Origin of basalt microlapilli in lower Miocene pelagic sediment, northeastern Pacific Ocean
T. L. VALLIER U.S. Geological Survey, Menlo Park, California 94025 DENNIS BOHRER Deep Sea Drilling Project, Scripps Institution of Oceanography, La Jolla, California 92037 GROVER MORELAND Smithsonian Institution, Washington, D.C. 20560 EDWIN H. McKEE U.S. Geological Survey, Menlo Park, California 94025
ABSTRACT 4,785 m. Seismic data show acoustic basement overlain by pelagic sediment that in turn is overlain by horizontally stratified turbidite Basalt microlapilli consisting of glassy particles and rock frag- deposits that form part of the Delgada Fan. The hole was drilled ments were recovered in lower Miocene pelagic sediment from and discontinuously cored to a depth of 217 m; 214 m of sediment Deep Sea Drilling Project site 32 in the northeastern Pacific Ocean. and 3 m of basalt were penetrated. The basalt contains Major-element chemistry indicates that the parent tholeiitic magma microphenocrysts of plagioclase, clinopyroxene, and titanomagne- was high in iron and titanium, similar to many of the abyssal tite set in a mesostasis of iron-rich glass and palagonite (Vallier, basalts from the eastern Pacific Ocean, and the relatively high K20 1970). The sediment-basalt contact was not recovered. content suggests additional fractionation before eruption. The mic- The sediment column is subdivided into four units. Unit 1, of late rolapilli most likely were erupted subaerially from an island vol- Pliocene and Quaternary age, consists of 90 m of silty clay with cano, although a shallow submarine source cannot be discounted. some silt and silty sand. Unit 2, of late Miocene and early Pliocene If wind were the only method of dispersal, then the volcano was age, is a 66-m-thick silty clay unit with very rare silty sand beds. probably within 100 km of site 32. If surface or density currents Unit 3 consists of 20 m of pale-yellow fossiliferous pelagic clay of played a role in transport, however, the volcano may have been at a middle and late Miocene age. Unit 4, the lowest 38 m of sediment, greater distance. consists of pelagic brown clay of early Oligocene to middle Miocene age. The early Oligocene paleontologic age of oldest sed- INTRODUCTION iments is consistent with the magnetic anomaly (anomaly 13) age for the site of about 38 m.y., based on the time scale by Heirtzler Basaltic microlapilli (sand-sized lapilli) consisting of glassy parti- and others (1968). cles and rock fragments were recovered from lower Miocene Smear slides of sediments were studied from the interval between pelagic sediments in cores at site 32 of the Deep Sea Drilling Project 185 m and the base of the sediment column at 214 m. Major (Fig. 1). Preliminary studies of glassy particles by von der Borch lithologies are pelagic brown clay and zeolitic brown clay. Minor (1970, 1971) led him to conclude that they are tholeiitic and prob- ably formed during submarine eruptions. Our studies confirm the tholeiitic composition, but suggest subaerial rather than submarine eruptions. Reports on basaltic microlapilli in pelagic sediments are rare compared with those on terrestrial microspherules, microtektites, and lunar glassy particles. Yet all these particles have characteris- tics in common, particularly size and shape. This paper provides new data on the sedimentology, petrography, and chemistry of basaltic microlapilli that are used for comparative studies and for interpretations of their origin. Similar microlapilli have been de- scribed by Heiken and Lofgren (1974) from nonmarine regions. Ten samples were selected (Fig. 2) from an interval where the microlapilli are most abundant. Samples were sieved for grain-size analysis, and 12 doubly polished thin sections of the microlapilli were prepared for petrographic studies. Scanning electron micro- scopy was used to determine surface textures. Refractive indices of fresh glass were measured by oil immersion methods, and age de- termination was by the potassium-argon method. Chemical analyses were by x-ray fluorescence, atomic absorption, and elec- tron probe techniques.
STRATIGRAPHY
Stratigraphic relations at site 32 are described by McManus, Burns, and others (1970, p. 16-18) and von der Borch (1971, p. 5—9). Site 32 is located about 385 km west of San Francisco on Figure 1. Location map showing site 32 of the Deep Sea Drilling Project, the distal part of the Delgada submarine fan at a water depth of northeastern Pacific Ocean (base map is modified from Menard, 1964).
Geological Society of America Bulletin, v. 88, p. 787-796, 16 figs., June 1977, Doc. no. 70605.
