Geology topics

R.V. Fodor

Publications and source records attributed to R.V. Fodor.

5 recordsLinked to original sources

USGS2006

Perspectives on basaltic magma crystallization and differentiation: Lava-lake blocks erupted at Mauna Loa volcano summit, Hawaii

Explosive eruptions at Mauna Loa summit ejected coarse-grained blocks (free of lava coatings) from Moku'aweoweo caldera. Most are gabbronorites and gabbros that have 0–26 vol.% olivine and 1–29 vol.% oikocrystic orthopyroxene. Some blocks are ferrogabbros and diorites with micrographic matrices, and diorite veins (≤2 cm) cross-cut some gabbronorites and gabbros. One block is an open-textured dunite. The MgO of the gabbronorites and gabbros ranges ∼ 7–21 wt.%. Those with MgO >10 wt.% have some incompatible-element abundances (Zr, Y, REE; positive Eu anomalies) lower than those in Mauna Loa lavas of comparable MgO; gabbros (MgO <10 wt.%) generally overlap lava compositions. Olivines range Fo 83–58 , clinopyroxenes have Mg#s ∼83–62, and orthopyroxene Mg#s are 84–63 — all evolved beyond the mineral-Mg#s of Mauna Loa lavas. Plagioclase is An 75–50 . Ferrogabbro and diorite blocks have ∼ 3–5 wt.% MgO (TiO 2 3.2–5.4%; K 2 O 0.8–1.3%; La 16–27 ppm), and a diorite vein is the most evolved (SiO 2 59%, K 2 O 1.5%, La 38 ppm). They have clinopyroxene Mg#s 67–46, and plagioclase An 57–40 . The open-textured dunite has olivine ∼ Fo 83.5 . Seven isotope ratios are 87 Sr/ 86 Sr 0.70394–0.70374 and 143 Nd/ 144 Nd 0.51293–0.51286, and identify the suite as belonging to the Mauna Loa system. Gabbronorites and gabbros originated in solidification zones of Moku'aweoweo lava lakes where they acquired orthocumulate textures and incompatible-element depletions. These features suggest deeper and slower cooling lakes than the lava lake paradigm, Kilauea Iki, which is basalt and picrite. Clinopyroxene geobarometry suggests crystallization at <1 kbar P. Highly evolved mineral Mg#s, <75, are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional ‘shifts.’ Ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes. Clinopyroxene geobarometry suggests <300 bar P. Open-textured dunite represents olivine-melt mush, precursor to vertical olivine-rich bodies (as in Kilauea Iki). Its Fo 83.5 identifies the most primitive lake magma as ∼8.3 wt.% MgO. Mass balancing and MELTS show that such a magma could have yielded both ferrogabbro and diorite by ≥50% fractional crystallization, but under different fO 2 : < FMQ (250 bar) led to diorite, and FMQ (250 bar) yielded ferrogabbro. These segregation veins, documented as similar to those of Kilauea, testify to appreciable volumes of ‘rhyolitic’ liquid forming in oceanic environments. Namely, SiO 2 -rich veins are intrinsic to all shields that reached caldera stage to accommodate various-sized cooling, differentiating lava lakes.

Hawaii↗

USGS1994

Petrology of gabbroic xenoliths in 1960 Kilauea basalt: crystalline remnants of prior (1955) magmatism

