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Geochemical evidence for the Trindade hotspot trace: Columbia seamount ankaramite
Lithos 51 Ž2000. 293–304 www.elsevier.nlrlocaterlithos
Geochemical evidence for the Trindade hotspot trace: Columbia seamount ankaramite R.V. Fodor a,) , B.B. Hanan b a
Department of Marine, Earth and Atmospheric Sciences, North Carolina State UniÕersity, P.O.Box 8208, Raleigh, NC 27695, USA b Department of Geological Sciences, San Diego State UniÕersity, San Diego, CA 92182, USA Received 5 August 1999; accepted 26 November 1999
Abstract The Columbia seamount ; 825 km offshore from Brazil at ; 208S lies on the east–west ‘trace’ of the Trindade hotspot. Continental and oceanic magmatism believed to have originated with this hotspot is alkalic and SiO 2-undersaturated, and dates from ; 85 Ma in southern Brazil to - 3 Ma on the islands of Trindade and Martin Vaz ; 1100 km offshore. An ankaramite Žclinopyroxene ; 16 vol%. dredged from Columbia seamount Žest. 10 Ma. conforms to this geochemistry with SiO 2-undersaturated Al-rich clinopyroxene Ž8–13 wt.% Al 2 O 3 . and rhonite. Clinopyroxene isotopic compositions are ¨ 87 Srr86 Sr s 0.703900, 143 Ndr144 Nd s 0.512786, 206 Pbr204 Pb s 19.190, 207 Pbr204 Pb s 15.045, and 208 Pbr204 Pb s 39.242 — resembling those for Trindade, except for slightly higher 207 Pbr204 Pb. The isotopic composition and abundance ratios among weathering-resistant Nb, La, and Yb suggest that Columbia seamount magmatism represents the present-day Trindade plume, but ; 10 million years earlier and perhaps when the plume manifested a signature of ‘contamination’ from subducted sediments. The Columbia seamount analyses provide the first quantitative assessment for the Trindade hotspot trace existing between the Brazil margin and Trindade, strengthening the case for a continuum of magmatism extending from the ; 85 Ma Brazilian igneous provinces of Poxoreu ´ and Alto Paranaiba. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Trindade hotspot; Brazil magmatism; Columbia seamount; Ankaramite; Pb isotopes
1. Introduction Several studies assign the origins of alkalic igneous provinces in southern Brazil ŽUlbrich and Gomes, 1981. to the Trindade Island Žand Martin Vaz Island. hotspot located ; 1100 km east of Brazil at ; 208S ŽFig. 1.. As first proposed by Herz Ž1977., Cretaceous Brazilian alkalic and carbonatitic )
Corresponding author. fax: q1-919-515-7802. E-mail addresses: rfodor@ ncsu.edu Ž R.V. [email protected] ŽB.B. Hanan..
Fodor . ,
provinces in southeastern Brazil developed when the South American plate passed westward over the Trindade hotspot. Crough et al. Ž1980., Hartnandy and Le Roex Ž1985., O’Connor and Duncan Ž1990., and Toyoda et al. Ž1994. all acknowledged this model. Gibson et al. Ž1995; 1997. interpreted the high-K, high-Mg Alto Paranaiba igneous province and the Poxoreu ´ alkalic basalt province — both in Brazil, northwest of Rio de Janeiro ŽFig. 1. — as having origins attributed to the Trindade hotspot ; 85 million years ago when the Trindade plume head impacted subcontinental lithosphere. More re-
0024-4937r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 Ž 0 0 . 0 0 0 0 2 - 5
294 R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304 Fig. 1. Map showing the Trindade hotspot ‘trace’ as depicted by the chain of seamounts extending westward from Trindade and Martin Vaz islands — the present location of the Trindade hotspot — to the Brazil margin, and into Brazil to the two ;85 Ma igneous provinces of Alto Paranaiba and Poxoreu ´ ŽGibson et al., 1995, 1997.. The triangle on Columbia seamount is the location of the dredge that recovered ankaramite from the seamount. ESS-9 is a diabase sample, ; 43 Ma, from beneath the Abrolhos platform and near the basaltic Abrohlos Islands Ž40–50 Ma. ŽFodor et al., 1983, 1989., a province also relevant to the Trindade hotspot. A portion of the Parana´ continental flood-basalt province Ž ;130–135 Ma. coincides with the western end of the ‘trace’.
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
cently Thompson et al. Ž1998. assigned Eocene magmatism in southern Brazil to deflection of the Trindade plume head. The present-day Trindade hotspot manifests highly undersaturated alkalic lavas and plutonic rocks such as basanites, nephelinites, phonolites, and ankaratrites ŽHerz, 1977; Weaver, 1990.. An east–west line of seamounts extending westward from Trindade to the Brazil margin at ; 208S ŽFig. 1. forms a convincing hotspot ‘trace’ created by continuous igneous activity from the time when southeastern Brazil passed over a mantle plume fixed at the location of Trindade until present times. The ‘trace’ accordingly suggests a genetic relationship between late Cretaceous igneous provinces in Brazil, produced from the plume-head, and a string of seamounts and islands produced from the plume-tail. In contrast, VanDecar et al. Ž1995. suggest that the late Cretaceous alkalic province in southeastern Brazil represents final magmatism associated with the Parana´ flood basalts ; 130–135 Ma. They conjecture that ; 85 Ma, the dying heat source yielded alkalic magmas in the northern extremity of the Parana´ province, at ; 208S ŽFig. 1.. In this view, VanDecar et al. Ž1995. discount any hotspot role in the origins of igneous rocks - 85 Ma that form part of the trace between Trindade and Brazil. Instead, the seamounts west of Trindade and Martin Vaz ŽFig. 1. islands formed along a ‘leaky’ transform fault. Knowing the magma types among the seamounts offshore Brazil and their isotopic compositions are necessary for documenting the role of the Trindade hotspot in magmatism within and offshore Brazil ; 208S. We provide data for the compositional gap between the two end-points of the ; 2500 km Trindade trace Žfrom Poxoreu ´ province in Brazil to Trindade Island; Fig. 1. by characterizing a lava from Columbia seamount in terms of whole-rock, mineral, and Sr, Nd, and Pb isotopic compositions. Columbia seamount is on the Trindade trace ; 825 km offshore at ; 208S ŽFig. 1..
2. Background information The chain of seamounts between Trindade and Brazil is called the Vitoria-Trindade seamount chain
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or the Vitoria-Trindade lineament ŽAsmus and Guazelli, 1981.. Samples are essentially nonexistent. There is only one known dredging and that was done on Feburary 22, 1966 by the Research Vessel Vema Žcruise 22, dredge 2.. The dredge location is 20842X S and 31851X W, on Columbia seamount, which is located ; 275 km west of Trindade ŽFig. 1.. The recovered material is in the Lamont-Doherty Earth Observatory repository; it is mainly coral but includes one volcanic sampling that serves as the focus of this study. Based on an estimated 3 cmryear rate of plate motion ŽGripp and Gordon, 1990., and assuming an origin at Trindade hotspot, Columbia seamount is ; 10 million years old. Another igneous province relevant to this study of Trindade-related magmatism is the Abrolhos Islands ŽFodor et al., 1989. located offshore Brazil at ; 188S. The location of this Eocene Ž; 40–50 Ma; K–Ar ages, Cordani and Blazekovic, 1970. oceanic province of mildly alkalic basalts places it north of the middle portion of the Trindade trace ŽFig. 1. and therefore makes it a candidate for origin in the Trindade hotspot. 3. Petrography The sample of Columbia seamount is a ; 10-cm ankaramite with about 16 vol.% clinopyroxene and 4 vol.% olivine phenocrysts and microphenocrysts ŽTable 1.. Clinopyroxene grains are subhedral to euhedral, have optically distinct rims, and are generally 0.5–4 mm in size; there are, however, megacrysts ; 1 cm in size. Olivine grains are also subhedral to euhedral and typically - 1 mm in size; some are partially or completely replaced by calcite and smectite. Rhonite, a silica–deficient Fe–Ca–Mg–Ti sili¨ cate, makes up ; 0.5 vol.% as tabular grains - 0.2 mm in size. Titaniferous magnetite comprises ; 3 vol.% of the lava with grains from ; 0.1 to 0.5 mm in size. About 23 vol.% of the ankaramite is vesicles Ž0.5–3mm. lined with greenish-yellow smectite, and filled with a clear, fibrous MgO–Al 2 O 3 –SO 3 hydrous phase. The groundmass appears to have been largely glass containing clinopyroxene microlites and Fe–Ti oxide grains. Any original glass is now hydrated and altered to brownish-colored smectite, but the clinopyroxene microlites within remain fresh.
