Preliminary Investigations of the Heavy Mineral Record from CRP-3 Drillcore, Victoria Land Basin, Antarctica
Received 2 February 200 1 : accepted in r e ~ , i s c ' i l f o r m 15 May 200 1
Abstract - Cenozoic and Devonian sediments obtained in CRP-3 drillcorc recovered o n the continental shelf of McMurdo Sound in Ross Sea, Antarctica, have been investigated for their heavy mineral content. T h e heavy mineral spectrum of the CRP-3 sedirnents is dominated by pyroxenes. amphiboles. stable minerals, alterites
;ind o p a q u e grains. Within the g r o u p of stable heavy minerals, zircon. garnet, titanite. apatite and tourmaline are quantitatively most important. Several sections of Ihe CRP-3 core contain considerable amounts of carbonate minerals or topaz.
'The c o r e section above c. 150 mbsf contains o n average twice as many heavy minerals as the section below 150 mbsf including the Devonian Beacon sandstone.
T h e Cenozoic sediments can be distinguished from the Devonian sediments by the presence of pyroxenes and garnet in the Cenozoic sediments, which are absent in the Devonian Beacon sediments.
This paper is confined 10 the presentation of the results of heavy mineral analyses. Only short discussions or references to other investigations and results of the CRP-3 core are made. More detailed interpretations require a higher resolution of analysis, which is scheduled to be carried out.
INTRODUCTION
The CRP-3 core was recovered from the last drill hole of the international Cape Roberts Project in the Victoria Land Basin on the continental Antarctic shelf of McMurdo Sound in the Ross Sea. T h e history, objectives and results of the Cape Roberts Project and CRP-3 are given in detail in Cape Roberts Science Team (1998, 1999) Hambrey & Wise (1998), Barrett
& Ricci (2000a. b) and in Neumann & Ehrmann (this
volume).
Major objectives of the Cape Roberts Project are to study changes in the Cenozoic Antarctic climate a n d in the dynamics of the Antarctic i c e masses.
T h e s e goals a r e approached w i t h different s e d i m e n t o l o g i c a l , geochemical. petrological, palaeontological and geophysical methods. Within that scope. the results of the initial investigations on the heavy mineral record of CRP-3 are presented here.
W e use heavy mineral analysis a s a m e t h o d f o r reconstructing relationships between sediments and their source areas. It has to be considered, however.
t h a t not only the s o u r c e rocks control t h e heavy mineral distribution in sediments. The heavy mineral content of sediments will be affected by climate and the kind of erosion in the source area, the chemical and physical conditions during transport processes, sorting and hydraulic effects and mineral solution and 1-ecrystallisation during diagenesis (Boenigk, 1983;
Mange & Maurer, 1991; Morton & Hallsworth, 1999).
METHODS
Ninety six samples were available f o r heavy mineral separation, taken at almost constant intervals from the Cenozoic section of the CRP-3 core. Fifty f o u r of the 9 6 samples w e r e selected f o r heavy mineral optical analysis. T h e average d i s t a n c e between the selected samples was about 1 6 m . Additionally, 7 samples of the Devonian Beacon S a n d s t o n e section f r o m 9 3 9 . 4 0 to 8 2 3 . 1 1 mbsf (metres below seafloor) of C R P - 3 were used f o r heavy mineral identification.
The preparation of the samples was done in the same way as for the heavy mineral investigation of CRP- 1 and CRP-2/2A (Polozek & Ehrmann, 1998;
Polozek. 2000). T h e very f i n e sand fraction (63-125 pm) of each sample was gained by sieving.
The samples were not treated with any chemicals oi- subjected to a n y other t y p e of separation ( e . g . magnetic separation) before or after the density separation. A sodium metatungstate solution with a density of 2.85 gem^ was used for the separation of the light and heavy minerals. After 2 0 minutes centrifuging at 3000 rpm (rotations per minute), the heavy minerals at the bottom of the test-tubes were fixed by freezing with liquid nitrogen. Then the light mineral fraction was washed with distilled water into a filter and after defreezing the heavy mineral fraction was washed into another filter. T h e dried heavy mineral fraction of each sample was weighed and mounted in Meltmount (refractive index=1.68) onto
glass slides under maximum temperatures of 120°C Identification a n d c o ~ ~ n t i n g of thc heavy mineral r a i n s was exclusively pcrf"or~iieci with a polarising microscope. A minimum of" 300 grains in each sample were counted and identified along different traverses.
