OF MT. EREBUS
The rocks that form the summit of Mt. Erebus consist primarily of anorthoclase phonolite described by Kyle (1977). Kyle and Rankin (1976) and Goldich et al. (1975) published chemical and trace-element analyses of anorthoclase phonolite from the summit of Mt. Erebus, including an average Sr content of ~890 ppm (see also notes (*1) and (*2), Table I). The mineral composition of the salts from the summit of Mt. Erebus was studied by XRD. Sample 81007 is a yellow salt composed of fine, needlelike crystals too small to obtain an optic figure. Impurities amount to less than 5% and probably constitute only 1 or 2% of the sample. A qualitative analysis by energy dispersion of X-ray spectra generated in a scanning electron microscope indicates major concentrations of Na, Al and CI, and lesser amounts of K, Ca, Si, S and Fe. A precise XRD pattern of sample 0 81007 was obtained by counting for 20 s at 0.01 (28) intervals. Values of 28 were corrected for goniometer errors by reference to silicon and tetra0 decanol standards. The probable error is assumed to be ± 0.02 (28). The dspacings for sample 81007, listed in Table II, do not fit those of jarosite, natrojarosite, fibroferrite, alunogen, burkeite, sulfohalite, malladrite, chloraluminate, aluminum trifluoride or any other mineral currently listed in the Search Manuals of the Joint Committee on Powder Diffraction Standards. However, the mineral alunite does occur as a minor impurity. Sample 81006 is also a yellow salt and has a diffraction pattern similar to that of 81007. The other two samples (81008 and 810010) are white and tan-colored, respectively, but their mineral compositions are uncertain. Sample 81008 may be a mixture of minerals including a carbonate. Sample 810010 yields
62 TABLE II Listing of d-spacing and relative peak intensities for sample 81007, summit of Mt. Erebus, Ross Island, Antarctica 0
d (A)
Intensity
8 .507 (17) 6 .219 (9) 5.021 (6) 4 .539 (5) 4.253 (4) 3 .623 (3) 3.515 (3) 3 .116 (2) 2.922 (2) 2.837 (2) 2.759 (2) 2.551 (1)
10 4 3 2 2 2 1 2 6 1 3 1
0
d (A)
2.472 (1) 2.356 (1) 2.331 (1) 2.128 (1) 2.096 (1) - 2.061 (1 ) 2.015 (1) 1.980(1) 1.758 (1) 1.462 .(1)
Intensity 1 1 1 1 1 1 1 1 1 1
Values in parentheses are probable errors of the last figure.
only a diffuse pattern with peaks at 28 (Cu) values near 15.5, 29.7 and 31.20 • Four samples of salt and one anorthoclase crystal from the summit of Mt . Erebus were analyzed as described before. Three samples (81006,7, 10) have very similar 87Sr/86Sr ratios (Table I) whose average is 0.70345. This value lies between the 87Sr/ 86Sr ratios of the anorthoclase phonolite from Cape Royds and the anorthoclase crystal from the summit of Mt. Erebus and probably represents Sr derived predominantly from the plume of volcanic gases and from the volcanic rocks exposed at the summit. The white salt from the ice cave (81008) is anomalous because the 87Sr/86Sr ratio of this sample is 0.70460 which is significantly higher than the 87Sr/86Sr ratios of volcanic rocks on Ross Island. Evidently this salt contains a component of Sr that is enriched in 87Sr compared to Sr derived from volcanic sources. The ice caves occur in an area of high heat flow and steam discharge where large amounts of snow are being melted. This snow probably contains salt crystals of marine origin that may have contributed to the nucleation of snow flakes. The presence of marine Sr in Antarctic snow was reported by Jones and Faure (1967) who obtained an 87Sr/86Sr ratio of 0.7090 (relative to 0 .7080 for the Eimer and Amend'" Sr-isotope standard) for snow collected from the surface of the Meserve Glacier in Wright Valley, ~40 km from the coast. Therefore, the melting of snow in the ice caves over a period of time may have contributed sufficient amounts of marine Sr to cause an increase of the 87Sr/86Sr ratio above the value of volcanic Sr which remains the dominant component in the salt.