787
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CORE 9 10 10 analysis was made by standard isotope-dilution techniques with a Nier-type 6-in.-radius, 60° sector mass spectrometer operated in SECTION 6 2 the static mode. The potassium analysis was obtained by flame — 0 photometer using a lithium internal standard. The precision of the date, shown as the plus or minus value, is the analytical uncertainty in measurement of radiogenic argon-40 and potassium in the sam- ple. The value is determined on the basis of experience with dupli- cate analyses in the Menlo Park laboratories. In general, a sample that contains a low percentage of radiogenically produced argon- 40 has a high analytical uncertainty. The glassy objects yielded more than 90 percent atmospheric argon-40 (8.13 percent radio- genic 40Ar), and the estimated error is about 12 percent of the age.
Analytical results for the age determination are as follows: KaO = 40 n 40 LEGEND 0.466 percent; Arrad = 1.1284 x 10~ moles/g; Artot = 8.1 percent. Decay constants used for 40K are Xe = 0.585 X 10"10 yr-1 Sample location and \/3 = 4.72 x 10~10 yr"1, and the atomic abundance of 40K is 1.19 x 10~4 mole/mole. The age calculated from these data is 16.3 Smear slide location X ± 2 m.y., which is early Miocene, consistent with the age deduced — 50 Drilling Breccia from Radiolaria.
Brown clay PHYSICAL CHARACTERISTICS v WW v v v Silicic volcanic ash Samples were taken from parts of the cores that had been dis- turbed by the coring process. Sample integrity is thereby suspect, Altered volcanic ash and all are thought to be mixtures of pelagic clay, microlapilli, and — 75 Zones with other materials that were not necessarily closely associated before SS :-xi- glassy objects coring. The clay and silt fractions (<0.062 mm) were removed by wet sieving and discarded. The sand fraction was dry sieved through 11 sieves with openings between +40 (0.062 mm) and -1<£ (2.0 mm). Grain-size distributions are distinctly bimodal (Figs. 3, 4). Most microlapilli fall in the size range of 0.3 to 2.0 mm and account for the primary mode. The secondary mode in the finer grain sizes is —100 caused by the presence of silicified fossil fragments. Median diameters (Md<£) are between 0.75 and 0.80, and the samples are well sorted [that is, cr is 0.40 to 0.45, using the methods of Inman (1952)]. The relative percentage of glassy particles and rock frag- ments varies with grain size (Fig. 5); in the coarser grain sizes, rock fragments are prevalent. Round and smooth outlines of glassy particles contrast with an- —125 gular and jagged outlines of rock fragments. Shapes of glassy parti- cles include spheroids, ellipsoids, teardrops, dumbbells, discs, re- r Figure 2. Graphic columns of niforms, canoes, rollers, and irregular shapes (Figs. 6, 7). Spheroid core 9 (section 6) and core 10 and ellipsoid shapes are most common (Fig. 8). (sections 1 and 2), showing sed- Surface textures of most glassy particles are smooth. Others have iment types, sample localities, surface textures that resulted from one or more of the following and some smear slide locations. processes: (1) welding together of hot fragments on impact (Fig. 7), Depth below sea floor is 183.5 to Void" (2) shattering and cratering of cooled fragments upon impact, and -150 188.0 m. (3) implosion or explosion of near-surface gas bubbles. Welded particles have a nodular or studded appearance, and some show at lithologies mostly are silicic volcanic ash and thin-bedded least three distinct sequences of welding. Only rare impact craters montmorillonite-rich clay that probably is altered ash. were noted, which consist of small, dish-shaped, conchoidal frac- The basaltic microlapilli occur mostly in the 185- to 188-m tures. interval of core 10 (sections 1 and 2), in scattered clots and along Glassy particles are transparent to opaque and are pale brown, the sides of the core where they were injected during the coring dark brown, black, light gray, green, and amber. Both transparency process (Fig. 2). Because of the disturbance caused by coring, the and color depend on the type of cooling surface formed. For exam- original distribution of the microlapilli cannot be determined. Ap- ple, palagonite rinds impart very dark brown and black colors; parently they were concentrated either in a single layer or in several these particles generally are translucent or opaque. A minutely frac- thin layers in the 185- to 188-m depth interval. tured rim creates a gray opaque surface, as does a thin rind com- posed of a silica mineral. Transparent particles are generally pale AGE brown and rarely green or amber. Refractive indices were measured on sideromelane fragments of Fossils associated with the basaltic microlapilli consist of only a 20 different pale-brown glassy particles. The indices are remark- few poorly preserved Radiolaria that suggest a late Oligocene or ably uniform at 1.601 ± 0.002, which corresponds to a Si02 con- early Miocene age (McManus, Burns, and others, 1970, p. 48). tent of about 49.5 percent (George, 1924). This compares well with Fresh, glassy basaltic particles were hand-picked under a micro- an average Si02 content of 50.27 derived from chemical studies scope and dated by the potassium-argon method. The argon mass (see below).
