Phosphorite associations on seamounts in the tropical southwest Pacific Ocean

Marine Geology, 71 (1986) 215--236

215

Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands

PHOSPHORITE ASSOCIATIONS ON SEAMOUNTS IN THE TROPICAL SOUTHWEST PACIFIC OCEAN

DAVID J. CULLEN and WILLIAM C. BURNETT

New Zealand Oceanographic Institute, P.O. Box 12-346, Wellington North (New Zealand) Department of Oceanography, Florida State University, Tallahassee, FL 32306 (U.S.A.) (Received May 14, 1985; accepted August 5, 1985)

ABSTRACT Cullen, D.J. and Burnett, W.C., 1986. Phosphorite associations on seamounts in the tropical southwest Pacific Ocean. Mar. Geol., 71: 215--236. Recent surveys have revealed two distinct types of submarine phosphorite on seamounts between latitudes 6°--22°S in the southwest Pacific Ocean. The more widely distributed type, comprising carbonate--fluorapatite and calcite of typical marine facies, is found on a number of isolated seamount peaks at depths between 1000--1650 m. A markedly different assemblage, in which dolomite is associated with carbonate--fluorapatite, has been discovered at depths of 550--1100 m on two flat-topped guyots on the northern margin of the Fiji Plateau. The latter assemblage is strongly reminiscent of the "insular" type of phosphorite that occurs subaerially on several tropical Pacific islands and atolls. It is possible that similar phosphorite deposits may lie beneath biogenic carbonate sediments on a series of shallow banks in line with the guyots. The mineral associations, geochemistry, textures and geological settings indicate that the guyot phosphorites may well represent submerged insular deposits, while the phosphorites on the isolated seamounts are entirely of marine origin.

INTRODUCTION

Within the last decade or so, phosphorites have been widely reported and described from seamounts in each of the major non-polar oceans. The earliest records, however, date back to the 1950's when phosphatised Tertiary foraminiferal limestones were sampled on Horizon and Cape Johnson guyots in the Mid-Pacific Mountains (Hamilton, 1956), and on Sylvania Guyot, near Bikini in the Marshall Islands {Hamilton and Rex, 1959). A summarised account of subsequent discoveries and studies of seamount phosphorites in the Pacific and elsewhere has been published by Baturin {1982). The only detailed description of seamount phosphorites in the southwest Pacific region, however, remains that of Slater and Goodwin (1973), dealing with the phosphate occurrences on guyots in the Tasman Sea. Three fundamentally different processes have been proposed to explain the divers occurrences of phosphates on seamounts: (1) inundation of "guanoderived" insular phosphate deposits; (2) metasomatic replacement of biogenic 0025-3227/86/$03.50

© 1986 Elsevier Science Publishers B.V.

216 calcium carbonate by carbonate--fluorapatite, the principal mineral phase of sedimentary phosphorites; and (3) emplacement of phosphate resulting directly from volcanic or hydrothermal activity. With regard to the inundation hypothesis, attention is drawn to the planed, presumably wave-truncated, crests of many seamounts, and the not infrequent occurrence on such surfaces of shallow-water fossil biota (Jones and Goddard, 1979). The conclusion seems inescapable that, at an earlier stage in their evolution, the crests of such seamounts probably lay at or just above sea level as emerged islands or atolls. Baturin {1982) clearly infers, from the proximity of some phosphorite-bearing seamounts to "guano" islands in the northeast Indian Ocean and the southern Caribbean Sea, a possible c o m m o n origin for these submerged and subaerial phosphatic deposits. As yet, however, no petrographic, geochemical or other form of data have been presented to substantiate such a relationship. On the other hand, there is widespread and convincing evidence, from uneroded deep seamount peaks, of diagenetic replacement of calcium carbonate by carbonate--fluorapatite at depths and in circumstances where there can be no question of involvement of avian guano (Heezen et al., 1973) or of direct volcanism. Evidently also, these seamount phosphorites formed through processes different from those that have led to phosphogenesis in areas of high productivity and coastal upwelling {Baturin, 1982; Burnett et al., 1983). Although the factors responsible for the replacement of carbonate by phosphate on oceanic seamounts are imperfectly understood, it is clear that the only likely source of phosphorus is the dissolved phosphate in the seawater itself. Since most calcium carbonate exposed to seawater is n o t converted to apatite, the replacement is here tentatively related to periods when the dissolved phosphate concentration below the superficial zone of mixing in the oceans was abnormally high. In this context, Arthur and Jenkyns (1981) note that the ages of seamount phosphorites, unlike those of major "phosphate events", may correspond in a general way to episodes dominated by widespread "oceanic anoxic events". It is possible that the coating and impregnation of the phosphorites by iron and manganese oxides may also relate to past widespread anoxia in the oceans (Frakes and Bolton, 1984). The concept of volcanogenic formation of phosphorites (Kharin, 1974) has not gained wide acceptance, and it would appear that the association of phosphates with pyroclasts and basalts, found n o t infrequently on seamounts, is little more than fortuitous. The association probably has no deeper significance than the relationship of phosphates on seamounts to their more c o m m o n l y encountered parent -- calcareous ooze - in that a suitable medium or microenvironment is provided for phosphate accumulation and diagenesis by the volcanic rocks. METHODS Echo-sounding and seismic profiling records were obtained during three cruises aboard the R.V. "Tangaroa", operated by the New Zealand