63 SUMMARY
The results of this study suggest that marine Sr is the dominant component of Sr in secondary salts at Cape Royds and Cape Evans on Ross Island. The abundance of the marine component decreases with elevation above sea level and possibly with the seasonal duration of open water along the coast of Ross Island. Chem ical weathering of the volcanic rocks contributed detectable amounts of Sr to salts deposited along the coast and is the dominant source of Sr at higher elevations on the summits of Castle Rock and Mt. Erebus. An exception to this generalization may occur in places of excessive snow melting, as in the ice caves at the summit of Mt . Erebus, where sufficient Sr of marine origin is released to increase the 87Sr/86Sr ratio of salts above that of the volcanic sources. The yellow salts that coat rocks surfaces near Main Crater at the summit of Mt. Erebus contain volcanic Sr derived both from the plume of volcanic gases and by acid-rock interactions of the local anorthoclase phonolite. ACKNOWLEDGEMENTS
The authors are very grateful to P.R. Kyle for collecting the salt samples from the summit of Mt. Erebus and for reviewing the manuscript. The isotopic analyses were made in the Geochronology Laboratory , Research and Development Department, CONOCO, Inc., whose support through use of its facilities is greatly appreciated. This research was supported by the National Science Foundation through Grant DPP-7920407.
REFERENCES Armstrong, R.L., 1978 . K-Ar dating : Late Cenozoic McMurdo Volcanic Group and dry valley glacial history, Victoria Land, Antarctica. N.Z. J . Geol. Geophys., 21: 685-698. Bowser, C.J ., Rafter, T .A. and Black, R .F., 1970. Geochemical evidence for the origin of mirabilite deposits near Hobbs Glacier, Victoria Land, Antarctica . Mineral. Soc. Am., Spec. Pap. No.3, pp. 261-272. Dort, Jr., W. and Dort, D .S., 1970. Sodium sulfate deposits in Antarctica. Modem Geol., 1: 97-117. Faure, G., 1977. Principles of Isotope Geology. Wiley , New York, N. Y., 468 pp. Faure, G., 1982 . The marine-strontium geochronometer. In: G.S. Odin (Editor), Numerical Absolute Dating in Stratigraphy. Wiley, New York, N.Y., pp . 73-79 . Faure , G. and Powell, J .L., 1972. Strontium Isotope Geology. Springer, New York, N.Y. , 166 pp. Giggenbach, W.F ., Kyle, P.R. and Lyon, G.L., 1973. Present volcanic activity on Mt . Erebus, Ross Island, Antarctica. Geology, 1: 135-136. Goldich, 8:S ., Treves, S.B ., Suhr, N.H. and Stuckless, J .S ., 1975. Geochemistry of the Cenozoic volcanic rocks of the Ross Island and vicinity, Antarctica. J . Geol. , 83 : 415-435.
64
Jones, L.M. and Faure, G., 1967. Origin of the salts in Lake Vanda, Wright Valley, southern Victoria Land, Antarctica. Earth Planet. Sci. Lett." 3: 101-10.6. Jones, L.M. and Faure, G., 1978. A study of strontium isotopes in lakes and surficial deposits of the ice-free valleys, southern Victoria Land, Antarctica. Chern. Geo!., 22: 107-120. Jones, L.M. and Walker, RL., 1972. Geochemistry of the McMurdo volcanics, Victoria Land, Part 1. Strontium isotope composition. Antarct. J. U.S., 7(5): 142-144. Keys, J.R. and Williams, K., 1981. Origin of crystalline, cold desert salts in the McMurdo region, Antarctica. Geochim. Cosmochim. Acta, 45: 2299-2309. Kyle, P.R., 1977. Mineralogy and glass chemistry of recent volcanic ejecta from Mt. Erebus, Ross Island, Antarctica. N.Z. J. Geo!. Geophys., 20: 1123-1146. Kyle, P.R., 1981. Mineralogy and geochemistry of a basanite to phonolite sequence at Hut Point Peninsula, Antarctica, based on core from Dry Valley Drilling Project drillholes 1, 2 and 3. J. Petrol., 22: 451-500. Kyle, P.R. and Cole, J.W., 1974. Structural control of volcanism in the McMurdo Volcanic Group, Antarctica. Bull. Volcano!., 38: 16-25. Kyle, P.R. and Rankin, P.C., 1976. Rare earth element geochemistry of Late Cenozoic alkaline lavas of the McMurdo Volcanic Group, Antarctica. Geochim. Cosmochim. Acta, 40: 1497-1507. Kyle, P.R. and Treves, S.B., 1974. Geology of the Hut Point Peninsula, Ross Island. Antarct. J. U.S., 9: 232-234. Treves, S.B., 1962. The geology of Cape Evans and Cape Royds, Ross Island, Antarctica. In: H. Wexler, M.J. Rubin and J.E. Caskey, Jr. (Editors), Antarctic Research. Am. Geophys. Monogr. 7, Pub!. No. 1036, pp. 40-46. Vennum, W.R., 1979. Evaporite encrustations and yellow and green surficial salts from Orville Coast and eastern Ellsworth Land. Antarct. J. U.S., 14(5): 22-24.