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SIZE IN PHI UNITS
SIZE IN PHI (if) UNITS
PETROGRAPHY
Major components of the microlapilli are (1) pale-brown sideromelane; (2) dark-brown to black tachylite (volcanic glass with abundant crystallites); (3) orange-brown to dark-brown palagonite as variolites, rims, veins, and streaks; (4) plumose var- iolites of clinopyroxene and feldspar; (5) feldspar, olivine, and clinopyroxene as skeletal laths, crystallites, and microphenocrysts; (6) euhedral opaque minerals; (7) smectite; (8) chalcedonic miner- als as rims and vesicle fillings; and (9) zeolite minerals, mostly as replacement of feldspar in the lithic fragments.
Glassy Particles
Glassy particles have a wide range of textures and mineral com- positions (Figs. 9, 10). Pale-brown transparent particles are com- posed of fresh sideromelane, and cooling rinds and other rims are either very thin or absent. Some have small crystallites scattered ir- regularly and arranged in rows. Almost all have spheroid vesicles amounting to 10 to 40 vol percent. A gradation of compositions and textures exists between the Figure 4. Frequency diagrams of samples 2, 4, and 10. transparent and translucent particles. Translucent particles are composed mainly of tachylite that may be partly or wholly are clinopyroxene, olivine, plagioclase, and an opaque mineral that palagonidzed. Many are rimmed by palagonite, and crystallites are is either ilmenite or titanomagnetite. Olivine crystallites progress common. The vesicles generally are not lined with secondary min- from a simple cross, to a complex orthorhombic cross with hairlike erals, although rarely they do have thin rinds of either palagonite or projections perpendicular to the cross axes, to olivine skeletal crys- a silica mineral (optical properties suggest cristobalite). Crystallites tals. Plagioclase crystallites and microphenocrysts generally occur
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PERCENT SIZE RANGE >1.0 mm 0.84 - 1.0 mm 0.60 - 0.84 mm 0.42 - 0.60 ml a) 0.30 - 0.42 mm b) >.30 mm c)
Basaltic rock ¡Glassy objects fragments Figure 5. Relative percentages of glassy particles and rock fragments (1,250 counts in each size fraction) as a function of size in sample 10. a, Three percent of total size fraction consists of siliceous rock fragments; b, 23 percent of total size fraction consists of siliceous rock fragments; and c, less than 3 percent is basaltic rock fragments and glassy objects; the re- mainder is siliceous rock fragments.
Figure 7. Scanning electron microscope photograph of irregular glassy particle with small teardrop particle welded to surface.
PERCENT SIZE RANGE 0
>1.0 mm 0.84 - 1.0 mm 0.60 - 0.84 mm 0.42 - 0.60 mm 0.30 - 0.42 mm
Spheroid shapes lllllllll Irregular shapes
Ellipsoid shapes ^MOther shapes (e.g. Teardrop, dumbbell, reniform, canoe shape) Figure 8. Relative percentages of glassy particle shapes in sample 10.
in glomerols; in some glassy objects they are flow-oriented. Euhed- ral opaque minerals commonly occur in weak flow lines. Glassy particles that appeared opaque during studies with a stereoscopic microscope also show a wide range of compositions and textures when observed in polished sections with a polarizing microscope. Opaqueness is primarily the result of thick rims around the glass that are formed by inward-growing microjoints or by concentrations of crystallites, silica minerals, or palagonite. Some of these opaque glassy particles, however, are not rimmed but are composed entirely of either tachylite, palagonite, feldspar variolites, or a mixture of tachylite, feldspar, palagonite, and clay.
Rock Fragments
Basaltic rock fragments have a wide range of textures and mineralogies (Fig. 11). Most fragments are hypohyaline with either (1) hyalopilitic textures that have feldspar microlites aligned in a tachylite or sideromelane mesostasis or (2) textures that have feldspar and clinopyroxene microphenocrysts and opaque mineral euhedra set randomly in a tachylite mesostasis or in a mesostasis that is a combination of tachylite, palagonite, and clay. Primary constituents are plagioclase, clinopyroxene, opaque minerals, sideromelane, and tachylite. The rare olivine microphenocrysts have been replaced by brown or brownish-green smectite. Second- ary constitutents are smectite, palagonite, phillipsite, analcite(P), and a silica mineral (cristobalite?). Smectite completely replaces the glassy parts of many rock fragments.