T h e results of the hcavy mineral distribution a r e presented as grain percentages (Fig. l ) based o n the number of counted heavy mineral grains in each siimple.
All raw data are stored i n the Pangaea data bank of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. and are available via www.pangaea.de.
RESULTS
The heavy mineral concentrations range from < l wt% up to 30 wt% (Fig. 1 ) . The section above 150 mbsf contains an average heavy mineral content of 5 to 2 0 wt%. Below 150 mbsf, the sediments contain only 0-5 wt% of heavy minerals, with some isolated peak values reaching up to 25 wt%.
Twenty four minerals and mineral groups were identified during optical analysis. The heavy mineral content of the CRP-3 core is dominated by pyroxenes, s t a b l e minerals, alterites, o p a q u e g r a i n s a n d amphiboles. A strikingly high content of carbonates ( u p t o 8 5 %) was recorded between c. 5 5 0 a n d 450 mbsf.
CENOZOIC SECTION (0 - 823.1 1 n~bsf)
Py roxenes
T h e pyroxenes in s e d i m e n t s f r o m CRP-3 are mainly clinopyroxenes. T h e a v e r a g e clinopyroxene/orthopyroxene ratio is 11.3 and there are no significant differences between the distribution pattern of clinopyroxenes a n d o r t h o p y r o x e n e s throughout the Cenozoic interval of the core. A more detailed distinction of single pyroxene minerals is not possible by optical means. All varieties of pyroxenes are combined in a single "pyroxenes" group. T h e following main intervals can be distinguished in the pyroxene distribution. From 7 8 0 to 6 3 0 mbsf t h e pyroxene content ranges between 20% and SO%, from 823 to 780 mbsf and from 630 to 200 mbsf between 0 % a n d 1 0 % with f o u r i s o l a t e d , u n u s u a l h i g h e r U
abundances up to 90% at 542.73, 361.92, 297.31 and 227.90 mbsf. The highest pyroxene concentration was recorded above 200 mbsf with 60% to 90% (Fig. 1).
Almost all clinopyroxenes and orthopyroxenes show cleavage. Grains without a visible cleavage are very rare. T h e mineral grains often show solution m a r k s , h e n c e t h e s u b r o u n d e d to r o u n d e d g r a i n s appear angular or broken. Besides the pale green to pale brownish-green pyroxenes without pleochroism, m o r e intensively coloured g r a i n s s h o w w e a k
Stable miiiwals/giirnet
Scvcriil minerals. which a r e r e l a t i v e l y si;ililc against weathering, mechanical a b r a s i o n :iiid diagcncsis, form the group of the "stable ~ ~ i i ~ i c r a l s " . The group comprises minerals of the epidoic group (epidote, clinozoisite). zircon, titanite, rutilc. npiiliie and tourmaline. Above 200 mbsf the stable inii~cr;ils occur in minor amounts < 5%. Below 200 mhst' :ill ol' the stable minerals b e c o m e more ahinnlaiii.
Proportions of individual stable minerals reach u p to
70 % ( e . g . garnet at 505 mbsf and 570 mhsl'). The,
content of stable minerals (stable minerals -t giiriii-t) between 823.11 and 200 mbsf fluctuates from S(X io 90%. Because of the differing distribution paticrii of garnet (no garnet in the Devonian s e c t i o n below 823.11 mbsf), this stable mineral is shown separately in figure 1 . The occurrence curve of garnet i n tlic Cenozoic section resembles essentially the curve of the stable minerals. Some more important divcrgenccs of the distribution of garnet compared w i t h oilier s t a b l e minerals a r e recorded at 4 0 6 . 0 9 mhsl', 443.05 mbsf, 571.10 nibsf and at 774.84 mbsf. I n general the amounts of garnet (0-70%) fluctuate more strongly than the content of other stable minerals (0-30%).
