Geochemistry and Petrology of Emeishan Basalts and Subcontinental Mantle Evolution in Southwestern China

Three major volcanic rock sequences in the P2β formation(Emeishan basalts)were sampled dur-ing a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China .Two of the three sections-Emei and Tangfang are composed of continental flood basalts(CFB) while the third-Ertan is an alkalic center.Multi-element chemical analyses indi-cate a predominance of low MgO transitional quartz tholeiites at Emei and Tangfang,whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites.Consanguineity of the rocks from the three sections is sug-gested by consistently high TiO2 ,K2O,incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions,but antypical of CFB.Sr isotope data for ten Emei basalt samples(^87Sr/^86Sr=0.7066-0.7082)which show no correla-tion with Rb/Sr ratios (0.02-0.12) and Nd isotopes for two of the samples(^143Nd/^144Nd=0.51171-0.51174)are interpreted as being related to the mantle evolution.The primary magmas re-sponsible for all the three sequences have been modeled in terms of a uniformly metasomatized man-tle source.Trace element models support the derivation of the Emei and Tangfang primary magmas from 10-15 percent partial melting of spinel lherzolite,followed by fractional crystallization of olivive and clinopyroxene.The primary alkaline olivine basalts at Ertan are generated by 7-10 percent par-tial melting of a chemically equivalent source in the garnet-peridodite stability region.The assumed mantle composition is characterixzed by Rb=3.8-5.5 ppm,Sr=62-83ppm,Ba=45-64 ppm,La=3.8-5.6ppm,and Yb=0.46-0.57ppm.The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par-tial melting.Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diaper acts as the agent for upward enrichment as well as uplift and extension of the crust.     

Petrochemistry of the volcanic rocks of the Island of Principe,Gulf of Guinea

Principe is one of the volcanic centres comprising the Cameroun line in West Africa. The volcanic rocks can be divided into two stratigraphic units:

1. Younger lava series — basanite and nephelinite overlying.
2. Older lava series — transitional to mildly alkaline basalt and hawaiite.

These units lie on a basement of palagonite breccias of tholeiitic affinities. The basic lavas are intruded by plugs ranging in composition from tristanite to phonolite and are overlain by phonolite lavas. These rocks form two chemically and mineralogically distinct suites:

1. Phonolites which evolved by low pressure crystal fractionation of the younger lava series basanitic magma, and
2. Tristanite — trachyte — trachyphonolite suite which may have evolved by high pressure crystal fractionation of the older lava series magma.

     

Whole-basin,mass-balance approach for identifying critical phosphorus-loading thresholds in shallow lakes

Lake Lochloosa, Florida (USA) recently underwent a shift from macrophyte to phytoplankton dominance, offering us the opportunity to use a whole-basin, mass-balance approach to investigate the influence of phosphorus loading on ecosystem change in a shallow, sub-tropical lake. We analyzed total phosphorus (TP) sedimentation in the basin to improve our understanding of the forcing factor responsible for the recent shift to phytoplankton dominance. We measured ²¹⁰Pb activity, organic matter (OM), organic carbon (OC) and TP in short sediment cores from 20 locations to develop a comprehensive, whole-basin estimate of recent mass sedimentation rates (MSR) for bulk sediment, OM, OC and TP. The whole-basin sedimentation models provided insights into historic lake processes that were not evident from the limited, historic water quality data. We used Akaike’s Information Criteria to differentiate statistically between constant MSR and exponentially increasing MSR. An eightfold, exponential increase in TP accumulation over the past century provided evidence for the critical role of increased P loading as a forcing factor in the recent shift to phytoplankton dominance. Model results show increased TP retention and decreased TP residence time were in-lake responses to increased TP loading and the shift from macrophyte to phytoplankton dominance in Lake Lochloosa. Comparison of TP loading with TP retention and historic, diatom-inferred limnetic TP concentrations identified the TP loading threshold that was exceeded to trigger the shift to phytoplankton dominance.     

Preliminary study on the preservation of giant clam (Tridacnidae) shells from the Balobok Rockshelter archaeological site,south Philippines

Tridacnidae shells, a valuable archive of past environments, are common in the Balobok Rockshelter archaeological site on Sanga‐Sanga Island in the south Philippines. This site was occupied during the mid‐Holocene (ca. 5000–8800 ¹⁴C yr B.P.), a period of Neolithic cultural expansion in the Philippines. This paper focuses on the preservation of two shell specimens, Hippopus hippopus and Tridacna maxima, unearthed from two mid‐Holocene layers within the rockshelter. The shells' mineralogy and microstructure (prismatic and crossed‐lamellar) were studied using micro‐Fourier transform infrared spectroscopy and scanning electron microscopy to determine if the samples were suitable as paleoenvironmental records. Both shells are still aragonitic but aragonite crystals of both microstructure types are partly dissolved. This dissolution, characteristic of meteoric water alteration, precludes their utility in paleoenvironmental geochemical studies. Nevertheless, these shells are abundant in archaeological sites in the region and may be better preserved in other depositional contexts; more studies on Philippine Tridacnidae shell diagenesis are needed. © 2011 Wiley Periodicals, Inc.