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Table 1 Whole-rock and modal Žvol.%. compositions of ankaramite from Columbia seamount, South Atlantic Ocean Analysis is volatile-free, after 10008C ignition. SiO 2 Žwt.%. TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2O P2 O5 Total
33.82 4.67 11.90 15.70 0.45 15.11 15.05 0.70 0.40 1.79 99.59
Mgaa FeOrMgO a L.O.I.b
68.2 0.83 15.1
Rb Žppm. Sr Ba Zr Nb Y Ni
1 985 385 161 57 33 265
La Ce Nd Sm Eu Tb Yb Lu
57.2 144.4 60.2 11.2 3.56 1.13 2.05 0.27
Olivine Clinopyroxene Rhonite ¨ Fe–Ti oxide Vugs Smectite lining Mg–Al–S filling Groundmass general smectite Smectite after ol Calcite after ol 87
Srr86 Sr 143 Ndr144 Nd 206 Pbr204 Pb 207 Pbr204 Pb 208 Pbr204 Pb
3.7 vol.% 16.3 0.5 2.9 8.9 14.1 50.2 2.4 0.3 0.7 Leached 0.703900 0.512786 19.190 15.045 39.242
4. Analytical techniques Whole-rock composition was determined by X-ray fluorescence spectrometry except for rare-earth element abundances acquired by neutron activation. An ARL-SEMQ microprobe was used for mineral analyses. Analytical instruments and the reactor and radiation lab are at North Carolina State University. Clinopyroxene isotope compositions were determined by standard thermal ionization mass spectrometry at the San Diego State University. Two clinopyroxene separates were prepared. One was leached with a HNO 3 q HF mixture to remove possible surface seawater contamination, and both were acid-washed in cold 2N HCl, followed by washing in 1 N H Br and rinse in quartz-distilled H 2 O. We used a VGSector 54 mass spectrometer, and samples were analyzed as metal species. Data were collected in static mode and corrected for mass fractionation and machine bias based on replicate analyses of NBS SRM981 for Pb ŽTodt et al., 1984.. The Pb isotope ratios were normalized on the basis of replicate analyses of NBS SRM981. The discrimination factor averages 0.97 " 0.006 Ž2se.‰ per mass unit. Mass discrimination for Pb averaged 0.125% " 0.0042% Ž2sm. per mass unit. Uncertainties in the Pb isotope ratios are - 0.05% per amu, computed by error propagation of sample and standard analyses. Pb blanks were - 100 pg, or negligible. Sr data were collected in multi-dynamic mode. Repeated measurements for NIST SRM987 gave 0.710273 " 3 Ž2sm, n s 78.; sample measurements were normalized to 88 Srr86 Sr s 0.1194 and standard referenced to 0.71025. Nd data were collected in multidynamic mode. Repeated measurements for the La Jolla Nd standard gave 0.511841 " 2 Ž2sm, n s 41.; sample measurements were normalized to 146 Ndr144 Nd s 0.7219 and standard referenced to 0.511858. Procedural blanks for Sr and Nd are - 50 pg, negligible for the analyses.
Unleached
18.855 15.666 38.890
a Calculated as Fe 2q is 80% of all Fe as FeO; Mgasatomic MgrŽMgqFe 2q.=100; for FeOrMgO, FeOs80% of all Fe. b Loss on ignition; volatile content.
5. Analytical results 5.1. Mineral compositions Clinopyroxene is a zoned, high-Al variety, where average Al 2 O 3 and TiO 2 are, respectively, ; 9 and
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
3 wt.%, and highest values Ž; 13 wt.% Al 2 O 3 ; 5.5 wt.% TiO 2 . are in the optically distinct rims ŽTable 2.. The Mga averages ; 76.5 wt.%. Olivine is reasonably consistent in composition from grain to grain and within grains. The average composition is Feo 82.3 , but one grain was noted to be Fo 80.3 ŽTable 2.. Rhonite composition is in Table 2. Rhonite is a ¨ ¨ rare SiO 2-undersaturated mineral restricted to undersaturated alkalic magmas. Johnston and Stout Ž1985. assess the few available analyses of rhonite world¨ wide, and our analysis for rhonite in the Columbia ¨ seamount ankaramite is like those most often observed for this phase. Titaniferous magnetite is the Fe-Ti oxide observed in the ankaramite ŽTable 2.. Table 3 lists compositions for the smectite that lines vesicles and for the MgO–Al 2 O 3 –SO 3 hydrous phase that fills the vesicles. Based on the formula for the SO 3-bearing phase, it has a gibbsite-like structure ŽAlŽOH. 3 .. This suggests that it is a hydrous Al 2 O 3 phase that accommodated Mg and S in its crystalline structure.
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5.2. Whole-rock composition Table 1 lists the ankaramite composition. Most notable are the low SiO 2 Ž; 34 wt.%. and relatively high TiO 2 and P2 O5 contents. However, relative deficiency in SiO 2 may be exaggerated due to alteration and to the presence of the SiO 2-free vesiclefilling MgO–Al 2 O 3 –SO 3 hydrous phase ŽTable 3.. High MgO is expected for ankaramite, but in this case the ; 15 wt.% MgO may be somewhat higher than the original value because of the secondary phases. That is, smectite and the gibbsite-like phases have ; 19 and 32 wt.% MgO, respectively ŽTable 3., and they collectively form nearly a quarter of the rock volume. Whereas some of the MgO contents in these secondary phases may have been derived from alteration of olivine in the ankaramite, some MgO may have originated externally and consequently added to the original MgO content of the lava. Ankaramite Na 2 O and K 2 O contents are low for an SiO 2-undersaturated rock, probably due to leaching in the marine environment. On the other hand,
Table 2 Representative mineral compositions Žin wt.%. for clinopyroxene, olivine, rhonite, and magnetite in ankaramite from Columbia seamount, ¨ South Altantic Ocean clinopyroxene core
a
rim
SiO 2 TiO 2 Al 2 O 3 Cr2 O 3 FeO MnO MgO CaO Na 2 O K 2O NiO Total
45.6 2.7 8.9 0.14 6.8 0.07 12.7 21.8 0.65 - 0.01
44.5 3.4 10.3 0.24 6.7 0.09 12.1 21.2 0.58 - 0.01
99.36
Mga FeO Fe 2 O 3 usp Total
76.9
a
rhonite ¨
olivine a
observed range Ž47.5–40.0. Ž2.0–5.5. Ž6.8–10.7. Ž0.01 –0.34. Ž6.0–8.0. Ž0.02–0.14. Ž11.5–15.2. Ž20.5–23.9. Ž0.32–0.94.
38.5
38.3
16.6 0.24 43.5 0.33
18.5 0.27 42.2 0.30
99.11
0.18 99.35
0.15 99.72
76.3
82.4
80.3
Average compositions for 8 point-analyses taken from core and rim areas of a typical phenocryst. Range for multiple point-analyses on several phenocrysts.
b
magnetite
b
23.7 12.0 16.5 - 0.01 21.6 0.16 13.3 12.2 0.88 0.01 100.35
14.7 7.0 0.20 66.8 0. 42 7.3
96.42 30.1 38.0 27.5 97.72
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Table 3 Compositions Žin wt.%. of smectite and an Mg–Al–S hydrous phase that occupy the vugs of the Columbia seamount ankaramite Smectite
Mg–Al–S phase
SiO 2 TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2O P2 O5 SO 3 Sum
36.0 0.09 20.0 7.9 0.01 18.8 0.43 0.24 0.77 0.29 0.49 85.02
– 0.02 21.8 0.08 0.04 31.7 0.13 1.9 0.03 0.28 14.7 70.68
H 2 O Žby difference. Total
15.0 100.02
29.3 99.98
Os 20 a
Os 3 b
4.966 3.034 8 0.215 0.009 0.911 0.001 3.865 0.064 0.064 0.135 0.034 0.051 5.349 6.903
– 0.351
Si Al iv Total Al vi Ti Fe Mn Mg Ca Na K P S Total H 2O ŽOH. SO 3 Total a b
– 0.001 – 0.646 0.002 0.050 – 0.003 1
2.675 0.151 2.826
H 2 O calculated independent of structure. Based on gibbsite structure, AlŽOH. 3 .
alteration-resistant incompatible elements, namely Zr, Nb, P, and light rare-earth elements, are relatively high for a clinopyroxene-rich lava and accordingly depict ‘alkalic’ characteristics for this lava. We surveyed for a P2 O5-bearing phase by using an expanded electron microprobe beam, but did not observe apatite or any other phosphate. Rather, the groundmass yielded ; 3.5 wt.% P2 O5 , suggesting that P2 O5 is largely within the ankaramite’s original glassy groundmass, now altered to smectite.