57
THE ORIGIN OF SALTS ON MOUNT EREBUS AND ALONG THE COAST OF ROSS ISLAND, ANTARCTICA*
LOIS M. JONES', GUNTER FAURE' , KAREN S. TAYLOR' CORBATO'
and CHARLES E.
I Research and Development Department, CONOCO, Inc ., Ponca City, OK 74603 (U.S.A.) , Department of Geology and Mineralogy and Institute of Polar S tud ies, The Ohio Statio Un iversity, Columbus, OH 43210 (U .S .A.)
(Received February 9,1982; revised and accepted February 23 , 1982)
ABSTRACT Jones, L.M. , Faure, G., Taylor, K.S . and Corbat6, C.E., 1983 . The origin of salts on Mount Erebus and along the co ast of Ross Island, Antarctica . Isot. Geosci., 1: 57-64 . Salt deposits in the form of crusts a nd efflorescences are a co m m o n feature of rock exposures along the coast of Ross Island and on the summit of Mt. Erebus, an active volcano that dominates the island. The 8'Sr/ 8· Sr ratios of salts from coastal sites decrease with increasing elevation from 0.70861 at Cape Royds to 0.70 344 at the top of Castle Rock on Hut Point penin sula, only 413 m above sea level, bu t may also vary along the coast in response to the seasonal duration of open water. The variation of ·'Sr / 8· Sr ratios ca n be attributed to mixing of marine Sr witb Sr derived from the volcanic rocks and confirms the importance of chemical weathering as a source of Sr in these secondary salts. YeHow salts of complex mineralogical composition from the summit of Mt. Erebus have 8' Sr/ 8· Sr ratios whose average is 0 .70345. This value is consistent with the hypothesis that these salts formed both from the plume of volcanic gases emanating from a convecting lava lake of phonolitic composition and by chemical weathering of the volcanic rocks. On e sample of white salt from ice caves at the summit has an anomalously high 8'Sr/ 8·Sr ratio of 0.70460 perhaps because it contains a component of marine Sr released by melting of large quantities of snow in the ice caves.
INTRODUCTION
Ross Island is part of a large volcanic province located in southern Victoria Land of Antarctica (Kyle and Cole, 1974) and is composed of at least four overlapping volcanic cones, the largest of which is Mount Erebus with an elevation of 3794 m (Fig. 1). The rocks of Ross Island consist primarily of basanite and basanitoid with lesser amounts of hawaiite, phono*Laboratory for Isotope Geology and Geochemistry (Isotopia) Contribution No. 60.
