Geotimes - July 2004 - Highlights

Geotimes

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Highlights

Igneous Petrology
Lang Farmer

How mafic rocks in Earth's mantle are involved in the production of magmas and how we determine the lifespans of magma bodies in Earth's crust remained important and controversial issues in igneous petrology last year.

One of the more provocative papers of 2003 provided a new perspective on the generation of ocean island alkali basalts. Many workers have conceded that ocean island basalts are derived in some measure by partial melting of subducted oceanic crust in Earth's mantle and not solely from small degrees of melting of ultramafic-composition mantle rocks. But until now the consensus has been that partial melting of mafic lithologies in the mantle, such as pyroxenites, cannot directly generate the magmas parental to alkali basalts. An experimental study published by Marc Hirschmann and coworkers in the June 2003 Geology, however, demonstrated that the composition of liquids produced from the melting of garnet pyroxenites may strongly depend on the bulk composition of the pyroxenite itself. Hirschmann and colleagues demonstrated that partial melting of "silica-deficient" garnet pyroxenites (those containing accessory olivine rather than quartz, feldspar or kyanite) produces magnesian silica-undersaturated melts compositionally similar to alkali basalts, even at depths as shallow as 60 kilometers. These results imply that highly magnesian and alkalic basaltic melts can be produced in the upper mantle without the involvement of the ultramafic rocks, hitherto one of the basic tenets of alkali basalt petrogenesis.

Adakites, which are calc-alkaline, high-magnesium andesites, are another possible product of partial melting of mafic rocks in the upper mantle. For these rocks, an origin through melting of subducted oceanic crust is often invoked, given that Phanerozoic adakites are associated with volcanism at convergent plate margins. But in the November Geology, Sun-Lin Chung and coworkers described a group of "post-orogenic" adakites from the Cenozoic in southern Tibet, the first Phanerozoic adakites identified outside active subduction zones. The researchers suggest that the Tibetan adakites were derived from partial melting of lower continental crustal eclogites or garnet amphibolites, with the melting triggered by convective removal of the underlying continental lithosphere during the formation of the Tibetan Plateau. One interesting implication of their discovery is that adakite-like rocks found in Archean crustal terranes (trondhjemite-tonalite-granodiorite suites) can no longer be considered as unequivocal evidence of Archean subduction zones, as these rocks may instead be the products of Archean lithosphere delamination events.

Of course, all of this depends on whether adakites actually derive from the melting of mafic rocks. Tim Grove and coworkers provide an alternative view in Contributions to Mineralogy and Petrology (v. 145, p. 515-533). Their experimental studies of adakite-like rocks from Mount Shasta, Calif., suggest that these rocks ultimately represent the product of hydrous melting of ultramafic (albeit refractory) portions of the mantle wedge above the subducting oceanic crust. Their study illustrates that basic questions remain regarding the origin of adakites and the range of magma types produced from hydrated ultramafic mantle.

How long silicic magmas reside in the continental crust before erupting remained a debate issue in 2003. In the June Geology, John Wolff and Frank Ramos provided the first high-precision lead isotopic analyses of individual sanidine phenocrysts in ash-flow tuffs. Working on the 1.6-million-year-old Otowi Member of the Bandelier Tuff in New Mexico, the researchers demonstrated that the sanidines have variable lead and strontium isotopic compositions. The lead isotopic variations, however, could not have been generated within the sanidine grains or their host magmas prior to eruption, at least not in any geologically reasonable amount of time. Instead, the lead isotopic heterogeneities must reflect wall rock assimilation, a likely origin for the strontium isotopic variations as well. These results imply that attempts to use radiogenic isotopic variations in phenocrysts to determine magmatic residence times may be stymied by wall rock interaction and call into question past strontium isotope studies suggesting that some silicic magmas fester in the upper crust for up to 1 million years before erupting.

By contrast, the zircon oxygen isotope study presented by Ilya Bindeman and John Valley in the May 2003 Geological Society of America Bulletin may have more robust implications for crustal residence times of silicic magmas. These authors found that zircon crystals in a low-oxygen-isotope ash flow at Timber Mountain in southern Nevada preserve core-to-rim variations. These isotopic variations identify the zircon crystals as xenocrysts inherited from earlier volcanic rocks that melted to form the ash itself. Furthermore, because of the preservation of isotopic zonations in zircon but not in any other phenocryst mineral phases in the tuff, the inherited zircon grains were likely immersed in the silicic magma for 10,000 to 15,000 years before erupting in their host ash-flow tuff.

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Farmer is a professor in the Department of Geological Sciences and at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, Boulder. E-mail: farmer@cires.colorado.edu.

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