Alteration notwithstanding, we compare the ankaramite whole-rock analysis to compositions available for other rocks believed to have origins with the Trindade hotspot. These are rocks of Trindade Island and the Abrolhos Islands Žoffshore Brazil at ; 188S. and the continental rocks of the Poxoreu ´ and Alto Paranaiba provinces ŽFig. 1.. Plots of MgO against some incompatible elements show that the ankaramite feasibly creates an extension of a compositional trend formed by the primitive and some fractionated Trindade lavas ŽFig. 2.. Its composition also could conform to a trend created by Mg-rich Trindade lavas and the alkalic basalts of Poxoreu ´ and Abrolhos Islands ŽFig. 2., which have similar compositions. However, the seamount ankaramite is too low in Nb, Zr, and La to reasonably form a compositional trend with continental Alto Paranaiba high-K, high-Mg rocks ŽFig. 2.. The ankaramite rare-earth element ŽREE. pattern ŽFig. 3. displays light-REE enrichment that is characteristic of alkalic magmas. The pattern is identical to that for a Trindade basanite ŽFig. 3.. The LarYb and LarSm ratios distinguish the ankaramite from Alto Paranaiba high-K, high-Mg rocks and from Abrolhos Islands and Poxoreu ´ alkalic basalts ŽFig. 3. 5.3. Isotopic composition Clinopyroxene in the ankaramite has 87 Srr86 Sr and 143 Ndr144 Nd ratios ŽTable 1. similar to those known for basaltic rocks of Trindade and Abrolhos islands ŽFig. 4.. They are also compositionally close to isotopic values for basalts in the continental Poxoreu ´ province, which marks the extreme western end of the Trindade trace ŽFig. 1.. The continental Alto Paranaiba province, on the other hand, differs notably from the Columbia seamount data by its isotopically enriched composition ŽFig. 4.. Ankaramite Pb isotopic ratios are in Fig. 5 and in Table 1. The 206 Pbr204 Pb ratio for leached Columbia seamount clinopyroxene overlaps with those for both Trindade and Abrolhos islands ŽFig. 5a,b.. Additionally, the 208 Pbr204 Pb ratio is comparable to those of some Trindade samples, but higher than Abrolhos ratios, and 207 Pbr204 Pb is slightly higher than those ratios for Trindade and most Abrolhos samples. Overall, however, the Pb isotopic ratios for all three
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
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Fig. 2. MgO variation diagrams comparing Columbia seamount ankaramite to primitive Ž) 8 wt.% MgO; basanites. and fractionated Žnephelinites, mugearite, phonolites. lavas of Trindade Island ŽWeaver, 1990.. Diagrams include compositions for high-K, high-Mg lavas of Alto Paranaiba igneous province ŽGibson et al., 1995. and basaltic lavas of Poxoreu ´ igneous province ŽGibson et al., 1997., both ; 85 Ma and in southern Brazil, and for lavas of the Abrolhos platform ŽFodor et al., 1989. ŽFig. 1..
300
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Fig. 3. A rare-earth element pattern for Columbia seamount ankaramite compared to those of lavas from Trindade ŽWeaver, 1990., Poxoreu ´ ŽGibson et al., 1997., Alto Paranaiba ŽGibson et al., 1995., and Abrolhos ŽFodor et al., 1989..
seemingly trace-related provinces — Columbia seamount, Trindade, and Abrolhos — are remarkably similar against the distinctly different compositions of some geographically distant hotspot representations in the South Atlantic, such as Ascension, the Walvis ridge, the Rio Grande rise, Tristan da Cunha, and St. Helena ŽFigs. 4, 5a,b..
6. Discussion
suggests an equilibrium liquid with FeOrMgO ; 1.3 Žassuming FerMg olrliq K D of 0.3., higher than the 0.83 FeOrMgO observed ŽTable 1. Ži.e., alteration andror accumulative clinopyroxene probably modified original MgO.. Even though the major-element composition of Columbia seamount lava may not lend itself for quantitative comparison to other apparent Trindadeplume related oceanic and continental lavas, we can nonetheless conclude that the ankaramite represents SiO 2-undersaturated magma like those associated with the hotspot. For example, its Ti- and Al-rich clinopyroxene is characteristic of highly alkalic, undersaturated magmas Že.g., Dobosi and Fodor, 1992; Fodor et al., 1995., and rhonite always occurs in ¨ rocks described as melilitite or nephelinite basalts or alkalic syenites Že.g., Johnston and Stout, 1985.. This rhonite is the first noted for any continental or ¨ oceanic alkalic rocks assigned to Trindade hotspot. Rhonite is probably present in other lavas along the ¨ ; 208S trace but overlooked during petrographic examinations; its discovery here was during microprobe analyses of oxide phases. The highly SiO 2-undersaturated composition, then, as interpreted from these phases, is consistent with the basanites and nephelinites of Trindade Island. However, we cannot decisively assign the seamount composition to any MgO-trend ŽFig. 2. defined by Trindade or other provinces along the trace.
6.1. Ankaramite assessment in Õiew of other rock types along the hotspot trace Assessing whether a lava with ) 15 vol.% clinopyroxene represents a magma with cumulative pyroxene or a primary melt requires comparing Al 2 O 3rCaO ratios to those of lavas of similar composition — undersaturated in this case — that unambiguously represent magmas. It also requires evaluating whole-rock FeOrMgO ratios in terms of FerMg olivinerrock distribution. However, secondary smectite, calcite, and MgO–Al 2 O 3 –SO 3 hydrous phase, which collectively comprise about one quarter of the rock ŽTable 1., have likely left whole-rock Al 2 O 3 , CaO, and MgO abundances different from their original values and therefore obscure such straightforward compositional assessments. As it stands, however, the olivine composition ŽTable 2.
Fig. 4. Sr–Nd isotopic compositions for the Columbia seamount ankaramite compared to those for a variety of South Atlantic provinces, including high-P2 O5 , high-TiO2 ŽHPT. Parana´ continental flood basalts in southern Brazil ŽFig. 1.. Data sources as in Fig. 3 and in Hanan and Graham Ž1996..
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
301
Fig. 5. Ža, b. Pb-isotope ratios for Columbia seamount, Trindade Island ŽOversby, 1971., and 40–50 Ma basaltic rocks of Abrolhos platform offshore Brazil ŽFodor et al., 1989. compared to compositional fields for South Atlantic MORB, hotspot-related ocean island basalt from Rio Grande rise ŽR., Tristan da Cunha, Gough, and Discovery islands ŽTGD., Bouvet, Ascension, Fernando de Noronha, Walvis ridge, South Atlantic oceanic sediments, and for Parana´ basalts Ždata from compilations of Hanan and Graham, 1996.. SrK line represents Stacey-Kramers Ž1975. isotopic compositions for the geologic past in 250-million-year intervals. Because Oversby Ž1971. Trindade data are from PbI 2 mass spectrometry and may not be reliably compared to modern analyses, we corrected Oversby’s reported BCR-1 composition to an acceptable BCR-1 Pb isotopic composition ŽWooden et al., 1988. and normalized her Trindade Pb ratios accordingly. Halliday et al. Ž1992. also present Trindade data, but two of their three compositions have low 207 Pbr204 Pb ratios relative to Oversby Ž1971. data; because their data may not be reproducible ŽHalliday et al., 1993., we did not include them. Žc. Pb-isotope ratio diagrams comparing all three isotopic ratios simultaneously. This expression has the advantage of projecting the relative proportions of the most abundant Pb isotopes, 206 Pb, 207 Pb, and 208 Pb, in a single projection. Columbia seamount, Trindade, and Abrolhos are all on a linear, binary-like trend formed by the ‘common’ plume component C ŽHanan and Graham, 1996. that does not lead to the composition of Tristan plume ŽVanDecar et al., 1995. — which has its composition represented by Parana, ´ TGD ŽTristan da Cunha, Gough, Discovery., and Walvis. This projection illustrates a common mantle-plume source among Columbia seamount ankaramite, Trindade lavas, and Abrolhos platform basalts, but excludes the Tristan da Cunha plume.