In order to circumvent problems of dilution causcil by abundant pyroxene grains above 200 mbsf. the pyroxenes were excluded by statistic calculations. The results of the additional s t a t i s t i c s s h o w that the contents of stable minerals and garnet are not greatly diminished in the section above 200 nibsf compared with lower levels in the core.
Alterites & opaque grains
All the minerals l a c k i n g d e f i n i t e optical characteristics were counted as altei-ites. They are always dark coloured and, as a result of progressive alteration, at most translucent. T h e nontransparent mineral grains were counted as opaque grains.
The content of alterites and opaque grains in the Cenozoic CRP-3 section ranges up to 10% above 200 mbsf and fluctuates from 5% to 45% below 200 mbsf (Fig. 1 ) . Opaque minerals a r e significantly more abundant than the alterites in the intervals below c. 5 2 0 mbsf and f r o m c. 4 1 0 t o 2 0 0 m b s f . T h e c o m b i n e d alterites a n d o p a q u e g r a i n s reach a relatively c o n s t a n t c o n c e n t r a t i o n of a b o u t 25%
between 450 and 200 mbsf and below 750 mbsf. The lowered values between 750 and 430 mbsf and above 200 mbsf are a result of the diluting effect of high carbonate or pyroxene content at the s a m e depth interval.
A few grains of the opaque minerals in different samples are magnetic.
P- . - . . . -. --
Devonian Early Oligocene - (? Late Eocene) Â ¥ Â Early Oligocene
W I-?
520 M . Neumann
Amphiboles
T h e amphiboles are green, brown, red or blue coloured, colourless, with or without pleochroism and show cleavage or no cleavage. The amphiboles total includes all varieties of amphiboles. Amphiboles occur in nearly all samples of the Cenozoic CRP-3 core (Fig. 1). Above 200 mbsf and below 400 nibsf (lie amphibole content is quite low (<5%). From 400 t o 2 0 0 mbsf the amphibole proportion ranges from a r o u n d 5 % up to 10%. At 6 0 1 . 7 5 mbsf a sample containing 18.1 % amphiboles was recorded. The most abundant amphiboles are greenish-brown or greenish- blue hornblendes with maximum contents of c. 5%.
Although occurring only in small amounts < 2.5%, some amphiboles differ in their distribution patterns.
Thus, blue amphiboles (glaucophane or riebeckite) are restricted to a depth interval from 770 to 200 mbsf and colourless amphiboles (treinolite ?) occur almost exclusively above 4 10 mbsf. Identification of the blue aniphiboles is only by o p t i c a l meth.ods, not by c h e m i c a l or microprobe a n a l y s e s , and all a r e provisionally classified as blue, Na-bearing or alkaline a m p h i b o l e s (cf. M a n g e & M a u r e r , 1 9 9 1 ) . B l u e alkaline amphiboles can be used as a specific source- r e l a t e d mineral like t i t a n a u g i t e . A f t e r G h e n t &
Henderson (1968), these amphiboles are present in the P r e c a m b r i a n and lower P a l a e o z o i c r o c k s of t h e b a s e m e n t and as a r f v e d s o n i t e i n t h e M c M u r d o Volcanic Group. Skinner & Ricker (1968a, 1968b) d e s c r i b e d b l u e a m p h i b o l e s f r o m t h e B e a c o n Supergroup and from the granitoids of the Granite Harbour Intrusive Complex. No significant distribution of cleaved and uncleaved amphiboles was observed.
Because of their provenance-specific nature, further investigations shall i n c l u d e a m o r e detailed classification of amphiboles.
Carbonates
A s t r i k i n g f e a t u r e of t h e heavy m i n e r a l assemblage in the CRP-3 record is the erratic but generally high abundance of carbonate minerals in the interval from c. 650 to 450 mbsf (up to 84% of the heavy mineral fraction). T h e m a i n l y c o l o u r l e s s , sometimes reddish-brown-coated carbonates have an a n g u l a r to subrounded s h a p e a n d a r e partly contaminated with dirty coatings. T h e carbonate grains often have an internal nucleus consisting of pyroxene. This could indicate that the carbonate is authigenic. A reaction between the pyroxenes and the sodium metatungstate solution during the separation of heavy and light minerals can be excluded. Such a reaction might produce scheelite. Since scheelite was not observed in the samples, it is not probable that the m i n e r a l s were i n f l u e n c e d b y t h e d e n s i t y separation. Taking all optical characteristics of the carbonate grains into account, it is likely that the samples i n the interval 6 5 0 t o 4 5 0 mbsf c o n t a i n minerals of the dolomite-ankerite series.
