Tephra glasses retrieved from 10 Deep Sea Drilling Project (DSDP) cores around the Mariana arc system make up a remarkable record of explosive volcanism in the Marianas over the past 40 m.y. Major element compositions for and used to examine the nature and history of this activity. Three maxima of volcanic explosivity, presumably related to especially vigorous volcanism in the Mariana arc, are identified, with the biggest maximum at around 18--11 Ma and two other maxima at 35--24 and 6--0 Ma; the two younger maxima are contemporaneous with peaks in explosive volcanism observed for other western Pacific arcs. Explosive arc volcanism has been predominantly tholeiitic since shortly after arc inception; no boninitic glasses were found. The tephra glasses belong to the low- to medium-K suite, except during an enigmatic phase of medium- to high-K explosive volcanism during the late Miocene (11--7 Ma). Even though Mariana tephra glasses are largely similar in composition to Mariana arc lavas, the tephra show a much larger compositional range than that found for subaerial Mariana arc lavas, which totally lack dacitic to rhyolitic volcanic products.

The tephra glasses define a bimodal population in terms of silica content, with a pronounced minimum, or ''Daly gap,'' around 65--66% SiO2. Mariana tephra glasses are fractionated, with the least evolved glass being Fe-rich and having a magnesium number (Mg ♯) (100Mg/Mg+Fe2+) of 55.

Basaltic tephra glasses contain ≤16% Al2O3 (average 14.3% Al2O3) and are not high-alumina basalts; this contrasts with the observation that modern Mariana arc lavas contain 15--21% (average 17.4%) Al2O3. The high-alumina basalt lavas of the Mariana arc probably reflect plagioclase accumulation and not liquid compositions. All tephra glasses plot near low-pressure cotectics and reaction curves on the subprojection olivine-clinopyroxene-quartz; mafic and felsic samples define distinct trends. The mafic trend reflects fractional crystallization of mantle-derived basaltic magma, whereas the felsic trend may be due either to anatexis of Mariana arc crust or to fractionation of mafic melts. The tephra glass data reinforce the model that the magmatic evolution of the Mariana arc has been dominated by low-pressure fractionation, perhaps accompanied by anatexis. Episodic changes in melting regime to generate Miocene potassic tephra may be related to changing mantle sources and processes related to episodes of back arc basin spreading. These episodic changes are superimposed on a long-term increase in potassium that reflects progressive metasomatism of the mantle source. Long-term increases in K2O contents for Mariana arc magmas inferred from the tephra glass record are 0.004 wt % m.y.-1 (mafic), 0.011 wt % m.y.-1 (intermediate), and 0.023 wt % m.y.-1 (felsic). ¿ American Geophysical Union 1995