6.2. Source area with respect to the hotspot trace We can evaluate the Columbia seamount source area by comparing the ankaramite’s clinopyroxene isotopic composition and its abundance ratios for alteration-resistant incompatible elements to those of provinces believed to be part of the Trindade trace
ŽFig. 1.. However, whereas the ; 85 Ma age and location of the Alto Paranaiba province suggest that it was generated by Trindade plume impact on lithosphere, it largely represents melts from subcontinental lithosphere rather than from asthenospheric plume ŽGibson et al., 1995.. On the other hand, the ; 84 Ma Poxoreu ´ province mainly represents astheno-
302
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spheric melts from the Trindade plume ŽGibson et al., 1997.. Accordingly, our assessment of Columbia seamount source characteristics focusses on those known for the two ‘end-points’ of the plume trace — Poxoreu ´ and Trindade Island — and for those at mid-point, Abrolhos Islands ŽFig. 1.. The essentially identical Sr and Nd isotopic compositions of Columbia seamount ankaramite, Trindade basanites and nephelinites, and Abrolhos alkalic basalts ŽFig. 4. point to a common Trindade plume source for all three oceanic provinces. Additionally, Poxoreu ´ isotopic ratios substantiate a Tristan plume origin because they are only slightly more ‘enriched’, differing from the Trindade signature in a
way that can be attributed to small contributions from subcontinental lithosphere that Poxoreu ´ magmas transcended Že.g., Gibson et al., 1997.. The Pb isotopic compositions — in particular, the 206 Pbr204 Pb ratio — of the seamount ankaramite support commonality with the Trindade plume source. In detail, however, the slightly higher 207 Pbr204 Pb for Columbia seamount, which, along with variances among Trindade data ŽFig. 5a,b., suggest at least a small level of isotopic heterogeniety along the plume trace. Higher 207 Pbr204 Pb ratios can be interpreted as signatures of marine sediments having contaminated mantle sources during ancient subduction events. That is, small percentages of
Fig. 6. Nb variation diagrams for Columbia seamount ankaramite, Trindade primitive Ž) 8 wt.% MgO; basanite. and fractionated lavas ŽWeaver, 1990., and basaltic rocks from Abrolhos platform ŽFodor et al.,1989., Poxoreu ´ Ž1997., and Alto Paranaiba ŽGibson et al., 1995..
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
sediments introduced into mantle source areas provide the mantle with Pb isotopic ratios characteristic of sediments because sediment Pb abundances overwhelm those in mantle rock Že.g., ; 25 ppm vs 0.05 ppm; e.g., Ben Othman et al., 1989.. Temporal and spatial differences in 207 Pbr204 Pb of the Trindade plume can therefore be intrinsic to the plume if it comprises upwelling mantle material that includes a regional subduction history. Among trace elements, the LarNb ratios also depict geochemical similarity among the four provinces ŽFig. 6.. The abundance ratios of Sr and P2 O5 with Nb, however, show differences among the provinces ŽFig. 6.. Unlike the essentially isotopic homogeniety along the plume trace, then, there are some minor geochemical differences which do not seem attributable to varying percentages of melting and probably reflect trace-element source heterogeniety. The alternative model suggested by VanDecar et al. Ž1995. for magmatism at ; 208S can be evaluated as Pb isotopic compositions in relation to a mantle component, C, identified by Hanan and Graham Ž1996. as an endmember common to mid-ocean ridge and many hotspot basalt compositional arrays ŽFig. 5c.. The VanDecar et al. Ž1995. model invokes entrainment of the Tristan da Cunha plume head Žresponsible for Parana´ basalts. in the subcontinental mantle during the ; 130 Ma opening of the South Atlantic to remain beneath Brazil at present-day ; 208S since ; 85 Ma. It accordingly occupies approximately the same place postulated by Gibson et al. Ž1995, 1997. for the impact of the Trindade plume. Widening of the Atlantic since that time may have dispersed the plume-head isotopic signature to ‘contaminate’ suboceanic mantle Že.g., a´ la Hanan et al., 1986; Kincaid et al., 1996., resulting in the Tristan da Cunha plume contributing to and becoming manifested during subsequent magmatism associated with the Trindade plume. However, Fig. 5c shows that the isotopic ratios of Pb for Columbia, Trindade, and Abrolhos all plot on a linear trend away from the common mantle plume component and, most importantly, not toward compositions that represent the Tristan da Cunha plume. Our assessment, then, is that the Columbia seamount ankaramite represents suboceanic asthenosphere with isotopic compositions — and, to some
303
extent, trace element characteristics — that strengthen the case for a ; 2500-km continuum of Trindade plume magmatism. Consistency of Sr, Nd, and some Pb isotopic ratios from ; 84 Ma magmatism ŽPoxoreu ´ province., into and through mid-range 50–10 Ma magmatism ŽAbrolhos and Columbia., through to ‘present-day’ Trindade attest to an overall compositional uniformity from plume head to tail.
7. Summary The overall composition of the Columbia seamount ankaramite is consistent with those of Trindade lavas and offers the seamount a place along the ‘trace’ of an oceanic lithosphere manifestation of the Trindade hotspot. Isotopic compositions of Abrolhos Islands and the Poxoreu ´ province in Brazil are additional geologic reference points for the Trindade plume. Specifically, the Columbia seamount ankaramite Ži. suggests that the seamount presently located ; 275 km west of the hotspot formed in response to its oceanic lithosphere having passed over the Trindade plume ; 10 million years ago, and Žii. it reduces the ; 1100-km distance between the Brazilian margin and the present-day hotspot to - 900 km for which the ‘trace’ must be inferred until compositions are obtained for additional seamounts.
Acknowledgements This report is part of larger study of Brazil margin magmatism supported by NSF OCE8509894. We thank LDEO for loan of the sample. G. Fitton, J.-G. Schilling, R.N. Thompson, and L. Larsen provided valuable reviews.
References Asmus, H.E., Guazelli, W., 1981. Sumario dos estruturas da margem continental brasileira e das areas continentais e occeanicas adjacentes, hipoteses sobre o tectonismo causador e implicacoes no prognostico de seu potencial em recursos minerais. Serie Projeto REMAC 9, 269 p. Ben Othman, D., White, W.M., Patchett, J., 1989. The geochemistry of marine sediments, island arc magma genesis, and crust–mantle recycling. Earth Planet. Sci. Lett. 84, 1–21.
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Cordani, U.G., Blazekovic, A., 1970. Idades radiometricas das rochas vulcanicas dos Abrolhos. Anais XXIV Congr. Bras. Geol. Soc. Bras. Geol. Brasilia, 1970, 265–270. Crough, S.T., Morgan, W.J., Hargreaves, R.B., 1980. Kimberlites: their relation to mantle hot spots. Earth Planet. Sci. Lett. 50, 260–274. Dobosi, G., Fodor, R.V., 1992. Magma fractionation, replenishment, and mixing as inferred from greencore clinopyroxenes in Pliocene basanite, southern Slovakia. Lithos 28, 113–150. Fodor, R.V., Dobosi, G, Sial, A.N., 1995. Zoned clinopyroxenes in alkalic basalt: clues to fractionation and magma-mixing histories for seemingly primitive magmas. Chem. Erde 55, 133–148. Fodor, R.V., McKee, E.H., Asmus, H.E., 1983. K–Ar ages and the opening of the South Atlantic Ocean: basaltic rock from the Brazilian margin. Marine Geol. 54, M1–M8. Fodor, R.V., Mukasa, S.B., Gomes, C.B., Cordani, U.G., 1989. Ti-rich Eocene basaltic rocks, Abrolhos platform, offshore Brazil, 188S: petrology with respect to South Atlantic magmatism. J. Petrol. 30, 763–786. Gibson, S.A., Thompson, R.N., Leonardos, O.H., Dickin, A.P., Mitchell, J.G., 1995. The Late Cretaceous impact of the Trindade mantle plume: evidence from large-volume, mafic, postassic magmatism in SE Brazil. J. Petrol. 36, 189–229. Gibson, S.A., Thompson, R.N., Weska, R.K., Dickin, A.P., Leonardos, O.H., 1997. Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil. Contrib. Mineral. Petrol. 126, 303– 314. Gripp, A.E., Gordon, R.G., 1990. Current plate velocities relative to the hotspot incorporating the NUVEL1 global plate motion model. Geophys. Res. Lett. 17, 1109–1112. Halliday, A.N., Davies, G.R., Lee, D.C., Tommasini, S., Paslick, C.R., Fitton, J.G., James, D.E., 1992. Lead isotope evidence for young trace element enrichment in the oceanic upper mantle. Nature 359, 623–627. Halliday, A.N., Davies, G.R., Lee, D.C., Tommasini, S, Paslick, C.R., Fitton, J.G., James, D.E., 1993. Correction: lead isotope evidence for young trace-element enrichment in the oceanic upper mantle. Nature 362, 184. Hanan, B.B., Graham, D.W., 1996. Lead and helium isotope evidence from oceanic basalts for a common deep source of mantle plumes. Science 272, 991–995. Hanan, B.B., Kingsley, R.H., Schilling, J.G., 1986. Pb isotopic evidence in the South Atlantic for migrating ridge–hotspot interactions. Nature 322, 137–144.