The 1960 Kapoho lavas of Kilauea's east rift zone contain 1-10 cm xenoliths of olivine gabbro, olivine gabbro-norite, and gabbro norite. Textures are poikilitic (ol+sp+cpx in pl) and intergranular (cpx+pl??ol??opx). Poikilitic xenoliths, which we interpret as cumulates, have the most primitive mineral compositions, Fo82.5, cpx Mg# 86.5, and An80.5. Many granular xenoliths (ol and noritic gabbro) contain abundant vesicular glass that gives them intersertal, hyaloophitic, and overall 'open' textures to suggest that they represent 'mush' and 'crust' of a magma crystallization environment. Their phase compositions are more evolved (Fo80-70, cpx Mg# 82-75, and An73-63) than those of the poikilitic xenoliths. Associated glass is basaltic, but evolved (MgO 5 wt%; TiO2 3.7-5.8 wt%). The gabbroic xenolith mineral compositions fit existing fractional crystallization models that relate the origins of various Kilauea lavas to one another. FeO/MgO crystal-liquid partitioning is consistent with the poikilitic ol-gabbro assemblage forming as a crystallization product from Kilauea summit magma with ???8 wt% MgO that was parental to evolved lavas on the east rift zone. For example, least squares calculations link summit magmas to early 1955 rift-zone lavas (???5 wt% MgO) through ???28-34% crystallization of the ol+sp+cpx+pl that comprise the poikilitic ol-gabbros. The other ol-gabbro assemblages and the olivine gabbro-norite assemblages crystallized from evolved liquids, such as represented by the early 1955 and late 1955 lavas (???6.5 wt% MgO) of the east rift zone. The eruption of 1960 Kapoho magmas, then, scoured the rift-zone reservoir system to entrain portions of cumulate and solidification zones that had coated reservoir margins during crystallization of prior east rift-zone magmas. ?? 1994 Springer-Verlag.

Bulletin of Volcanology↗

USGS1992

Ages, rare-earth element enrichment, and petrogenesis of tholeiitic and alkalic basalts from Kahoolawe Island, Hawaii

Kahoolawe Island, Hawaii (18×11 km), is a basaltic shield volcano with caldera-filling lavas, seven identified postshield vents, and at least two occurrences of apparent rejuvenated-stage eruptive. We examined 42 samples that represent all stages of Kahoolawe volcano stratigraphy for their petrography, whole-rock major-and trace-element contents, mineral compositions, and K−Ar ages. The two oldest shield samples have an average age of 1.34±0.08 Ma, and four postshield samples (3 are alkalic) average 1.15±0.03 Ma; ages of 1.08 and 0.99 Ma for two additional tholeiitic samples probably are minimum ages. Whole-rock major- and trace-element and mineral compositions of Kahoolawe shield and caldera-fill laves are generally similar to the lavas forming Kilauea and Mauna Loa tholeiitic shields, but in detail, Kahoolawe shield lavas have distinctive compositions. An unusual aspect of many postshield Ka-hoolawe lavas is anomalously high REE and Y abundances (up to 200 ppm La and 175 ppm Y) and negative Ce anomalies. These enrichments reflect surficial processes, where weathering and soil development promoted REE-Y transport at the weathering front. Major element abundances (MgO, 10–6 wt.%) for shield and caldera-fill basalts are consistent with fractionation of ol+px+pl in frequently replenished magma reservoirs. In general, tholeiitic basalts erupted from late vents are higher in SiO 2 than the shield lavas, and temporal differences in parental magma compositions are the likely explanation. Alkalic basalts that erupted from vents are comparable in composition to those at other Hawaiian volcanoes. Trace-element abundance ratios indicate that alkalic basalts represent either relatively lower degrees of melting of the shield source or a distinct source. Apparent rejuvenated-stage basalts (i.e., emplaced after substantial Kahoolawe erosion) are tholeiitic, unlike the rejuvenated-stages at other Hawaiian volcanoes (alkalic). Kahoolawe, like several other Hawaiian volcanoes, has intercalated tholeiitic and alkalic basalts in the postshield stage, but it is the only volcano that appears to have produced tholeiitic rejuvenated-stage lavas.