MAJOR-ELEMENT CHEMISTRY Figure 6. Scanning electron microscope photographs of glassy particles: a, representative shapes; b, representative shapes; c, teardrop shape; d, Major-element analyses were made on 14 glassy particles by dumbell shape with a slaggy surface; e, irregular shape; f, slaggy sphere electron probe methods and on bulk samples of both glassy parti- with nodular projection and a possible impact scar. cles and rock fragments (Table 1). These results were compared
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/6/787/3429333/i0016-7606-88-6-787.pdf by guest on 28 September 2021 Figure 9. Photomicrographs of glassy particles (plane-polarized light): a, clear sideromelane spherule with palagonite rind; b, spherule with vesicles, sideromelane, palagonite, and feldspar microlites; c, part of an ellipsoid glassy particle composed of palagonite; d, ellipsoid glassy particle with sidermelane surrounded by a rind composed of cooling joints and palagonite.
TABLE 1. CHEMICAL ANALYSES OF GLASSY PARTICLES AND ROCK FRAGMENTS, DSDP SITE 32
1 2 3 4 5 6 7 8 9 10 11 12 13 14 A B C
Si02 49.08 49.60 50.45 50.01 50.12 50.34 51.39 50.18 49.94 50.22 50.49 50.30 50.79 50.83 50.27 50.10 51.80 AI2O3 12.93 14.09 14.28 14.01 14.01 14.23 14.16 14.08 14.27 14.20 14.13 14.04 14.18 14.25 14.06 13.98 14.62 a FeO 13.50 12.95 13.08 12.79 12.86 12.97 13.30 13.01 12.95 12.91 13.05 12.84 12.90 13.00 13.00 11.74 10.76 MgO 8.48 6.72 6.87 6.65 6.79 6.80 7.16 6.90 6.69 6.78 7.09 6.85 6.92 6.84 6.97 5.75 4.62 CaO 10.07 10.60 10.62 10.59 10.28 10.47 10.56 10.70 10.67 10.69 10.32 10.77 10.73 10.67 10.55 10.05 10.43 Na20 2.93 2.57 2.60 2.72 2.64 2.67 2.44 2.67 2.75 2.65 2.61 2.72 2.80 2.58 2.67 3.02 2.82 K2O 0.38 0.37 0.37 0.38 0.34 0.34 0.18 0.41 0.41 0.38 0.31 0.40 0.45 0.38 0.36 0.31 0.28 Ti02 1.71 2.15 2.20 2.13 2.16 2.10 2.16 2.17 2.14 2.16 2.17 2.11 2.12 2.17 2.12 2.33 2.38 p2o5 0.12 0.12 0.12 0.15 0.15 0.13 0.13 0.17 0.15 0.14 0.12 0.17 0.15 0.14 0.14 0.17 0.18 MnO 0.23 0.21 Total 99.20 99.17 100.59 99.43 99.35 100.05 101.48 100.29 99.97 100.13 100.29 100.20 101.04 100.86 100.14 97.68 98.10 Note: 1 to 14 are analyses of individual glassy particles; E. Jarosewich, analyst using electron probe. A = average of 14 analyses of individual glassy particles (also A of Tables 2 and 3). B = bulk sample analysis of glassy particles; R. Batiza, analyst using x-ray fluorescence and atomic absorption tech- niques. C = bulk sample analysis of rock fragments; R. Batiza, analyst using x-ray fluorescence and atomic absorption techniques. " Total iron as FeO.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/6/787/3429333/i0016-7606-88-6-787.pdf by guest on 28 September 2021 Figure 10. Photomicrographs of glassy particles (plane-polarized light): a, small glassy particle with bulbous protrusion (note vesicle filled with ver- micular chalcedonic mineral; cristobalite?); b, glassy particle with line of pyroxene crystallites; c, part of glassy spherule with palagonite rind and olivine crystallites in sideromelane; d, higher magnification of c, showing olivine crystallites.