Hartnandy, C.J.H., LeRoex, A.P., 1985. Southern Ocean hot spot tracks and Cenozoic absolute motion of the African, Antarctic and South American plates. Earth Planet. Sci Lett. 75, 245– 257. Herz, N., 1977. Timing and spreading in the South Atlantic: information from Brazillian alkaline rocks. Geol. Soc. Am. Bull. 88, 101–112. Johnston, A.D., Stout, J.H., 1985. Compositional variation of naturally occurring rhonite. Am. Mineral. 70, 1216. ¨ Kincaid, C., Schilling, J.-G., Cable, C., 1996. The dynamics of off-axis plume-ridge interaction in the uppermost mantle. Earth Planet. Sci. Lett. 137, 29–43. O’Connor, J.M., Duncan, R.A., 1990. Evolution of the Walvis Ridge–Rio Grande Rise hot spot system: implications for African and South American plate motions over plumes. J. Geophys. Res. 95, 17475–17502. Oversby, V.M., 1971. Lead in ocean islands, Faial, Azores, and Trindade. Earth Planet. Sci. Lett. 11, 401–406. Stacey, J.S.D., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26 207–221. Thompson, R.N., Gibson, S.A., Mitchell, J.G., Dickin, P., Leonardos, O.H., Brod, J.A., Greenwood, J.C., 1998. Migrating Cretaceous–Eocene magmatism in the Serra do Mar alkaline province, SE Brazil: melts from the deflected Trindade mantle plume? J. Petrol. 39, 1493–1526. Todt, W., Cliff, R.A., Hanser, A., Hofmann, A.W., 1984. 202 Pb q205 Pb double spike for lead isotopic analyses. Terra Cognita 4, 209. Toyoda, K., Horiuchi, H., Tokonami, M., 1994. Dupal anomaly of Brazilian carbonatite: geochemical correlation with hotspots in the South Atlantic and implications for the mantle source. Earth Planet. Sci. Lett. 126, 315–331. Ulbrich, H.G.J., Gomes, C.B., 1981. Alkaline rocks from Continental Brazil. Earth Sci. Rev. 17, 135–154. VanDecar, J.C., James, D.E., Assumpcao, M., 1995. Seismic evidence for a fossil mantle plume beneath SouthAmerica and implications for plate driving forces. Nature 378, 25–31. Weaver, B.L., 1990. Geochemistry of highly undersaturated ocean island basalt suites from the South Atlantic Ocean: Fernando de Noronha and Trindade islands. Contrib. Mineral. Petrol. 105, 502–515. Wooden, J.L., Mueller, P.A., 1988. Pb, Sr, and Nd isotopic compositions of a suite of Late Archean igneous rock, eastern Beartooth Mountains: implications for crust–mantle evolution. Earth Planet. Sci. Lett. 87, 59–72.
Geochemical evidence for the Trindade hotspot trace: Columbia seamount ankaramite R.V. Fodor a,) , B.B. Hanan b a
Department of Marine, Earth and Atmospheric Sciences, North Carolina State UniÕersity, P.O.Box 8208, Raleigh, NC 27695, USA b Department of Geological Sciences, San Diego State UniÕersity, San Diego, CA 92182, USA Received 5 August 1999; accepted 26 November 1999
Abstract The Columbia seamount ; 825 km offshore from Brazil at ; 208S lies on the east–west ‘trace’ of the Trindade hotspot. Continental and oceanic magmatism believed to have originated with this hotspot is alkalic and SiO 2-undersaturated, and dates from ; 85 Ma in southern Brazil to - 3 Ma on the islands of Trindade and Martin Vaz ; 1100 km offshore. An ankaramite Žclinopyroxene ; 16 vol%. dredged from Columbia seamount Žest. 10 Ma. conforms to this geochemistry with SiO 2-undersaturated Al-rich clinopyroxene Ž8–13 wt.% Al 2 O 3 . and rhonite. Clinopyroxene isotopic compositions are ¨ 87 Srr86 Sr s 0.703900, 143 Ndr144 Nd s 0.512786, 206 Pbr204 Pb s 19.190, 207 Pbr204 Pb s 15.045, and 208 Pbr204 Pb s 39.242 — resembling those for Trindade, except for slightly higher 207 Pbr204 Pb. The isotopic composition and abundance ratios among weathering-resistant Nb, La, and Yb suggest that Columbia seamount magmatism represents the present-day Trindade plume, but ; 10 million years earlier and perhaps when the plume manifested a signature of ‘contamination’ from subducted sediments. The Columbia seamount analyses provide the first quantitative assessment for the Trindade hotspot trace existing between the Brazil margin and Trindade, strengthening the case for a continuum of magmatism extending from the ; 85 Ma Brazilian igneous provinces of Poxoreu ´ and Alto Paranaiba. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Trindade hotspot; Brazil magmatism; Columbia seamount; Ankaramite; Pb isotopes
1. Introduction Several studies assign the origins of alkalic igneous provinces in southern Brazil ŽUlbrich and Gomes, 1981. to the Trindade Island Žand Martin Vaz Island. hotspot located ; 1100 km east of Brazil at ; 208S ŽFig. 1.. As first proposed by Herz Ž1977., Cretaceous Brazilian alkalic and carbonatitic )
Corresponding author. fax: q1-919-515-7802. E-mail addresses: rfodor@ ncsu.edu Ž R.V. [email protected] ŽB.B. Hanan..
Fodor . ,
provinces in southeastern Brazil developed when the South American plate passed westward over the Trindade hotspot. Crough et al. Ž1980., Hartnandy and Le Roex Ž1985., O’Connor and Duncan Ž1990., and Toyoda et al. Ž1994. all acknowledged this model. Gibson et al. Ž1995; 1997. interpreted the high-K, high-Mg Alto Paranaiba igneous province and the Poxoreu ´ alkalic basalt province — both in Brazil, northwest of Rio de Janeiro ŽFig. 1. — as having origins attributed to the Trindade hotspot ; 85 million years ago when the Trindade plume head impacted subcontinental lithosphere. More re-
0024-4937r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 Ž 0 0 . 0 0 0 0 2 - 5
294 R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304 Fig. 1. Map showing the Trindade hotspot ‘trace’ as depicted by the chain of seamounts extending westward from Trindade and Martin Vaz islands — the present location of the Trindade hotspot — to the Brazil margin, and into Brazil to the two ;85 Ma igneous provinces of Alto Paranaiba and Poxoreu ´ ŽGibson et al., 1995, 1997.. The triangle on Columbia seamount is the location of the dredge that recovered ankaramite from the seamount. ESS-9 is a diabase sample, ; 43 Ma, from beneath the Abrolhos platform and near the basaltic Abrohlos Islands Ž40–50 Ma. ŽFodor et al., 1983, 1989., a province also relevant to the Trindade hotspot. A portion of the Parana´ continental flood-basalt province Ž ;130–135 Ma. coincides with the western end of the ‘trace’.
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
cently Thompson et al. Ž1998. assigned Eocene magmatism in southern Brazil to deflection of the Trindade plume head. The present-day Trindade hotspot manifests highly undersaturated alkalic lavas and plutonic rocks such as basanites, nephelinites, phonolites, and ankaratrites ŽHerz, 1977; Weaver, 1990.. An east–west line of seamounts extending westward from Trindade to the Brazil margin at ; 208S ŽFig. 1. forms a convincing hotspot ‘trace’ created by continuous igneous activity from the time when southeastern Brazil passed over a mantle plume fixed at the location of Trindade until present times. The ‘trace’ accordingly suggests a genetic relationship between late Cretaceous igneous provinces in Brazil, produced from the plume-head, and a string of seamounts and islands produced from the plume-tail. In contrast, VanDecar et al. Ž1995. suggest that the late Cretaceous alkalic province in southeastern Brazil represents final magmatism associated with the Parana´ flood basalts ; 130–135 Ma. They conjecture that ; 85 Ma, the dying heat source yielded alkalic magmas in the northern extremity of the Parana´ province, at ; 208S ŽFig. 1.. In this view, VanDecar et al. Ž1995. discount any hotspot role in the origins of igneous rocks - 85 Ma that form part of the trace between Trindade and Brazil. Instead, the seamounts west of Trindade and Martin Vaz ŽFig. 1. islands formed along a ‘leaky’ transform fault. Knowing the magma types among the seamounts offshore Brazil and their isotopic compositions are necessary for documenting the role of the Trindade hotspot in magmatism within and offshore Brazil ; 208S. We provide data for the compositional gap between the two end-points of the ; 2500 km Trindade trace Žfrom Poxoreu ´ province in Brazil to Trindade Island; Fig. 1. by characterizing a lava from Columbia seamount in terms of whole-rock, mineral, and Sr, Nd, and Pb isotopic compositions. Columbia seamount is on the Trindade trace ; 825 km offshore at ; 208S ŽFig. 1..