217 Oceanographic Institute. Excellent profiles of the guyots and some shallow banks were provided by sequential traverses using a Bolt single-channel airgun system and a 12 kHz echo-sounder. Open-boat traverses across the shallow banks and in atoll lagoons were also attempted with an E.G.&G. 300-J Uniboom system, but, on these occasions, recorder problems prevented the acquisition o f records of comparable high quality. Mineralogical analysis of samples recovered on the cruises was mostly by standard X-ray diffraction (XRD) procedures, with a few analyses by JEOL733 electron-probe microanalyser. Cell dimensions of apatites were determined by the XRD method described by McClellan (1980), and structural carbonate concentrations in the carbonate--fluorapatites were derived by Gulbrandsen's (1970) method. Microscale textures were observed using a Cambridge Stereoscan 250 Mk2 scanning electron microscope, with a LINK electron dispersive X-ray analyser for elemental determinations. Chemical analyses of several phosphatic samples were carried out using an automated Philips X-ray spectrometer system. To minimise matrix effects, glass discs were prepared from the samples, using lithium tetraborate, after ignition to 1000°C (Norrish and Hutton, 1969). Uranium-series isotopes were determined in the acid-soluble portions of selected samples by isotope dilution alpha spectrometry, using separation methods described elsewhere by Burnett and Veeh (1977). O B S E R V A T I O N S IN T H E S O U T H W E S T

PACIFIC

The general nature and distribution of insular phosphorites in the tropical southwest Pacific Ocean have been known for several decades, and have been ably summarised by Hutchinson (1950) and White and Warin (1964). Virtually unknown, however, have been the character and extent of submarine phosphatic deposits in this region. It was left to Cook (1975) to collate available data on the present-day bathymetry, physical oceanography, biological productivity and marine chemistry of the southwest Pacific in an attempt to determine the most favourable circumstances and localities for phosphogenesis, and hence to furnish some initial guidelines for prospecting for offshore phosphorite deposits. Although unable to cite specific examples, Cook (1975) acknowledged the probable occurrence on seamounts in the tropical southwest Pacific, of both submerged guano deposits and replacement phosphates, and it was indeed his synthesis that provided the essential stimulus for the present investigation. The present study started in 1980, with a survey of isolated seamounts in the general vicinity of the Samoa Basin, between latitudes 21°S (NiueRarotonga) and 7°S (i.e. to the north of the Tokelau and Northern Cook island groups). The summits and upper slopes of several rugged, peaked seamounts were dredged, and ferromanganese-encrusted fragments of phosphatised foraminiferal ooze were recovered from many. Low phosphorus values (<:5%) were encountered on seamounts in the southern part of the region, for example on Falealupo Seamount (1280 m) west of Western

218

Samoa, on Eclipse Seamount (1655 m) near Aitutaki, and on Tom Davis Seamount (1032 m) west-southwest of Rarotonga (Fig.l). Surprisingly, no phosphates were recovered from flat-topped Capricorn Seamount, on the subducting eastern margin of the Tonga Trench west of Niue, although Miocene limestone does occur on this feature (Brodie, 1965). Higher phosphorus values (>10%) were found only on seamounts north of latitude 10°S, as on Kalolo Seamount (1325 m) northwest of the Tokelau Islands, and on Albert Henry Seamount (1373 m) north of the Northern Cook Islands. At the latter location, incidentally, the ferromanganese encrustation attains the maximum thickness (~ 40 mm) encountered in the entire sampling programme. A second survey in 1982 concentrated on seamounts and shallow banks on the Fiji Plateau, and especially along its northern margin where a Kal~o Seamount Q551

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219

discontinuous chain of shallow ( 1 8 - 2 7 m), flat-topped banks extends for some 1400 km between longitudes 172°W and 176°E (Fig.2). Because of a thick mantle of unconsolidated foraminiferal sand, pteropod ooze and algal (Halimeda) grit, it proved impossible to obtain indurated rock samples from the summits and upper slopes of the shallow banks. However, it was discovered that, interspersed among the banks, were occasional deeper, fiatt o p p e d seamounts (or guyots) clearly analogous to the shallow banks but tectonically lowered to depths of 600--1300 m. At these depths the guyots had largely escaped blanketing by biogenic debris, and it proved possible to recover small samples of coral and indurated coralline grit from them, much of which proved to be highly phosphatic. As on the peaked seamounts, the phosphorite is encrusted with iron and manganese oxides, but the encrustations on the guyots tend to be noticeably thinner. A third and final survey, in 1983, concentrated on a detailed bathymetric and structural study of MacLeod and Solomoni guyots, on the northern margin of the Fiji Plateau. The objective of this survey was the structural and morphological comparison of the guyots with neighbouring flat-topped shallow banks and with nearby modern atolls, in order to determine whether these three types of oceanic feature could have had a c o m m o n origin. BATHYMETRY