Hawaii↗

USGS1990

Petrology, isotope characteristics, and K-Ar ages of the Maranhão, northern Brazil, Mesozoic basalt province

Northern Brazil contains remnants of Mesozoic flood basalts and hypabyssal rocks that were apparently emplaced during tectonism related to opening of the Atlantic Ocean. Analyses and new K-Ar ages reveal that this &sim;700x250 km Maranhão province (5°–8°S) has low-Ti basalts (&sim;1.1 wt% TiO 2 ) in the western part that range about 160 to 190 Ma, and high-Ti basalts (3.4–4.4 wt% TiO 2 ) in the eastern part about 115–122 Ma. Low-Ti basalt compositions are less evolved and have a smaller range, Mg# 62-56, than the high-Ti basalts, Mg# 44–33. General characteristics of the least evolved members of low- and high-Ti groups include, respectively, Zr 100 and 250 ppm, Sr 225 and 475 ppm, Ba 200 and 500 ppm, Nb 10 and 26 ppm, Y 29 and 36 ppm, La/Yb (n) 4.2 and 8.8, where La (n) is 30 and 90. Overall compositions resemble the low- and high-Ti basaltic rocks of the Mesozoic Serra Geral (Paraná) province in southern Brazil. The Maranhão low-Ti basalts have more radiogenic Sr and Pb and higher δ 18 O than the high-Ti basalts. Respectively, low- vs high-Ti: ɛ Sr 26−54 vs 15−18; 206 Pb/ 204 Pb=18.25–.78 vs 18.22–.24; and δ 18 O 8.9–12.6 vs 6.5–8.6. Nd isotopes overlap: ɛ Nd −1.6 to −3.8 vs −2.1 to −3. Ages, compositions, and isotopes indicate that the low- and high-Ti groups had independent parentages from enriched subcontinental mantle. However, both groups can be modeled from one source composition if low-Ti basalt isotopes reflect crustal contamination, and if the parentages for each group were picritic liquids that represent either higher (for low-Ti) or lower (for high-Ti) percentages of melting of that single source. When comparing Pb isotopes of Maranhão and Serra Geral high-Ti basalts (uncontaminated) to evaluate the DUPAL anomaly, Maranhão has Pb Δ7/4=4.6–11, and Pb Δ8/4=72–87; Serra Geral has Pb Δ7/4=10–13, and Pb Δ8/4=95–125. The small difference is not enough to conform to DUPAL contours, and is inconsistent with large-scale isotopic heterogeneity of mantle beneath Brazil prior to rifting of South America from Africa. Maranhão low-Ti magmas probably relate to the opening of central North Atlantic, and high-Ti magmas to the opening of equatorial Atlantic. The proposed greater percentage of source melting for low-Ti basalts may reflect a Triassic-Jurassic hotspot, while lesser melting for high-Ti magmas may relate to Cretaceous decompressional (rifting) melting.

Contributions to Mineralogy and Petrology↗

USGS1983

K-Ar ages and the opening of the South Atlantic Ocean: Basaltic rock from the Brazilian margin

New K-Ar ages for 13 samples of basalt, gabbro, wehrlite, and trachyandesite drilled from the Brazilian continental shelf and coastline yield information about the timing of the opening of the South Atlantic and the nature of the crust seaward from the Brazil margin. The oldest basalt is 138.1 ± 3.5 m.y. "> 138.1 ± 3.5m.y. old and from offshore at 24.5°S; it represents Serra Geral flood basalt on attenuated crust in the Santos basin. Basalts from the coastline at 19.5°S are mixed with terrigenous graben sediments, and their ages confirm that rifting was underway before 130 m.y. Offshore sites in the Campos basin, ∼22°–23°S, 41°W, have basalt ranging from 124 to 112 m.y. in age and mostly continental flood basalt in composition. One 112-m.y. basalt, however, is MORB-like and could therefore represent oceanic lithosphere mixed with continental crust about 50 km from the shoreline at ∼22.5°S. Other samples reveal compositionally varied intraplate, passive-margin magmatism occurring 75-43 m.y. ago. They correlate with profuse contemporary alkalic magmatism on the southeastern Brazil coast and probably represent reactivation of zones of “weakness” (i.e., fracture zone-lineaments).

Marine Geology↗