TABLE 2. CHEMICAL ANALYSES OF ABYSSAL BASALT GLASSES, ATLANTIC AND PACIFIC OCEANS
1 2 3 4 5 6 7 A D 50.16 50.27 51.17 Si02 51.16 49.38 50.51 49.60 50.46 50.22 14.14 15.10 14.06 12.30 ai2o, 15.28 15.08 14.00 14.92 13.54 FeO* 10.36 11.34 12.19 11.35 12.60 11.77 10.13 13.00 15.74 MgO 7.02 6.84 6.70 7.26 5.99 6.17 8.42 6.97 4.83 CaO 10.80 10.37 11.26 10.59 10.75 10.98 11.58 10.55 9.26 NajO 2.95 2.98 2.54 2.97 2.75 3.11 2.39 2.67 2.57 K„0 0.12 0.34 0.16 0.32 0.21 0.17 0.10 0.36 0.16 1.53 2.12 2.93 TiOz 1.84 2.16 1.89 2.21 2.32 2.14 PA 0.15 0.22 0.15 0.22 0.18 0.18 0.11 0.14 0.19 Total 99.68 98.71 99.40 99.44 98.80 98.88 99.52 100.14 99.15 FeO' /MgO 1.47 1.66 1.82 1.56 2.08 1.92 1.22 1.85 3.22
Note: Analyses 1 to 7 are from Melson and others (1976). All analyses were done by Gary Byerly and J. Nelen, Smithsonian Institution, using electron probe. 1 = average of 27 basalt glasses from the Mid-Atlantic Ridge dredge haul, lat 09.60°N, long 40.65°W; 2 = average of 3 basalt glasses from a fracture zone in the Atlantic Ocean, lat 52.67°N, long 34.94°W; 3 = average of 74 analyses from the Juan de Fuca Ridge, lat 44.66°N, long 130.33°W; 4 = average of 13 analyses from a fracture zone on the Juan de Fuca Ridge, lat 44.27°N, long 129.750W; 5 = group 1 glass (4 analyses) from the East Pacific Rise, lat 13.22°S, long 112.33°W; 6 = group 2 glass (8 analyses) from the East Pacific Rise, lat 13.22°S, long 112.33°W; 7 = group 3 glass (3 analyses) from the East Pacific Rise, lat 13.22°S, long 112.33°W; A = average of 14 glassy particles from site 32 (see Table 1); and D = site 32 "basement" basalt glass. * Total iron as FeO.
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A
m *r ?OOu
Figure 11. Photomicrographs of basaltic rock fragments (plane-polarized light): a, tachylite mesostasis with abundant vesicles; b, typical rock fragment with tachylitic and palagonitic mesostasis, vesicles, and feldspar microphenocrysts; c, rock fragment with pyroxene and feldspar.
with selected analyses of abyssal, oceanic island, seamount, and site 32. K20 values are lower in abyssal basalts erupted along the plateau tholeiites and with bottle-green Ivory Coast microtektites mid-oceanic ridges and higher in basalts from fracture zones and in (Figs. 12 through 15). The chemical compositions of glassy parti- the glassy particles from site 32. cles are similar except for sample 1, which has lower amounts of Compared with the "average oceanic tholeiite" of Engel and Si02, A1203, and Ti02 but higher FeO* (total iron as FeO), MgO, others (1965), the glassy particles (A in Table 2) have higher Si02, and Na20 values. Analyses of all particles are in the tholeiite fields FeO*, K20, and Ti02 values but lower amounts of A1203, MgO, (Figs. 13, 14) of Macdonald (1968) and Bass and others (1973). All CaO, and Na20. Glass from the underlying basalt "basement" at have relatively high FeO*, and most have Ti02 contents greater site 32 (D in Table 2), when compared with the "average oceanic than 2.0. The average composition of individual glassy particles (A tholeiite" of Engel and others (1965), contains more Si02, FeO*, in tables and figures), when compared with compositions of bulk and Ti02 and less A1203, MgO, CaO, and Na20; the K20 amounts samples (B and C in Table 1 and figures), shows higher FeO* and are about the same. MgO and lower Na20 and Ti02 values. These differences may re- The average composition of glassy particles, when compared flect the presence of phenocrysts and altered glass in the bulk sam- with the composition of glass from the "basement" basalt at site ples. It is also possible that alteration of rock fragments caused the 32, indicates that the glassy particles have higher contents of relatively higher Si02 and A1203 values and lower amounts of A1203, MgO, CaO, Na20, and K20 but lower amounts of Si02, FeO* and MgO. Furthermore, differences between the average FeO*, Ti02, and P205. Both are relatively high in iron and analysis (A in Table 1) and the bulk glassy particle sample (B) — titanium (FeTi group of abyssal basalts). There are striking particularly in the higher amounts of Si02, FeO*, and MgO and similarities between all glasses from site 32 and abyssal basaltic lower values of NazO in the average analysis — may be related to glasses from many parts of the East Pacific Rise and the Juan de the conversion of glass to palagonite, as pointed out by Frey and Fuca Ridge (Table 2), particularly in high iron and titanium values. others (1974, p. 5519). FeTi group basalts are characteristic of large parts of the East There is a wide range of major-element chemistries in abyssal Pacific Rise and differ from basalts of the Mid-Atlantic Ridge by basalt glasses (Melson and others, 1976). Table 2 presents analyses having significantly higher FeO* and Ti02 values. Few glass from selected sites in the Pacific and Atlantic Oceans, including analyses of Atlantic Ocean abyssal basalts show similar iron and dredge hauls from the mid-ocean ridge system and from fracture titanium enrichments (Melson and others, 1976). In fact, this zones that cut the ridge system. Major apparent differences among major difference between glass chemistries of the Atlantic and the analyses are in FeO*, Ti02, and KzO contents. In general, Pacific Oceans suggests that regional "average oceanic tholeiite" FeO* and TiOz values are lower in dredge hauls from the Atlantic compositions are of more value than an "average composition" of and higher in dredge hauls from the Pacific, including glasses from abyssal tholeiites from all ocean basins. Compositions of glassy particles from site 32 are somewhat simi- lar to those of basalts from Mendocino Ridge and Cobb Seamount FeO (Table 3) in KzO and Si02 contents. There are noticeable dif- Figure 12. AMF dia- ferences in total iron and CaO between the glassy particles and gram from analyses listed in Mendocino Ridge basalt, and tholeiite from Cobb Seamount has Tables 1, 2, and 3. FeO* for analyses S and T calculated as FeO + 0.9 FejOg. 6.0 r 1 i — i
ALKALI BASALT FIELD
Na20 4.0 +
K20 3-0
(2) S 8 2.0 THOLEIITE FIELD 1.0
0.0 45.0 46.0 47.0 48.0 49.0 50.0 51.0
NooO Figure 13. Alkali-silica diagram (Macdonald, 1968) from analyses listed KoO MgO in Tables 1 and 2. Glassy particle field outlined by solid line.
