Petrology and alteration of selected units of Mid-Atlantic Ridge basalts sampled from sites 332 and 335, DSDP

Baragar, W R A; Plant, A. G.; Pringle, Gavin; Schau, M

doi:10.1139/e77-081

Cited by 49 publications

(19 citation statements)

References 16 publications

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“…This concentric zoning of the pillow lavas has already been described in samples from Leg 37 (Baragar et al, 1977), and observed by the authors in samples from Leg 46 (Lofgren, 1971). It is a primary structure indicating fast cooling conditions.…”

Section: Mineralogy Of Alteration In Basalts From Site 417supporting

confidence: 78%

“…The varioles are generally centered around a microlite or microphenocryst of plagioclase (An 50 to 75), which is surrounded by a thin rim of fibrous, brown, and anisotropic minerals; after Baragar et al (1977), these tiny rims would be made of plagioclase and pyroxene. In the spherulitic zone (SZ), 1 cm thick, the varioles join together, forming a polygonal net.…”

Section: Mineralogy Of Alteration In Basalts From Site 417mentioning

confidence: 99%

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Mineralogy and Geochemistry of Alteration Products in Holes 417A and 417D Basement Samples (Deep Sea Drilling Project Leg 51)

Juteau¹,

Noack²,

Whitechurch³

et al. 1980

Initial Reports of the Deep Sea Drilling Project

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The striking difference between the basalts of Hole 417A (strongly altered) and those of Hole 417D (slightly altered) is one of the main discoveries of Legs 51-52. After a macroscopic description of the two basaltic sections, the authors describe the mineralogy of alteration in the two holes. In Hole 417D, the pillow margins exhibit a nice concentric zoning, with a hyaloclastic zone, a glassy zone, a variolitic zone, a spherolitic zone, and the pillow core. Fresh black and brilliant glass is abundant; it is partly altered mainly in the hyaloclastic zone (delicate concentric rims of brown palagonite) and in the variolitic zone (yellow fibropalagonite). X-ray diffraction patterns indicate a dioctahedral smectite (montmorillonite type) for the palagonite to a depth of 475 meters, and a trioctahedral smectite (saponite type) for deeper samples. SEM chemical profiles in the palagonite rims of hyaloclastic fragment indicate a strong enrichment in K and Fe in the outermost layers, suggesting a "protoceladonite" composition for the corresponding smectites. The glassy zone remains remarkably fresh: palagonitic layers developed along cracks are quite similar to the palagonite rims in the hyaloclastic zone. Clinopyroxene and plagioclase phenocrysts are generally devoid of alteration; olivine phenocrysts are completely replaced by calcite, iron oxides, and brown to green smectites. Vesicles and veinlets are filled by green smectites, brown smectites, and calcite, in order of abundance. No notable evolution with depth was observed.In Hole 417A, the pillow margins exhibit the same concentric structure. The glass is completely palagonitized, with two types of development, according to the density of fractures in the glassy zone. Olivine phenocrysts are completely replaced by iddingsite, calcite, and green and brown smectites. Plagioclase phenocrysts are strongly altered and replaced by analcite, calcite, potash feldspar, and/or chabazite. Clinopyroxenes are not affected by alteration. Vesicles and veinlets are filled by the same minerals described in Hole 417D, in the same order of succession. Green smectites are dioctahedral and brown smectites are trioctahedral. No notable variations with depth have been noticed, except a strong oxidation in the first 10 meters of the basaltic pile.Comparison between chemistry of fresh glasses and palagonites shows a complete depletion in CaO; loss in SiC 2, total FeO, MgO, CaO, Na2θ, and TiCh; strong enrichment in K2O and H2O; and a strong increase in the Fe2θ.j/FeO ratio. With these data and those available in previous studies of palagonites, the authors tried to estimate the balance of chemical exchange for the major oxides during the palagonitization process, and the average rates of chemical variation per year for each oxide. The comparison of the chemistry of fresh and altered basaltic pillow cores leads to similar conclusions concerning the behavior of major elements during alteration, but the quantities involved and rates of exchanges are quite smaller, especially for CaO.Rar...