AND STRUCTURE

Peaked seamounts The seamounts investigated during the present survey are perfectly normal, isolated, oceanic seamounts that rise abruptly from the abyssal sea floor in depths exceeding 5000 m, to peak between 1000--1400 m below sea level. The lower slopes -- probably mantled by volcaniclastic talus - - t e n d to be generally smooth with gradients in the order of 6--9 ° (1:10--1:6.5): the summits are more irregular and interspersed with sharp to rounded volcanicrock protuberances. B o t t o m photography (Fig.3a) and sampling reveal that the only sediment accumulation in these shallower areas of uneven relief is in the form of small pockets of foraminiferal ooze. These accumulations provide the only calcareous parent sediment available for phosphatisation. Nowhere was the ooze sufficiently thick to register as even a restricted planar surface on echograms traversing the seamount summits; nor could superficial sediment accumulations be detected in low-resolution profiling records. The sediment trapped in areas of irregular microrelief on the seamount summits probably does not exceed a few tens of centimetres in thickness.

Guyots Although morphologically the guyots on the northern margin of the Fiji Plateau have m u c h in c o m m o n with the peaked seamounts, it is not merely their planed, truncated summits that render them so different. Both the bases

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Fig.3. B o t t o m p h o t o g r a p h s f r o m a t r o p i c a l Pacific s e a m o u n t a n d a guyot, a. S t a t i o n Q 5 2 8 o n Kalolo S e a m o u n t ( 1 3 2 5 m), s h o w i n g t h e u n e v e n sea-floor surface w i t h an o u t c r o p p i n g mass o f M n - c o a t e d volcanic r o c k a n d a p o c k e t o f r i p p l e - m a r k e d pale o o z e . b. S t a t i o n T 3 2 1 o n S o l o m o n i G u y o t ( 7 0 0 m ) , s h o w i n g t h e relatively s m o o t h surface of i n d u r a t e d r e e f - t y p e s e d i m e n t a n d t h e very t h i n d i s c o n t i n u o u s m a n t l e o f pale u n c o n s o l i d a t e d ooze. T h e base of each photograph represents approximately one metre.

223 and the summits of the guyots lie at appreciably shallower depths than those of their peaked counterparts, the guyot bases rising from the sea floor in a b o u t 3000--4000 m of water, and the summits attaining minimum depths of 600--1000 m. The planed summit surfaces are roughly equidimensional. The "diameter" of MacLeod G u y o t is about 35 km; Solomoni G u y o t is somewhat larger, and a b o u t 40 km across. Both have subsidiary plateaux (approximately 18 × 6 km) to the north (Fig. 2). Moreover, the guyots are not isolated like the peaked seamounts, but lie within a well-defined belt of shallow banks (Fig.2) -- also characterised by planed summits -- apparently sited along an upward-bulged segment of oceanic crust on the northern flank of the Fiji Plateau. Indeed, there seems little d o u b t that the guyots are subsided analogues of the associated shallow banks. Gentle tilting during subsidence -- downward to the southwest on 8olomoni G u y o t and to the southeast on MacLeod G u y o t -- is evident from both echograms and acoustic profiles, as is the geomorphological expression of faulting that presumably also accompanied the subsidence. Another significant feature that differentiates the guyots from the peaked seamounts is the occurrence, on the planed summits of the former, of thick sediment accumulations. These are readily recognised in airgun profiles (Fig.4) traversing both MacLeod and Solomoni guyots, where they attain a maximum thickness of about 800--1000 m. Two sedimentary units can be distinguished in the profiles. The lower unit has an irregular base, and is up to 700 m thick. It probably comprises volcaniclastic debris, infilling the original volcanic relief as a consequence of the planation process. Superposed u p o n it is an upper unit, some 120--200 m thick on MacLeod G u y o t and up to 400 m thick on Solomoni Guyot, overlain by a thin discontinuous blanket of unconsolidated ooze. The upper unit's surface, which outcrops locally as a slightly raised rim to the plateau, is smooth and extremely hard (Fig.3b), like the wave-swept reef-flat bordering modern emerged atolls. Such small samples as have been recovered appear to be of massive reef-building coral. So far as can be judged from a comparison of the seismic profiling records from the guyots with those from neighbouring banks -- such as Combe (18 m), Tuscarora (26 m), Bayonnaise (18--29 m), Kosciusko (18--22 m) and Macaw (18 m) -- and also from Funafuti Lagoon, the superficial structures in all of these features are similar (Fig.4). Indeed, there can be little d o u b t that, before subsidence and inundation, both the guyot summits and the nearby shallow banks comprised typical coral atolls enclosing large lagoons. ANALYTICAL RESULTS