2. Background information The chain of seamounts between Trindade and Brazil is called the Vitoria-Trindade seamount chain
295
or the Vitoria-Trindade lineament ŽAsmus and Guazelli, 1981.. Samples are essentially nonexistent. There is only one known dredging and that was done on Feburary 22, 1966 by the Research Vessel Vema Žcruise 22, dredge 2.. The dredge location is 20842X S and 31851X W, on Columbia seamount, which is located ; 275 km west of Trindade ŽFig. 1.. The recovered material is in the Lamont-Doherty Earth Observatory repository; it is mainly coral but includes one volcanic sampling that serves as the focus of this study. Based on an estimated 3 cmryear rate of plate motion ŽGripp and Gordon, 1990., and assuming an origin at Trindade hotspot, Columbia seamount is ; 10 million years old. Another igneous province relevant to this study of Trindade-related magmatism is the Abrolhos Islands ŽFodor et al., 1989. located offshore Brazil at ; 188S. The location of this Eocene Ž; 40–50 Ma; K–Ar ages, Cordani and Blazekovic, 1970. oceanic province of mildly alkalic basalts places it north of the middle portion of the Trindade trace ŽFig. 1. and therefore makes it a candidate for origin in the Trindade hotspot. 3. Petrography The sample of Columbia seamount is a ; 10-cm ankaramite with about 16 vol.% clinopyroxene and 4 vol.% olivine phenocrysts and microphenocrysts ŽTable 1.. Clinopyroxene grains are subhedral to euhedral, have optically distinct rims, and are generally 0.5–4 mm in size; there are, however, megacrysts ; 1 cm in size. Olivine grains are also subhedral to euhedral and typically - 1 mm in size; some are partially or completely replaced by calcite and smectite. Rhonite, a silica–deficient Fe–Ca–Mg–Ti sili¨ cate, makes up ; 0.5 vol.% as tabular grains - 0.2 mm in size. Titaniferous magnetite comprises ; 3 vol.% of the lava with grains from ; 0.1 to 0.5 mm in size. About 23 vol.% of the ankaramite is vesicles Ž0.5–3mm. lined with greenish-yellow smectite, and filled with a clear, fibrous MgO–Al 2 O 3 –SO 3 hydrous phase. The groundmass appears to have been largely glass containing clinopyroxene microlites and Fe–Ti oxide grains. Any original glass is now hydrated and altered to brownish-colored smectite, but the clinopyroxene microlites within remain fresh.
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
296
Table 1 Whole-rock and modal Žvol.%. compositions of ankaramite from Columbia seamount, South Atlantic Ocean Analysis is volatile-free, after 10008C ignition. SiO 2 Žwt.%. TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2O P2 O5 Total
33.82 4.67 11.90 15.70 0.45 15.11 15.05 0.70 0.40 1.79 99.59
Mgaa FeOrMgO a L.O.I.b
68.2 0.83 15.1
Rb Žppm. Sr Ba Zr Nb Y Ni
1 985 385 161 57 33 265
La Ce Nd Sm Eu Tb Yb Lu
57.2 144.4 60.2 11.2 3.56 1.13 2.05 0.27
Olivine Clinopyroxene Rhonite ¨ Fe–Ti oxide Vugs Smectite lining Mg–Al–S filling Groundmass general smectite Smectite after ol Calcite after ol 87
Srr86 Sr 143 Ndr144 Nd 206 Pbr204 Pb 207 Pbr204 Pb 208 Pbr204 Pb
3.7 vol.% 16.3 0.5 2.9 8.9 14.1 50.2 2.4 0.3 0.7 Leached 0.703900 0.512786 19.190 15.045 39.242
4. Analytical techniques Whole-rock composition was determined by X-ray fluorescence spectrometry except for rare-earth element abundances acquired by neutron activation. An ARL-SEMQ microprobe was used for mineral analyses. Analytical instruments and the reactor and radiation lab are at North Carolina State University. Clinopyroxene isotope compositions were determined by standard thermal ionization mass spectrometry at the San Diego State University. Two clinopyroxene separates were prepared. One was leached with a HNO 3 q HF mixture to remove possible surface seawater contamination, and both were acid-washed in cold 2N HCl, followed by washing in 1 N H Br and rinse in quartz-distilled H 2 O. We used a VGSector 54 mass spectrometer, and samples were analyzed as metal species. Data were collected in static mode and corrected for mass fractionation and machine bias based on replicate analyses of NBS SRM981 for Pb ŽTodt et al., 1984.. The Pb isotope ratios were normalized on the basis of replicate analyses of NBS SRM981. The discrimination factor averages 0.97 " 0.006 Ž2se.‰ per mass unit. Mass discrimination for Pb averaged 0.125% " 0.0042% Ž2sm. per mass unit. Uncertainties in the Pb isotope ratios are - 0.05% per amu, computed by error propagation of sample and standard analyses. Pb blanks were - 100 pg, or negligible. Sr data were collected in multi-dynamic mode. Repeated measurements for NIST SRM987 gave 0.710273 " 3 Ž2sm, n s 78.; sample measurements were normalized to 88 Srr86 Sr s 0.1194 and standard referenced to 0.71025. Nd data were collected in multidynamic mode. Repeated measurements for the La Jolla Nd standard gave 0.511841 " 2 Ž2sm, n s 41.; sample measurements were normalized to 146 Ndr144 Nd s 0.7219 and standard referenced to 0.511858. Procedural blanks for Sr and Nd are - 50 pg, negligible for the analyses.
Unleached
18.855 15.666 38.890
a Calculated as Fe 2q is 80% of all Fe as FeO; Mgasatomic MgrŽMgqFe 2q.=100; for FeOrMgO, FeOs80% of all Fe. b Loss on ignition; volatile content.
5. Analytical results 5.1. Mineral compositions Clinopyroxene is a zoned, high-Al variety, where average Al 2 O 3 and TiO 2 are, respectively, ; 9 and
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
3 wt.%, and highest values Ž; 13 wt.% Al 2 O 3 ; 5.5 wt.% TiO 2 . are in the optically distinct rims ŽTable 2.. The Mga averages ; 76.5 wt.%. Olivine is reasonably consistent in composition from grain to grain and within grains. The average composition is Feo 82.3 , but one grain was noted to be Fo 80.3 ŽTable 2.. Rhonite composition is in Table 2. Rhonite is a ¨ ¨ rare SiO 2-undersaturated mineral restricted to undersaturated alkalic magmas. Johnston and Stout Ž1985. assess the few available analyses of rhonite world¨ wide, and our analysis for rhonite in the Columbia ¨ seamount ankaramite is like those most often observed for this phase. Titaniferous magnetite is the Fe-Ti oxide observed in the ankaramite ŽTable 2.. Table 3 lists compositions for the smectite that lines vesicles and for the MgO–Al 2 O 3 –SO 3 hydrous phase that fills the vesicles. Based on the formula for the SO 3-bearing phase, it has a gibbsite-like structure ŽAlŽOH. 3 .. This suggests that it is a hydrous Al 2 O 3 phase that accommodated Mg and S in its crystalline structure.