Others
Other minerals occur in very low amounts o r witll palcliy distribution. Micas, represented m:iinly h!
biotite a n d chlorite, were detected above 200 mhsl' only in a m o u n t s < 5 % . Below that d e p t h . micii.-i become m o r e abundant in few s a m p l e s will1 concentrations up to 19%. Although not :I Iivav\
mineral, rare grains of glass with a gencral abund:iiic~~
< 1%; were recorded at 756 mbsf. 450 mbsf and I'roiii
330 to 150 mbsf.
BEACON SANDSTONES (823.1 1 to 939.42 mhslj
T h e heavy mineral distribution of the Beiicon Sandstones of the CRP-3 core is based on 7 samples (Fig. 1). The heavy mineral content of the sandsloiu-s is dominated by alterite and opaque minerals, stiihlr minerals and amphiboles (Fig. 1). Pyroxenes are very rare and g a r n e t is absent. Alterite and o p a q u e minerals range from 10 to 50% with a decreasing downcore trend. The stable minerals alternate with t11c alterite and opaque minerals and show an increasing downcore trend from 10% up to 80%. Amphiboles an.' more abundant in the Beacon Sandstones than i n thc Cenozoic section. The amphibole content reaches u p to 22% with no obvious trend.
The alteriteslopaque grains ratio in the sandstones is rather irregular. Whereas alterites dominate i n tlic upper s a m p l e s , very well rounded o p a q u e grains clearly preponderate in the lowermost s a m p l e at 935.32 mbsf. The appearance of the alterite minerals changes throughout the section. In particular, the s a m p l e at 8 2 8 . 0 8 m b s f . c l o s e to the lowermost Cenozoic conglomerates, contains reddish-brown angular a l t e r i t e s w i t h very angular r i m s , which resemble the carbonate minerals in the C e n o z o i c section from c. 650 to 450 mbsf. These alterites can be distinguished from the carbonates, which are also present in that sample. The other alterites are usually dark grey t o b r o w n i s h black and s u b r o u n d e d to angular.
Zircon, tourmaline, titanite and apatite are the most abundant stable minerals. T h e ratios of the single stable minerals vary throughout the Devonian section. Zircon dominates the sample at 858.12 mbsf, tourmaline the sample at 840.27 mbsf - each mineral forming 2 7 % of the total heavy mineral fraction.
Apatite reaches 35% in sample 935.12 mbsf. In that sample strongly weathered apatites with solution marks also occur uniquely, together with well rounded and fresh apatites and nearly isotropic apatites with a hexagonal shape. Zircon occurs as colourless, clear and sub- to well rounded grains and as idiomorphic to subrounded turbid grains with many concentric growth lines. The subrounded titanites are commonly strongly w e a t h e r e d a n d show a dark and scarred surface.
Tin' iimphibole group mainly consists of brownish green iiiul reddish brown hornhlciidcs. Amphibolcs without c l e a v a g e are slightly more abundant than grains vvilli cleavage. A small number of blue sodic ainpliilioles (glaucopliane or ricbeckite) were recorded in five of the seven Beacon sandstone samples.
Other minerals occurring in the Beacon section of CRP-3 iirc t o p a z , micas (mainly biotitc) a n d rare c l i n o x o i s i t e s . Topaz c o n t e n t s range u p to 1 1 % . Carboi~iit~, between 20% and 25%- is contained i n the samples ;it 8 2 8 . 0 8 , 880.46 and 9 2 5 . 9 7 mbsf. T h e other samples have low (<10%) or no carbonate.
