REE fractionation and Ce anomalies in weathered Karoo dolerite
- Authors: Marsh, Julian S
- Date: 1991
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/145819 , vital:38469 , https://doi.org/10.1016/0009-2541(91)90099-D
- Description: Analyses of samples from a weathering profile on Karoo dolerite allow elements to be divided into three groups depending on their behaviour. Si, K, Na, Mg, Ca, Sr, Ba and V are mobilized and removed from weathered products. Fe, Al. Ti, Zr, Hf, Zn, Cu, Sc, Co and Ni are immobile. REE, Y, and to a lesser extent Cr, are mobile and redistributed within the profile without a net loss of these elements from the profile. Large positive Ce anomalies are developed in oxidized weathered products by preferential leaching of the other REE's. Negative Ce anomalies and REE enrichment is a feature of less altered dolerite.
- Full Text: false
- Date Issued: 1991
The petrogenesis of the Kirwan Basalts of Dronning Maud Land, Antarctica
- Authors: Harris, Chris , Marsh, Julian S , Duncan, Andrew R , Erlank, Anthony J
- Date: 1990
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/145492 , vital:38443 , https://doi.org/10.1093/petrology/31.2.341
- Description: The 420 m thick sequence of Kirwan basalt crops out along the southernmost 50 km of the Kirwanveggen Escarpment (74°S, 6°W). There is little variation in major element chemistry of these basalts (SiO2 49·3–51·6 wt.%; MgO 5·1–6·6 wt.%), but the concentrations of certain incompatible elements (e.g., Zr) vary by factors of approximately two or more.
- Full Text: false
- Date Issued: 1990
The petrogenesis of the Kirwan Basalts of Dronning Maud Land, Antarctica
- Authors: Harris, Chris , Marsh, Julian S , Duncan, Andrew R , Erlank, Anthony J
- Date: 1990
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/145472 , vital:38441 , https://doi.org/10.1093/petrology/31.2.341
- Description: The 420 m thick sequence of Kirwan basalt crops out along the southernmost 50 km of the Kirwanveggen Escarpment (74°S, 6°W). There is little variation in major element chemistry of these basalts (SiO2 49·3–51·6 wt.%; MgO 5·1–6·6 wt.%), but the concentrations of certain incompatible elements (e.g., Zr) vary by factors of approximately two or more.
- Full Text: false
- Date Issued: 1990
Oxygen isotope geochemistry of the Mesozoic volcanics of the Etendeka Formation, Namibia
- Authors: Harris, Chris , Smith, H Stuart , Milner, Simon C , Erlank, Anthony J , Duncan, Andrew R , Marsh, Julian S , Ikin, Nicholas P
- Date: 1989
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/143272 , vital:38219 , https://doi.org/10.1007/BF00371087
- Description: The Etendeka Formation volcanics consist of a bimodal association of basalts and quartz latites. Forty three new whole rock oxygen isotope analyses are reported for all the major magma types. All the rocks except a minor suite of dolerites have higher δ18O values than normal mantle. The basic rocks (average of 29=8.8‰) have significantly different δ18O to the acid rocks (average of 10=14.4‰). These data are apparently consistent with previously published petrogenetic models, which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived.
- Full Text: false
- Date Issued: 1989
The geochemistry of potassic lavas from Vulsini, central Italy and implications for mantle enrichment processes beneath the Roman region
- Authors: Rogers, N W , Hawkesworth, C J , Parker, R J , Marsh, Julian S
- Date: 1985
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/136034 , vital:37330 , https://doi.org/10.1007/BF00378265
- Description: Major and trace element and 143Nd/144Nd (0.51209–0.51216) and 87Sr/86Sr (0.70879–0.71105) isotope analyses are presented on a representative group of lavas from the Vulsini district of the Roman magmatic province. Three distinct series are identified; the high-K and low-K series are similar to those described from other Italian volcanoes, while the third is represented by a group of relatively undifferentiated leucite basanites which are thought to be near-primary mantle melts.
