The geology and geochemistry of Isla Marchena, Galapagos Archipelago: An ocean island adjacent to a mid-ocean ridge

[1] We have measured Ni, Ca, and Mn in olivine phenocrysts from volcanoes in the Gal apagos Archipelago to infer the mantle source lithologies. Results show that peridotite is the dominant source lithology for Fernandina, Floreana, Genovesa, Wolf Island, and Darwin Island. These volcanoes largely characterize the PLUME, WD, FLO, and DUM Nd, Sr, and Pb isotopic endmembers of Harpp and White (2001). Volcan Wolf, Alcedo, Marchena, and Cerro Azul, also produced from the melting of peridotite sources, have isotopic compositions that can be defined by mixing of the four isotopic endmembers. Our analysis suggests that peridotite was present in the sources of the volcanoes covered in this study and therefore is the dominant source lithology of the Gal apagos plume. Pyroxenite melting is generally focused in two isotopically distinct domains: Roca Redonda, Volcan Ecuador, and Sierra Negra in the enriched western part of the archipelago and Santiago, Santa Cruz, and Santa Fe in the depleted east. One implication of this finding is that the Western and Eastern Pyroxenite Domains represent two separate bodies of recycled crust within the Gal apagos mantle plume. Furthermore, both isotopically enriched and depleted domains of the archipelago were generated from mixtures of peridotite and pyroxenite. This suggests that there is no relationship between the source lithology of the Gal apagos plume and its isotopic characteristics. The identification of peridotitesource melting in volcanoes with isotopic characteristics that have been attributed to recycled crust points to the importance of mixing in OIB genesis, consistent with studies in the Canary Islands.

Section: Methodsmentioning

confidence: 99%

Lithological structure of the Galápagos Plume

Vidito

Herzberg

Gazel

et al. 2013

“…Location descriptions enclosed in double quotes are copied from original publications; others are from references 1 and 4 and sources for this publication. References are numbered as follows: 1, Cox and Dalrymple [1966]; 2, Nordlie [1973]; 3, Swanson et al [1974]; 4, Bailey [1976]; 5, Geist et al [1986]; 6, Cullen and McBirney [1987]; 7, Vicenzi et al [1990] (for specimen descriptions); 8, Bow and Geist [1992]; 9, White et al [1993]; 10, Kurz and Geist [1999]; 11, Geist et al [1994].…”

Section: Islands Ages and Plate Motionmentioning

confidence: 99%

“…Figure 7 is modified from McBirney and Williams [1969, Figure 22, p. 85], which had been traced from U.S. HO chart 5845. The island is a low shield volcano, partly mantled by tephra and pyroclastic cones [ Vicenzi et al , 1990]. Cox was able to collect only three paleomagnetic sites, probably because of the ubiquity of tephra along the coast [cf.…”

Section: Island Descriptionsmentioning

confidence: 99%

“…[17] Figure 7 is modified from McBirney and Williams [1969, Figure 22, p. 85], which had been traced from U.S. HO chart 5845. The island is a low shield volcano, partly mantled by tephra and pyroclastic cones [Vicenzi et al, 1990] [1969] characterized the western part as an "older, ruined volcano" and showed as Figure 2 of their Plate 20 an aerial photograph of the spectacular fault scarp forming the western margin of the island. Cox took advantage of this unusual (for the Galapagos) shoreline exposure of lavas in stratigraphic sequence.…”

Section: Isla Marchenamentioning

confidence: 99%

See 1 more Smart Citation

Time‐averaged paleomagnetic field at the equator: Complete data and results from the Galapagos Islands, Ecuador