DISCUSSION
The p r e l i m i n a r y results of t h e heavy mineral distribution of t h e C R P - 3 s e d i m e n t s show s o m e significant characteristics. The Cenozoic sediments are distinguished from the Devonian Beacon Sandstone by the a b s e n c e of garnet and the near-absence of pyroxenes in the Beacon Sandstones in the analysed samples. Amphiboles seem to be more abundant in t h e Beacon S a n d s t o n e s than in t h e C e n o z o i c sediments. T h e differences in t h e heavy mineral assemblages of Cenozoic and Devonian sediments will be investigated further in the future. In particular, m o r e samples from the Beacon Sandstones will be investigated for their heavy mineral content in order to determine specific characteristics and to compare them with samples of Beacon Sandstones from tlie Transantarctic Mountains.
The Cenozoic interval of the CRP-3 core shows two sections with differing heavy mineral distribution patterns. Above 200 mbsf, the heavy mineral fraction consists mainly of pyroxenes and minor alterites and opaque minerals. Below 200 mbsf, the heavy mineral a s s e m b l a g e m a i n l y c o m p r i s e s s t a b l e m i n e r a l s , alterites, o p a q u e m i n e r a l s and s m a l l a m o u n t s of amphiboles. M i n e r a l s of t h e pyroxene g r o u p and carbonate minerals are dominant in various intervals between 200 mbsf and at the base of the Cenozoic section of the CRP-3 core.
The upper heavy mineral assemblage (200-0 mbsf) of CRP-3 is similar to lower part of CRP-2/2A with a comparable high pyroxene content. T h e pyroxene g r o u p of C R P - 2 / 2 A c o n t a i n s m i n o r but d i s t i n c t a m o u n t s of t i t a n a u g i t e a n d p y r o x e n e s w i t h o u t a visible cleavage (Polozek & Ehrmann, 1998; Polozek, 2000). Titanaugite and pyroxenes without a visible cleavage are closely related to the McMurdo Volcanic Group, which is their source rock (Nathan & Schulte, 1968, Weiblein et al.,1981). In CRP-3, titanaugite and pyroxenes w i t h o u t a v i s i b l e c l e a v a g e a r e totally a b s e n t . T h e l a c k of t i t a n a u g i t e a n d u n c l e a v e d pyroxenes in C R P - 3 indicates that t h e M c M u r d o Volcanics did not contribute to the sediments of CRP- 3. This is in accordance with the preliminary clast a n d sand grain analyses, which detected n o input
f r o m the McMurdo Volc;mic Group to the C R P - 3 sediments (Sandroni & '1'iil;irico. tliis volume: Smellie, this volume). 'l'lnis, ~ l i c rocks ol" the western adjacent r . Ir~i~~siintiirctic Mountiiins are the likeliest source area.
I n general. all of (lie recorded heavy minerals of t h e Cenoxoic CRP-3 section ciin originate from rocks o f the Traiisiintarctic Monntiiins. Polozek & Elirmann
( 1998) and Polo/.ek (2000) indicate different sources for the heavy minerals f o u n d i n the CRP-l and t h e CRP-2/2A core.
Previous iiivestisatioiis (Neumann & El~rmaiin, this volume: Neumann & Ehrmann. 2000; Smellie, 2000:
Smellie, this volume) docurncnt a gradual change i n the litliology of the rocks eroded in the Transantarctic Mountains. I n an early phase, mainly sediments of the Beacon Supergroup acted as source rocks. This w a s followed by a phase with a dominance of basement- derived sediments i n CRP-212.4. The heavy mineral distribution of the CRP-3 core does not clearly reflect the postulated uplift history of the Transantarctic Mountains. For example, if we assume a stronger i n f l u e n c e of t h e b a s e m e n t rocks relative to t h e younger Cenozoic sediments of tlie CRP-3 core, t h e content of stable minerals should show an increasing, or at least a constant trend throughout the upper 200 mbsf. The origin of tlie abundant garnet below 200 m b s f a l s o g e n e r a t e s q u e s t i o n s . A l t h o u g h t h e investigated Beacon Sandstones samples from t h e bottom of CRP-3 and samples from the Transantarctic Mountains (unpublished data of the author) represent a minor section of tlie Beacon Supergroup. which i s believed to be a main source rock for the Cenozoic s e d i m e n t s of C R P - 3 b e t w e e n 7 9 0 a n d 4 2 0 mbsf' (Neumann & Ehrn~ann, this volunle), they contain n o o r at most very rare garnet. Smellie (this volume) pointed out that t h e basement rocks as t h e o t h e r source for garnet (Skinner & Ricker, 1968a, 1968b) represent <20% of the provenance area for the lower C e n o z o i c part of t h e C R P - 3 s e d i m e n t s . S u c h a restricted provenance area represent a contrast to the recorded garnet concentrations up to 70%. It will be a main goal of the further investigations to balance inconsistent trends and to solve these contradictions.