297
5.2. Whole-rock composition Table 1 lists the ankaramite composition. Most notable are the low SiO 2 Ž; 34 wt.%. and relatively high TiO 2 and P2 O5 contents. However, relative deficiency in SiO 2 may be exaggerated due to alteration and to the presence of the SiO 2-free vesiclefilling MgO–Al 2 O 3 –SO 3 hydrous phase ŽTable 3.. High MgO is expected for ankaramite, but in this case the ; 15 wt.% MgO may be somewhat higher than the original value because of the secondary phases. That is, smectite and the gibbsite-like phases have ; 19 and 32 wt.% MgO, respectively ŽTable 3., and they collectively form nearly a quarter of the rock volume. Whereas some of the MgO contents in these secondary phases may have been derived from alteration of olivine in the ankaramite, some MgO may have originated externally and consequently added to the original MgO content of the lava. Ankaramite Na 2 O and K 2 O contents are low for an SiO 2-undersaturated rock, probably due to leaching in the marine environment. On the other hand,
Table 2 Representative mineral compositions Žin wt.%. for clinopyroxene, olivine, rhonite, and magnetite in ankaramite from Columbia seamount, ¨ South Altantic Ocean clinopyroxene core
a
rim
SiO 2 TiO 2 Al 2 O 3 Cr2 O 3 FeO MnO MgO CaO Na 2 O K 2O NiO Total
45.6 2.7 8.9 0.14 6.8 0.07 12.7 21.8 0.65 - 0.01
44.5 3.4 10.3 0.24 6.7 0.09 12.1 21.2 0.58 - 0.01
99.36
Mga FeO Fe 2 O 3 usp Total
76.9
a
rhonite ¨
olivine a
observed range Ž47.5–40.0. Ž2.0–5.5. Ž6.8–10.7. Ž0.01 –0.34. Ž6.0–8.0. Ž0.02–0.14. Ž11.5–15.2. Ž20.5–23.9. Ž0.32–0.94.
38.5
38.3
16.6 0.24 43.5 0.33
18.5 0.27 42.2 0.30
99.11
0.18 99.35
0.15 99.72
76.3
82.4
80.3
Average compositions for 8 point-analyses taken from core and rim areas of a typical phenocryst. Range for multiple point-analyses on several phenocrysts.
b
magnetite
b
23.7 12.0 16.5 - 0.01 21.6 0.16 13.3 12.2 0.88 0.01 100.35
14.7 7.0 0.20 66.8 0. 42 7.3
96.42 30.1 38.0 27.5 97.72
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
298
Table 3 Compositions Žin wt.%. of smectite and an Mg–Al–S hydrous phase that occupy the vugs of the Columbia seamount ankaramite Smectite
Mg–Al–S phase
SiO 2 TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2O P2 O5 SO 3 Sum
36.0 0.09 20.0 7.9 0.01 18.8 0.43 0.24 0.77 0.29 0.49 85.02
– 0.02 21.8 0.08 0.04 31.7 0.13 1.9 0.03 0.28 14.7 70.68
H 2 O Žby difference. Total
15.0 100.02
29.3 99.98
Os 20 a
Os 3 b
4.966 3.034 8 0.215 0.009 0.911 0.001 3.865 0.064 0.064 0.135 0.034 0.051 5.349 6.903
– 0.351
Si Al iv Total Al vi Ti Fe Mn Mg Ca Na K P S Total H 2O ŽOH. SO 3 Total a b
– 0.001 – 0.646 0.002 0.050 – 0.003 1
2.675 0.151 2.826
H 2 O calculated independent of structure. Based on gibbsite structure, AlŽOH. 3 .
alteration-resistant incompatible elements, namely Zr, Nb, P, and light rare-earth elements, are relatively high for a clinopyroxene-rich lava and accordingly depict ‘alkalic’ characteristics for this lava. We surveyed for a P2 O5-bearing phase by using an expanded electron microprobe beam, but did not observe apatite or any other phosphate. Rather, the groundmass yielded ; 3.5 wt.% P2 O5 , suggesting that P2 O5 is largely within the ankaramite’s original glassy groundmass, now altered to smectite.
Alteration notwithstanding, we compare the ankaramite whole-rock analysis to compositions available for other rocks believed to have origins with the Trindade hotspot. These are rocks of Trindade Island and the Abrolhos Islands Žoffshore Brazil at ; 188S. and the continental rocks of the Poxoreu ´ and Alto Paranaiba provinces ŽFig. 1.. Plots of MgO against some incompatible elements show that the ankaramite feasibly creates an extension of a compositional trend formed by the primitive and some fractionated Trindade lavas ŽFig. 2.. Its composition also could conform to a trend created by Mg-rich Trindade lavas and the alkalic basalts of Poxoreu ´ and Abrolhos Islands ŽFig. 2., which have similar compositions. However, the seamount ankaramite is too low in Nb, Zr, and La to reasonably form a compositional trend with continental Alto Paranaiba high-K, high-Mg rocks ŽFig. 2.. The ankaramite rare-earth element ŽREE. pattern ŽFig. 3. displays light-REE enrichment that is characteristic of alkalic magmas. The pattern is identical to that for a Trindade basanite ŽFig. 3.. The LarYb and LarSm ratios distinguish the ankaramite from Alto Paranaiba high-K, high-Mg rocks and from Abrolhos Islands and Poxoreu ´ alkalic basalts ŽFig. 3. 5.3. Isotopic composition Clinopyroxene in the ankaramite has 87 Srr86 Sr and 143 Ndr144 Nd ratios ŽTable 1. similar to those known for basaltic rocks of Trindade and Abrolhos islands ŽFig. 4.. They are also compositionally close to isotopic values for basalts in the continental Poxoreu ´ province, which marks the extreme western end of the Trindade trace ŽFig. 1.. The continental Alto Paranaiba province, on the other hand, differs notably from the Columbia seamount data by its isotopically enriched composition ŽFig. 4.. Ankaramite Pb isotopic ratios are in Fig. 5 and in Table 1. The 206 Pbr204 Pb ratio for leached Columbia seamount clinopyroxene overlaps with those for both Trindade and Abrolhos islands ŽFig. 5a,b.. Additionally, the 208 Pbr204 Pb ratio is comparable to those of some Trindade samples, but higher than Abrolhos ratios, and 207 Pbr204 Pb is slightly higher than those ratios for Trindade and most Abrolhos samples. Overall, however, the Pb isotopic ratios for all three
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
299
Fig. 2. MgO variation diagrams comparing Columbia seamount ankaramite to primitive Ž) 8 wt.% MgO; basanites. and fractionated Žnephelinites, mugearite, phonolites. lavas of Trindade Island ŽWeaver, 1990.. Diagrams include compositions for high-K, high-Mg lavas of Alto Paranaiba igneous province ŽGibson et al., 1995. and basaltic lavas of Poxoreu ´ igneous province ŽGibson et al., 1997., both ; 85 Ma and in southern Brazil, and for lavas of the Abrolhos platform ŽFodor et al., 1989. ŽFig. 1..
300
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
Fig. 3. A rare-earth element pattern for Columbia seamount ankaramite compared to those of lavas from Trindade ŽWeaver, 1990., Poxoreu ´ ŽGibson et al., 1997., Alto Paranaiba ŽGibson et al., 1995., and Abrolhos ŽFodor et al., 1989..
seemingly trace-related provinces — Columbia seamount, Trindade, and Abrolhos — are remarkably similar against the distinctly different compositions of some geographically distant hotspot representations in the South Atlantic, such as Ascension, the Walvis ridge, the Rio Grande rise, Tristan da Cunha, and St. Helena ŽFigs. 4, 5a,b..
6. Discussion
suggests an equilibrium liquid with FeOrMgO ; 1.3 Žassuming FerMg olrliq K D of 0.3., higher than the 0.83 FeOrMgO observed ŽTable 1. Ži.e., alteration andror accumulative clinopyroxene probably modified original MgO.. Even though the major-element composition of Columbia seamount lava may not lend itself for quantitative comparison to other apparent Trindadeplume related oceanic and continental lavas, we can nonetheless conclude that the ankaramite represents SiO 2-undersaturated magma like those associated with the hotspot. For example, its Ti- and Al-rich clinopyroxene is characteristic of highly alkalic, undersaturated magmas Že.g., Dobosi and Fodor, 1992; Fodor et al., 1995., and rhonite always occurs in ¨ rocks described as melilitite or nephelinite basalts or alkalic syenites Že.g., Johnston and Stout, 1985.. This rhonite is the first noted for any continental or ¨ oceanic alkalic rocks assigned to Trindade hotspot. Rhonite is probably present in other lavas along the ¨ ; 208S trace but overlooked during petrographic examinations; its discovery here was during microprobe analyses of oxide phases. The highly SiO 2-undersaturated composition, then, as interpreted from these phases, is consistent with the basanites and nephelinites of Trindade Island. However, we cannot decisively assign the seamount composition to any MgO-trend ŽFig. 2. defined by Trindade or other provinces along the trace.
6.1. Ankaramite assessment in Õiew of other rock types along the hotspot trace Assessing whether a lava with ) 15 vol.% clinopyroxene represents a magma with cumulative pyroxene or a primary melt requires comparing Al 2 O 3rCaO ratios to those of lavas of similar composition — undersaturated in this case — that unambiguously represent magmas. It also requires evaluating whole-rock FeOrMgO ratios in terms of FerMg olivinerrock distribution. However, secondary smectite, calcite, and MgO–Al 2 O 3 –SO 3 hydrous phase, which collectively comprise about one quarter of the rock ŽTable 1., have likely left whole-rock Al 2 O 3 , CaO, and MgO abundances different from their original values and therefore obscure such straightforward compositional assessments. As it stands, however, the olivine composition ŽTable 2.
