Background Inhalation of asbestos and other mineral fibers are known causes of malignant mesothelioma (MM) and lung cancers. In a setting of occupational exposure to asbestos, MM occurs 4–8 times more frequently in men than in women, at the median age of 74 years, while an environmental exposure to asbestos causes the same number of MMs in men and women, at younger ages. Methods We studied the geology of Nevada to identify mineral fibers in the environment. We compared MM mortality in different Nevada Counties, per sex and age group, for the 1999–2010 period. Results We identified the presence of carcinogenic minerals in Nevada, including actinolite asbestos, erionite, winchite, magnesioriebeckite and richterite. We discovered that, compared with the US and other Nevada counties, Clark and Nye counties, in southern Nevada, had a significantly higher proportion of MM that occurred in young individuals (<55 years) and in women. Conclusions The elevated percentage of women and individuals younger than 55 years old, combined with a sex ratio of 1:1 in this age group and the presence of naturally occurring asbestos, suggests that environmental exposure to mineral fibers in southern Nevada may be contributing to some of these mesotheliomas. Further research to assess environmental exposures should allow the development of strategies to minimize exposure, as the development of rural areas continues in Nevada, and to prevent MM and other asbestos-related diseases.

Suprasubduction-zone ophiolites have been recognized in the geologic record for over thirty years. These ophiolites are essentially intact structurally and stratigraphically, show evidence for synmagmatic extension, and contain lavas with geochemical characteristics of arc-volcanic rocks. They are now inferred to have formed by hinge retreat in the forearc of nascent or reconfi gured island arcs. Emplacement of these forearc assemblages onto the leading edge of partially subducted continental margins is a normal part of their evolution. A recent paper has challenged this interpretation. The authors assert that the "ophiolite conundrum" (seafl oor spreading shown by dike complexes versus arc geochemistry) can be resolved by a model called "historical contingency," which holds that most ophiolites form at mid-ocean ridges that tap upper-mantle sources previously modifi ed by subduction. They support this model with examples of modern mid-ocean ridges where suprasubduction zone-like compositions have been detected (e.g., ridge-trench triple junctions). The historical contingency model is fl awed for several reasons: (1) the majorand trace-element compositions of magmatic rocks in suprasubduction-zone ophiolites strongly resemble rocks formed in primitive island-arc settings and exhibit distinct differences from rocks formed at mid-ocean-ridge spreading centers; (2) slab-infl uenced compositions reported from modern ridge-trench triple junctions and subduction reversals are subtle and/or do not compare favorably with either modern subduction zones or suprasubduction-zone ophiolites; (3) crystallization sequences, hydrous minerals, miarolitic cavities, and reaction textures in suprasubduction-zone ophiolites imply crystallization from magmas with high water activities, rather than mid-ocean-ridge systems; (4) models of whole Earth convection, subduction recycling, and ocean-island basalt isotopic compositions ignore the fact that these components represent the residue of slab melting, not the low fi eld strength element-enriched component found in active arc-volcanic suites and suprasubduction-zone ophiolites; and (5) isotopic components indicative of mantle heterogeneities

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Abstract. The steeply tilted Mount Perkins block, northwestern Arizona, exposes a cross section of a magmatic system that evolved through the onset of regional extension. New 4øAr/39Ar ages of variably tilted (0-90 ø) volcanic strata bracket extension between 15.7 and 11.3 Ma. Preextensional intrusive activity included emplacement of a composite Miocene laccolith and stock, trachydacite dome complex, and east striking rhyolite dikes. Related volcanic activity produced an -18-16 Ma stratovolcano, cored by trachydacite domes and flanked by trachydacite-trachyandesite flows, and -16 Ma rhyolite flows. Similar compositions indicate a genetic link between the stratovolcano and granodioritic phase of the laccolith. Magmatic activity synchronous with early regional extension (15.7-14.5 Ma) generated a thick, felsic volcanic sequence, a swarm of northerly striking subvertical rhyolite dikes, and rhyolite domes. Field relations and compositions indicate that the dike swarm and felsic volcanic sequence are cogenetic. Modes of magma emplacement changed during the onset of extension from subhorizontal sheets, east striking dikes, and stocks to northerly striking, subvertical dike swarms, as the regional stress field shifted from nearly isotropic to decidedly anisotropic with an east-west trending, horizontal least principal stress. Preextensional trachydacitic and preextensional to synextensional rhyolitic magmas were part of an evolving system, which involved the ponding of mantle-derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower levels. Major extension halted this system by generating abundant pathways to the surface (fractures), which flushed out preexisting crustal melts and hybrid magmas. Remaining silicic melts were quenched by rapid, upper crustal cooling induced by tectonic denudation. These processes facilitated eruption of mafic magmas. Accordingly, silicic magmatism at Mount Perkins ended abruptly during peak extension -14.5 Ma and gave way to mafic magmatism, which continued until extension ceased.

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