Geology Reports⌕ Search

Geology topics

J.S. Noller

Publications and source records attributed to J.S. Noller.

3 recordsLinked to original sources
USGS1991

Petrology of lower crustal and upper mantle xenoliths from the Cima Volcanic Field, California

Basaltic rocks of the Cima volcanic field in the southern Basin and Range province contain abundant gabbro, pyroxenite, and peridotite xenoliths. Composite xenoliths containing two or more rock types show that upper-mantle spinel peridotite was enriched by multiple dike intrusions in at least three episodes; the mantle was further enriched by intergranular and shear-zone melt infiltration in at least two episodes. The oldest dikes, now metamorphosed, are Cr-diopside websterite. Dikes of intermediate age are most abundant at Cima and consist of igneous-textured websterite and two-pyroxene gabbro and microgabbro of tholeiitic or calcalkalic parentage. The youngest dikes are igneous-textured clinopyroxenite, gabbro, and olivine microgabbro of alkalic parentage. The dikes in peridotite are interpreted as parts of a system of conduits through which tholeiitic (or calcalkalic) and alkalic magmas fed lower-crustal intrusions, which are represented by abundant xenoliths of the same igneous rock types as observed in the dikes. Mineral assemblages of dikes in peridotite indicate that an enriched uppermost mantle zone no thicker than 15 km could have been sampled. Because of their high densities, the gabbros and pyroxenites can occupy the zone immediately above the present Moho (modeled on seismic data as 10-13 km thick, with V p 6.8 km/s) only if their seismic velocities are reduced by the joints, partial melts, and fluid inclusions that occur in them. Alternatively, these xenoliths may have been derived entirely from beneath the Moho, in which case the Moho is not the local crust-mantle boundary.

Journal of Petrology↗
USGS1986

Solid and fluid inclusions in mantle xenoliths: An analytical dilemma?

Two Cr-diopside Iherzolite xenoliths with kaersutite selvages, from Dish Hill, California, contain four types of solid and fluid inclusions that can be used to discriminate between actual mantle processes and processes acting upon a fragment of mantle as it is entrained and carried to the earth surface. On the basis of distribution, early formed inclusions are assigned to a process associated with emplacement of dikes in the mantle. The other types of inclusions were generated during ascent in the host basalt magma. Solid and fluid inclusions are important sites of incompatible element concentrations in mantle xenoliths. Scientists assigning specific chemical signatures to mantle processes are confronted with an analytical dilemma: leaching of xenolith samples before analysis may destroy inclusions that are carrying intrinsic mantle components, whereas failure to leach the samples probably leaves a host-rock contaminant. Thin-section maps of these xenoliths show that the distribution and abundance of two types of solid inclusions are systematic. Through the use of these maps it is demonstrated that zones in xenoliths with the least amount of postentrainment contamination can be pinpointed, and the problems resulting from random leaching can be minimized.

Geology↗