Grommé

Mankinen

Prévot

2010

[1] We present here the complete paleomagnetic laboratory results from a collection of approximately 1500 oriented cores from all 16 of the Galapagos Islands, Ecuador, collected by Allan Cox in 1964-1965 but nearly all previously unpublished. The islands are located in the eastern Pacific Ocean within 1.4°of latitude from the equator and range in age from historically erupted to 3 Ma, mostly determined by published K-Ar and 3 He isotopic dating. The number of sites collected on each island ranges from 1 to 28, for a total of 186. After combining duplicate site mean directions, 149 are used for an overall mean direction and 8 represent excursions and one reversal path. Divided by geomagnetic polarity chron, 110 site means are Brunhes or Jaramillo (normal polarity), 27 are Matuyama (reversed polarity), and 12 are Gauss (both polarities). We have completed the magnetic cleaning that was commenced in the late 1960s. Secondary (mostly viscous) magnetizations were nearly all removed by alternating field demagnetization at 10 mT. We have used the so-called blanket cleaning method, generally at 10 mT. All sites were in basalt flows and gave good paleomagnetic results; none was rejected in toto, and only a few core specimens were magnetically unsatisfactory. Nearly all sites had eight independently oriented cores, and within-site angular standard deviations of directions range from 1°to 8°. We used both Fisher and Bingham statistics to analyze the data and found that many of the direction populations are strongly elongate along the paleomagnetic meridian, while the corresponding virtual pole (VGP) populations are essentially circularly distributed. The paleomagnetic poles, calculated as the means of VGPs, are as follows: Brunhes and Jaramillo, north latitude = 86.9°, east longitude = 245.1°, and 95% confidence radius A 95 = 1.9°; Matuyama, latitude = 87.2°, longitude = 158.2°, and A 95 = 3.8°; Gauss, latitude = 83.0°, longitude = 204.7°, and A 95 = 7.0°. These paleomagnetic poles do not differ significantly from one another, but the Brunhes and Jaramillo combined pole is significantly near-sided with respect to the Galapagos, as is the overall mean pole, which is at latitude = 87.1°and longitude = 227.6°, with A 95 = 1.7°. Omitting the excursion and reversal path data, the overall angular standard deviation of VGPs is 11.7°with lower and upper 95% confidence limits of 10.8°and 12.7°, respectively, in good agreement with previously published values for near-equatorial sampling latitudes.

“…One of the petrologic features that distinguishes the northern lavas from those of the main Galápagos Archipelago is the abundance of plagioclase‐ultraphyric basalts. These lavas contain up to 50% anorthitic plagioclase megacrysts that can reach 3 cm in length [ Cullen et al , 1989; Vicenzi et al , 1990; Harpp et al , 2002; Sinton et al , 1993]. Every island in the northern region has erupted these lavas, whereas they are rare in the main archipelago [ Cullen et al , 1989].…”

Section: Wolf–darwin Lineament: Background and Observationsmentioning

confidence: 99%

Wolf–Darwin lineament and plume–ridge interaction in northern Galápagos

Harpp

Geist

2002

[1] The Wolf-Darwin Lineament (WDL), located in the northwestern sector of the Galápagos Archipelago, lies between the focus of the Galápagos hot spot and the Galápagos Spreading Center. Consequently, most researchers have attributed its origin to the interaction between the plume and the adjacent ridge. We propose that the WDL is caused only partially by the plume-ridge interaction, and instead that it is primarily the result of tensional stresses emanating from the inside corner of the transform fault at 91°W. An additional factor that amplifies the tension in this region is the oblique orientation of the major transform fault with respect to the Nazca plate's spreading direction. This setting creates a transtensional zone whereby strain is partitioned into strike-slip motion along the transform and extension throughout the inside corner of the ridge-transform system. The area under tension is magmatic owing to the overlapping effects of the ridge and the Galápagos plume. The extensional model predicts no age-progressive volcanism, which is supported by observed age relationships. The WDL volcanoes define two distinct chemical groups: lavas erupted south of Wolf Island have compositions similar to those produced along the GSC west of 93°W, while those from the northern WDL resemble GSC lavas from the segment directly north of the lineament. This geographic distribution implies that the WDL is supplied by the same type of plume-affected mantle as the segment of the GSC that produced the lithosphere underlying the volcanoes. The observed WDL geochemical gradients are consistent with the extension model; the region under tension simply taps hybrid products of mixing at the margins of the subridge convection system and the periphery of the plume. Essentially, the stress field around the transform fault, normally not observable in a typical midocean ridge setting, is illuminated by the presence of melt from the adjacent hot spot.Components: 8821 words, 8 figures.