0167·6695/83/$03 .00
©
1983 Elsevier Science Publishers B.V.
58
lite and benmoreite. The chemical composition and petrologic evolution of these rocks have been studied by Goldich et al. (1975), Kyle and Rankin (1976), and by Kyle (1981). Age determinations by the K-Ar method indicate that volcanic activity on Ross Island started less than 5.0 Ma ago (Armstrong, 1978). Fumarolic activity at the summit of Mt. Erebus has occurred continuously at least since 1841 when the volcano was first sighted by Captain James Ross. The presence of a small lava lake within the Inner Crater inside the Main Crater was first reported by Giggenbach et al. (1973) and was described in detail by Kyle (1977). Salt efflorescences and crusts occur widely along the coast of Ross Island and in the summit area of Mt. Erebus. The most common minerals are halite (NaCI), mirabilite (Na2S04·10H20), thenardite (Na2S04), gypsum (CaS04·2H20), alunite (K 2AI6(OH)dS04)4), and calcite (CaC0 3 ) . Keys and Williams (1981) collected samples of salt from the summit of Mt. Erebus and tentatively identified several additional salts, including malladrite (NazSiF6), alunogen (AI2(S04h·18H20) and jarosite «K,Na)(Fe,AI)(S04h(OH)6). Many of the salt deposits on Mt. Erebus are yellow in color and superficially resemble native sulfur which has not been found on Mt. Erebus. Yellow salts composed of natrojarosite, alunite and fibroferrite (Fe(S04)OH· 5H zO) were also reported by Vennum (1979) from rock exposures in the area of the Orville Coast and Ellsworth Land. He attributed the occurrence of these minerals to the oxidation of pyrite in shale, slate and granodiorite. The salts on Ross Island are derived both from marine and volcanic sources. Salts of marine origin dominate along the coast where they form primarily from marine aerosols, by melting of snow containing marine salts, and by evaporation of the resulting meltwater ponds (Dort and Dort, 1970; Bowser et al., 1970; Keys and Williams, 1981). In the summit area of Mt. Erebus salts may be deposited directly from the plume of volcanic gases and may form by acid-rock interactions. Salt at locations on Ross Island that are intermediate in elevation between the coast and the summit of Mt. Erebus should be composed of material derived both from marine and volcanic sources. The samples for this study were collected at Cape Royds, Cape Evans and on Hut Point peninsula along the coast of Ross Island and from the summit of Mt. Erebus (Fig. 1). The primary objective of this study is to determine the origin of Sr in the salts on Ross Island by means of measurements of its isotopic composition which is an effective parameter for this purpose because the 87Srj86Sr ratios in the oceans and in young volcanic rocks have distinctly different values (Faure and Powell, 1972; Faure, 1977). The 87Srj86Sr ratio of seawater from the Ross Sea is 0.7090 (Jones and Faure, 1967), in good agreement with the average for modem marine Sr which is 0.70906 ± 0.0003 (20) (Faure, 1982) . The average 87Srj86Sr ratio of 38 volcanic rocks on Ross Island and in southern Victoria Land is 0.70366 ± 0.00025 (2a) (Jones and Walker , 1972). Therefore, the 87Srj86Sr ratios of salts collected along the coast of Ross Island and from the summit of Mt.
59 Cape B! rd
Cape Royds
Cape Crozier
169'E
78'45'S
Contour Intervol: 1000 meters!
ROSS ISLAN D
T
Scale
N
6 5 166'E
161'E
168'E
10 15 2'0 2'5 kilometers
j
Fig. 1. Topographic map of Ross Island, Antarctica, showing sample localities.
Erebus should indicate the relative importance of Sr of marine and of volcanic origin at each of these sites. SALTS FROM COASTAL LOCATIONS ON ROSS ISLAND
Five samples of salts were collected from rock surfaces at Cape Royds, Cape Evans and Hut Point peninsula (Treves, 1962; Kyle and Treves, 1974) shown in Fig. 1. The salts were dissolved in demineralized water whereas rock and mineral samples were treated with a mixture of hydrofluoric, nitric and perchloric acid. Sr was separated from the solutions by cationexchange chromatography in HCI medium. The 87Sr/86Sr ratios were measured on a Varian MAT® 260 mass spectrometer, using double filaments (Re) and double Faraday collectors. The salts from sites along the coast are composed of thenardite, halite and calcite based on X-ray diffraction (XRD) scans, using a Cu-target X-ray source (Table I). The 87Sr/86Sr ratios of the salts from the coastal sites vary from 0.70861 at Cape Royds to 0.70344 at the summit of Castle Rock and thus encompass virtually the entire range from nearly pure marine to dominantly volcanic sources. Sr of marine origin prevails in the salts from Cape Royds and Cape Evans close to sea level, but its abundance in the salts appears to decline with increasing elevation as shown in Fig. 2. The salt on
60 T ABLE I Is o topic co m p ositio n o f salt ef fl oresce nc es and volcanic rocks, Ross ISland, Antarctica Sample
Location
Ele vation (m)
Description
" Sr,8·Sr ± 2a
thenardite and halite anorthoclase phonolite thenardite and halite thenardite calcit e
0 .70861 0 .70363 0.70849 0 .70729 0 .70344
± ± ± ± ±
0.00002 0.00004 0.00005 0.00003 0 .00002
yell o w salt yellow salt carbonates*3 tan-colored salt anorthoclase
0 .70353 0 .70341 0 .70460 0.70342 0 .70330
± ± ± ± ±
0 .00003 0.00002 0.00003 0.00002 0.00003
SrC0 3
0.70811 ± 0.00002
Coastal loea tions: 24 25 * ' 42 26 29
Cap e R o y ds Cap e Royds Cap e Ev ans Hut Point p eninsula Castle Rock
15 15 15 302 413
Su m m it of M t. Ere bus :
8 1 0 06 8 1 0 07 8 1008 8 1 0 0 10 G4 *'
summit summit ice cave summit summit
- 3, 6 0 0 - 3 ,6 0 0 - 3,60 0 -3,600 -3,6 0 0
Is otop e s tan dard : Elmer and Amend®
* I Rb ; 108 ± 1.5 ppm; Sr > 1260 ± 22 ppm. * 2 Rb > 40 ± 0.8 ppm; Sr > 2302 ± 12 ppm . * 3Thls sample effe rv esced strongly wh en a n aqueous solu t ion of it was acidified. An XRD pattern is not d efinitive bu t suggests that the sample con t ain s a mixture o f Na-bicarbonate s and N a-c arbon a te .