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Section: Mineralogy Of Alteration In Basalts From Site 417supporting

confidence: 78%

Section: Mineralogy Of Alteration In Basalts From Site 417mentioning

confidence: 99%

Mineralogy and Geochemistry of Alteration Products in Holes 417A and 417D Basement Samples (Deep Sea Drilling Project Leg 51)

Juteau¹,

Noack²,

Whitechurch³

et al. 1980

Initial Reports of the Deep Sea Drilling Project

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“…A study of pillows from the seafloor weathering zone and low-temperature zone of hole CY1/1A shows that pillow interiors are either unaltered or have the same trends in element mobility as pillow rims but to a lesser degree [Baragar et al, 1991]. Similar trends of element mobility have also been documented for rim-core pairs of pillows altered at low-temperatures [Baragar et al, 1977] and at greenschist facies conditions [Hump hris and Thompson, 1978]. Thus qualitative compositional trends are discussed in following section for each alteration zone in hole CY1/1A.…”

Section: Chemical Effects Of Alterationmentioning

confidence: 83%

Patterns and processes of alteration in the lavas and dykes of the Troodos Ophiolite, Cyprus

Gillis¹,

Robinson²

1990

J. Geophys. Res.

114

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Alteration panenu in the lavas and dykes of the Troodos Ophiolite, Cypnu, record a compte~ ~st~ of axial hydrochennal alteration, crustal aging, and.subsequent uplift ~d c;rnptac~ent of ~e ophiolite. ~1eld mappina shows that distribution of five alteration zooes, each ~ith duunct rruneralog•cal, gcochermcal, and hydroloaic characteristics, is influenced by igneous stratigraphy, st~cture~ a~d the nature a.nd thickness of the overlying aedimenu. Paragenetic aequences of ICCOlldary ~erals indicate .that alteration conditions changed proareasively as the crust cooled and moved o(f~Uls. Along spreading axes, low temperatures ~SO"C) were mainuined by the rapid flow of seawater m and out of the lavas, and .only minimal alteration took place. In contrast, lower water/rock ratios and hlgh~r te.mperarures (>200°C) m the dykes promoted extensive seawater·rock interaction. Althou~~ the sharp rue .m ~perarure between the two reJ.imes generally coincides with the lava-dyke transJUoo, late· stage mtrus1ons or hydrothermal upwellina zooes locally cause high·temperarure alteration to extend upward .into the lavas. As a segment of crust moved off·axis, temperatures remained tow in the lavas and proaress1vety decreased, ~rom. >250" 10 <80"C in the dykes. High permeability in the uppermost lavas led to the downward rruarauon of an oxidati~e alteration front whose thickness and spatial distribution was dependent upon the. rate and nature of sedimentation and, thus, the original seafloor morphology. Although field relations sho.w that alteration has a consistent vertical pattern in Troodos, the alteration zooes are not laterally oonunuous, and the stratigraphic depth of their boundaries varies considerably. IN1ROOUC110NEarly studies of the massive sulfide deposits [Constantinou and Govett, 1973) and alteration patterns within the Troodos ophiolite [Gass and Smewing, 1973) contributed greatly to the development of models for oceanic hydrothermal systems [SpooMr et al., 1977; C haprno.n and SpooMr, 1977] and subseafloor metamorphism [Coleman, 1977]. Subsequent discovery of high-temperature vents at mid-ocean ridges [e.g., Edmond et al., 1979), seamounts [MalaJwff et al., 1982], and back arc basins [Horbie et al., 1986] showed that hydrothermal circulation is a fundamental process in many oceanic environments.Mineralogical and chemical zonations within the Troodos massive sulfide ore deposits indicate that they formed from hydrothermal flu1ds whose compositions were remarkably similar to those venting on the seafloor today [Adamides, 1987;Herzig and Friedrich , 1987;Richards et al., 1989). Fluid compositions in Troodos were buffered by epidosites at conditions similar to those inferred for the root-zones of active systems at mid-ocean ridges [Bowers and Taylor, 1985;Richardson et al., 1987;Schiffman et al., 1987;Schiffman and Smith, 1988;Seyfried et al., 1988;Von Damm, 1988] . Because of these and other similarities, the upflow zones of the Troodos hydrothermal systems are thought to be ancient analogues of modem active systems [Oud...