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TABLE 3. COMPOSITIONS OF SITE 32 GLASSES, MENDOCINO RIDGE BASALT, COBB SEAMOUNT BASALT, MAUNA LOA AND KILAUEA BASALTS, COLUMBIA RIVER BASALT GROUP, AND BOTTLE-GREEN MICROTEKTITES
A S TUVWXY
Si02 50.27 49.94 49.82 52.18 50.63 51.17 50.99 51.80 AI2O3 14.06 14.85 15.57 14.17 14.01 14.82 14.36 15.70 FeO* 13.00 8.07 6.58 10.94 11.07 11.84 13.00 9.53 Fe203 2.17 3.13 MgO 6.97 6.42 5.82 7.00 7.00 5.39 4.97 18.10 CaO 10.55 11.92 12.82 10.58 11.43 9.32 8.91 3.27 Na20 2.67 2.70 3.24 2.29 2.30 3.20 2.89 0.52 K2O 0.36 0.26 0.38 0.37 0.51 0.96 1.21 0.23 TI02 2.12 2.27 1.85 2.07 2.64 2.40 2.55 0.61 P2O5 0.14 0.18 0.16 0.21 0.26 0.34 0.54 MnO 0.22 0.11 0.17 0.17 0.21 0.22 0.13 Total 100.14 98.96 99.48 99.98 100.02 99.65 99.64 99.89 FeO/MgO 1.85 1.56 1.58 2.22 2.63 0.53
Note: A = average composition of 14 analyses of glassy particles (see Table 1). S = top of Mendocino Ridge, depth 1,260 m, lat 40°23'N, long 127° 59'W; total H20 is 1.30 (Engel and Engel, 1963). T = top of Cobb Seamount, depth 125 m, lat 47°05'N, long 130°45'W; total HzO is 0.54 (Engel and Engel, 1963). U = composition of historic Mauna Loa flows, Hawaii (Wright, 1971). V = average composition of prehistoric flows, Kilauea caldera, Hawaii; MgO averages are normalized to 7.0 (Wright, 1971). W = high-titanium Picture Gorge Basalt (Wright and others, 1973). X = nonweigh- ted average composition of all Columbia River Basalt groups (averaged from those given by Wright and others, 1973). Y = average composition of ten Ivory Coast bottle-green microtektites (Glass, 1972). * Total iron as FeO, except for S and T.
significantly higher amounts of A1203, CaO, and Na20, and lower Microtektites have many physical similarities to the glassy parti- iron, MgO, and Ti02 values compared with the glassy particles. cles from site 32 (Glass, 1967, 1968, 1969, 1972; Cassiday and Glassy particles from site 32, when compared with "historic" others, 1969). Most microtektites, however, have much higher Mauna Loa flows, have similar A1203, MgO, CaO, K20, and Ti02 Si02 and KaO values and are depleted in the other major oxides. values, lower SiOz and P205, and higher amounts of FeO* and The average composition of ten bottle-green microtektites from the Na20. Compared with the average composition of Kilauea caldera Ivory Coast (Glass, 1972) is similar to that of the glassy particles in basalt, glassy particles from site 32 have lower amounts of CaO, Si02, A1203, and K20 values, but the microtektites have much :: K20, Ti02, and P205 and higher FeO ' and Na20 values (Wright, higher MgO values and lower amounts of the other major oxides 1971). Chemical analyses of Columbia River Basalt show higher (Table 3). K20 and P205 contents and lower MgO and CaO values (Wright and others, 1973). DISCUSSION
l.Or Stratigraphy, age data, physical characteristics, and major- element chemistry give clues to the origin and transportation of the
16.0 0.8
ALKALI BASALT 15.0 0.6
AI2O3
14.0 0.4 -M) OCEAN RIDGE BASALT V - * 1 * II. 0.2 OCEAN ISLAND 13.0 TH0LEIITE 0
Site 32 • D 12.0 1.5 2.0 2.5 3.0 Ti02 (%) Ti0o Figure 14. P205-Ti02 diagram using fields of Bass and others (1973). Letters are same as those in Tables 1, 2, and 3. Site 32 glasses are outlined Figure 15. Al203-Ti02 diagram. Letters correspond to those in Tables 1, by solid line. 2, and 3. Solid lines outline fields of glassy particles (from Table 1).