It is expected that erosion, transport, weathering and sorting have affected the heavy mineral distribution of the CRP-3 sediments.
T h e clearly subdivided absolute heavy mineral content of the samples (Fig. 1) could be a hint to the c h a r a c t e r of t h e main source rocks. T h e y o u n g e r sediments above 150 mbsf with an higher total heavy mineral content are probably immature first-cycle sediments and a result of mainly non sedimentary s o u r c e rocks (i.e. likely basement). L o w e r heavy m i n e r a l c o n t e n t s in t h e C e n o z o i c s e c t i o n b e l o w 150 mbsf are caused by multiple recycled sedimentary source rocks. Thus, the significantly decreased content of weathering - and transport-resistant pyroxenes between 650 and 200 mbsf can be interpreted as a result of repeated recycling of former recycled rocks.
.5 22 M . Neumann
Â¥nriche concentrations of the stable minerals can be also explained by multiple recycling. The contents of stable minerals and garnet are increased in tlic section i c l o w 2 0 0 m b s f , which is c o n t e r n p o n i ~ ~ e o ~ ~ s l y characterized by lower total heavy mineral amounts.
Interpreting these three distribution p i i t t c r ~ l ~ of the section 6 5 0 - 2 0 0 mbsf (lower total heavy mineral concentrations, decreased pyroxenes. increased stable minerals and garnet contents) as a result of repeated recycling, suggests that sedimentary rocks acted as the main source rock. Therefore the sandstones of the Beacon Supergroup are the likeliest source rock for that section of CRP-3. Lower pyroxene concentrations m a y be c a u s e d by s o u r c e r o c k s , which c o n t a i n primarily no or reduced pyroxene. The sandstones of the Taylor Group as part of the Beacon Supergroup are the most important rocks containing nearly n o pyroxene, respectively the lowest amount of Ferrar Dolerite s i l l s a s main s o u r c e f o r p y r o x e n e s . Continuing this idea the Taylor Group is the most probably source for the sediments of CRP-3 between 6 5 0 mbsf a n d 2 0 0 m b s f , troodly similar t o interpretations of CRP-3 sand grain modes by Smellie (this volun~e).
The carbonate-rich samples at 650 mbsf to 450 mbsf are relatively close to t h e fault zone at 539 mbsf. Probably the carbonates are authigenic and were precipitated from circulating water provided by the tectonic structures. The authigenic nature of the carbonates is supported by the fact that, in samples below 180 mbsf, pyroxenes are invariably replaced by smectite or rare carbonate (Cape Roberts Science Team, 2 0 0 0 ) . O n t h e other h a n d the B e a c o n Sandstones contain considerable amounts of carbonate in their heavy mineral fraction and are a alternative potential source rock for the Cenozoic section.
A C K N O W L E D G E M E N T S - T h e s a m p l e preparation f o r t h e d e n s i t y s e p a r a t i o n a n d h e a v y m i n e r a l i d e n t i f i c a t i o n w a s u n d e r t a k e n b y H . R h o d e s a t t h e A l f r e d W e g e n e r I n s t i t u t e f o r P o l a r a n d M a r i n e R e s e a r c h i n B r e m e r h a v e n . G e r m a n y a n d C . T s c h r i t t e r at t h e I n s t i t u t e o f G e o p h y s i c s a n d G e o l o g y i n L e i p z i g , G e r m a n y - t h a n k s f o r y o u r a s s i s t a n c e . S p e c i a l t h a n k s t o B . D i e k m a n n , A.C. M o r t o n . K. P o l o z e k . P. S c h r e c k a n d J.L. S m e l l i e f o r r e v i e w o r helpful hints a n d W. E h r m a n n f o r s o u r c i n g o f t h e f i n a n c i a l s u p p o r t b y t h e Deiifsche Fo~~sclz~i~~fsgemeinschaff.
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