Jorullo Volcano, Michoacan, Mexico: the earliest stages of fractionation in calc-alkaline magmas

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6453125

Luhr, J; Carmichael, I S.E.

Between 1759 and 1774, Jorullo Volcano and four associated cinder cones erupted approximately 2 km/sup 3/ of magma which evolved progressively with time from early ol-hy-normative primitive basalts (Mgnumber=73, 516 ppm Cr, 260 ppm Ni) to late-stage qtz-hy-normative basaltic andesites. All lavas contain <6 vol% phenocrysts of magnesian olivine (fO/sub 90-70/) with Cr-Al-Mg spinel inclusions, and microphenocrysts of plagioclase and augite. Smooth whole-rock major and trace element compositional trends through the suite can be largely modeled by simple crystal fractionation of olivine, augite, plagioclase, and minor spinel. In order for augite and plagioclase to have been near-liquidus phases, the modeled crystal fractionation event must have occurred at lower-crustal to upper-mantle pressures (8-15 Kb). The crystals actually present in the Jorullo lavas, however, formed at low pressures. Compared to modeled abundances, La, Ce, Rb, Sr, Ba, Hf, Th, and Ta are anomalously enriched in the late-stage basaltic andesites and Dy, Yb, and Lu are anomalously depleted. These discrepancies apparently indicate the importance of other magma-chamber processes. Most high-alumina basalts reported in the literature have 18 to 21 wt% Al/sub 2/O/sub 3/, but are too depleted in MgO, Cr, and Ni to be direct mantle products. These high-alumina basalts have probably undergone significant fractionation of olivine, augite, plagioclase, and spinel from primitive parental basalt similar to the early Jorullo magmas. For hydraulic reasons, such primitive basalts are rarely erupted in mature arcs, and may be completely absent from mature stratovolcanoes.

• Research Organization: Franklin and Marshall College, Lancaster, PA (USA)
• OSTI ID: 6453125
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580300 -- Mineralogy
Petrology
&amp; Rock Mechanics-- (-1989)
580400* -- Geochemistry-- (-1989)
ALKALINE EARTH METAL COMPOUNDS
ALUMINIUM COMPOUNDS
ALUMINIUM OXIDES
ANDESITES
BASALT
CHALCOGENIDES
CHEMICAL COMPOSITION
DEVELOPING COUNTRIES
ERUPTION
FELDSPARS
FRACTIONATION
IGNEOUS ROCKS
INCLUSIONS
IRON COMPOUNDS
IRON SILICATES
LATIN AMERICA
LAVA
MAGMATISM
MAGNESIUM COMPOUNDS
MAGNESIUM OXIDES
MAGNESIUM SILICATES
MEXICO
MINERALOGY
MINERALS
NORTH AMERICA
OLIVINE
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PETROCHEMISTRY
PETROGENESIS
ROCKS
SEPARATION PROCESSES
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SPINELS
TRANSITION ELEMENT COMPOUNDS
VOLCANIC ROCKS
VOLCANISM
VOLCANOES