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/6/787/3429333/i0016-7606-88-6-787.pdf by guest on 28 September 2021 ORIGIN OF BASALT MICROLAPILLI IN LOWER MIOCENE PELAGIC SEDIMENT 795
microlapilli. Stratigraphie and age data show that the microlapilli (2) density, pressure, and humidity of the atmosphere; and (3) pre- were deposited in a sequence of lower Miocene pelagic brown clay. vailing winds. Other variables may include the initial trajectories of Shapes, surface textures, and petrography suggest that the parti- fragments and the height of the surrounding terrain. We know of cles were erupted subaerially, but we do not know how far they no work that adequately discusses the size distribution and wind were transported in the atmosphere before deposition or if marine transport distances of particles similar to those described here. density currents redistributed the microlapilli after their initial de- Even the eruption height is not known, although heights of other position. The teardrop shapes of some glassy particles and the basaltic eruptions (Strombolian type) have been reported (see presence of welded composite particles and impact craters strongly Heiken, 1974). For example, the 1968 eruption of Fuego in suggest a subaerial origin. The particles were elongated as they Guatemala sent up an ash cloud that reached an elevation of simultaneously cooled and passed through the atmosphere. Weld- 12,000 m, and a 1968 eruption of Cerro Negro in Nicaragua also ing took place during early phases of eruption while droplets were sent up a cloud several thousand metres high. These probably are still plastic. Furthermore, the sizes of vesicles in the microlapilli maxima for Strombolian-type eruptions, which definitely are much suggest that hydrostatic pressure could not have been great. more explosive than Hawaiian-type eruptions. Ash clouds from Although there are no known descriptions of similar microlapilli Hawaiian-type eruptions generally reach heights of only a few formed during submarine eruptions, that origin cannot be dis- hundred metres (W. Duffield, 1975, personal commun.). counted. If the microlapilli, particularly the glassy particles, were For a first approximation, some comparisons can be made with erupted in the sea, however, then the liquid droplets would have data from grain-size distribution studies of undifferentiated tephra shattered during contact with the water unless they were somehow samples to determine wind transport distances (Fisher, 1964; initially protected from direct contact with water by gas bubbles Walker, 1971). Fisher (1964, p. 350) plotted median diameter that formed during eruption. (Md$) versus log distance of tephra samples, and his best-fit curve In low-viscosity magmas, droplet shape is, in part, controlled by shows that samples with median diameters of 0$ to l
Higher KzO values of glassy objects and rock fragments, com- pared with K20 contents of abyssal tholeiites, suggest that additional fractionation occurred. The relatively high K20 is very similar to K20 values from Hawaiian lavas, Cobb Seamount, and basalt from fracture zones such as the Mendocino Ridge (Table 3, analysis S) and one that intersects the Juan de Fuca Ridge (Table 2, analysis 4), which coincidentally also shows iron and titanium en- richment. Apparently, the microlapilli were erupted from a magma relatively rich in iron and titanium that probably had undergone additional fractionation beneath an oceanic island volcano or sea- mount. Compilations by Chase and others (1975) and Wilde and others (1976) give detailed bathymetry around site 32 and provide some data on the distribution of seamounts, one of which might have been the source volcano (Fig. 16). Their ages are not known nor are the lengths of time they were active. The most likely source volcano is either "DJ" Seamount, about 120 km northwest, or an unnamed seamount that lies south of "DJ" and about 90 km from site 32. A line of seamounts that begins 180 km southeast of site 32 also Figure 16. Sketch map of area around site 32 showing the major sea- should be considered as a possible source. The closest one, at a mounts. Map is modified from Chase and others (1975) and Wilde and present depth of 2,226 m, appears to be flat-topped, which implies others (1976). that it reached the surface and was subsequently eroded and sub-
