Amounts Of Olivine Research Articles

A generalized garnet-forming reaction for metaigneous rocks in the Adirondacks

A generalized reaction is presented to account for garnet formation in a variety of Adirondack metaigneous rocks. This reaction, which is the sum of five partial reactions written in aluminum-fixed frames of reference, is given by: 4(y+1+w)Anorthite+4k(y+1+2w)Olivine +4(1−k)(y+1+2w)Fe-oxide+(8(y+1) −4k(y+1+2w))Orthopyroxene = 2(y+1)Garnet +2(y+1+2w)Clinopyroxene+4wSpinel where y is a function of plagioclase composition, k refers to the relative amounts of olivine and Fe-oxide participating in the reaction, and w is a measure of silicon mobility. When mass balanced for Mg and Fe, this reaction is found to be consistent with analyzed mineral compositions in a wide range of Adirondack metaigneous rocks. The reaction applies equally well whether the garnets were formed directly from the rectants given above or went through an intermadiate stage involving the formation of spinel, orthopyroxene, and clinopyroxene.

四国中央部，三波川変成帯の藤原苦鉄質‐超苦鉄質複合岩体 Ｉ 岩石化学と造岩鉱物

The Fujiwara plutonic complex emplaced as largely solid state at the present position in the Sanbagawa metamorphic belt of central Shikoku, about 400m long and 200m wide, consists of a mafic unit and an ultramafic unit. The former appears to be in contact with the latter also tectonically. In the mafic unit the main rock-type is a Mg-rich medium-grained gabbro which is penetrated by iron-rich fine-grained gabbro. The ultramafic unit comprises serpentinites derived from dunite and wehrlite. The Sanbagawa metamorphism in epidote amphibolite facies has intensely superimposed on the whole complex. The metamorphic overprinting is characterized by associations with hornblende, zoisite, albite, garnet and chlorite. Not only wehrlitic serpentinite but also most gabbroic rocks still contain some relic clinopyroxenes. 18 whole rock chemical analyses were performed. The chemical compositions of olivines, clinopyroxenes, hornblendes, garnet and albite were determined by means of EPMA and the classic wet method. The whole rock chemistry suggests that the complex is mainly composed of meta-cumulates and the minor fine-grained gabbro represents a part of residual liquid. Probably some gabbroic rocks primarily contained a considerable amount of olivine. The ultramafics are relatively rich in iron and Al2O3. A significant gap in Mg/(Mg+Fe) ratio between olivine and its host rock is notable in the serpentinite and this fact could be indicative of the recrystallization of this mineral. The analysed iron-rich hornblendes are relatively rich in Na in M4, but they are also rich in (Na+K) in A. This peculiarity may be a reflex an iron-rich environment and a low oxidation state during metamorphism.

Petrogenesis of the Boundary intrusions in the Flin Flon area of Saskatchewan and Manitoba

The Aphebian Boundary intrusions are a group of lensoid, ultramafic to felsic rocks which occur in a N–NW trending zone 10 km long by 4 km wide centred on the town of Flin Flon. The intrusions were emplaced into Amisk metavolcanic rocks and Missi metasedimentary rocks. Field relationships, petrography, and chemical characteristics of the Boundary intrusions indicate that they are composed of three compositionally distinct, sequentially emplaced groups. From oldest to youngest, these are (1) a mafic augite- and biotite-bearing mela-dioritic group, (2) a felsic group ranging from leucodiorite to granodiorite, and (3) an olivine-bearing (wehrlite to olivine gabbro) group. The mafic group crystallized at relatively high [Formula: see text] and [Formula: see text], such that successive differentiates have increasing MgO/FeO ratios. Molar ratio diagrams clearly indicate that fractionation of augite, minor magnetite, and possibly subordinate olivine can account for the observed chemical variation of approximately 80% of this group, whereas the olivine-bearing group could only have formed by crystal fractionation of subequal amounts of olivine and clinopyroxene, and minor magnetite. The felsic group is chemically similar to the post-Missi granodioritic plutons and cannot represent SiO2-rich residual liquids produced solely by fractionation of augite and olivine from a basaltic magma.

Distinct Cooling Histories of Apollo 15 Basalts

THE ridges and valleys of the Fra Mauro region on the Moon consist of a blanket of ejecta from the impact which created the Imbrium basin approximately 1,200 km away. The estimated thickness of the blanket at the Apollo 14 landing site is 40 to 80 m (ref. 1). The rocks returned from the landing site are predominantly friable breccias which include fragments of Imbrium basalt mixed with regolith. Some of the fragments may be older breccias which were formed by impacts before the Imbrium event. The two largest basaltic rocks returned are 14310 (3,439 g) and 14053 (251 g). Basalt 14310 has an intersertal texture (grain size 0.1 to 0.5 mm) and is at least 60% plagioclase (Ab5–20Or1–4 An75–95), 35 to 40% ortho and clinopyroxene; there are also small amounts of ilmenite, troilite, ulvospinel, glass metallic iron and so on3,4. This rock is chemically similar to the feldspathic basalts which may be lunar highland material5. It has also been suggested that it may have been crystallized at Fra Mauro in near-surface conditions from a remolten pocket in the ejecta blanket6. Basalt 14053 has an ophitic texture (grain size 0.5 to 2 mm) and contains 40 to 45% plagioclase (Ab10–20Or1–5 An75–90), 45 to 50% clinopyroxene and small amounts of olivine, ilmenite, cristobalite, troilite, ulvospinel, metallic iron, glass and so on3. This rock is probably a clast from a boulder of diameter 1 to 2 m which is a breccia2. Texture and composition are similar to those of Mare basalts as returned, for example, from Oceanus Procellarum.

XXII.—The Petrology of Picritic Rocks in Minor Intrusions—a Hebridean Group

SynopsisThe results are presented of a detailed petrological reconnaissance of a group of picritic minor intrusions in the Hebrides. A substantial amount of new factual data is subjected to a unified treatment as a basis for reference and discussion. Olivine phenocrysts are not appreciably zoned and there is no evidence that they have a reaction relation with the liquid represented by the groundmass. Variations in the size and amount of olivine in individual intrusions are examined in detail and attributed to composite intrusion of differentiated material. A distinctive non-porphyritic facies found in several sills and in one dyke is chemically analyzed. Four analyses from widely separated localities establish this facies as a remarkably invariant, eucritic rock-type. The composition of the groundmass of the picritic rocks is variable and there is no evidence whatever of the participation of basaltic magma in their formation. Although no attempt is made to explain the new data in detail, a comprehensive working hypothesis is formulated. The origin of such picritic intrusions is believed to be due to selective fusion of pre-existing ultrabasic rock. Liquid more basic than normal basalt magmas can be formed by this process. Some re-precipitation of olivine may have preceded final emplacement of a magnesia-rich liquid which contained xenocrysts, mainly of olivine, from the source rock.