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“…Available data on the chemical composition of such secondary products were obtained from Baragar et al(1977), Banks (1972), Melson and Thompson (1973), Bass (1976), Seyfried et al (1976), Scott and Swanson (1976), Scheidegger and Stakes (1977), and the present study. The analyzed samples in these papers are believed to be representative of the compositions of alteration products found in oceanic crust, both in regard to age (0 to 110 m.y.B.P.)…”

Section: Evidence For Evolutionary Changes In the Composition Of Secomentioning

confidence: 99%

X-Ray Diffraction and Chemical Study of Secondary Minerals from Deep Sea Drilling Project Leg 51, Holes 417A and 417D

Scheidegger¹,

Stakes²

1980

Initial Reports of the Deep Sea Drilling Project

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Secondary minerals found in fracture fillings and in fragments of altered basalt from Holes 417A and 417D were studied by both X-ray diffraction and chemical techniques. Minerals found in fracture fillings from Hole 417A are dominated by montmorillonite, "protoceládonite," analcite, and lesser saponite; celadonite and ferrosaponite are the characteristic secondary minerals in Hole 417D fracture fillings. Assuming that minerals found in such fracture fillings reflect the composition of the secondary fluids that produced them, it is apparent that those from Hole 417A were dominantly Al-rich, while those from Hole 417D were more enriched in Fe, Mg, and K. X-ray diffraction study of bulk samples support such fundamental differences in secondary mineralogy. In addition, the X-ray data on bulk samples suggest that primary plagioclase is the feldspar in Hole 417D rocks, and secondary potassium feldspar is the feldspar in Hole 417A altered rocks.Using available published data on secondary miner? 1 " found in other altered oceanic crust, it is possible to interpret the differences in secondary mineralogy that exist between the two sites. Secondary minerals present in Hole 417D rocks are believed to have formed under hydrothermally influenced, low temperature, nonoxidative diagenesis; whereas, those present in Hole 417A were produced under similarly low temperatures, but much more highly oxidizing conditions. The fundamental differences in secondary mineralogy between the two sites can be best explained by the accompanying remobilization of elements that involved plagioclase alteration in Hole 417A rocks. A comparison of the composition of Hole 417A and 417D secondary minerals with those found in younger crust suggests that the age of crust, influenced by the changing conditions of alteration, control the chemistry of secondary minerals found in available pore spaces in altered rocks. Minerals found in young crust (<15 m.y.B.P.) are highly Mg-rich; minerals found in crust of intermediate age (-15-50 m.y.B.P.) are dominantly enriched in Fe and Mg; and those found in older crust have higher contents of Al and K.

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Petrology and alteration of selected units of Mid-Atlantic Ridge basalts sampled from sites 332 and 335, DSDP

Cited by 49 publications

References 16 publications

Mineralogy and Geochemistry of Alteration Products in Holes 417A and 417D Basement Samples (Deep Sea Drilling Project Leg 51)

Mineralogy and Geochemistry of Alteration Products in Holes 417A and 417D Basement Samples (Deep Sea Drilling Project Leg 51)

Patterns and processes of alteration in the lavas and dykes of the Troodos Ophiolite, Cyprus

X-Ray Diffraction and Chemical Study of Secondary Minerals from Deep Sea Drilling Project Leg 51, Holes 417A and 417D

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