Cast le Roe l< E
400
.-----
-
Hu t Poin t Penin sul a
OJ
>
OJ ...J
~
•
300
Cinder Cone "302"
Vl
~ o
.0
'" c 200 .... o
'" > OJ
L:j 100
..
s;
Vl
Vl
U
OJ C
C
u '"
s,
'"
:E:
'0 z>
!
Ca pe Evans+Royds
~
0.706
0. 704
••
!
0.708
87Sri 86 Sr 8
Fi g. 2. Plo t of " Sr/ · Sr ratios of salts, collected near t he coast of Ross Island, vs. t he ele vat ion o f the collecting site.
the summit of Castle Ro ck at an elevation of only 413 m above sea level (a.s.l.) contains Sr that was derived en tirely from the hyaloclastite of which that volcanic feature is composed . Fig. 2 also illustrates the point that the
61
87Sr/86Sr ratios of the salts collected from rock exposures at Cape Royds and Cape Evans (~15 m a.s.l.) are significantly less than the marine value, presumably because they contain Sr derived from the volcanic substrate by water-rock interactions. Salt from a cinder cone on Hut Point peninsula (302 m a.s.l.) contains still more volcanic Sr, whereas the salt on Castle Rock contains Sr that appears to be entirely of volcanic origin. These results suggest that the effective transport of marine Sr decreases with increasing elevation above sea level and that chemical weathering of silicate minerals is a significant source of Sr under the climatic conditions prevailing in this region of Antarctica. These conclusions regarding the possible sources of Sr in salts on Ross Island are consistent with the results of earlier studies by Jones and Faure (1967, 1978) on soil salts and brines in the Wright and Taylor valleys of southern Victoria Land. The effect of ice-free conditions along the coast of Ross Island on the 87Sr/86Sr ratio of salt efflorescences has not yet been studied. Both Cape Royds and Cape Evans experience ice-free conditions during the summer. Open water probably exists longer at Cape Royds than at Cape Evans which may account for the higher 87SrjB6Sr ratio of salts at Cape Royds. SALTS FROM THE SUMMIT OF MT. EREBUS
The rocks that form the summit of Mt. Erebus consist primarily of anorthoclase phonolite described by Kyle (1977). Kyle and Rankin (1976) and Goldich et al. (1975) published chemical and trace-element analyses of anorthoclase phonolite from the summit of Mt. Erebus, including an average Sr content of ~890 ppm (see also notes (*1) and (*2), Table I). The mineral composition of the salts from the summit of Mt. Erebus was studied by XRD. Sample 81007 is a yellow salt composed of fine, needlelike crystals too small to obtain an optic figure. Impurities amount to less than 5% and probably constitute only 1 or 2% of the sample. A qualitative analysis by energy dispersion of X-ray spectra generated in a scanning electron microscope indicates major concentrations of Na, Al and CI, and lesser amounts of K, Ca, Si, S and Fe. A precise XRD pattern of sample 0 81007 was obtained by counting for 20 s at 0.01 (28) intervals. Values of 28 were corrected for goniometer errors by reference to silicon and tetra0 decanol standards. The probable error is assumed to be ± 0.02 (28). The dspacings for sample 81007, listed in Table II, do not fit those of jarosite, natrojarosite, fibroferrite, alunogen, burkeite, sulfohalite, malladrite, chloraluminate, aluminum trifluoride or any other mineral currently listed in the Search Manuals of the Joint Committee on Powder Diffraction Standards. However, the mineral alunite does occur as a minor impurity. Sample 81006 is also a yellow salt and has a diffraction pattern similar to that of 81007. The other two samples (81008 and 810010) are white and tan-colored, respectively, but their mineral compositions are uncertain. Sample 81008 may be a mixture of minerals including a carbonate. Sample 810010 yields