Mineralogy The observed environmental and geological differences between the isolated, peaked seamounts and the somewhat shallower, flat-topped guyots are reflected by contrasts in the mineralogical facies of the phosphorite deposits occurring in the t w o situations. In particular, the differences involve

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225

the crystal size and crystal habit of the apatite itself, and the nature of its association with other minerals. In the case of phosphorites on the peaked seamounts, c a r b o n a t e - f l u o r apatite is associated with calcite in the form of coccolith platelets and foraminiferal tests, and as subhedral to anhedral interstitial crystalline aggregates. The apatite itself tends to occur as masses of relatively coarse subhedral crystals, 1.5--3.0 ~m across, intergrown with the interstitial calcite. In cavities and less c o m p a c t areas of the phosphorite, well
Na 9.57

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Fig.5. SEM micrographs o f carbonate--fluorapatite from peaked seamounts and guyots. a. Large, squat, euhedral crystals in a cavity (Stn. Q556 - Albert Henry Seamount). Scale bar denotes 2 ~m. b. Similar crystals, intergrown (Stn. Q555 - Albert Henry Seamount). c. Smaller, more equidimensional crystals (Stn. T303 -- MacLeod Guyot). d. Microcrystalline carbonate--fluorapatite forming rhabdolithic structures (Stn. T309 -- MacLeod Guyot).

226 with a structural CO2 content a little over 5% by weight. The mineral is clearly a typical carbonate--fluorapatite of the type that normally occurs in sedimentary marine deposits. In contrast, the apatite in phosphorites retrieved from MacLeod and Solomoni guyots is manifestly more finely crystalline, with dimensions almost exclusively less than 1 pro. Scattered patches occur of short, stubby apatite crystals with roughly equidimensional a and c axes, i.e., with proportionately longer c axes than in the apatites on peaked seamounts (Fig.5c). In their smaller size and more equidimensional shape, the guyot apatites approach the form of apatite that typifies the insular phosphorites (Fig.6d). Botryoidal aggregates of this form of apatite are c o m m o n as rims around the other minerals p r e s e n t - especially dolomite (Fig.6b) - - a n d infrequent rhabdolithic structures have also been observed (Fig.5d) in the guyot samples. Minerals associated with the guyot apatite include anhedral to subhedral calcite grains in modest amounts, and -- most significantly - dolomite. The latter occurs as extensive patches of relatively large (20--30 ~m), euhedral to subhedral r h o m b o h e d r a (Fig.6a).

Fig.6.8EM micrographs o f dolomite and carbonate--fluorapatite from guyot and "insular" deposits, a. Large euhedral dolomite crystals (Stn. T303 --MacLeod Guyot). b. Subhedral dolomite rimmed by small botryoidal aggregates of carbonate--fluorapatite (Stn. T309 -MacLeod Guyot). c. Large euhedral dolomite crystals, with minor calcite and apatite (Stn. T325 -- Alexa Bank). d. Minute acicular apatite crystals, seen mostly in cross-section (insular phosphorite from Nauru, equatorial western Pacific Ocean).

227 Rather surprisingly, in view of the differences in crystal size and morphology, the estimated composition -- based on cell dimension data for guyot sample T315 -- indicates the following formula: Ca

Na 9.49

Mg 0.36

(PO4) 0.14

(CO3) 4.71

F 1.29 2.52

The mineral is again clearly a carbonate--fluorapatite with a CO2 content just below 6% by weight, and chemical composition not significantly different from that of the apatite on peaked seamounts.