Fig. 4. Sr–Nd isotopic compositions for the Columbia seamount ankaramite compared to those for a variety of South Atlantic provinces, including high-P2 O5 , high-TiO2 ŽHPT. Parana´ continental flood basalts in southern Brazil ŽFig. 1.. Data sources as in Fig. 3 and in Hanan and Graham Ž1996..
R.V. Fodor, B.B. Hananr Lithos 51 (2000) 293–304
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Fig. 5. Ža, b. Pb-isotope ratios for Columbia seamount, Trindade Island ŽOversby, 1971., and 40–50 Ma basaltic rocks of Abrolhos platform offshore Brazil ŽFodor et al., 1989. compared to compositional fields for South Atlantic MORB, hotspot-related ocean island basalt from Rio Grande rise ŽR., Tristan da Cunha, Gough, and Discovery islands ŽTGD., Bouvet, Ascension, Fernando de Noronha, Walvis ridge, South Atlantic oceanic sediments, and for Parana´ basalts Ždata from compilations of Hanan and Graham, 1996.. SrK line represents Stacey-Kramers Ž1975. isotopic compositions for the geologic past in 250-million-year intervals. Because Oversby Ž1971. Trindade data are from PbI 2 mass spectrometry and may not be reliably compared to modern analyses, we corrected Oversby’s reported BCR-1 composition to an acceptable BCR-1 Pb isotopic composition ŽWooden et al., 1988. and normalized her Trindade Pb ratios accordingly. Halliday et al. Ž1992. also present Trindade data, but two of their three compositions have low 207 Pbr204 Pb ratios relative to Oversby Ž1971. data; because their data may not be reproducible ŽHalliday et al., 1993., we did not include them. Žc. Pb-isotope ratio diagrams comparing all three isotopic ratios simultaneously. This expression has the advantage of projecting the relative proportions of the most abundant Pb isotopes, 206 Pb, 207 Pb, and 208 Pb, in a single projection. Columbia seamount, Trindade, and Abrolhos are all on a linear, binary-like trend formed by the ‘common’ plume component C ŽHanan and Graham, 1996. that does not lead to the composition of Tristan plume ŽVanDecar et al., 1995. — which has its composition represented by Parana, ´ TGD ŽTristan da Cunha, Gough, Discovery., and Walvis. This projection illustrates a common mantle-plume source among Columbia seamount ankaramite, Trindade lavas, and Abrolhos platform basalts, but excludes the Tristan da Cunha plume.
6.2. Source area with respect to the hotspot trace We can evaluate the Columbia seamount source area by comparing the ankaramite’s clinopyroxene isotopic composition and its abundance ratios for alteration-resistant incompatible elements to those of provinces believed to be part of the Trindade trace
ŽFig. 1.. However, whereas the ; 85 Ma age and location of the Alto Paranaiba province suggest that it was generated by Trindade plume impact on lithosphere, it largely represents melts from subcontinental lithosphere rather than from asthenospheric plume ŽGibson et al., 1995.. On the other hand, the ; 84 Ma Poxoreu ´ province mainly represents astheno-
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spheric melts from the Trindade plume ŽGibson et al., 1997.. Accordingly, our assessment of Columbia seamount source characteristics focusses on those known for the two ‘end-points’ of the plume trace — Poxoreu ´ and Trindade Island — and for those at mid-point, Abrolhos Islands ŽFig. 1.. The essentially identical Sr and Nd isotopic compositions of Columbia seamount ankaramite, Trindade basanites and nephelinites, and Abrolhos alkalic basalts ŽFig. 4. point to a common Trindade plume source for all three oceanic provinces. Additionally, Poxoreu ´ isotopic ratios substantiate a Tristan plume origin because they are only slightly more ‘enriched’, differing from the Trindade signature in a
way that can be attributed to small contributions from subcontinental lithosphere that Poxoreu ´ magmas transcended Že.g., Gibson et al., 1997.. The Pb isotopic compositions — in particular, the 206 Pbr204 Pb ratio — of the seamount ankaramite support commonality with the Trindade plume source. In detail, however, the slightly higher 207 Pbr204 Pb for Columbia seamount, which, along with variances among Trindade data ŽFig. 5a,b., suggest at least a small level of isotopic heterogeniety along the plume trace. Higher 207 Pbr204 Pb ratios can be interpreted as signatures of marine sediments having contaminated mantle sources during ancient subduction events. That is, small percentages of
Fig. 6. Nb variation diagrams for Columbia seamount ankaramite, Trindade primitive Ž) 8 wt.% MgO; basanite. and fractionated lavas ŽWeaver, 1990., and basaltic rocks from Abrolhos platform ŽFodor et al.,1989., Poxoreu ´ Ž1997., and Alto Paranaiba ŽGibson et al., 1995..
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sediments introduced into mantle source areas provide the mantle with Pb isotopic ratios characteristic of sediments because sediment Pb abundances overwhelm those in mantle rock Že.g., ; 25 ppm vs 0.05 ppm; e.g., Ben Othman et al., 1989.. Temporal and spatial differences in 207 Pbr204 Pb of the Trindade plume can therefore be intrinsic to the plume if it comprises upwelling mantle material that includes a regional subduction history. Among trace elements, the LarNb ratios also depict geochemical similarity among the four provinces ŽFig. 6.. The abundance ratios of Sr and P2 O5 with Nb, however, show differences among the provinces ŽFig. 6.. Unlike the essentially isotopic homogeniety along the plume trace, then, there are some minor geochemical differences which do not seem attributable to varying percentages of melting and probably reflect trace-element source heterogeniety. The alternative model suggested by VanDecar et al. Ž1995. for magmatism at ; 208S can be evaluated as Pb isotopic compositions in relation to a mantle component, C, identified by Hanan and Graham Ž1996. as an endmember common to mid-ocean ridge and many hotspot basalt compositional arrays ŽFig. 5c.. The VanDecar et al. Ž1995. model invokes entrainment of the Tristan da Cunha plume head Žresponsible for Parana´ basalts. in the subcontinental mantle during the ; 130 Ma opening of the South Atlantic to remain beneath Brazil at present-day ; 208S since ; 85 Ma. It accordingly occupies approximately the same place postulated by Gibson et al. Ž1995, 1997. for the impact of the Trindade plume. Widening of the Atlantic since that time may have dispersed the plume-head isotopic signature to ‘contaminate’ suboceanic mantle Že.g., a´ la Hanan et al., 1986; Kincaid et al., 1996., resulting in the Tristan da Cunha plume contributing to and becoming manifested during subsequent magmatism associated with the Trindade plume. However, Fig. 5c shows that the isotopic ratios of Pb for Columbia, Trindade, and Abrolhos all plot on a linear trend away from the common mantle plume component and, most importantly, not toward compositions that represent the Tristan da Cunha plume. Our assessment, then, is that the Columbia seamount ankaramite represents suboceanic asthenosphere with isotopic compositions — and, to some
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extent, trace element characteristics — that strengthen the case for a ; 2500-km continuum of Trindade plume magmatism. Consistency of Sr, Nd, and some Pb isotopic ratios from ; 84 Ma magmatism ŽPoxoreu ´ province., into and through mid-range 50–10 Ma magmatism ŽAbrolhos and Columbia., through to ‘present-day’ Trindade attest to an overall compositional uniformity from plume head to tail.
7. Summary The overall composition of the Columbia seamount ankaramite is consistent with those of Trindade lavas and offers the seamount a place along the ‘trace’ of an oceanic lithosphere manifestation of the Trindade hotspot. Isotopic compositions of Abrolhos Islands and the Poxoreu ´ province in Brazil are additional geologic reference points for the Trindade plume. Specifically, the Columbia seamount ankaramite Ži. suggests that the seamount presently located ; 275 km west of the hotspot formed in response to its oceanic lithosphere having passed over the Trindade plume ; 10 million years ago, and Žii. it reduces the ; 1100-km distance between the Brazilian margin and the present-day hotspot to - 900 km for which the ‘trace’ must be inferred until compositions are obtained for additional seamounts.
Acknowledgements This report is part of larger study of Brazil margin magmatism supported by NSF OCE8509894. We thank LDEO for loan of the sample. G. Fitton, J.-G. Schilling, R.N. Thompson, and L. Larsen provided valuable reviews.
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