[1] The data and analysis presented in this paper provide an assessment of lava morphologies and the geochemistry of lavas from the Oman ophiolite. In order to provide detailed constraints on the construction of the upper oceanic crust, a continuous volcanic transect (300 m-thick) was sampled at high-frequency in the Semail ophiolite along Wadi Shaffan. The Wadi Shaffan section is composed mainly of pillow lavas interbedded with massive flows and occasional hyaloclastites. The sampling performed along Wadi Shaffan implies temporal variations in the activity of the ridge. The section is characterized by chemical compositions consistent with those of V1-Geotimes volcanism. The Wadi Shaffan transect was built through two main petrological and geochemical sequences of volcanic activity. Trace element ratios (e.g. Zr/Nb and La/Yb) allow us to distinguish two main sequences with two different parental magmas. Differences in the degree of partial melting are required to explain these trace element ratio variations. Beyond these differences in parent melt composition, variations in trace element abundances (TiO2, Zr, REE) involve differentiation processes prior to emplacement. In the lower sequence, less differentiated lavas are in the upper part of the cycle. Magma mixing is proposed to explain this reversed geochemical evolution through time. In the upper sequence, geochemical analysis suggests a different magma chamber process. This sequence consists of multiple events of magma emplacement. Variations in trace element abundance suggest four magmatic cycles. Each magmatic cycle is characterized by primitive lavas evolving to more differentiated lavas with time. The upper sequence lavas appear to be in equilibrium with clinopyroxene and lower sills from the MTZ (Mantle-Crust Transition Zone) and with lower gabbros. We propose a model in which the upper sequence lavas were directly derived from the MTZ and lower gabbro sills and then transported to the surface without interaction with higher crustal levels. G Geochemistry Geophysics Geosystems
The two main islands of the Juan Fernandez group, some 180 km apart are intra-plate volcanoes related to a single hot spot. K-Ar determinations suggest an age difference of c. 3 Ma which is in accord with a spreading rate of c. 6 cm a −1 in this part of the Nazca plate. The younger island of Alexander Selkirk is a relatively recent volcano (<1 Ma) composed of olivine tholeiites, many of them picrites, together with occasional quartz tholeiites and rare trachytes. Robinson Crusoe is an older, deeply dissected island with several separate volcanic centres. The older lavas are predominantly olivine tholeiites, again often picritic, with an occasional quartz tholeiite flow. Alkali basalt flows, never abundant, become more common in the higher part of the succession. Basanite flows are restricted to younger parasitic centres, but basanite dykes and sills cut the older formations. The tholeiites of Alexander Selkirk have a low pressure phenocryst assemblage and their low Ba contents, low Nb/Zr ratios and REE patterns suggest higher degrees of mantle partial melting than their closest counterparts on Robinson Crusoe. The basanites of the latter island have a higher pressure phenocryst assemblage and have higher La/Sm ratios than the tholeiitic lavas. The basanites are not always distinct from the Robinson Crusoe tholeiites on the basis of incompatible trace elements. 86 Sr/ 87 Sr ratios are fairly uniform ( c. 0.7035) in all the rock types but 143 Nd/ 144 Nd ratios are more variable, attaining their lowest values in some of the basanites. Trace element and isotopic data suggest remelting of source regions modified by migration of small degree partial melts over a long period.
Report of a joint Mineral Deposit Studies Group and Geosciences Information Group meeting in association with the Institution of Mining and Metallurgy (Midlands Section), held on 6 and 7 April 1993 in the Geology Department at Leicester University. The meeting was organized by Michael K.G. Whateley and Peter K. Harvey. After the first Mineral Resource Evaluation meeting held in Cardiff in March 1991, it became clear that there was a large, interested group of geologists, engineers and financial experts who have a common interest in mineral resource evaluation. It appeared timely to repeat the conference; this second meeting therefore was convened to discuss methods in mineral resource evaluation and describe case histories. This successful international meeting brought together all those who are involved in the day to day exploration and evaluation of mineral deposits in addition to those who are responsible for their economic evaluation and for mine feasibility and design studies. The two-day meeting included 31 papers, six posters and commercial displays by nine companies. A Special Publication of the Geological Society will be published within about a year from the date of the meeting. The meeting attracted over 100 speakers and delegates from Ireland, the Netherlands, USA, Portugal, Germany, Malawi, Canada, Ghana, Sweden, Bulgaria, Belgium, England, Scotland and even Wales. The theme of the conference was to discuss methods of mineral deposit evaluation, and to follow this with case histories from around the world. The conference started with a keynote address by G. Riddler (British Geological Survey) on
Abstract The inversion of chemical data derived from geochemical and related nuclear spectroscopy tools to provide mineralogy logs is now a widespread approach to the interpretation of these data. The most valuable of such transforms attempt to provide the percentages of actual phases present (minerals and fluids) at each depth interval rather than the ideal minerals occurring in simple theoretical models. Of the numerous problems involved in the inversion for a particular phase assemblage, the most serious is probably that of compositional colinearity in which three or more of the phases sought lie on, or close to, the same compositional plane. Depending upon the algorithm used for the inversion the effects of such compositional constraints may vary between a failure to find any (numerical) solution, failure to find a unique solution, or a solution which may be significantly in error for only a very small deviation in the chemical log or phase compositions. These effects are illustrated using geochemical logs from sedimentary environments together with examples from laboratory derived and numerically simulated geochemical data.
Calculating elemental mass balance across subduction zones enhances our understanding of global geochemical budgets and large‐scale Earth processes. However, to accurately constrain the input flux, it is critical to know the lithological diversity and chemical characteristics of the downgoing oceanic plate. The west Pacific altered ocean crust that was drilled during Ocean Drilling Program (ODP) Leg 185 represents a significant component of the input to the Mariana Subduction Factory. The lithological sequence in Hole 801C consists of aphyric basalt, occurring as thick massive units, pillow units, and breccia units. The shallowest basalts are intercalated with sediments and two hydrothermal deposits. Core recovery was good for a basement hole (average 47%); however, over half the lithological section was unaccounted for. Downhole logging data provide a continuous record of physical, chemical, and structural properties of the rocks at the borehole wall, thus, when calibrated using available cored material, they can be used to reconstruct lithology in unrecovered intervals. Core‐log integration results reveal a significant bias in core recovery, with infrequent retrieval of delicate breccia units and preferential recovery of more massive, competent, and less altered flow units. This is important because the breccia units are host to many of the key tracer elements used in mass balance calculations. The massive basalts exhibit high density, resistivity and velocity values and low porosity and gamma ray values. Formation MicroScanner (FMS) images of massive basalts are bright (reflecting their resistive nature) with a homogenous texture and regular fracture pattern. Breccia or pillow basalts are characterised by low resistivity, density and velocity, and high porosity and gamma ray values and unrecovered intervals displayed these same characteristics. The reconstructed log‐based lithological sequence consists of thick massive flow units (27.4%), pillow units (33%), breccia units (31%), sediments (1.4%), and hydrothermal deposits (1.3%), with 5.9% unclassified due to unreliable tool response in intervals where hole conditions were poor. These findings have a significant bearing on the Subduction Factory recycling equation. The proportion of pillow basalts doubled and the amount of breccia increased six‐fold from that reported using core description alone, demonstrating convincingly that core‐log integration is essential to provide an accurate representation of the input flux. The log‐based stratigraphy reconstructed for Hole 801C represents the first example of Jurassic fast‐spread (160 km/m.y.) ocean crust and provides constraints on the relationships between crustal structure, age, alteration, and spreading rate.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