Geochemistry Samples from stations Q551 (Kalolo Seamount) and Q556 (Albert Henry Seamount} were partially analysed chemically and found to contain normal marine sedimentary apatites with F/P2Os.weight ratios between 0.124-0.127 and structural carbonate contents of between 5.5--6.5 wt.% as CO2. Electron microprobe analysis has also provided a sequential series of partial analyses for a sample from Station Q551 (Table 1). Samples from the guyots and banks along the northern margin of the Fiji Plateau were more completely analysed (Table 2). The major element analyses of these samples reflect the varying proportions of the major mineral phases present, i.e., calcite, carbonate--fluorapatite, dolomite, and ferromanganese oxides. A few samples from the guyots proved to be almost pure dolomite, although most had at least some associated phosphatic material as well. Interestingly, a sample (T325H) from the southern slopes of Alexa and Turpie banks, west of Solomoni Guyot, is also composed of practically pure dolomite (Fig.6c), and serves as a strong indication that a comparable apatite-dolomite association may occur beneath the thick biogenic sediments that mantle the aligned shallow banks. Because of the extreme paucity of "basement" samples from the shallow banks, however, the presence of phosphorite in that setting remains to be confirmed. Concentrations of a few trace elements were also determined in the guyot samples (Table 2). Of those determined, uranium seems to be the only one whose concentration is mainly controlled by the amount of carbonate-fluorapatite. Thorium is probably present to some degree in the apatite, but is also influenced by the occurrence of ferromanganese material. The remaining elements, Y, La, and Ce, are apparently most affected by the relative abundances of Fe and Mn oxides. In T310, the purest sample with respect to carbonate--fluorapatite, iron and manganese reach relatively low values, second only to sample T325H which is virtually pure dolomite with an extremely low trace-element content. The rare-earth element (REE) content of guyot apatites appears to be quite low compared to REE abundances reported from isolated seamounts (Burnett et al., 1983}, but not as low as the values in insular phosphorite (D.Z. Piper, pets. commun., 1985}. While isolated, peaked seamount samples tend to have Ce/La ratios comparable to that of seawater, the guyot apatite appears to be enriched in Ce relative to La.

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229 TABLE 2 Chemical composition of selected samples from North Fiji Plateau guyots. All major oxides given as weight percent. Trace elements given as parts per million (ppm). Loss on ignition (LOI) represents weight loss at 1000° C MacLeod Guyot T-303

T-308

Solomoni Guyot T-309

T-310

T-313

T-315

Alexa Bank T-319

T-325H

sio 2 TiO 2 Al203 Fe203 MnO MgO CaO Na:O K:O P2Os LOI

1.30 0.55 0.69 9.76 6.40 8.70 27.16 0.36 0.07 6.82 35.35

1.79 0.66 0.57 13.53 11.82 4.64 20.26 0.67 0.13 6.19 35.18

1.66 0.61 1.29 7.24 4.58 5.98 37.48 0.28 0.05 14.57 23.75

<0.01 0.06 0.08 1.09 0.72 5.37 45.69 0.67 0.13 16.42 27.66

0.17 0.28 0.67 4.64 2.20 7.63 38.58 0.17 0.05 9.25 34.61

0.30 0.34 0.63 4.35 2.61 6.17 40.37 0.39 0.09 14.54 27.36

0.44 0.33 0.62 5.33 2.75 10.08 34.94 0.03 0.03 0.56 44.13

0.07 0.02 0.07 0.34 0.20 17.63 35.17 <0.04 0.02 0.13 46.58

Total:

97.16

95.06

97.49

97.90

98.25

97.15

99.24

100.27

U Th Y La Ce U/P:O s (Xl0-')

11 3.8 101 192 335 1.6

15 4.7 169 449 581 2.4

14 <2 166 97 224 1.0

28 <2 62 26 41 1.7

11 3.1 220 78 93 1.2

19 2.9 69 51 131 1.3

<2 2.7 60 46 123 --

<2 <2 4.3 4 5 --

The f a c t t h a t c a r b o n a t e - - f l u o r a p a t i t e appears t o be t h e o n l y significant h o s t f o r u r a n i u m in the g u y o t samples is s h o w n b y the relatively n a r r o w range in their U / P : O s ratios. T h e s e ratios, w h i c h range b e t w e e n 1 . 0 - - 2 . 4 × 10 -4 , are a p p r o x i m a t e l y o n e o r d e r o f m a g n i t u d e higher t h a n t h o s e m e a s u r e d f o r apatite samples f r o m t h e isolated s e a m o u n t s (Table 3). S e a m o u n t samples invariably have u r a n i u m c o n c e n t r a t i o n s o f a r o u n d 5 p p m , while t h e phosp h a t i c g u y o t samples a n a l y s e d all c o n t a i n m o r e t h a n 11 p p m . It is interesting t h a t t h e range in U/P2Os values m e a s u r e d f o r t h e g u y o t samples is q u i t e close t o t h a t o b s e r v e d for insular p h o s p h o r i t e s f r o m the intensely p h o s p h a t i s e d islands o f N a u r u a n d M a k a t e a ( R o e and B u r n e t t , 1 9 8 5 ) .

Uranium-series isotopes and ages In o r d e r t o d e t e r m i n e w h e t h e r a n y o f t h e s e a m o u n t or g u y o t p h o s p h a t i c r o c k s were f o r m e d r e c e n t l y , a few characteristic samples were analysed b y uranium-series d a t i n g t e c h n i q u e s . I s o t o p e d a t a f r o m t h r e e samples f r o m this s t u d y (one f r o m a p e a k e d s e a m o u n t and t w o f r o m g u y o t s ) , and f r o m a sample f r o m N e c k e r Bank, are p r e s e n t e d in Table 3 t o g e t h e r with t h e range and average o f values f r o m seven samples o f n o r t h Pacific s e a m o u n t p h o s p h o r i t e s . All m e a s u r e d 234U/2~sU activity ratios are indistinguishable f r o m o r slightly

230 TABLE3 Uranium-series isotopes in samples from one isolated seamount, two guyots, and one from the Necker Bank, Central Pacific. Also shown for comparison is the range and average of seven samples from the Musicians Seamounts, North Pacific. Quoted errors are based on counting statistics Type/Sample No.