- Full Text: false
- Date Issued: 1985
The mineralogy, petrology, and origin of the Merensky cyclic unit in the western Bushveld Complex
- Authors: Kruger, Floris J , Marsh, Julian S
- Date: 1985
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/136024 , vital:37329 , https://doi.org/10.2113/gsecongeo.80.4.958
- Description: The Merensky cyclic unit of the Bushveld Complex represents the first products to crystallize after the influx and mixing of a large new batch of magma in the chamber. Excluding the Merensky pegmatoid, the Merensky cyclic unit grades upward from an orthopyroxenite at the base, through norite to anorthosite at the top of the sequence. It is followed by the very similar Bastard cyclic unit.
- Full Text: false
- Date Issued: 1985
Al/Cr ratios of coexisting pyroxenes and spinellids in some ultramafic rocks
- Authors: Eales, Hugh V , Marsh, Julian S
- Date: 1983
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/133534 , vital:36987 , https://doi.org/10.1016/0009-2541(83)90045-1
- Description: Al/Cr atomic ratios of coexisting spinellids and ortho- and clinopyroxenes show a pattern of sympathetic variation that persists through ultramafic rocks of layered mafic complexes of upper-crustal type, Alpine complexes, and the nodules found in kimberlites and alkaline basalts. Simple expressions are empirically derived to link (Al/Cr) orthopyroxene and (Al/Cr)spinel ratios in putatively equilibrated rocks. Equivalence of spinel compositions in shallow layered complexes and kimberlite nodules of deep-seated origin negates direct crystallochemical control of Al/Cr ratios of spinels by pressure.
- Full Text: false
- Date Issued: 1983
Significance of 87Sr/86Sr ratios in the Merensky cyclic unit of the Bushveld Complex
- Authors: Kruger, Floris J , Marsh, Julian S
- Date: 1982
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/133524 , vital:36986 , https://doi.org/10.1038/298053a0
- Description: In a number of mafic layered intrusions, layers with platinum group element (PGE) mineralization coincide with breaks in cumulus mineral composition and isotopic variation trends through the layered sequence1,2. For example, in the Bushveld Complex in South Africa, abrupt increases in initial 87Sr/86Sr ratios occur at the UG-2 and Merensky pegmatoid (or ‘reef’) horizons, both of which are exploited for PGEs. We report here the results of a detailed Sr-isotope study across the Merensky unit and its immediate foot- and hangingwall-rocks which, when considered together with new ideas on processes operating in magma chambers3–5, suggest that magma mixing and post-cumulus infiltration of liquids displaced from below were important during crystallization of the Merensky pegmatoid.
- Full Text: false
- Date Issued: 1982
Distribution of Ca in highly fractionated peralkaline magmas
- Authors: Marsh, Julian S
- Date: 1976
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/132974 , vital:36914 , https://doi.org/10.1016/0012-821X(76)90106-0
- Description: Many peralkaline rhyolites and granites contain less than 0.15 wt.% CaO. In contrast, strongly fractionated peralkaline nepheline syenites and phonolites usually contain greater than 0.5 wt.% CaO. Consideration of known distributions of Ca between crystals and liquid in conjunction with crystal fractionation does not provide an adequate explanation of the contrasting levels of Ca depletion observed. Examination of the suites of late-crystallizing accessory phases in peralkaline rocks suggests that Ca is more soluble in undersaturated magmas than in over-saturated magmas. Activities for CaO in silicic and phonolitic rocks are calculated and the latter have higher CaO activities than the former and this may manifest itself in the different suites of accessory phases and levels of Ca depletion noted in natural rocks.
- Full Text: false
- Date Issued: 1976
Aenigmatite stability in silica-undersaturated rocks
- Authors: Marsh, Julian S
- Date: 1975
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/132942 , vital:36911 , https://doi.org/10.1007/BF00373333
- Description: Aenigmatite is common in many trachytes, phonolites and agpaitic nepheline syenites. Petrographic evidence suggests that the aenigmatite in these rocks arises by the reaction of Ti-magnetite with a peralkaline silica-undersaturated liquid, and it is postulated that a no-oxide field, where aenigmatite is stable, exists in alkaline undersaturated magmas.
- Full Text: false
- Date Issued: 1975