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sided. Additional holes in the area and dredging of the seamounts Sea Drilling Project: Jour. Geophys. Research, v. 79, p. 5507-5527. to determine their ages and petrology would help solve the prob- George, W. O., 1924, The relation of the physical properties of natural lem. glasses to their chemical composition: Jour. Geology, v. 32, p. 353- 372. Glass, B. P., 1967, Microtektites in deep-sea sediments: Nature, v. 214, CONCLUSIONS p. 372-374. 1968, Glassy objects (microtektites?) from deep sea sediments near the The basaltic microlapilli were erupted in early Miocene time Ivory Coast: Science, v. 161, p. 891-893. when both the east-central Pacific and the North American conti- 1969, Chemical composition of Ivory Coast microtektites: Geochim. et nent were undergoing significant tectonic adjustments. Most evi- Cosmochim. Acta, v. 33, p. 1135-1147. dence indicates a subaerial eruption from a low viscosity magma. 1972, Bottle green microtektites: Jour. Geophys. Research, v. 77, Glassy particles formed when liquid droplets cooled and solidified p. 7057-7064. during transport through the atmosphere, but subsequent dispersal Heiken, Grant, 1974, An atlas of volcanic ash: Smithsonian Contr. Earth by submarine density currents cannot be ruled out. The distance Sciences, no. 12, 101 p. between the volcano and site 32 is not known, but assuming a sub- Heiken, Grant, and Lofgren, Gary, 1974, Terrestrial glass spheres: Geol. Soc. America Bull., v. 82, p. 1045-1050. aerial eruption and dispersal by wind, and knowing the approxi- Heirtzler, J. R., Dickson, G. O., Herrón, E. M., Pitman, W. C., III, and Le mate maximum height of low-viscosity magma eruption clouds, it Pichón, X., 1968, Marine magnetic anomalies, geomagnetic field re- probably was no more than 100 km. If surface or density currents versals, and motions of the ocean floor and continents: Jour. Geophys. played roles in the dispersal, then the distance could be much Research, v. 73, p. 2119-2136. greater. Major-element chemistry shows that the parent magma Inman, D. L., 1952, Measures for describing the size distribution of sedi- was a high-iron and high-titanium tholeiite similar to a large part ments: Jour. Sed. Petrology, v. 22, p. 125-145. of the abyssal basalts from the eastern Pacific and suggests that Kay, R. N., Hubbard, N. J., and Gast, P. W., 1970, Chemical characteris- additional fractionation occurred beneath the volcano before erup- tics and origin of oceanic ridge volcanic rocks: Jour. Geophys. Re- tion. search, v. 75, p. 1585-1613. Macdonald, G. A., 1968, Composition and origin of Hawaiian lavas, in Coats, R. R., Hay, R. L., and Anderson, C. A., eds., Studies in vol- ACKNOWLEDGMENTS canology: Geol. Soc. America Mem. 116, p. 477-522. McManus, D. A., Burns, R. E., and others, 1970, Initial reports of the Deep Samples were provided by the Deep Sea Drilling Project, which is Sea Drilling Project, Vol. 5: Washington, D.C., U.S. Govt. Printing funded by the National Science Foundation. Space and equipment Office, 827 p. were made available by Scripps Institution of Oceanography, the Melson, W. G., Vallier, T. L., Wright, T. L., Byerly, Gary, and Nelen, J., Smithsonian Institution, and the U.S. Geological Survey in Menlo 1976, Chemical diversity of abyssal volcanic glass erupted along Park, California. We are particularly grateful to E. Jarosewich of Pacific, Atlantic, and Indian Ocean sea-floor spreading centers, in Sut- ton, G. H., Manghnani, M. H., and Moberly, R., eds., The geophysics the Smithsonian Institution and R. Batiza of the Scripps Institution of the Pacific Ocean Basin and its margin: Am. Geophys. Union of Oceanography, who provided analyses of the microlapilli, and Geophys. Mon. 19, p. 351-369. to W. Duffield and J. Hein, U.S. Geological Survey, and B. Glass, Menard, H. W., 1964, Marine geology of the Pacific: New York, University of Delaware, who patiently reviewed early drafts and McGraw-Hill Book Co., 271 p. made many helpful suggestions that improved the manuscript. Miyashiro, A., Shido, F., and Ewing, M., 1970, Crystallization and dif- Wally Charm provided SEM photographs. ferentiation in abyssal tholeiites and gabbros from mid-ocean ridges: Earth and Planetary Sci. Letters, v. 7, p. 361-365. Scheidegger, K. F., 1973, Temperatures and compositions of magmas as- REFERENCES CITED cending along mid-ocean ridges: Jour. Geophys. Research, v. 78, p. 3340-3355. Anderson, R. N., Clague, D. A., Klitgord, K. D., Marshall, Monte, and Schilling, J. G., 1973, Iceland mantle plume existence, and influence along Nishimori, R. K., 1975, Magnetic and petrological variations along the Reykjanes Ridge: I. 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Printed in U.S.A.
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