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0.32

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1.15

1.014+0.005 1.00 ± 0.01

1.52-+ 0.03

1.69

1.07 -+ 0.03

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0.94-3.01 1.45

0.16--0.30 0.24

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n=7 Range: Average:

3.7--7.8 5.8

0.5--5.8 2.5

b e l o w secular e q u i l i b r i u m , indicating t h a t t h e s e s a m p l e s are at least several h u n d r e d s o f t h o u s a n d s o f y e a r s old. T h e " e x c e s s " 23°Th i n d i c a t e d b y 23°Th/ 234U activity r a t i o s g r e a t e r t h a n 1.0 is a c o n s e q u e n c e o f c o n t a m i n a t i n g ferrom a n g a n e s e m a t e r i a l e n r i c h e d in 23°Th. A l t h o u g h we c a n n o t calculate ages f r o m t h e s e d a t a , t h e u r a n i u m - s e r i e s analyses did c o n f i r m t h e d i s t i n c t l y differe n t c h a r a c t e r s o f t h e s e a m o u n t and g u y o t p h o s p h o r i t e s in t e r m s o f t h e i r U/P2Os w e i g h t ratios. All of t h e s e a m o u n t samples, including t h o s e f r o m t h e n o r t h Pacific, s h o w low ratios, while t h e s a m p l e s f r o m t h e g u y o t s a n d N e c k e r B a n k display relatively high U/P2Os ratios. U n f o r t u n a t e l y , m o s t o f t h e s a m p l e s e x a m i n e d so far are d e v o i d o f m i c r o fossils t h a t c o u l d be diagnostic f o r establishing m a x i m u m ages. T h e o n l y s a m p l e ( Q 5 5 1 ) f o r w h i c h we h a v e m a n a g e d t o d e t e r m i n e a b i o s t r a t i g r a p h i c age is f r o m A l b e r t H e n r y S e a m o u n t , the isolated s e a m o u n t n e a r P u k a p u k a , in t h e N o r t h e r n C o o k Islands. T h e p r e s e n c e o f t h e f o r a m i n i f e r , Pulleniatina obliquiloculata, in this s a m p l e indicates a m a x i m u m age o f 2 . 8 - 3 . 0 m . y . {i.e. L a t e Pliocene). This is m a r k e d l y y o u n g e r t h a n t h e L a t e C r e t a c e o u s a n d E o c e n e s e a m o u n t p h o s p h o r i t e s d r e d g e d f r o m t h e n o r t h Pacific ( H e e z e n et al., 1973). T H E DOLOMITE---APATITE

ASSOCIATION

T h e paragenesis o f t h e m i n e r a l d o l o m i t e in t h e m a r i n e e n v i r o n m e n t has long b e e n t h e s u b j e c t o f e n q u i r y a n d c o n t r o v e r s y , a n d , in so far as an understanding o f it m a y h e l p t o clarify t h e origin o f t h e a s s o c i a t e d p h o s p h o r i t e , it w a r r a n t s f u r t h e r appraisal here.