62 TABLE II Listing of d-spacing and relative peak intensities for sample 81007, summit of Mt. Erebus, Ross Island, Antarctica 0
d (A)
Intensity
8 .507 (17) 6 .219 (9) 5.021 (6) 4 .539 (5) 4.253 (4) 3 .623 (3) 3.515 (3) 3 .116 (2) 2.922 (2) 2.837 (2) 2.759 (2) 2.551 (1)
10 4 3 2 2 2 1 2 6 1 3 1
0
d (A)
2.472 (1) 2.356 (1) 2.331 (1) 2.128 (1) 2.096 (1) - 2.061 (1 ) 2.015 (1) 1.980(1) 1.758 (1) 1.462 .(1)
Intensity 1 1 1 1 1 1 1 1 1 1
Values in parentheses are probable errors of the last figure.
only a diffuse pattern with peaks at 28 (Cu) values near 15.5, 29.7 and 31.20 • Four samples of salt and one anorthoclase crystal from the summit of Mt . Erebus were analyzed as described before. Three samples (81006,7, 10) have very similar 87Sr/86Sr ratios (Table I) whose average is 0.70345. This value lies between the 87Sr/ 86Sr ratios of the anorthoclase phonolite from Cape Royds and the anorthoclase crystal from the summit of Mt. Erebus and probably represents Sr derived predominantly from the plume of volcanic gases and from the volcanic rocks exposed at the summit. The white salt from the ice cave (81008) is anomalous because the 87Sr/86Sr ratio of this sample is 0.70460 which is significantly higher than the 87Sr/86Sr ratios of volcanic rocks on Ross Island. Evidently this salt contains a component of Sr that is enriched in 87Sr compared to Sr derived from volcanic sources. The ice caves occur in an area of high heat flow and steam discharge where large amounts of snow are being melted. This snow probably contains salt crystals of marine origin that may have contributed to the nucleation of snow flakes. The presence of marine Sr in Antarctic snow was reported by Jones and Faure (1967) who obtained an 87Sr/86Sr ratio of 0.7090 (relative to 0 .7080 for the Eimer and Amend'" Sr-isotope standard) for snow collected from the surface of the Meserve Glacier in Wright Valley, ~40 km from the coast. Therefore, the melting of snow in the ice caves over a period of time may have contributed sufficient amounts of marine Sr to cause an increase of the 87Sr/86Sr ratio above the value of volcanic Sr which remains the dominant component in the salt.
63 SUMMARY
The results of this study suggest that marine Sr is the dominant component of Sr in secondary salts at Cape Royds and Cape Evans on Ross Island. The abundance of the marine component decreases with elevation above sea level and possibly with the seasonal duration of open water along the coast of Ross Island. Chem ical weathering of the volcanic rocks contributed detectable amounts of Sr to salts deposited along the coast and is the dominant source of Sr at higher elevations on the summits of Castle Rock and Mt. Erebus. An exception to this generalization may occur in places of excessive snow melting, as in the ice caves at the summit of Mt . Erebus, where sufficient Sr of marine origin is released to increase the 87Sr/86Sr ratio of salts above that of the volcanic sources. The yellow salts that coat rocks surfaces near Main Crater at the summit of Mt. Erebus contain volcanic Sr derived both from the plume of volcanic gases and by acid-rock interactions of the local anorthoclase phonolite. ACKNOWLEDGEMENTS
The authors are very grateful to P.R. Kyle for collecting the salt samples from the summit of Mt. Erebus and for reviewing the manuscript. The isotopic analyses were made in the Geochronology Laboratory , Research and Development Department, CONOCO, Inc., whose support through use of its facilities is greatly appreciated. This research was supported by the National Science Foundation through Grant DPP-7920407.
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