231 In association with carbonate--fluorapatite, dolomite characterises the insular phosphorite deposits that have formed - s u p p o s e d l y as a reaction p r o d u c t of guano on underlying reef limestone - on Pacific islands such as Nauru, Ocean Island and Makatea, b u t it is found also in non-phosphatic deposits elsewhere -- as, for instance, on Niue. Schlanger (1965) has linked the occurrence of dolomite to diagenesis by Mg-rich brines generated by evaporation in small (diameter < 6 miles) atoll lagoons. In this context, it is interesting to note that more recently Aharon and Veeh (1984) have advocated, on the basis of stable oxygen and carbon isotope studies, increased evaporation and diminished rainfall as important factors in the formation of insular phosphorites. Aridity and periodic desiccation also feature largely in the depositional environment of dolomite and p r o t o d o l o m i t e in the "classic" sabka localities in the Persian Gulf (Curtis et al., 1963; McKenzie et al., 1980} and the Red Sea (Friedman, 1980), and in the South Australian Coorong (Von der Borch, 1965; Von der Borch et al., 1975). Gebelein et al. {1980) have reported a somewhat different occurrence of p r o t o d o l o m i t e in Holocene calcareous sediments beneath (and in lithified crusts surrounding) vegetated metre-high ridges on the extensive tidal flats of Andros Island in the Bahamas. The dolomitisation here may be related to mixing of fresh water, in lenses beneath the ridges, with surrounding groundwater of more normal marine salinity, although such an interpretation would probably be disputed by Friedman (1980). While all of these occurrences formed either above or just below sea level, several researchers have pointed to evidence that could imply a deep marine origin for some d o l o m i t e - a p a t i t e occurrences. Marlowe (1971), for instance, regards the intimate association of dolomitic phosphorites in the eastern Caribbean with iron and manganese oxides as indicative of formation in deep water. Other features of the occurrence in this region however, such as the coarse conglomeratic texture of the deposits and an abundant flora of shallow-water algae naturally enriched in magnesium (see Chave, 1954), are more consistent with emergent or very shallow conditions and with dolomitisation in an "intra-reef" environment as postulated by Fairbridge (1957). In this case, the impregnation by iron and manganese oxides may merely reflect sea-floor subsidence, after primary deposition of the dolomite and apatite at shallower depths. Perhaps it is pertinent to mention here the empirical results of Gulbrandsen et al. (1984) in synthesising carbonate--fluorapatite in sea water. An intermediate phase in the process involved the spontaneous crystallisation of a magnesium phosphate, and it became evident that time is a major factor in suppressing the inhibiting effect of magnesium upon apatite formation. The sequential recrystallisations also illustrate potential intimate inter-relationships of calcium, magnesium and phosphorus in the generation of phosphate minerals, and m a y help to explain the d o l o m i t e - a p a t i t e association encountered in insular and marine deposits. Different mechanisms have been advanced by Repellin (1977), Rodgers et al. {1982)and Saller ( 1 9 8 4 ) t o explain subsurface dolomitisation, unrelated

232 to phosphatisation, on Mururoa Atoll, Niue, and Enewetak Atoll, respectively. The dolomite on Mururoa and Niue is believed to have crystallised in underground contact zones between fresh-water lenses (presumably of meteoric origin) and percolating sea water. Similar diagenetic processes have also been reviewed by Matthews (1974). On Enewetak, Saller (1984) envisages dolomite emplacement in Eocene strata (1250--.1400 m below sea level) by cold oceanic water, supersaturated with respect to dolomite, permeating the base of the sediment pile since Miocene times. He cites correspondence between tidal fluctuations and temperature profiles in onshore wells and in the open sea as support for his interpretation. However, Saller provides no comparison of the salinity or chemistry of water in the wells with the ambient sea water: he does not reconcile his finding with that of Gomberg and Bonatti (1970), who describe leaching of magnesium from calcite during recrystallisation by deep oceanic water: and he does n o t explain the occurrence of dolomite in Miocene strata at shallower levels, up to - 3 8 0 m, on Enewetak. Although the dolomite at both levels on the atoll is regarded by Schlanger (1957) as the end-product of crystallographic re-ordering of the Ca--Mg solid solution that comprised the original high-magnesium algal calcite, Saller (1984) points out that coralline algae tend to lose their original magnesium with burial, and that on Enewetak up to 80% may have dissipated by the time the algae reach a depth of 400 m. Dolomitisation in quite different environments - on open marine slopes in water depths between 200--2000 m -- has recently been described by Kelts and McKenzie (1982, 1984) in the Gulf of California, and by Mullins et al. (1984) off Little Bahama Bank. In both instances, diagenetic dolomite or protodolomite occurs, 0.9--1.2 m below the sediment--water interface, in hemipelagic oozes -- an anoxic diatomaceous m u d d y ooze rich in organic carbon in the Gulf of California, and a calcareous ooze off Little Bahama Bank. The geographic proximity of the latter occurrence to the locus of tidalflat dolomitisation on Andros Island (Gebelein et al., 1980) may be worthy of note. However, it may be that dolomite was able to form in both the California and Bahamas deep-water environments because the inhibiting effect of dissolved sulphate had been removed during microbial sulphate reduction (Baker and Kastner, 1981). Although this t y p e of marine environm e n t could also produce diagenetic carbonate--fluorapatite, the facies associations (high organic matter, silica, etc.) bear little resemblance to the dolomite--apatite association encountered on the southwest Pacific guyots. CONCLUSIONS Although Baker and Kastner (1981) have demonstrated experimentally that there is little to inhibit dolomitisation in the deep marine environment provided sulphate concentrations are low and concurrent silica diagenesis is minimal, the fact remains t h a t known instances of deep-sea dolomitisation are extremely rare, and most Neogene dolomites formed in very shallow or even supratidal environments. The chances are, therefore, that the dolomite

233

part at least of the dolomite--apatite association on MacLeod and Solomoni guyots in the southwest Pacific relates to shallow-water/lagoonal or actual subaerial conditions of deposition that predated the subsidence and final inundation of the guyots. To summarise, t w o distinct phosphorite associations are recognised on elevated areas in the southwest Pacific. The first comprises phosphatised indurated and semi-indurated foraminiferal oozes that occur on the summits and upper slopes of isolated peaked seamounts, in depths between 1000 and 1650 m. These phosphorites are assumed to have formed in the open marine environment at their present or shallower depths, by partial replacement of pre-existing calcium carbonate by carbonate--fluorapatite. It is possible that the phosphate, as well as some of the iron and manganese oxides, were emplaced during past periods of an expanded oxygen-minimum layer in the ocean. The other association, of carbonate--fluorapatite and dolomite, occurs at somewhat shallower depths (550--1100 m) on two guyots lying within a line of shallow banks north of Fiji. It seems likely that the dolomite of MacLeod and Solomoni guyots relates to shallow-water lagoonal or actual subaerial conditions of deposition that predated the subsidence and final inundation of these guyots. It is difficult to tell how great a role evaporation and desiccation in partly enclosed lagoons played in the formation of the apatite, but we do note from our SEM studies that the relationship between the two phases is such that the apatite emplacement obviously postdated crystallisation of the dolomite. We suggest, therefore, that the dolomite--apatite assemblage discovered on these guyots is akin to the "insular" t y p e of phosphorite TABLE4 General characteristics o f tropical s o u t h w e s t Pacific Ocean Seamount, insular, and g u y o t phosphorites. Features listed represent those m o s t c o m m o n l y observed Characteristic

Microtextures: Crystal size (urn) Shape

Associated minerals:

Geochemistry: F/P20 s CARFAP-CO 2 REE U / P 2 0 s (10 -4)

Environment Seamount

Island

0.X--X short, hexagonal platelets

0.0X--0.X various, m a n y secondary forms

calcite volcanics F e / M n crusts

calcite dolomite gypsum

~ 0.12 high ( ~ 6%) high a 0.2--0.3

a B u r n e t t et ai. (1983); bpiper (unpublished).

<0.089 low ( - 1% ) low b ~2.0

Guyot

<0.X stubby, e q u a n t crystals; rhabdoliths calcite dolomite F e / M n crusts ~ 0.1 high ( - 6% ) low 1.0--2.4

234

deposit, and formed in an atoll environment close to sea level prior to subsidence and final inundation. The respective characteristics of the seamount and guyot phosphorites discussed in this paper, together with those of insular phosphates, are summarised in Table 4. Our surveys indicate that many of the shallow banks may also represent submerged atolls that could bear "insular" phosphorite deposits. The recovery of a highly dolomitic sample from the flanks of one of the banks supports this concept. Further investigation of the line of shallow banks along the northern margin of the Fiji Plateau is therefore recommended to establish whether "insular" phosphorites do occur on any of them, and whether, in view of the extremely shallow depths of many of the banks, deposits exist that could have economic potential. ACKNOWLEDGEMENTS

The authors are indebted to the officers and crew of the New Zealand Oceanographic Institute's research vessel "Tangaroa", for their invaluable assistance throughout the sea-borne phases of this investigation. Analytical expertise was provided by Kenneth Palmer (Victoria University, Wellington), Graham Walker (Physics and Engineering Laboratory, DSIR), Mark Bowden and Ian Brown (Chemistry Division, DSIR), Alva Challis (N.Z. Geological Survey), and Philippa Black (University of Auckland). Marlene Clark (Florida State University) identified the foraminifera in sample Q551. Supplemental financial support was provided through the US-NZ Cooperative Science Programme (NSF Grants NSF INT8111783 and NZ84-110). This study is a contribution to the International Geological Correlation Programme Project 156 (Phosphorites). REFERENCES Aharon, P. and Veeh, H.H., 1984. Isotope studies of insular phosphates explain atoll phosphatisation. Nature, 309: 614--617. Arthur, M.A. and Jenkyns, H.C., 1981. Phosphorites and paleoceanography. Oceanol. Acta, 4 (Suppl.): 83--96. Baker, P.A. and Kastner, M., 1981. Constraints on the formation of sedimentary dolomite. Science, 213: 214--216. Baturin, G.N., 1982. Phosphorites on the Seafloor: Origin, Composition and Distribution. Elsevier, Amsterdam, 343 pp. Brodie, J.W., 1965. Capricorn Seamount, south-west Pacific Ocean. Trans. R. Soc. N.Z., Geol., 3: 151--158. Burnett, W.C. and Veeh, H.H., 1977. Uranium-series disequilibrium studies in phosphorite nodules from the west coast of South America. Geochim. Cosmochim. Acta, 4: 755-764. Burnett, W.C., Roe, K.K. and Piper, D.Z., 1983. Upwelling and phosphorite formation in the ocean. In: E. Suess and J. Thiede (Editors), Coastal Upwelling -- its Sediment Record. Plenum, New York, N.Y., pp.377--397. Chave, K.E., 1954. Aspects of the biogeochemistry of magnesium. 1. Calcareous marine organisms. J. Geol., 62: 266--283.

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